Nnano.2012.168 Microfluidic Technologies for Accelerating the Clinical Translation AnaLygia

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NATURE NANOTECHNOLOGY | VOL 7 | OCTOBER 2012 | www.nature.com/naturenanotechnology 623

Nanomedicine is the application o nanotechnology to medi-cine, specically involving the use o engineered nanoma-terials or therapy and diagnosis o major diseases such as

cancer, cardiovascular and inectious diseases1. Te rst genera-tion o nanoparticles with applications in medicine dates back tothe 1970s, when drug-loaded nanoscale liposomes were developedto deliver their cargo to diseased cells in a rojan horse ashion2.Since then, a new generation o targeted drug delivery vehicles (orexample, polymeric nanoparticles)3, contrast agents (such as ironoxide nanoparticles)4, diagnostic tools5, and antennas or photo-thermal therapy (or example, gold nanoparticles)6 have emerged.

Tis is driven in part by urther understanding o the biology o dis-eased states, and by technological advances in imaging techniquesand synthesis o novel biocompatible and biodegradable materials7.Now, nanomedicine promises the precise delivery o drugs to dis-ease sites (such as tumours and atherosclerotic plaques) without o-target toxicities, and the early detection o diseases using selectivecontrast agents and sensitive diagnostic tools8.

Nanoparticles are attractive in medicine because their suracescan be chemically modied or targeting specic disease tissues, oror in vivo stability. For therapy, drugs can be encapsulated insidenanoparticles and released in a controlled manner over time. Forimaging, nanoparticles can provide higher contrast (or exam-ple, iron oxide nanoparticles or magnetic resonance imaging) orhigher brightness (or example, quantum dots (QDs) or uores-

cence imaging) than conventional small-molecule agents9

. Despitethese advantages and several decades o research, only a handul onanoparticles have received approval rom the US Food and DrugAdministration. Examples include iron oxide nanoparticles ormagnetic resonance imaging (or example, Feridex and Resovist),liposomes encapsulating the anticancer drug, doxorubicin, orchemotherapy (known as Doxil), and the protein-based nanopar-ticle encapsulating paclitaxel or chemotherapy (called Abraxane)10.

Microfuidic technologies or accelerating the

clinical translation o nanoparticlesPedro M. Valencia1, Omid C. Farokhzad2,3*, Rohit Karnik4* and Robert Langer1,3*

Using nanoparticles or therapy and imaging holds tremendous promise or the treatment o major diseases such ascancer. However, their translation into the clinic has been slow because it remains dicult to produce nanoparticles thatare consistent batch-to-batch, and in sucient quantities or clinical research. Moreover, platorms or rapid screening onanoparticles are still lacking. Recent microuidic technologies can tackle some o these issues, and oer a way to acceleratethe clinical translation o nanoparticles. In this Progress Article, we highlight the advances in microuidic systems that cansynthesize libraries o nanoparticles in a well-controlled, reproducible and high-throughput manner. We also discuss theuse o microuidics or rapidly evaluating nanoparticles in vitro under microenvironments that mimic the in vivo conditions.Furthermore, we highlight some systems that can manipulate small organisms, which could be used or evaluating the in vivotoxicity o nanoparticles or or drug screening. We conclude with a critical assessment o the near- and long-term impact o

microuidics in the feld o nanomedicine.

In act, translation o nanoparticles to the clinic has been slowcompared with small-molecule drugs, with the majority o nano-particles not even reaching the point oin vivo evaluation, and evenewer reaching clinical trials. Tis is due to a combination o actors.It remains dicult to reproducibly synthesize batches o nanopar-ticles that have identical properties and in sucient quantities orclinical applications11. Moreover, knowledge on the ate o nano-particles at the body-, organ- and cell-level remains limited12; thismakes rational design o nanoparticles dicult and necessitates theuse o screening-based approaches or synthesis. Furthermore, thereare ew platorms that can rapidly evaluate the biological behaviour

o nanoparticles in vitro under conditions that can be correlatedwith their perormance in vivo11. For example, there is a need orhigh-throughput methods or evaluating the binding and internali-zation o nanoparticles by cells, or the interaction o nanoparticleswith plasma proteins and the complement system, among others.Finally, there is insucient understanding o the biophysical andchemical interactions o nanoparticles with proteins, membranes,DNA and organelles. Tese interactions could have either bene-cial or adverse outcomes13. It is expected that technologies tacklingsome o these challenges could signicantly accelerate the discoveryand clinical translation o nanomedicines.

Microuidics the science and technology o manipulating nano-litre volumes in microscale uidic channels has impacted a rangeo applications, including biological analysis, chemical synthesis, sin-

gle-cell analysis and tissue engineering14

. Building on its origins insemiconductor technology and chemical separations, the expansiono microuidics has been driven by its ability to process small samplevolumes and access biologically relevant length scales and microscaletransport phenomena. Tis expansion has been largely acilitated bytechniques, such as sof lithography, that enable rapid design and pro-totyping o microuidic devices using a variety o materials14. Recentadvances and innovations in microuidics are expected to improve

1Department o Chemical Engineering, Massachusetts Institute o Technology, Cambridge, Massachusetts 02139, USA, 2Laboratory o Nanomedicine and

Biomaterials and Department o Anaesthesiology, Brigham and Womens Hospital, Harvard Medical School, Boston, Massachusetts 02115, USA, 3MIT-

Harvard Center or Cancer Nanotechnology Excellence, Massachusetts Institute o Technology, Cambridge, Massachusetts 02139, USA 4Department o

Mechanical Engineering, Massachusetts Institute o Technology, Cambridge, Massachusetts 02139, USA. *e-mail: [email protected];

[email protected]; [email protected]

PROGRESS ARTICLEPUBLISHED ONLINE: 8 OCTOBER 2012 |DOI: 10.1038/NNANO.2012.168

2012 Macmillan Publishers Limited. All rights reserved


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624 NATURE NANOTECHNOLOGY | VOL 7 | OCTOBER 2012 | www.nature.com/naturenanotechnology

the synthesis o nanoparticles and accelerate their transition to clini-cal evaluation (Fig. 1). Although many o these microuidic systemsare still being developed, they have the potential to become widelyadopted because they are economical, reproducible, amenable tomodications and can be integrated with other technologies15. In thisProgress Article, we highlight some o these technologies and discusstheir impact on accelerating the clinical translation o nanoparticles.

Well-controlled synthesis o nanoparticlesAmphiphilic molecules such as block copolymers and lipids cansel-assemble into nanoparticles when they experience a change

in solvent quality (or example, rom organic solvent to aqueous)(Fig. 2a). A common and exible way to accomplish a change insolvent quality is by mixing the solvent with the anti-solvent, wheremixing time directly inuences the nal size and size distribu-tion o the nanoparticles ormed16. I the mixing timescale, mix, islonger than the characteristic timescale or chains to nucleate andgrow (agg, ~10100 ms depending on the molecular weight o thechain), the nanoparticles begin to assemble under varying degreeso solvent quality. Tis heterogeneous environment prevents eec-tive stabilization o the nanoparticles by the hydrophilic portion othe amphiphilic molecule and acilitates their aggregation, leadingto the ormation o larger, polydisperse nanoparticles. However,imix < agg, particle sel-assembly occurs primarily when the sol-vent change is complete. Tis homogenous solvent environmentor nanoparticle assembly allows the hydrophilic portion o the

molecule to stabilize the nanoparticles more eectively, and thisyields smaller nanoparticles with uniorm size17. Although conven-tional bulk mixing occurs at the timescale o seconds, in microu-idic devices the mixing time o solvents is controllable and tunablerom the millisecond to microsecond scale (reaching mix < agg)

16,18.In recent years, several microuidic systems that enable rapid

mixing without the need o external actuators, such as stirrers orelectric elds, have been developed19. Te most widely used includeow-ocusing mixers20, droplet mixers21 and those with micromixingstructures embedded inside the channel22. Flow ocusing squeezesthe solvent stream between two anti-solvent streams, resulting in

rapid solvent exchange via diusion (Fig. 2b). Droplets and three-dimensional microchannel geometries result in complex olding ouid ows, which can completely mix two or more streams in mil-liseconds (Fig. 2b). Te implementation o these mixing techniquesor the ormation o organic nanoparticles in continuous ow hasresulted in polymeric and lipid nanoparticles with tunable nano-particle size, narrower size distribution, higher drug loadings andgreater batch-to-batch reproducibility relative to those made withconventional bulk techniques23 (Fig. 2c).

Similarly, inorganic nanoparticles comprising transition met-als such as gold, iron and cadmium, among others, undergo sel-assembly where metal solutes nucleate, grow and agglomerate intonanoclusters (Fig. 2d)24. Obtaining narrow particle-size distributionrequires rapid nucleation ollowed by growth o nanoparticles tothe desired size in the absence o urther nucleation, which can be

Characterization Scale-upBulktechnologies

+

Current

Time

12 years 25 years 58 years

Benchtop

synthesis

In vitro

evaluation

In vivo

evaluation

Clinical

trials

Nanoparticles in

clinical development

Quantum dots

GoldIron oxide

LiposomesPolymer

nanoparticles

Figure 1 | Nanoparticles in clinical development, steps or their translation (with average timescales) and microfuidic methods (green boxes) that could

improve or complement current technologies. Synthesis is carried out in large reaction asks, whereas microuidic synthesis is carried out at micro and nano

length scales that allow or improved control over reaction conditions. Characterization oten involves taking a small sample o nanoparticles and measuring

their properties oine, whereas nanopores embedded in microuidic devices allow or real-time, in-line characterization. In vitro evaluation in plate wells

produces a microenvironment ar rom that in vivo, whereas continuous ow in microuidic systems result in conditions closer to those in vivo. In vivo evaluation

in large animals is helpul or estimating the pharmacology o nanoparticles. To complement these studies microuidic systems could enable real-time tracking

o nanoparticles in large numbers o small organisms. Scale-up is generally carried out in reactor vessels several times larger than benchtop asks, whereas

parallelization o microuidic channels can increase the production rate o nanoparticles with properties identical to the one at bench scale.

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accomplished by controlling the mixing time o reagents, reactiontemperature and reaction time25. In bulk, these parameters are di-cult to control, leading to uneven mixing, local temperature uc-tuations and uncontrolled reaction times25. In contrast, microuidicdevices allow or control over the mixing time by varying solventow rates or channel geometry. Moreover, better heat transer owingto large surace areas enables better temperature control, preventingthe ormation o large temperature gradients. Finally, as the channellength directly corresponds to the time taken by the reactants toow through it in continuous ow synthesis, the reaction time canbe controlled by tuning the channel length or by adding reagents atprecise downstream locations during the particle ormation processto quench the reaction26.

wo-phase droplet mixers where reagents are encapsulated indroplets and separated by inert uids are commonly used or the

synthesis o inorganic nanoparticles (Fig. 2e)27

. In this congura-tion, rapid mixing o the solutions occurs inside the droplets, whichserve as identical microscale reactors providing homogeneousconditions or nanoparticle nucleation and growth. Both droplet-based and single-phase systems have been used to synthesize QDsthat exhibit narrow size distributions, which translates into sharperabsorption peaks and better luminescence qualities27, with controlover size to tune the absorption spectra28 (Fig. 2). Finally, similarsystems have been implemented in the controlled synthesis o goldnanoparticles o dened size and shape, iron oxide nanoparticleswith higher magnetization, and QDs with controlled size and bio-compatible coatings29.

Over the past our years, several examples showing the use omicrouidics or the synthesis o nanoparticles with dierent size,shape and surace compositions have emerged30. At present, there

I

I nA

+

An

II III

II An

+ A An+1

+

III An

+ Am

An+m

IV An

+ Abulk Abulk

a

d e

b c

f

Water

Water

Nanoparticle

precursors

Nanoparticle precursors

Other precursorsMixing

MixReaction 2

Reaction

droplet

100 mMixing

Inert

phase

Reaction

1

Oil

Nanoparticles

out

25

1.2

0.8

0.4

0.0

20

15

10

5

00

20

1.0

16

12

Normalized

fluorescence

8

4

0420 470 520 570

h (nm)

620 670 720

0.8

0.6

250 300 350 400 450 500

0.4

0.20.0

30 40 50

Geometric radius (nm)60 70 80

20 40 60

Nanoparticle diameter (nm)

Flow ratio = 0.03

Membrane extrusion

Microfluidics

On chip

3 min

10 min

20 min

Benchtop

Flow ratio = 0.1

Bulk mixing

Vol.fraction(%)

Normalized

numberdensity

A

bsorbance

80 100Nanoprecipitation

Lipidpolymer

hybrid

nanoparticle

LipidLipidPEG

LipidLipidPEGPLGA

Controlled

microvortex

Figure 2 | Microfuidic synthesis o nanoparticles. a, Schematic o the sel-assembly mechanism o organic nanoparticles. On mixing with anti-solvent,

polymers (or lipids) are brought to the vicinity o each other (I) then nucleate (II), subsequently aggregating into nanoparticles (III). b, Schematic o

microuidic synthesis o organic nanoparticles by rapid mixing through hydrodynamic ow ocusing (top) and microvortices (bottom). Red and dark blue

indicate organic and aqueous streams, respectively, while pink and light blue indicate their degree o mixing. PEG, polyethylene glycol; PLGA, poly(lactic-

co-glycolic acid). c, Size distribution o polymeric nanoparticles (top) and liposomes (bottom) prepared in microuidics compared with bulk synthesis.

In both cases, narrower particle-size distributions are produced through microuidics. d, Schematic o the sel-assembly mechanism o inorganic

nanoparticles. Individual molecules rst nucleate (I and II), ollowed by aggregation o nuclei into nanoparticles (III). I the reaction is not quenched or

stabilized, nanoparticles tend to agglomerate into bulk material (IV). A reers to individual molecules orming the nanoparticle, and An and Am reer to

nuclei ormed o n and m number o A molecules, respectively. e, Microuidic synthesis o inorganic nanoparticles by rapid mixing through two-phase

ow where reagents are embedded in uid droplets carried by an inert uid. , Top: sharp versus broad absorption maximum o QDs synthesized in

microchannels and bulk, respectively. Bottom: control o the absorption spectra o QDs as unction o reaction time. Figure reproduced with permission

rom: a, re. 16, 2003 APS; b, Top: re. 18, 2008 ACS; Bottom: re. 35, 2012 ACS; c, Top: re. 18, 2008 ACS; Bottom: re. 23, 2008 Springer;

d, re. 24, 2005 ACS; e, re. 27, 2004 RSC; , Top: re. 28, 2010 Wiley; Bottom: re. 27, 2004 RSC.

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are microuidic systems capable o characterizing the nanopar-ticle size and stability ollowing synthesis in a single platorm 31,32.Moreover, large numbers o distinct nanoparticles can be obtainedthrough combinatorial synthesis33, and production rates o identicalnanoparticles can be increased through parallelization or re-designo the devices34,35. Similar to high-throughput synthesis o librar-ies o small molecules, these advantages could potentially enablescreening and optimization o libraries o nanoparticles with distinctproperties. One o the challenges in gene therapy, or example, isnding the right ormulation or delivering nucleic acids to specicsites in the body. By mixing dierent precursors at varying ratios,microuidic systems have enabled a one-step combinatorial synthe-sis o libraries o polymeric and lipid nanoparticles that encapsu-late DNA and small-interering RNA, respectively. Screening these

libraries o nanoparticles has helped identiy superior ormulationsor gene transection, compared with conventional transectionagents such as lipoectamine 2000. Furthermore, using this method,potent lipid-based small-interering RNA ormulations or in vivodelivery to the liver have also been discovered33,34.

Evaluation and screening o nanoparticlesAnother challenge in nanoparticle development is the lack o in vitromodels capable o predicting in vivo behaviour36. Conventionally,nanoparticles are evaluated in vitro using cells cultured in well plates,which does not capture the complexity o nanoparticlecell interac-tions in vivo. For instance, a recent study showed that sedimentationo gold nanoparticles in well plates could lead to misinterpretationo results, such as increased nanoparticle uptake37. Microuidics

Vacuum

chamber

acuum

chamber

Static conditions in conventional plates

Flow conditions in microfluidic devices

Quantum dots

a

c d

b

Flow

Quantum dots

Sedimentation

Epithelium Air

Endothelium Membrane

Side chambers

Capillaries

1. Load

3. Isolate 4. Clean

2. Capture

5. Orient

7. Microsurgery

6. Immobilize 8. Unload

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Cell

Aggregation

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InletFlow

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Gutepithelium Porousmembrane

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chamber

acuumV

chamber

Channel array

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Control valve

Immobilization

membrane

Single

aspiration

Vacuum

chamber5 mm

Vacuum

controller

Cell channel

bilayer

Alveolus

Figure 3 | Microfuidic systems or in vitro evaluation and screening o nanoparticles. a, Schematic o nanoparticle sedimentation in conventional plates,

which could result in misinterpretation o results. In contrast, ow conditions in microuidics provide a more-accurate method or evaluating nanoparticles

in vitro. b, Let: schematic o the lung-on-a-chip that reconstitutes the critical unctional alveolar-capillary interace o the human lung through a stretchable

membrane containing an epithelium layer on one side and an endothelium layer on the other. Right: photograph o actual device. c, Top: schematic o

the gut-on-a-chip made by exible, porous, extracellular matrix-coated membrane lined by gut epithelial cells. The blue and brown arrows indicate two

diferent streams o culture medium separated by a membrane, entering the channel rom the top and bottom, respectively. Bottom: photograph o the

gut-on-a-chip device made o polydimethylsiloxane elastomer. A syringe pump was used to peruse dyes (red and blue) or channel visualization.

d, Let: photograph o a dye-lled microuidic system designed to handle C. elegans worms. Red, control valve layer; yellow, ow layer; blue, immobilization

layer. Scale bar, 1 mm. Right: schematic showing load, capture, orient, immobilization and unload o the worm. Figure reproduced with permission rom:

a, re. 39, 2010 AIP; b, re. 41, 2010 AAAS; c, re. 42, 2012 RSC; d, re. 48, 2010 NAS.




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provides signicant advantages over conventional methods or celland tissue culture by displaying structures and networks at relevantphysiological length scales, and by incorporating uid ow andmechanical orces that bring the cell-based assays a step closer tomimicking the in vivo microenvironment38 (Fig. 3a). Tis is espe-cially advantageous, or instance, when investigating the toxic eects

related to the cell uptake o nanoparticles. A recently developedmicrouidic system or evaluating QD toxicity on mouse broblastsrevealed increased cell viability under ow conditions compared withstatic incubation, possibly due to the absence o QD sedimentation39.

Recently, researchers have ocused on developing biomimeticmicrouidic technologies capable o portraying organ-level unc-tions on a chip, such as those observed in the lung, liver and kidneys,among others38,40. For instance, microuidic systems that reconsti-tute the critical unctional alveolar-capillary liquid/air interace othe human lung have been recently abricated by growing alveolarepithelial cells and microvascular endothelial cells on dierent sideso a perorated silicone membrane. Te membrane was pneumati-cally actuated to mimic the physiological expansioncontractionmotion due to breathing. It was ound that cyclic mechanical strainaccentuates toxic and inammatory responses in the lung when

exposed to silica nanoparticles, which could not have been observedwith other conventional in vitro systems41 (Fig. 3b). Using a similardesign approach, a gut-on-a-chip was developed by coating bothsides o a membrane separating two microuidic devices with extra-cellular matrix and lined by human intestinal epithelial cells. It wasdemonstrated that by subjecting the membrane to ow and cyclic

strains similar to those encountered in the gut, villi-like structureswere ormed and the co-culture o the intestinal microbes was madepossible42 (Fig. 3c).

Expansion o these technologies to other organs, or instanceliver-on-a-chip43, could lead to platorms or evaluating andscreening nanoparticle toxicity in organs where nanoparticles tendto accumulate and toxicity is likely to be a major concern (or exam-ple, liver, spleen and kidney). Although nanoparticles would stillneed to be evaluated in animals, such microuidic systems couldtake in vitro nanoparticle screening to a new level o utility by select-ing promising candidates with higher probabilities o success rom alarge pool o nanoparticle ormulations, and eliminating those thatwould otherwise have ailed in larger animal studies. Furthermore,coupling these technologies with microuidic devices used ornanoparticle synthesis opens the possibility o rapid combinatorial

Table 1 | Advantages, disadvantages/challenges, stage o development and potential impact o microuidic systems on dierent

steps in the clinical translation o nanoparticles.

Advantages Disadvantages/challenges Stage o development Potential impact

Synthesis Tunable nanoparticle

size

Narrower size

distribution

Reproducible synthesis

Potential or high-

throughput synthesis

and optimization o

nanoparticles

Solvent and high-

temperature

incompatibility or low-

cost polydimethylsiloxane

microchannels

Higher costs and

complexities in the

abrication o glass and

silicon microdevices

***** Rapid combinatorial,

controlled and reproducible

synthesis o libraries o

distinct nanoparticles or a

speciic application, and/or

reerence nanoparticles or

toxicology studies

Characterization Label-ree

characterization

Potential or eedback

control and real-

time nanoparticle

optimization

Current methods are not

applicable to all classes o

nanoparticles

Not all properties can be

characterized, such as drug

encapsulation and release,

and signal-to-noise ratio

* In-line rapid characterization

and optimization o

nanoparticles

In vitro Biological conditions

closer to in vivo

microenvironments

Potential or high-throughout screening

o a large number

o nanoparticles at

dierent concentrations

Higher costs and

complexities in the

abrication and operation

compared with well plates Might not be reusable and i

reusable, it would be diicult

to keep sterile

**** High-throughput studies o

nanoparticle toxicity, eicacy,

tumour penetration and organ

distribution, using organ-on-a-chip systems

In vivo Large number o

organisms could

be used or a single

measurement

High-throughput

evaluation o toxicity

or a large number o

nanoparticles

Lack o methods to translate

data rom small-scale

organisms to other species

Pharmacokinetics or

biodistribution cannot be

determined

** Real-time tracking o the

distribution or toxicity o

nanoparticles on small-scale

organisms

Large-scale synthesis Continuous synthesis

Bench-scale to clinical-scale reproducibility

Parallelization allows

or tuning scale o

production

Diicult to build systems

at low-cost that arecomparable to a batch

reactor able to prepare

grams or kilograms o

nanoparticles

*** Synthesis o nanoparticles or

human administration usingstackable parallel microluidic

units

Rank:Most advanced in development (*****) to least advanced in development (*), based on the amount o research carried out on each category, as well as the potential ease o adoption by industry.




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screening o a large number o dierent nanoparticles under variousconditions (or example, concentrations, pH values, under the pres-ence o specic proteins and so on).

Nanoparticles exhibiting promising results in vitro are subse-quently evaluated in vivo, which is considerably more expensive andresource intensive, especially in non-human primates. Althoughmost o the parameters, such as pharmacokinetics, biodistribu-tion and ecacy, are evaluated in mice and larger animals, trackingphysiological eects o nanoparticles on animal development could

potentially be obtained using a large number o smaller organisms.Te zebrash and Caenorhabditis elegans worms are well-knownmodels or studying undamental mechanisms and progressiono human diseases, and or drug screening44. For example, thezebrash was recently used as an in vivo model to develop a haz-ard ranking or engineered nanoparticles based on their impacton mortality rate and morphological deects in zebrash embryosexposed to these materials45. However, current methods or manip-ulating these organisms generally suer rom low throughput, lowautomation and imprecise delivery o external stimuli46. o solvethese challenges, engineered microuidic systems with dimensionscomparable to small organisms and containing valves and suctionpoints have been developed. Tese systems enable precise manipu-lation o these organisms with respect to placement and orienta-

tion or high-throughput screening46,47

(Fig. 3d). Other microuidicsystems are being developed that are capable o imaging dynamiccellular processes in small organisms, such as cell division andmigration, degeneration, aging and regeneration48. With such tech-nologies in place, it might be possible to use real-time microscopyto track physiological responses to uorescently labelled nanothera-peutics and nano-imaging agents, as well as assess the distribu-tion and ecacy o nanoparticles at both the organ and body level.Furthermore, real-time tracking o nanoparticle-induced toxicity atdierent concentrations and conditions in small organisms couldenable rapid selection o nanoparticles (especially those made withnovel synthetic materials) that are more likely to be non-toxic inlarger animals.

Future prospectsAt present, the eld o microuidics applied to nanomedicine is stillin its inancy. Although nanoparticles have a relatively small oot-print in the pharmaceutical industry, it is anticipated that as theseproducts bring in revenue, industry-led research and developmenteorts would probably adopt technologies, such as microuidics, toaccelerate their development. Nevertheless, microuidic technolo-gies, such as organ-on-a-chip and small-animal screening, are likelyto be adopted rst or the screening o small-molecule drug candi-dates, where the need or such tools is evident.

Tere are a ew key directions at the intersection o microu-idics and nanomedicine that are likely to be pursued in the nearuture (able 1). Although the quantity o nanoparticles synthe-sized by microuidic devices is ofen in the micro- to milligram

range, parallel and stackable microuidic systems could continu-ously produce nanoparticles on the gram to kilogram scale withthe same properties as those prepared at the bench scale. Similarly,the use o microuidic platorm technologies to reproducibly syn-thesize and screen libraries o nanoparticles with dierent chemi-cal compositions and/or physical and chemical properties couldpotentially advance nanoparticle discovery analogously to how thehigh-throughput screening o small molecules in medicinal chem-istry advanced small-molecule discovery. With respect to the designand development o novel nanoparticle constructs, the use o micro-uidics could enable the synthesis o nanoparticles with propertiesnot accessible by conventional synthesis, similar to what has alreadyoccurred or microparticle synthesis49.

Another avenue o uture research will be the integration o di-erent steps o nanoparticle development into a single system (or

example, nanoparticle synthesis characterization and evaluation),together with eedback control through a combination o microu-idics, robotics and automation, thus signicantly cutting the timeand cost o nanoparticle development. Finally, mass-producedmicrouidic devices and well-dened nanoparticle precursors canaid in the synthesis o identical batches o nanoparticles with little tono variations introduced by user handling. Tis could lead to the useand commercialization o nanoparticle synthesis kits composed ocalibrated devices that can reproducibly synthesize a specic class

o nanoparticle with well-dened properties or use as standards inconventional toxicological assays. Considering the large number onanoparticles being made o novel synthetic materials or o unusualshapes, such standardization would be highly useul or regulatorypurposes, among others.

Microuidic technologies are capable o accelerating the dis-covery and translation o nanoparticles, and could serve as a toolor nanotherapeutics to reach a similar tipping point reachedby genome sequencing in the past decade afer high-throughputsequencing technologies were developed50. Among all the microu-idic technologies, those developed or synthesis and in vitro screen-ing o nanoparticleshave the highest probability o making an impactin the near uture (able 1). Specically, microuidic synthesis maybe adopted as a second-generation manuacturing technology afer

the initial success o US Food and Drug Administration-approvednanoparticles in cases where the advantages o microuidic synthe-sis are signicant. Microuidic synthesis may also be adopted as ascreening tool to identiy optimal nanoparticles in academic andindustrial research laboratories. Alternatively, the impact o micro-uidics might be observable in the medium- to long-term utureor nanoparticle characterization and in vivo evaluation. For nano-particle characterization, the use o microuidics would probablyincrease once more-advanced technologies are developed to charac-terize several nanoparticle properties (or example, size, charge, sur-ace composition and stability) in a single system. Similarly, in vivoevaluation o nanoparticles in microuidics would probably matureonce both easily adoptable microuidic systems or manipulatingsmall organisms and methods or translating data obtained rom

these organisms to larger animals are developed.Overall, the use o microuidic technologies in nanomedicinebrings exciting opportunities to expand the body o knowledge inthe eld, advance the clinical translation o nano-based therapeuticsand imaging agents, and demonstrate innovative ways to developother classes o drugs.

Received 20 July 2012; accepted 31 August 2012; published online8 October 2012

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AcknowledgementsTis work was supported by the Koch-Prostate Cancer Foundation Award in

Nanotherapeutics (R.L. and O.C.F.), the National Cancer Institute Center o Cancer

Nanotechnology Excellence at MI-Harvard (U54-CA151884, R.L. and O.C.F.), and the

National Heart, Lung, and Blood Institute Programs o Excellence in Nanotechnology

(HHSN268201000045C; R.L. and O.C.F.). P.M.V. is supported by the National Science

Foundation graduate research ellowship. We thank B. imko and F. Karim or assistance

in drafing Figs 1 and 2, respectively. We also thank A. Radovic-Moreno, C. Alabi and

E. Pridgen or their insightul comments.

Additional inormationTe authors declare competing nancial interests: O.C.F. and R.L. disclose nancial

interest in BIND Biosciences and Selecta Biosciences, two biotechnology companies

developing nanoparticle technologies or medical applications. BIND and Selecta did not

support the aorementioned work, and at present these companies have no rights to anytechnology or intellectual property developed as part o this work.

Reprints and permissions inormation is available online at www.nature.com/reprints.

Correspondence and requests or materials should be addressed to R.L., R.K. and O.C.F.


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