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Apparatus for preparing electrospunnanofibres a comparative review

J-H He13 Y Liu2 and L Xu13

A brief introduction to the development of electrospinning is given and various apparatus to

fabricate nanofibres or nanoporous materials are systematically reviewed with emphasis on the

vibration electrospinning and melt electrospinning for enlarging electrospinability siro-electro-

spinning for mimicking the spinning procedure of a spider magneto-electrospinning for

controlling the instability arising in the electrospinning process and bubble electrospinning for

mass production of nanofibres Electrospinning for producing nanoporous materials and the

nano-effect for improving the properties of nanofibres are also introduced

Keywords Nanofibres Vibration electrospinning Siro-electrospinning Magneto-electrospinning Bubble electrospinning

This paper is part of a special issue on lsquoElectrospun nanofibres and electrospun nanoporous materialsrsquo

IntroductionSeventy-five years after the discovery of the principle ofelectrospinning by Formhals in 19341 we are only justbeginning to understand the depth and complexity of hownanofibres behave unusually well in many aspects forexample remarkable strength high surface energy andsurface reactivity as well as excellent thermal and electricconductivity and to consider their roles in the scientific andeconomical revival especially of the developing world2

Although there are many methods of fabricatingnanofibres electrospinning is perhaps the simplest thecheapest and the most straightforward way to producenanofibres by forcing a polymer melt or solutionthrough a spinnerette with an electric field Owing toits ultrahigh specific surface electrospun nanofibres andelectrospun nanoporous microspheres have caughtmuch attention as the most promising material innanotechnology and served as a highly versatile plat-form for a broad rang of applications in widely differentareas such as photonic structures microfluid channels(nanofluidics) catalysis sensors medicine pharmacydrug deliver invisibility device (eg stealth plane stealthclothes) radioprotection and tissue engineering etcElectrospinning technology also raises new hope forspinal cord repair and prevention of virus infection3

Electrospinning becomes a hot topic in both academicand industrial communities According to Web of Sciencepublications on electrospinning are rocketing (see Fig 1)

Recently a number of excellent reviews and commentsdealing with specific aspects of electrospinning4 nano-fibres5 and their applications67 properties8 assemblies

and mathematical models9ndash16 of electrospinning havealready been published However a systematic andcontemporary review of the development on electrospin-ning apparatus is still lacking Recently many newapparatus have appeared to prepare for various nano-fibres such as vibration electrospinning melt electro-spinning siro-electrospinning magneto-electrospinningand bubble electrospinning A comparative review onapparatus is therefore much needed

Brief history of electrospinningtechnologyThe basic principle of electrospinning goes back to as earlyas 1882 when Rayleigh17 published an article on the thinliquid jets in electric fields and their stability The articlemight be the first literature on the electrohydrodynamicsIn 1902 Cooley18 and Morton19 issued their patents aboutapparatus which is now called electrospraying forelectrically dispersing fluids Though their inventions werenot called electrostatic spinning or electrospinning thefundamental principle is the same as that of electrospin-ning patented by Formhals1 in 1934 The essentialdifference between electrospraying and electrospinning isthat the former used a low molecular weight solutionwhile the latter used a polymer solution (see Fig 2)

Formhals was considered as the originator of electro-spinning1 because he presented systematically theprocess for the production of artificial fibres frompolymer solutions under the application of a highelectrical field in his 1934 patent The schematic drawingof Formhalsrsquo apparatus was shown in Fig 3 He calledthe process as the lsquoelectrical spinningrsquo of fibres

From the mid-1940s to the early 1990s Formhalsrsquoinvention did not catch much attention In 1971Baumgarten20 obtained experimentally the acrylicmicrofibres via electrostatic spinning The term lsquoelectro-spinningrsquo was first used in Baumgartenrsquos article20

The research on electrospinning took a big leap duringthe 1990s with the development of nanoscience and

1Modern Textile Institute Donghua University 1882 Yanrsquoan Xilu RoadShanghai 200051 China2School of Textiles Tianjin Polytechnic University 63 Chenglin RoadTianjin 300160 China3National Engineering Laboratory of Modern Silk Soochow UniversitySuzhou 215021 China

Corresponding author email jhhedhueducn

2010 Institute of Materials Minerals and MiningPublished by Maney on behalf of the InstituteReceived 1 October 2008 accepted 2 December 2009DOI 101179026708310X12798718274430 Materials Science and Technology 2010 VOL 26 NO 11 1275

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technology Up to now electrospun nanofibres havebecome one of the fast moving fields of science andtechnology that have undergone a variable revolution overthe last two decades leading to the major advances in theunderstanding and application at nano- and microlevels

Classical electrospinning apparatusGenerally the simplest electrospinning apparatus consistsof a syringe with a metal needle a syringe pump a DChigh voltage generator and a metal plate collector21ndash28

Figure 4 shows a schematic diagram of this traditional

electrospinning apparatus which can be divided into fourregions including solution storage and feeding systemspinneret spinning area and collector system The syringeand the syringe pump make up of the solution storage andfeeding system A metal needle y1 mm in diameter isusually used as the spinneret A charged pendent drop ofpolymer solution at the tip of the spinneret is elongated toa conical shape which is known as the Taylor cone Agrounded collector was placed beneath the electrospinningspinneret A DC high voltage generator15 though fewexperiments were conducted successfully using an ACgenerator is employed and usually connected with thespinneret and produces an electric field in spinning areabetween the spinneret and the collector Once the electricfield exceeds the critical value needed to overcome thesurface tension a fluid jet ejects from the apex of the cone

The traditional electrospinning is subject to instabilityin the electrospinning process low electrospinabilitylow properties compared with nature fibres and lowproductivity therefore to overcome the shortcomingsnew apparatus have appeared

Vibration electrospinningThe traditional electrospinning has to overcome thesurface tension of the polymer solution while thesurface tension depends strongly upon the viscosity ofthe polymer which might be too high to be electrospuninto nano- or microfibres

The threshold voltage scales with the surface tensionin the form

2 Schematic of two processes electrospinning and electrospraying

1 Rise of publications on electrospinning indexed in Web

of Science

3 Schematic drawing of apparatus for production of artificial fibres by Formhals

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Ethresholdc1=2

where Ethreshold is the critical voltage needed for theejection of the jet from the Taylor cone and c is thesurface tension of the solution which is the intrinsicproperty of the solution and strongly depends upon thesolution viscosity

A facile strategy for preparing electrospun nanofibresby vibration technology is therefore suggested by Wanet al26 Vibration is applied to the polymer solutions ormelts leading to a dramatic reduction in viscosity as aresult a moderate voltage is needed to produce finenanofibres that are commonly observed during conven-tional electrospinning procedure at an elevated voltageThe novel strategy produces finer nanofibres than thoseobtained without vibration and expands the range ofthis technique by making fibres from macromoleculeswhich cannot be electrospun by traditional ways

Effect of viscosity on diameter of electrospunfibre2629ndash31

Even under very high voltage not every solution can beelectrospun and the jet might break up into droplets asa result of the surface tension in the case of smallmolecules with low viscosity liquids of small molecules(known as electrospraying) For high viscosity liquids ofmacromolecules the jet does not break up but travels asa jet to the grounded target (known as electrospinning)For very high viscous solutions or melts the process canbe used in electro-extrusion in plastic industry32

The experimental data and theoretical analysis29 showthat the solution viscosity has great effects on thediameter of electrospun fibres initiating droplet shapeand the jet instability The increase in solution viscosityhas been associated with the production of largerdiameter fibres and it was shown that the fibre diameterdepends allometrically on the solution viscosity in theform29

dga (1)

where d is the average diameter of the electrospun fibreg is the viscosity and a is the scaling exponent Theexponent value might differ between different polymers

For acrylic solution Baumgarten20 found that thescaling exponent is y12

Effect of vibration on viscosityVibration technology33ndash35 has been introduced intopolymer processing for many years Initially it was onlyapplied in researches for polymer melt viscosity mea-surement Subsequently the principle of melt vibrationwas introduced into practical applications includinginjection moulding extrusion and compression mould-ingthermoforming for the reduction of viscosity tolowering processing temperature and pressure to elim-inate melt defects and weld lines and for the enhance-ment of mechanical properties by modifying theamorphous and semicrystalline texture and orientationalstate

Ibar36 observed the effect of low frequency vibrationduring processing for polymethyl methacrylate(PMMA) (Fig 5) The result showed that increasingfrequency makes the solution viscosity decreased Thelower the frequency the greater the differences ofviscosity at various temperatures At low frequenciesthe solution viscosity decreased quickly while atfrequencies ranging from 100 to 500 rad s21 theincrease in frequency was not obvious However

4 Schematic diagram of traditional electrospinning apparatus

5 Solution viscosity g of PMMA versus vibration

frequency v (Ref 36)

He et al Apparatus for preparing electrospun nanofibres

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frequencies 200 rad s21 would decrease the viscositygradient due to temperature Ibar explained the effect ofvibration frequency and amplitude on melt viscosity interms of shear thinning criteria

When a vibrating force is applied to a concentratedand entangled polymer solution or melt the weak vander Waals force connecting with macromoleculesbecomes weaker and the entanglement is relaxed sothat the viscous force between the macromoleculesdecreases dramatically resulting in the reduction inviscosity

According to the experimental observation thefollowing allometric scaling can be obtained

gvb (2)

where b is a scaling exponent that varies with thecharacteristics of polymers For PMMA solution at239uC gv225 can be obtained from the experiment byIbar36

Application of vibration technology to polymerelectrospinningWhen an additional vibrating force is applied toconducting polymer solutions or melts a dramaticreduction in viscosity occurs

The voltage is applied to polarise dielectrics where thecharges are not completely free to move but the positiveand negative charges that compose the body may bedisplaced in relation to one another when a vibratingforce is applied (see Fig 6)

Owing to its lower viscosity a relatively lower voltageis needed to eject jet from the spinnerette

From equations (1) and (2) the following power lawcan be obtained29

dvd (3)

where d is the scaling exponent that varies amongdifferent polymers

The scheme of the vibration electrospinning set-upwas shown in Fig 7 The needle tip was connected to aDC high voltage generator via an alligator clip Anultrasonic generator was used to provide vibration invibration electrospinning set-up

The main advantages of vibration electrospinning are

(i) dramatic reduction in viscosity so that the jetlength can be controlled by frequency

(ii) finer fibres can be produced compared withtraditional electrospinning apparatus under thesame conditions or the patented apparatus caneven spin such fibres whereas the traditionalapparatus cannot or can spin fibres with thesame diameter at much lower voltage

(iii) the patented electrospinning process as that inpolymer extrusion can enhance the mechanicalproperties of electrospun fibres by modifying theamorphous and semicrystalline texture andorientational state

Thermo-electrospinning apparatusIt is well known that most melting polymers can beelectrospun into nanofibres however research efforts onmelt electrospinning are relatively scarce37 largelybecause a melt electrospinning apparatus is complexand expensive for the heater and the cooler are a must inthis apparatus While it is not as prevalent as polymersolution electrospinning melt electrospinning eliminatesthe need for organic solvents and has the potential toincrease throughput due to no loss in mass by solventevaporation37

Melt blowing process is a process which producesmicrofibres directly from polymer resin Melt blowing isused commercially as a one step process for convertingpolymer resin directly into a non-woven mat of fibres Inthe process a very high velocity gas impacts a moltenpolymer stream as illustrated in Fig 8 As a result thepolymer stream is attenuated rapidly by the highvelocity hot air and the resultant fibres are collectedupon a screen as a non-woven mat The fibre diametercan be reduced from 700 to 1 mm in 50 ms and thisdramatic change in diameter significantly weakens itsmechanical strength and affects other fibre character-istics such as pore size air permeability loft drape feeletc

Controlling stability by temperatureIn order to control the solvent evaporation a furnace isadded as illustrated in Fig 9 In the furnace thetemperature field can be adjusted and the temperature

a without electric field b with electric field plus vibrating force which leads to high molecular orientation due to electro-nic force and high mechanical properties

6 Effect of electronic field on entangled macromolecule

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near the orifice is lower than that of the melts and higherthan that near collector

Owing to the gradient of temperature cold air flowsup along the furnace wall and comes down along thefibre and the low velocity gas further impacts thepolymer jet as illustrated in Fig 9 Convection currentsoccurred in the upper half of the furnace and wereenhanced by the injection of comparatively cool feed-stock down the axis It is believed that the gas movedupward beside the furnace walls and downward in thecentre of the tube As a result the polymer jet is furtherattenuated and the jet length can be increased so thatthe instability can be controlled

It would be more effective if a set-up can be devised bytaking into account the controllable temperature (Fig 9)

High temperature can also dramatically reduce theviscosity and the electrospinning of polymer melts canoffer an advantage over solution electrospinning38

Figure 10 illustrates an apparatus of melt electrospin-ning system The effects of various melt electrospinningparameters on the morphology and fibre diameter ofpolypropylene of different tacticities were studied39

Recently He et al40 suggested some novel melt electro-spinning apparatus such as vibrationndashmelt electrospinningand aero-melt electrospinning as shown in Figs 11ndash13

Hot air is applied in aero-electrospinning (seeFig 13) Hot air further attenuates the filamentsmaking the electrospun fibres ever smaller Melts areextruded by static electronic force through small orificeinto convergent streams of hot air that rapidly attenuatethe charged jet into small diameter fibres The airstreams can also control the instability in the electro-spinning process and transport fibres to a collectorwhere they bond at fibrendashfibre contact points to producea cohesive non-woven web

Additionally McCann et al41 developed a meltcoaxial electrospinning similar to coelectrospinning insolution electrospinning for the encapsulation of solidmaterials and fabrication of phase change nanofibresOgata et al42 presented a new melt electrospinningsystem equipped with a laser melting device Theyemployed the laser beams from three directions toirradiate the end of rod-like sample The fibres obtainedwere collected onto a grounded rotating disc

7 Schematic of vibration technology in polymer electro-

spinning This apparatus was patented (China Patent

2004200205963) To use this principle to prepare elec-

trospun fibres transfer agreement must be made29

8 Schematic of thermo-electrospinning

9 Schematic of electrospinning process with controllable

temperature field

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Magneto-electrospinningAs it is well known that during the electrospinningprocess the charged jet is of intrinsic instability thatleads to the uneven construction of nanofibres and wasteof most energy which otherwise can be used to furtherstretch the jet into even smaller fibres (Fig 14) Theinstability also leads to low molecular orientation as aresult low mechanical properties arise

Our group suggests a completely novel approach tocontrol the instability in electrospinning and thetechnology is termed as megnetio-electrospinning4043

It can be concluded that the magnetic approach is themost effective and economical way to control instabilityand improve the mechanical properties as well

If a magnetic field is applied in the electrospinningprocess as illustrated in Fig 15 an Ampere force isgenerated due to the current in the polymer jet

FI

~LII

BI

(4)

where I is the current floating inside the conductor B isthe value of magnetic field induction and L is theconductor length

The resultant force of electric force and the viscousforce of the jet flow enlarges the whipping circle If amagnetic field is applied in the electrospinning theproblem can be completely overcome The current in thejet under the magnetic field produces a centripetalforce ie the direction of the Ampere force is alwaystowards the initial equilibrium point (see Fig 15)leading to the shrinking of the radius of whipping circleAs a result the stability condition is enormouslyimproved

The magnetic force is perpendicular to the velocity soit contributes no work to the moving jet the shrunkencircle means less energy waste in the instability processand the saved energy is used to increase the kineticenergy of the moving jet According to the conservationof mass pr2ru5Q thus r2lt1u the radius becomesmuch smaller than that without magnetic fieldMacromolecule chains of polymer jet are subject to thecoupled forces of electronic force and magnetic forceand arranged again (see Fig 16)

The produced magnetic force can ameliorate the innerstructure of macromolecules resulting in a remarkableamelioration of the strength of nanofibre

Siro-electrospinningTwo strand spun or Sirofil yarns4445 have now beenwidely used in the worsted industry The strands aretexturised to improve the bulk of the resultant yarnswhich have been demonstrated to possess more desirableproperties For example the weaveability of the fabricformed by the Sirofil yarns is significantly improved overits counterpart yarns Dragline silk is made of manynanofibres similar to siro-spinning To mimic the spider

10 Schematic illustration of set-up for melt electrospinning

12 Vibrationndashmelt electrospinning40

13 Aero-electrospinning40

11 Melt electrospinning does not require solvent tempera-

ture of melts can be controlled by electronic heater40

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spinning (see Fig 17) siro-electrospinning is created(see Fig 18) Li et al patented a conjugate electrospin-ning apparatus46ndash48 as illustrated in Fig 19 Two ormore fibres are combined together by weak staticelectronic force An apparatus is devised where a twistis added (see Fig 18) causing the fibres to have atendency to twist and crimp Fabrics so produced canimprove its mechanical characteristics furthermore itsstrength toughness softness and feel can be improveddramatically

The present technology can also be taken in conjunc-tion with other ones For example the vibrationtechnology can be added

Using this technique it is now possible to spinconductive polymer fibres under ambient conditions bydepositing nanometre metal particle in the polymersolutions or melts If some titanium dioxide is added inthe solutions or melts the electrospun fibres areself-clearing

Spider spinningAfter 400 million years of evolution nature endowsspiders with genius of spinning flexible lightweightfibres which have the strongest strength in the world atleast five times stronger by weight than steel and haveremarkable toughness and elasticity4950 Even in themodern times it is difficult to synthesise a materialhaving advantages of strength and toughness exceptcarbon nanotube fibres which is spun from solution invery hot temperature or pressure51 while the spider silksare produced at room temperature and from aqueoussolutions Spider silk is the only natural material thatcombines the properties unmatched by any knownsynthetic high performance fibres

Spider silks are protein based lsquobiopolymerrsquo filamentsor threads secreted by specialised epithelial cells asconcentrated soluble precursors of highly repetitiveprimary sequences52 Many experiments have beenconducted and much research has focused on gene

14 Instability in electrospinning most energy was wasted

due to instability

a application of magnetic field to electrospinning b Ampere force in electronic jet induced by magnetic field15 Effect of magnetic field on electrospinning43

16 Macromolecules under coupled forces of electronic

force and magnetic force

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sequencing of spider and a biomimicry technology hasbeen developed53ndash55 however theoretical analysis is notyet dealt with it and our understanding of themechanism of spider spinning is rare and primitive Ifthe mystery in its mechanism can be solved then themechanism can be applied to synthetic high performancefibres such as electrospun fibres with combination ofgreat strength and stretch The devised method must bemuch economical so this could be the beginning of anew materials revolution

Mystery in spider spinning processIt is still an enigma how a spider produces a silk whichhuman needs ten thousands voltage to do One possibleanswer is that a spider applies sufficiently the nano-effect of bubble dynamics

The pressure difference between the inside and outsideof a bubble depends upon the surface tension and theradius of the bubble

For a bubble with two surfaces providing tension thenet upward force on the top hemisphere of the bubble isjust the pressure difference times the area of theequatorial circle

Fupward~pr2(PiPo) (5)

The surface tension force downward around circle istwice the surface tension times the circumference sincetwo surfaces contribute to the force

Fdownward~2pr(2T) (6)

This gives

T~1

4r(PiPo) (7)

Hendricks et al56 calculated the minimum sprayingpotential of a suspended hemispherical conductingdrop in air as

Ethreshold~300 20pcreth THORN1=2(8)

Buchko et al57 modified equation (8) in the form

E2threshold~

4H2

L2( ln

2L

R

3

2)0117pcR (9)

Ethresholdc1=2 (10)

where Ethreshold is the critical voltage needed for theejection of the jet from the Taylor cone H is theseparation distance between the needle and the collectorL is the length of the needle (or capillary) R is the radiusof the needle and c is the surface tension of the solution

From Fig 20 it can be clearly observed that draglinesilk is made of many nanofibres with a diameter ofy20 nm Therefore it can be assumed that a bubble canbe produced at the apex of each tube with a diameter ofy20 nm According to equation (7) the surface tensionof such bubbles is extremely small thus it can be easilyovercome either by the body weight of spider or tensioncreated by the rear legs

Most natural spider silk is only 25ndash4 mm in diameterand the weight of most spiders varies from 90 to1500 mg Consider a dragline silk with a diameter of361026 m which consists of many nanoscale fibrils witha diameter of y20 nm (see Fig 20) Therefore thenumber of nanofibres in the assembly can be estimatedwhich reads

n~(3|106

20|109)2amp2|104 (11)

It means that there are tens of thousands of nanofibresin the assembly

Similar to the HallndashPetch relationship the nanofibrestrength depends upon the fibre diameter in nanoscale(from few nanometres to tens of nanometres)

t~t0zkt

d1=2(12)

where kt is the fitting parameters (material constants) t0

is the strength of the bulk material and d is the fibrediameter as shown in Fig 21

Comparing the strength of a single fibre tSF with thediameter of 361026 m with the strength of a draglineassembly tDA consisting of 26104 nanofibres with thediameter of 2061029 m the following can be estimated

tDAt0

tSFt0

~24|106 (13)

Extraordinarily high strength of a dragline assembly ispredicted compared with a single fibre with the samesection area and same material

The finding shows that it is a challenge to developtechnologies capable of preparing for nanofibres within50 nm

It means that there are tens of thousands of nanotubesin spinneret of spider A weight of G51024 kg of aspider is assumed In the spinning process the spider can

17 Sirofil technology to improve mechanical properties of

mother fibres

18 Siro-electrospinning40

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use its weight to overcome the surface tension of allbubbles produced at apex of nanotubes which requires

T~G (14)

from which the pressure difference between the insideand outside of a bubble can be determined which reads

DP~PiP0~4G

nr~

4|104

(2|104)(20|109)~1 N m2 (15)

Extremely small force is needed in the spider spinning

Bubble electrospinningFrom the hint of the mechanism of the spider spinning anew approach is designed to overcome the bottleneck inpresent electrospinning technology which is to minimisethe surface tension of the electrospun solutions mimick-ing the spider spinning Compressed gas is widely used intextile industry59 and this technology will be used toreduce the surface tension Our system60ndash62 consists of avertical solution reservoir with a gas tube feeding fromthe bottom in which a metal electrode is fixed along thecentreline of the tube and a grounded collector over thereservoir (see Fig 22) It has been found that manysmall bubbles with different sizes were produced on thesolution surface

The mechanism of the new electrospinning process isdeceptively simple in the absence of an electric fieldthe aerated solution forms various bubbles on thesurface When an electric field is present it inducescharges into the bubble surface and these quickly relaxto the bubble surface The coupling of surface chargeand the external electric field creates a tangential stressresulting in the deformation of the small bubble into aprotuberance induced upward directed reentrant jetOnce the electric field exceeds the critical value neededto overcome the surface tension a fluid jet ejects fromthe apex of the conical bubble (see Fig 23) Thethreshold voltage needed to overcome the surfacetension depends upon the size of the bubble and inletair pressure

Although the maximal or minimal size of a bubblemight depend upon the solution viscosity the surfacetension of bubbles is independent of properties of thespun solutions such as viscosity which is the mainobstruction in traditional electrospinning Temperaturecan be used to adjust the bubble sizes in practicalapplication of the new electrospinning process The keyadvantages of this electrospinning process used herecompared with the traditional electrospinning processare that it can produce smaller nanofibres withoutrequirement of nozzles which are the main shortcomingof the traditional electrospinning technology further-more in our method with a relatively low voltagemillions of protruded bubbles can be easily producedwhich can be electrospun into nanofibres simulta-neously and electrospinnability does not strongly

21 E modulus of nanofibres redrawn according to

experimental data by Gu et al58 dots are experimental

data

19 Schematic set-up used for conjugate electrospinning experiments46ndash48

20 Spider spinning process reproduced with the permis-

sion of Dennis Kunkel Microscopy Inc The diameter

of a single nanofibre is y20 nm its spinneret on the

posterior portion of the abdomen of spider consists

of millions of nanoscale tubes and a bubble can be

produced at apex of each nanotube The surface ten-

sion of each bubble is extremely small such that it

can be spun into nanofibres with an awfully small

force either by the body weight of spider or tension

created by the rear legs

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depend upon the solution viscosity overcoming com-pletely the main shortcoming of the traditional electro-spinning The average diameter of nanofibres can beeasily controllable and the minimum diameter in ourexperiment reached as small as 50 nm (see Fig 24)

This new technology is of critical importance for thenew generation of electrospinning especially forthe specialists in design manufacturing and usingnanofibres

Electrospinning dilation and electrospunnanoporous microspheresDuring the electrospinning process the charged jet isaccelerated by a constant external electric field and thespinning velocity probably reaches maximum andperhaps exceeds the velocity of sound in air in a veryshort time before the spinning becomes instableAccording to the conservation of mass equation

pr2ru~Q (16)

where r is the radius of the jet u is the velocity Q is theflowrate and r is the density The radius of the jetdecreases with increasing velocity of the incompressiblejet When the velocity reaches its maximum the radius

of the jet is minimal and macromoleculars of thepolymers are compacted together tighter and tighter asillustrated in Fig 25 There must exist a critical minimalradius rcr for all electrospun jet r(rcr for continuousultrafine fibres and the critical maximal velocity is

ucr~Q=prr2cr However the velocity can exceed this

critical value ucr if a higher voltage is applied

When the radius of the jet reaches the critical valuer5rcr and the jet speed exceeds its critical value uucrin order to keep conservation of mass equation the jetdilates by decreasing its density leading to porosity ofthe electrospun fibres (see Figs 26 and 27) which iscalled electrospinning dilation

Nanoporous microspheres are especially useful forinvisibility device (eg stealth plane)6413 and drugdeliver6566

New electrospinning apparatus for massproduction of nanofibresIn recent development and application of electrospunnanofibres one key bottleneck is the lack of capacity tothe mass production of nanofibres It is limited for atraditional electrospinning apparatus to increase theability of nanofibre throughout because there are manyseemingly insurmountable difficulties One is the smalldiameter (about 01ndash2 mm) of the spinneret and the lowflowrate (from 01 to dozens of mm h21) of the solutionIf the diameter is enlarged the suspended droplet may

22 Experimental set-up of aerated solution electrospin-

ning This principle to prepare for nanoproducts was

patented (China Patent 2007100364474) To use this

principle to prepare for nanoporous products transfer

agreement must be made63

23 Ejected jets captured by digital camera63

24 Minimum diameter of nanofibres was 50 nm (Ref 63)

25 Macromolecular chains are compacted during electro-

spinning

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not be formed as a stable Taylor cone at the orificebecause of the gravity and other factors and so is theincreasing flowrate Unlike the traditional dry spinningprocess or wet spinning process in which the jet isextruded from the spinneret with high speed thetraditional electrospinning process depends on theelectrostatic force which just provides a stable and slowliquid flow to form a stable Taylor cone at the tip of thespinneret rather than the extrusion pressure Thespinning speed is much less than that of the dry spinningor wet spinning The other shortcomings are easilyjammed difficult clean etc

As a result the possible technique of the use oftraditional electrospinning apparatus to produce nano-fibres of mass production is to fit the multiple spinneretstogether with a line or matrix pattern Many researchersadopted this technique to produce nanofibres from theearly period of the invention of this technology to nowIt is prevailing until Theron et al67 found bothexperimentally and in the simulations that the jets werepushed away from their neighbours by the Coulombicforces applied by the latter A non-uniform non-wovenmat would be obtained as an immediate consequence ofthis phenomenon Fang et al68 once reported that theysolved the problem of the mutual Coulombic interac-tions by the incorporation of secondary electrodes toisolate the electric field distribution of the primaryelectrode spinnerets But the problem of the smalldiameter of the spinneret and the low flowrate still

limited the fibre output Dosunmu et al69 suggested anew multiple porous tube apparatus in which thepolymer solution was electrified and pushed by airpressure through the walls of a large diameter cylindricalporous tube (see Fig 28a) Multiple jets formed on theporous surface and electrospun into nanofibres in theirprocess However the existing problem of this processmight be also the non-uniform fibre mat (see Fig 5 inRef 69)

In order to eliminate the problems of increasing thenanofibres of mass production of traditional electro-spinning apparatus some researchers demonstratedvarious novel electrospinning apparatus in recent yearsYet these apparatus different as they seem have onething in common The Taylor cone was formed on thefree surface of the solution and the jets were ejectedupward Yarin and Zussman70 proposed an upwardneedleless electrospinning which consisted of a two layersystem with the lower layer being a ferromagneticsuspension and the upper layer a polymer solution (seeFig 28b) In the process many vertical spikes ofmagnetic suspension perturbed the interlayer interfaceas the Taylor cones Though the production rate ishigher than that of traditional electrospinning processthere are some flaws such as not easy operation of twolayers liquids

So far only one electrospinning process Nanospiderspinning technology is claimed to be a commercialsuccess in the mass production of nanofibres all over theworld The principle of Nanospider is to use a cylinderto form the fibres The cylinder is partially immersed in

26 Electrospinning dilation phenomenon of poly

(butylenes succinate) (PBS) electrospun fibres64

27 Electrospun nanoporous microspheres where kind of

traditional Chinese drugs called lsquoYunnan Baiyorsquo was

used as additive65

a Dosunmursquos apparatus b upward needleless electro-spinning

28 New electrospinning apparatus for produing mass

production of nanofibres

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the polymer solution and as it rotates the polymersolution is carried to the top part of the cylinder whereTaylor cones are created The Taylor streams are formednext to each other throughout the entire length of thecylinder resulting in the high production capacity of thisapparatus71

The above novel approaches are exciting for theresearchers and the engineers When we cheer and applaudfor these much progress of electrospinning however a newproblem caused by the opened liquid surface has risen upIn every electrospinning process the volatilisation of thesolvent from the flying jet is a must In the above newupward electrospinning apparatus the free liquid surfacecan be used to create many Taylor cones and increase thethroughput of nanofibres at the same time can befacilitated to the escape of the volatilising solvent Withthe process proceeding the viscosity of the polymersolution will increase which will introduce some newproblems such as increasing the difficulty of spinning andchanging the properties of the solution

ConclusionsIt is clear that an understanding of the electrospinningand its nanofibres was greatly advanced by thenanoscience and technology in the 1990s and by thesubsequent development of the many effective applica-tions available in many fields today Remarkableadvances in electrospinning technology have beenpossible due to developments in modern technologyVarious novel and useful electrospinning techniques arereviewed in this work according to their differentmodifications Some modified apparatus were used toproduce different nanofibre mats with special structuresfor some special applications and some for massproduction of nanofibres Most of the advancesachieved to date have been made in the collector systemof the traditional electrospinning apparatus but otherparts such as solution storage and feeding systemspinneret and spinning area have been studied as wellMore recently emphasis has been placed on creatingnew apparatus to mass production of nanofibres Newtechniques have been designed and improved and moreelectrospinning apparatus have been developed to betterproduce nanofibres

Collectively the studies about electrospinning andnanofibres carried out so far have furnished a usefuldatabase for the electrohydrodynamics and some otherfields Although this field is in its infancy it is becomingincreasingly clear that there are multiple forms of newtechniques Identifying the similarities and differences invarious apparatus will enhance our understanding ofthis technology The next decades are expected to notonly enrich the existing knowledge but also significantlyadvance the technology into the commercial domain

Acknowledgements

The present work is supported financially by NationalNatural Science Foundation of China under Grant Nos10802021 and 10972053 National Natural ScienceFoundation of Shanghai under Grant No08ZR1400300 Shanghai Rising-Star Program underGrant No09QA1400100 Key Project of ChineseMinistry of Education No210006 National ScienceFoundation of Tianjin No10JCYBJC02200 Tianjin

Universities Science and Technology DevelopmentFund Project No20080322

References1 A Formhals lsquoProcess and apparatus for preparing artifical

threadsrsquo US Patent 1975504 1934

2 M S El Naschie Chaos Solitons Fract 2006 30 769ndash773

3 J-H He Chaos Solitons Fract 2008 38 1390ndash1393

4 J Lyons and F Ko Polym News 2005 30 170ndash178

5 D Li and Y-N Xia Adv Mater 2004 16 1151ndash1170

6 Z-M Huang Y-Z Zhang M Kotaki and S Ramakrishna

Compos Sci Technol 2003 63 2223ndash2253

7 Q P Pham U Sharma and A G Mikos Tissue Eng 2006 12

1197ndash1211

8 E P S Tan and C T Lim Compos Sci Technol 2006 66 1102ndash

1111

9 J-H He and H-M Liu Nonlinear Analy 2005 63 e919ndashe929

10 J-H He and Y-Q Wan Polymer 2004 45 6731ndash6734

11 J-H He Y-Q Wan and J-Y Yu Polymer 2005 46 2799ndash2801

12 J-H He Y-Q Wan and J-Y Yu Fibers Polym 2008 9 140ndash

142

13 J-H He L Xu Y Wu and Y Liu Polym Int 2007 56 1323ndash

1329

14 J-H He Y-Q Wan and J-Y Yu Int J Nonlinear Sci 2004 5

243ndash252

15 J-H He Y Wu and N Pan Int J Nonlinear Sci 2005 6 243ndash

248

16 Y-Q Wan J-H He and J-Y Yu Iran Polym J 2006 15 265ndash

268

17 L Rayleigh Philos Mag 1882 14 184ndash186

18 J F Cooley lsquoApparatus for electrically dispensing fluidsrsquo US

Patent 692631 1902

19 W J Morton lsquoMethod of dispensing fluidsrsquo US Patent 705691

1902

20 P K Baumgarten J Colloid Interface Sci 1971 36 71ndash79

21 J-H He and Y Liu J Polym Eng 2008 28 101ndash114

22 Y Liu J-H He L Xu and J-Y Yu Int J Electrospun Nanofibers

Appl 2007 1 7ndash15

23 Y Liu J-H He and J-Y Yu Fibres Text East Eur 2007 15

30ndash33

24 X-H Qin Y-Q Wan J-H He J Zhang J-Y Yu and

S-Y Wang Polymer 2004 45 6409ndash6413

25 D-H Shou and J-H He J Polym Eng 2008 28 115ndash118

26 Y-Q Wan J-H He Y Wu and J-Y Yu Mater Lett 2006 60

3296ndash3300

27 S-Q Wang J-H He and L Xu Polym Int 2008 57 1079ndash1082

28 Y Wu J-H He L Xu and J-Y Yu Int J Electrospun

Nanofibers Appl 2007 1 1ndash6

29 J-H He Y-Q Wan and J-Y Yu Int J Nonlinear Sci 2004 5

253ndash262

30 Y-Q Wan J-H He L Xu and J-Y Yu Int J Electrospun

Nanofibers Appl 2007 1 17ndash28

31 Y-Q Wan J-H He J-Y Yu and Y Wu J Appl Polym Sci

2007 103 3840ndash3843

32 J Zhang Y-Q Wan J-H He and Z-Y Zhu China Patent

2004100156853 2004

33 B H Bersted J Appl Polym Sci 2003 28 2777ndash2791

34 A I Isayev C M Wong and X Zeng Adv Polym Technol 2003

10 31ndash45

35 H Wu S Guo G Chen J Lin W Chen and H Wang J Appl

Polym Sci 2003 90 1873ndash1878

36 J P Ibar Polym Eng Sci 1998 38 1ndash20

37 S Sell C Barnes M Smith M McClure P Madurantakam

J Grant M McManus and G Bowlin Polym Int 2007 56 1349ndash

1360

38 P D Dalton K Klinkhammer J Salber D Klee and M Moller

Biomacromolecules 2006 7 686ndash690

39 J Lyons C Li and F Ko Polymer 2004 45 7597ndash7603

40 J-H He Y-Q Wan and L Xu Chaos Solitons Fract 2007 33

26ndash37

41 J T McCann M Marquez and Y Xia Nano Lett 2006 6 2868ndash

2872

42 N Ogata S Yamaguchi N Shimada G Lu T Iwata K Nakane

and T Ogihara J Appl Polym Sci 2007 104 1640ndash1645

43 Y Wu J-Y Yu J-H He and Y-Q Wan Chaos Solitons Fract

2007 32 5ndash7

44 J-H He Y-P Yu J-Y Yu W-R Li S-Y Wang and N Pan

Text Res J 2005 75 181ndash184

He et al Apparatus for preparing electrospun nanofibres

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technology Up to now electrospun nanofibres havebecome one of the fast moving fields of science andtechnology that have undergone a variable revolution overthe last two decades leading to the major advances in theunderstanding and application at nano- and microlevels

Classical electrospinning apparatusGenerally the simplest electrospinning apparatus consistsof a syringe with a metal needle a syringe pump a DChigh voltage generator and a metal plate collector21ndash28

Figure 4 shows a schematic diagram of this traditional

electrospinning apparatus which can be divided into fourregions including solution storage and feeding systemspinneret spinning area and collector system The syringeand the syringe pump make up of the solution storage andfeeding system A metal needle y1 mm in diameter isusually used as the spinneret A charged pendent drop ofpolymer solution at the tip of the spinneret is elongated toa conical shape which is known as the Taylor cone Agrounded collector was placed beneath the electrospinningspinneret A DC high voltage generator15 though fewexperiments were conducted successfully using an ACgenerator is employed and usually connected with thespinneret and produces an electric field in spinning areabetween the spinneret and the collector Once the electricfield exceeds the critical value needed to overcome thesurface tension a fluid jet ejects from the apex of the cone

The traditional electrospinning is subject to instabilityin the electrospinning process low electrospinabilitylow properties compared with nature fibres and lowproductivity therefore to overcome the shortcomingsnew apparatus have appeared

Vibration electrospinningThe traditional electrospinning has to overcome thesurface tension of the polymer solution while thesurface tension depends strongly upon the viscosity ofthe polymer which might be too high to be electrospuninto nano- or microfibres

The threshold voltage scales with the surface tensionin the form

2 Schematic of two processes electrospinning and electrospraying

1 Rise of publications on electrospinning indexed in Web

of Science

3 Schematic drawing of apparatus for production of artificial fibres by Formhals

He et al Apparatus for preparing electrospun nanofibres

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Ethresholdc1=2

where Ethreshold is the critical voltage needed for theejection of the jet from the Taylor cone and c is thesurface tension of the solution which is the intrinsicproperty of the solution and strongly depends upon thesolution viscosity

A facile strategy for preparing electrospun nanofibresby vibration technology is therefore suggested by Wanet al26 Vibration is applied to the polymer solutions ormelts leading to a dramatic reduction in viscosity as aresult a moderate voltage is needed to produce finenanofibres that are commonly observed during conven-tional electrospinning procedure at an elevated voltageThe novel strategy produces finer nanofibres than thoseobtained without vibration and expands the range ofthis technique by making fibres from macromoleculeswhich cannot be electrospun by traditional ways

Effect of viscosity on diameter of electrospunfibre2629ndash31

Even under very high voltage not every solution can beelectrospun and the jet might break up into droplets asa result of the surface tension in the case of smallmolecules with low viscosity liquids of small molecules(known as electrospraying) For high viscosity liquids ofmacromolecules the jet does not break up but travels asa jet to the grounded target (known as electrospinning)For very high viscous solutions or melts the process canbe used in electro-extrusion in plastic industry32

The experimental data and theoretical analysis29 showthat the solution viscosity has great effects on thediameter of electrospun fibres initiating droplet shapeand the jet instability The increase in solution viscosityhas been associated with the production of largerdiameter fibres and it was shown that the fibre diameterdepends allometrically on the solution viscosity in theform29

dga (1)

where d is the average diameter of the electrospun fibreg is the viscosity and a is the scaling exponent Theexponent value might differ between different polymers

For acrylic solution Baumgarten20 found that thescaling exponent is y12

Effect of vibration on viscosityVibration technology33ndash35 has been introduced intopolymer processing for many years Initially it was onlyapplied in researches for polymer melt viscosity mea-surement Subsequently the principle of melt vibrationwas introduced into practical applications includinginjection moulding extrusion and compression mould-ingthermoforming for the reduction of viscosity tolowering processing temperature and pressure to elim-inate melt defects and weld lines and for the enhance-ment of mechanical properties by modifying theamorphous and semicrystalline texture and orientationalstate

Ibar36 observed the effect of low frequency vibrationduring processing for polymethyl methacrylate(PMMA) (Fig 5) The result showed that increasingfrequency makes the solution viscosity decreased Thelower the frequency the greater the differences ofviscosity at various temperatures At low frequenciesthe solution viscosity decreased quickly while atfrequencies ranging from 100 to 500 rad s21 theincrease in frequency was not obvious However

4 Schematic diagram of traditional electrospinning apparatus

5 Solution viscosity g of PMMA versus vibration

frequency v (Ref 36)

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frequencies 200 rad s21 would decrease the viscositygradient due to temperature Ibar explained the effect ofvibration frequency and amplitude on melt viscosity interms of shear thinning criteria

When a vibrating force is applied to a concentratedand entangled polymer solution or melt the weak vander Waals force connecting with macromoleculesbecomes weaker and the entanglement is relaxed sothat the viscous force between the macromoleculesdecreases dramatically resulting in the reduction inviscosity

According to the experimental observation thefollowing allometric scaling can be obtained

gvb (2)

where b is a scaling exponent that varies with thecharacteristics of polymers For PMMA solution at239uC gv225 can be obtained from the experiment byIbar36

Application of vibration technology to polymerelectrospinningWhen an additional vibrating force is applied toconducting polymer solutions or melts a dramaticreduction in viscosity occurs

The voltage is applied to polarise dielectrics where thecharges are not completely free to move but the positiveand negative charges that compose the body may bedisplaced in relation to one another when a vibratingforce is applied (see Fig 6)

Owing to its lower viscosity a relatively lower voltageis needed to eject jet from the spinnerette

From equations (1) and (2) the following power lawcan be obtained29

dvd (3)

where d is the scaling exponent that varies amongdifferent polymers

The scheme of the vibration electrospinning set-upwas shown in Fig 7 The needle tip was connected to aDC high voltage generator via an alligator clip Anultrasonic generator was used to provide vibration invibration electrospinning set-up

The main advantages of vibration electrospinning are

(i) dramatic reduction in viscosity so that the jetlength can be controlled by frequency

(ii) finer fibres can be produced compared withtraditional electrospinning apparatus under thesame conditions or the patented apparatus caneven spin such fibres whereas the traditionalapparatus cannot or can spin fibres with thesame diameter at much lower voltage

(iii) the patented electrospinning process as that inpolymer extrusion can enhance the mechanicalproperties of electrospun fibres by modifying theamorphous and semicrystalline texture andorientational state

Thermo-electrospinning apparatusIt is well known that most melting polymers can beelectrospun into nanofibres however research efforts onmelt electrospinning are relatively scarce37 largelybecause a melt electrospinning apparatus is complexand expensive for the heater and the cooler are a must inthis apparatus While it is not as prevalent as polymersolution electrospinning melt electrospinning eliminatesthe need for organic solvents and has the potential toincrease throughput due to no loss in mass by solventevaporation37

Melt blowing process is a process which producesmicrofibres directly from polymer resin Melt blowing isused commercially as a one step process for convertingpolymer resin directly into a non-woven mat of fibres Inthe process a very high velocity gas impacts a moltenpolymer stream as illustrated in Fig 8 As a result thepolymer stream is attenuated rapidly by the highvelocity hot air and the resultant fibres are collectedupon a screen as a non-woven mat The fibre diametercan be reduced from 700 to 1 mm in 50 ms and thisdramatic change in diameter significantly weakens itsmechanical strength and affects other fibre character-istics such as pore size air permeability loft drape feeletc

Controlling stability by temperatureIn order to control the solvent evaporation a furnace isadded as illustrated in Fig 9 In the furnace thetemperature field can be adjusted and the temperature

a without electric field b with electric field plus vibrating force which leads to high molecular orientation due to electro-nic force and high mechanical properties

6 Effect of electronic field on entangled macromolecule

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near the orifice is lower than that of the melts and higherthan that near collector

Owing to the gradient of temperature cold air flowsup along the furnace wall and comes down along thefibre and the low velocity gas further impacts thepolymer jet as illustrated in Fig 9 Convection currentsoccurred in the upper half of the furnace and wereenhanced by the injection of comparatively cool feed-stock down the axis It is believed that the gas movedupward beside the furnace walls and downward in thecentre of the tube As a result the polymer jet is furtherattenuated and the jet length can be increased so thatthe instability can be controlled

It would be more effective if a set-up can be devised bytaking into account the controllable temperature (Fig 9)

High temperature can also dramatically reduce theviscosity and the electrospinning of polymer melts canoffer an advantage over solution electrospinning38

Figure 10 illustrates an apparatus of melt electrospin-ning system The effects of various melt electrospinningparameters on the morphology and fibre diameter ofpolypropylene of different tacticities were studied39

Recently He et al40 suggested some novel melt electro-spinning apparatus such as vibrationndashmelt electrospinningand aero-melt electrospinning as shown in Figs 11ndash13

Hot air is applied in aero-electrospinning (seeFig 13) Hot air further attenuates the filamentsmaking the electrospun fibres ever smaller Melts areextruded by static electronic force through small orificeinto convergent streams of hot air that rapidly attenuatethe charged jet into small diameter fibres The airstreams can also control the instability in the electro-spinning process and transport fibres to a collectorwhere they bond at fibrendashfibre contact points to producea cohesive non-woven web

Additionally McCann et al41 developed a meltcoaxial electrospinning similar to coelectrospinning insolution electrospinning for the encapsulation of solidmaterials and fabrication of phase change nanofibresOgata et al42 presented a new melt electrospinningsystem equipped with a laser melting device Theyemployed the laser beams from three directions toirradiate the end of rod-like sample The fibres obtainedwere collected onto a grounded rotating disc

7 Schematic of vibration technology in polymer electro-

spinning This apparatus was patented (China Patent

2004200205963) To use this principle to prepare elec-

trospun fibres transfer agreement must be made29

8 Schematic of thermo-electrospinning

9 Schematic of electrospinning process with controllable

temperature field

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Magneto-electrospinningAs it is well known that during the electrospinningprocess the charged jet is of intrinsic instability thatleads to the uneven construction of nanofibres and wasteof most energy which otherwise can be used to furtherstretch the jet into even smaller fibres (Fig 14) Theinstability also leads to low molecular orientation as aresult low mechanical properties arise

Our group suggests a completely novel approach tocontrol the instability in electrospinning and thetechnology is termed as megnetio-electrospinning4043

It can be concluded that the magnetic approach is themost effective and economical way to control instabilityand improve the mechanical properties as well

If a magnetic field is applied in the electrospinningprocess as illustrated in Fig 15 an Ampere force isgenerated due to the current in the polymer jet

FI

~LII

BI

(4)

where I is the current floating inside the conductor B isthe value of magnetic field induction and L is theconductor length

The resultant force of electric force and the viscousforce of the jet flow enlarges the whipping circle If amagnetic field is applied in the electrospinning theproblem can be completely overcome The current in thejet under the magnetic field produces a centripetalforce ie the direction of the Ampere force is alwaystowards the initial equilibrium point (see Fig 15)leading to the shrinking of the radius of whipping circleAs a result the stability condition is enormouslyimproved

The magnetic force is perpendicular to the velocity soit contributes no work to the moving jet the shrunkencircle means less energy waste in the instability processand the saved energy is used to increase the kineticenergy of the moving jet According to the conservationof mass pr2ru5Q thus r2lt1u the radius becomesmuch smaller than that without magnetic fieldMacromolecule chains of polymer jet are subject to thecoupled forces of electronic force and magnetic forceand arranged again (see Fig 16)

The produced magnetic force can ameliorate the innerstructure of macromolecules resulting in a remarkableamelioration of the strength of nanofibre

Siro-electrospinningTwo strand spun or Sirofil yarns4445 have now beenwidely used in the worsted industry The strands aretexturised to improve the bulk of the resultant yarnswhich have been demonstrated to possess more desirableproperties For example the weaveability of the fabricformed by the Sirofil yarns is significantly improved overits counterpart yarns Dragline silk is made of manynanofibres similar to siro-spinning To mimic the spider

10 Schematic illustration of set-up for melt electrospinning

12 Vibrationndashmelt electrospinning40

13 Aero-electrospinning40

11 Melt electrospinning does not require solvent tempera-

ture of melts can be controlled by electronic heater40

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spinning (see Fig 17) siro-electrospinning is created(see Fig 18) Li et al patented a conjugate electrospin-ning apparatus46ndash48 as illustrated in Fig 19 Two ormore fibres are combined together by weak staticelectronic force An apparatus is devised where a twistis added (see Fig 18) causing the fibres to have atendency to twist and crimp Fabrics so produced canimprove its mechanical characteristics furthermore itsstrength toughness softness and feel can be improveddramatically

The present technology can also be taken in conjunc-tion with other ones For example the vibrationtechnology can be added

Using this technique it is now possible to spinconductive polymer fibres under ambient conditions bydepositing nanometre metal particle in the polymersolutions or melts If some titanium dioxide is added inthe solutions or melts the electrospun fibres areself-clearing

Spider spinningAfter 400 million years of evolution nature endowsspiders with genius of spinning flexible lightweightfibres which have the strongest strength in the world atleast five times stronger by weight than steel and haveremarkable toughness and elasticity4950 Even in themodern times it is difficult to synthesise a materialhaving advantages of strength and toughness exceptcarbon nanotube fibres which is spun from solution invery hot temperature or pressure51 while the spider silksare produced at room temperature and from aqueoussolutions Spider silk is the only natural material thatcombines the properties unmatched by any knownsynthetic high performance fibres

Spider silks are protein based lsquobiopolymerrsquo filamentsor threads secreted by specialised epithelial cells asconcentrated soluble precursors of highly repetitiveprimary sequences52 Many experiments have beenconducted and much research has focused on gene

14 Instability in electrospinning most energy was wasted

due to instability

a application of magnetic field to electrospinning b Ampere force in electronic jet induced by magnetic field15 Effect of magnetic field on electrospinning43

16 Macromolecules under coupled forces of electronic

force and magnetic force

He et al Apparatus for preparing electrospun nanofibres

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sequencing of spider and a biomimicry technology hasbeen developed53ndash55 however theoretical analysis is notyet dealt with it and our understanding of themechanism of spider spinning is rare and primitive Ifthe mystery in its mechanism can be solved then themechanism can be applied to synthetic high performancefibres such as electrospun fibres with combination ofgreat strength and stretch The devised method must bemuch economical so this could be the beginning of anew materials revolution

Mystery in spider spinning processIt is still an enigma how a spider produces a silk whichhuman needs ten thousands voltage to do One possibleanswer is that a spider applies sufficiently the nano-effect of bubble dynamics

The pressure difference between the inside and outsideof a bubble depends upon the surface tension and theradius of the bubble

For a bubble with two surfaces providing tension thenet upward force on the top hemisphere of the bubble isjust the pressure difference times the area of theequatorial circle

Fupward~pr2(PiPo) (5)

The surface tension force downward around circle istwice the surface tension times the circumference sincetwo surfaces contribute to the force

Fdownward~2pr(2T) (6)

This gives

T~1

4r(PiPo) (7)

Hendricks et al56 calculated the minimum sprayingpotential of a suspended hemispherical conductingdrop in air as

Ethreshold~300 20pcreth THORN1=2(8)

Buchko et al57 modified equation (8) in the form

E2threshold~

4H2

L2( ln

2L

R

3

2)0117pcR (9)

Ethresholdc1=2 (10)

where Ethreshold is the critical voltage needed for theejection of the jet from the Taylor cone H is theseparation distance between the needle and the collectorL is the length of the needle (or capillary) R is the radiusof the needle and c is the surface tension of the solution

From Fig 20 it can be clearly observed that draglinesilk is made of many nanofibres with a diameter ofy20 nm Therefore it can be assumed that a bubble canbe produced at the apex of each tube with a diameter ofy20 nm According to equation (7) the surface tensionof such bubbles is extremely small thus it can be easilyovercome either by the body weight of spider or tensioncreated by the rear legs

Most natural spider silk is only 25ndash4 mm in diameterand the weight of most spiders varies from 90 to1500 mg Consider a dragline silk with a diameter of361026 m which consists of many nanoscale fibrils witha diameter of y20 nm (see Fig 20) Therefore thenumber of nanofibres in the assembly can be estimatedwhich reads

n~(3|106

20|109)2amp2|104 (11)

It means that there are tens of thousands of nanofibresin the assembly

Similar to the HallndashPetch relationship the nanofibrestrength depends upon the fibre diameter in nanoscale(from few nanometres to tens of nanometres)

t~t0zkt

d1=2(12)

where kt is the fitting parameters (material constants) t0

is the strength of the bulk material and d is the fibrediameter as shown in Fig 21

Comparing the strength of a single fibre tSF with thediameter of 361026 m with the strength of a draglineassembly tDA consisting of 26104 nanofibres with thediameter of 2061029 m the following can be estimated

tDAt0

tSFt0

~24|106 (13)

Extraordinarily high strength of a dragline assembly ispredicted compared with a single fibre with the samesection area and same material

The finding shows that it is a challenge to developtechnologies capable of preparing for nanofibres within50 nm

It means that there are tens of thousands of nanotubesin spinneret of spider A weight of G51024 kg of aspider is assumed In the spinning process the spider can

17 Sirofil technology to improve mechanical properties of

mother fibres

18 Siro-electrospinning40

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use its weight to overcome the surface tension of allbubbles produced at apex of nanotubes which requires

T~G (14)

from which the pressure difference between the insideand outside of a bubble can be determined which reads

DP~PiP0~4G

nr~

4|104

(2|104)(20|109)~1 N m2 (15)

Extremely small force is needed in the spider spinning

Bubble electrospinningFrom the hint of the mechanism of the spider spinning anew approach is designed to overcome the bottleneck inpresent electrospinning technology which is to minimisethe surface tension of the electrospun solutions mimick-ing the spider spinning Compressed gas is widely used intextile industry59 and this technology will be used toreduce the surface tension Our system60ndash62 consists of avertical solution reservoir with a gas tube feeding fromthe bottom in which a metal electrode is fixed along thecentreline of the tube and a grounded collector over thereservoir (see Fig 22) It has been found that manysmall bubbles with different sizes were produced on thesolution surface

The mechanism of the new electrospinning process isdeceptively simple in the absence of an electric fieldthe aerated solution forms various bubbles on thesurface When an electric field is present it inducescharges into the bubble surface and these quickly relaxto the bubble surface The coupling of surface chargeand the external electric field creates a tangential stressresulting in the deformation of the small bubble into aprotuberance induced upward directed reentrant jetOnce the electric field exceeds the critical value neededto overcome the surface tension a fluid jet ejects fromthe apex of the conical bubble (see Fig 23) Thethreshold voltage needed to overcome the surfacetension depends upon the size of the bubble and inletair pressure

Although the maximal or minimal size of a bubblemight depend upon the solution viscosity the surfacetension of bubbles is independent of properties of thespun solutions such as viscosity which is the mainobstruction in traditional electrospinning Temperaturecan be used to adjust the bubble sizes in practicalapplication of the new electrospinning process The keyadvantages of this electrospinning process used herecompared with the traditional electrospinning processare that it can produce smaller nanofibres withoutrequirement of nozzles which are the main shortcomingof the traditional electrospinning technology further-more in our method with a relatively low voltagemillions of protruded bubbles can be easily producedwhich can be electrospun into nanofibres simulta-neously and electrospinnability does not strongly

21 E modulus of nanofibres redrawn according to

experimental data by Gu et al58 dots are experimental

data

19 Schematic set-up used for conjugate electrospinning experiments46ndash48

20 Spider spinning process reproduced with the permis-

sion of Dennis Kunkel Microscopy Inc The diameter

of a single nanofibre is y20 nm its spinneret on the

posterior portion of the abdomen of spider consists

of millions of nanoscale tubes and a bubble can be

produced at apex of each nanotube The surface ten-

sion of each bubble is extremely small such that it

can be spun into nanofibres with an awfully small

force either by the body weight of spider or tension

created by the rear legs

He et al Apparatus for preparing electrospun nanofibres

Materials Science and Technology 2010 VOL 26 NO 11 1283

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depend upon the solution viscosity overcoming com-pletely the main shortcoming of the traditional electro-spinning The average diameter of nanofibres can beeasily controllable and the minimum diameter in ourexperiment reached as small as 50 nm (see Fig 24)

This new technology is of critical importance for thenew generation of electrospinning especially forthe specialists in design manufacturing and usingnanofibres

Electrospinning dilation and electrospunnanoporous microspheresDuring the electrospinning process the charged jet isaccelerated by a constant external electric field and thespinning velocity probably reaches maximum andperhaps exceeds the velocity of sound in air in a veryshort time before the spinning becomes instableAccording to the conservation of mass equation

pr2ru~Q (16)

where r is the radius of the jet u is the velocity Q is theflowrate and r is the density The radius of the jetdecreases with increasing velocity of the incompressiblejet When the velocity reaches its maximum the radius

of the jet is minimal and macromoleculars of thepolymers are compacted together tighter and tighter asillustrated in Fig 25 There must exist a critical minimalradius rcr for all electrospun jet r(rcr for continuousultrafine fibres and the critical maximal velocity is

ucr~Q=prr2cr However the velocity can exceed this

critical value ucr if a higher voltage is applied

When the radius of the jet reaches the critical valuer5rcr and the jet speed exceeds its critical value uucrin order to keep conservation of mass equation the jetdilates by decreasing its density leading to porosity ofthe electrospun fibres (see Figs 26 and 27) which iscalled electrospinning dilation

Nanoporous microspheres are especially useful forinvisibility device (eg stealth plane)6413 and drugdeliver6566

New electrospinning apparatus for massproduction of nanofibresIn recent development and application of electrospunnanofibres one key bottleneck is the lack of capacity tothe mass production of nanofibres It is limited for atraditional electrospinning apparatus to increase theability of nanofibre throughout because there are manyseemingly insurmountable difficulties One is the smalldiameter (about 01ndash2 mm) of the spinneret and the lowflowrate (from 01 to dozens of mm h21) of the solutionIf the diameter is enlarged the suspended droplet may

22 Experimental set-up of aerated solution electrospin-

ning This principle to prepare for nanoproducts was

patented (China Patent 2007100364474) To use this

principle to prepare for nanoporous products transfer

agreement must be made63

23 Ejected jets captured by digital camera63

24 Minimum diameter of nanofibres was 50 nm (Ref 63)

25 Macromolecular chains are compacted during electro-

spinning

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not be formed as a stable Taylor cone at the orificebecause of the gravity and other factors and so is theincreasing flowrate Unlike the traditional dry spinningprocess or wet spinning process in which the jet isextruded from the spinneret with high speed thetraditional electrospinning process depends on theelectrostatic force which just provides a stable and slowliquid flow to form a stable Taylor cone at the tip of thespinneret rather than the extrusion pressure Thespinning speed is much less than that of the dry spinningor wet spinning The other shortcomings are easilyjammed difficult clean etc

As a result the possible technique of the use oftraditional electrospinning apparatus to produce nano-fibres of mass production is to fit the multiple spinneretstogether with a line or matrix pattern Many researchersadopted this technique to produce nanofibres from theearly period of the invention of this technology to nowIt is prevailing until Theron et al67 found bothexperimentally and in the simulations that the jets werepushed away from their neighbours by the Coulombicforces applied by the latter A non-uniform non-wovenmat would be obtained as an immediate consequence ofthis phenomenon Fang et al68 once reported that theysolved the problem of the mutual Coulombic interac-tions by the incorporation of secondary electrodes toisolate the electric field distribution of the primaryelectrode spinnerets But the problem of the smalldiameter of the spinneret and the low flowrate still

limited the fibre output Dosunmu et al69 suggested anew multiple porous tube apparatus in which thepolymer solution was electrified and pushed by airpressure through the walls of a large diameter cylindricalporous tube (see Fig 28a) Multiple jets formed on theporous surface and electrospun into nanofibres in theirprocess However the existing problem of this processmight be also the non-uniform fibre mat (see Fig 5 inRef 69)

In order to eliminate the problems of increasing thenanofibres of mass production of traditional electro-spinning apparatus some researchers demonstratedvarious novel electrospinning apparatus in recent yearsYet these apparatus different as they seem have onething in common The Taylor cone was formed on thefree surface of the solution and the jets were ejectedupward Yarin and Zussman70 proposed an upwardneedleless electrospinning which consisted of a two layersystem with the lower layer being a ferromagneticsuspension and the upper layer a polymer solution (seeFig 28b) In the process many vertical spikes ofmagnetic suspension perturbed the interlayer interfaceas the Taylor cones Though the production rate ishigher than that of traditional electrospinning processthere are some flaws such as not easy operation of twolayers liquids

So far only one electrospinning process Nanospiderspinning technology is claimed to be a commercialsuccess in the mass production of nanofibres all over theworld The principle of Nanospider is to use a cylinderto form the fibres The cylinder is partially immersed in

26 Electrospinning dilation phenomenon of poly

(butylenes succinate) (PBS) electrospun fibres64

27 Electrospun nanoporous microspheres where kind of

traditional Chinese drugs called lsquoYunnan Baiyorsquo was

used as additive65

a Dosunmursquos apparatus b upward needleless electro-spinning

28 New electrospinning apparatus for produing mass

production of nanofibres

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the polymer solution and as it rotates the polymersolution is carried to the top part of the cylinder whereTaylor cones are created The Taylor streams are formednext to each other throughout the entire length of thecylinder resulting in the high production capacity of thisapparatus71

The above novel approaches are exciting for theresearchers and the engineers When we cheer and applaudfor these much progress of electrospinning however a newproblem caused by the opened liquid surface has risen upIn every electrospinning process the volatilisation of thesolvent from the flying jet is a must In the above newupward electrospinning apparatus the free liquid surfacecan be used to create many Taylor cones and increase thethroughput of nanofibres at the same time can befacilitated to the escape of the volatilising solvent Withthe process proceeding the viscosity of the polymersolution will increase which will introduce some newproblems such as increasing the difficulty of spinning andchanging the properties of the solution

ConclusionsIt is clear that an understanding of the electrospinningand its nanofibres was greatly advanced by thenanoscience and technology in the 1990s and by thesubsequent development of the many effective applica-tions available in many fields today Remarkableadvances in electrospinning technology have beenpossible due to developments in modern technologyVarious novel and useful electrospinning techniques arereviewed in this work according to their differentmodifications Some modified apparatus were used toproduce different nanofibre mats with special structuresfor some special applications and some for massproduction of nanofibres Most of the advancesachieved to date have been made in the collector systemof the traditional electrospinning apparatus but otherparts such as solution storage and feeding systemspinneret and spinning area have been studied as wellMore recently emphasis has been placed on creatingnew apparatus to mass production of nanofibres Newtechniques have been designed and improved and moreelectrospinning apparatus have been developed to betterproduce nanofibres

Collectively the studies about electrospinning andnanofibres carried out so far have furnished a usefuldatabase for the electrohydrodynamics and some otherfields Although this field is in its infancy it is becomingincreasingly clear that there are multiple forms of newtechniques Identifying the similarities and differences invarious apparatus will enhance our understanding ofthis technology The next decades are expected to notonly enrich the existing knowledge but also significantlyadvance the technology into the commercial domain

Acknowledgements

The present work is supported financially by NationalNatural Science Foundation of China under Grant Nos10802021 and 10972053 National Natural ScienceFoundation of Shanghai under Grant No08ZR1400300 Shanghai Rising-Star Program underGrant No09QA1400100 Key Project of ChineseMinistry of Education No210006 National ScienceFoundation of Tianjin No10JCYBJC02200 Tianjin

Universities Science and Technology DevelopmentFund Project No20080322

References1 A Formhals lsquoProcess and apparatus for preparing artifical

threadsrsquo US Patent 1975504 1934

2 M S El Naschie Chaos Solitons Fract 2006 30 769ndash773

3 J-H He Chaos Solitons Fract 2008 38 1390ndash1393

4 J Lyons and F Ko Polym News 2005 30 170ndash178

5 D Li and Y-N Xia Adv Mater 2004 16 1151ndash1170

6 Z-M Huang Y-Z Zhang M Kotaki and S Ramakrishna

Compos Sci Technol 2003 63 2223ndash2253

7 Q P Pham U Sharma and A G Mikos Tissue Eng 2006 12

1197ndash1211

8 E P S Tan and C T Lim Compos Sci Technol 2006 66 1102ndash

1111

9 J-H He and H-M Liu Nonlinear Analy 2005 63 e919ndashe929

10 J-H He and Y-Q Wan Polymer 2004 45 6731ndash6734

11 J-H He Y-Q Wan and J-Y Yu Polymer 2005 46 2799ndash2801

12 J-H He Y-Q Wan and J-Y Yu Fibers Polym 2008 9 140ndash

142

13 J-H He L Xu Y Wu and Y Liu Polym Int 2007 56 1323ndash

1329

14 J-H He Y-Q Wan and J-Y Yu Int J Nonlinear Sci 2004 5

243ndash252

15 J-H He Y Wu and N Pan Int J Nonlinear Sci 2005 6 243ndash

248

16 Y-Q Wan J-H He and J-Y Yu Iran Polym J 2006 15 265ndash

268

17 L Rayleigh Philos Mag 1882 14 184ndash186

18 J F Cooley lsquoApparatus for electrically dispensing fluidsrsquo US

Patent 692631 1902

19 W J Morton lsquoMethod of dispensing fluidsrsquo US Patent 705691

1902

20 P K Baumgarten J Colloid Interface Sci 1971 36 71ndash79

21 J-H He and Y Liu J Polym Eng 2008 28 101ndash114

22 Y Liu J-H He L Xu and J-Y Yu Int J Electrospun Nanofibers

Appl 2007 1 7ndash15

23 Y Liu J-H He and J-Y Yu Fibres Text East Eur 2007 15

30ndash33

24 X-H Qin Y-Q Wan J-H He J Zhang J-Y Yu and

S-Y Wang Polymer 2004 45 6409ndash6413

25 D-H Shou and J-H He J Polym Eng 2008 28 115ndash118

26 Y-Q Wan J-H He Y Wu and J-Y Yu Mater Lett 2006 60

3296ndash3300

27 S-Q Wang J-H He and L Xu Polym Int 2008 57 1079ndash1082

28 Y Wu J-H He L Xu and J-Y Yu Int J Electrospun

Nanofibers Appl 2007 1 1ndash6

29 J-H He Y-Q Wan and J-Y Yu Int J Nonlinear Sci 2004 5

253ndash262

30 Y-Q Wan J-H He L Xu and J-Y Yu Int J Electrospun

Nanofibers Appl 2007 1 17ndash28

31 Y-Q Wan J-H He J-Y Yu and Y Wu J Appl Polym Sci

2007 103 3840ndash3843

32 J Zhang Y-Q Wan J-H He and Z-Y Zhu China Patent

2004100156853 2004

33 B H Bersted J Appl Polym Sci 2003 28 2777ndash2791

34 A I Isayev C M Wong and X Zeng Adv Polym Technol 2003

10 31ndash45

35 H Wu S Guo G Chen J Lin W Chen and H Wang J Appl

Polym Sci 2003 90 1873ndash1878

36 J P Ibar Polym Eng Sci 1998 38 1ndash20

37 S Sell C Barnes M Smith M McClure P Madurantakam

J Grant M McManus and G Bowlin Polym Int 2007 56 1349ndash

1360

38 P D Dalton K Klinkhammer J Salber D Klee and M Moller

Biomacromolecules 2006 7 686ndash690

39 J Lyons C Li and F Ko Polymer 2004 45 7597ndash7603

40 J-H He Y-Q Wan and L Xu Chaos Solitons Fract 2007 33

26ndash37

41 J T McCann M Marquez and Y Xia Nano Lett 2006 6 2868ndash

2872

42 N Ogata S Yamaguchi N Shimada G Lu T Iwata K Nakane

and T Ogihara J Appl Polym Sci 2007 104 1640ndash1645

43 Y Wu J-Y Yu J-H He and Y-Q Wan Chaos Solitons Fract

2007 32 5ndash7

44 J-H He Y-P Yu J-Y Yu W-R Li S-Y Wang and N Pan

Text Res J 2005 75 181ndash184

He et al Apparatus for preparing electrospun nanofibres

1286 Materials Science and Technology 2010 VOL 26 NO 11

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Ethresholdc1=2

where Ethreshold is the critical voltage needed for theejection of the jet from the Taylor cone and c is thesurface tension of the solution which is the intrinsicproperty of the solution and strongly depends upon thesolution viscosity

A facile strategy for preparing electrospun nanofibresby vibration technology is therefore suggested by Wanet al26 Vibration is applied to the polymer solutions ormelts leading to a dramatic reduction in viscosity as aresult a moderate voltage is needed to produce finenanofibres that are commonly observed during conven-tional electrospinning procedure at an elevated voltageThe novel strategy produces finer nanofibres than thoseobtained without vibration and expands the range ofthis technique by making fibres from macromoleculeswhich cannot be electrospun by traditional ways

Effect of viscosity on diameter of electrospunfibre2629ndash31

Even under very high voltage not every solution can beelectrospun and the jet might break up into droplets asa result of the surface tension in the case of smallmolecules with low viscosity liquids of small molecules(known as electrospraying) For high viscosity liquids ofmacromolecules the jet does not break up but travels asa jet to the grounded target (known as electrospinning)For very high viscous solutions or melts the process canbe used in electro-extrusion in plastic industry32

The experimental data and theoretical analysis29 showthat the solution viscosity has great effects on thediameter of electrospun fibres initiating droplet shapeand the jet instability The increase in solution viscosityhas been associated with the production of largerdiameter fibres and it was shown that the fibre diameterdepends allometrically on the solution viscosity in theform29

dga (1)

where d is the average diameter of the electrospun fibreg is the viscosity and a is the scaling exponent Theexponent value might differ between different polymers

For acrylic solution Baumgarten20 found that thescaling exponent is y12

Effect of vibration on viscosityVibration technology33ndash35 has been introduced intopolymer processing for many years Initially it was onlyapplied in researches for polymer melt viscosity mea-surement Subsequently the principle of melt vibrationwas introduced into practical applications includinginjection moulding extrusion and compression mould-ingthermoforming for the reduction of viscosity tolowering processing temperature and pressure to elim-inate melt defects and weld lines and for the enhance-ment of mechanical properties by modifying theamorphous and semicrystalline texture and orientationalstate

Ibar36 observed the effect of low frequency vibrationduring processing for polymethyl methacrylate(PMMA) (Fig 5) The result showed that increasingfrequency makes the solution viscosity decreased Thelower the frequency the greater the differences ofviscosity at various temperatures At low frequenciesthe solution viscosity decreased quickly while atfrequencies ranging from 100 to 500 rad s21 theincrease in frequency was not obvious However

4 Schematic diagram of traditional electrospinning apparatus

5 Solution viscosity g of PMMA versus vibration

frequency v (Ref 36)

He et al Apparatus for preparing electrospun nanofibres

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frequencies 200 rad s21 would decrease the viscositygradient due to temperature Ibar explained the effect ofvibration frequency and amplitude on melt viscosity interms of shear thinning criteria

When a vibrating force is applied to a concentratedand entangled polymer solution or melt the weak vander Waals force connecting with macromoleculesbecomes weaker and the entanglement is relaxed sothat the viscous force between the macromoleculesdecreases dramatically resulting in the reduction inviscosity

According to the experimental observation thefollowing allometric scaling can be obtained

gvb (2)

where b is a scaling exponent that varies with thecharacteristics of polymers For PMMA solution at239uC gv225 can be obtained from the experiment byIbar36

Application of vibration technology to polymerelectrospinningWhen an additional vibrating force is applied toconducting polymer solutions or melts a dramaticreduction in viscosity occurs

The voltage is applied to polarise dielectrics where thecharges are not completely free to move but the positiveand negative charges that compose the body may bedisplaced in relation to one another when a vibratingforce is applied (see Fig 6)

Owing to its lower viscosity a relatively lower voltageis needed to eject jet from the spinnerette

From equations (1) and (2) the following power lawcan be obtained29

dvd (3)

where d is the scaling exponent that varies amongdifferent polymers

The scheme of the vibration electrospinning set-upwas shown in Fig 7 The needle tip was connected to aDC high voltage generator via an alligator clip Anultrasonic generator was used to provide vibration invibration electrospinning set-up

The main advantages of vibration electrospinning are

(i) dramatic reduction in viscosity so that the jetlength can be controlled by frequency

(ii) finer fibres can be produced compared withtraditional electrospinning apparatus under thesame conditions or the patented apparatus caneven spin such fibres whereas the traditionalapparatus cannot or can spin fibres with thesame diameter at much lower voltage

(iii) the patented electrospinning process as that inpolymer extrusion can enhance the mechanicalproperties of electrospun fibres by modifying theamorphous and semicrystalline texture andorientational state

Thermo-electrospinning apparatusIt is well known that most melting polymers can beelectrospun into nanofibres however research efforts onmelt electrospinning are relatively scarce37 largelybecause a melt electrospinning apparatus is complexand expensive for the heater and the cooler are a must inthis apparatus While it is not as prevalent as polymersolution electrospinning melt electrospinning eliminatesthe need for organic solvents and has the potential toincrease throughput due to no loss in mass by solventevaporation37

Melt blowing process is a process which producesmicrofibres directly from polymer resin Melt blowing isused commercially as a one step process for convertingpolymer resin directly into a non-woven mat of fibres Inthe process a very high velocity gas impacts a moltenpolymer stream as illustrated in Fig 8 As a result thepolymer stream is attenuated rapidly by the highvelocity hot air and the resultant fibres are collectedupon a screen as a non-woven mat The fibre diametercan be reduced from 700 to 1 mm in 50 ms and thisdramatic change in diameter significantly weakens itsmechanical strength and affects other fibre character-istics such as pore size air permeability loft drape feeletc

Controlling stability by temperatureIn order to control the solvent evaporation a furnace isadded as illustrated in Fig 9 In the furnace thetemperature field can be adjusted and the temperature

a without electric field b with electric field plus vibrating force which leads to high molecular orientation due to electro-nic force and high mechanical properties

6 Effect of electronic field on entangled macromolecule

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near the orifice is lower than that of the melts and higherthan that near collector

Owing to the gradient of temperature cold air flowsup along the furnace wall and comes down along thefibre and the low velocity gas further impacts thepolymer jet as illustrated in Fig 9 Convection currentsoccurred in the upper half of the furnace and wereenhanced by the injection of comparatively cool feed-stock down the axis It is believed that the gas movedupward beside the furnace walls and downward in thecentre of the tube As a result the polymer jet is furtherattenuated and the jet length can be increased so thatthe instability can be controlled

It would be more effective if a set-up can be devised bytaking into account the controllable temperature (Fig 9)

High temperature can also dramatically reduce theviscosity and the electrospinning of polymer melts canoffer an advantage over solution electrospinning38

Figure 10 illustrates an apparatus of melt electrospin-ning system The effects of various melt electrospinningparameters on the morphology and fibre diameter ofpolypropylene of different tacticities were studied39

Recently He et al40 suggested some novel melt electro-spinning apparatus such as vibrationndashmelt electrospinningand aero-melt electrospinning as shown in Figs 11ndash13

Hot air is applied in aero-electrospinning (seeFig 13) Hot air further attenuates the filamentsmaking the electrospun fibres ever smaller Melts areextruded by static electronic force through small orificeinto convergent streams of hot air that rapidly attenuatethe charged jet into small diameter fibres The airstreams can also control the instability in the electro-spinning process and transport fibres to a collectorwhere they bond at fibrendashfibre contact points to producea cohesive non-woven web

Additionally McCann et al41 developed a meltcoaxial electrospinning similar to coelectrospinning insolution electrospinning for the encapsulation of solidmaterials and fabrication of phase change nanofibresOgata et al42 presented a new melt electrospinningsystem equipped with a laser melting device Theyemployed the laser beams from three directions toirradiate the end of rod-like sample The fibres obtainedwere collected onto a grounded rotating disc

7 Schematic of vibration technology in polymer electro-

spinning This apparatus was patented (China Patent

2004200205963) To use this principle to prepare elec-

trospun fibres transfer agreement must be made29

8 Schematic of thermo-electrospinning

9 Schematic of electrospinning process with controllable

temperature field

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Magneto-electrospinningAs it is well known that during the electrospinningprocess the charged jet is of intrinsic instability thatleads to the uneven construction of nanofibres and wasteof most energy which otherwise can be used to furtherstretch the jet into even smaller fibres (Fig 14) Theinstability also leads to low molecular orientation as aresult low mechanical properties arise

Our group suggests a completely novel approach tocontrol the instability in electrospinning and thetechnology is termed as megnetio-electrospinning4043

It can be concluded that the magnetic approach is themost effective and economical way to control instabilityand improve the mechanical properties as well

If a magnetic field is applied in the electrospinningprocess as illustrated in Fig 15 an Ampere force isgenerated due to the current in the polymer jet

FI

~LII

BI

(4)

where I is the current floating inside the conductor B isthe value of magnetic field induction and L is theconductor length

The resultant force of electric force and the viscousforce of the jet flow enlarges the whipping circle If amagnetic field is applied in the electrospinning theproblem can be completely overcome The current in thejet under the magnetic field produces a centripetalforce ie the direction of the Ampere force is alwaystowards the initial equilibrium point (see Fig 15)leading to the shrinking of the radius of whipping circleAs a result the stability condition is enormouslyimproved

The magnetic force is perpendicular to the velocity soit contributes no work to the moving jet the shrunkencircle means less energy waste in the instability processand the saved energy is used to increase the kineticenergy of the moving jet According to the conservationof mass pr2ru5Q thus r2lt1u the radius becomesmuch smaller than that without magnetic fieldMacromolecule chains of polymer jet are subject to thecoupled forces of electronic force and magnetic forceand arranged again (see Fig 16)

The produced magnetic force can ameliorate the innerstructure of macromolecules resulting in a remarkableamelioration of the strength of nanofibre

Siro-electrospinningTwo strand spun or Sirofil yarns4445 have now beenwidely used in the worsted industry The strands aretexturised to improve the bulk of the resultant yarnswhich have been demonstrated to possess more desirableproperties For example the weaveability of the fabricformed by the Sirofil yarns is significantly improved overits counterpart yarns Dragline silk is made of manynanofibres similar to siro-spinning To mimic the spider

10 Schematic illustration of set-up for melt electrospinning

12 Vibrationndashmelt electrospinning40

13 Aero-electrospinning40

11 Melt electrospinning does not require solvent tempera-

ture of melts can be controlled by electronic heater40

He et al Apparatus for preparing electrospun nanofibres

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spinning (see Fig 17) siro-electrospinning is created(see Fig 18) Li et al patented a conjugate electrospin-ning apparatus46ndash48 as illustrated in Fig 19 Two ormore fibres are combined together by weak staticelectronic force An apparatus is devised where a twistis added (see Fig 18) causing the fibres to have atendency to twist and crimp Fabrics so produced canimprove its mechanical characteristics furthermore itsstrength toughness softness and feel can be improveddramatically

The present technology can also be taken in conjunc-tion with other ones For example the vibrationtechnology can be added

Using this technique it is now possible to spinconductive polymer fibres under ambient conditions bydepositing nanometre metal particle in the polymersolutions or melts If some titanium dioxide is added inthe solutions or melts the electrospun fibres areself-clearing

Spider spinningAfter 400 million years of evolution nature endowsspiders with genius of spinning flexible lightweightfibres which have the strongest strength in the world atleast five times stronger by weight than steel and haveremarkable toughness and elasticity4950 Even in themodern times it is difficult to synthesise a materialhaving advantages of strength and toughness exceptcarbon nanotube fibres which is spun from solution invery hot temperature or pressure51 while the spider silksare produced at room temperature and from aqueoussolutions Spider silk is the only natural material thatcombines the properties unmatched by any knownsynthetic high performance fibres

Spider silks are protein based lsquobiopolymerrsquo filamentsor threads secreted by specialised epithelial cells asconcentrated soluble precursors of highly repetitiveprimary sequences52 Many experiments have beenconducted and much research has focused on gene

14 Instability in electrospinning most energy was wasted

due to instability

a application of magnetic field to electrospinning b Ampere force in electronic jet induced by magnetic field15 Effect of magnetic field on electrospinning43

16 Macromolecules under coupled forces of electronic

force and magnetic force

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sequencing of spider and a biomimicry technology hasbeen developed53ndash55 however theoretical analysis is notyet dealt with it and our understanding of themechanism of spider spinning is rare and primitive Ifthe mystery in its mechanism can be solved then themechanism can be applied to synthetic high performancefibres such as electrospun fibres with combination ofgreat strength and stretch The devised method must bemuch economical so this could be the beginning of anew materials revolution

Mystery in spider spinning processIt is still an enigma how a spider produces a silk whichhuman needs ten thousands voltage to do One possibleanswer is that a spider applies sufficiently the nano-effect of bubble dynamics

The pressure difference between the inside and outsideof a bubble depends upon the surface tension and theradius of the bubble

For a bubble with two surfaces providing tension thenet upward force on the top hemisphere of the bubble isjust the pressure difference times the area of theequatorial circle

Fupward~pr2(PiPo) (5)

The surface tension force downward around circle istwice the surface tension times the circumference sincetwo surfaces contribute to the force

Fdownward~2pr(2T) (6)

This gives

T~1

4r(PiPo) (7)

Hendricks et al56 calculated the minimum sprayingpotential of a suspended hemispherical conductingdrop in air as

Ethreshold~300 20pcreth THORN1=2(8)

Buchko et al57 modified equation (8) in the form

E2threshold~

4H2

L2( ln

2L

R

3

2)0117pcR (9)

Ethresholdc1=2 (10)

where Ethreshold is the critical voltage needed for theejection of the jet from the Taylor cone H is theseparation distance between the needle and the collectorL is the length of the needle (or capillary) R is the radiusof the needle and c is the surface tension of the solution

From Fig 20 it can be clearly observed that draglinesilk is made of many nanofibres with a diameter ofy20 nm Therefore it can be assumed that a bubble canbe produced at the apex of each tube with a diameter ofy20 nm According to equation (7) the surface tensionof such bubbles is extremely small thus it can be easilyovercome either by the body weight of spider or tensioncreated by the rear legs

Most natural spider silk is only 25ndash4 mm in diameterand the weight of most spiders varies from 90 to1500 mg Consider a dragline silk with a diameter of361026 m which consists of many nanoscale fibrils witha diameter of y20 nm (see Fig 20) Therefore thenumber of nanofibres in the assembly can be estimatedwhich reads

n~(3|106

20|109)2amp2|104 (11)

It means that there are tens of thousands of nanofibresin the assembly

Similar to the HallndashPetch relationship the nanofibrestrength depends upon the fibre diameter in nanoscale(from few nanometres to tens of nanometres)

t~t0zkt

d1=2(12)

where kt is the fitting parameters (material constants) t0

is the strength of the bulk material and d is the fibrediameter as shown in Fig 21

Comparing the strength of a single fibre tSF with thediameter of 361026 m with the strength of a draglineassembly tDA consisting of 26104 nanofibres with thediameter of 2061029 m the following can be estimated

tDAt0

tSFt0

~24|106 (13)

Extraordinarily high strength of a dragline assembly ispredicted compared with a single fibre with the samesection area and same material

The finding shows that it is a challenge to developtechnologies capable of preparing for nanofibres within50 nm

It means that there are tens of thousands of nanotubesin spinneret of spider A weight of G51024 kg of aspider is assumed In the spinning process the spider can

17 Sirofil technology to improve mechanical properties of

mother fibres

18 Siro-electrospinning40

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use its weight to overcome the surface tension of allbubbles produced at apex of nanotubes which requires

T~G (14)

from which the pressure difference between the insideand outside of a bubble can be determined which reads

DP~PiP0~4G

nr~

4|104

(2|104)(20|109)~1 N m2 (15)

Extremely small force is needed in the spider spinning

Bubble electrospinningFrom the hint of the mechanism of the spider spinning anew approach is designed to overcome the bottleneck inpresent electrospinning technology which is to minimisethe surface tension of the electrospun solutions mimick-ing the spider spinning Compressed gas is widely used intextile industry59 and this technology will be used toreduce the surface tension Our system60ndash62 consists of avertical solution reservoir with a gas tube feeding fromthe bottom in which a metal electrode is fixed along thecentreline of the tube and a grounded collector over thereservoir (see Fig 22) It has been found that manysmall bubbles with different sizes were produced on thesolution surface

The mechanism of the new electrospinning process isdeceptively simple in the absence of an electric fieldthe aerated solution forms various bubbles on thesurface When an electric field is present it inducescharges into the bubble surface and these quickly relaxto the bubble surface The coupling of surface chargeand the external electric field creates a tangential stressresulting in the deformation of the small bubble into aprotuberance induced upward directed reentrant jetOnce the electric field exceeds the critical value neededto overcome the surface tension a fluid jet ejects fromthe apex of the conical bubble (see Fig 23) Thethreshold voltage needed to overcome the surfacetension depends upon the size of the bubble and inletair pressure

Although the maximal or minimal size of a bubblemight depend upon the solution viscosity the surfacetension of bubbles is independent of properties of thespun solutions such as viscosity which is the mainobstruction in traditional electrospinning Temperaturecan be used to adjust the bubble sizes in practicalapplication of the new electrospinning process The keyadvantages of this electrospinning process used herecompared with the traditional electrospinning processare that it can produce smaller nanofibres withoutrequirement of nozzles which are the main shortcomingof the traditional electrospinning technology further-more in our method with a relatively low voltagemillions of protruded bubbles can be easily producedwhich can be electrospun into nanofibres simulta-neously and electrospinnability does not strongly

21 E modulus of nanofibres redrawn according to

experimental data by Gu et al58 dots are experimental

data

19 Schematic set-up used for conjugate electrospinning experiments46ndash48

20 Spider spinning process reproduced with the permis-

sion of Dennis Kunkel Microscopy Inc The diameter

of a single nanofibre is y20 nm its spinneret on the

posterior portion of the abdomen of spider consists

of millions of nanoscale tubes and a bubble can be

produced at apex of each nanotube The surface ten-

sion of each bubble is extremely small such that it

can be spun into nanofibres with an awfully small

force either by the body weight of spider or tension

created by the rear legs

He et al Apparatus for preparing electrospun nanofibres

Materials Science and Technology 2010 VOL 26 NO 11 1283

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depend upon the solution viscosity overcoming com-pletely the main shortcoming of the traditional electro-spinning The average diameter of nanofibres can beeasily controllable and the minimum diameter in ourexperiment reached as small as 50 nm (see Fig 24)

This new technology is of critical importance for thenew generation of electrospinning especially forthe specialists in design manufacturing and usingnanofibres

Electrospinning dilation and electrospunnanoporous microspheresDuring the electrospinning process the charged jet isaccelerated by a constant external electric field and thespinning velocity probably reaches maximum andperhaps exceeds the velocity of sound in air in a veryshort time before the spinning becomes instableAccording to the conservation of mass equation

pr2ru~Q (16)

where r is the radius of the jet u is the velocity Q is theflowrate and r is the density The radius of the jetdecreases with increasing velocity of the incompressiblejet When the velocity reaches its maximum the radius

of the jet is minimal and macromoleculars of thepolymers are compacted together tighter and tighter asillustrated in Fig 25 There must exist a critical minimalradius rcr for all electrospun jet r(rcr for continuousultrafine fibres and the critical maximal velocity is

ucr~Q=prr2cr However the velocity can exceed this

critical value ucr if a higher voltage is applied

When the radius of the jet reaches the critical valuer5rcr and the jet speed exceeds its critical value uucrin order to keep conservation of mass equation the jetdilates by decreasing its density leading to porosity ofthe electrospun fibres (see Figs 26 and 27) which iscalled electrospinning dilation

Nanoporous microspheres are especially useful forinvisibility device (eg stealth plane)6413 and drugdeliver6566

New electrospinning apparatus for massproduction of nanofibresIn recent development and application of electrospunnanofibres one key bottleneck is the lack of capacity tothe mass production of nanofibres It is limited for atraditional electrospinning apparatus to increase theability of nanofibre throughout because there are manyseemingly insurmountable difficulties One is the smalldiameter (about 01ndash2 mm) of the spinneret and the lowflowrate (from 01 to dozens of mm h21) of the solutionIf the diameter is enlarged the suspended droplet may

22 Experimental set-up of aerated solution electrospin-

ning This principle to prepare for nanoproducts was

patented (China Patent 2007100364474) To use this

principle to prepare for nanoporous products transfer

agreement must be made63

23 Ejected jets captured by digital camera63

24 Minimum diameter of nanofibres was 50 nm (Ref 63)

25 Macromolecular chains are compacted during electro-

spinning

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not be formed as a stable Taylor cone at the orificebecause of the gravity and other factors and so is theincreasing flowrate Unlike the traditional dry spinningprocess or wet spinning process in which the jet isextruded from the spinneret with high speed thetraditional electrospinning process depends on theelectrostatic force which just provides a stable and slowliquid flow to form a stable Taylor cone at the tip of thespinneret rather than the extrusion pressure Thespinning speed is much less than that of the dry spinningor wet spinning The other shortcomings are easilyjammed difficult clean etc

As a result the possible technique of the use oftraditional electrospinning apparatus to produce nano-fibres of mass production is to fit the multiple spinneretstogether with a line or matrix pattern Many researchersadopted this technique to produce nanofibres from theearly period of the invention of this technology to nowIt is prevailing until Theron et al67 found bothexperimentally and in the simulations that the jets werepushed away from their neighbours by the Coulombicforces applied by the latter A non-uniform non-wovenmat would be obtained as an immediate consequence ofthis phenomenon Fang et al68 once reported that theysolved the problem of the mutual Coulombic interac-tions by the incorporation of secondary electrodes toisolate the electric field distribution of the primaryelectrode spinnerets But the problem of the smalldiameter of the spinneret and the low flowrate still

limited the fibre output Dosunmu et al69 suggested anew multiple porous tube apparatus in which thepolymer solution was electrified and pushed by airpressure through the walls of a large diameter cylindricalporous tube (see Fig 28a) Multiple jets formed on theporous surface and electrospun into nanofibres in theirprocess However the existing problem of this processmight be also the non-uniform fibre mat (see Fig 5 inRef 69)

In order to eliminate the problems of increasing thenanofibres of mass production of traditional electro-spinning apparatus some researchers demonstratedvarious novel electrospinning apparatus in recent yearsYet these apparatus different as they seem have onething in common The Taylor cone was formed on thefree surface of the solution and the jets were ejectedupward Yarin and Zussman70 proposed an upwardneedleless electrospinning which consisted of a two layersystem with the lower layer being a ferromagneticsuspension and the upper layer a polymer solution (seeFig 28b) In the process many vertical spikes ofmagnetic suspension perturbed the interlayer interfaceas the Taylor cones Though the production rate ishigher than that of traditional electrospinning processthere are some flaws such as not easy operation of twolayers liquids

So far only one electrospinning process Nanospiderspinning technology is claimed to be a commercialsuccess in the mass production of nanofibres all over theworld The principle of Nanospider is to use a cylinderto form the fibres The cylinder is partially immersed in

26 Electrospinning dilation phenomenon of poly

(butylenes succinate) (PBS) electrospun fibres64

27 Electrospun nanoporous microspheres where kind of

traditional Chinese drugs called lsquoYunnan Baiyorsquo was

used as additive65

a Dosunmursquos apparatus b upward needleless electro-spinning

28 New electrospinning apparatus for produing mass

production of nanofibres

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the polymer solution and as it rotates the polymersolution is carried to the top part of the cylinder whereTaylor cones are created The Taylor streams are formednext to each other throughout the entire length of thecylinder resulting in the high production capacity of thisapparatus71

The above novel approaches are exciting for theresearchers and the engineers When we cheer and applaudfor these much progress of electrospinning however a newproblem caused by the opened liquid surface has risen upIn every electrospinning process the volatilisation of thesolvent from the flying jet is a must In the above newupward electrospinning apparatus the free liquid surfacecan be used to create many Taylor cones and increase thethroughput of nanofibres at the same time can befacilitated to the escape of the volatilising solvent Withthe process proceeding the viscosity of the polymersolution will increase which will introduce some newproblems such as increasing the difficulty of spinning andchanging the properties of the solution

ConclusionsIt is clear that an understanding of the electrospinningand its nanofibres was greatly advanced by thenanoscience and technology in the 1990s and by thesubsequent development of the many effective applica-tions available in many fields today Remarkableadvances in electrospinning technology have beenpossible due to developments in modern technologyVarious novel and useful electrospinning techniques arereviewed in this work according to their differentmodifications Some modified apparatus were used toproduce different nanofibre mats with special structuresfor some special applications and some for massproduction of nanofibres Most of the advancesachieved to date have been made in the collector systemof the traditional electrospinning apparatus but otherparts such as solution storage and feeding systemspinneret and spinning area have been studied as wellMore recently emphasis has been placed on creatingnew apparatus to mass production of nanofibres Newtechniques have been designed and improved and moreelectrospinning apparatus have been developed to betterproduce nanofibres

Collectively the studies about electrospinning andnanofibres carried out so far have furnished a usefuldatabase for the electrohydrodynamics and some otherfields Although this field is in its infancy it is becomingincreasingly clear that there are multiple forms of newtechniques Identifying the similarities and differences invarious apparatus will enhance our understanding ofthis technology The next decades are expected to notonly enrich the existing knowledge but also significantlyadvance the technology into the commercial domain

Acknowledgements

The present work is supported financially by NationalNatural Science Foundation of China under Grant Nos10802021 and 10972053 National Natural ScienceFoundation of Shanghai under Grant No08ZR1400300 Shanghai Rising-Star Program underGrant No09QA1400100 Key Project of ChineseMinistry of Education No210006 National ScienceFoundation of Tianjin No10JCYBJC02200 Tianjin

Universities Science and Technology DevelopmentFund Project No20080322

References1 A Formhals lsquoProcess and apparatus for preparing artifical

threadsrsquo US Patent 1975504 1934

2 M S El Naschie Chaos Solitons Fract 2006 30 769ndash773

3 J-H He Chaos Solitons Fract 2008 38 1390ndash1393

4 J Lyons and F Ko Polym News 2005 30 170ndash178

5 D Li and Y-N Xia Adv Mater 2004 16 1151ndash1170

6 Z-M Huang Y-Z Zhang M Kotaki and S Ramakrishna

Compos Sci Technol 2003 63 2223ndash2253

7 Q P Pham U Sharma and A G Mikos Tissue Eng 2006 12

1197ndash1211

8 E P S Tan and C T Lim Compos Sci Technol 2006 66 1102ndash

1111

9 J-H He and H-M Liu Nonlinear Analy 2005 63 e919ndashe929

10 J-H He and Y-Q Wan Polymer 2004 45 6731ndash6734

11 J-H He Y-Q Wan and J-Y Yu Polymer 2005 46 2799ndash2801

12 J-H He Y-Q Wan and J-Y Yu Fibers Polym 2008 9 140ndash

142

13 J-H He L Xu Y Wu and Y Liu Polym Int 2007 56 1323ndash

1329

14 J-H He Y-Q Wan and J-Y Yu Int J Nonlinear Sci 2004 5

243ndash252

15 J-H He Y Wu and N Pan Int J Nonlinear Sci 2005 6 243ndash

248

16 Y-Q Wan J-H He and J-Y Yu Iran Polym J 2006 15 265ndash

268

17 L Rayleigh Philos Mag 1882 14 184ndash186

18 J F Cooley lsquoApparatus for electrically dispensing fluidsrsquo US

Patent 692631 1902

19 W J Morton lsquoMethod of dispensing fluidsrsquo US Patent 705691

1902

20 P K Baumgarten J Colloid Interface Sci 1971 36 71ndash79

21 J-H He and Y Liu J Polym Eng 2008 28 101ndash114

22 Y Liu J-H He L Xu and J-Y Yu Int J Electrospun Nanofibers

Appl 2007 1 7ndash15

23 Y Liu J-H He and J-Y Yu Fibres Text East Eur 2007 15

30ndash33

24 X-H Qin Y-Q Wan J-H He J Zhang J-Y Yu and

S-Y Wang Polymer 2004 45 6409ndash6413

25 D-H Shou and J-H He J Polym Eng 2008 28 115ndash118

26 Y-Q Wan J-H He Y Wu and J-Y Yu Mater Lett 2006 60

3296ndash3300

27 S-Q Wang J-H He and L Xu Polym Int 2008 57 1079ndash1082

28 Y Wu J-H He L Xu and J-Y Yu Int J Electrospun

Nanofibers Appl 2007 1 1ndash6

29 J-H He Y-Q Wan and J-Y Yu Int J Nonlinear Sci 2004 5

253ndash262

30 Y-Q Wan J-H He L Xu and J-Y Yu Int J Electrospun

Nanofibers Appl 2007 1 17ndash28

31 Y-Q Wan J-H He J-Y Yu and Y Wu J Appl Polym Sci

2007 103 3840ndash3843

32 J Zhang Y-Q Wan J-H He and Z-Y Zhu China Patent

2004100156853 2004

33 B H Bersted J Appl Polym Sci 2003 28 2777ndash2791

34 A I Isayev C M Wong and X Zeng Adv Polym Technol 2003

10 31ndash45

35 H Wu S Guo G Chen J Lin W Chen and H Wang J Appl

Polym Sci 2003 90 1873ndash1878

36 J P Ibar Polym Eng Sci 1998 38 1ndash20

37 S Sell C Barnes M Smith M McClure P Madurantakam

J Grant M McManus and G Bowlin Polym Int 2007 56 1349ndash

1360

38 P D Dalton K Klinkhammer J Salber D Klee and M Moller

Biomacromolecules 2006 7 686ndash690

39 J Lyons C Li and F Ko Polymer 2004 45 7597ndash7603

40 J-H He Y-Q Wan and L Xu Chaos Solitons Fract 2007 33

26ndash37

41 J T McCann M Marquez and Y Xia Nano Lett 2006 6 2868ndash

2872

42 N Ogata S Yamaguchi N Shimada G Lu T Iwata K Nakane

and T Ogihara J Appl Polym Sci 2007 104 1640ndash1645

43 Y Wu J-Y Yu J-H He and Y-Q Wan Chaos Solitons Fract

2007 32 5ndash7

44 J-H He Y-P Yu J-Y Yu W-R Li S-Y Wang and N Pan

Text Res J 2005 75 181ndash184

He et al Apparatus for preparing electrospun nanofibres

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frequencies 200 rad s21 would decrease the viscositygradient due to temperature Ibar explained the effect ofvibration frequency and amplitude on melt viscosity interms of shear thinning criteria

When a vibrating force is applied to a concentratedand entangled polymer solution or melt the weak vander Waals force connecting with macromoleculesbecomes weaker and the entanglement is relaxed sothat the viscous force between the macromoleculesdecreases dramatically resulting in the reduction inviscosity

According to the experimental observation thefollowing allometric scaling can be obtained

gvb (2)

where b is a scaling exponent that varies with thecharacteristics of polymers For PMMA solution at239uC gv225 can be obtained from the experiment byIbar36

Application of vibration technology to polymerelectrospinningWhen an additional vibrating force is applied toconducting polymer solutions or melts a dramaticreduction in viscosity occurs

The voltage is applied to polarise dielectrics where thecharges are not completely free to move but the positiveand negative charges that compose the body may bedisplaced in relation to one another when a vibratingforce is applied (see Fig 6)

Owing to its lower viscosity a relatively lower voltageis needed to eject jet from the spinnerette

From equations (1) and (2) the following power lawcan be obtained29

dvd (3)

where d is the scaling exponent that varies amongdifferent polymers

The scheme of the vibration electrospinning set-upwas shown in Fig 7 The needle tip was connected to aDC high voltage generator via an alligator clip Anultrasonic generator was used to provide vibration invibration electrospinning set-up

The main advantages of vibration electrospinning are

(i) dramatic reduction in viscosity so that the jetlength can be controlled by frequency

(ii) finer fibres can be produced compared withtraditional electrospinning apparatus under thesame conditions or the patented apparatus caneven spin such fibres whereas the traditionalapparatus cannot or can spin fibres with thesame diameter at much lower voltage

(iii) the patented electrospinning process as that inpolymer extrusion can enhance the mechanicalproperties of electrospun fibres by modifying theamorphous and semicrystalline texture andorientational state

Thermo-electrospinning apparatusIt is well known that most melting polymers can beelectrospun into nanofibres however research efforts onmelt electrospinning are relatively scarce37 largelybecause a melt electrospinning apparatus is complexand expensive for the heater and the cooler are a must inthis apparatus While it is not as prevalent as polymersolution electrospinning melt electrospinning eliminatesthe need for organic solvents and has the potential toincrease throughput due to no loss in mass by solventevaporation37

Melt blowing process is a process which producesmicrofibres directly from polymer resin Melt blowing isused commercially as a one step process for convertingpolymer resin directly into a non-woven mat of fibres Inthe process a very high velocity gas impacts a moltenpolymer stream as illustrated in Fig 8 As a result thepolymer stream is attenuated rapidly by the highvelocity hot air and the resultant fibres are collectedupon a screen as a non-woven mat The fibre diametercan be reduced from 700 to 1 mm in 50 ms and thisdramatic change in diameter significantly weakens itsmechanical strength and affects other fibre character-istics such as pore size air permeability loft drape feeletc

Controlling stability by temperatureIn order to control the solvent evaporation a furnace isadded as illustrated in Fig 9 In the furnace thetemperature field can be adjusted and the temperature

a without electric field b with electric field plus vibrating force which leads to high molecular orientation due to electro-nic force and high mechanical properties

6 Effect of electronic field on entangled macromolecule

He et al Apparatus for preparing electrospun nanofibres

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near the orifice is lower than that of the melts and higherthan that near collector

Owing to the gradient of temperature cold air flowsup along the furnace wall and comes down along thefibre and the low velocity gas further impacts thepolymer jet as illustrated in Fig 9 Convection currentsoccurred in the upper half of the furnace and wereenhanced by the injection of comparatively cool feed-stock down the axis It is believed that the gas movedupward beside the furnace walls and downward in thecentre of the tube As a result the polymer jet is furtherattenuated and the jet length can be increased so thatthe instability can be controlled

It would be more effective if a set-up can be devised bytaking into account the controllable temperature (Fig 9)

High temperature can also dramatically reduce theviscosity and the electrospinning of polymer melts canoffer an advantage over solution electrospinning38

Figure 10 illustrates an apparatus of melt electrospin-ning system The effects of various melt electrospinningparameters on the morphology and fibre diameter ofpolypropylene of different tacticities were studied39

Recently He et al40 suggested some novel melt electro-spinning apparatus such as vibrationndashmelt electrospinningand aero-melt electrospinning as shown in Figs 11ndash13

Hot air is applied in aero-electrospinning (seeFig 13) Hot air further attenuates the filamentsmaking the electrospun fibres ever smaller Melts areextruded by static electronic force through small orificeinto convergent streams of hot air that rapidly attenuatethe charged jet into small diameter fibres The airstreams can also control the instability in the electro-spinning process and transport fibres to a collectorwhere they bond at fibrendashfibre contact points to producea cohesive non-woven web

Additionally McCann et al41 developed a meltcoaxial electrospinning similar to coelectrospinning insolution electrospinning for the encapsulation of solidmaterials and fabrication of phase change nanofibresOgata et al42 presented a new melt electrospinningsystem equipped with a laser melting device Theyemployed the laser beams from three directions toirradiate the end of rod-like sample The fibres obtainedwere collected onto a grounded rotating disc

7 Schematic of vibration technology in polymer electro-

spinning This apparatus was patented (China Patent

2004200205963) To use this principle to prepare elec-

trospun fibres transfer agreement must be made29

8 Schematic of thermo-electrospinning

9 Schematic of electrospinning process with controllable

temperature field

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Magneto-electrospinningAs it is well known that during the electrospinningprocess the charged jet is of intrinsic instability thatleads to the uneven construction of nanofibres and wasteof most energy which otherwise can be used to furtherstretch the jet into even smaller fibres (Fig 14) Theinstability also leads to low molecular orientation as aresult low mechanical properties arise

Our group suggests a completely novel approach tocontrol the instability in electrospinning and thetechnology is termed as megnetio-electrospinning4043

It can be concluded that the magnetic approach is themost effective and economical way to control instabilityand improve the mechanical properties as well

If a magnetic field is applied in the electrospinningprocess as illustrated in Fig 15 an Ampere force isgenerated due to the current in the polymer jet

FI

~LII

BI

(4)

where I is the current floating inside the conductor B isthe value of magnetic field induction and L is theconductor length

The resultant force of electric force and the viscousforce of the jet flow enlarges the whipping circle If amagnetic field is applied in the electrospinning theproblem can be completely overcome The current in thejet under the magnetic field produces a centripetalforce ie the direction of the Ampere force is alwaystowards the initial equilibrium point (see Fig 15)leading to the shrinking of the radius of whipping circleAs a result the stability condition is enormouslyimproved

The magnetic force is perpendicular to the velocity soit contributes no work to the moving jet the shrunkencircle means less energy waste in the instability processand the saved energy is used to increase the kineticenergy of the moving jet According to the conservationof mass pr2ru5Q thus r2lt1u the radius becomesmuch smaller than that without magnetic fieldMacromolecule chains of polymer jet are subject to thecoupled forces of electronic force and magnetic forceand arranged again (see Fig 16)

The produced magnetic force can ameliorate the innerstructure of macromolecules resulting in a remarkableamelioration of the strength of nanofibre

Siro-electrospinningTwo strand spun or Sirofil yarns4445 have now beenwidely used in the worsted industry The strands aretexturised to improve the bulk of the resultant yarnswhich have been demonstrated to possess more desirableproperties For example the weaveability of the fabricformed by the Sirofil yarns is significantly improved overits counterpart yarns Dragline silk is made of manynanofibres similar to siro-spinning To mimic the spider

10 Schematic illustration of set-up for melt electrospinning

12 Vibrationndashmelt electrospinning40

13 Aero-electrospinning40

11 Melt electrospinning does not require solvent tempera-

ture of melts can be controlled by electronic heater40

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spinning (see Fig 17) siro-electrospinning is created(see Fig 18) Li et al patented a conjugate electrospin-ning apparatus46ndash48 as illustrated in Fig 19 Two ormore fibres are combined together by weak staticelectronic force An apparatus is devised where a twistis added (see Fig 18) causing the fibres to have atendency to twist and crimp Fabrics so produced canimprove its mechanical characteristics furthermore itsstrength toughness softness and feel can be improveddramatically

The present technology can also be taken in conjunc-tion with other ones For example the vibrationtechnology can be added

Using this technique it is now possible to spinconductive polymer fibres under ambient conditions bydepositing nanometre metal particle in the polymersolutions or melts If some titanium dioxide is added inthe solutions or melts the electrospun fibres areself-clearing

Spider spinningAfter 400 million years of evolution nature endowsspiders with genius of spinning flexible lightweightfibres which have the strongest strength in the world atleast five times stronger by weight than steel and haveremarkable toughness and elasticity4950 Even in themodern times it is difficult to synthesise a materialhaving advantages of strength and toughness exceptcarbon nanotube fibres which is spun from solution invery hot temperature or pressure51 while the spider silksare produced at room temperature and from aqueoussolutions Spider silk is the only natural material thatcombines the properties unmatched by any knownsynthetic high performance fibres

Spider silks are protein based lsquobiopolymerrsquo filamentsor threads secreted by specialised epithelial cells asconcentrated soluble precursors of highly repetitiveprimary sequences52 Many experiments have beenconducted and much research has focused on gene

14 Instability in electrospinning most energy was wasted

due to instability

a application of magnetic field to electrospinning b Ampere force in electronic jet induced by magnetic field15 Effect of magnetic field on electrospinning43

16 Macromolecules under coupled forces of electronic

force and magnetic force

He et al Apparatus for preparing electrospun nanofibres

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sequencing of spider and a biomimicry technology hasbeen developed53ndash55 however theoretical analysis is notyet dealt with it and our understanding of themechanism of spider spinning is rare and primitive Ifthe mystery in its mechanism can be solved then themechanism can be applied to synthetic high performancefibres such as electrospun fibres with combination ofgreat strength and stretch The devised method must bemuch economical so this could be the beginning of anew materials revolution

Mystery in spider spinning processIt is still an enigma how a spider produces a silk whichhuman needs ten thousands voltage to do One possibleanswer is that a spider applies sufficiently the nano-effect of bubble dynamics

The pressure difference between the inside and outsideof a bubble depends upon the surface tension and theradius of the bubble

For a bubble with two surfaces providing tension thenet upward force on the top hemisphere of the bubble isjust the pressure difference times the area of theequatorial circle

Fupward~pr2(PiPo) (5)

The surface tension force downward around circle istwice the surface tension times the circumference sincetwo surfaces contribute to the force

Fdownward~2pr(2T) (6)

This gives

T~1

4r(PiPo) (7)

Hendricks et al56 calculated the minimum sprayingpotential of a suspended hemispherical conductingdrop in air as

Ethreshold~300 20pcreth THORN1=2(8)

Buchko et al57 modified equation (8) in the form

E2threshold~

4H2

L2( ln

2L

R

3

2)0117pcR (9)

Ethresholdc1=2 (10)

where Ethreshold is the critical voltage needed for theejection of the jet from the Taylor cone H is theseparation distance between the needle and the collectorL is the length of the needle (or capillary) R is the radiusof the needle and c is the surface tension of the solution

From Fig 20 it can be clearly observed that draglinesilk is made of many nanofibres with a diameter ofy20 nm Therefore it can be assumed that a bubble canbe produced at the apex of each tube with a diameter ofy20 nm According to equation (7) the surface tensionof such bubbles is extremely small thus it can be easilyovercome either by the body weight of spider or tensioncreated by the rear legs

Most natural spider silk is only 25ndash4 mm in diameterand the weight of most spiders varies from 90 to1500 mg Consider a dragline silk with a diameter of361026 m which consists of many nanoscale fibrils witha diameter of y20 nm (see Fig 20) Therefore thenumber of nanofibres in the assembly can be estimatedwhich reads

n~(3|106

20|109)2amp2|104 (11)

It means that there are tens of thousands of nanofibresin the assembly

Similar to the HallndashPetch relationship the nanofibrestrength depends upon the fibre diameter in nanoscale(from few nanometres to tens of nanometres)

t~t0zkt

d1=2(12)

where kt is the fitting parameters (material constants) t0

is the strength of the bulk material and d is the fibrediameter as shown in Fig 21

Comparing the strength of a single fibre tSF with thediameter of 361026 m with the strength of a draglineassembly tDA consisting of 26104 nanofibres with thediameter of 2061029 m the following can be estimated

tDAt0

tSFt0

~24|106 (13)

Extraordinarily high strength of a dragline assembly ispredicted compared with a single fibre with the samesection area and same material

The finding shows that it is a challenge to developtechnologies capable of preparing for nanofibres within50 nm

It means that there are tens of thousands of nanotubesin spinneret of spider A weight of G51024 kg of aspider is assumed In the spinning process the spider can

17 Sirofil technology to improve mechanical properties of

mother fibres

18 Siro-electrospinning40

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use its weight to overcome the surface tension of allbubbles produced at apex of nanotubes which requires

T~G (14)

from which the pressure difference between the insideand outside of a bubble can be determined which reads

DP~PiP0~4G

nr~

4|104

(2|104)(20|109)~1 N m2 (15)

Extremely small force is needed in the spider spinning

Bubble electrospinningFrom the hint of the mechanism of the spider spinning anew approach is designed to overcome the bottleneck inpresent electrospinning technology which is to minimisethe surface tension of the electrospun solutions mimick-ing the spider spinning Compressed gas is widely used intextile industry59 and this technology will be used toreduce the surface tension Our system60ndash62 consists of avertical solution reservoir with a gas tube feeding fromthe bottom in which a metal electrode is fixed along thecentreline of the tube and a grounded collector over thereservoir (see Fig 22) It has been found that manysmall bubbles with different sizes were produced on thesolution surface

The mechanism of the new electrospinning process isdeceptively simple in the absence of an electric fieldthe aerated solution forms various bubbles on thesurface When an electric field is present it inducescharges into the bubble surface and these quickly relaxto the bubble surface The coupling of surface chargeand the external electric field creates a tangential stressresulting in the deformation of the small bubble into aprotuberance induced upward directed reentrant jetOnce the electric field exceeds the critical value neededto overcome the surface tension a fluid jet ejects fromthe apex of the conical bubble (see Fig 23) Thethreshold voltage needed to overcome the surfacetension depends upon the size of the bubble and inletair pressure

Although the maximal or minimal size of a bubblemight depend upon the solution viscosity the surfacetension of bubbles is independent of properties of thespun solutions such as viscosity which is the mainobstruction in traditional electrospinning Temperaturecan be used to adjust the bubble sizes in practicalapplication of the new electrospinning process The keyadvantages of this electrospinning process used herecompared with the traditional electrospinning processare that it can produce smaller nanofibres withoutrequirement of nozzles which are the main shortcomingof the traditional electrospinning technology further-more in our method with a relatively low voltagemillions of protruded bubbles can be easily producedwhich can be electrospun into nanofibres simulta-neously and electrospinnability does not strongly

21 E modulus of nanofibres redrawn according to

experimental data by Gu et al58 dots are experimental

data

19 Schematic set-up used for conjugate electrospinning experiments46ndash48

20 Spider spinning process reproduced with the permis-

sion of Dennis Kunkel Microscopy Inc The diameter

of a single nanofibre is y20 nm its spinneret on the

posterior portion of the abdomen of spider consists

of millions of nanoscale tubes and a bubble can be

produced at apex of each nanotube The surface ten-

sion of each bubble is extremely small such that it

can be spun into nanofibres with an awfully small

force either by the body weight of spider or tension

created by the rear legs

He et al Apparatus for preparing electrospun nanofibres

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depend upon the solution viscosity overcoming com-pletely the main shortcoming of the traditional electro-spinning The average diameter of nanofibres can beeasily controllable and the minimum diameter in ourexperiment reached as small as 50 nm (see Fig 24)

This new technology is of critical importance for thenew generation of electrospinning especially forthe specialists in design manufacturing and usingnanofibres

Electrospinning dilation and electrospunnanoporous microspheresDuring the electrospinning process the charged jet isaccelerated by a constant external electric field and thespinning velocity probably reaches maximum andperhaps exceeds the velocity of sound in air in a veryshort time before the spinning becomes instableAccording to the conservation of mass equation

pr2ru~Q (16)

where r is the radius of the jet u is the velocity Q is theflowrate and r is the density The radius of the jetdecreases with increasing velocity of the incompressiblejet When the velocity reaches its maximum the radius

of the jet is minimal and macromoleculars of thepolymers are compacted together tighter and tighter asillustrated in Fig 25 There must exist a critical minimalradius rcr for all electrospun jet r(rcr for continuousultrafine fibres and the critical maximal velocity is

ucr~Q=prr2cr However the velocity can exceed this

critical value ucr if a higher voltage is applied

When the radius of the jet reaches the critical valuer5rcr and the jet speed exceeds its critical value uucrin order to keep conservation of mass equation the jetdilates by decreasing its density leading to porosity ofthe electrospun fibres (see Figs 26 and 27) which iscalled electrospinning dilation

Nanoporous microspheres are especially useful forinvisibility device (eg stealth plane)6413 and drugdeliver6566

New electrospinning apparatus for massproduction of nanofibresIn recent development and application of electrospunnanofibres one key bottleneck is the lack of capacity tothe mass production of nanofibres It is limited for atraditional electrospinning apparatus to increase theability of nanofibre throughout because there are manyseemingly insurmountable difficulties One is the smalldiameter (about 01ndash2 mm) of the spinneret and the lowflowrate (from 01 to dozens of mm h21) of the solutionIf the diameter is enlarged the suspended droplet may

22 Experimental set-up of aerated solution electrospin-

ning This principle to prepare for nanoproducts was

patented (China Patent 2007100364474) To use this

principle to prepare for nanoporous products transfer

agreement must be made63

23 Ejected jets captured by digital camera63

24 Minimum diameter of nanofibres was 50 nm (Ref 63)

25 Macromolecular chains are compacted during electro-

spinning

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not be formed as a stable Taylor cone at the orificebecause of the gravity and other factors and so is theincreasing flowrate Unlike the traditional dry spinningprocess or wet spinning process in which the jet isextruded from the spinneret with high speed thetraditional electrospinning process depends on theelectrostatic force which just provides a stable and slowliquid flow to form a stable Taylor cone at the tip of thespinneret rather than the extrusion pressure Thespinning speed is much less than that of the dry spinningor wet spinning The other shortcomings are easilyjammed difficult clean etc

As a result the possible technique of the use oftraditional electrospinning apparatus to produce nano-fibres of mass production is to fit the multiple spinneretstogether with a line or matrix pattern Many researchersadopted this technique to produce nanofibres from theearly period of the invention of this technology to nowIt is prevailing until Theron et al67 found bothexperimentally and in the simulations that the jets werepushed away from their neighbours by the Coulombicforces applied by the latter A non-uniform non-wovenmat would be obtained as an immediate consequence ofthis phenomenon Fang et al68 once reported that theysolved the problem of the mutual Coulombic interac-tions by the incorporation of secondary electrodes toisolate the electric field distribution of the primaryelectrode spinnerets But the problem of the smalldiameter of the spinneret and the low flowrate still

limited the fibre output Dosunmu et al69 suggested anew multiple porous tube apparatus in which thepolymer solution was electrified and pushed by airpressure through the walls of a large diameter cylindricalporous tube (see Fig 28a) Multiple jets formed on theporous surface and electrospun into nanofibres in theirprocess However the existing problem of this processmight be also the non-uniform fibre mat (see Fig 5 inRef 69)

In order to eliminate the problems of increasing thenanofibres of mass production of traditional electro-spinning apparatus some researchers demonstratedvarious novel electrospinning apparatus in recent yearsYet these apparatus different as they seem have onething in common The Taylor cone was formed on thefree surface of the solution and the jets were ejectedupward Yarin and Zussman70 proposed an upwardneedleless electrospinning which consisted of a two layersystem with the lower layer being a ferromagneticsuspension and the upper layer a polymer solution (seeFig 28b) In the process many vertical spikes ofmagnetic suspension perturbed the interlayer interfaceas the Taylor cones Though the production rate ishigher than that of traditional electrospinning processthere are some flaws such as not easy operation of twolayers liquids

So far only one electrospinning process Nanospiderspinning technology is claimed to be a commercialsuccess in the mass production of nanofibres all over theworld The principle of Nanospider is to use a cylinderto form the fibres The cylinder is partially immersed in

26 Electrospinning dilation phenomenon of poly

(butylenes succinate) (PBS) electrospun fibres64

27 Electrospun nanoporous microspheres where kind of

traditional Chinese drugs called lsquoYunnan Baiyorsquo was

used as additive65

a Dosunmursquos apparatus b upward needleless electro-spinning

28 New electrospinning apparatus for produing mass

production of nanofibres

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the polymer solution and as it rotates the polymersolution is carried to the top part of the cylinder whereTaylor cones are created The Taylor streams are formednext to each other throughout the entire length of thecylinder resulting in the high production capacity of thisapparatus71

The above novel approaches are exciting for theresearchers and the engineers When we cheer and applaudfor these much progress of electrospinning however a newproblem caused by the opened liquid surface has risen upIn every electrospinning process the volatilisation of thesolvent from the flying jet is a must In the above newupward electrospinning apparatus the free liquid surfacecan be used to create many Taylor cones and increase thethroughput of nanofibres at the same time can befacilitated to the escape of the volatilising solvent Withthe process proceeding the viscosity of the polymersolution will increase which will introduce some newproblems such as increasing the difficulty of spinning andchanging the properties of the solution

ConclusionsIt is clear that an understanding of the electrospinningand its nanofibres was greatly advanced by thenanoscience and technology in the 1990s and by thesubsequent development of the many effective applica-tions available in many fields today Remarkableadvances in electrospinning technology have beenpossible due to developments in modern technologyVarious novel and useful electrospinning techniques arereviewed in this work according to their differentmodifications Some modified apparatus were used toproduce different nanofibre mats with special structuresfor some special applications and some for massproduction of nanofibres Most of the advancesachieved to date have been made in the collector systemof the traditional electrospinning apparatus but otherparts such as solution storage and feeding systemspinneret and spinning area have been studied as wellMore recently emphasis has been placed on creatingnew apparatus to mass production of nanofibres Newtechniques have been designed and improved and moreelectrospinning apparatus have been developed to betterproduce nanofibres

Collectively the studies about electrospinning andnanofibres carried out so far have furnished a usefuldatabase for the electrohydrodynamics and some otherfields Although this field is in its infancy it is becomingincreasingly clear that there are multiple forms of newtechniques Identifying the similarities and differences invarious apparatus will enhance our understanding ofthis technology The next decades are expected to notonly enrich the existing knowledge but also significantlyadvance the technology into the commercial domain

Acknowledgements

The present work is supported financially by NationalNatural Science Foundation of China under Grant Nos10802021 and 10972053 National Natural ScienceFoundation of Shanghai under Grant No08ZR1400300 Shanghai Rising-Star Program underGrant No09QA1400100 Key Project of ChineseMinistry of Education No210006 National ScienceFoundation of Tianjin No10JCYBJC02200 Tianjin

Universities Science and Technology DevelopmentFund Project No20080322

References1 A Formhals lsquoProcess and apparatus for preparing artifical

threadsrsquo US Patent 1975504 1934

2 M S El Naschie Chaos Solitons Fract 2006 30 769ndash773

3 J-H He Chaos Solitons Fract 2008 38 1390ndash1393

4 J Lyons and F Ko Polym News 2005 30 170ndash178

5 D Li and Y-N Xia Adv Mater 2004 16 1151ndash1170

6 Z-M Huang Y-Z Zhang M Kotaki and S Ramakrishna

Compos Sci Technol 2003 63 2223ndash2253

7 Q P Pham U Sharma and A G Mikos Tissue Eng 2006 12

1197ndash1211

8 E P S Tan and C T Lim Compos Sci Technol 2006 66 1102ndash

1111

9 J-H He and H-M Liu Nonlinear Analy 2005 63 e919ndashe929

10 J-H He and Y-Q Wan Polymer 2004 45 6731ndash6734

11 J-H He Y-Q Wan and J-Y Yu Polymer 2005 46 2799ndash2801

12 J-H He Y-Q Wan and J-Y Yu Fibers Polym 2008 9 140ndash

142

13 J-H He L Xu Y Wu and Y Liu Polym Int 2007 56 1323ndash

1329

14 J-H He Y-Q Wan and J-Y Yu Int J Nonlinear Sci 2004 5

243ndash252

15 J-H He Y Wu and N Pan Int J Nonlinear Sci 2005 6 243ndash

248

16 Y-Q Wan J-H He and J-Y Yu Iran Polym J 2006 15 265ndash

268

17 L Rayleigh Philos Mag 1882 14 184ndash186

18 J F Cooley lsquoApparatus for electrically dispensing fluidsrsquo US

Patent 692631 1902

19 W J Morton lsquoMethod of dispensing fluidsrsquo US Patent 705691

1902

20 P K Baumgarten J Colloid Interface Sci 1971 36 71ndash79

21 J-H He and Y Liu J Polym Eng 2008 28 101ndash114

22 Y Liu J-H He L Xu and J-Y Yu Int J Electrospun Nanofibers

Appl 2007 1 7ndash15

23 Y Liu J-H He and J-Y Yu Fibres Text East Eur 2007 15

30ndash33

24 X-H Qin Y-Q Wan J-H He J Zhang J-Y Yu and

S-Y Wang Polymer 2004 45 6409ndash6413

25 D-H Shou and J-H He J Polym Eng 2008 28 115ndash118

26 Y-Q Wan J-H He Y Wu and J-Y Yu Mater Lett 2006 60

3296ndash3300

27 S-Q Wang J-H He and L Xu Polym Int 2008 57 1079ndash1082

28 Y Wu J-H He L Xu and J-Y Yu Int J Electrospun

Nanofibers Appl 2007 1 1ndash6

29 J-H He Y-Q Wan and J-Y Yu Int J Nonlinear Sci 2004 5

253ndash262

30 Y-Q Wan J-H He L Xu and J-Y Yu Int J Electrospun

Nanofibers Appl 2007 1 17ndash28

31 Y-Q Wan J-H He J-Y Yu and Y Wu J Appl Polym Sci

2007 103 3840ndash3843

32 J Zhang Y-Q Wan J-H He and Z-Y Zhu China Patent

2004100156853 2004

33 B H Bersted J Appl Polym Sci 2003 28 2777ndash2791

34 A I Isayev C M Wong and X Zeng Adv Polym Technol 2003

10 31ndash45

35 H Wu S Guo G Chen J Lin W Chen and H Wang J Appl

Polym Sci 2003 90 1873ndash1878

36 J P Ibar Polym Eng Sci 1998 38 1ndash20

37 S Sell C Barnes M Smith M McClure P Madurantakam

J Grant M McManus and G Bowlin Polym Int 2007 56 1349ndash

1360

38 P D Dalton K Klinkhammer J Salber D Klee and M Moller

Biomacromolecules 2006 7 686ndash690

39 J Lyons C Li and F Ko Polymer 2004 45 7597ndash7603

40 J-H He Y-Q Wan and L Xu Chaos Solitons Fract 2007 33

26ndash37

41 J T McCann M Marquez and Y Xia Nano Lett 2006 6 2868ndash

2872

42 N Ogata S Yamaguchi N Shimada G Lu T Iwata K Nakane

and T Ogihara J Appl Polym Sci 2007 104 1640ndash1645

43 Y Wu J-Y Yu J-H He and Y-Q Wan Chaos Solitons Fract

2007 32 5ndash7

44 J-H He Y-P Yu J-Y Yu W-R Li S-Y Wang and N Pan

Text Res J 2005 75 181ndash184

He et al Apparatus for preparing electrospun nanofibres

1286 Materials Science and Technology 2010 VOL 26 NO 11

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near the orifice is lower than that of the melts and higherthan that near collector

Owing to the gradient of temperature cold air flowsup along the furnace wall and comes down along thefibre and the low velocity gas further impacts thepolymer jet as illustrated in Fig 9 Convection currentsoccurred in the upper half of the furnace and wereenhanced by the injection of comparatively cool feed-stock down the axis It is believed that the gas movedupward beside the furnace walls and downward in thecentre of the tube As a result the polymer jet is furtherattenuated and the jet length can be increased so thatthe instability can be controlled

It would be more effective if a set-up can be devised bytaking into account the controllable temperature (Fig 9)

High temperature can also dramatically reduce theviscosity and the electrospinning of polymer melts canoffer an advantage over solution electrospinning38

Figure 10 illustrates an apparatus of melt electrospin-ning system The effects of various melt electrospinningparameters on the morphology and fibre diameter ofpolypropylene of different tacticities were studied39

Recently He et al40 suggested some novel melt electro-spinning apparatus such as vibrationndashmelt electrospinningand aero-melt electrospinning as shown in Figs 11ndash13

Hot air is applied in aero-electrospinning (seeFig 13) Hot air further attenuates the filamentsmaking the electrospun fibres ever smaller Melts areextruded by static electronic force through small orificeinto convergent streams of hot air that rapidly attenuatethe charged jet into small diameter fibres The airstreams can also control the instability in the electro-spinning process and transport fibres to a collectorwhere they bond at fibrendashfibre contact points to producea cohesive non-woven web

Additionally McCann et al41 developed a meltcoaxial electrospinning similar to coelectrospinning insolution electrospinning for the encapsulation of solidmaterials and fabrication of phase change nanofibresOgata et al42 presented a new melt electrospinningsystem equipped with a laser melting device Theyemployed the laser beams from three directions toirradiate the end of rod-like sample The fibres obtainedwere collected onto a grounded rotating disc

7 Schematic of vibration technology in polymer electro-

spinning This apparatus was patented (China Patent

2004200205963) To use this principle to prepare elec-

trospun fibres transfer agreement must be made29

8 Schematic of thermo-electrospinning

9 Schematic of electrospinning process with controllable

temperature field

He et al Apparatus for preparing electrospun nanofibres

Materials Science and Technology 2010 VOL 26 NO 11 1279

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Magneto-electrospinningAs it is well known that during the electrospinningprocess the charged jet is of intrinsic instability thatleads to the uneven construction of nanofibres and wasteof most energy which otherwise can be used to furtherstretch the jet into even smaller fibres (Fig 14) Theinstability also leads to low molecular orientation as aresult low mechanical properties arise

Our group suggests a completely novel approach tocontrol the instability in electrospinning and thetechnology is termed as megnetio-electrospinning4043

It can be concluded that the magnetic approach is themost effective and economical way to control instabilityand improve the mechanical properties as well

If a magnetic field is applied in the electrospinningprocess as illustrated in Fig 15 an Ampere force isgenerated due to the current in the polymer jet

FI

~LII

BI

(4)

where I is the current floating inside the conductor B isthe value of magnetic field induction and L is theconductor length

The resultant force of electric force and the viscousforce of the jet flow enlarges the whipping circle If amagnetic field is applied in the electrospinning theproblem can be completely overcome The current in thejet under the magnetic field produces a centripetalforce ie the direction of the Ampere force is alwaystowards the initial equilibrium point (see Fig 15)leading to the shrinking of the radius of whipping circleAs a result the stability condition is enormouslyimproved

The magnetic force is perpendicular to the velocity soit contributes no work to the moving jet the shrunkencircle means less energy waste in the instability processand the saved energy is used to increase the kineticenergy of the moving jet According to the conservationof mass pr2ru5Q thus r2lt1u the radius becomesmuch smaller than that without magnetic fieldMacromolecule chains of polymer jet are subject to thecoupled forces of electronic force and magnetic forceand arranged again (see Fig 16)

The produced magnetic force can ameliorate the innerstructure of macromolecules resulting in a remarkableamelioration of the strength of nanofibre

Siro-electrospinningTwo strand spun or Sirofil yarns4445 have now beenwidely used in the worsted industry The strands aretexturised to improve the bulk of the resultant yarnswhich have been demonstrated to possess more desirableproperties For example the weaveability of the fabricformed by the Sirofil yarns is significantly improved overits counterpart yarns Dragline silk is made of manynanofibres similar to siro-spinning To mimic the spider

10 Schematic illustration of set-up for melt electrospinning

12 Vibrationndashmelt electrospinning40

13 Aero-electrospinning40

11 Melt electrospinning does not require solvent tempera-

ture of melts can be controlled by electronic heater40

He et al Apparatus for preparing electrospun nanofibres

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spinning (see Fig 17) siro-electrospinning is created(see Fig 18) Li et al patented a conjugate electrospin-ning apparatus46ndash48 as illustrated in Fig 19 Two ormore fibres are combined together by weak staticelectronic force An apparatus is devised where a twistis added (see Fig 18) causing the fibres to have atendency to twist and crimp Fabrics so produced canimprove its mechanical characteristics furthermore itsstrength toughness softness and feel can be improveddramatically

The present technology can also be taken in conjunc-tion with other ones For example the vibrationtechnology can be added

Using this technique it is now possible to spinconductive polymer fibres under ambient conditions bydepositing nanometre metal particle in the polymersolutions or melts If some titanium dioxide is added inthe solutions or melts the electrospun fibres areself-clearing

Spider spinningAfter 400 million years of evolution nature endowsspiders with genius of spinning flexible lightweightfibres which have the strongest strength in the world atleast five times stronger by weight than steel and haveremarkable toughness and elasticity4950 Even in themodern times it is difficult to synthesise a materialhaving advantages of strength and toughness exceptcarbon nanotube fibres which is spun from solution invery hot temperature or pressure51 while the spider silksare produced at room temperature and from aqueoussolutions Spider silk is the only natural material thatcombines the properties unmatched by any knownsynthetic high performance fibres

Spider silks are protein based lsquobiopolymerrsquo filamentsor threads secreted by specialised epithelial cells asconcentrated soluble precursors of highly repetitiveprimary sequences52 Many experiments have beenconducted and much research has focused on gene

14 Instability in electrospinning most energy was wasted

due to instability

a application of magnetic field to electrospinning b Ampere force in electronic jet induced by magnetic field15 Effect of magnetic field on electrospinning43

16 Macromolecules under coupled forces of electronic

force and magnetic force

He et al Apparatus for preparing electrospun nanofibres

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sequencing of spider and a biomimicry technology hasbeen developed53ndash55 however theoretical analysis is notyet dealt with it and our understanding of themechanism of spider spinning is rare and primitive Ifthe mystery in its mechanism can be solved then themechanism can be applied to synthetic high performancefibres such as electrospun fibres with combination ofgreat strength and stretch The devised method must bemuch economical so this could be the beginning of anew materials revolution

Mystery in spider spinning processIt is still an enigma how a spider produces a silk whichhuman needs ten thousands voltage to do One possibleanswer is that a spider applies sufficiently the nano-effect of bubble dynamics

The pressure difference between the inside and outsideof a bubble depends upon the surface tension and theradius of the bubble

For a bubble with two surfaces providing tension thenet upward force on the top hemisphere of the bubble isjust the pressure difference times the area of theequatorial circle

Fupward~pr2(PiPo) (5)

The surface tension force downward around circle istwice the surface tension times the circumference sincetwo surfaces contribute to the force

Fdownward~2pr(2T) (6)

This gives

T~1

4r(PiPo) (7)

Hendricks et al56 calculated the minimum sprayingpotential of a suspended hemispherical conductingdrop in air as

Ethreshold~300 20pcreth THORN1=2(8)

Buchko et al57 modified equation (8) in the form

E2threshold~

4H2

L2( ln

2L

R

3

2)0117pcR (9)

Ethresholdc1=2 (10)

where Ethreshold is the critical voltage needed for theejection of the jet from the Taylor cone H is theseparation distance between the needle and the collectorL is the length of the needle (or capillary) R is the radiusof the needle and c is the surface tension of the solution

From Fig 20 it can be clearly observed that draglinesilk is made of many nanofibres with a diameter ofy20 nm Therefore it can be assumed that a bubble canbe produced at the apex of each tube with a diameter ofy20 nm According to equation (7) the surface tensionof such bubbles is extremely small thus it can be easilyovercome either by the body weight of spider or tensioncreated by the rear legs

Most natural spider silk is only 25ndash4 mm in diameterand the weight of most spiders varies from 90 to1500 mg Consider a dragline silk with a diameter of361026 m which consists of many nanoscale fibrils witha diameter of y20 nm (see Fig 20) Therefore thenumber of nanofibres in the assembly can be estimatedwhich reads

n~(3|106

20|109)2amp2|104 (11)

It means that there are tens of thousands of nanofibresin the assembly

Similar to the HallndashPetch relationship the nanofibrestrength depends upon the fibre diameter in nanoscale(from few nanometres to tens of nanometres)

t~t0zkt

d1=2(12)

where kt is the fitting parameters (material constants) t0

is the strength of the bulk material and d is the fibrediameter as shown in Fig 21

Comparing the strength of a single fibre tSF with thediameter of 361026 m with the strength of a draglineassembly tDA consisting of 26104 nanofibres with thediameter of 2061029 m the following can be estimated

tDAt0

tSFt0

~24|106 (13)

Extraordinarily high strength of a dragline assembly ispredicted compared with a single fibre with the samesection area and same material

The finding shows that it is a challenge to developtechnologies capable of preparing for nanofibres within50 nm

It means that there are tens of thousands of nanotubesin spinneret of spider A weight of G51024 kg of aspider is assumed In the spinning process the spider can

17 Sirofil technology to improve mechanical properties of

mother fibres

18 Siro-electrospinning40

He et al Apparatus for preparing electrospun nanofibres

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use its weight to overcome the surface tension of allbubbles produced at apex of nanotubes which requires

T~G (14)

from which the pressure difference between the insideand outside of a bubble can be determined which reads

DP~PiP0~4G

nr~

4|104

(2|104)(20|109)~1 N m2 (15)

Extremely small force is needed in the spider spinning

Bubble electrospinningFrom the hint of the mechanism of the spider spinning anew approach is designed to overcome the bottleneck inpresent electrospinning technology which is to minimisethe surface tension of the electrospun solutions mimick-ing the spider spinning Compressed gas is widely used intextile industry59 and this technology will be used toreduce the surface tension Our system60ndash62 consists of avertical solution reservoir with a gas tube feeding fromthe bottom in which a metal electrode is fixed along thecentreline of the tube and a grounded collector over thereservoir (see Fig 22) It has been found that manysmall bubbles with different sizes were produced on thesolution surface

The mechanism of the new electrospinning process isdeceptively simple in the absence of an electric fieldthe aerated solution forms various bubbles on thesurface When an electric field is present it inducescharges into the bubble surface and these quickly relaxto the bubble surface The coupling of surface chargeand the external electric field creates a tangential stressresulting in the deformation of the small bubble into aprotuberance induced upward directed reentrant jetOnce the electric field exceeds the critical value neededto overcome the surface tension a fluid jet ejects fromthe apex of the conical bubble (see Fig 23) Thethreshold voltage needed to overcome the surfacetension depends upon the size of the bubble and inletair pressure

Although the maximal or minimal size of a bubblemight depend upon the solution viscosity the surfacetension of bubbles is independent of properties of thespun solutions such as viscosity which is the mainobstruction in traditional electrospinning Temperaturecan be used to adjust the bubble sizes in practicalapplication of the new electrospinning process The keyadvantages of this electrospinning process used herecompared with the traditional electrospinning processare that it can produce smaller nanofibres withoutrequirement of nozzles which are the main shortcomingof the traditional electrospinning technology further-more in our method with a relatively low voltagemillions of protruded bubbles can be easily producedwhich can be electrospun into nanofibres simulta-neously and electrospinnability does not strongly

21 E modulus of nanofibres redrawn according to

experimental data by Gu et al58 dots are experimental

data

19 Schematic set-up used for conjugate electrospinning experiments46ndash48

20 Spider spinning process reproduced with the permis-

sion of Dennis Kunkel Microscopy Inc The diameter

of a single nanofibre is y20 nm its spinneret on the

posterior portion of the abdomen of spider consists

of millions of nanoscale tubes and a bubble can be

produced at apex of each nanotube The surface ten-

sion of each bubble is extremely small such that it

can be spun into nanofibres with an awfully small

force either by the body weight of spider or tension

created by the rear legs

He et al Apparatus for preparing electrospun nanofibres

Materials Science and Technology 2010 VOL 26 NO 11 1283

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depend upon the solution viscosity overcoming com-pletely the main shortcoming of the traditional electro-spinning The average diameter of nanofibres can beeasily controllable and the minimum diameter in ourexperiment reached as small as 50 nm (see Fig 24)

This new technology is of critical importance for thenew generation of electrospinning especially forthe specialists in design manufacturing and usingnanofibres

Electrospinning dilation and electrospunnanoporous microspheresDuring the electrospinning process the charged jet isaccelerated by a constant external electric field and thespinning velocity probably reaches maximum andperhaps exceeds the velocity of sound in air in a veryshort time before the spinning becomes instableAccording to the conservation of mass equation

pr2ru~Q (16)

where r is the radius of the jet u is the velocity Q is theflowrate and r is the density The radius of the jetdecreases with increasing velocity of the incompressiblejet When the velocity reaches its maximum the radius

of the jet is minimal and macromoleculars of thepolymers are compacted together tighter and tighter asillustrated in Fig 25 There must exist a critical minimalradius rcr for all electrospun jet r(rcr for continuousultrafine fibres and the critical maximal velocity is

ucr~Q=prr2cr However the velocity can exceed this

critical value ucr if a higher voltage is applied

When the radius of the jet reaches the critical valuer5rcr and the jet speed exceeds its critical value uucrin order to keep conservation of mass equation the jetdilates by decreasing its density leading to porosity ofthe electrospun fibres (see Figs 26 and 27) which iscalled electrospinning dilation

Nanoporous microspheres are especially useful forinvisibility device (eg stealth plane)6413 and drugdeliver6566

New electrospinning apparatus for massproduction of nanofibresIn recent development and application of electrospunnanofibres one key bottleneck is the lack of capacity tothe mass production of nanofibres It is limited for atraditional electrospinning apparatus to increase theability of nanofibre throughout because there are manyseemingly insurmountable difficulties One is the smalldiameter (about 01ndash2 mm) of the spinneret and the lowflowrate (from 01 to dozens of mm h21) of the solutionIf the diameter is enlarged the suspended droplet may

22 Experimental set-up of aerated solution electrospin-

ning This principle to prepare for nanoproducts was

patented (China Patent 2007100364474) To use this

principle to prepare for nanoporous products transfer

agreement must be made63

23 Ejected jets captured by digital camera63

24 Minimum diameter of nanofibres was 50 nm (Ref 63)

25 Macromolecular chains are compacted during electro-

spinning

He et al Apparatus for preparing electrospun nanofibres

1284 Materials Science and Technology 2010 VOL 26 NO 11

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not be formed as a stable Taylor cone at the orificebecause of the gravity and other factors and so is theincreasing flowrate Unlike the traditional dry spinningprocess or wet spinning process in which the jet isextruded from the spinneret with high speed thetraditional electrospinning process depends on theelectrostatic force which just provides a stable and slowliquid flow to form a stable Taylor cone at the tip of thespinneret rather than the extrusion pressure Thespinning speed is much less than that of the dry spinningor wet spinning The other shortcomings are easilyjammed difficult clean etc

As a result the possible technique of the use oftraditional electrospinning apparatus to produce nano-fibres of mass production is to fit the multiple spinneretstogether with a line or matrix pattern Many researchersadopted this technique to produce nanofibres from theearly period of the invention of this technology to nowIt is prevailing until Theron et al67 found bothexperimentally and in the simulations that the jets werepushed away from their neighbours by the Coulombicforces applied by the latter A non-uniform non-wovenmat would be obtained as an immediate consequence ofthis phenomenon Fang et al68 once reported that theysolved the problem of the mutual Coulombic interac-tions by the incorporation of secondary electrodes toisolate the electric field distribution of the primaryelectrode spinnerets But the problem of the smalldiameter of the spinneret and the low flowrate still

limited the fibre output Dosunmu et al69 suggested anew multiple porous tube apparatus in which thepolymer solution was electrified and pushed by airpressure through the walls of a large diameter cylindricalporous tube (see Fig 28a) Multiple jets formed on theporous surface and electrospun into nanofibres in theirprocess However the existing problem of this processmight be also the non-uniform fibre mat (see Fig 5 inRef 69)

In order to eliminate the problems of increasing thenanofibres of mass production of traditional electro-spinning apparatus some researchers demonstratedvarious novel electrospinning apparatus in recent yearsYet these apparatus different as they seem have onething in common The Taylor cone was formed on thefree surface of the solution and the jets were ejectedupward Yarin and Zussman70 proposed an upwardneedleless electrospinning which consisted of a two layersystem with the lower layer being a ferromagneticsuspension and the upper layer a polymer solution (seeFig 28b) In the process many vertical spikes ofmagnetic suspension perturbed the interlayer interfaceas the Taylor cones Though the production rate ishigher than that of traditional electrospinning processthere are some flaws such as not easy operation of twolayers liquids

So far only one electrospinning process Nanospiderspinning technology is claimed to be a commercialsuccess in the mass production of nanofibres all over theworld The principle of Nanospider is to use a cylinderto form the fibres The cylinder is partially immersed in

26 Electrospinning dilation phenomenon of poly

(butylenes succinate) (PBS) electrospun fibres64

27 Electrospun nanoporous microspheres where kind of

traditional Chinese drugs called lsquoYunnan Baiyorsquo was

used as additive65

a Dosunmursquos apparatus b upward needleless electro-spinning

28 New electrospinning apparatus for produing mass

production of nanofibres

He et al Apparatus for preparing electrospun nanofibres

Materials Science and Technology 2010 VOL 26 NO 11 1285

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the polymer solution and as it rotates the polymersolution is carried to the top part of the cylinder whereTaylor cones are created The Taylor streams are formednext to each other throughout the entire length of thecylinder resulting in the high production capacity of thisapparatus71

The above novel approaches are exciting for theresearchers and the engineers When we cheer and applaudfor these much progress of electrospinning however a newproblem caused by the opened liquid surface has risen upIn every electrospinning process the volatilisation of thesolvent from the flying jet is a must In the above newupward electrospinning apparatus the free liquid surfacecan be used to create many Taylor cones and increase thethroughput of nanofibres at the same time can befacilitated to the escape of the volatilising solvent Withthe process proceeding the viscosity of the polymersolution will increase which will introduce some newproblems such as increasing the difficulty of spinning andchanging the properties of the solution

ConclusionsIt is clear that an understanding of the electrospinningand its nanofibres was greatly advanced by thenanoscience and technology in the 1990s and by thesubsequent development of the many effective applica-tions available in many fields today Remarkableadvances in electrospinning technology have beenpossible due to developments in modern technologyVarious novel and useful electrospinning techniques arereviewed in this work according to their differentmodifications Some modified apparatus were used toproduce different nanofibre mats with special structuresfor some special applications and some for massproduction of nanofibres Most of the advancesachieved to date have been made in the collector systemof the traditional electrospinning apparatus but otherparts such as solution storage and feeding systemspinneret and spinning area have been studied as wellMore recently emphasis has been placed on creatingnew apparatus to mass production of nanofibres Newtechniques have been designed and improved and moreelectrospinning apparatus have been developed to betterproduce nanofibres

Collectively the studies about electrospinning andnanofibres carried out so far have furnished a usefuldatabase for the electrohydrodynamics and some otherfields Although this field is in its infancy it is becomingincreasingly clear that there are multiple forms of newtechniques Identifying the similarities and differences invarious apparatus will enhance our understanding ofthis technology The next decades are expected to notonly enrich the existing knowledge but also significantlyadvance the technology into the commercial domain

Acknowledgements

The present work is supported financially by NationalNatural Science Foundation of China under Grant Nos10802021 and 10972053 National Natural ScienceFoundation of Shanghai under Grant No08ZR1400300 Shanghai Rising-Star Program underGrant No09QA1400100 Key Project of ChineseMinistry of Education No210006 National ScienceFoundation of Tianjin No10JCYBJC02200 Tianjin

Universities Science and Technology DevelopmentFund Project No20080322

References1 A Formhals lsquoProcess and apparatus for preparing artifical

threadsrsquo US Patent 1975504 1934

2 M S El Naschie Chaos Solitons Fract 2006 30 769ndash773

3 J-H He Chaos Solitons Fract 2008 38 1390ndash1393

4 J Lyons and F Ko Polym News 2005 30 170ndash178

5 D Li and Y-N Xia Adv Mater 2004 16 1151ndash1170

6 Z-M Huang Y-Z Zhang M Kotaki and S Ramakrishna

Compos Sci Technol 2003 63 2223ndash2253

7 Q P Pham U Sharma and A G Mikos Tissue Eng 2006 12

1197ndash1211

8 E P S Tan and C T Lim Compos Sci Technol 2006 66 1102ndash

1111

9 J-H He and H-M Liu Nonlinear Analy 2005 63 e919ndashe929

10 J-H He and Y-Q Wan Polymer 2004 45 6731ndash6734

11 J-H He Y-Q Wan and J-Y Yu Polymer 2005 46 2799ndash2801

12 J-H He Y-Q Wan and J-Y Yu Fibers Polym 2008 9 140ndash

142

13 J-H He L Xu Y Wu and Y Liu Polym Int 2007 56 1323ndash

1329

14 J-H He Y-Q Wan and J-Y Yu Int J Nonlinear Sci 2004 5

243ndash252

15 J-H He Y Wu and N Pan Int J Nonlinear Sci 2005 6 243ndash

248

16 Y-Q Wan J-H He and J-Y Yu Iran Polym J 2006 15 265ndash

268

17 L Rayleigh Philos Mag 1882 14 184ndash186

18 J F Cooley lsquoApparatus for electrically dispensing fluidsrsquo US

Patent 692631 1902

19 W J Morton lsquoMethod of dispensing fluidsrsquo US Patent 705691

1902

20 P K Baumgarten J Colloid Interface Sci 1971 36 71ndash79

21 J-H He and Y Liu J Polym Eng 2008 28 101ndash114

22 Y Liu J-H He L Xu and J-Y Yu Int J Electrospun Nanofibers

Appl 2007 1 7ndash15

23 Y Liu J-H He and J-Y Yu Fibres Text East Eur 2007 15

30ndash33

24 X-H Qin Y-Q Wan J-H He J Zhang J-Y Yu and

S-Y Wang Polymer 2004 45 6409ndash6413

25 D-H Shou and J-H He J Polym Eng 2008 28 115ndash118

26 Y-Q Wan J-H He Y Wu and J-Y Yu Mater Lett 2006 60

3296ndash3300

27 S-Q Wang J-H He and L Xu Polym Int 2008 57 1079ndash1082

28 Y Wu J-H He L Xu and J-Y Yu Int J Electrospun

Nanofibers Appl 2007 1 1ndash6

29 J-H He Y-Q Wan and J-Y Yu Int J Nonlinear Sci 2004 5

253ndash262

30 Y-Q Wan J-H He L Xu and J-Y Yu Int J Electrospun

Nanofibers Appl 2007 1 17ndash28

31 Y-Q Wan J-H He J-Y Yu and Y Wu J Appl Polym Sci

2007 103 3840ndash3843

32 J Zhang Y-Q Wan J-H He and Z-Y Zhu China Patent

2004100156853 2004

33 B H Bersted J Appl Polym Sci 2003 28 2777ndash2791

34 A I Isayev C M Wong and X Zeng Adv Polym Technol 2003

10 31ndash45

35 H Wu S Guo G Chen J Lin W Chen and H Wang J Appl

Polym Sci 2003 90 1873ndash1878

36 J P Ibar Polym Eng Sci 1998 38 1ndash20

37 S Sell C Barnes M Smith M McClure P Madurantakam

J Grant M McManus and G Bowlin Polym Int 2007 56 1349ndash

1360

38 P D Dalton K Klinkhammer J Salber D Klee and M Moller

Biomacromolecules 2006 7 686ndash690

39 J Lyons C Li and F Ko Polymer 2004 45 7597ndash7603

40 J-H He Y-Q Wan and L Xu Chaos Solitons Fract 2007 33

26ndash37

41 J T McCann M Marquez and Y Xia Nano Lett 2006 6 2868ndash

2872

42 N Ogata S Yamaguchi N Shimada G Lu T Iwata K Nakane

and T Ogihara J Appl Polym Sci 2007 104 1640ndash1645

43 Y Wu J-Y Yu J-H He and Y-Q Wan Chaos Solitons Fract

2007 32 5ndash7

44 J-H He Y-P Yu J-Y Yu W-R Li S-Y Wang and N Pan

Text Res J 2005 75 181ndash184

He et al Apparatus for preparing electrospun nanofibres

1286 Materials Science and Technology 2010 VOL 26 NO 11

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Magneto-electrospinningAs it is well known that during the electrospinningprocess the charged jet is of intrinsic instability thatleads to the uneven construction of nanofibres and wasteof most energy which otherwise can be used to furtherstretch the jet into even smaller fibres (Fig 14) Theinstability also leads to low molecular orientation as aresult low mechanical properties arise

Our group suggests a completely novel approach tocontrol the instability in electrospinning and thetechnology is termed as megnetio-electrospinning4043

It can be concluded that the magnetic approach is themost effective and economical way to control instabilityand improve the mechanical properties as well

If a magnetic field is applied in the electrospinningprocess as illustrated in Fig 15 an Ampere force isgenerated due to the current in the polymer jet

FI

~LII

BI

(4)

where I is the current floating inside the conductor B isthe value of magnetic field induction and L is theconductor length

The resultant force of electric force and the viscousforce of the jet flow enlarges the whipping circle If amagnetic field is applied in the electrospinning theproblem can be completely overcome The current in thejet under the magnetic field produces a centripetalforce ie the direction of the Ampere force is alwaystowards the initial equilibrium point (see Fig 15)leading to the shrinking of the radius of whipping circleAs a result the stability condition is enormouslyimproved

The magnetic force is perpendicular to the velocity soit contributes no work to the moving jet the shrunkencircle means less energy waste in the instability processand the saved energy is used to increase the kineticenergy of the moving jet According to the conservationof mass pr2ru5Q thus r2lt1u the radius becomesmuch smaller than that without magnetic fieldMacromolecule chains of polymer jet are subject to thecoupled forces of electronic force and magnetic forceand arranged again (see Fig 16)

The produced magnetic force can ameliorate the innerstructure of macromolecules resulting in a remarkableamelioration of the strength of nanofibre

Siro-electrospinningTwo strand spun or Sirofil yarns4445 have now beenwidely used in the worsted industry The strands aretexturised to improve the bulk of the resultant yarnswhich have been demonstrated to possess more desirableproperties For example the weaveability of the fabricformed by the Sirofil yarns is significantly improved overits counterpart yarns Dragline silk is made of manynanofibres similar to siro-spinning To mimic the spider

10 Schematic illustration of set-up for melt electrospinning

12 Vibrationndashmelt electrospinning40

13 Aero-electrospinning40

11 Melt electrospinning does not require solvent tempera-

ture of melts can be controlled by electronic heater40

He et al Apparatus for preparing electrospun nanofibres

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spinning (see Fig 17) siro-electrospinning is created(see Fig 18) Li et al patented a conjugate electrospin-ning apparatus46ndash48 as illustrated in Fig 19 Two ormore fibres are combined together by weak staticelectronic force An apparatus is devised where a twistis added (see Fig 18) causing the fibres to have atendency to twist and crimp Fabrics so produced canimprove its mechanical characteristics furthermore itsstrength toughness softness and feel can be improveddramatically

The present technology can also be taken in conjunc-tion with other ones For example the vibrationtechnology can be added

Using this technique it is now possible to spinconductive polymer fibres under ambient conditions bydepositing nanometre metal particle in the polymersolutions or melts If some titanium dioxide is added inthe solutions or melts the electrospun fibres areself-clearing

Spider spinningAfter 400 million years of evolution nature endowsspiders with genius of spinning flexible lightweightfibres which have the strongest strength in the world atleast five times stronger by weight than steel and haveremarkable toughness and elasticity4950 Even in themodern times it is difficult to synthesise a materialhaving advantages of strength and toughness exceptcarbon nanotube fibres which is spun from solution invery hot temperature or pressure51 while the spider silksare produced at room temperature and from aqueoussolutions Spider silk is the only natural material thatcombines the properties unmatched by any knownsynthetic high performance fibres

Spider silks are protein based lsquobiopolymerrsquo filamentsor threads secreted by specialised epithelial cells asconcentrated soluble precursors of highly repetitiveprimary sequences52 Many experiments have beenconducted and much research has focused on gene

14 Instability in electrospinning most energy was wasted

due to instability

a application of magnetic field to electrospinning b Ampere force in electronic jet induced by magnetic field15 Effect of magnetic field on electrospinning43

16 Macromolecules under coupled forces of electronic

force and magnetic force

He et al Apparatus for preparing electrospun nanofibres

Materials Science and Technology 2010 VOL 26 NO 11 1281

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sequencing of spider and a biomimicry technology hasbeen developed53ndash55 however theoretical analysis is notyet dealt with it and our understanding of themechanism of spider spinning is rare and primitive Ifthe mystery in its mechanism can be solved then themechanism can be applied to synthetic high performancefibres such as electrospun fibres with combination ofgreat strength and stretch The devised method must bemuch economical so this could be the beginning of anew materials revolution

Mystery in spider spinning processIt is still an enigma how a spider produces a silk whichhuman needs ten thousands voltage to do One possibleanswer is that a spider applies sufficiently the nano-effect of bubble dynamics

The pressure difference between the inside and outsideof a bubble depends upon the surface tension and theradius of the bubble

For a bubble with two surfaces providing tension thenet upward force on the top hemisphere of the bubble isjust the pressure difference times the area of theequatorial circle

Fupward~pr2(PiPo) (5)

The surface tension force downward around circle istwice the surface tension times the circumference sincetwo surfaces contribute to the force

Fdownward~2pr(2T) (6)

This gives

T~1

4r(PiPo) (7)

Hendricks et al56 calculated the minimum sprayingpotential of a suspended hemispherical conductingdrop in air as

Ethreshold~300 20pcreth THORN1=2(8)

Buchko et al57 modified equation (8) in the form

E2threshold~

4H2

L2( ln

2L

R

3

2)0117pcR (9)

Ethresholdc1=2 (10)

where Ethreshold is the critical voltage needed for theejection of the jet from the Taylor cone H is theseparation distance between the needle and the collectorL is the length of the needle (or capillary) R is the radiusof the needle and c is the surface tension of the solution

From Fig 20 it can be clearly observed that draglinesilk is made of many nanofibres with a diameter ofy20 nm Therefore it can be assumed that a bubble canbe produced at the apex of each tube with a diameter ofy20 nm According to equation (7) the surface tensionof such bubbles is extremely small thus it can be easilyovercome either by the body weight of spider or tensioncreated by the rear legs

Most natural spider silk is only 25ndash4 mm in diameterand the weight of most spiders varies from 90 to1500 mg Consider a dragline silk with a diameter of361026 m which consists of many nanoscale fibrils witha diameter of y20 nm (see Fig 20) Therefore thenumber of nanofibres in the assembly can be estimatedwhich reads

n~(3|106

20|109)2amp2|104 (11)

It means that there are tens of thousands of nanofibresin the assembly

Similar to the HallndashPetch relationship the nanofibrestrength depends upon the fibre diameter in nanoscale(from few nanometres to tens of nanometres)

t~t0zkt

d1=2(12)

where kt is the fitting parameters (material constants) t0

is the strength of the bulk material and d is the fibrediameter as shown in Fig 21

Comparing the strength of a single fibre tSF with thediameter of 361026 m with the strength of a draglineassembly tDA consisting of 26104 nanofibres with thediameter of 2061029 m the following can be estimated

tDAt0

tSFt0

~24|106 (13)

Extraordinarily high strength of a dragline assembly ispredicted compared with a single fibre with the samesection area and same material

The finding shows that it is a challenge to developtechnologies capable of preparing for nanofibres within50 nm

It means that there are tens of thousands of nanotubesin spinneret of spider A weight of G51024 kg of aspider is assumed In the spinning process the spider can

17 Sirofil technology to improve mechanical properties of

mother fibres

18 Siro-electrospinning40

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1282 Materials Science and Technology 2010 VOL 26 NO 11

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use its weight to overcome the surface tension of allbubbles produced at apex of nanotubes which requires

T~G (14)

from which the pressure difference between the insideand outside of a bubble can be determined which reads

DP~PiP0~4G

nr~

4|104

(2|104)(20|109)~1 N m2 (15)

Extremely small force is needed in the spider spinning

Bubble electrospinningFrom the hint of the mechanism of the spider spinning anew approach is designed to overcome the bottleneck inpresent electrospinning technology which is to minimisethe surface tension of the electrospun solutions mimick-ing the spider spinning Compressed gas is widely used intextile industry59 and this technology will be used toreduce the surface tension Our system60ndash62 consists of avertical solution reservoir with a gas tube feeding fromthe bottom in which a metal electrode is fixed along thecentreline of the tube and a grounded collector over thereservoir (see Fig 22) It has been found that manysmall bubbles with different sizes were produced on thesolution surface

The mechanism of the new electrospinning process isdeceptively simple in the absence of an electric fieldthe aerated solution forms various bubbles on thesurface When an electric field is present it inducescharges into the bubble surface and these quickly relaxto the bubble surface The coupling of surface chargeand the external electric field creates a tangential stressresulting in the deformation of the small bubble into aprotuberance induced upward directed reentrant jetOnce the electric field exceeds the critical value neededto overcome the surface tension a fluid jet ejects fromthe apex of the conical bubble (see Fig 23) Thethreshold voltage needed to overcome the surfacetension depends upon the size of the bubble and inletair pressure

Although the maximal or minimal size of a bubblemight depend upon the solution viscosity the surfacetension of bubbles is independent of properties of thespun solutions such as viscosity which is the mainobstruction in traditional electrospinning Temperaturecan be used to adjust the bubble sizes in practicalapplication of the new electrospinning process The keyadvantages of this electrospinning process used herecompared with the traditional electrospinning processare that it can produce smaller nanofibres withoutrequirement of nozzles which are the main shortcomingof the traditional electrospinning technology further-more in our method with a relatively low voltagemillions of protruded bubbles can be easily producedwhich can be electrospun into nanofibres simulta-neously and electrospinnability does not strongly

21 E modulus of nanofibres redrawn according to

experimental data by Gu et al58 dots are experimental

data

19 Schematic set-up used for conjugate electrospinning experiments46ndash48

20 Spider spinning process reproduced with the permis-

sion of Dennis Kunkel Microscopy Inc The diameter

of a single nanofibre is y20 nm its spinneret on the

posterior portion of the abdomen of spider consists

of millions of nanoscale tubes and a bubble can be

produced at apex of each nanotube The surface ten-

sion of each bubble is extremely small such that it

can be spun into nanofibres with an awfully small

force either by the body weight of spider or tension

created by the rear legs

He et al Apparatus for preparing electrospun nanofibres

Materials Science and Technology 2010 VOL 26 NO 11 1283

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depend upon the solution viscosity overcoming com-pletely the main shortcoming of the traditional electro-spinning The average diameter of nanofibres can beeasily controllable and the minimum diameter in ourexperiment reached as small as 50 nm (see Fig 24)

This new technology is of critical importance for thenew generation of electrospinning especially forthe specialists in design manufacturing and usingnanofibres

Electrospinning dilation and electrospunnanoporous microspheresDuring the electrospinning process the charged jet isaccelerated by a constant external electric field and thespinning velocity probably reaches maximum andperhaps exceeds the velocity of sound in air in a veryshort time before the spinning becomes instableAccording to the conservation of mass equation

pr2ru~Q (16)

where r is the radius of the jet u is the velocity Q is theflowrate and r is the density The radius of the jetdecreases with increasing velocity of the incompressiblejet When the velocity reaches its maximum the radius

of the jet is minimal and macromoleculars of thepolymers are compacted together tighter and tighter asillustrated in Fig 25 There must exist a critical minimalradius rcr for all electrospun jet r(rcr for continuousultrafine fibres and the critical maximal velocity is

ucr~Q=prr2cr However the velocity can exceed this

critical value ucr if a higher voltage is applied

When the radius of the jet reaches the critical valuer5rcr and the jet speed exceeds its critical value uucrin order to keep conservation of mass equation the jetdilates by decreasing its density leading to porosity ofthe electrospun fibres (see Figs 26 and 27) which iscalled electrospinning dilation

Nanoporous microspheres are especially useful forinvisibility device (eg stealth plane)6413 and drugdeliver6566

New electrospinning apparatus for massproduction of nanofibresIn recent development and application of electrospunnanofibres one key bottleneck is the lack of capacity tothe mass production of nanofibres It is limited for atraditional electrospinning apparatus to increase theability of nanofibre throughout because there are manyseemingly insurmountable difficulties One is the smalldiameter (about 01ndash2 mm) of the spinneret and the lowflowrate (from 01 to dozens of mm h21) of the solutionIf the diameter is enlarged the suspended droplet may

22 Experimental set-up of aerated solution electrospin-

ning This principle to prepare for nanoproducts was

patented (China Patent 2007100364474) To use this

principle to prepare for nanoporous products transfer

agreement must be made63

23 Ejected jets captured by digital camera63

24 Minimum diameter of nanofibres was 50 nm (Ref 63)

25 Macromolecular chains are compacted during electro-

spinning

He et al Apparatus for preparing electrospun nanofibres

1284 Materials Science and Technology 2010 VOL 26 NO 11

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Ltd

not be formed as a stable Taylor cone at the orificebecause of the gravity and other factors and so is theincreasing flowrate Unlike the traditional dry spinningprocess or wet spinning process in which the jet isextruded from the spinneret with high speed thetraditional electrospinning process depends on theelectrostatic force which just provides a stable and slowliquid flow to form a stable Taylor cone at the tip of thespinneret rather than the extrusion pressure Thespinning speed is much less than that of the dry spinningor wet spinning The other shortcomings are easilyjammed difficult clean etc

As a result the possible technique of the use oftraditional electrospinning apparatus to produce nano-fibres of mass production is to fit the multiple spinneretstogether with a line or matrix pattern Many researchersadopted this technique to produce nanofibres from theearly period of the invention of this technology to nowIt is prevailing until Theron et al67 found bothexperimentally and in the simulations that the jets werepushed away from their neighbours by the Coulombicforces applied by the latter A non-uniform non-wovenmat would be obtained as an immediate consequence ofthis phenomenon Fang et al68 once reported that theysolved the problem of the mutual Coulombic interac-tions by the incorporation of secondary electrodes toisolate the electric field distribution of the primaryelectrode spinnerets But the problem of the smalldiameter of the spinneret and the low flowrate still

limited the fibre output Dosunmu et al69 suggested anew multiple porous tube apparatus in which thepolymer solution was electrified and pushed by airpressure through the walls of a large diameter cylindricalporous tube (see Fig 28a) Multiple jets formed on theporous surface and electrospun into nanofibres in theirprocess However the existing problem of this processmight be also the non-uniform fibre mat (see Fig 5 inRef 69)

In order to eliminate the problems of increasing thenanofibres of mass production of traditional electro-spinning apparatus some researchers demonstratedvarious novel electrospinning apparatus in recent yearsYet these apparatus different as they seem have onething in common The Taylor cone was formed on thefree surface of the solution and the jets were ejectedupward Yarin and Zussman70 proposed an upwardneedleless electrospinning which consisted of a two layersystem with the lower layer being a ferromagneticsuspension and the upper layer a polymer solution (seeFig 28b) In the process many vertical spikes ofmagnetic suspension perturbed the interlayer interfaceas the Taylor cones Though the production rate ishigher than that of traditional electrospinning processthere are some flaws such as not easy operation of twolayers liquids

So far only one electrospinning process Nanospiderspinning technology is claimed to be a commercialsuccess in the mass production of nanofibres all over theworld The principle of Nanospider is to use a cylinderto form the fibres The cylinder is partially immersed in

26 Electrospinning dilation phenomenon of poly

(butylenes succinate) (PBS) electrospun fibres64

27 Electrospun nanoporous microspheres where kind of

traditional Chinese drugs called lsquoYunnan Baiyorsquo was

used as additive65

a Dosunmursquos apparatus b upward needleless electro-spinning

28 New electrospinning apparatus for produing mass

production of nanofibres

He et al Apparatus for preparing electrospun nanofibres

Materials Science and Technology 2010 VOL 26 NO 11 1285

Pub

lishe

d by

Man

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ublis

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(c)

IOM

Com

mun

icat

ions

Ltd

the polymer solution and as it rotates the polymersolution is carried to the top part of the cylinder whereTaylor cones are created The Taylor streams are formednext to each other throughout the entire length of thecylinder resulting in the high production capacity of thisapparatus71

The above novel approaches are exciting for theresearchers and the engineers When we cheer and applaudfor these much progress of electrospinning however a newproblem caused by the opened liquid surface has risen upIn every electrospinning process the volatilisation of thesolvent from the flying jet is a must In the above newupward electrospinning apparatus the free liquid surfacecan be used to create many Taylor cones and increase thethroughput of nanofibres at the same time can befacilitated to the escape of the volatilising solvent Withthe process proceeding the viscosity of the polymersolution will increase which will introduce some newproblems such as increasing the difficulty of spinning andchanging the properties of the solution

ConclusionsIt is clear that an understanding of the electrospinningand its nanofibres was greatly advanced by thenanoscience and technology in the 1990s and by thesubsequent development of the many effective applica-tions available in many fields today Remarkableadvances in electrospinning technology have beenpossible due to developments in modern technologyVarious novel and useful electrospinning techniques arereviewed in this work according to their differentmodifications Some modified apparatus were used toproduce different nanofibre mats with special structuresfor some special applications and some for massproduction of nanofibres Most of the advancesachieved to date have been made in the collector systemof the traditional electrospinning apparatus but otherparts such as solution storage and feeding systemspinneret and spinning area have been studied as wellMore recently emphasis has been placed on creatingnew apparatus to mass production of nanofibres Newtechniques have been designed and improved and moreelectrospinning apparatus have been developed to betterproduce nanofibres

Collectively the studies about electrospinning andnanofibres carried out so far have furnished a usefuldatabase for the electrohydrodynamics and some otherfields Although this field is in its infancy it is becomingincreasingly clear that there are multiple forms of newtechniques Identifying the similarities and differences invarious apparatus will enhance our understanding ofthis technology The next decades are expected to notonly enrich the existing knowledge but also significantlyadvance the technology into the commercial domain

Acknowledgements

The present work is supported financially by NationalNatural Science Foundation of China under Grant Nos10802021 and 10972053 National Natural ScienceFoundation of Shanghai under Grant No08ZR1400300 Shanghai Rising-Star Program underGrant No09QA1400100 Key Project of ChineseMinistry of Education No210006 National ScienceFoundation of Tianjin No10JCYBJC02200 Tianjin

Universities Science and Technology DevelopmentFund Project No20080322

References1 A Formhals lsquoProcess and apparatus for preparing artifical

threadsrsquo US Patent 1975504 1934

2 M S El Naschie Chaos Solitons Fract 2006 30 769ndash773

3 J-H He Chaos Solitons Fract 2008 38 1390ndash1393

4 J Lyons and F Ko Polym News 2005 30 170ndash178

5 D Li and Y-N Xia Adv Mater 2004 16 1151ndash1170

6 Z-M Huang Y-Z Zhang M Kotaki and S Ramakrishna

Compos Sci Technol 2003 63 2223ndash2253

7 Q P Pham U Sharma and A G Mikos Tissue Eng 2006 12

1197ndash1211

8 E P S Tan and C T Lim Compos Sci Technol 2006 66 1102ndash

1111

9 J-H He and H-M Liu Nonlinear Analy 2005 63 e919ndashe929

10 J-H He and Y-Q Wan Polymer 2004 45 6731ndash6734

11 J-H He Y-Q Wan and J-Y Yu Polymer 2005 46 2799ndash2801

12 J-H He Y-Q Wan and J-Y Yu Fibers Polym 2008 9 140ndash

142

13 J-H He L Xu Y Wu and Y Liu Polym Int 2007 56 1323ndash

1329

14 J-H He Y-Q Wan and J-Y Yu Int J Nonlinear Sci 2004 5

243ndash252

15 J-H He Y Wu and N Pan Int J Nonlinear Sci 2005 6 243ndash

248

16 Y-Q Wan J-H He and J-Y Yu Iran Polym J 2006 15 265ndash

268

17 L Rayleigh Philos Mag 1882 14 184ndash186

18 J F Cooley lsquoApparatus for electrically dispensing fluidsrsquo US

Patent 692631 1902

19 W J Morton lsquoMethod of dispensing fluidsrsquo US Patent 705691

1902

20 P K Baumgarten J Colloid Interface Sci 1971 36 71ndash79

21 J-H He and Y Liu J Polym Eng 2008 28 101ndash114

22 Y Liu J-H He L Xu and J-Y Yu Int J Electrospun Nanofibers

Appl 2007 1 7ndash15

23 Y Liu J-H He and J-Y Yu Fibres Text East Eur 2007 15

30ndash33

24 X-H Qin Y-Q Wan J-H He J Zhang J-Y Yu and

S-Y Wang Polymer 2004 45 6409ndash6413

25 D-H Shou and J-H He J Polym Eng 2008 28 115ndash118

26 Y-Q Wan J-H He Y Wu and J-Y Yu Mater Lett 2006 60

3296ndash3300

27 S-Q Wang J-H He and L Xu Polym Int 2008 57 1079ndash1082

28 Y Wu J-H He L Xu and J-Y Yu Int J Electrospun

Nanofibers Appl 2007 1 1ndash6

29 J-H He Y-Q Wan and J-Y Yu Int J Nonlinear Sci 2004 5

253ndash262

30 Y-Q Wan J-H He L Xu and J-Y Yu Int J Electrospun

Nanofibers Appl 2007 1 17ndash28

31 Y-Q Wan J-H He J-Y Yu and Y Wu J Appl Polym Sci

2007 103 3840ndash3843

32 J Zhang Y-Q Wan J-H He and Z-Y Zhu China Patent

2004100156853 2004

33 B H Bersted J Appl Polym Sci 2003 28 2777ndash2791

34 A I Isayev C M Wong and X Zeng Adv Polym Technol 2003

10 31ndash45

35 H Wu S Guo G Chen J Lin W Chen and H Wang J Appl

Polym Sci 2003 90 1873ndash1878

36 J P Ibar Polym Eng Sci 1998 38 1ndash20

37 S Sell C Barnes M Smith M McClure P Madurantakam

J Grant M McManus and G Bowlin Polym Int 2007 56 1349ndash

1360

38 P D Dalton K Klinkhammer J Salber D Klee and M Moller

Biomacromolecules 2006 7 686ndash690

39 J Lyons C Li and F Ko Polymer 2004 45 7597ndash7603

40 J-H He Y-Q Wan and L Xu Chaos Solitons Fract 2007 33

26ndash37

41 J T McCann M Marquez and Y Xia Nano Lett 2006 6 2868ndash

2872

42 N Ogata S Yamaguchi N Shimada G Lu T Iwata K Nakane

and T Ogihara J Appl Polym Sci 2007 104 1640ndash1645

43 Y Wu J-Y Yu J-H He and Y-Q Wan Chaos Solitons Fract

2007 32 5ndash7

44 J-H He Y-P Yu J-Y Yu W-R Li S-Y Wang and N Pan

Text Res J 2005 75 181ndash184

He et al Apparatus for preparing electrospun nanofibres

1286 Materials Science and Technology 2010 VOL 26 NO 11

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

spinning (see Fig 17) siro-electrospinning is created(see Fig 18) Li et al patented a conjugate electrospin-ning apparatus46ndash48 as illustrated in Fig 19 Two ormore fibres are combined together by weak staticelectronic force An apparatus is devised where a twistis added (see Fig 18) causing the fibres to have atendency to twist and crimp Fabrics so produced canimprove its mechanical characteristics furthermore itsstrength toughness softness and feel can be improveddramatically

The present technology can also be taken in conjunc-tion with other ones For example the vibrationtechnology can be added

Using this technique it is now possible to spinconductive polymer fibres under ambient conditions bydepositing nanometre metal particle in the polymersolutions or melts If some titanium dioxide is added inthe solutions or melts the electrospun fibres areself-clearing

Spider spinningAfter 400 million years of evolution nature endowsspiders with genius of spinning flexible lightweightfibres which have the strongest strength in the world atleast five times stronger by weight than steel and haveremarkable toughness and elasticity4950 Even in themodern times it is difficult to synthesise a materialhaving advantages of strength and toughness exceptcarbon nanotube fibres which is spun from solution invery hot temperature or pressure51 while the spider silksare produced at room temperature and from aqueoussolutions Spider silk is the only natural material thatcombines the properties unmatched by any knownsynthetic high performance fibres

Spider silks are protein based lsquobiopolymerrsquo filamentsor threads secreted by specialised epithelial cells asconcentrated soluble precursors of highly repetitiveprimary sequences52 Many experiments have beenconducted and much research has focused on gene

14 Instability in electrospinning most energy was wasted

due to instability

a application of magnetic field to electrospinning b Ampere force in electronic jet induced by magnetic field15 Effect of magnetic field on electrospinning43

16 Macromolecules under coupled forces of electronic

force and magnetic force

He et al Apparatus for preparing electrospun nanofibres

Materials Science and Technology 2010 VOL 26 NO 11 1281

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

sequencing of spider and a biomimicry technology hasbeen developed53ndash55 however theoretical analysis is notyet dealt with it and our understanding of themechanism of spider spinning is rare and primitive Ifthe mystery in its mechanism can be solved then themechanism can be applied to synthetic high performancefibres such as electrospun fibres with combination ofgreat strength and stretch The devised method must bemuch economical so this could be the beginning of anew materials revolution

Mystery in spider spinning processIt is still an enigma how a spider produces a silk whichhuman needs ten thousands voltage to do One possibleanswer is that a spider applies sufficiently the nano-effect of bubble dynamics

The pressure difference between the inside and outsideof a bubble depends upon the surface tension and theradius of the bubble

For a bubble with two surfaces providing tension thenet upward force on the top hemisphere of the bubble isjust the pressure difference times the area of theequatorial circle

Fupward~pr2(PiPo) (5)

The surface tension force downward around circle istwice the surface tension times the circumference sincetwo surfaces contribute to the force

Fdownward~2pr(2T) (6)

This gives

T~1

4r(PiPo) (7)

Hendricks et al56 calculated the minimum sprayingpotential of a suspended hemispherical conductingdrop in air as

Ethreshold~300 20pcreth THORN1=2(8)

Buchko et al57 modified equation (8) in the form

E2threshold~

4H2

L2( ln

2L

R

3

2)0117pcR (9)

Ethresholdc1=2 (10)

where Ethreshold is the critical voltage needed for theejection of the jet from the Taylor cone H is theseparation distance between the needle and the collectorL is the length of the needle (or capillary) R is the radiusof the needle and c is the surface tension of the solution

From Fig 20 it can be clearly observed that draglinesilk is made of many nanofibres with a diameter ofy20 nm Therefore it can be assumed that a bubble canbe produced at the apex of each tube with a diameter ofy20 nm According to equation (7) the surface tensionof such bubbles is extremely small thus it can be easilyovercome either by the body weight of spider or tensioncreated by the rear legs

Most natural spider silk is only 25ndash4 mm in diameterand the weight of most spiders varies from 90 to1500 mg Consider a dragline silk with a diameter of361026 m which consists of many nanoscale fibrils witha diameter of y20 nm (see Fig 20) Therefore thenumber of nanofibres in the assembly can be estimatedwhich reads

n~(3|106

20|109)2amp2|104 (11)

It means that there are tens of thousands of nanofibresin the assembly

Similar to the HallndashPetch relationship the nanofibrestrength depends upon the fibre diameter in nanoscale(from few nanometres to tens of nanometres)

t~t0zkt

d1=2(12)

where kt is the fitting parameters (material constants) t0

is the strength of the bulk material and d is the fibrediameter as shown in Fig 21

Comparing the strength of a single fibre tSF with thediameter of 361026 m with the strength of a draglineassembly tDA consisting of 26104 nanofibres with thediameter of 2061029 m the following can be estimated

tDAt0

tSFt0

~24|106 (13)

Extraordinarily high strength of a dragline assembly ispredicted compared with a single fibre with the samesection area and same material

The finding shows that it is a challenge to developtechnologies capable of preparing for nanofibres within50 nm

It means that there are tens of thousands of nanotubesin spinneret of spider A weight of G51024 kg of aspider is assumed In the spinning process the spider can

17 Sirofil technology to improve mechanical properties of

mother fibres

18 Siro-electrospinning40

He et al Apparatus for preparing electrospun nanofibres

1282 Materials Science and Technology 2010 VOL 26 NO 11

Pub

lishe

d by

Man

ey P

ublis

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(c)

IOM

Com

mun

icat

ions

Ltd

use its weight to overcome the surface tension of allbubbles produced at apex of nanotubes which requires

T~G (14)

from which the pressure difference between the insideand outside of a bubble can be determined which reads

DP~PiP0~4G

nr~

4|104

(2|104)(20|109)~1 N m2 (15)

Extremely small force is needed in the spider spinning

Bubble electrospinningFrom the hint of the mechanism of the spider spinning anew approach is designed to overcome the bottleneck inpresent electrospinning technology which is to minimisethe surface tension of the electrospun solutions mimick-ing the spider spinning Compressed gas is widely used intextile industry59 and this technology will be used toreduce the surface tension Our system60ndash62 consists of avertical solution reservoir with a gas tube feeding fromthe bottom in which a metal electrode is fixed along thecentreline of the tube and a grounded collector over thereservoir (see Fig 22) It has been found that manysmall bubbles with different sizes were produced on thesolution surface

The mechanism of the new electrospinning process isdeceptively simple in the absence of an electric fieldthe aerated solution forms various bubbles on thesurface When an electric field is present it inducescharges into the bubble surface and these quickly relaxto the bubble surface The coupling of surface chargeand the external electric field creates a tangential stressresulting in the deformation of the small bubble into aprotuberance induced upward directed reentrant jetOnce the electric field exceeds the critical value neededto overcome the surface tension a fluid jet ejects fromthe apex of the conical bubble (see Fig 23) Thethreshold voltage needed to overcome the surfacetension depends upon the size of the bubble and inletair pressure

Although the maximal or minimal size of a bubblemight depend upon the solution viscosity the surfacetension of bubbles is independent of properties of thespun solutions such as viscosity which is the mainobstruction in traditional electrospinning Temperaturecan be used to adjust the bubble sizes in practicalapplication of the new electrospinning process The keyadvantages of this electrospinning process used herecompared with the traditional electrospinning processare that it can produce smaller nanofibres withoutrequirement of nozzles which are the main shortcomingof the traditional electrospinning technology further-more in our method with a relatively low voltagemillions of protruded bubbles can be easily producedwhich can be electrospun into nanofibres simulta-neously and electrospinnability does not strongly

21 E modulus of nanofibres redrawn according to

experimental data by Gu et al58 dots are experimental

data

19 Schematic set-up used for conjugate electrospinning experiments46ndash48

20 Spider spinning process reproduced with the permis-

sion of Dennis Kunkel Microscopy Inc The diameter

of a single nanofibre is y20 nm its spinneret on the

posterior portion of the abdomen of spider consists

of millions of nanoscale tubes and a bubble can be

produced at apex of each nanotube The surface ten-

sion of each bubble is extremely small such that it

can be spun into nanofibres with an awfully small

force either by the body weight of spider or tension

created by the rear legs

He et al Apparatus for preparing electrospun nanofibres

Materials Science and Technology 2010 VOL 26 NO 11 1283

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depend upon the solution viscosity overcoming com-pletely the main shortcoming of the traditional electro-spinning The average diameter of nanofibres can beeasily controllable and the minimum diameter in ourexperiment reached as small as 50 nm (see Fig 24)

This new technology is of critical importance for thenew generation of electrospinning especially forthe specialists in design manufacturing and usingnanofibres

Electrospinning dilation and electrospunnanoporous microspheresDuring the electrospinning process the charged jet isaccelerated by a constant external electric field and thespinning velocity probably reaches maximum andperhaps exceeds the velocity of sound in air in a veryshort time before the spinning becomes instableAccording to the conservation of mass equation

pr2ru~Q (16)

where r is the radius of the jet u is the velocity Q is theflowrate and r is the density The radius of the jetdecreases with increasing velocity of the incompressiblejet When the velocity reaches its maximum the radius

of the jet is minimal and macromoleculars of thepolymers are compacted together tighter and tighter asillustrated in Fig 25 There must exist a critical minimalradius rcr for all electrospun jet r(rcr for continuousultrafine fibres and the critical maximal velocity is

ucr~Q=prr2cr However the velocity can exceed this

critical value ucr if a higher voltage is applied

When the radius of the jet reaches the critical valuer5rcr and the jet speed exceeds its critical value uucrin order to keep conservation of mass equation the jetdilates by decreasing its density leading to porosity ofthe electrospun fibres (see Figs 26 and 27) which iscalled electrospinning dilation

Nanoporous microspheres are especially useful forinvisibility device (eg stealth plane)6413 and drugdeliver6566

New electrospinning apparatus for massproduction of nanofibresIn recent development and application of electrospunnanofibres one key bottleneck is the lack of capacity tothe mass production of nanofibres It is limited for atraditional electrospinning apparatus to increase theability of nanofibre throughout because there are manyseemingly insurmountable difficulties One is the smalldiameter (about 01ndash2 mm) of the spinneret and the lowflowrate (from 01 to dozens of mm h21) of the solutionIf the diameter is enlarged the suspended droplet may

22 Experimental set-up of aerated solution electrospin-

ning This principle to prepare for nanoproducts was

patented (China Patent 2007100364474) To use this

principle to prepare for nanoporous products transfer

agreement must be made63

23 Ejected jets captured by digital camera63

24 Minimum diameter of nanofibres was 50 nm (Ref 63)

25 Macromolecular chains are compacted during electro-

spinning

He et al Apparatus for preparing electrospun nanofibres

1284 Materials Science and Technology 2010 VOL 26 NO 11

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not be formed as a stable Taylor cone at the orificebecause of the gravity and other factors and so is theincreasing flowrate Unlike the traditional dry spinningprocess or wet spinning process in which the jet isextruded from the spinneret with high speed thetraditional electrospinning process depends on theelectrostatic force which just provides a stable and slowliquid flow to form a stable Taylor cone at the tip of thespinneret rather than the extrusion pressure Thespinning speed is much less than that of the dry spinningor wet spinning The other shortcomings are easilyjammed difficult clean etc

As a result the possible technique of the use oftraditional electrospinning apparatus to produce nano-fibres of mass production is to fit the multiple spinneretstogether with a line or matrix pattern Many researchersadopted this technique to produce nanofibres from theearly period of the invention of this technology to nowIt is prevailing until Theron et al67 found bothexperimentally and in the simulations that the jets werepushed away from their neighbours by the Coulombicforces applied by the latter A non-uniform non-wovenmat would be obtained as an immediate consequence ofthis phenomenon Fang et al68 once reported that theysolved the problem of the mutual Coulombic interac-tions by the incorporation of secondary electrodes toisolate the electric field distribution of the primaryelectrode spinnerets But the problem of the smalldiameter of the spinneret and the low flowrate still

limited the fibre output Dosunmu et al69 suggested anew multiple porous tube apparatus in which thepolymer solution was electrified and pushed by airpressure through the walls of a large diameter cylindricalporous tube (see Fig 28a) Multiple jets formed on theporous surface and electrospun into nanofibres in theirprocess However the existing problem of this processmight be also the non-uniform fibre mat (see Fig 5 inRef 69)

In order to eliminate the problems of increasing thenanofibres of mass production of traditional electro-spinning apparatus some researchers demonstratedvarious novel electrospinning apparatus in recent yearsYet these apparatus different as they seem have onething in common The Taylor cone was formed on thefree surface of the solution and the jets were ejectedupward Yarin and Zussman70 proposed an upwardneedleless electrospinning which consisted of a two layersystem with the lower layer being a ferromagneticsuspension and the upper layer a polymer solution (seeFig 28b) In the process many vertical spikes ofmagnetic suspension perturbed the interlayer interfaceas the Taylor cones Though the production rate ishigher than that of traditional electrospinning processthere are some flaws such as not easy operation of twolayers liquids

So far only one electrospinning process Nanospiderspinning technology is claimed to be a commercialsuccess in the mass production of nanofibres all over theworld The principle of Nanospider is to use a cylinderto form the fibres The cylinder is partially immersed in

26 Electrospinning dilation phenomenon of poly

(butylenes succinate) (PBS) electrospun fibres64

27 Electrospun nanoporous microspheres where kind of

traditional Chinese drugs called lsquoYunnan Baiyorsquo was

used as additive65

a Dosunmursquos apparatus b upward needleless electro-spinning

28 New electrospinning apparatus for produing mass

production of nanofibres

He et al Apparatus for preparing electrospun nanofibres

Materials Science and Technology 2010 VOL 26 NO 11 1285

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the polymer solution and as it rotates the polymersolution is carried to the top part of the cylinder whereTaylor cones are created The Taylor streams are formednext to each other throughout the entire length of thecylinder resulting in the high production capacity of thisapparatus71

The above novel approaches are exciting for theresearchers and the engineers When we cheer and applaudfor these much progress of electrospinning however a newproblem caused by the opened liquid surface has risen upIn every electrospinning process the volatilisation of thesolvent from the flying jet is a must In the above newupward electrospinning apparatus the free liquid surfacecan be used to create many Taylor cones and increase thethroughput of nanofibres at the same time can befacilitated to the escape of the volatilising solvent Withthe process proceeding the viscosity of the polymersolution will increase which will introduce some newproblems such as increasing the difficulty of spinning andchanging the properties of the solution

ConclusionsIt is clear that an understanding of the electrospinningand its nanofibres was greatly advanced by thenanoscience and technology in the 1990s and by thesubsequent development of the many effective applica-tions available in many fields today Remarkableadvances in electrospinning technology have beenpossible due to developments in modern technologyVarious novel and useful electrospinning techniques arereviewed in this work according to their differentmodifications Some modified apparatus were used toproduce different nanofibre mats with special structuresfor some special applications and some for massproduction of nanofibres Most of the advancesachieved to date have been made in the collector systemof the traditional electrospinning apparatus but otherparts such as solution storage and feeding systemspinneret and spinning area have been studied as wellMore recently emphasis has been placed on creatingnew apparatus to mass production of nanofibres Newtechniques have been designed and improved and moreelectrospinning apparatus have been developed to betterproduce nanofibres

Collectively the studies about electrospinning andnanofibres carried out so far have furnished a usefuldatabase for the electrohydrodynamics and some otherfields Although this field is in its infancy it is becomingincreasingly clear that there are multiple forms of newtechniques Identifying the similarities and differences invarious apparatus will enhance our understanding ofthis technology The next decades are expected to notonly enrich the existing knowledge but also significantlyadvance the technology into the commercial domain

Acknowledgements

The present work is supported financially by NationalNatural Science Foundation of China under Grant Nos10802021 and 10972053 National Natural ScienceFoundation of Shanghai under Grant No08ZR1400300 Shanghai Rising-Star Program underGrant No09QA1400100 Key Project of ChineseMinistry of Education No210006 National ScienceFoundation of Tianjin No10JCYBJC02200 Tianjin

Universities Science and Technology DevelopmentFund Project No20080322

References1 A Formhals lsquoProcess and apparatus for preparing artifical

threadsrsquo US Patent 1975504 1934

2 M S El Naschie Chaos Solitons Fract 2006 30 769ndash773

3 J-H He Chaos Solitons Fract 2008 38 1390ndash1393

4 J Lyons and F Ko Polym News 2005 30 170ndash178

5 D Li and Y-N Xia Adv Mater 2004 16 1151ndash1170

6 Z-M Huang Y-Z Zhang M Kotaki and S Ramakrishna

Compos Sci Technol 2003 63 2223ndash2253

7 Q P Pham U Sharma and A G Mikos Tissue Eng 2006 12

1197ndash1211

8 E P S Tan and C T Lim Compos Sci Technol 2006 66 1102ndash

1111

9 J-H He and H-M Liu Nonlinear Analy 2005 63 e919ndashe929

10 J-H He and Y-Q Wan Polymer 2004 45 6731ndash6734

11 J-H He Y-Q Wan and J-Y Yu Polymer 2005 46 2799ndash2801

12 J-H He Y-Q Wan and J-Y Yu Fibers Polym 2008 9 140ndash

142

13 J-H He L Xu Y Wu and Y Liu Polym Int 2007 56 1323ndash

1329

14 J-H He Y-Q Wan and J-Y Yu Int J Nonlinear Sci 2004 5

243ndash252

15 J-H He Y Wu and N Pan Int J Nonlinear Sci 2005 6 243ndash

248

16 Y-Q Wan J-H He and J-Y Yu Iran Polym J 2006 15 265ndash

268

17 L Rayleigh Philos Mag 1882 14 184ndash186

18 J F Cooley lsquoApparatus for electrically dispensing fluidsrsquo US

Patent 692631 1902

19 W J Morton lsquoMethod of dispensing fluidsrsquo US Patent 705691

1902

20 P K Baumgarten J Colloid Interface Sci 1971 36 71ndash79

21 J-H He and Y Liu J Polym Eng 2008 28 101ndash114

22 Y Liu J-H He L Xu and J-Y Yu Int J Electrospun Nanofibers

Appl 2007 1 7ndash15

23 Y Liu J-H He and J-Y Yu Fibres Text East Eur 2007 15

30ndash33

24 X-H Qin Y-Q Wan J-H He J Zhang J-Y Yu and

S-Y Wang Polymer 2004 45 6409ndash6413

25 D-H Shou and J-H He J Polym Eng 2008 28 115ndash118

26 Y-Q Wan J-H He Y Wu and J-Y Yu Mater Lett 2006 60

3296ndash3300

27 S-Q Wang J-H He and L Xu Polym Int 2008 57 1079ndash1082

28 Y Wu J-H He L Xu and J-Y Yu Int J Electrospun

Nanofibers Appl 2007 1 1ndash6

29 J-H He Y-Q Wan and J-Y Yu Int J Nonlinear Sci 2004 5

253ndash262

30 Y-Q Wan J-H He L Xu and J-Y Yu Int J Electrospun

Nanofibers Appl 2007 1 17ndash28

31 Y-Q Wan J-H He J-Y Yu and Y Wu J Appl Polym Sci

2007 103 3840ndash3843

32 J Zhang Y-Q Wan J-H He and Z-Y Zhu China Patent

2004100156853 2004

33 B H Bersted J Appl Polym Sci 2003 28 2777ndash2791

34 A I Isayev C M Wong and X Zeng Adv Polym Technol 2003

10 31ndash45

35 H Wu S Guo G Chen J Lin W Chen and H Wang J Appl

Polym Sci 2003 90 1873ndash1878

36 J P Ibar Polym Eng Sci 1998 38 1ndash20

37 S Sell C Barnes M Smith M McClure P Madurantakam

J Grant M McManus and G Bowlin Polym Int 2007 56 1349ndash

1360

38 P D Dalton K Klinkhammer J Salber D Klee and M Moller

Biomacromolecules 2006 7 686ndash690

39 J Lyons C Li and F Ko Polymer 2004 45 7597ndash7603

40 J-H He Y-Q Wan and L Xu Chaos Solitons Fract 2007 33

26ndash37

41 J T McCann M Marquez and Y Xia Nano Lett 2006 6 2868ndash

2872

42 N Ogata S Yamaguchi N Shimada G Lu T Iwata K Nakane

and T Ogihara J Appl Polym Sci 2007 104 1640ndash1645

43 Y Wu J-Y Yu J-H He and Y-Q Wan Chaos Solitons Fract

2007 32 5ndash7

44 J-H He Y-P Yu J-Y Yu W-R Li S-Y Wang and N Pan

Text Res J 2005 75 181ndash184

He et al Apparatus for preparing electrospun nanofibres

1286 Materials Science and Technology 2010 VOL 26 NO 11

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

sequencing of spider and a biomimicry technology hasbeen developed53ndash55 however theoretical analysis is notyet dealt with it and our understanding of themechanism of spider spinning is rare and primitive Ifthe mystery in its mechanism can be solved then themechanism can be applied to synthetic high performancefibres such as electrospun fibres with combination ofgreat strength and stretch The devised method must bemuch economical so this could be the beginning of anew materials revolution

Mystery in spider spinning processIt is still an enigma how a spider produces a silk whichhuman needs ten thousands voltage to do One possibleanswer is that a spider applies sufficiently the nano-effect of bubble dynamics

The pressure difference between the inside and outsideof a bubble depends upon the surface tension and theradius of the bubble

For a bubble with two surfaces providing tension thenet upward force on the top hemisphere of the bubble isjust the pressure difference times the area of theequatorial circle

Fupward~pr2(PiPo) (5)

The surface tension force downward around circle istwice the surface tension times the circumference sincetwo surfaces contribute to the force

Fdownward~2pr(2T) (6)

This gives

T~1

4r(PiPo) (7)

Hendricks et al56 calculated the minimum sprayingpotential of a suspended hemispherical conductingdrop in air as

Ethreshold~300 20pcreth THORN1=2(8)

Buchko et al57 modified equation (8) in the form

E2threshold~

4H2

L2( ln

2L

R

3

2)0117pcR (9)

Ethresholdc1=2 (10)

where Ethreshold is the critical voltage needed for theejection of the jet from the Taylor cone H is theseparation distance between the needle and the collectorL is the length of the needle (or capillary) R is the radiusof the needle and c is the surface tension of the solution

From Fig 20 it can be clearly observed that draglinesilk is made of many nanofibres with a diameter ofy20 nm Therefore it can be assumed that a bubble canbe produced at the apex of each tube with a diameter ofy20 nm According to equation (7) the surface tensionof such bubbles is extremely small thus it can be easilyovercome either by the body weight of spider or tensioncreated by the rear legs

Most natural spider silk is only 25ndash4 mm in diameterand the weight of most spiders varies from 90 to1500 mg Consider a dragline silk with a diameter of361026 m which consists of many nanoscale fibrils witha diameter of y20 nm (see Fig 20) Therefore thenumber of nanofibres in the assembly can be estimatedwhich reads

n~(3|106

20|109)2amp2|104 (11)

It means that there are tens of thousands of nanofibresin the assembly

Similar to the HallndashPetch relationship the nanofibrestrength depends upon the fibre diameter in nanoscale(from few nanometres to tens of nanometres)

t~t0zkt

d1=2(12)

where kt is the fitting parameters (material constants) t0

is the strength of the bulk material and d is the fibrediameter as shown in Fig 21

Comparing the strength of a single fibre tSF with thediameter of 361026 m with the strength of a draglineassembly tDA consisting of 26104 nanofibres with thediameter of 2061029 m the following can be estimated

tDAt0

tSFt0

~24|106 (13)

Extraordinarily high strength of a dragline assembly ispredicted compared with a single fibre with the samesection area and same material

The finding shows that it is a challenge to developtechnologies capable of preparing for nanofibres within50 nm

It means that there are tens of thousands of nanotubesin spinneret of spider A weight of G51024 kg of aspider is assumed In the spinning process the spider can

17 Sirofil technology to improve mechanical properties of

mother fibres

18 Siro-electrospinning40

He et al Apparatus for preparing electrospun nanofibres

1282 Materials Science and Technology 2010 VOL 26 NO 11

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

use its weight to overcome the surface tension of allbubbles produced at apex of nanotubes which requires

T~G (14)

from which the pressure difference between the insideand outside of a bubble can be determined which reads

DP~PiP0~4G

nr~

4|104

(2|104)(20|109)~1 N m2 (15)

Extremely small force is needed in the spider spinning

Bubble electrospinningFrom the hint of the mechanism of the spider spinning anew approach is designed to overcome the bottleneck inpresent electrospinning technology which is to minimisethe surface tension of the electrospun solutions mimick-ing the spider spinning Compressed gas is widely used intextile industry59 and this technology will be used toreduce the surface tension Our system60ndash62 consists of avertical solution reservoir with a gas tube feeding fromthe bottom in which a metal electrode is fixed along thecentreline of the tube and a grounded collector over thereservoir (see Fig 22) It has been found that manysmall bubbles with different sizes were produced on thesolution surface

The mechanism of the new electrospinning process isdeceptively simple in the absence of an electric fieldthe aerated solution forms various bubbles on thesurface When an electric field is present it inducescharges into the bubble surface and these quickly relaxto the bubble surface The coupling of surface chargeand the external electric field creates a tangential stressresulting in the deformation of the small bubble into aprotuberance induced upward directed reentrant jetOnce the electric field exceeds the critical value neededto overcome the surface tension a fluid jet ejects fromthe apex of the conical bubble (see Fig 23) Thethreshold voltage needed to overcome the surfacetension depends upon the size of the bubble and inletair pressure

Although the maximal or minimal size of a bubblemight depend upon the solution viscosity the surfacetension of bubbles is independent of properties of thespun solutions such as viscosity which is the mainobstruction in traditional electrospinning Temperaturecan be used to adjust the bubble sizes in practicalapplication of the new electrospinning process The keyadvantages of this electrospinning process used herecompared with the traditional electrospinning processare that it can produce smaller nanofibres withoutrequirement of nozzles which are the main shortcomingof the traditional electrospinning technology further-more in our method with a relatively low voltagemillions of protruded bubbles can be easily producedwhich can be electrospun into nanofibres simulta-neously and electrospinnability does not strongly

21 E modulus of nanofibres redrawn according to

experimental data by Gu et al58 dots are experimental

data

19 Schematic set-up used for conjugate electrospinning experiments46ndash48

20 Spider spinning process reproduced with the permis-

sion of Dennis Kunkel Microscopy Inc The diameter

of a single nanofibre is y20 nm its spinneret on the

posterior portion of the abdomen of spider consists

of millions of nanoscale tubes and a bubble can be

produced at apex of each nanotube The surface ten-

sion of each bubble is extremely small such that it

can be spun into nanofibres with an awfully small

force either by the body weight of spider or tension

created by the rear legs

He et al Apparatus for preparing electrospun nanofibres

Materials Science and Technology 2010 VOL 26 NO 11 1283

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

depend upon the solution viscosity overcoming com-pletely the main shortcoming of the traditional electro-spinning The average diameter of nanofibres can beeasily controllable and the minimum diameter in ourexperiment reached as small as 50 nm (see Fig 24)

This new technology is of critical importance for thenew generation of electrospinning especially forthe specialists in design manufacturing and usingnanofibres

Electrospinning dilation and electrospunnanoporous microspheresDuring the electrospinning process the charged jet isaccelerated by a constant external electric field and thespinning velocity probably reaches maximum andperhaps exceeds the velocity of sound in air in a veryshort time before the spinning becomes instableAccording to the conservation of mass equation

pr2ru~Q (16)

where r is the radius of the jet u is the velocity Q is theflowrate and r is the density The radius of the jetdecreases with increasing velocity of the incompressiblejet When the velocity reaches its maximum the radius

of the jet is minimal and macromoleculars of thepolymers are compacted together tighter and tighter asillustrated in Fig 25 There must exist a critical minimalradius rcr for all electrospun jet r(rcr for continuousultrafine fibres and the critical maximal velocity is

ucr~Q=prr2cr However the velocity can exceed this

critical value ucr if a higher voltage is applied

When the radius of the jet reaches the critical valuer5rcr and the jet speed exceeds its critical value uucrin order to keep conservation of mass equation the jetdilates by decreasing its density leading to porosity ofthe electrospun fibres (see Figs 26 and 27) which iscalled electrospinning dilation

Nanoporous microspheres are especially useful forinvisibility device (eg stealth plane)6413 and drugdeliver6566

New electrospinning apparatus for massproduction of nanofibresIn recent development and application of electrospunnanofibres one key bottleneck is the lack of capacity tothe mass production of nanofibres It is limited for atraditional electrospinning apparatus to increase theability of nanofibre throughout because there are manyseemingly insurmountable difficulties One is the smalldiameter (about 01ndash2 mm) of the spinneret and the lowflowrate (from 01 to dozens of mm h21) of the solutionIf the diameter is enlarged the suspended droplet may

22 Experimental set-up of aerated solution electrospin-

ning This principle to prepare for nanoproducts was

patented (China Patent 2007100364474) To use this

principle to prepare for nanoporous products transfer

agreement must be made63

23 Ejected jets captured by digital camera63

24 Minimum diameter of nanofibres was 50 nm (Ref 63)

25 Macromolecular chains are compacted during electro-

spinning

He et al Apparatus for preparing electrospun nanofibres

1284 Materials Science and Technology 2010 VOL 26 NO 11

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

not be formed as a stable Taylor cone at the orificebecause of the gravity and other factors and so is theincreasing flowrate Unlike the traditional dry spinningprocess or wet spinning process in which the jet isextruded from the spinneret with high speed thetraditional electrospinning process depends on theelectrostatic force which just provides a stable and slowliquid flow to form a stable Taylor cone at the tip of thespinneret rather than the extrusion pressure Thespinning speed is much less than that of the dry spinningor wet spinning The other shortcomings are easilyjammed difficult clean etc

As a result the possible technique of the use oftraditional electrospinning apparatus to produce nano-fibres of mass production is to fit the multiple spinneretstogether with a line or matrix pattern Many researchersadopted this technique to produce nanofibres from theearly period of the invention of this technology to nowIt is prevailing until Theron et al67 found bothexperimentally and in the simulations that the jets werepushed away from their neighbours by the Coulombicforces applied by the latter A non-uniform non-wovenmat would be obtained as an immediate consequence ofthis phenomenon Fang et al68 once reported that theysolved the problem of the mutual Coulombic interac-tions by the incorporation of secondary electrodes toisolate the electric field distribution of the primaryelectrode spinnerets But the problem of the smalldiameter of the spinneret and the low flowrate still

limited the fibre output Dosunmu et al69 suggested anew multiple porous tube apparatus in which thepolymer solution was electrified and pushed by airpressure through the walls of a large diameter cylindricalporous tube (see Fig 28a) Multiple jets formed on theporous surface and electrospun into nanofibres in theirprocess However the existing problem of this processmight be also the non-uniform fibre mat (see Fig 5 inRef 69)

In order to eliminate the problems of increasing thenanofibres of mass production of traditional electro-spinning apparatus some researchers demonstratedvarious novel electrospinning apparatus in recent yearsYet these apparatus different as they seem have onething in common The Taylor cone was formed on thefree surface of the solution and the jets were ejectedupward Yarin and Zussman70 proposed an upwardneedleless electrospinning which consisted of a two layersystem with the lower layer being a ferromagneticsuspension and the upper layer a polymer solution (seeFig 28b) In the process many vertical spikes ofmagnetic suspension perturbed the interlayer interfaceas the Taylor cones Though the production rate ishigher than that of traditional electrospinning processthere are some flaws such as not easy operation of twolayers liquids

So far only one electrospinning process Nanospiderspinning technology is claimed to be a commercialsuccess in the mass production of nanofibres all over theworld The principle of Nanospider is to use a cylinderto form the fibres The cylinder is partially immersed in

26 Electrospinning dilation phenomenon of poly

(butylenes succinate) (PBS) electrospun fibres64

27 Electrospun nanoporous microspheres where kind of

traditional Chinese drugs called lsquoYunnan Baiyorsquo was

used as additive65

a Dosunmursquos apparatus b upward needleless electro-spinning

28 New electrospinning apparatus for produing mass

production of nanofibres

He et al Apparatus for preparing electrospun nanofibres

Materials Science and Technology 2010 VOL 26 NO 11 1285

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

the polymer solution and as it rotates the polymersolution is carried to the top part of the cylinder whereTaylor cones are created The Taylor streams are formednext to each other throughout the entire length of thecylinder resulting in the high production capacity of thisapparatus71

The above novel approaches are exciting for theresearchers and the engineers When we cheer and applaudfor these much progress of electrospinning however a newproblem caused by the opened liquid surface has risen upIn every electrospinning process the volatilisation of thesolvent from the flying jet is a must In the above newupward electrospinning apparatus the free liquid surfacecan be used to create many Taylor cones and increase thethroughput of nanofibres at the same time can befacilitated to the escape of the volatilising solvent Withthe process proceeding the viscosity of the polymersolution will increase which will introduce some newproblems such as increasing the difficulty of spinning andchanging the properties of the solution

ConclusionsIt is clear that an understanding of the electrospinningand its nanofibres was greatly advanced by thenanoscience and technology in the 1990s and by thesubsequent development of the many effective applica-tions available in many fields today Remarkableadvances in electrospinning technology have beenpossible due to developments in modern technologyVarious novel and useful electrospinning techniques arereviewed in this work according to their differentmodifications Some modified apparatus were used toproduce different nanofibre mats with special structuresfor some special applications and some for massproduction of nanofibres Most of the advancesachieved to date have been made in the collector systemof the traditional electrospinning apparatus but otherparts such as solution storage and feeding systemspinneret and spinning area have been studied as wellMore recently emphasis has been placed on creatingnew apparatus to mass production of nanofibres Newtechniques have been designed and improved and moreelectrospinning apparatus have been developed to betterproduce nanofibres

Collectively the studies about electrospinning andnanofibres carried out so far have furnished a usefuldatabase for the electrohydrodynamics and some otherfields Although this field is in its infancy it is becomingincreasingly clear that there are multiple forms of newtechniques Identifying the similarities and differences invarious apparatus will enhance our understanding ofthis technology The next decades are expected to notonly enrich the existing knowledge but also significantlyadvance the technology into the commercial domain

Acknowledgements

The present work is supported financially by NationalNatural Science Foundation of China under Grant Nos10802021 and 10972053 National Natural ScienceFoundation of Shanghai under Grant No08ZR1400300 Shanghai Rising-Star Program underGrant No09QA1400100 Key Project of ChineseMinistry of Education No210006 National ScienceFoundation of Tianjin No10JCYBJC02200 Tianjin

Universities Science and Technology DevelopmentFund Project No20080322

References1 A Formhals lsquoProcess and apparatus for preparing artifical

threadsrsquo US Patent 1975504 1934

2 M S El Naschie Chaos Solitons Fract 2006 30 769ndash773

3 J-H He Chaos Solitons Fract 2008 38 1390ndash1393

4 J Lyons and F Ko Polym News 2005 30 170ndash178

5 D Li and Y-N Xia Adv Mater 2004 16 1151ndash1170

6 Z-M Huang Y-Z Zhang M Kotaki and S Ramakrishna

Compos Sci Technol 2003 63 2223ndash2253

7 Q P Pham U Sharma and A G Mikos Tissue Eng 2006 12

1197ndash1211

8 E P S Tan and C T Lim Compos Sci Technol 2006 66 1102ndash

1111

9 J-H He and H-M Liu Nonlinear Analy 2005 63 e919ndashe929

10 J-H He and Y-Q Wan Polymer 2004 45 6731ndash6734

11 J-H He Y-Q Wan and J-Y Yu Polymer 2005 46 2799ndash2801

12 J-H He Y-Q Wan and J-Y Yu Fibers Polym 2008 9 140ndash

142

13 J-H He L Xu Y Wu and Y Liu Polym Int 2007 56 1323ndash

1329

14 J-H He Y-Q Wan and J-Y Yu Int J Nonlinear Sci 2004 5

243ndash252

15 J-H He Y Wu and N Pan Int J Nonlinear Sci 2005 6 243ndash

248

16 Y-Q Wan J-H He and J-Y Yu Iran Polym J 2006 15 265ndash

268

17 L Rayleigh Philos Mag 1882 14 184ndash186

18 J F Cooley lsquoApparatus for electrically dispensing fluidsrsquo US

Patent 692631 1902

19 W J Morton lsquoMethod of dispensing fluidsrsquo US Patent 705691

1902

20 P K Baumgarten J Colloid Interface Sci 1971 36 71ndash79

21 J-H He and Y Liu J Polym Eng 2008 28 101ndash114

22 Y Liu J-H He L Xu and J-Y Yu Int J Electrospun Nanofibers

Appl 2007 1 7ndash15

23 Y Liu J-H He and J-Y Yu Fibres Text East Eur 2007 15

30ndash33

24 X-H Qin Y-Q Wan J-H He J Zhang J-Y Yu and

S-Y Wang Polymer 2004 45 6409ndash6413

25 D-H Shou and J-H He J Polym Eng 2008 28 115ndash118

26 Y-Q Wan J-H He Y Wu and J-Y Yu Mater Lett 2006 60

3296ndash3300

27 S-Q Wang J-H He and L Xu Polym Int 2008 57 1079ndash1082

28 Y Wu J-H He L Xu and J-Y Yu Int J Electrospun

Nanofibers Appl 2007 1 1ndash6

29 J-H He Y-Q Wan and J-Y Yu Int J Nonlinear Sci 2004 5

253ndash262

30 Y-Q Wan J-H He L Xu and J-Y Yu Int J Electrospun

Nanofibers Appl 2007 1 17ndash28

31 Y-Q Wan J-H He J-Y Yu and Y Wu J Appl Polym Sci

2007 103 3840ndash3843

32 J Zhang Y-Q Wan J-H He and Z-Y Zhu China Patent

2004100156853 2004

33 B H Bersted J Appl Polym Sci 2003 28 2777ndash2791

34 A I Isayev C M Wong and X Zeng Adv Polym Technol 2003

10 31ndash45

35 H Wu S Guo G Chen J Lin W Chen and H Wang J Appl

Polym Sci 2003 90 1873ndash1878

36 J P Ibar Polym Eng Sci 1998 38 1ndash20

37 S Sell C Barnes M Smith M McClure P Madurantakam

J Grant M McManus and G Bowlin Polym Int 2007 56 1349ndash

1360

38 P D Dalton K Klinkhammer J Salber D Klee and M Moller

Biomacromolecules 2006 7 686ndash690

39 J Lyons C Li and F Ko Polymer 2004 45 7597ndash7603

40 J-H He Y-Q Wan and L Xu Chaos Solitons Fract 2007 33

26ndash37

41 J T McCann M Marquez and Y Xia Nano Lett 2006 6 2868ndash

2872

42 N Ogata S Yamaguchi N Shimada G Lu T Iwata K Nakane

and T Ogihara J Appl Polym Sci 2007 104 1640ndash1645

43 Y Wu J-Y Yu J-H He and Y-Q Wan Chaos Solitons Fract

2007 32 5ndash7

44 J-H He Y-P Yu J-Y Yu W-R Li S-Y Wang and N Pan

Text Res J 2005 75 181ndash184

He et al Apparatus for preparing electrospun nanofibres

1286 Materials Science and Technology 2010 VOL 26 NO 11

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

use its weight to overcome the surface tension of allbubbles produced at apex of nanotubes which requires

T~G (14)

from which the pressure difference between the insideand outside of a bubble can be determined which reads

DP~PiP0~4G

nr~

4|104

(2|104)(20|109)~1 N m2 (15)

Extremely small force is needed in the spider spinning

Bubble electrospinningFrom the hint of the mechanism of the spider spinning anew approach is designed to overcome the bottleneck inpresent electrospinning technology which is to minimisethe surface tension of the electrospun solutions mimick-ing the spider spinning Compressed gas is widely used intextile industry59 and this technology will be used toreduce the surface tension Our system60ndash62 consists of avertical solution reservoir with a gas tube feeding fromthe bottom in which a metal electrode is fixed along thecentreline of the tube and a grounded collector over thereservoir (see Fig 22) It has been found that manysmall bubbles with different sizes were produced on thesolution surface

The mechanism of the new electrospinning process isdeceptively simple in the absence of an electric fieldthe aerated solution forms various bubbles on thesurface When an electric field is present it inducescharges into the bubble surface and these quickly relaxto the bubble surface The coupling of surface chargeand the external electric field creates a tangential stressresulting in the deformation of the small bubble into aprotuberance induced upward directed reentrant jetOnce the electric field exceeds the critical value neededto overcome the surface tension a fluid jet ejects fromthe apex of the conical bubble (see Fig 23) Thethreshold voltage needed to overcome the surfacetension depends upon the size of the bubble and inletair pressure

Although the maximal or minimal size of a bubblemight depend upon the solution viscosity the surfacetension of bubbles is independent of properties of thespun solutions such as viscosity which is the mainobstruction in traditional electrospinning Temperaturecan be used to adjust the bubble sizes in practicalapplication of the new electrospinning process The keyadvantages of this electrospinning process used herecompared with the traditional electrospinning processare that it can produce smaller nanofibres withoutrequirement of nozzles which are the main shortcomingof the traditional electrospinning technology further-more in our method with a relatively low voltagemillions of protruded bubbles can be easily producedwhich can be electrospun into nanofibres simulta-neously and electrospinnability does not strongly

21 E modulus of nanofibres redrawn according to

experimental data by Gu et al58 dots are experimental

data

19 Schematic set-up used for conjugate electrospinning experiments46ndash48

20 Spider spinning process reproduced with the permis-

sion of Dennis Kunkel Microscopy Inc The diameter

of a single nanofibre is y20 nm its spinneret on the

posterior portion of the abdomen of spider consists

of millions of nanoscale tubes and a bubble can be

produced at apex of each nanotube The surface ten-

sion of each bubble is extremely small such that it

can be spun into nanofibres with an awfully small

force either by the body weight of spider or tension

created by the rear legs

He et al Apparatus for preparing electrospun nanofibres

Materials Science and Technology 2010 VOL 26 NO 11 1283

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

depend upon the solution viscosity overcoming com-pletely the main shortcoming of the traditional electro-spinning The average diameter of nanofibres can beeasily controllable and the minimum diameter in ourexperiment reached as small as 50 nm (see Fig 24)

This new technology is of critical importance for thenew generation of electrospinning especially forthe specialists in design manufacturing and usingnanofibres

Electrospinning dilation and electrospunnanoporous microspheresDuring the electrospinning process the charged jet isaccelerated by a constant external electric field and thespinning velocity probably reaches maximum andperhaps exceeds the velocity of sound in air in a veryshort time before the spinning becomes instableAccording to the conservation of mass equation

pr2ru~Q (16)

where r is the radius of the jet u is the velocity Q is theflowrate and r is the density The radius of the jetdecreases with increasing velocity of the incompressiblejet When the velocity reaches its maximum the radius

of the jet is minimal and macromoleculars of thepolymers are compacted together tighter and tighter asillustrated in Fig 25 There must exist a critical minimalradius rcr for all electrospun jet r(rcr for continuousultrafine fibres and the critical maximal velocity is

ucr~Q=prr2cr However the velocity can exceed this

critical value ucr if a higher voltage is applied

When the radius of the jet reaches the critical valuer5rcr and the jet speed exceeds its critical value uucrin order to keep conservation of mass equation the jetdilates by decreasing its density leading to porosity ofthe electrospun fibres (see Figs 26 and 27) which iscalled electrospinning dilation

Nanoporous microspheres are especially useful forinvisibility device (eg stealth plane)6413 and drugdeliver6566

New electrospinning apparatus for massproduction of nanofibresIn recent development and application of electrospunnanofibres one key bottleneck is the lack of capacity tothe mass production of nanofibres It is limited for atraditional electrospinning apparatus to increase theability of nanofibre throughout because there are manyseemingly insurmountable difficulties One is the smalldiameter (about 01ndash2 mm) of the spinneret and the lowflowrate (from 01 to dozens of mm h21) of the solutionIf the diameter is enlarged the suspended droplet may

22 Experimental set-up of aerated solution electrospin-

ning This principle to prepare for nanoproducts was

patented (China Patent 2007100364474) To use this

principle to prepare for nanoporous products transfer

agreement must be made63

23 Ejected jets captured by digital camera63

24 Minimum diameter of nanofibres was 50 nm (Ref 63)

25 Macromolecular chains are compacted during electro-

spinning

He et al Apparatus for preparing electrospun nanofibres

1284 Materials Science and Technology 2010 VOL 26 NO 11

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

not be formed as a stable Taylor cone at the orificebecause of the gravity and other factors and so is theincreasing flowrate Unlike the traditional dry spinningprocess or wet spinning process in which the jet isextruded from the spinneret with high speed thetraditional electrospinning process depends on theelectrostatic force which just provides a stable and slowliquid flow to form a stable Taylor cone at the tip of thespinneret rather than the extrusion pressure Thespinning speed is much less than that of the dry spinningor wet spinning The other shortcomings are easilyjammed difficult clean etc

As a result the possible technique of the use oftraditional electrospinning apparatus to produce nano-fibres of mass production is to fit the multiple spinneretstogether with a line or matrix pattern Many researchersadopted this technique to produce nanofibres from theearly period of the invention of this technology to nowIt is prevailing until Theron et al67 found bothexperimentally and in the simulations that the jets werepushed away from their neighbours by the Coulombicforces applied by the latter A non-uniform non-wovenmat would be obtained as an immediate consequence ofthis phenomenon Fang et al68 once reported that theysolved the problem of the mutual Coulombic interac-tions by the incorporation of secondary electrodes toisolate the electric field distribution of the primaryelectrode spinnerets But the problem of the smalldiameter of the spinneret and the low flowrate still

limited the fibre output Dosunmu et al69 suggested anew multiple porous tube apparatus in which thepolymer solution was electrified and pushed by airpressure through the walls of a large diameter cylindricalporous tube (see Fig 28a) Multiple jets formed on theporous surface and electrospun into nanofibres in theirprocess However the existing problem of this processmight be also the non-uniform fibre mat (see Fig 5 inRef 69)

In order to eliminate the problems of increasing thenanofibres of mass production of traditional electro-spinning apparatus some researchers demonstratedvarious novel electrospinning apparatus in recent yearsYet these apparatus different as they seem have onething in common The Taylor cone was formed on thefree surface of the solution and the jets were ejectedupward Yarin and Zussman70 proposed an upwardneedleless electrospinning which consisted of a two layersystem with the lower layer being a ferromagneticsuspension and the upper layer a polymer solution (seeFig 28b) In the process many vertical spikes ofmagnetic suspension perturbed the interlayer interfaceas the Taylor cones Though the production rate ishigher than that of traditional electrospinning processthere are some flaws such as not easy operation of twolayers liquids

So far only one electrospinning process Nanospiderspinning technology is claimed to be a commercialsuccess in the mass production of nanofibres all over theworld The principle of Nanospider is to use a cylinderto form the fibres The cylinder is partially immersed in

26 Electrospinning dilation phenomenon of poly

(butylenes succinate) (PBS) electrospun fibres64

27 Electrospun nanoporous microspheres where kind of

traditional Chinese drugs called lsquoYunnan Baiyorsquo was

used as additive65

a Dosunmursquos apparatus b upward needleless electro-spinning

28 New electrospinning apparatus for produing mass

production of nanofibres

He et al Apparatus for preparing electrospun nanofibres

Materials Science and Technology 2010 VOL 26 NO 11 1285

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

the polymer solution and as it rotates the polymersolution is carried to the top part of the cylinder whereTaylor cones are created The Taylor streams are formednext to each other throughout the entire length of thecylinder resulting in the high production capacity of thisapparatus71

The above novel approaches are exciting for theresearchers and the engineers When we cheer and applaudfor these much progress of electrospinning however a newproblem caused by the opened liquid surface has risen upIn every electrospinning process the volatilisation of thesolvent from the flying jet is a must In the above newupward electrospinning apparatus the free liquid surfacecan be used to create many Taylor cones and increase thethroughput of nanofibres at the same time can befacilitated to the escape of the volatilising solvent Withthe process proceeding the viscosity of the polymersolution will increase which will introduce some newproblems such as increasing the difficulty of spinning andchanging the properties of the solution

ConclusionsIt is clear that an understanding of the electrospinningand its nanofibres was greatly advanced by thenanoscience and technology in the 1990s and by thesubsequent development of the many effective applica-tions available in many fields today Remarkableadvances in electrospinning technology have beenpossible due to developments in modern technologyVarious novel and useful electrospinning techniques arereviewed in this work according to their differentmodifications Some modified apparatus were used toproduce different nanofibre mats with special structuresfor some special applications and some for massproduction of nanofibres Most of the advancesachieved to date have been made in the collector systemof the traditional electrospinning apparatus but otherparts such as solution storage and feeding systemspinneret and spinning area have been studied as wellMore recently emphasis has been placed on creatingnew apparatus to mass production of nanofibres Newtechniques have been designed and improved and moreelectrospinning apparatus have been developed to betterproduce nanofibres

Collectively the studies about electrospinning andnanofibres carried out so far have furnished a usefuldatabase for the electrohydrodynamics and some otherfields Although this field is in its infancy it is becomingincreasingly clear that there are multiple forms of newtechniques Identifying the similarities and differences invarious apparatus will enhance our understanding ofthis technology The next decades are expected to notonly enrich the existing knowledge but also significantlyadvance the technology into the commercial domain

Acknowledgements

The present work is supported financially by NationalNatural Science Foundation of China under Grant Nos10802021 and 10972053 National Natural ScienceFoundation of Shanghai under Grant No08ZR1400300 Shanghai Rising-Star Program underGrant No09QA1400100 Key Project of ChineseMinistry of Education No210006 National ScienceFoundation of Tianjin No10JCYBJC02200 Tianjin

Universities Science and Technology DevelopmentFund Project No20080322

References1 A Formhals lsquoProcess and apparatus for preparing artifical

threadsrsquo US Patent 1975504 1934

2 M S El Naschie Chaos Solitons Fract 2006 30 769ndash773

3 J-H He Chaos Solitons Fract 2008 38 1390ndash1393

4 J Lyons and F Ko Polym News 2005 30 170ndash178

5 D Li and Y-N Xia Adv Mater 2004 16 1151ndash1170

6 Z-M Huang Y-Z Zhang M Kotaki and S Ramakrishna

Compos Sci Technol 2003 63 2223ndash2253

7 Q P Pham U Sharma and A G Mikos Tissue Eng 2006 12

1197ndash1211

8 E P S Tan and C T Lim Compos Sci Technol 2006 66 1102ndash

1111

9 J-H He and H-M Liu Nonlinear Analy 2005 63 e919ndashe929

10 J-H He and Y-Q Wan Polymer 2004 45 6731ndash6734

11 J-H He Y-Q Wan and J-Y Yu Polymer 2005 46 2799ndash2801

12 J-H He Y-Q Wan and J-Y Yu Fibers Polym 2008 9 140ndash

142

13 J-H He L Xu Y Wu and Y Liu Polym Int 2007 56 1323ndash

1329

14 J-H He Y-Q Wan and J-Y Yu Int J Nonlinear Sci 2004 5

243ndash252

15 J-H He Y Wu and N Pan Int J Nonlinear Sci 2005 6 243ndash

248

16 Y-Q Wan J-H He and J-Y Yu Iran Polym J 2006 15 265ndash

268

17 L Rayleigh Philos Mag 1882 14 184ndash186

18 J F Cooley lsquoApparatus for electrically dispensing fluidsrsquo US

Patent 692631 1902

19 W J Morton lsquoMethod of dispensing fluidsrsquo US Patent 705691

1902

20 P K Baumgarten J Colloid Interface Sci 1971 36 71ndash79

21 J-H He and Y Liu J Polym Eng 2008 28 101ndash114

22 Y Liu J-H He L Xu and J-Y Yu Int J Electrospun Nanofibers

Appl 2007 1 7ndash15

23 Y Liu J-H He and J-Y Yu Fibres Text East Eur 2007 15

30ndash33

24 X-H Qin Y-Q Wan J-H He J Zhang J-Y Yu and

S-Y Wang Polymer 2004 45 6409ndash6413

25 D-H Shou and J-H He J Polym Eng 2008 28 115ndash118

26 Y-Q Wan J-H He Y Wu and J-Y Yu Mater Lett 2006 60

3296ndash3300

27 S-Q Wang J-H He and L Xu Polym Int 2008 57 1079ndash1082

28 Y Wu J-H He L Xu and J-Y Yu Int J Electrospun

Nanofibers Appl 2007 1 1ndash6

29 J-H He Y-Q Wan and J-Y Yu Int J Nonlinear Sci 2004 5

253ndash262

30 Y-Q Wan J-H He L Xu and J-Y Yu Int J Electrospun

Nanofibers Appl 2007 1 17ndash28

31 Y-Q Wan J-H He J-Y Yu and Y Wu J Appl Polym Sci

2007 103 3840ndash3843

32 J Zhang Y-Q Wan J-H He and Z-Y Zhu China Patent

2004100156853 2004

33 B H Bersted J Appl Polym Sci 2003 28 2777ndash2791

34 A I Isayev C M Wong and X Zeng Adv Polym Technol 2003

10 31ndash45

35 H Wu S Guo G Chen J Lin W Chen and H Wang J Appl

Polym Sci 2003 90 1873ndash1878

36 J P Ibar Polym Eng Sci 1998 38 1ndash20

37 S Sell C Barnes M Smith M McClure P Madurantakam

J Grant M McManus and G Bowlin Polym Int 2007 56 1349ndash

1360

38 P D Dalton K Klinkhammer J Salber D Klee and M Moller

Biomacromolecules 2006 7 686ndash690

39 J Lyons C Li and F Ko Polymer 2004 45 7597ndash7603

40 J-H He Y-Q Wan and L Xu Chaos Solitons Fract 2007 33

26ndash37

41 J T McCann M Marquez and Y Xia Nano Lett 2006 6 2868ndash

2872

42 N Ogata S Yamaguchi N Shimada G Lu T Iwata K Nakane

and T Ogihara J Appl Polym Sci 2007 104 1640ndash1645

43 Y Wu J-Y Yu J-H He and Y-Q Wan Chaos Solitons Fract

2007 32 5ndash7

44 J-H He Y-P Yu J-Y Yu W-R Li S-Y Wang and N Pan

Text Res J 2005 75 181ndash184

He et al Apparatus for preparing electrospun nanofibres

1286 Materials Science and Technology 2010 VOL 26 NO 11

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

depend upon the solution viscosity overcoming com-pletely the main shortcoming of the traditional electro-spinning The average diameter of nanofibres can beeasily controllable and the minimum diameter in ourexperiment reached as small as 50 nm (see Fig 24)

This new technology is of critical importance for thenew generation of electrospinning especially forthe specialists in design manufacturing and usingnanofibres

Electrospinning dilation and electrospunnanoporous microspheresDuring the electrospinning process the charged jet isaccelerated by a constant external electric field and thespinning velocity probably reaches maximum andperhaps exceeds the velocity of sound in air in a veryshort time before the spinning becomes instableAccording to the conservation of mass equation

pr2ru~Q (16)

where r is the radius of the jet u is the velocity Q is theflowrate and r is the density The radius of the jetdecreases with increasing velocity of the incompressiblejet When the velocity reaches its maximum the radius

of the jet is minimal and macromoleculars of thepolymers are compacted together tighter and tighter asillustrated in Fig 25 There must exist a critical minimalradius rcr for all electrospun jet r(rcr for continuousultrafine fibres and the critical maximal velocity is

ucr~Q=prr2cr However the velocity can exceed this

critical value ucr if a higher voltage is applied

When the radius of the jet reaches the critical valuer5rcr and the jet speed exceeds its critical value uucrin order to keep conservation of mass equation the jetdilates by decreasing its density leading to porosity ofthe electrospun fibres (see Figs 26 and 27) which iscalled electrospinning dilation

Nanoporous microspheres are especially useful forinvisibility device (eg stealth plane)6413 and drugdeliver6566

New electrospinning apparatus for massproduction of nanofibresIn recent development and application of electrospunnanofibres one key bottleneck is the lack of capacity tothe mass production of nanofibres It is limited for atraditional electrospinning apparatus to increase theability of nanofibre throughout because there are manyseemingly insurmountable difficulties One is the smalldiameter (about 01ndash2 mm) of the spinneret and the lowflowrate (from 01 to dozens of mm h21) of the solutionIf the diameter is enlarged the suspended droplet may

22 Experimental set-up of aerated solution electrospin-

ning This principle to prepare for nanoproducts was

patented (China Patent 2007100364474) To use this

principle to prepare for nanoporous products transfer

agreement must be made63

23 Ejected jets captured by digital camera63

24 Minimum diameter of nanofibres was 50 nm (Ref 63)

25 Macromolecular chains are compacted during electro-

spinning

He et al Apparatus for preparing electrospun nanofibres

1284 Materials Science and Technology 2010 VOL 26 NO 11

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

not be formed as a stable Taylor cone at the orificebecause of the gravity and other factors and so is theincreasing flowrate Unlike the traditional dry spinningprocess or wet spinning process in which the jet isextruded from the spinneret with high speed thetraditional electrospinning process depends on theelectrostatic force which just provides a stable and slowliquid flow to form a stable Taylor cone at the tip of thespinneret rather than the extrusion pressure Thespinning speed is much less than that of the dry spinningor wet spinning The other shortcomings are easilyjammed difficult clean etc

As a result the possible technique of the use oftraditional electrospinning apparatus to produce nano-fibres of mass production is to fit the multiple spinneretstogether with a line or matrix pattern Many researchersadopted this technique to produce nanofibres from theearly period of the invention of this technology to nowIt is prevailing until Theron et al67 found bothexperimentally and in the simulations that the jets werepushed away from their neighbours by the Coulombicforces applied by the latter A non-uniform non-wovenmat would be obtained as an immediate consequence ofthis phenomenon Fang et al68 once reported that theysolved the problem of the mutual Coulombic interac-tions by the incorporation of secondary electrodes toisolate the electric field distribution of the primaryelectrode spinnerets But the problem of the smalldiameter of the spinneret and the low flowrate still

limited the fibre output Dosunmu et al69 suggested anew multiple porous tube apparatus in which thepolymer solution was electrified and pushed by airpressure through the walls of a large diameter cylindricalporous tube (see Fig 28a) Multiple jets formed on theporous surface and electrospun into nanofibres in theirprocess However the existing problem of this processmight be also the non-uniform fibre mat (see Fig 5 inRef 69)

In order to eliminate the problems of increasing thenanofibres of mass production of traditional electro-spinning apparatus some researchers demonstratedvarious novel electrospinning apparatus in recent yearsYet these apparatus different as they seem have onething in common The Taylor cone was formed on thefree surface of the solution and the jets were ejectedupward Yarin and Zussman70 proposed an upwardneedleless electrospinning which consisted of a two layersystem with the lower layer being a ferromagneticsuspension and the upper layer a polymer solution (seeFig 28b) In the process many vertical spikes ofmagnetic suspension perturbed the interlayer interfaceas the Taylor cones Though the production rate ishigher than that of traditional electrospinning processthere are some flaws such as not easy operation of twolayers liquids

So far only one electrospinning process Nanospiderspinning technology is claimed to be a commercialsuccess in the mass production of nanofibres all over theworld The principle of Nanospider is to use a cylinderto form the fibres The cylinder is partially immersed in

26 Electrospinning dilation phenomenon of poly

(butylenes succinate) (PBS) electrospun fibres64

27 Electrospun nanoporous microspheres where kind of

traditional Chinese drugs called lsquoYunnan Baiyorsquo was

used as additive65

a Dosunmursquos apparatus b upward needleless electro-spinning

28 New electrospinning apparatus for produing mass

production of nanofibres

He et al Apparatus for preparing electrospun nanofibres

Materials Science and Technology 2010 VOL 26 NO 11 1285

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

the polymer solution and as it rotates the polymersolution is carried to the top part of the cylinder whereTaylor cones are created The Taylor streams are formednext to each other throughout the entire length of thecylinder resulting in the high production capacity of thisapparatus71

The above novel approaches are exciting for theresearchers and the engineers When we cheer and applaudfor these much progress of electrospinning however a newproblem caused by the opened liquid surface has risen upIn every electrospinning process the volatilisation of thesolvent from the flying jet is a must In the above newupward electrospinning apparatus the free liquid surfacecan be used to create many Taylor cones and increase thethroughput of nanofibres at the same time can befacilitated to the escape of the volatilising solvent Withthe process proceeding the viscosity of the polymersolution will increase which will introduce some newproblems such as increasing the difficulty of spinning andchanging the properties of the solution

ConclusionsIt is clear that an understanding of the electrospinningand its nanofibres was greatly advanced by thenanoscience and technology in the 1990s and by thesubsequent development of the many effective applica-tions available in many fields today Remarkableadvances in electrospinning technology have beenpossible due to developments in modern technologyVarious novel and useful electrospinning techniques arereviewed in this work according to their differentmodifications Some modified apparatus were used toproduce different nanofibre mats with special structuresfor some special applications and some for massproduction of nanofibres Most of the advancesachieved to date have been made in the collector systemof the traditional electrospinning apparatus but otherparts such as solution storage and feeding systemspinneret and spinning area have been studied as wellMore recently emphasis has been placed on creatingnew apparatus to mass production of nanofibres Newtechniques have been designed and improved and moreelectrospinning apparatus have been developed to betterproduce nanofibres

Collectively the studies about electrospinning andnanofibres carried out so far have furnished a usefuldatabase for the electrohydrodynamics and some otherfields Although this field is in its infancy it is becomingincreasingly clear that there are multiple forms of newtechniques Identifying the similarities and differences invarious apparatus will enhance our understanding ofthis technology The next decades are expected to notonly enrich the existing knowledge but also significantlyadvance the technology into the commercial domain

Acknowledgements

The present work is supported financially by NationalNatural Science Foundation of China under Grant Nos10802021 and 10972053 National Natural ScienceFoundation of Shanghai under Grant No08ZR1400300 Shanghai Rising-Star Program underGrant No09QA1400100 Key Project of ChineseMinistry of Education No210006 National ScienceFoundation of Tianjin No10JCYBJC02200 Tianjin

Universities Science and Technology DevelopmentFund Project No20080322

References1 A Formhals lsquoProcess and apparatus for preparing artifical

threadsrsquo US Patent 1975504 1934

2 M S El Naschie Chaos Solitons Fract 2006 30 769ndash773

3 J-H He Chaos Solitons Fract 2008 38 1390ndash1393

4 J Lyons and F Ko Polym News 2005 30 170ndash178

5 D Li and Y-N Xia Adv Mater 2004 16 1151ndash1170

6 Z-M Huang Y-Z Zhang M Kotaki and S Ramakrishna

Compos Sci Technol 2003 63 2223ndash2253

7 Q P Pham U Sharma and A G Mikos Tissue Eng 2006 12

1197ndash1211

8 E P S Tan and C T Lim Compos Sci Technol 2006 66 1102ndash

1111

9 J-H He and H-M Liu Nonlinear Analy 2005 63 e919ndashe929

10 J-H He and Y-Q Wan Polymer 2004 45 6731ndash6734

11 J-H He Y-Q Wan and J-Y Yu Polymer 2005 46 2799ndash2801

12 J-H He Y-Q Wan and J-Y Yu Fibers Polym 2008 9 140ndash

142

13 J-H He L Xu Y Wu and Y Liu Polym Int 2007 56 1323ndash

1329

14 J-H He Y-Q Wan and J-Y Yu Int J Nonlinear Sci 2004 5

243ndash252

15 J-H He Y Wu and N Pan Int J Nonlinear Sci 2005 6 243ndash

248

16 Y-Q Wan J-H He and J-Y Yu Iran Polym J 2006 15 265ndash

268

17 L Rayleigh Philos Mag 1882 14 184ndash186

18 J F Cooley lsquoApparatus for electrically dispensing fluidsrsquo US

Patent 692631 1902

19 W J Morton lsquoMethod of dispensing fluidsrsquo US Patent 705691

1902

20 P K Baumgarten J Colloid Interface Sci 1971 36 71ndash79

21 J-H He and Y Liu J Polym Eng 2008 28 101ndash114

22 Y Liu J-H He L Xu and J-Y Yu Int J Electrospun Nanofibers

Appl 2007 1 7ndash15

23 Y Liu J-H He and J-Y Yu Fibres Text East Eur 2007 15

30ndash33

24 X-H Qin Y-Q Wan J-H He J Zhang J-Y Yu and

S-Y Wang Polymer 2004 45 6409ndash6413

25 D-H Shou and J-H He J Polym Eng 2008 28 115ndash118

26 Y-Q Wan J-H He Y Wu and J-Y Yu Mater Lett 2006 60

3296ndash3300

27 S-Q Wang J-H He and L Xu Polym Int 2008 57 1079ndash1082

28 Y Wu J-H He L Xu and J-Y Yu Int J Electrospun

Nanofibers Appl 2007 1 1ndash6

29 J-H He Y-Q Wan and J-Y Yu Int J Nonlinear Sci 2004 5

253ndash262

30 Y-Q Wan J-H He L Xu and J-Y Yu Int J Electrospun

Nanofibers Appl 2007 1 17ndash28

31 Y-Q Wan J-H He J-Y Yu and Y Wu J Appl Polym Sci

2007 103 3840ndash3843

32 J Zhang Y-Q Wan J-H He and Z-Y Zhu China Patent

2004100156853 2004

33 B H Bersted J Appl Polym Sci 2003 28 2777ndash2791

34 A I Isayev C M Wong and X Zeng Adv Polym Technol 2003

10 31ndash45

35 H Wu S Guo G Chen J Lin W Chen and H Wang J Appl

Polym Sci 2003 90 1873ndash1878

36 J P Ibar Polym Eng Sci 1998 38 1ndash20

37 S Sell C Barnes M Smith M McClure P Madurantakam

J Grant M McManus and G Bowlin Polym Int 2007 56 1349ndash

1360

38 P D Dalton K Klinkhammer J Salber D Klee and M Moller

Biomacromolecules 2006 7 686ndash690

39 J Lyons C Li and F Ko Polymer 2004 45 7597ndash7603

40 J-H He Y-Q Wan and L Xu Chaos Solitons Fract 2007 33

26ndash37

41 J T McCann M Marquez and Y Xia Nano Lett 2006 6 2868ndash

2872

42 N Ogata S Yamaguchi N Shimada G Lu T Iwata K Nakane

and T Ogihara J Appl Polym Sci 2007 104 1640ndash1645

43 Y Wu J-Y Yu J-H He and Y-Q Wan Chaos Solitons Fract

2007 32 5ndash7

44 J-H He Y-P Yu J-Y Yu W-R Li S-Y Wang and N Pan

Text Res J 2005 75 181ndash184

He et al Apparatus for preparing electrospun nanofibres

1286 Materials Science and Technology 2010 VOL 26 NO 11

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

not be formed as a stable Taylor cone at the orificebecause of the gravity and other factors and so is theincreasing flowrate Unlike the traditional dry spinningprocess or wet spinning process in which the jet isextruded from the spinneret with high speed thetraditional electrospinning process depends on theelectrostatic force which just provides a stable and slowliquid flow to form a stable Taylor cone at the tip of thespinneret rather than the extrusion pressure Thespinning speed is much less than that of the dry spinningor wet spinning The other shortcomings are easilyjammed difficult clean etc

As a result the possible technique of the use oftraditional electrospinning apparatus to produce nano-fibres of mass production is to fit the multiple spinneretstogether with a line or matrix pattern Many researchersadopted this technique to produce nanofibres from theearly period of the invention of this technology to nowIt is prevailing until Theron et al67 found bothexperimentally and in the simulations that the jets werepushed away from their neighbours by the Coulombicforces applied by the latter A non-uniform non-wovenmat would be obtained as an immediate consequence ofthis phenomenon Fang et al68 once reported that theysolved the problem of the mutual Coulombic interac-tions by the incorporation of secondary electrodes toisolate the electric field distribution of the primaryelectrode spinnerets But the problem of the smalldiameter of the spinneret and the low flowrate still

limited the fibre output Dosunmu et al69 suggested anew multiple porous tube apparatus in which thepolymer solution was electrified and pushed by airpressure through the walls of a large diameter cylindricalporous tube (see Fig 28a) Multiple jets formed on theporous surface and electrospun into nanofibres in theirprocess However the existing problem of this processmight be also the non-uniform fibre mat (see Fig 5 inRef 69)

In order to eliminate the problems of increasing thenanofibres of mass production of traditional electro-spinning apparatus some researchers demonstratedvarious novel electrospinning apparatus in recent yearsYet these apparatus different as they seem have onething in common The Taylor cone was formed on thefree surface of the solution and the jets were ejectedupward Yarin and Zussman70 proposed an upwardneedleless electrospinning which consisted of a two layersystem with the lower layer being a ferromagneticsuspension and the upper layer a polymer solution (seeFig 28b) In the process many vertical spikes ofmagnetic suspension perturbed the interlayer interfaceas the Taylor cones Though the production rate ishigher than that of traditional electrospinning processthere are some flaws such as not easy operation of twolayers liquids

So far only one electrospinning process Nanospiderspinning technology is claimed to be a commercialsuccess in the mass production of nanofibres all over theworld The principle of Nanospider is to use a cylinderto form the fibres The cylinder is partially immersed in

26 Electrospinning dilation phenomenon of poly

(butylenes succinate) (PBS) electrospun fibres64

27 Electrospun nanoporous microspheres where kind of

traditional Chinese drugs called lsquoYunnan Baiyorsquo was

used as additive65

a Dosunmursquos apparatus b upward needleless electro-spinning

28 New electrospinning apparatus for produing mass

production of nanofibres

He et al Apparatus for preparing electrospun nanofibres

Materials Science and Technology 2010 VOL 26 NO 11 1285

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

the polymer solution and as it rotates the polymersolution is carried to the top part of the cylinder whereTaylor cones are created The Taylor streams are formednext to each other throughout the entire length of thecylinder resulting in the high production capacity of thisapparatus71

The above novel approaches are exciting for theresearchers and the engineers When we cheer and applaudfor these much progress of electrospinning however a newproblem caused by the opened liquid surface has risen upIn every electrospinning process the volatilisation of thesolvent from the flying jet is a must In the above newupward electrospinning apparatus the free liquid surfacecan be used to create many Taylor cones and increase thethroughput of nanofibres at the same time can befacilitated to the escape of the volatilising solvent Withthe process proceeding the viscosity of the polymersolution will increase which will introduce some newproblems such as increasing the difficulty of spinning andchanging the properties of the solution

ConclusionsIt is clear that an understanding of the electrospinningand its nanofibres was greatly advanced by thenanoscience and technology in the 1990s and by thesubsequent development of the many effective applica-tions available in many fields today Remarkableadvances in electrospinning technology have beenpossible due to developments in modern technologyVarious novel and useful electrospinning techniques arereviewed in this work according to their differentmodifications Some modified apparatus were used toproduce different nanofibre mats with special structuresfor some special applications and some for massproduction of nanofibres Most of the advancesachieved to date have been made in the collector systemof the traditional electrospinning apparatus but otherparts such as solution storage and feeding systemspinneret and spinning area have been studied as wellMore recently emphasis has been placed on creatingnew apparatus to mass production of nanofibres Newtechniques have been designed and improved and moreelectrospinning apparatus have been developed to betterproduce nanofibres

Collectively the studies about electrospinning andnanofibres carried out so far have furnished a usefuldatabase for the electrohydrodynamics and some otherfields Although this field is in its infancy it is becomingincreasingly clear that there are multiple forms of newtechniques Identifying the similarities and differences invarious apparatus will enhance our understanding ofthis technology The next decades are expected to notonly enrich the existing knowledge but also significantlyadvance the technology into the commercial domain

Acknowledgements

The present work is supported financially by NationalNatural Science Foundation of China under Grant Nos10802021 and 10972053 National Natural ScienceFoundation of Shanghai under Grant No08ZR1400300 Shanghai Rising-Star Program underGrant No09QA1400100 Key Project of ChineseMinistry of Education No210006 National ScienceFoundation of Tianjin No10JCYBJC02200 Tianjin

Universities Science and Technology DevelopmentFund Project No20080322

References1 A Formhals lsquoProcess and apparatus for preparing artifical

threadsrsquo US Patent 1975504 1934

2 M S El Naschie Chaos Solitons Fract 2006 30 769ndash773

3 J-H He Chaos Solitons Fract 2008 38 1390ndash1393

4 J Lyons and F Ko Polym News 2005 30 170ndash178

5 D Li and Y-N Xia Adv Mater 2004 16 1151ndash1170

6 Z-M Huang Y-Z Zhang M Kotaki and S Ramakrishna

Compos Sci Technol 2003 63 2223ndash2253

7 Q P Pham U Sharma and A G Mikos Tissue Eng 2006 12

1197ndash1211

8 E P S Tan and C T Lim Compos Sci Technol 2006 66 1102ndash

1111

9 J-H He and H-M Liu Nonlinear Analy 2005 63 e919ndashe929

10 J-H He and Y-Q Wan Polymer 2004 45 6731ndash6734

11 J-H He Y-Q Wan and J-Y Yu Polymer 2005 46 2799ndash2801

12 J-H He Y-Q Wan and J-Y Yu Fibers Polym 2008 9 140ndash

142

13 J-H He L Xu Y Wu and Y Liu Polym Int 2007 56 1323ndash

1329

14 J-H He Y-Q Wan and J-Y Yu Int J Nonlinear Sci 2004 5

243ndash252

15 J-H He Y Wu and N Pan Int J Nonlinear Sci 2005 6 243ndash

248

16 Y-Q Wan J-H He and J-Y Yu Iran Polym J 2006 15 265ndash

268

17 L Rayleigh Philos Mag 1882 14 184ndash186

18 J F Cooley lsquoApparatus for electrically dispensing fluidsrsquo US

Patent 692631 1902

19 W J Morton lsquoMethod of dispensing fluidsrsquo US Patent 705691

1902

20 P K Baumgarten J Colloid Interface Sci 1971 36 71ndash79

21 J-H He and Y Liu J Polym Eng 2008 28 101ndash114

22 Y Liu J-H He L Xu and J-Y Yu Int J Electrospun Nanofibers

Appl 2007 1 7ndash15

23 Y Liu J-H He and J-Y Yu Fibres Text East Eur 2007 15

30ndash33

24 X-H Qin Y-Q Wan J-H He J Zhang J-Y Yu and

S-Y Wang Polymer 2004 45 6409ndash6413

25 D-H Shou and J-H He J Polym Eng 2008 28 115ndash118

26 Y-Q Wan J-H He Y Wu and J-Y Yu Mater Lett 2006 60

3296ndash3300

27 S-Q Wang J-H He and L Xu Polym Int 2008 57 1079ndash1082

28 Y Wu J-H He L Xu and J-Y Yu Int J Electrospun

Nanofibers Appl 2007 1 1ndash6

29 J-H He Y-Q Wan and J-Y Yu Int J Nonlinear Sci 2004 5

253ndash262

30 Y-Q Wan J-H He L Xu and J-Y Yu Int J Electrospun

Nanofibers Appl 2007 1 17ndash28

31 Y-Q Wan J-H He J-Y Yu and Y Wu J Appl Polym Sci

2007 103 3840ndash3843

32 J Zhang Y-Q Wan J-H He and Z-Y Zhu China Patent

2004100156853 2004

33 B H Bersted J Appl Polym Sci 2003 28 2777ndash2791

34 A I Isayev C M Wong and X Zeng Adv Polym Technol 2003

10 31ndash45

35 H Wu S Guo G Chen J Lin W Chen and H Wang J Appl

Polym Sci 2003 90 1873ndash1878

36 J P Ibar Polym Eng Sci 1998 38 1ndash20

37 S Sell C Barnes M Smith M McClure P Madurantakam

J Grant M McManus and G Bowlin Polym Int 2007 56 1349ndash

1360

38 P D Dalton K Klinkhammer J Salber D Klee and M Moller

Biomacromolecules 2006 7 686ndash690

39 J Lyons C Li and F Ko Polymer 2004 45 7597ndash7603

40 J-H He Y-Q Wan and L Xu Chaos Solitons Fract 2007 33

26ndash37

41 J T McCann M Marquez and Y Xia Nano Lett 2006 6 2868ndash

2872

42 N Ogata S Yamaguchi N Shimada G Lu T Iwata K Nakane

and T Ogihara J Appl Polym Sci 2007 104 1640ndash1645

43 Y Wu J-Y Yu J-H He and Y-Q Wan Chaos Solitons Fract

2007 32 5ndash7

44 J-H He Y-P Yu J-Y Yu W-R Li S-Y Wang and N Pan

Text Res J 2005 75 181ndash184

He et al Apparatus for preparing electrospun nanofibres

1286 Materials Science and Technology 2010 VOL 26 NO 11

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

the polymer solution and as it rotates the polymersolution is carried to the top part of the cylinder whereTaylor cones are created The Taylor streams are formednext to each other throughout the entire length of thecylinder resulting in the high production capacity of thisapparatus71

The above novel approaches are exciting for theresearchers and the engineers When we cheer and applaudfor these much progress of electrospinning however a newproblem caused by the opened liquid surface has risen upIn every electrospinning process the volatilisation of thesolvent from the flying jet is a must In the above newupward electrospinning apparatus the free liquid surfacecan be used to create many Taylor cones and increase thethroughput of nanofibres at the same time can befacilitated to the escape of the volatilising solvent Withthe process proceeding the viscosity of the polymersolution will increase which will introduce some newproblems such as increasing the difficulty of spinning andchanging the properties of the solution

ConclusionsIt is clear that an understanding of the electrospinningand its nanofibres was greatly advanced by thenanoscience and technology in the 1990s and by thesubsequent development of the many effective applica-tions available in many fields today Remarkableadvances in electrospinning technology have beenpossible due to developments in modern technologyVarious novel and useful electrospinning techniques arereviewed in this work according to their differentmodifications Some modified apparatus were used toproduce different nanofibre mats with special structuresfor some special applications and some for massproduction of nanofibres Most of the advancesachieved to date have been made in the collector systemof the traditional electrospinning apparatus but otherparts such as solution storage and feeding systemspinneret and spinning area have been studied as wellMore recently emphasis has been placed on creatingnew apparatus to mass production of nanofibres Newtechniques have been designed and improved and moreelectrospinning apparatus have been developed to betterproduce nanofibres

Collectively the studies about electrospinning andnanofibres carried out so far have furnished a usefuldatabase for the electrohydrodynamics and some otherfields Although this field is in its infancy it is becomingincreasingly clear that there are multiple forms of newtechniques Identifying the similarities and differences invarious apparatus will enhance our understanding ofthis technology The next decades are expected to notonly enrich the existing knowledge but also significantlyadvance the technology into the commercial domain

Acknowledgements

The present work is supported financially by NationalNatural Science Foundation of China under Grant Nos10802021 and 10972053 National Natural ScienceFoundation of Shanghai under Grant No08ZR1400300 Shanghai Rising-Star Program underGrant No09QA1400100 Key Project of ChineseMinistry of Education No210006 National ScienceFoundation of Tianjin No10JCYBJC02200 Tianjin

Universities Science and Technology DevelopmentFund Project No20080322

References1 A Formhals lsquoProcess and apparatus for preparing artifical

threadsrsquo US Patent 1975504 1934

2 M S El Naschie Chaos Solitons Fract 2006 30 769ndash773

3 J-H He Chaos Solitons Fract 2008 38 1390ndash1393

4 J Lyons and F Ko Polym News 2005 30 170ndash178

5 D Li and Y-N Xia Adv Mater 2004 16 1151ndash1170

6 Z-M Huang Y-Z Zhang M Kotaki and S Ramakrishna

Compos Sci Technol 2003 63 2223ndash2253

7 Q P Pham U Sharma and A G Mikos Tissue Eng 2006 12

1197ndash1211

8 E P S Tan and C T Lim Compos Sci Technol 2006 66 1102ndash

1111

9 J-H He and H-M Liu Nonlinear Analy 2005 63 e919ndashe929

10 J-H He and Y-Q Wan Polymer 2004 45 6731ndash6734

11 J-H He Y-Q Wan and J-Y Yu Polymer 2005 46 2799ndash2801

12 J-H He Y-Q Wan and J-Y Yu Fibers Polym 2008 9 140ndash

142

13 J-H He L Xu Y Wu and Y Liu Polym Int 2007 56 1323ndash

1329

14 J-H He Y-Q Wan and J-Y Yu Int J Nonlinear Sci 2004 5

243ndash252

15 J-H He Y Wu and N Pan Int J Nonlinear Sci 2005 6 243ndash

248

16 Y-Q Wan J-H He and J-Y Yu Iran Polym J 2006 15 265ndash

268

17 L Rayleigh Philos Mag 1882 14 184ndash186

18 J F Cooley lsquoApparatus for electrically dispensing fluidsrsquo US

Patent 692631 1902

19 W J Morton lsquoMethod of dispensing fluidsrsquo US Patent 705691

1902

20 P K Baumgarten J Colloid Interface Sci 1971 36 71ndash79

21 J-H He and Y Liu J Polym Eng 2008 28 101ndash114

22 Y Liu J-H He L Xu and J-Y Yu Int J Electrospun Nanofibers

Appl 2007 1 7ndash15

23 Y Liu J-H He and J-Y Yu Fibres Text East Eur 2007 15

30ndash33

24 X-H Qin Y-Q Wan J-H He J Zhang J-Y Yu and

S-Y Wang Polymer 2004 45 6409ndash6413

25 D-H Shou and J-H He J Polym Eng 2008 28 115ndash118

26 Y-Q Wan J-H He Y Wu and J-Y Yu Mater Lett 2006 60

3296ndash3300

27 S-Q Wang J-H He and L Xu Polym Int 2008 57 1079ndash1082

28 Y Wu J-H He L Xu and J-Y Yu Int J Electrospun

Nanofibers Appl 2007 1 1ndash6

29 J-H He Y-Q Wan and J-Y Yu Int J Nonlinear Sci 2004 5

253ndash262

30 Y-Q Wan J-H He L Xu and J-Y Yu Int J Electrospun

Nanofibers Appl 2007 1 17ndash28

31 Y-Q Wan J-H He J-Y Yu and Y Wu J Appl Polym Sci

2007 103 3840ndash3843

32 J Zhang Y-Q Wan J-H He and Z-Y Zhu China Patent

2004100156853 2004

33 B H Bersted J Appl Polym Sci 2003 28 2777ndash2791

34 A I Isayev C M Wong and X Zeng Adv Polym Technol 2003

10 31ndash45

35 H Wu S Guo G Chen J Lin W Chen and H Wang J Appl

Polym Sci 2003 90 1873ndash1878

36 J P Ibar Polym Eng Sci 1998 38 1ndash20

37 S Sell C Barnes M Smith M McClure P Madurantakam

J Grant M McManus and G Bowlin Polym Int 2007 56 1349ndash

1360

38 P D Dalton K Klinkhammer J Salber D Klee and M Moller

Biomacromolecules 2006 7 686ndash690

39 J Lyons C Li and F Ko Polymer 2004 45 7597ndash7603

40 J-H He Y-Q Wan and L Xu Chaos Solitons Fract 2007 33

26ndash37

41 J T McCann M Marquez and Y Xia Nano Lett 2006 6 2868ndash

2872

42 N Ogata S Yamaguchi N Shimada G Lu T Iwata K Nakane

and T Ogihara J Appl Polym Sci 2007 104 1640ndash1645

43 Y Wu J-Y Yu J-H He and Y-Q Wan Chaos Solitons Fract

2007 32 5ndash7

44 J-H He Y-P Yu J-Y Yu W-R Li S-Y Wang and N Pan

Text Res J 2005 75 181ndash184

He et al Apparatus for preparing electrospun nanofibres

1286 Materials Science and Technology 2010 VOL 26 NO 11

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

45 J-H He Y-P Yu and N Pan Mech Res Commun 2005 32

197ndash200

46 X Li F Sun C Yao and T Song lsquoConjugate electrospinning

continuous yarns from oppositely charged nanofibersrsquo 231st

National Meeting of the American-Chemical-Society MAR 26ndash

30 2006 Atlanta GA ABSTRACTS OF PAPERS OF THE

AMERICAN CHEMICAL SOCIETY 213 18-PMSE 2006

47 X-S Li C Yao and F-Q Sun lsquoApparatus and methods for the

preparation of continuous nanofiber yarnsrsquo China Patent

2005100953840 2005

48 X-S Li C Yao and T-Y Song lsquoMethod for the preparation of

continuous nanofiber yarnsrsquo China Patent 2005100385715 2005

49 F I Bell I J McEwen and C Viney Nature 2002 416 37

50 F Vollrath and D P Knight Nature 2001 410 541ndash548

51 Y-L Li I A Kinloch and A H Windle Science 2004 304 276ndash278

52 A Lazaris S Arcidiacono Y Huang J-F Zhou F Duguay

N Chretien E A Welsh J W Soares and C N Karatzas

Science 2002 295 472ndash476

53 J Gatesy C Hayashi D Motriuk J Woods and R Lewis

Science 2001 291 2603ndash2605

54 H-J Jin and D L Kaplan Nature 2003 424 1057ndash1061

55 R F Service Science 2002 295 419ndash421

56 C D Hendricks Jr R S Carson J J Hogan and J M Schneider

AIAA J 1964 2 733ndash737

57 C J Buchko L-C Chen Y Shen and D C Martin Polymer

1999 40 7397ndash7407

58 S-Y Gu Q-L Wu J Ren and G J Vancso Macromol Rapid

Commun 2005 26 716ndash720

59 Y Liu and L Xu Int J Nonlinear Sci 2006 7 389ndash392

60 Y Liu and J-H He Int J Nonlinear Sci 2007 8 393ndash396

61 Y Liu J-H He L Xu and J-Y Yu J Polym Eng 2008 28 55ndash

66

62 Y Liu J-H He and J-Y Yu J Phys Conf Ser 2008 96

012001

63 J-H He Y Liu L Xu J-Y Yu and G Sun Chaos Solitons

Fract 2008 37 643ndash651

64 J-H He Y Liu L Xu and J-Y Yu Chaos Solitons Fract 2007

32 1096ndash1100

65 L Xu J-H He and Y Liu Int J Nonlinear Sci 2007 8 199

ndash202

66 L Xu and Z-F Ren J Polym Eng 2008 28 27ndash31

67 S A Theron A L Yarin E Zussman and E Kroll Polymer

2005 46 2889ndash2899

68 D Fang C Charles S H Benjamin and C Benjamin Polym

Nanofibers (ACS Symp Ser) 2006 918 91ndash105

69 O O Dosunmu G G Chase W Kataphinan and D H Reneker

Nanotechnology 2006 17 1123ndash1127

70 A L Yarin and E Zussman Polymer 2004 45 2977ndash2980

71 httpwwwnanopeuticsnetnanospiderhtml

He et al Apparatus for preparing electrospun nanofibres

Materials Science and Technology 2010 VOL 26 NO 11 1287