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ViewC Online Lab on a Chip Dynamic Article Links < Cite this: DOI: 10.1039/c0lc00392a TECHNICAL NOTE www.rsc.org/loc UV polymerization of hydrodynamically shaped fibers† Abel L. Thangawng, Peter B. Howell, Jr, Christopher M. Spillmann, Jawad Naciri and Frances S. Ligler* Received9thSeptember2010,Accepted15thDecember2010 DOI:10.1039/c0lc00392a A 12 19 Mostnaturalandman-madefibershavecircularcross-sections;thusthepropertiesofmaterials 03 20 anuary C0LC0 cfiobmerpsowseitdhopfrnedoent-ecrimrciunleadrficbroesrss-aserectliaorngaellyshuanpeex.pPloarsesdiv.eWheyddreomdoynnsatmraitceftohceutseicnhgnaonlodgUyfVorfabricating n 28 J1039/ pchoalyrmacetreirziazatitoionno.fashapedacrylatestreamproducedmetre-longfibersforstructuralandmechanical n o10. hingtog | doi: Materialscomposedofstackedplates,suchasabaloneshellsor to create cross-sectional stream shapes more complex than asor evenbrickwalls,arestrongerthanthesamematerialsinbulk.1 simplycircularorrectangular.19,20Mottetal.developedsoftware Wc. y - s.rs However,suchstackedplatematerialsaregenerallycomposedof thatselectsthenumberandshapeofgroovesthatcanbeplaced orub hard, inorganic materials cemented together by a more flexible onoppositesidesofachanneltodirecttwolaminarflowstreams Laborathttp://p sourbmstaannc-meaanddedmoantoertioaclscuarriencloonmgpfiobseerdshoafpsetsa.cVkesryoffewplantaet-ulirkael twoitahcrheisepveectatofineaacl,hportehdeert.e2r1mWineehdadveisutrsiebduttihoinssoofftwthaeretwtoodfleusiigdns search 011 on fiasbseerms.blIefdsinutcohuflltarat,-stsrtoanckgafibbleerfibbuenrdsleesxoisrtefdab,rcicosu?ld they be mmiicxreorsfluoidrictodecvoicmesplpertoeldyuceinnsghheaigthhlyoinnetersdtriegaitmatewdistthreaanmosthfoerr e2 val Ruary theMyanar-me afdaebrpicoaltyemderusfiibnegrstwaroe-pnheaasrelysaylswteamyss;rofourndexbaemcapulsee, ewtithalo.utdemmixoinnsgtrfaotredmitchraotflofiwvecysttoremaemtrsycaopuplldicabtieonsse.q22ueHnotiwalellyl an aded by Nd on 19 Ja aiHssyiondltruroobddilyuiznceaedmdpiiconltyfoomaceuirrsibpnyrgeecluuerscsitnorgorsopmriniacnrmionoflgnu,oiedmxitcerrus–syiinsotinetimaotsroprhusaloslilnubgtei.oe2–nn8 idsnturtecroaedmdussc.t2re3edaTmbheetcwpooemteenpnletgitraeollyotovseuprosreloytsumnseodreitzdheatsthueecahpcrcheovsniuocceucnsetlsyrsiciivntetulryboedisnutacrneodd- oe ownlblish raenpdorstuedb-mfoircrmonakidnigamsoetliedrso.9r–1h5oLllaonw,ertoualn.dfafibbreircsatwedith3m00i–c9ro0n0 oshthaeprescoomffpelresxisnhtaripgeusiningtopfiobsseirbsilwitiitehswfoelrl-mdeafitnereidalcsrowsist-hsecatiwonidael Du P micron tubes with adjustable wall thicknesses using multiple varietyofapplications,includinghighstrengthmaterials,wound annular flows.16 The fibers and tubes have been polymerized healingdressings,controlledreleasematerials,tissueengineering using both chemical initiators and UV polymerization. Hydro- supports,oranti-ballisticfabrics. dynamic focusing has also been used to precipitate fibers in In an initial demonstration of production of polymer fibers microfluidic channels as two reactive streams intersect. These withpre-designedcross-sectionalshapes,wecastacrylatefibers fibers are roughly rectangular, as a result of the shape of the using a hydrodynamic focusing device with grooves in the top interfacebetweenthetwostreams.Similarly,thebarcodedfiber and bottom of a microfluidic channel that focused a miscible strips produced by Kim et al. are rectangular because they are sheath fluid around the solubilized polymer core solution.24 derived from laminar streams aligned in parallel.17 One other Laminar flow minimized mixing of the sheath and core fluids, example of non-round fibers was originally reported in 199518 and hydrodynamic focusing of the sheath fluid controlled the and was produced by co-extrusion of polymers with different cross-sectional shape of the core. The cross-sectional size was melting temperatures (www.fitfibers.com). Removal of one determined by the flow-rate ratio of the core and sheath polymer produced tri-lobed and deep-grooved fibers advanta- streams. The fiber hardened as the solvent diffused out of the geous for wicking textiles and for thermal and acoustic insu- core into the sheath fluid after the hydrodynamic shaping lations. process and during subsequent flow through the microfluidic Hydrodynamicfocusingcanbeperformedusinggroovesinthe channel. However, the range of fiber diameters that could be wallsofmicrofluidicchannelstofocusonestreamwithanother produced using this casting process was limited. The larger fibers tended to harden first on the outside, leading to collapse of the fibers as the internal regions shrunk during casting. The Naval Research Laboratory, 4555 Overlook Ave, SW Washington, DC, smallestdiameterthatcouldbeachieved((cid:2)300nm)waslimited USA20375.E-mail:[email protected];Fax:+12024048897; by the ability of the pumps to transport the viscous fluids Tel:+12024046002 withoutpulsing.Theslowrateofhardeningalsomeantthatthe †Electronic supplementary information (ESI) available: Experimental fibers tended to become rounder after exiting the microfluidic details, COMSOL models of fluid flow in the fabrication device, and rawdatafromthemechanicalanalysis.SeeDOI:10.1039/c0lc00392a channel. ThisjournalisªThe RoyalSociety ofChemistry 2011 Lab Chip Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE 3. DATES COVERED 2011 2. REPORT TYPE 00-00-2011 to 00-00-2011 4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER UV polymerization of hydrodynamically shaped fibers 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 6. AUTHOR(S) 5d. PROJECT NUMBER 5e. TASK NUMBER 5f. WORK UNIT NUMBER 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION Naval Research Laboratory,4555 Overlook Ave, SW, REPORT NUMBER ,Washington,DC,20375 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR’S ACRONYM(S) 11. SPONSOR/MONITOR’S REPORT NUMBER(S) 12. DISTRIBUTION/AVAILABILITY STATEMENT Approved for public release; distribution unlimited 13. SUPPLEMENTARY NOTES 14. ABSTRACT Most natural and man-made fibers have circular cross-sections; thus the properties of materials composed of non-circular fibers are largely unexplored. We demonstrate the technology for fabricating fibers with predetermined cross-sectional shape. Passive hydrodynamic focusing and UV polymerization of a shaped acrylate stream produced metre-long fibers for structural and mechanical characterization. 15. SUBJECT TERMS 16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF 18. NUMBER 19a. NAME OF ABSTRACT OF PAGES RESPONSIBLE PERSON a. REPORT b. ABSTRACT c. THIS PAGE Public Release 4 unclassified unclassified unclassified Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std Z39-18 View Online Toovercometheselimitations,hereweexploreUVpolymer- izationasafaster,moreuniformmethodofpolymerizingfibers duringthehydrodynamicshapingprocess.Theinitialchallenge wastodetermineconditionsunderwhichtheUVpolymerization couldbeaccomplishedfastenoughtolockinthecross-sectional shapeofthefiberbeforeitexited themicrofluidicchannel.The rolesofflowvelocity,flow-rateratio,andUVpowerincreating fiberswithrectangularcross-sectionalshapeswereexplored.The microfluidic sheath flow system successfully shaped an acrylate mixture into flat fibers using hydrodynamic focusing and UV polymerization. Initial measurements of the structural and mechanical properties demonstrated shape control and fiber integrity. A 12 19 03 20 anuary C0LC0 EThxepdeersimigne,nftaablrication and operation of the microfabrication n 28 J1039/ device and COMSOL simulations of the fluid flows within the n o10. device are described in the ESI†. The acrylate solution was hingtog | doi: pacreidpa(r1e1d awst%fo)l,loewths:yl4e-nheydgrloyxcyobludtyiml aetchryalcartyela(t8e5(w1t%w)t,%a)craynlidc asor photoinitiator (2,2-dimethoxy-2-phenylacetophenone, 3 wt%). Wc. y - s.rs All components were in liquid form when combined with the orub photoinitiator dissolved in dichloromethane at a stock concen- Laborathttp://p tAraldtiroicnh.ofP1rigormLto(cid:3)1.mAixllincghemthiecalssolwuetiroenos,btathineedhyfrdormoqSuiignmonae- search 011 on iinnhhiibbiittoorr wreamsorevmalovceodlufmronm(cthoeluamcnrylSaDteHcoRm-4pofnroenmtsSucsiienngtiafinc e2 val Ruary PproelpymareerdPbryodmucixtsin,gIngcl.y,cOenrotalr(i5o0,Nv%Y)).,Tmheethsahneaotlh(2so5luvt%io)n, awnads an aded by Nd on 19 Ja wa(DpapVter-roEx(,2im5Bavrot%eol)ky.fiT1eh0ldecvPEi,sncwgoiesnireteieermsinoegafstLuhareebsdohreuaasttiohnrgaienasdIdsniagcmi.t,apMllevidsisodcllouemtbiooerntoesr,, oe ownlblish MchAar)a.ctDereitzaaitlisonofartehepromveitdheoddisnftohreEstSruI†c.tural and mechanical Fig. 1 Scanning electron microscope images of shaped, polymerized Du acrylatefibersatsheath-to-sampleflow-rateratiosof(A)300:12[25:1], P (B) 300:6 [50:1], (C) 300:3 [100:1], (D) 600:3 [200:1], and (E) Results and discussion 900:3 [300:1]. Numbers indicate actual flow rates of the sheath:acrylateinmLmin(cid:3)1and[reducedratio].Scalebars:100mm(A– Fig. 1 shows scanning electron microscope images of represen- C, D2, E2), 20 mm (D1, E1). Fibers were manually wound onto glass tative fibers obtained at different sheath-to-sample flow-rate coverslips. ratios. The dimensions of the fibers are tabulated in Table 1. Fiberswithdimensions(width(cid:4)height)from198mm(cid:4)45mm to12.2mm(cid:4)2.6mmweredemonstrated.Asevidentfromthese flowinpartaccountedforthedecreaseintheconsistencyandthe images, the actual cross-sectional shapes of the fibers obtained greatervariationinfiberdimensions.Thesecondfactorthatmay were in general agreement with those of the COMSOL simula- havecontributedtothevariabilitywasthedecreasedUVexpo- tions(Figure2S,ESI†),bothintermsofshapeandsizeforthose suretimeforthetwosmallerfibers(Table1). obtained at slower flow-rates. It should also be noted that the TheUVexposuretimewasdefinitelyafactorinchoosingthe cross-sections of the three larger fibers were very uniform for experimental conditions, especially the sheath flow rate. Inter- eachsetofflowconditions,withcoefficientsofvariation(CVs)of estingly, we observed that a minimum exposure time was 7–14%inwidthandheightmeasurements.Thetwosmallerfibers required to initiate the polymerization process for the acrylate exhibited more variation in cross-sectional measurements (25– solution.EvenwhentheUVintensitywasincreasedto8Wcm(cid:3)2 38% CVs). Two factors account for the differences in the by placing the light guides directly on the channel surface, no consistencyofthefiberdimensions.First,the3largerfiberswere polymerization was observed unless the exposure time was fabricatedbyadjustingtheflowrateoftheacrylatesolutionfrom approximately0.5secondsormore.Thisis,however,notoutof 3 to 12m L min(cid:3)1, while maintaining the sheath flow rate at theordinaryasthereisaminimumrequiredexposuretoinitiate 300 mL min(cid:3)1. Since 3 mL min(cid:3)1 was the minimum pump rate, polymerizationasshownwithotherpolymers.25Itwasdesirable further increases in the flow-rate ratios required increasing the tomaintainthesameexposuretime(UVdosage)forallfibersif sheathflowrateto600mLmin(cid:3)1and900mLmin(cid:3)1.Pumpingthe theirmaterialpropertiesweretobecompared.Forthatreason, coreagainstthebackpressurefromanincreasinglystrongsheath the first three samples were fabricated with a constant sheath LabChip Thisjournal isªTheRoyalSociety ofChemistry2011 View Online Table1 Dimensionsoffibersasafunctionofsheathandsampleflowrates FlowRates Fiber Fiber UVExposure SimulatedFiber (Sheath:Sample)[mL/min] Width Height timea Dimensionsb (reducedratio) [mm] [mm] [s] Width(cid:4)Height[mm] 300:12(25:1) 198(cid:6)27 45(cid:6)6 5.8 188(cid:4)52 300:6(50:1) 173(cid:6)17 30(cid:6)2 5.9 177(cid:4)48 300:3(100:1) 95(cid:6)10 31(cid:6)4 5.9 132(cid:4)39 600:3(200:1) 21(cid:6)8 7(cid:6)2 3.0 84(cid:4)32 900:3(300:1) 12(cid:6)3 2.6(cid:6)0.7 2.0 70(cid:4)28 aApproximateUVexposuretimeduringthepolymerizationprocessusinga1000mm[W](cid:4)750mm[H](cid:4)4000mm[L]channel.bFibersizeswere measuredfromtheCOMSOLconcentrationplotsatthetransitionfromcoretosheathatapproximatelythe85%diffusionboundary. A 12 0139 flow rate of 300 mL min(cid:3)1. At 300 mL min(cid:3)1 and a channel curve). Such an observation suggests polymer training or 20 anuary C0LC0 agpeopmroextirmyaotfely1060.09mmmms(cid:4)(cid:3)1,7w50himchmt,rathneslalitneedatroflaonwexvpeoloscuirteytwimaes apnonlyemaleinrgbaocfktbhoenefibiserrse,lieavewdelol-feisntatebrlnisahlesdtrepsrsoecse.sAsnwahlyesriesboyftthhee n 28 J1039/ roaft(cid:2)e5o.f8bse(lToawbl3em1L).Smininc(cid:3)e1t,htehpeusmhepatdhidflnoowtaralltoewwtahseisnacmrepalseefldotwo vreisvceoaulesdloasssigonfitfihceafinbtenrosnw-liitnheaeirgdhetcsruecacseesfsriovemp1a2s.s2ivJekwgo(cid:3)1rktolo8o.6pJs n o10. obtainsmallerfibers.Furtherincreasesinthesheathflowrateto kg(cid:3)1(Fig.2B).Thelevelingoffofthecalculatedviscouslosswith hingtog | doi: rfiebdeurcsetwheerfiebenrostizepodleycmreearsiezdedtheadexepqousauterleytimtoetomtahinetpaoininttthheairt raepppelaictaedtiownoorfktleonospilsesfutrretshsecrosnutpinpuoerdtstthheeacnonnecaelpintgthoaftthreepfiebaetresd. asor Wc. integrity. Taken together, the mechanical evaluation of these materials y - s.rs The impact of the reduced exposure time on shape retention indicateswehavesuccessfullycreatedstable,flatpolymericfibers esearch Laborator2011 on http://pub ciacsnlaoidnmFseaepi.gmllsTe.oitr-1ecablDyynelsipspnaeonodeldnrrytimceiEnade.lrciioTnzsmethhodepaapaafitrebitt,sewhoreosnsut-ipiogmnhfgeateFhsstietehgine.seyhgn1awvEpietrehraosearnoetwmfroetothunhuentendys(dmeoe.gandw.lltetehgroreeefilfg/boalnaierromrs)sst, vCHiayodnUrcoVldupysnhioaomntosipcofolycmuseirnizgautisoinngupsainssgivaemwiaclrlosfltruuicdtiucraepspwraosauchse.dto al Rary the fiber interface would adjust to minimize the interfacial shapeaprepolymerstream,whichwassubsequentlypolymerized vu using UV exposure. The shape designed using flow simulations an tension,causingthefibertobecomerounderonthesideexposed Na oaded by ed on 19 J tfoabTtrhhiceeaatmierd.ecahtaaniflcoalwprraotpeeorftie3s00omfLthmeinp(cid:3)h1osthoepaotlhymfloerwizetod1fi2bmerLs wtfihbaeesrrsmataieoixnhotiafbitinhteeeddfl,oraewnpdrroatdtheuescsoiibfzletehoesfhstahhpeeaestfihboaevnresdrwtmhaesetpcrroeenplteornolygllmtehdesr.u.sTTinhhgee DownlPublish mfibiner(cid:3)s1taocrfyolarcteeswohluilteionmowneirteordinetgertmheinestdrabiny.suTbhjeecitninitgiaml tuelntispillee fipdhealsietymofattchheinsghapoef wthaes ashfeuantchtioanndofpbroetpholeyxmpeorsuflreuitdims.eTahnids modulusofthefibersat30(cid:5)Cwasfoundtobe6.1MPa,asshown microfluidic approach for production of fibers with defined cross-sectionalshapecanproducefibersforfurtherdevelopment in Fig. 2A (solid black curve). This value is on par with the ofmaterialswithneworimprovedperformancecharacteristics. estimated tensile modulus of round acrylate-based fibers also created using a microfluidic photopolymerization method.12 Thus the in situ polymerization process described here is suffi- cienttoproducerobustfiberswithviscoelasticproperties. Acknowledgements Subsequentstress/straincycleswereperformedtoevaluatethe Thisworkwasfunded byNRL/ONRMA042-06-41WU9899 viscous loss of the fibers as they are subjected repeatedly to andtheDefenseThreatReductionAgency.ATwasaNational tensilestress.Animmediateobservationwasanotabledecrease Research Council Postdoctoral Fellow. The views are those of inthetensilemodulusofthefiberbundleto(cid:2)2.8MPawiththe theauthorsanddonotrepresenttheopinionorpolicyoftheUS successive application of a tensile stress (Fig. 2A, dashed red NavyortheDepartmentofDefense. References 1 F. D. Fleischli, M. Dietiker, C. Borgia and R. Spolenak, Acta Biomater.,2008,4,1694–1706. 2 Y. 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