Polymer nanofibers assembled by electrospinning - Molecules and Code

Current Opinion in Colloid and Interface Science 8(2003) 64–75

1359-0294/03/$ - see front matter� 2003 Elsevier Science Ltd. All rights reserved.doi:10.1016/S1359-0294Ž03.00004-9

Polymer nanofibers assembled by electrospinning

Audrey Frenot, Ioannis S. Chronakis*

IFP Research, Swedish Institute for Fiber and Polymer Research, P.O. Box 104, S-431 22 Molndal, Sweden¨

Abstract

Electrospinning is a process by which polymer nanofibers(with diameter lower than 100 nm and lengths up to kilometres) canbe produced using an electrostatically driven jet of polymer solution(or polymer melt). Simple alignment of electrospunnanofibers constructs unique functional nanostructures such as nanotubes and nanowires. Significant progress has been made inthis area throughout the past few years and this technology has been exploited to a wide range of applications. Most of the recentwork on electrospinning has focused either on trying to understand deeper the fundamental aspects of the process in order to gaincontrol of nanofiber morphology, structure, surface functionality, and strategies for assembling them or on determining appropriateconditions for electrospinning of various polymers and biopolymers.� 2003 Elsevier Science Ltd. All rights reserved.

Keywords: Electrospinning; Electrospun; Nanofibers; Nanostructured material; Nanotechnology; Polymer fibers; Biopolymer fibers; Blends;Electrified fluid jet

1. Introduction

Unlike conventional fiber spinning techniques(wetspinning, dry spinning, melt spinning, gel spinning),which are capable of producing polymer fibers withdiameters down to the micrometer range, electrostaticspinning, or ‘electrospinning’ is a process capable ofproducing polymer fibers in the nanometer diameterrange. Electrospinning is a novel and efficient fabrica-tion process that can be utilised to assemble fibrouspolymer mats composed of fiber diameters ranging fromseveral microns down to fibers with diameter lower than100 nm (Fig. 1). This electrostatic processing methoduses a high-voltage electric field to form solid fibersfrom a polymeric fluid stream(solution or melt) deliv-ered through a millimeter-scale nozzle.Nanofibers are the ultra-fine solid fibers notable for

their very small diameters(lower than 100 nm), theirlarge surface area per unit mass and small pore size.Due to the inherent properties of the electrospinningprocess, which can control the deposition of polymerfibers onto a target substrate, nanofibers with complex,and seamless three-dimensional shapes could be formed.Construction of nanoscale composite fibers by electros-

*Corresponding author. Tel.:q46-31-706-63-00; fax:q46-31-706-63-65.

E-mail address: [email protected](I.S. Chronakis).

pinning from a mixture of rigid rod polymers andflexible polymers is also feasible. The electrospun nan-ofibers can even be aligned to construct unique func-tional nanostructures such as nanotubes and nanowires.Furthermore, depending on the specific polymer beingused, a wide range of fabric properties such as strength,weight and porosity, surface functionality etc. can beachieved. This novel fiber spinning technique providesas well the capacity to lace together a variety of typesof polymers, fibers, and particles to produce ultrathinlayers. Small insoluble particles can be added to thepolymer solution and encapsulated in the dry nanofibers.Soluble drugs or bacterial agents can be added andelectrospun into nonwoven mats.Although the process of electrospinning has been

known for almost 70 years and the first patent wasissued to Formhals in 1934(US Patent, 1-975-504),polymeric nanofibers produced by electrospinning havebecome a topic of great interest for the past few years.Reneker and Chun, who revived interest in this technol-ogy in the early 1990s, has shown the possibility toelectrospinw1x a wide range of polymer solutions in1996. Larrondo and Manley had performed a similarwork on polymer melts in 1981w2x. Typically, electros-pinning is applicable to a wide range of polymers likethose used in conventional spinning, i.e. polyolefine,polyamides, polyester, aramide, acrylic as well as bio-

65A. Frenot, I.S. Chronakis / Current Opinion in Colloid and Interface Science 8 (2003) 64–75

Fig. 1. (a) Electrospun PET from a solution of 13 wt.% in 1:1 dichloromethane and trifluoroacetic acid. Comparison with human hair;(b)Electrospun PEO fibers from a solution of 5 wt.% PEO in 50 wt.% H O and 45 wt.% ethanol. The diameter of the fibers lies between 50 and2

500 nm.(Audrey Frenot, unpublished work).

Fig. 2. Scheme of an electrospinning equipment. NB, several rawmaterial sources can be used(i.e. several capillaries).

polymers like proteins, DNA, polypeptides, or otherslike electric conducting and photonic polymers. Thehigh specific surface area and small pore size of elec-trospun nanofibers make them interesting candidates fora wide variety of applications. For instance nanofiberswith a diameter of 100 nm have a ratio of geometricalsurface area to mass of approximately 100 myg. Anoth-2

er interesting aspect of using electrospun fibers is thatthe fibers may dissipate or retain the electrostatic chargesdepending on the electrical properties of the polymer.The charges can be manipulated as well by electricalfields and the electrical polarity of the fibers is affectedby the polarity of the applied voltage. Nanofibers pro-vide also a connection between the nanoscale world andthe macroscale world, since the diameters are in thenanometer range and the lengths are kilometres. There-fore, current emphasis of research is to exploit suchproperties and focus on determining appropriate condi-tions for electrospinning various polymers and biopoly-mers for eventual applications including multifunctionalmembranes, biomedical structural elements(scaffoldingused in tissue engineering, wound dressing, drug deliv-ery, artificial organs, vascular grafts), protective shieldsin speciality fabrics, filter media for submicron particlesin separation industry, composite reinforcement, andstructures for nano-electronic machines among others.In this review we concentrate on current understand-

ing of the electrospinning process and those parameters,which influence the properties of the fibers producedfrom it. We also address issues of nanofiber structuresand characteristics and discuss models proposed forprocessing instabilities. Due to the growing interest onnanofibers processed with specific bulk morphologiesand surface topologies, we focus on such recent advanc-es and discuss some general considerations and remain-

ing challenges. The current utilisation of nanofibersmainly for biomedical applications, for developing nano-electronic machines and for designing potential struc-tures in order to develop functional nanostructuredmaterials, is another active research area that covered inthis review. The major developments are highlightedcovering the publications period from 1999 onwards.

2. The electrospinning process

The electrospinning equipment for polymer solutionand melt are well presented in the papersw3–7 ,8x and●●

w2x, respectively. Electrospinning is easily realised byapplying a high voltage to a capillary filled with thepolymer fluid to be spun with help of an electrode. The

66 A. Frenot, I.S. Chronakis / Current Opinion in Colloid and Interface Science 8 (2003) 64–75

resulting fibers are collected on a grounded plate, whichcan be covered with a fabric for example. The processis schematically illustrated in Fig. 2. The groups workingon electrospinning have adopted different solution forthe polymer feed. Some have simply opted for placingthe capillary perpendicularly, letting the polymer fluiddropping with help of gravitation and lying the collectorunderneathw9x. Sometimes the capillary can be tilted ata defined angle to control the floww5,10,11x. In othercases, the capillary was horizontalw12 x and a pump●●

was used to initiate the droplet, the pump being alsoused in the case of vertical feedingw3,4,6,12 x. The●●

electrode can be inserted either in the polymer fluid orplaced onto the tip of the capillary if a syringe with ametal needle is used.The collector is usually a plane metal sheet or a grid

that can be covered with a fabric. Kim and Renekerused a different set-up in a work on polybenzimidazole(PBI) w13x. Actually, they used a rotating cylindercovered with a grounded aluminium sheet. Rangpukanand Reneker have also reported the development ofelectrospinning from molten polymers in vacuumw14x.Electrospinning from polymer melts in vacuum is advan-tageous because higher electric field strength over largedistances and higher temperature can be used than inair. In the work by Theron et al. an electrostatic field-assisted assembly technique was described, combinedwith an electrospinning process used to position andalign individual nanofibers on a tapered and groundedwheel-like bobbinw15 x. The bobbin is able to wind a●●

continuously as-spun nanofiber at its tip-like edge. Thealignment approach has resulted in polyethylene oxide(PEO)-based nanofibers with diameters ranging from100 to 300 nm and lengths of up to hundreds of microns.The equipment developed by the University of Mas-

sachusetts Darmouth in collaboration with Massachu-setts Institute of Technology had also a differentconfiguration: parallel-plate geometry. Indeed a ‘point-plate’ geometry leads to curved electric field lines closeto the capillary, resulting thus in a non-uniform electricfield. The parallel-plate configuration allows the controlof the curvaturew6x. Moreover, they fed the solution tomultiple spinnerets placed in a radial geometry. Thecollector was rotating.Beneath, a brief description of the electrospinning

process is placed(Fig. 2). Electric field is subjected tothe end of a capillary tube that contains the polymerfluid held by its surface tension. This induces a chargeon the surface of the liquid. Mutual charge repulsioncauses a force directly opposite to the surface tensionw16x. As the intensity of the electric field is increased,the hemispherical surface of the fluid at the tip of thecapillary tube elongates to form a conical shape knownas the Taylor cone(the base region). With increasingfield, a critical value is attained when the repulsive

electrostatic force overcomes the surface tension and acharged jet of fluid is ejected from the tip of the Taylorcone. The discharged polymer solution jet undergoes awhipping process wherein the solvent evaporates, leav-ing behind a charged polymer fiber, which is highlystretched and reduced in diameter as it travels before itlays itself randomly on a grounded collecting metalscreen(the jet region). The rapidly whipping jet, thefluid instability, is an essential element of the processthat cause bending and stretching of the jet. In the caseof the melt the discharged jet solidifies when it travelsin the air and is collected on the grounded metal screen.Splaying occurs in a region in which the radical forcesfrom the electrical charges carried by the jet, becomelarger than the cohesive forces within the jet, and thesingle jet divides into many charged jets(with approx-imately equal diameters and charge per unit length)before fibers ‘landing’ on the collector(the collectionregion). Although little is currently known about thissplaying process of the primary jet into multiple fila-ments, it is thought to be responsible—together with theelongation due to the acceleration of the jet by electricalforces—for the unusually small diameter fibers whichcan produced by electrospinningw1x.

As mentioned, electrospun fibers can be electrostati-cally charged andyor the charge can be manipulated.The charge can be removed, when it is not wanted, byexposure to ions carried in the air(or the charge cansometimes be neutralised during contact with the collec-tor). If the fibers are electrically conducting, the chargecan also be adjusted by conduction through the fiberw1x. Tsai and Schreuder-Gibson, discuss the effect ofelectrospinning material and conditions upon the residualelectrostatic charge of polymer nanofibersw17x. Poly-mers such PEO, polycaprolactone(a polyester), poly-carbonate, and polystyrene have been evaluated for theircharge induction and charge retention characteristics andare ranked according to their inherent polarity.Besides circular fibers, a variety of cross-sectional

shapes and sizes can be obtained from different polymersolutions during electrospinning(Fig. 3). ActuallyKoombhongse, Wenxia and Reneker obtained branchedfibers, flat ribbons, bent ribbons, ribbons with othershapes, and fibers that were split longitudinally fromlarger fibers from electrospinning a polymer solutionw9x. Studies on the properties of fibers with these cross-sectional shapes from a number of different kinds ofpolymers and solvents indicates that fluid mechanicaleffects, electrical charge carried with the jet, and evap-oration of the solvent all contributed to the formationof the fibers. The influence of a skin on the jets ofpolymer solutions accounts as well for a number ofproperties observed. We address the issues of the elec-trospinning processing variables in the next paragraph.


Fig. 3. Scanning electron micrographs showing flat ribbons formation during electrospinning of PET from a solution of 13 wt.% in 1:1 dichlo-romethane and trifluoroacetic acid. Left magnification=1000, right: an enlarged image with magnification=3500.(Audrey Frenot, unpublishedwork).

3. Electrospinning processing variables and instabil-ities models

The following parameters and processing variablesaffect the electrospinning process:(i) System parameterssuch as molecular weight, molecular-weight distributionand architecture(branched, linear etc.) of the polymerand solution properties(viscosity, conductivity and sur-face tension), and (ii) Process parameters such aselectric potential, flow rate and concentration, distancebetween the capillary and collection screen, ambientparameters(temperature, humidity and air velocity inthe chamber) and finally motion of target screenw18x.For instance, the polymer solution must have a concen-tration high enough to cause polymer entanglements yetnot so high that the viscosity prevents polymer motioninduced by the electric field. The solution must alsohave a surface tension low enough, a charge densityhigh enough, and a viscosity high enough to prevent thejet from collapsing into droplets before the solvent hasevaporated. Morphological changes can occur upondecreasing the distance between the syringe needle andthe substrate. Increasing the distance or decreasing theelectrical field decreases the bead density, regardless ofthe concentration of the polymer in the solution. Appliedfields can, moreover, influence the morphology in peri-odic ways, creating a variety of new shapes on thesurface.Deitzel et al. have evaluated systematically the effects

of two important processing parameters, spinning volt-age and solution concentration, on the morphology ofthe fibers formedw19x. They found that spinning voltageis strongly correlated with the formation of bead defectsin the fibers, and their measurements can be used tosignal the onset of the processing voltage at which thebead defect density increases substantially. Solutionconcentration has also been found to most stronglyaffect fiber size, with fiber diameter increasing with

increasing solution concentration according to a powerlaw relationship. In addition, electrospinning from solu-tions of high concentration has been found to producea bimodal distribution of fiber sizes, reminiscent ofdistributions observed in the similar droplet generationprocess of electrospray. Moreover they found evidencethat electrostatic effects influence the macroscale mor-phology of electrospun fabrics and may result in theformation of heterogeneous or three-dimensionalstructures.Meanwhile, both electrostatic and fluid dynamic insta-

bilities can contribute to the basic operation of theprocessw3,7 ,9,20 ,21,22 ,23 ,24 ,25 ,26x. The bend-●● ● ●● ● ● ●

ing instabilities that occur during electrospinning havebeen studied and mathematically modelled by Renekeret al. w7 ,9,21x. After the jet travels straightforward,●●

unsteadiness appears under the form of loops. The jetdoes not only bend but it also forms lateral excursionsthat grow into spiralling loop. Every loop grows largerin diameter and the jet becomes thinnerw7 x. New●●

bending instabilities arise when the jet is thin enoughand enough stress relaxation of the viscoelastic stresshas taken place. An envelope cone(Fig. 4) defines theregion of these instabilities.Reneker et al.w7 x chose to model the instabilities●●

with viscoelastic dumbbells connected together(Fig. 5).The interactions between the beads forming the dumb-bells were following Coulomb’s law. The other para-meters taken into account were the electrical forces dueto the electric field, surface tension and the fact that thespring connecting the beads had a Maxwellian viscoe-lastic resistance to elongation, like the jet. Aerodynamicforces and gravity effect were neglected as well assolvent evaporation. This was taken into account in alater study w21x. In these articles, it is stated thatelectrospinning process and instabilities can be consid-ered as particular examples of the Earnshaw theorem in


Fig. 4. Schematic of the electrospinning equipment showing the enve-lope cone. With courtesy of the American Institute of Physicsw7 x.●●

Fig. 5. Bending jet modelled by a system of beads connected byviscoelastic elements. With courtesy of the American Institute ofPhysicsw7 x.●●

electrostatics. This means that one can not get a stablestructure if the elements of this one interact only byCoulomb’s law. There is a tendency of the charges inthe polymer fluid to reduce their Coulomb interactionenergy by moving the fluid in a complicated path.However, the challenges ahead lay in extending thislevel of fundamental knowledge to include non-lineareffects arising from viscoelasticity of the fluid andadditional fluid phenomena that occur downstream ofthe initial instability. This was attempted in a later studyby the same group, where they propose that in liquidswith a non-relaxing elastic force, that force also affectsthe shapes of the electrospun fibersw22 x. They showed●●

that the Taylor cone does not represent a unique criticalshape but half angles are much closer to this new shape.Another group by Hohman, Shin, Rutlege and Brenner

also studied electrospinning, with regard to electricallyforced jet and instabilities, and proposed a stabilitytheory for electrified fluid jetsw23 ,24 ,25 ,26x. A series● ● ●

of papers demonstrate that an essential mechanism ofelectrospinning is a rapidly whipping fluid jet. Theauthors analysed the mechanics of this whipping jet bystudying the instability of an electrically forced fluid jetwith increasing field strength. An asymptotic approxi-mation of the equations of electrohydrodynamics wasdeveloped so that quantitative comparisons with exper-

iments can be carried out. The approximation governsboth long wavelength axisymmetric distortions of thejet, as well as long wavelength oscillations of thecenterline of the jet. Three different instabilities wereidentified: the classical(axisymmetric) Rayleigh insta-bility and electric field induced axisymmetric and whip-ping instabilities. At increasing field strengths, theelectrical instabilities are enhanced whereas the Rayleighinstability is suppressed. Which instability dominatesdepends strongly on the surface charge density and theradius of the jet. In their later work, they have used thestability theory to develop a quantitative method forpredicting when electrospinning occursw23 x. First, a●

method for calculating the shape and charge density ofa steady jet as it thins from the nozzle was presentedand was shown to capture quantitative features of theexperiments. Then, this information was combined withthe stability analysis to predict scaling laws for the jetbehaviour and to produce operating diagrams for whenelectrospinning occurs, both as a function of experimen-tal parameters. Predictions for how the regime of elec-trospinning changes as a function of the fluidconductivity and viscosity were also presented.Furthermore, Vao Soongnern, Doruker and Mattice

analysed static and dynamic properties of amorphouspolyethylene nanofibers by using a Monte Carlo simu-lation techniquew27,28 x. They looked at how confine-●

ment and curvature of the surface of the nanofibersaffects chain properties compared to the bulk of a thin


Fig. 6. Example of bead formation during electrospinning of 13 wt.%PET in 1:1 dichloromethane and trifluoroacetic acid(Audrey Frenot,unpublished work).

film. Major findings of this study were(i) fibers ofdifferent sizes, i.e. consisting of different numbers ofparent chains, exhibit almost identical hyperbolic densityprofiles at the surfaces,(ii) the end beads are predomi-nant and the middle beads are depleted at the freesurfaces,(iii ) there is an anisotropy in the orientationof bonds and chains at the surface,(iv) the centre ofmass distribution of the chains exhibits oscillatorybehaviour across the fibers and(v) the mobility of thechain in nanofiber increases as the diameter of thenanofiber decreases. They concluded that these nanofi-ber trends, with its strongly curved surface, are similarto the results seen in simulations by the same techniqueof free-standing thin film, where there is no curvatureat the surface.

4. Polymer nanofibers with specific bulk morpholo-gies and surface topologies

Electrospinning process is applicable to many poly-mers, as a melt or a solution. Reneker’s group hasdone work on many different polymers, for example:polyethylene terephtalatew1x; PBI w13x; polystyrenew8x,poly(2-hydroxyethyl methacrylate), polyvinylidenefluoride, and poly(ether imide) w9x; styrene–buta-diene–styrene triblock copolymer w29x andpoly(ferrocenyldimethylsilane) w30x. They have evenspun DNA fibers with a diameter of approximately 30nm w31x and silk for which there is a patentw32x. Theyhave used PEO in order to study more deeply theelectrospinning process and the influential parameters.They also studiedw33x the formation of beads on thefibers (Fig. 6). The bead morphology has been welldocumented for electrospun fibers and is affected byprocessing conditionsw19,33x. Beads can be consideredas capillary break-up of the jets by surface tension.Thus, charge density carried by the jet, surface tensionand viscoelastic properties of the solution are significantparameters. The higher the solution viscosity is, the less

beads are formed. The net charge density influences inthe same way. Actually, adding salt(i.e. NaCl) is a wayto increase this density. Deitzel et al. investigated aswell the formation of beads and found out that thespinning voltage influences mainly the formation ofbeads while the polymer concentration has effect on thefiber sizew19x. They also noticed that at high concentra-tion a bimodal distribution of the fiber sizes was created.Wilkes at the Virginia Polytechnic Institutew18x has

studied the effect of solution concentration, capillary–screen distance, electric potential at the tip and flowrate on electrospinning Esthane 5720, a segmented�

polyether urethane. He found that bead-like structureappeared when the capillary–screen distance decreased,while the average fiber diameter was increased. Whenincreasing the concentration, the average diameter risedand the bead-like structure turned into blobs at lowercapillary–screen distance. A too high concentrationleaded to failure of the fibers instead of being drawn,as the viscosity was too high. This can also be causedby a too high flow rate at lower concentration. Finally,increasing the potential decreased the fiber diameter.Demir et al. studied the electrospinning behaviour of

an elastomeric polyurethane urea copolymer in solutionw34x. They observed that mainly the morphology ofelectrospun fibers is strongly correlated with viscosity,equivalently concentration and temperature. Actually,the morphological imperfections like formation of fiberswith beads or curly fibers can be improved by increasingthe solution temperature. This is also an importantadvantage from the aspect of industrial applicationssince high temperature makes the electrospinning pro-cess quick.Biopolymers such as poly(lactic acid) (PLA) fibers

have been electrospun by Bognitzki et al.w4x. The fibershad an average diameter of approximately 1mm. Theyused tetraethyl benzylammonium chloride(TEBAC) toobtain fibers with lower diameter. This is due to theincreased surface tension and electrical conductivity thatTEBAC procures. Adding PEO to the solution resultedin the same phenomenon because of an increase ofsurface tension only. PLA and polyvinilpyrrolidoneblends are materials that have also been used by theseauthorsw3x.

For some applications it is desirable to control notonly the diameter of the fibers and their internal mor-phology (and thus their intrinsic properties) but alsotheir surface structure. An interesting feature of nanofi-bers is that they display an internal phase morphologythat is evidently controlled by rapid phase separationand rapid solidification. By selective removal of onecomponent, fine porous fibers and functional nanofiberswith specific surface topologies are accessible. Forma-tion of such co-continuous phase morphologies wasstudied in a work by Bognitzki et al. where they usedelectrospinning of PLA to manufacture polymerymetal


hybrid nano- and mesotubesw12 x. The aim was to be●●

able to control not only the fiber diameter but also thesurface structure, which can be important for filtrationapplications or formation of functional nanotubes. Thetubes were formed with a process they call TUbes byfiber template(TUFT) process. This process consists incoating the PLA nanofibers with first poly(p-xylylene)by chemical vapour deposition and then with the desiredmetal (aluminum in that case). Then the PLA core isremoved by taking advantage of the ability to degradeof PLA. This shows a great potential for the productionof novel tubular devices. Most possible TUFT processwill gain significance with improvement of the controlof tube diameters, structuring and functionalization ofthe tubes, which should be possible by adjusting theparameters for the preparation of template fibers byelectrospinning. In another work by Caruso et al. poly(L-lactide) electrospun fibers were coated with amorphoustitanium dioxide using a sol–gel coating techniquew35x.On removal of the thermally degradable polymer, hollowtitania fibers were produced. The sol–gel coating wasable to mimic the finer details of the fiber, therebyforming nodules on the inner walls of the tubes. Nano-tubes are also the topic treated in the presentation of Koet al. at the ASC 16th Annual Technical Conferencew36x. In that work, they have co-electrospun a mixtureof carbon nanotubes and PEO solution. This processaligns the carbon nanotubes(if well dispersed in thepolymer solution) in the resulting nanocompositesfibrils. According to models, the mechanical propertiesof carbon nanotubes composites are higher than usualcomposites. Nanotubes and nanowires can also be con-structed by using an electrostatic field-assisted assemblytechnique combined with an electrospinning process toposition and align individual nanofibers on a taperedand ground wheel-like bobbinw15 x.●●

Cellulosic nanofiber membranes that present goodwettability and absorbency properties as well as theability to carry chemically reactive functionalities werereported by Liu and Hsiehw37x. In another work, Sunget al. studied the electrospinning of polycarbonates andtheir surface characteristicw38x. Electrospun fibers there-from show a wrinkled structure that was found to dependon the evaporation rate of the solvent from the surfacerelated to the rate of evaporation from the core. Indeed,as the solvent on the surface has evaporated and a ‘skin’has formed, the solvent entrapped in the core diffusesto the ambient atmosphere and causes what they callthe ‘raisin-like structure’. They also believe that this isa reason for the formation of flat bands.Surface functionalization of electrospun fibers has

been also studied recently by Deitzel et al.w20 ,39 x.● ●

This has been accomplished by electrospinning a seriesof random copolymers of PMMA-r-TAN(polymethylmethacrylate with varying tetrahydroperfluorooctyl acry-late concentration) from a mixed solvent of toluene and

dimethyl formamide. They observed that surface segre-gation of fluorine in electrospun fibers is similar inmagnitude to that measured for the thin film surface.Fluorinated surfaces are inherently of interest for stainresistant and water repellent materials and for coatingapplications.Furthermore, chemical crosslinking method can be

used to crosslink the pre-formed nanofibers. In a recentwork by Ding et al., nanoscale poly(vinyl alcohol)(PVA) fiber (100–500 nm) aggregates were preparedwith an electrospinning technique and then chemicallycrosslinkedw40x. The results showed that the properlycrosslinked PVA fiber aggregates had better antiwatersolubility and mechanical properties than the non-cross-linked PVA fiber aggregates.

5. Molecular nano-electronic machines

Electronic polymers have been electrospun with thepurpose to be able one day to create nano-electronicmachinesw10,11,41x. For example, these small fiberscan support arrays of nanomachines, and connect inte-grated arrays of nanomachines to larger scale systems.Nanoscale fibers are also containing special arrange-ments of polymer molecules, often called crystallograph-ic defects, which can themselves, function asnanomachines to translate and rotate polymer moleculew1x.Using electrospinning Diaz-de-Leon has prepared

nanofibers of doped polyaniline(an organic conductingpolymer) as itself or in blend with polystyrene or PEOand electrically characterise themw10x. The conductivityis found to be very sensitive to the morphology of thefibers(i.e. amount of defects, thickness) which is indeedrelated to the initial blend properties and solvent used.Therefore, better chain alignment in the polymer blendsyields higher overall conductivity of the blended nano-fibers. It is notable also, that the nanofibers obtainedshowed regions with polymer crosses; this polymer–polymer junctions are of interest in making nanoelec-tronic junction devices. Conductive nanofibers wereobtained from electrospinning using blends of spinnable(e.g. poly(ethylene oxide)) and non-spinnable but con-ductive (e.g. polyaniline doped with camphorsulfonicacid) polymers, by MacDiarmid et al.w11,42 x. The rate●

for the vapour phase de-dopingyre-doping of the elec-trospun fibers was at least one order of magnitude fasterthan to those for cast films. Again, this stresses theenormous effect of increase in the surface-to-volumeratio and selected chemical properties accomplished byelectrospinning the polymer material into nanofibers.The resulting nanofibers are conductive and are ofpotential interest for applications in micro- and optolec-tronics, for example, nanowires, LEDs, photocells, etc.Moreover, the Korea Institute of Science and Technology


owns a patent on the fabrication of a lithium secondarybattery comprising a fibrous film made by electrospin-ning w43x. Another interesting progress has been thedevelopment of a hybrid solar cell utilising electrospunconductive polymers ‘doped’ with photovoltaic dyes andnano-crystalline semiconductor particlesw44x. Ziegler etal. have prepared a flexible photovoltaic membrane byutilising electrospun polyacrylonitrile(PAN) nanofiberdyed with copper phthalocyanine, while nano-crystallineTiO semiconductor particles can be embedded as well2

within the nanofibers.The feasibility of electrospinning photo-responsive

polymeric materials with desirable functionalities isexpected to find wide application in future electronicdevice fabrication. Control of the polymer solutions,blending with other polymers and electrospinning con-ditions can provide a variety of pathways for function-alization of interesting photo-responsive electrospunnanofibers. Drew et al.w41x have demonstrated success-ful engineering of polymeric membranes by electrospin-ning a photonic polymer system PAN blended with thelight harvesting azo-dye, Congo red. The process ofelectrospun nanofibers can also be exploited for use invarious applications in the sensor technology. A recentattempt is the one reported by Zhang et al. where a highsurface area chemosensor material has been obtained,by electrospinning of fluorescent conjugated polymerw45x. Blends of PEO and fluorescent poly((p-phenyleneethynylene)-alt-(thienylene ethynylene)) were electros-pun from a chloroform solution. Furthermore, Wang etal. have prepared fluorescent electrospun polymer filmsensors valuable for the detection of explosivesw46x.

6. Polymer nanofibers for biomedical applications

The use of polymer nanofibers for biomedical appli-cations is another active research area. In a recentpublication, an electrospinning method was used byZong and co-workers to fabricate bioabsorbable amor-phous poly(D,L-lactic acid) and semi-crystalline poly(L-lactic acid) nanofiber non-woven membranes forbiomedical applicationsw47x. They showed that the fiberdiameter and the nanostructured morphology dependedon processing parameters such as solution viscosity(e.g.concentration and polymer molecular weight), appliedelectric field strength, solution feeding rate and ionicsalt addition. Concentration and salt addition were foundto have relatively larger effects on the fiber diameterthan the other parameters. The Fiber Society Spring2001 Conference treated as well widely the electrospin-ning of nanofibers for biomedical applications. A studyon the fabrication of a scaffold by electrospinningbiomaterials such as PLA, poly(glycolic acid),poly(ethylene-co-vinyl acetate) (PEVA), and Type Icollagen, was reported by Bowling et al.w48x. Fang etal. presented nano-structured electrospun poly-D,L-lac-

tide-co-glycolide membranes for anti-adhesion applica-tions w49x. Among them, there was also a presentationfrom Reneker’s group on encapsulation of particles intoelectrospun polymeric nanofibersw50x. This wasachieved by adding insoluble particles to the polymersolution. These particles were gold, pollen spore, algin-ate, and silver sulfadiozine for burns treatment, forexample. They also added biomaterials for wound heal-ing or other substances that functionalized the fiber, e.g.soluble drugs, antibacterial agents. Medical applicationof electrospinning is also the subject of a patent claimedby Reneker and co-workersw51x. They produced a skinmask by directly electrospinning fibers onto the skinsurface in order to protect or heal eventual wounds.Another patentw52x claims the production of electrospunfibers containing pH-adjusting compound for use indressing for wound treatment or protection from contam-ination. Electrospun fiber mats were also explored asdrug delivery vehicles with promising results. Mats weremade either from PLA, PEVA, or from a 50:50 blendof the two using tetracycline hydrochloride as a modeldrug w53x.The use of electrospinning for tissue engineering is

also another interesting field of application of polymernanofibers. Electrospun nanofibrous structures meet theessential design criteria of an ideal tissue engineeredscaffold based upon its acts to support and guide cellgrowth. Most publications support that the electrospunnanofibrous structure is capable of supporting cellattachment and proliferation. Cells seeded on this struc-ture tend to maintain phenotypic shape and guidedgrowth according to nanofiber orientation. Li et al. havedeveloped an electrospun structure for tissue-engineeringapplications, composed of poly(D,L-lactide-co-glyco-lide) PLGA fibers ranging from 500 to 800 nm indiameter w54x. The structure features a morphologicsimilarity to the extracellular matrix of natural tissue,which is characterised by a wide range of pore diameterdistribution, high porosity, and effective mechanicalproperties. Matthews et al. studied how electrospinningcan be adapted to produce tissue-engineering scaffoldscomposed of collagen nanofibers(a matrix composedof 100 nm fiber) w55 x. They found that the structural●

properties of electrospun collagen varied with the tissueof origin, the isotope, and the concentration of thecollagen solution, which were used to spin the fibers.Moreover, blend components with collagen have beenused as well for electrospinningw56x. Type I collagen-PEO fibers and non-woven fiber networks were pro-duced by the electrospinning of a weak acid solution ofpurified collagen at ambient temperature and pressure.The process proposed by Huang et al. provides aconvenient, non-toxic, non-denaturing approach for thegeneration collagen-containing nanofibers(diameterrange of 100–150 nm) and nonwoven fabrics that havepotential application in wound healing, tissue engineer-


ing, and as hemostatic agents. The same group has alsoproduced elastin-mimetic fibers from peptide polymerin order to mimic protein fibers that are found in arterialwall w57 x. Electrospinning can even be used to create●

biocompatible thin films with useful coating design andsurface structure that can be deposited on implantabledevices in order to facilitate the integration of thesedevices with the body. Silk-like polymer with fibronec-tine functionality (extracellular matrix proteins) havebeen electrospun for making biocompatible films usedon prosthetic devices aimed to be implanted in thecentral nervous systemw58 x.●

7. Other functions of nanofiber structures

Non-woven textiles composed of electrospun fibershave a large specific surface area and small pore sizecompared to commercial textiles, making them excellentcandidates for use in filtration and membrane applica-tions. Thus, electrospinning has been exploited to pro-duce nonwoven composites for application in filtersw59x. A nanofiber web was produced from nylon 66 ona substrate made of either meltblown or spunbondpolypropylene. The resulting product was then sand-wiched between material of the same substrate and pointbonded in a calender to form the composite structure.The filtration properties of microparticles were thenmeasured and showed efficiency similar to the bestcommercial filters even if only one-fifth filtering mediawas used. Electrospinning can also be used to producecharged fibers for use in filtration mediaw17x. Obviously,the charge induction and charge retention characteristicsare related to the polymer material used forelectrospinning.Kim and Reneker have investigated the use of PBI

nanofibers as reinforcement in an epoxy and a rubbermatrix (styrene–butadiene) w60x. They observed that theepoxy was toughened by the nanofibers and this rein-forcing effect was higher than that of PBI fibroids(whiskers-like particles). Moreover, the PBI nanofiberscan provide dramatic reinforcement of a rubber matrix(i.e. the Young modulus was ten times higher and thetear strength was twice as large as for the unfilledrubber). In another work, the reinforcement effect of theultrathin fibers of nylon-4,6 with a semi-infinite lengthprepared by electrospinning solutions of the polymer informic acid was clearly demonstrated by Bergshoef andVancso w61x. These fibers can be applied to preparetransparent composites with an epoxy matrix. Theynoticed that at certain given process parameters, the jetof nylon-4,6 solution pulled from the capillary tip duringthe electrospinning process can splay into several finerjets. The potential of using polymer nanocomposites asthe foundation for fabricating nanofibers structures, illus-trated by the work of Fong et al. by demonstrating thedissolution and reprocessing of exfoliated montmorillon-

ite–nylon 6 (NLS) nanocompositew62x. They alsoexerted hierarchical control of morphology and formthrough the combination of a nanostructured materialand a nanoscale fabrication technique. The electrospin-ning process resulted in highly aligned montmorillonitelayers(layer normal perpendicular to the fiber axis) andnylon 6 crystallites(layer normal parallel to fiber axis).

Of particular interest are electrospun membranes com-posed of elastomeric fibers, for the development forseveral protective clothing applications. A lot of workis done with aim to develop garments that reducessoldiers’ risks for chemical exposurew63x. The idea isto lace several type of polymers and fibers to makeprotective ultra-thin layers that would enhance for exam-ples chemical reactivity and environmental resistance.The US Army presented at the Eighth InternationalConference on Textile Coating and Laminating its wayto consider electrospinningw64,65x. They see a mannerto produce membranes for laminated fabrics as well asdirect sprayed-on fibrous coating structures. They pre-pared microporous membranes of PAN and PBI, andcompared their convective airflow resistance and watervapour diffusion resistance with conventional fabricsPTFE wind barrier(like Gore-Tex). The electrospunmembranes had both good water vapour transport prop-erties and convective flow resistance, meaning that theywill present remarkable ‘breathing’ properties, which arenowadays required in clothing applications.

8. Conclusions and future perspectives

Electrospinning is a very simple and versatile methodof creating polymer-based high functional and highperformance nanofibers that can revolutionise the worldof structural materials. The process is versatile in thatthere is a wide range of polymer and biopolymersolutions or melts that can be spun. This assembleapproach can be expanded even into a hierarchicalassemble of produced nanofibers in other well-definedfunctional nanostructures, such as nanotubes and nano-wires, etc. The various recent approaches that have beendeveloped for scientific understanding, engineering con-trol and then potential implementation in devices ofelectrospun nanofibers are summarised in this review.Essential studies, nevertheless, are still required and

many challenges remain to be faced. In particular,integration of nanofibers into useful devices requiresnanofibers of well-controlled orientation, size, and othertarget characteristics as well as reproducibility locatingthem in specific positions and orientations. The abilityto do so, however, remains a major challenge in thefield. The design and construction of process equipmentfor controllable and reproducible electrospinning coulddetermine the character of instabilities revealed, satisfypotential implementation and act as stimulus to providenew products. Moreover, to control over key perform-


ance parameters, the clarification of the fundamentalelectrodynamics of the electrospinning process and cor-relation to the polymer fluid characteristics must bereliably predicted and utilised. Work that address theevaluation of the fluid instabilities, postulated to becrucial for producing nanofibers, and precise character-isation of the morphology and material properties ofelectrospun polymer fibers themselves are in early stagesand systematic studies are still emerging. With thesevariables under control, the challenges of fully quanti-tative modelling of the electrospinning process shouldbe feasible, including both stable and unstable operatingregimes, starting from the existing equations of electro-dynamic theory. Productivity improvement of the elec-trospinning process is also an essential feature and meritsmore studies.

References and recommended reading

Papers of particular interest, published within the annual period ofreview, have been highlighted as:

● of special interest●● of outstanding interest

w1x Reneker DH, Chun I. Nanometre diameter fibres of polymerproduced by electrospinning. Nanotechnology 1996;7:216–23.

w2x Larrondo L, Manley RSJ. Electrostatic fiber spinning frompolymer melts. I. Experimental observations on fiber for-mation and properties. J Polym Sci Polym Phys Ed1981;19:909–20.

w3x Bognitzki M, Frese T, Steinhart M, Greiner A, Wendorff JH.Preparation of fibers with nanoscaled morphologies: elec-trospinning of polymer blends. Polym Eng Sci 2001;41:982–9.

w4x Bognitzki M, Wendorff JH, Greiner A. Submicrometershaped polylactide fibers by electrospinning. Polym Prep(ACS, PMSE) 2000;82:115–6.

w5x Buer A, Ugbolue SC, Warner SB. Electrospinning andproperties of some nanofibers. Textile Res J 2001;71:323–8.

w6x Warner SB, Buer A, Ugbolue SC, Ruteldge GC, Shin MY.M98-D01 A fundamental investigation of the formation andproperties of electrospun fibers. Department of Textile Sci-ences, University of Massachusetts Dartmouth; Departmentof Chemical Engineering and Material Sciences, Massachu-setts Institute of Technology Cambridge, 2000.

w7x●●

Reneker DH, Yarin AL, Fong H, Koombhonge S. Bendinginstability of electrically charged liquid jets of polymersolutions in electrospinning. J Appl Phys 2000;87:4531–47.

In this article the reasons for the instability are analyzed andexplained using a mathematical model. The theory accounts forthe non-linear effects that are characteristics of finite perturba-tions, as well as for the rheological behaviour of viscoelasticliquids. The authors give the three-dimensional equations describ-ing the dynamics of the bending of electrospun jets, both in thenearly straight region where the instability grew slowly and inthe region where the bending dominated the path of the jet.w8x Torres B. Ultrafine fibers of polystyrene dissolved in tetrah-

ydrofuran prepared using electrospinning method. NationalConference of Undergraduate Research, Lexington: Univer-sity of Kentucky; 2001.

w9x Koombhongse S, Liu W, Reneker DH. Flat ribbons and othershapes by electrospinning. J Polym Sci, Polym Phys Ed2001;39:2598–606.

w10x Diaz-de-Leon MJ. Electrospinning nanofibers of polyanilineand polyaniliney(polystyrene and polyethylene oxide)blends. National Conference of Undergraduate Research,Lexington: University of Kentucky; 2001.

w11x MacDiarmid AG, Jones WE, Noris ID, et al. Electrostatical-ly-generated nanofibers of electronic polymers. SyntheticMet 2001;119:27–30.

w12x●●

Bognitzki M, Hou H, Ishaque M, et al. Polymer, metal, andhybrid nano- and mesotubes by coating degradable polymertemplate fibers(TUFT process). Adv Mater 2000;12:637–40.

The paper introduce an efficient and simple general route, calledTUFT process(tubes by fiber templates), towards nano- andmesotubes using electrospun degradable polymer fiber templates.This approach offers a great potential for the design of noveltubular devices in the sense of control of dimensions and shapeand even with respect to the broad range of materials that can bemanufactured.w13x Kim J-S, Reneker DH. Polybenzimidazole nanofiber pro-

duced by electrospinning. Polym Eng Sci 1999;39:849–54.w14x Rangpukan R, Reneker DH. Development of electrospinning

from molten polymers in vacuum. J Textile Apparel, TechnolManage 2001, vol. 1. Special issue: The Fiber Society Spring2001 Conference, Raleigh NC.

w15x●●

Theron A, Zussman E, Yarin AL. Electrostatic fields-assistedalignment of electrospun nanofibres. Nanotechnology2001;12:384–90.

A new approach was described for assembling nanofibers in parallelarrays while being able to control the average separation betweenthe fibres using electrostatic forces during the manufacturingprocess.w16x Doshi J, Reneker DH. Electrospinning process and applica-

tions of electrospun fibers. J Electrostat 1995;35:151–60.w17x Tsai PP, Schreuder-Gibson H. Effect of electrospinning mate-

rial and conditions upon residual electrostatic charge ofpolymer nanofibers. J Textile Apparel, Technol Manage 2001,vol. 1. Special issue: The Fiber Society Spring 2001 Confer-ence, Raleigh NC.

w18x Wilkes GL. Available from: http:yywww.che.vt.eduyWilkesyelectrospinningyelectrspinning.html. 2001.

w19x Deitzel JM, Kleinmeyer J, Harris D, Tan NCB. The effectof processing variables on the morphology of electrospunnanofibers and textiles. Polymer 2001;42:261–72.

w20x●

Deitzel JM, Kleinmeyer D, Hirvanen JK, Tan NCB. Con-trolled deposition of electrospun poly(ethylene oxide) fibers.Polymers 2001;42:8163–70.

The onset of a chaotic oscillation of the electrospinning jet wascontrolled by the deposition of sub-micron polymer fibers on asubstrate through use of an electrostatic lens element and collectiontarget of opposite polarity.w21x Yarin AL, Koombhongse S, Reneker DH. Bending instability

in electrospinning nanofibers. J Appl Phys 2001;89:3018–26.

w22x●●

Yarin AL, Reneker DH. Taylor cone and jetting from liquiddroplets in electrospinning of nanofibers. J Appl Phys2001;90:4836–46.

An interesting theory of the stable shapes of droplets as affectedby an electric field is proposed in this paper. The theory comparedwith data acquired in experimental work on electrospinning ofnanofibers from polymer solutions and melts.w23x

●Shin MY, Hohman MM, Brenner M, Ruteldge GC. Experi-mental characterization of electrospinning: the electricallyforced jet and instabilities. Polymer 2001;42:9955–67.


The electrospinning process was described by a small set ofoperating parameters and summarized through the use of operatingdiagrams of electric field versus flow rate.w24x

●Hohman MM, Shin MY, Ruteldge GC, Brenner M. Electros-pinning and electrically forced jets. I. Stability theory. PhysFluids 2001;13:2201–20.

A complete stability analysis of a charged fluid jet in a tangentialelectric field as a function of all fluid parameters was performed.w25x

●Hohman MM, Shin MY, Ruteldge GC, Brenner M. Electro-spinning and electrically forced jets. II. Applications. PhysFluids 2001;13:2221–36.

The stability analysis, in conjunction with an analysis of the chargedistribution and radius of the jets in electrospinning experiments,were used to derive quantitative thresholds for the onset ofelectrospinning as a function of all experimental parameters.w26x Shin MY, Hohman MM, Brenner M, Ruteldge GC. Electro-

spinning: a whipping fluid jet generates submicron polymerfibers. Appl Phys Lett 2001;78:1149–51.

w27x Vao-Soongnern V, Mattice WL. Dynamic properties of anamorphous polyethylne nanofiber. Langmuir 2000;16:6757–8.

w28x●

Vao-Soongnern V, Doruker P, Mattice WL. Simulation of anamorphous polyethylene nanofiber on a high coordinationlattice. Macromol Theor Simul 2000;9:1–13.

A Monte Carlo simulation that employs a bridging technique isused to allow interconversion between a fully atomistic represen-tation of a structure in continuous space and a coarse-grainedversion of the same structure in the discrete space of a high co-ordination lattice.w29x Fong H, Reneker DH. Elastomeric nanofibers of styren–

butadiene–styrene triblock copolymer. J Polym Sci: Part B:Polym Phys 1999;37:3488–93.

w30x Zhihao C, Foster MD, Wensheng Z, et al. Structure ofpoly(ferrocenyldimethylsilane) in electrospun nanofibers.Macromolecules 2001;34:6156–8.

w31x Fang X, Reneker DH. DNA fibers by electrospinning. JMacromol Sci Phys 1997;B36(2):169–73.

w32x Reneker DH, Eby RK, Ertley D, Hudson SD, Zarkoob S.Synthetically spun silk nanofibers and a process for makingthe same. Patent US6110590. University of Akron; 2000.

w33x Fong H, Chun I, Reneker DH. Beaded nanofibers formedduring electrospinning. Polymer 1999;40:4585–92.

w34x Demir MM, Yilgor I, Yilgor E, Erman B. Electrospinning ofpolyurethane fibers. Polymer 2002;43:3303–9.

w35x Caruso RA, Schattka JH, Greiner A. Titanium dioxide tubesfrom sol–gel coating of electrospun polymer fibers. AdvMater 2001;13:1577–9.

w36x Ko FK, Kahn S, Rahman A, Zhou O. Carbon nanotubereinforced nanocomposites by the electrospinning process.ASC 16th Annual Technical Conference, Blacksburg, VA:2001.

w37x Liu H, Hsieh Y-L. Cellulosic nanofiber membranes for liquidwettingyabsorbency and chemical reactivity. J Textile Appar-el, Technol Manage 2001, vol. 1. Special issue: The FiberSociety Spring 2001 Conference, Raleigh NC.

w38x Sung C, Gibson H, Varma RNK. Electrospinning of polycar-bonates and their surface characteristics. J Textile Apparel,Technol Manage 2001, vol. 1. Special issue: The FiberSociety Spring 2001 Conference, Raleigh NC.

w39x●

Deitzel JM, Kosik W, McKnight SH, Tan NCB, DesimoneJM, Crette S. Electrospinning of polymer nanofibers withspecific surface chemistry. Polymer 2002;43:1025–9.

This paper demonstrate a pathway for surface functionalization ofelectrospun nanofibers which may be exploited directly in thedesign of fibers for various applications.

w40x Ding B, Kim H-K, Lee S-C, et al. Preparation and charac-terization of a nanoscale poly(vinyl alcohol) fiber aggregateproduced by an electrospinning method. J Polym Sci: PartB: Polym Phys 2002;40:1261–8.

w41x Drew C, Wang X, Senecal K, et al. Electrospun nanofibersof electronic and photonic polymer systems, Orlando, FL:TANDEC; 2000.

w42x●

MacDiarmid AG, Ko FK, Lec RM, Noris ID, Shaker M.Electrospinning ultrafine conductive polymeric fibers, 2001,Patent WO0151690 and Norris ID, Shaker MM, Ko FK,MacDiarmid AG. Electrostatic fabrication of ultrafine con-ducting fibres: polyanilineypolyethylene oxide blends. Syn-thetic Met 2000; 114:109–14. This paper is one of theseveral papers published by the authors. They have madesubstantial progress for fabrication of electronic polymerfibers. The group reproducibly and controllably fabricatesnanofibers of pure electronic polymers(in their semicon-ducting and metallic regimes) andyor their blends in conven-tional organic polymers for the purpose of ascertaining theirapplicability in the fabrication of nano-electronic devices.

w43x Cho WB, Choi SW, Mu JS, Kim HS, Kim US, Cho WI. ALithium secondary battery comprising a super fine fibrouspolymer separator film and its fabrication method. PatentWO0189022 and WO0189023. Korea Institute of ScienceTechnology; 2001.

w44x Ziegler D, Senecal KJ, Dew C, Samuelson L. Electrospunfibrous membranes of photovoltaic and conductive polymers.J Textile Apparel, Technol Manage 2001, vol. 1. Specialissue: The Fiber Society Spring 2001 Conference, RaleighNC.

w45x Zhang Y, Dong H, Norris ID, MacDiarmid AG, Jones WE.High surface area chemosensor material by electrospinningof fluorescent conjugated polymer. Abstracts of papers ofthe American Chemical Society 2001; 222(2) August.

w46x Wang XY, Lee SH, Drew C, Senecal KJ, Kumar J, SamuelsonL. Fluorescent electrospun polymer films for the detectionof explosives. Abstracts of papers of the American ChemicalSociety 2002; 223(2) April :D44.

w47x Xinhua Z, Kim K, Shaofeng R, Hsiao BS, Chu B. Structureand process relationship of electrospun bioabsorbable nano-fiber membranes. Polymer 2002;43:4403–12.

w48x Bowling GL, Matthews JA, Simpson DG, Kenawy ER,Wnek GE. Electrospinning biomaterials. J Textile Apparel,Technol Manage 2001, vol. 1. Special issue: The FiberSociety Spring 2001 Conference, Raleigh NC.

w49x Fang D, Zong X, Chen W, Cruz S, Hsiao B, Chu B. Nano-structured electrospun poly-D,L-lactide-co-glycolide mem-branes for anti-adhesion applications. J Textile Apparel,Technol Manage 2001, vol. 1. Special issue: The FiberSociety Spring 2001 Conference, Raleigh NC.

w50x Kataphinan W, Dabney S, Smith D, Reneker DH. Fabricationof electrospun and encapsulation into polymer nanofibers. JTextile Apparel, Technol Manage 2001, vol. 1. Special issue:The Fiber Society Spring 2001 Conference, Raleigh NC.

w51x Kataphinan W, Dabney S, Reneker DH, Smith D, AkronUO. Electrospun skin masks and uses thereof. PatentWO0126610. University of Akron; 2001.

w52x Gibson P, McManus A, Mello C, et al. Electrospun fibersand an apparatus therefor. Patent WO0127365. University ofAkron; 2001.

w53x Kenawy ER, Bowlin GL, Mansfield K, et al. Release oftetracycline hydrochloride from electrospun poly(ethylene-vo-vinylacetate), poly(lactic acid), and a blend. J ControlRelease 2002;81:57–64.


w54x Li WJ, Laurencin CT, Caterson EJ, Tuan RS, Ko FK.Electrospun nanofibrous structure: a novel scaffold for tissueengineering. J Biomed Mater Res 2002;60:613–21.

w55x●

Matthews JA, Wnek GE, Simpson DG, Bowlin GL. Elec-trospinning of electrospinning of collagen nanofibers. Bio-macromolecules 2002;3:232–8.

Careful experiments demonstrate that it is possible to tailor subtlemechanical properties into a matrix by controlling fiber orientation.It is highlighted that electrospun collagen promotes cell growthand the penetration of cells into the engineered matrix.w56x Huang L, Nagapundi K, Chaikof EL. Engineered collagen-

PEO nanofibers and fabrics. J Biomater Sci Polym Ed2001;12:979–93.

w57x●

Huang L, McMillan RA, Apkarian RP, Pourdeyhimi B,Conticello VP, Chaikof EL. Generation of synthetic elastin-mimetic small diameter fibers and fiber networks. Macro-molecules 2000;33:2899–997.

The first report of fiber formation from an elastin-like analogue.Electrospinning techniques were employed to produce fibers in aform that mimics native elastin fiber diameter, utilising a 81 kDasynthetic elastin peptide polymer based upon the elastin-mimeticrepeat sequence(Val–Pro–Gly–Val–Gly) (Val–Pro–Gly–Lys–4

Gly).w58x

●Buchko CJ, Chen LC, Shen Y, Martin DC. Processing andmicrostructural characterization of porous biocompatible pro-tein polymer thin films. Polymer 1999;40:7397–407.

The electrospinning process is shown to produce protein polymercoatings. Control can be exerted over the morphology and thicknessof the coatings by varying process parameters(solution concentra-tion, applied field strength, deposition distance and depositiontime). The electrospinning process was shown to be a means ofcreating porous thin films with structural gradients and controlledmorphology that could enhance biocompatibility.w59x Doshi J. Nanofiber-based nonwoven composites: properties

and applications. Nonwovens World. 2001; augusti–septem-ber:64–68.

w60x Kim J-S, Reneker DH. Mechanical properties of compositesusing ultrafine electrospun fibers. Polym Comp1999;20:124–31.

w61x Bergshoef MM, Vancso GJ. Transparent nanocomposites withultrathin electrospun nylon-4,6 fiber reinforcement. AdvMater 1999;11:1362–5.

w62x Hao F, Weidong L, Wang C-S, Vaia RA. Generation ofelectrospun fibers of nylon 6 and nylon 6-montmorillonitenanocomposite. Polymer 2002;43:775–80.

w63x Ross SE. Electrospinning: the quest for nanofibers. InterFiber J 2001;50–3.

w64x Schreuder-Gibson H. Electrospinning technology—a novelway to apply polymers directly to a substrate, Fabrics JoC,editor. The Eighth International Conference on Textile Coat-ing and Laminating, Frankfurt Germany: 1998.

w65x Gibson P, Schreuder-Gibson H, Rivin D. Transport propertiesof porous membranes based on electrospun nanofibers. Col-loids Surface A: Phys. Eng. Aspects 2001;187:469–81.

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