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SensorsandActuatorsA211(2014)121–130

ContentslistsavailableatScienceDirect

SensorsandActuatorsA:Physical

journalhomepage:www.elsevier.com/locate/sna

Ahydrogel-basedintravascularmicrogrippermanipulatedusingmagneticﬁeldsଝ

Jui-ChangKuo,Hen-WeiHuang,Shu-WeiTung,Yao-JoeYang∗

DepartmentofMechanicalEngineering,NationalTaiwanUniversity,Taipei,Taiwan

article

info

abstract

Articlehistory:

Received15September2013

Receivedinrevisedform15January2014Accepted11February2014

Availableonline15March2014

Keywords:Hydrogel

IntravasculardeviceMagneticnanoparticleMicrogripper

Thisstudypresentsamagnetichydrogel-basedmicrogripperthatcanbewirelesslymanipulatedusingmagneticﬁelds.Theproposeddevicecanmovefreelyinliquidswhendrivenbydirectcurrent(dc)mag-neticﬁelds,andperformagrippingmotionbyusingalternatingcurrent(ac)magneticﬁelds.Thedeviceisfabricatedfromabiocompatiblehydrogelmaterialthatcanbeemployedforintravascularapplica-tions.Theactuationmechanismforgrippingmotionsisrealizedbycontrollingtheexposuredoseonthehydrogelcompositeduringthelithographyprocess.Thepreliminarycharacterizationofthedeviceisalsopresented.Themeasurementresultsshowthatthegrippingmotionreachedafullstrokeatapproximately38◦C.Bydispersingmultiwallcarbonnanotubes(MWCNT)intothematerial,theoverallresponsetimeofthegrippingmotiondecreasesbyapproximately2-fold.Devicemanipulationssuchasthegrippingmotion,translationalmotion,androtationalmotionarealsosuccessfullydemonstratedonapolyvinylchloride(PVC)tubeandinapolydimethylsiloxane(PDMS)microﬂuidicchannel.

1.Introduction

Cardiovasculardiseaseshavebecomeincreasinglycommonworldwide.Thecoronaryarteriesarecriticalvesselsforsupplyingtheheartwithnutrients,andcoronaryarteryanomaliesoftencausecardioplegiaanddeath[1,2].Therefore,bloodvesseltherapyhasrecentlybecomeapopularmedicalpractice.Intravascularsurgeryisoneofthepossiblemethodsofbloodvesseltherapy[3,4].Ingen-eral,intravascularsurgeryrequiresassistancefrommicrodevicestodeliverdiagnosticandtherapeuticmodalities[5,6],andseveraltypesofuntetheredmicrodevicehavebeendeveloped.

Typically,untetheredmicrodevicesscavengeenergyfromtheenvironmentandconvertthatenergyintomechanicalenergyforinducinglocomotionbyusingcertainprinciples[7].Donaldetal.proposedanuntetheredmicrorobot,operatedusingelectrostaticforce.Thedevicewasequippedwithacurvedsteeringarmthatwasmountedonanuntetheredscratchdriveactuator.Theproposeddevicecouldberemotelycontrolledtotravelthroughcomplexpaths[8].Fukutaetal.presentedamicromachinedpneumaticactu-atorthatcanbeemployedforair-jetplanarmicromanipulation,and

ଝSelectedPaperbasedonthepaperpresentedatThe17thInternationalCon-ferenceonSolid-StateSensors,ActuatorsandMicrosystems,June16-20,2013,Barcelona.Spain.

∗Correspondingauthor.Tel.:+886233662712.E-mailaddress:yjy@ntu.edu.tw(Y.-J.Yang).

alsoproposedapull-involtageminimizationmethodforreducingthevoltagerequiredforelectrostaticactuation[9].Erdemetal.proposedamicrorobotthatwaspropelledbycilia-likethermalbimorphactuatorarrays.Groupsofciliawerecontrolledindepen-dentlyforgeneratingplanarmotionwiththreedegreesoffreedom[10].Huetal.proposedahydrogel-basedmicrorobotthatwasoptothermallyactuatedusinglaser-inducedbubbles.Theproposeddevicedidnotconsistofsolidmaterials,butratheremployedagasbubbleinaliquidmediumforphysicallymanipulatingobjects[11].Recently,magneticallydrivenmicrodeviceshaveattractedattentionbecausetheycanbewirelesslydrivenandprovidearelativelylargeactuationforce[12–19].Ingeneral,theycouldbeoperatedinmagneticallytransparentmedia,suchasair,vacuum,conductingliquids,andnon-conductingliquids.Frutigeretal.pro-posedmicrorobotsthatwereoperatedusingwirelessmagneticmicro-actuators.Themicrorobotcouldbeactuatedusingalternat-ingcurrent(ac)magneticﬁelds,whichdirectlytransformedintomechanicalactuationwithoutrequiringintermediateconversionbyusinganelectroniccircuit[20].Leongetal.proposedamass-producible,tetherlessmicrogripper.Thelocomotionofthedevicecouldbemanipulatedmagnetically,andthegrippingmotioncouldbetriggeredbycontrollingthetemperature[21].Jiangetal.pro-posedaball-shapedmicrorobotwithrollingcapabilities.Drivenbymagneticﬁelds,thedevicecouldfreelyrollon3Dsurfacesinair,water,orsiliconoil[22].Tottorietal.proposedamagneticheli-calmicroswimmer,fabricatedusing3Ddirectlaserwriting.Thedevicewascapableofperformingsteerablecorkscrewmotions

http://dx.doi.org/10.1016/j.sna.2014.02.028

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Fig.1.Theschematicoftheproposedmicrogripper.Thedevicecanmovefreelyinliquidwhendrivenbydcmagneticﬁelds,andperformgrippingmotionsbyusingacmagneticﬁelds.

inwater[23].Kimetal.proposedamicromachinedmagnetichydrogelcellcarrierfabricatedusingasingleultraviolet(UV)expo-sure.Theprimaryapplicationoftheproposeddevicewastheactiveseparationofcellcarriersfromtheoriginalsolution[24].Bergelesetal.developedservoingmagneticintraocularmicrodevices,whichwereproposedforapplicationssuchastargeteddrugdeliveryandtheretinalveincannulationprocedure.Analgorithmthatlocalizesthemicromachineddevicebasedontheshapeofdeviceshasalsobeenproposed[25].

Forintravascularsurgery,itisdesirabletousemicrodevicestoperformclinicalactionssuchasdrugdelivery,sensing,andsurgery.Certainstudieshaveemployedsoftmaterialsthatrequiresimplefabricationprocesses,andhavedemonstratedthereversibilityofshapechanginginresponsetostimuli[26–32].However,becauseofchallengesinfabricationandmanipulation,mostmicrodevicescanonlybeoperatedaseitherafreelymovableunitoranendeffec-tor.Inthisstudy,ahydrogel-basedmicrogripperisproposedthatcanbewirelesslyactuatedfortranslational,rotational,andgrip-pingmotionsbyusingdirectcurrent(dc)andacmagneticﬁelds[33].Theproposeddevice,whichismadeofabiocompatiblehydro-gelmaterial,issuitableforintravascularapplications.Inaddition,themicrogrippercanbefabricatedusingasimplelithographytech-nique.

Theremainderofthisstudyisorganizedasfollows:theoper-ationalprinciplesanddesignarepresentedinSection2.TheproposedfabricationprocessofthemicrogripperisdescribedinSection3.ThemeasuredresultsofthefabricatedmicrogripperandthediscussionarepresentedinSection4.Finally,Section5drawstheconclusions.

2.Design

Fig.1showstheschematicoftheproposedmicrogripper.The

devicecanmovefreelyinliquidswhendrivenbydcmagneticﬁelds,andperformgrippingmotionsbyusingacmagneticﬁelds.Poten-tially,theproposeddevicecangripanobjectsuchasabloodclotinabloodvesselforintravasculartherapy.Inaddition,thepro-poseddevice,whichismadeofahydrogelmaterial,issuitableforintravascularapplicationsthatrequirebiocompatibility.

Fig.2(a)showstheoperationalprincipleofthedevicelocomo-tion.Fe3O4nanoparticles(NPs)andmultiwallcarbonnanotubes(MWCNTs)weredispersedinthethermoresponsivehydrogel.BecauseofthedispersedFe3O4NPsinthehydrogel,themove-mentofthedevicecanbewirelesslycontrolledbyapplyingdcmagneticﬁelds.AsshowninFig.2(a),withoutapplyingexternalmagneticﬁeldsduringthefabricationprocess,thenanoparti-clesarerandomlydispersedinthepre-gelsolutionofhydrogel.

Whenapplyinganexternalmagneticﬁeldduringprocess,thesuperparamagneticnanoparticlesformchain-likenanostructuresalongthedirectionoftheappliedmagneticﬁeld[15].Hence,thefabricatedhydrogel-basedmicrogripperpossessesaspeciﬁcmag-neticaxis,whichisexpectedtoenableamoreprecisemanipulation.Whenthedirectionoftheappliedmagneticﬁeldsischanged,thedevice,whichhasaspeciﬁcmagneticaxis,canrapidlyrotateorrealignalongthedirectionoftheﬁelds.

Fig.2(b)showsthatthegrippingmotioncanberealizedbythebimetallichydrogelcompositewithlayersofdifferentcross-links.Hydrogelpolymerscontainpendentbenzophenoneunitsthatallowthetuningofcross-linksbyusingirradiationdoses[26].Bycontrollingtheexposuredoseonthehydrogelcompositeduringthelithographyprocess,thefabricatedhydrogelwithlayersofdifferentcross-linkscaninducedifferentshrinkingresponsesatlowercriti-calsolutiontemperatures.Byapplyingacmagneticﬁelds,theFe3O4NPsareheatedbecauseoftheNéelandBrownianrelaxationpro-cess,whichinturninducestheinternaltemperatureelevationofthehydrogelmatrix[27].Becauseofthenon-homogeneousshrink-ingresponsesinthehydrogelstructure,thetemperatureelevationcreatesaninternalstressgradient,causingthedeformationofthestructure,thusgeneratingthegrippingmotion.Inaddition,bydis-persingMWCNTmoleculesinthehydrogel,thecompositeexhibitsashorterthermalresponsetimebecauseoftheenhancementofthemasstransportofwatermolecules[28–30].

Themagneticforce(Fm)exertedonthemicrogripperforatrans-lationalmotion(inx-direction)isproportionaltothegradientofthemagneticﬁeld[34]:

Fm=VbM∇Bx

(1)

whereVbisthevolumeofthemagnetizedobjectwithauniformmagnetizationM,andBxisthemagneticﬁeldinx-direction.

Asthemicrogrippermovesinx-directiondrivenbythemagneticﬁeldgradient,thedevicealsoexperiencesadragforceactingbytheﬂuid.Thetypicalformofthedragforceforanobjectimmersedinaﬂuidcanbewrittenas[35–37]:

Fd=

Cd󰀃fAv22m

(2)

where󰀃fisthedensityoftheﬂuid,vmistherelativevelocityoftheobjectwithrespecttotheﬂuidmedium,Aisthecrosssectionalareaoftheobject,andCdisthedragcoefﬁcient.Inlaminarﬂowregime,thedragcoefﬁcientanddragforceforasphereshapecanbeapproximatedas:

Cd,shpere=

2424Á

Re=󰀃;F(3)

fvmD

d,sphere=3󰀂DÁvm

Similarly,foracirculardiskparalleltotheﬂow,thedragcoefﬁ-cientanddragforcecanbewrittenas[35–37]:

Cd,disk=

13.613.6Á

Re=󰀃;FfvmD

d,disk=1.7󰀂DÁvm

(4)

where(Re=󰀃fvmD/Á)istheReynoldsnumber,andDisthediam-eteroftheobject.Also,thecrosssectionalareain(2)is:

2A=

󰀂D4

(5)

Sincethemicrogripperisneitheraspherenoracirculardisk,wemayemploythesimilarformshownin(3)and(4),andwritethedragforceforthemicrogripperas:

Fd,gripper=S󰀂DGÁvm

(6)

whereDGisthelengthofthegripper,andSistheshapecorrect-ingconstantofthemicrogripper.ThediscussionoftheupperandlowerlimitsofSisdescribedinAppendix.

J.-C.Kuoetal./SensorsandActuatorsA211(2014)121–130

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Fig.2.Theoperationalprinciplesof(a)themovingand(b)thegrippingmotions.

Asthemicrogripperisdrivenbythemagneticgradientatacon-stantvelocity,theforcebalancebetweenthemagneticforceandthedragforce(i.e.,Fd=Fm)givestheapproximaterelationshipbetweenvelocityandmagneticgradient:

VM∇Bx=b

S󰀂DÁ

(7)

3.Fabrication

Theproposedmicrogripperwasfabricatedusingathermore-sponsivehydrogelthathadFe3O4NPs(PVPcoated,NanostructuredandAmorphous)andMWCNTs(GoldenInnovationBusinessCom-panyLtd;averagediameter:30nm)dispersedthroughout.ThediameteroftheFe3O4NPswasapproximately20–30nm;therefore,thefabricatedmagneticstructuresaresuperparamagnetic.Fig.3showsthepreparationofthepre-gelsolution.Fortheoptimaldis-persionoftheMWCNTsinthehydrogel,anaqueous2wt%sodiumdeoxycholate(DOC)solutionwasusedasthesurfactanttodis-perseMWCNTsataconcentrationof0.5mg/mL[28].Subsequently,theaqueousDOC-MWCNTsolutionandFe3O4NPs(1mg/mL)werecompletelymixedusinganultrasonicagitatorat40kHzfor3h.Thepre-gelsolutionwascomposedof1.5gofN-isopropylacrylamide(monomers),57.1␮Lofacrylicacid(monomers),266.5mgofbenzophenoneacrylamide(crosslinker),and2.5mgofazobisisobu-tyronitrileasaphoto-initiator[26];allinthepreparedaqueousDOC-MWCNTs-Fe3O4solution.Byexposingthepre-gelsolutiontoUVlight,cross-linkingwasachieved.

Fig.4showsthefabricationprocessoftheproposedmagnetichydrogel-basedmicrogripper.Fig.4(a)showsthepre-gelsolutiondropsonaglasssubstrate.Atrenchwasformedonaglasssubstratebyusingahydroﬂuoricacidetchingprocess.Themagneticﬁeldswerethenappliedacrossthepre-gelsolution(Fig.4(b)),whichcausedthesuperparamagneticNPsinthepre-gelsolutiontoformchainsalongthedirectionoftheappliedmagneticﬁelds.Themag-nitudeoftheappliedmagneticﬁeldswas5mT.Figs.4(candd)showtheﬁrstUVexposureforpatterningtheprimarystructureofthemicrogripperwiththeﬁrstphotomask.TheUVexposuredosewas2J/cm2.Thepre-gelsolutionwasphoto-polymerizedusingthe

UVexposure,whichfrozethealignmentsofthesuperparamag-neticNPsinthepolymerizedregion.Theprimarystructureofthemicrogripperwasthenpatterned(Fig.4(e)).

Toavoidtheadhesionofthefabricatedhydrogeltothephotomask,thesurfaceofthephotomaskwastreatedwithoctade-cyltrichlorosilane(OTS)moleculessothatitssurfacepropertiescouldbecomemorehydrophobicandhavealowsurfaceenergy.ThemaskwasthoroughlyrinsedwithDIwateranddriedwithnitro-gengas.Itwasthenplacedina1mMOTSsolutionintoluenefor15minatroomtemperature.

Fig.4(fandg)showthesecondUVexposureforcreatinghighercross-linkingareawiththesecondphotomask.TheUVexposuredosewas4J/cm2.Fig.4(h)showsthebimetallicstructuresonthemicrogripperthatwascreatedusingthistwo-stepUV-exposureprocess.AsshowninFig.4,theﬁrsthydrogelandthesecondhydro-gelindicatethelowandhighlevelsofcross-linking,respectively.Fig.4(iandj)showthefabricateddeviceafteritwaswashedwithDIwater.

Thefabricatedhydrogelcompositeswereslicedusingacryostatmicrotomeinordertoobservethedistributionofnanoparticlesinthecomposites.ThephotographsoftheslicedhydrogelcompositesareshowninFig.5.Thewhiteareaisthehydrogelmatrix,andtheblackandbrownareasaretheclustersofmagneticnanoparticles.Fig.5(a)showsthehydrogelcompositethatwasnotmagnetizedbythemagneticﬁeldsduringthefabricationprocess.Themag-neticnanoparticlesofthiscompositearerandomlydispersedinthehydrogelmatrix.Fig.5(b)showsthehydrogelcompositethatwasmagnetizedbythemagneticﬁeldsduringthefabricationprocess.Thedirectionoftheappliedmagneticﬁeldduringtheprocessisalsoshownintheﬁgure.Themagneticnanoparticlesinthecompositeareself-assembledaschain-likestructuresalongthedirectionofthemagneticﬁeldsappliedduringthefabricationprocess.

Thecharge-coupleddevice(CCD)imagesofthefabricatedmicrogripperareshowninFig.6.Fig.6(a)isthetopviewofthemicrogripper.Thelengthofthefabricatedmicrogripperisapprox-imately700␮m.Thewidthofthegrippertipisapproximately100␮m.Fig.6(b)showsthesideviewofthemicrogripper.Thethicknessofthefabricateddeviceisapproximately100␮m.

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Fig.3.Thepreparationofthepre-gelsolution.

4.Measurementsanddiscussion

Tocharacterizethemagneticpolymerstructures,avibratingsamplemagnetometerwasusedtomeasurethehydrogelsam-pledispersedwithmagneticNPs.Fig.7showsthemagnetization

measurementofthefabricatedstructuredispersedwithFe3O4NPs.Theremanentmagnetizationatzeroappliedﬁeldsisconsiderablysmall,whichindicatesthatthesuperparamagneticcharacteristicoftheNPsastheparticlesizeissmallerthanthecriticalsizeforsuperparamagnetism.Moreover,thecurvesfortheforward

Fig.4.Thefabricationprocessoftheproposedmagnetichydrogel-basedmicrogripper.

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125

Fig.5.Thephotographsoftheslicedhydrogelcomposites.(a)Thehydrogelcompositethatwasnotmagnetizedbythemagneticﬁeldsduringthefabricationprocess.(b)Thehydrogelcompositethatwasmagnetizedbythemagneticﬁeldsduringthefabricationprocess.

Fig.6.TheCCDimagesofthefabricatedmicrogripper:(a)thetopview,and(b)thesideviewofthedevice.

andthebackwarddirectionsarecongruent,whichindicatestheorientation-independentproperties.TheinsetofFig.7showsthephotosofthepre-gelsolutionswithandwithoutdispersedmag-neticNPs.

TheexperimentalsetupforoperatingthemicrogripperisshowninFig.8.AsshowninFig.8(a),twopairsofHelmholtzCoils(HelmholtzcoilAandHelmholtzcoilB)werearrangedperpen-dicularlyandthecentersofthecoilsweretheoperationalregions(microﬂuidicchip).HelmholtzcoilAconsistsofcoilA-1andcoilA-2.HelmholtzcoilBconsistsofcoilB-1andcoilB-2.Theﬁg-ureontherightsideofFig.8(a)isthetopviewofthesystem.ThetwoHelmholtzcoilpairsgenerateduniformmagneticﬁeldsintheoperationalregion.TheHelmholtzcoilsareemployedtorotatethemicrogripper.Thetranslationalmotionofthemicrogripperisachievedbyturningononlyonecoilofeachcoilpair.Theacrylicframesofthecoilswerefabricatedusingalasermachiningsystem.Awindingmachine(SW-022,ShiningSun)wasemployedtowrapthecopperwiresontheframes.Thespeciﬁcationsoftheimple-mentedHelmholtzcoilswerelistedinTable1.ThephotosoftheimplementedHelmholtzcoilsareshowninFigs.8(bandc).

TheHelmholtzcoilswereconnectedtodcpowersupplies(GPD-4303S,GWInstek),controlledbyacomputer,usingtheGPIB®interface.Toremotelyactuatethegrippingmotionofthemicro-gripper,themicroﬂuidicchip,includingthemicrogripper,wasplacedontopoftheinductioncoilofa3kWinductionheater(LT-04-250,LantechIndustrialCo.,Ltd.)withanoperatingfrequencyof250kHz.TheobservationofthemicrogripperwasperformedusinganopticaltubeandaCCDcamera.AGaussmeter(Model6010,F.W.Bell)wasusedtomeasurethemagneticﬁelds.Thetemperatureofthemicrogripperwasmeasuredusinganinfraredthermalimager(Ti55FT,Fluke).

Themeasuredmovingdistancesofthemicrogripperwithdif-ferentmagneticgradientsareshowninFig.9.NotethatforMeasurement-A,themeasuredmagneticﬂuxdensityatthecen-teroftheoperationregionis5mT.Similarly,forMeasurement-BandMeasurement-C,themeasuredmagneticﬂuxdensitiesatthecenteroftheoperationregionare10mTand15mT,respectively.Themovingvelocitiesofthemicrogripperarelinearlyproportionaltotheappliedmagneticﬁeldsproducedbythemagneticcoil.Eachdatapointonthecurveistheaverageresultobtainedbymea-suringthreedeviceswiththesamedesignandconﬁguration.The

Table1

ThespeciﬁcationsoftheimplementedHelmholtzcoils.

Description

HelmholtzcoilA

HelmholtzcoilB

Fig.7.MagnetizationmeasurementofthefabricatedstructuredispersedwithFe3O4NPs.

CoilturnsRadius

DiameterofcopperwireDistancefromcenter

40090mm0.85mm90mm

400

110mm0.85mm110mm

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J.-C.Kuoetal./SensorsandActuatorsA211(2014)121–130

Fig.8.Experimentalsetupforoperatingthemicrogripper:(a)schematicofthemagneticdrivingsystemofthemicrogripper.(b)TheimplementedHelmholtzCoils.(c)Themicroﬂuidicchipwasplacedontopoftheinductioncoiloftheinductionheater.

80706050403020100-100 5 10 15 20 25 Measurement-A Measurement-B Measurement-CDistance (mm)

Fig.9.Themeasuredmovingdistancesofthemicrogripperwithdifferentmagneticgradients.

errorbarsindicatethemeasuredmaximalandminimalvalues.ThecurvesshowsthetranslationalmotionsofthemicrogrippercanbedrivenwithdifferentvelocitiesbycontrollingtheappliedelectriccurrenttocoilA-1.

Fig.10showsthegrippingmotionoftheproposedmicrogrip-per.Fig.10(a)showstheoriginalstateofthemicrogripper.Afterapplyingacmagneticﬁelds,themicrogripperwasheated,andthegrippingmotionwastheninducedbythenon-homogeneousshrinkingresponsesinthehydrogelstructure,asshowninFig.10(bandc).Thisgrippingmotioncanbepotentiallyusedfortheappli-cationsinsurgeryassistanceordrugdeliveryinthebloodvesselforintravascularapplications.

Fig.11presentsthemeasureddeformationofthemicrogrip-perindifferentdirectionsasthetemperatureincreased.Becauseofdifferenttemperature-dependentshrinkingresponsesinthegrip-perstructure,thegrippingmotionwasachievedbyelevatingthetemperature.Themeasurementresultsshowthatthedeforma-tionalongX1-directionwassubstantiallylargerthanthosealongtheX2-directionandY-direction,whichfacilitatesthedesirable

Time (sec)J.-C.Kuoetal./SensorsandActuatorsA211(2014)121–130

127

Fig.10.Thegrippingmotionoftheproposedmicrogripper.

Fig.11.Themeasureddeformationsofthemicrogripperindifferentdirectionsastemperatureincreased.

grippingmotionofthemicrogripper.Thegrippingmotionreachesafullstrokeatapproximately38◦C.Theresultsalsoindicatethatthetemperaturerangeforoperatingthemicrogripperisaboutfrom28◦Cto38◦C.Notethattheoperationtemperaturerangecanbeadjustedbycontrollingthequantitiesofcross-linkingandthedesignofbimetallicstructure.

Fig.12showsthetransientresponsetimesofthegrippingmotionsofthemicrogripper,obtainedbymeasuringthechangesinthesizeofthegapbetweenthegrippertips.Thegrippingmotionofthemicrogripperwasinducedbyapplyingacmagneticﬁelds.Thestrengthoftheacmagneticﬁeldwas50kA/m,andthefrequencywas250kHz.Theinsetshowsanopticalimageofthemicrogripper

Fig.12.Thetransientresponsetimesofthegrippingmotionsofthemicrogripper,obtainedbymeasuringthechangesinthesizeofthegapbetweenthegrippertips.Theinsetshowsanopticalimageofthemicrogripperbefore(left)andafter(right)activation.

before(left)andafter(right)activation.Thepurposeofintroduc-ingMWCNTsintothehydrogelistocreatemoreporousstructures,whichinturnenhanceswatermoleculartransportinthehydro-gel[28–30].AsshowninFig.12,thetimeconstantofthegrippingmotiondecreasesasthehydrogelwasdispersedwithMWCNTs.Theresponsetimeconstantofthegrippingmotiondecreasesbyapproximatelytwofold.

Fig.13showsthemanipulationofthemicrogripperonapolyvinylchloride(PVC)tube.Themagneticcoilscangeneratemagneticﬁeldsofdifferingmagnitudesindifferentdirections,andthereforeenablethemanipulationofthedirectionsoftheresultingmagneticﬁeldsintheoperationalregion.Themanipulationofthe

Fig.13.ThemanipulationofthemicrogripperonaPVCtube.

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Fig.14.Theoperationprocedureforrotationalandtranslationalmotionsusingcoils.

Fig.15.ThemanipulationofthemicrogripperinamicroﬂuidicchannelofthePDMSmicroﬂuidicchip.

J.-C.Kuoetal./SensorsandActuatorsA211(2014)121–130

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microgrippercanbefreelycontrolledusingdifferentdirectionsofmagneticﬁelds.

Fig.14showstheoperationprocedureofthemagneticcoils.InFig.14(a),allthecoilsareturnedoff,andthemicrogripperisinanarbitraryposition.InFig.14(b),bothcoilA-1andcoilA-2areturnedon(i.e.,Helmholtzcoilconﬁguration),andthegripperisrotatedtobealignedwithHelmholtzcoilAbecauseofthealignednanoparti-clechainsinthehydrogelcomposite.Fig.14(c)illustrateshowthetranslationalmotionofthedeviceisachieved.Inthiscase,coilA-2isonandcoilA-1isoff.Becauseofthegradientofmagneticﬁeldinthisconﬁguration,thedeviceisattractedtowardcoilA-2.Thismethodforactuatingtranslationalmotionshasbeenpresentedinvariousarticles([13,14,25]).Finally,asshowninFig.14(d),thedeviceisrotated180◦whenbothcoilA-1andcoilA-2(i.e.,Helmholtzcoilconﬁguration)weredrivenwiththepolaritywhichisoppositetothatinFig.14(b).NotethatasthemicrogripperispropelledbytheactuationconﬁgurationshowninFig.14(c),thegeneratedmagneticﬁeldgradientisinfactnotuniform.Therefore,thegeneratedmag-neticpropulsionforceisalsonotuniform,andingeneralcannotbeeasilycontrolled.

Fig.15showsthemanipulationofthemicrogripperinamicroﬂuidicchannelofthepolydimethylsiloxane(PDMS)microﬂu-idicchip.ThePDMSmicroﬂuidicchipwasfabricatedusingthesoftlithographyprocess.ThewidthofthechannelofthePDMSmicroﬂuidicchipis2mm.Byapplyingvariousdirectionsofmag-neticﬁelds,themicrogrippernavigatesfromthestartingpointtothedestination.

5.Conclusion

Amagnetichydrogel-basedmicrogripperwaspresentedinthisstudy.Theproposeddevicecanbewirelesslyactuatedfortransla-tional,rotational,andgrippingmotionsbyusingdcandacmagneticﬁelds.Thedeviceisfabricatedusingabiocompatiblehydrogelmaterial,andissuitableforintravascularapplicationsorothermedicalpurposes.BycontrollingtheUVexposuredoseonthehydrogelcompositeduringthelithographyprocess,theactuationmechanismforgrippingmotionswasrealized.Thepreliminarycharacterizationofthedevicewasalsopresented.Thegrippingmotionreachedafullstrokeatapproximately38◦C.Theoperationofthedevice,suchasthegrippingmotion,translationalmotion,androtationalmotion,wasdemonstratedonaPVCtubeandaPDMSmicroﬂuidicchannel.

Acknowledgement

ThisworkwassupportedinpartbytheNationalScienceCouncil,Taiwan,ROC(Contractno:NSC100-2221-E-002-075-MY3).

AppendixA.Appendix

InlowReynoldsnumberregime,theviscousdragforceactingonanobjectisstronglydependentonthetotalsurfaceareaimmersedintheﬂuid[36].Therefore,thedragforceactingonthecirculardisk(paralleltoﬂow)showninFig.A1(a)shouldbelargerthanthedragforceactingonthegrippershowninFig.A1(b),becausethetotalsurfaceareaofthegripperislessthanthatofacirculardiskwithadiameterwhichisthesameasthegripper’ssidelengthDG.Asaresult,comparingEq.(6)with(4),theshapecorrectingconstantSinEq.(6)shouldbelessthan1.7.

Inaddition,thelowerlimitofScanbeestimatedbyconsideringacirculardisk(seeFig.A1(c))whichhasthesametopsurfacearea(󰀂·D2e/4,andDe

isthediameterofthedisk)asthatofthemicrogripper.Thedragforceactingonthiscirculardisk(paralleltoﬂow)shouldbesmallerthanthatactingonthegripper,becausethegripper,

Fig.A1.(a)AcirculardiskwithdiameterofDG.(b)ThemicrogripperwithsidelengthofDG.(c)AcirculardiskwithdiameterofDewhichhasthesametopsurfaceareaasthatofthemicrogrippershownin(b).Notethatthecirculardisksandthemicrogripperhavethesamethickness.

whichhassamesurfaceareaasthedisk,hasamuchmorecomplexgeometry.

Thelengthofthegripper(DG)isabout700␮m.Also,basedonthemasklayout,thetotaltopsurfaceareaofthegripperisabout1.71×105␮m.Therefore,thediameterofthedisk(De)isabout467␮m,andthelowerlimitofSisabout1.13(=1.7×De/DG).Insummary,accordingtothissimpleanalysis,therangeoftheshapecorrectingconstantisestimatedtobe1.13AppendixB.Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound,intheonlineversion,athttp://dx.doi.org/10.1016/j.sna.2014.02.028.

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Biographies

Jui-ChangKuoreceivedtheB.S.degreeinCivilEngineeringandtheM.S.degreeinMechanicalEngineeringfromNationalChiaoTungUniversity,Hsinchu,Taiwan,in2008and2010,respectively.HeiscurrentlyworkingtowardthePh.D.degreeinMechanicalEngineeringatNationalTaiwanUniversity,Taipei,Taiwan.Hismainresearchinterestsincludebio-MEMSdevices,applicationsofhydrogelmaterial,andothertopicsinmicrosystemdevicedesignandfabrication.

Hen-WeiHuangreceivedhisB.S.degreeandM.S.degreeinMechanicalEngineeringatNationalTaiwanUniversity,Taiwanin2011and2012,respectively.Currently,heisaresearchassistantinResearchCenterofAppliedScience,AcademiaSinica.Hisresearchinterestsincludewirelesssensornetwork,embeddedsystemsforbiomed-icalapplications,bistablemicroactuators,microﬂuidicsandLabOnaChip.HeisamemberofIEEE.HeisalsotherecipientoftheoutstandingcreativityperformancescholarshipoftheCTCIFoundation.

Shu-WeiTungreceivedtheB.S.degreesindepartmentofMechanicalEngineeringfromNationalTaiwanUniversity,Taiwan,in2011.HeiscurrentlyworkingtowardtheM.S.degreeinDepartmentofMechanicalEngineeringatNationalTaiwanUni-versity,Taiwan.Hisresearchinterestsincludeapplicationsofhydrogelmaterials,micromachiningtechniques,andmicrotiparrays.

Yao-JoeYangreceivedtheB.S.degreeinMechanicalEngineeringfromNationalTaiwanUniversity(NTU),theM.S.degreeinMechanicalEngineeringfromtheUniversityofCalifornia,LosAngeles,andtheM.S.andPh.D.degreesinelectricalengi-neeringfromtheMassachusettsInstituteofTechnology,Cambridge.HewaswithCoventorInc.,Cambridge,asaSeniorApplicationEngineer.CurrentlyheisaDistin-guishedProfessoroftheDepartmentofMechanicalEngineering,NationalTaiwanUniversity,andservesastheDepartmentChairman.HisresearchinterestsincludeMEMS,nanotechnology,high-precisionmicromachining,ﬂexiblesensingarrays,biomedicaldevices,microﬂuidics,micromechanics,andsemiconductordevices.HeservesastheboarddirectoroftheChineseInstituteofAutomationEngineering(CIAE),theboarddirectoroftheChineseSocietyofMechanicalEngineers(CSME).HeisalsotheboarddirectorofASMETapeiChapter.HewastherecipientoftheOut-standingResearchAwardaswellastheDr.Da-YuWuMemorialAward(NationalOutstandingYoungResearcherAward)oftheNationalScienceCouncil.HewasalsotherecipientoftheNTUOutstandingResearcherAward.

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