A versatile plug microvalve for microfluidic applications

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ContentslistsavailableatScienceDirect

Sensors

and

Actuators

A:

Physical

jo u r n al h om ep a g e :w w w . e l s e v i e r . c o m / l o c a t e / s n a

A

versatile

plug

microvalve

for

microﬂuidic

applications

M. Tahsin

Guler

a

_,

_Pinar

_Beyazkilic

b

_,

_Caglar

_Elbuken

b,∗

a_Department_of_Physics,_Kirikkale_University,_71450,_Kirikkale,_Turkey

b_{UNAM—National}_{Nanotechnology}_Research_Center,_Institute_of_Materials_Science_and_{Nanotechnology,}_Bilkent_University,_06800,_Ankara,_Turkey

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received5May2017

Receivedinrevisedform28July2017 Accepted1September2017 Availableonline5September2017 Keywords: Valve Microvalve Portablemicroﬂuidics ColorimetricTNTassay

a

b

s

t

r

a

c

t

Mostoftheavailablemicrovalvesincludecomplicatedfabricationstepsandmultiplematerials.We presentamicrovalvewhichisinspiredfrommacroplugvalves.Theplugmicrovalveisfabricatedby boringaholethrougharigidcylindricalrodandinsertingitthroughamicrofluidicchip.Itsimply func-tionsbyrotatingtherodwhichalignsormisalignsthevalveportwiththemicrochannel.Therodismade upofarigidmaterialforapplyingthevalvetoanelasticpolydimethylsiloxane(PDMS)microchannel. Thevalvecanalsobeusedforarigidchannelbyinsertingtherodintoanelastictubing.Therefore,the presentedmicrovalvecanbeusedforbothelastomericandthermoplasticchannels.Theplugmicrovalve canbeappliedtoaprefabricatedmicrochannelanddoesnotrequiremodificationofthemolddesign. Wehaveverifiedtherepeatabilityandrobustnessofthevalvebyrepetitiveoperationcyclesusinga servomotor.Theplugmicrovalveisadaptabletonumerousmicrofluidicapplications.Wehaveshown threemodesofoperationforthemicrovalveincludingfluidflowcontrolacrossmultipleintersecting channels.Integratingthemicrovalvetosomecommonlyusedmicrofluidicdesigns,wedemonstrated theversatilityandthepracticalityofthemicrovalveforcontrollingflowfocusing,microdropletsorting andrapidchemicalagentdetection.Thislow-costmicrovalvesignificantlyminimizestheprototyping timeformicrofluidicsystems.

1. Introduction

Fullyintegrated microfluidic devices require several compo-nentssuchaschannels,pumps,mixersandvalves.Earlyyearsof thebirthofmicrofluidicswitnessedthedevelopmentofthesebasic componentsusingdifferentmethods.Intheliterature,avariety ofmicrovalveshavebeenreportedwhichemployelectroactive polymer [1], piezoelectricmaterials [2], ferrofluids[3], electro-staticactuation[4]orphasechangematerialssuchasparaffinwax [5],hydrogel[6],orthermosensitivematerials[7].Therearealso single-useburstvalvesthatarenormally-closedandopens irre-versibly[8].Thepassivecapillaryvalvescanalsobeconsideredas single-usesincereactivationofthevalverequirescomplete dry-ingofthechip[9].Onecanrefertoreviewarticlesfornumerous othertypesofmicrovalves[10,11].Amongstthesevalves,the pneu-maticmembranevalve hadaprofound impactin thefield [12]. Thehighscaleintegrationofthesevalvesallowedexquisitecontrol offluidsintinycompartments[13].Themulti-layermicrofluidic systemuses a control layer toadjust thepressure in order to activateordeactivateeachvalve.Thesimilardoormatvalves,which relyonSU-8molds,operateinasimilarway[14].However,these

∗ Correspondingauthor.

E-mailaddress:elbuken@unam.bilkent.edu.tr(C.Elbuken).

microvalvesrequireprecisemulti-layermicrofabricationandisnot suitableforrapidfabrication.Inordertoaddressthesechallenges thesimplistictwistvalveshavebeendeveloped[15].Thesevalves donotrequireanyoff-chipcomponentsandcanbefabricated rela-tivelyeasiercomparedtopneumaticmembranevalves.Theydonot needanypressureorpowertoretaintheirstate,thustheyare appli-cableforportableapplications.However,thesetwistvalvesstillrely ondeﬂectionofathinPDMSmembrane,whichisachievedeither bytwistingascreworbyactuationofaspringplunger[16].Their fabricationistediousandincludeseveraladditionalstepssuchas precisealignment,verticalintegrationofascrewandcuringofa secondmaterialtoformthethreads.Itisimportanttonotethat bothpneumaticmembranevalvesandtwistvalvesarepinchvalves thatfunctionbasedondeﬂectionofanelasticlayerwhichrequires microfabrication.

In recent years, microﬂuidic community has focused more on the application of integrated lab-on-a-chip (LOC) devices withincreasingnumberofresearchersfromdiversebackgrounds. Bringinginnewopportunitiesandnewresearchdirections,the increasingdemandfor more capablesystems posessome chal-lengessuchasthe needfor expertisein allthe componentsof anintegratedLOCsystem.Itisinterestingtonotethatalthough micropumpsandmicromixersarequitestandardizedand easy-to-reachformostresearchers,microvalveisstillatrickycomponent especiallyforpeoplewithoutmicrofabricationexpertise.Thereis http://dx.doi.org/10.1016/j.sna.2017.09.001

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stillnotagoldstandardmicrovalveavailableinthemarket. There-fore,thereisagapintheﬁeldforlow-cost,versatilemicrovalves thatcanbefabricatedandappliedbynon-experts.

In this study, we present arguably the simplest type of microvalvethatcanbeexploitedinmicrofluidicsystems.Wereport aplugmicrovalvethatisinspiredfrommacroscalequarterturn ball valves.The plug microvalve is fabricatedby boring a hole throughacylindricalrodwhichisinsertedintoa holepunched throughthemicrochannel.Thisvalveinheritsallthebenefitsof thetwist microvalve;(i) it doesnot requireanychange ofthe originalchanneldesign,(ii)itisverylow-cost,(iii)itistruly inte-gratedandoperateswithoutanyoff-chipcomponents,(iv)itonly requirespowerduringstatechange.Unlikethetwistvalve,theplug microvalvecanbefabricatedinlessthan5minwhichrequiresno skillsetotherthanpunchinganddrillingholes.Additionally,the presentedvalvecanbeintegratedtobothelastomericand thermo-plasticmicrofluidicdevicessuchaspolydimethylsiloxane(PDMS), polymethylmethacrylate(PMMA),acrylonitrilebutadienestyrene (ABS),polycarbonate(PC)andpolystyrene(PS)whichisaunique feature.Sinceitworksbycompletelyblockingthemicrochannel ratherthanpinchingthechannel,itworksforanychannelsizeor shape(rectangularorroundmicrochannel).

2. Fabricationandcharacterizationoftheplugmicrovalve TheoperationofthevalveisshowninFig.1.Asseen,thevalve iscomposedofarodwhichisinsertedtoachannel.Therod con-tainsaholethatisalignedwiththemicrochannelinitsONposition. Herein,weusedpolylacticacid(PLA)rodsowingtotheir mechan-icalproperties.PLAisacommonlyusedbiocompatible3Dprinting materialwhichis readilyavailable.It hasa Young’smodulusof 3.5GPawhichprovidestherigidityrequiredduringtheoperation ofthevalveandalsoeaseoffabricationduringholeopening. 2.1. Fabrication

Theplugmicrovalveisfabricatedinfoursimplesteps.First,we cutapieceofrodfromPLAfilamentstockandboreaholethrough theradialaxisoftherod(Fig.2a).Then,webendthetipofthe rod90◦ tothedirectionparalleltotheholeusingahotairgun setat200◦C(Fig.2b).Thisallowsease-of-handlingduringmanual operation.Then,athroughholeisopenedacrossthemicrofluidic chipintersectingthemicrochannelatthepointwhereweneedthe valve(Fig.2c).Finally,therodisinsertedintotheholeonaflat surfacesothattheholealignswiththemicrochannel(Fig.2d).

Thethroughholeoftherodisobtainedusingabenchtopmicro miller(ProxxonMF70)withtungstencarbidedrillbits(Proxxon 28321).PLArodisﬁxedintoaV-shapegroovealongametal

fix-ture(Fig.2a).Drillingisdonegraduallyby4–5rapidupanddown motionofthedrillbitinordernottoincreasethetemperatureof PLAandcausemelting.Inordertoachievethealignmentofthe holewiththemicrochannelinthefinaldevice,thedistancefrom thecenteroftheholetothebottomoftherodshouldmatchthe heightofthemicrochannelfromthebottomofthesubstrate.Inthis way,insertingtherodallthewayintothemicrofluidicchipona flatsurfaceensuresthealignment.Thehorizontalalignmentofthe rodwiththechannelisachievedwhenpunchingtheholethrough thedevicewhichisperformedunderstereomicroscope(Fig.2c). Samplerodswith600␮mand200␮mdiameterholesareshownin Fig.2ainset.Thedeadvolumesforthesemicrovalvesare0.48␮land 0.053␮l,respectively,whicharenegligibleformostapplications.

TherodsshowninFig.2aareusedforsoftPDMSmicrochannels. FormicrochannelsmadeoutofrigidmaterialssuchasPMMA,the rodshouldbeelastictoobtainasuccessfulsealing.Thisisachieved byinsertingthePLArodintoasiliconetubingandperformingthe holedrillingstepexplainedasabove.Aresultantvalvecanbeseen inFig.3c,forwhichweusedasiliconetubingof0.8mmIDand 2.4mmOD.(Versilicspx-50).Fig.3shows theplugmicrovalves integratedtoaPDMS(a–b)andaPMMA(c–d)microﬂuidicdevice. Fig.3aand3cshowtheOFFpositionofthevalveatwhichthebent sectionof therodisperpendicular tothemicrochannel.Fig.3b and 3dshow theON positionwiththe reddye solutionﬁlling the microchannels.PDMS channels are obtainedusing conven-tionalsoftlithographyusingSU-8moldsandsealedbybonding toablankPDMSpieceusingoxygenplasmaactivation[17].PMMA deviceswereobtainedbyCO2lasercutter(EpilogZing)andsealed

byaPMMApieceusingdoublesidedtape.Afterfabricationofthe microﬂuidicdevices,thelocationofthevalvesweremarkedusing amarkerpen.Then,holesarepunchedthroughthePDMSorPMMA deviceusingpunchingordrilling,respectively.Thelargerthehole throughthePLArod,themoretolerantthesystemto misalign-ments.

2.2. Leakagetest

Wetestedthesealingofthevalveusingapressurepump sys-tem(ElveFlowOB1).Weconnectedtheinletsofthemicrochannels showninFig.3toaDIwatersupplyandgraduallyincreasedthe inletpressure upto2bar.Thevalveswith200␮mand 600␮m holediameterswithstoodthemaximumpressureofourtestsetup (2bar)forboth thePDMSdeviceandthePMMAdevice.During thesetests,wehavenotobservedanyleakagearoundthevalvethus theplugmicrovalvesweresuccessfulinsealingaroundthehole. Itshouldbenotedthatsuccessfulsealingoftheplugmicrovalve depends on the geometrical parameters and also the chip/rod materialelasticity.Althoughitischallengingtodrawauniversal

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Fig.2.Fabricationsteps:(a)DrillingaholethroughthePLArod,(inset)close-upimagesofrodswith600␮m(top)and200␮m(bottom)diameterholes.(b)Bendingthetip oftheholeforease-of-handlingusingahotairgunsetat200◦Cforafewseconds.(c)Punchingofthethroughholefortheplacementoftherod.Thelocationofthevalveis determinedatthisstepandshowstheversatilityoftheplugmicrovalve.(d)Insertionoftherodtotheholeonaﬂatsurface.

Fig.3. Imagesofplugmicrovalveswith600␮mholediameterintegratedtoPDMSandPMMAmicrochannels.Plugmicrovalveat(a)OFFand(b)ONpositionacrossPDMS channelof200␮mwidthand50␮mheight.Plugmicrovalvewiththeadditionalsiliconeoutertubing,at(c)OFFand(d)ONpositionacrossPMMAchannelof1.5mmwidth and200␮mheight.Theﬂuidissuppliedataninletpressureof300mbarusingapressurepump.

conclusion,fromourrepetitiveexperimentswithrodsand chan-nelsofvaryingsizes,wecanprovideadesignruleofthumbfor successfulsealing:a)Thediameterofthethroughholeacrossthe rodshouldbeatleasttwicethesizeofthechannelwidth.b)The diameteroftherodshouldbeatleasttwicethesizeofthehole diameter.

2.3. Torquetest

The plug microvalve canbe operated by hand as shown in theSupplementarymovie1. We have characterizedthetorque required for closing/opening theplug microvalve using a digi-talforcemeter.A1.7mmdiameterrodvalvewith600␮mhole diameterhaving4.4cmactuationarmwasengagedtotheforce meterfromitsfarthermostpointandpulledretainingthe valve-integrated-microﬂuidicchip.Theforcevaluethatthevalvestarted totwistwasnoted.Torque(gcm)iscalculatedbymultiplyingthe armlengthandtheforce.Themeanvaluesofthreesuccessive mea-surementsareshowninFig.4.

WemeasuredthetorqueforPDMSandPMMAmicrochipsof twodifferentthickness.AsseeninFig.4a,the4.8mmthickPDMS devicerequiresapproximatelytwotimesthetorquerequiredfor 2.2mmthickPDMSdevice.Itisalsoevidentthattorquethat is requiredtoactivatethevalvedecreaseswithincreasinghole diam-eter.1.7mmdiameterPLArodwith600␮mdiameterholeshown inFig.2awasusedforPDMSdevicespunchedwith1.2,1.5and

2mmdiameterpunches.WehavealsotestedPDMSmicrodevices ofvaryingelasticitybyvaryingbasepolymer/curingagentratios duringfabrication.AsseeninFig.4a,stifferPDMSdevices(ratio of5:1)requirelesstorquefortheoperationofthevalve.Itshould benotedthatthesoftestPDMSdevice(ratioof20:1)with4.8mm thicknesswastornoffataround43gcmduringthetests.Therefore, thePLAplugmicrovalvecannotbeusedforveryelastic microde-vices.Forsuchadevice,theuseofotherrodmaterialscanbea solution.ForthePMMAdevice(Fig.4b),therodshowninFig.3c wasused.TheresultsweresimilartoPDMSdevice.Increasinghole diameterdecreasedtherequiredtorquewhileincreasingdevice thicknessincreasedit.

We have alsoinvestigated the sealing performance ofthese microvalves. We have seen that the diameter of the hole punched/drilledthroughthemicroﬂuidicdeviceisamore impor-tantparameterthanthedevicethicknessandelasticity(studied foronlyPDMSdevices).For thePDMSdevicesshowninFig.4a, thevalvesintegratedwith1.2and1.5mmdiameterpunchedholes withstood 2bar inlet pressure. However, when 2mm diameter punch was used tomake the through hole (which resulted in approximately1.55mmdiameterholewaistdue toelasticityof PDMS),thevalvestartedtoleakatapproximately240mbar. Sim-ilarresultswereobtainedforPMMAdevices.Forthevalvesmade by2.51mmdiameterhole,leakagewasobservedabove1.2bar, whereasotherdevicessurvivedtheleakagetestupto2bar.

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Fig.4.Experimentaltorquevaluesmeasuredfor1.7mmdiameterrodvalve actu-ationforvarioussizesofholespunchedthroughthedevice:(a)PDMSdevicewith varyingdevicethicknessandbase/curingagentratios,(b)PMMAdevicewithvarying devicethickness.

Fig.5. AutomatedoperationoftheplugmicrovalveusinganArduinocontrolled servomotor.Theﬂuidissuppliedbyapressure pumpataninletpressureof 300mbar.Themicrochannelwidthandheightare300and100␮m,respectively.

2.4. Automationandlong-termperformance

We havealso demonstratedtheautomated operationofthe microplugvalveandevaluatedits longtermperformance (sup-plementary movie2).For this purpose,we used aservo motor (PZ-15320).Inthisconfiguration,wedidnotbendthetipoftherod andinsertedthestraightrodwith600␮mholeintotheservo actua-tionarmusingacustom-madeadapterasshowninFig.5.Thevalve waspressfittotheservoarm.ThemotorwasfixedonthePDMS device(10:1mixed,4.8mmthick)forstand-aloneoperation.Itwas controlledusinganArduinomicrocontrollertomakea90◦ clock-wiserotationandthentogobacktohomeposition.Theresponse timeofthevalveismeasuredtobeapproximately200ms.We actu-atedtheservousingasquaretrainpulsewithaperiodof10s.This

Fig.6.Controloftheﬂowinmultiplemicrochannelswithasinglevalve.Three straightPDMS/PDMSchannelswhichintersectatajunctionwhereaplugmicrovalve isplaced.Intheshownstate,valveisONforthechannelwithyellowsolutionand OFFfortheremainingtwochannels.(Forinterpretationofthereferencestocolour inthisﬁgurelegend,thereaderisreferredtothewebversionofthisarticle.)

resultedinmorethan8000ON/OFFcyclesforonedayoperation. Aftertwodaysofoperation,wehavenotobservedany malfunction-inginthemicrovalveperformanceverifyingtherobustnessofthe valve.Forapplicationsdemandingfasterswitchingtimes,stronger servomotorsshouldbeemployed.

3. Modesofoperation

Mostmicrofluidicvalvescanonlybeusedatasingle configu-ration,whichissimplyforcontrollingthefluidflowalongasingle microchannel.Despiteitssimplicityplugmicrovalvecanbeused inmultipleconfigurations,whichisanotheruniquefeatureofthis versatilevalve.Inthisstudy,wedemonstratethreedifferentmodes ofoperationfortheplugmicrovalveusing10:1mixed4.8mmthick PDMSdevices.

ThebasicoperationofthemicrovalveisshowninFig.3,which istheON/OFFfluidcontrolofasinglechannel.Thismodeof opera-tionisanalogoustosinglepolesinglethrow(SPST)typeelectrical switches.Asasecondmodeofoperation,theplugmicrovalvecan beusedformultipleintersectingchannelsasshowninFig.6.In this configuration thevalve turns onone straight channel at a timewhileclosingtheothers.Fig.6showsaPDMS/PDMS microflu-idicdevicedemonstratingthisfeature.Threestraightchannelsof 300␮m-widthand100␮m-heightwerefabricated,whichintersect atasinglejunction.A1.7mmroddiametervalvewith600␮mhole isplacedattheintersection.Eachchannelisturnedonsequentially andfilledwithDIsolutionsofdifferentcolors.

Inanothernovelconfiguration,theplugmicrovalveisusedto controlplumbingforsingleinletmultipleoutletdesigns.InFig.7,a PDMS/PDMSdevicewithasingleinletandtwooutletsareshown. Themicrochannelwidthandheightare300and100␮m, respec-tively.Thechannelbifurcationisdesignedsothattheinletchannel firstopenstoawidereservoir(3mm×1mm)whichthensplitsinto twochannels.Themicrovalveisplacedatthisintersection.Asseen inFig.7,suchaconfigurationprovidesthreedifferentstates.First, thetwochannelsarekeptOFF,thentheyareturnedON sequen-tiallyallowingfluidflowwhichisindicatedbyfluidbulgingoutat thecorrespondingoutlets.Thismodeofoperationresemblesthe singlepoledoublethrow(SPDT)typeelectricalswitcheswithan OFFstateinbetween,whichisreferredassinglepolecenteroff (SPCO)configuration.

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Fig.7. Theuseoftheplugmicrovalveforbifurcatingjunctions.(a)Valveclosesboth channels.(b)and(c)OneofthetwooutletchannelsisturnedON.

4. Applicationsanddiscussion

Todemonstratethepracticalityofthevalveswehavedesigned three exemplary applications: a ﬂow focusing device, a micro-droplet sorting device and a portable TNT detection device. Hydrodynamic focusing, droplet-based systems and portable biosensors/detectorsarewidelyusedacrossseveralmicroﬂuidic applications.Theplugmicrovalvesprovidekeyfeaturesinallthese applicationsaspresentedinthissection.

First, we fabricated a 2D hydrodynamic flow focusing PDMS/PDMSdevicewith50␮mdepth.Weusedreddyesolution tosheathfocusyellowdyesolutionasshowninFig.8.Thewidth ofsheathflowchannelis100␮m,thewidthofthemainchannel is200␮m.Weintegratedtwoplugvalveswith600␮mhole dia-mater.Fig.8ashowsthevalvesattheirOFFposition.Forclarityof thechannels,firstthechipisfilledwithreddyesolutionwhenthe valvesareON.Then,thesystemistestedfortheotherthreestates ofthetwovalves.Thecorrespondingmicroscopeimagesforeach stateareshowninFig.8b–d.ThevalvescanbeswitchedONorOFF withoutanyneedforstoppingtheinletflows.Thisallowsrapid enablingordisablingofhydrodynamicfocusingforquickly switch-ingfromfocusedtounfocusedstate.Wehavealsotestedthepartial closureofthevalves.Sinceitischallengingtocontroltheposition ofthevalveforsuchanoperation,itisbesttousethepresented plugmicrovalvesinthebinarymode,i.e.asONorOFFvalves.

Foramicrodroplet-basedfluidicsystem,weusedthevalvein adualchannelconfigurationasshowninFig.9.Inthisway,we wereabletosortmicrodropletsataT-junctionwhichispreviously shownusingcomplicatedhydrodynamiccontroloractivesorting methods[18,19].Wedesignedadropletmicrofluidicdevicewitha

T-junctiongeneratorandaT-junctionsorter.Twochannels down-streamthesortingjunctionarecrossedtogether.Aplugmicrovalve isplacedtothisintersectionasshowninFig.9a,b.Inthis conﬁg-uration,thevalveopensonechannelwhileclosingtheotherone whichcausesthemicrodropletstobesortedrightorleftatthe sort-ingjunction(Fig.9c,d).Fortheseexperimentsweformedwater dropletsin100mPa.ssiliconeoilusinginletpressuresof210mbar foroiland200mbarforwater.Suchaconﬁgurationcanbeused whenmicrodropletsarerequiredtogothroughdifferent opera-tionsforapplicationssuchasbiochemicalsynthesisorsinglecell studies[20].

Finally, we exploited the microvalves in a portable colori-metricassayfor rapidtrinitrotoluene(TNT) moleculedetection, whichiscriticalforenvironmentalsafetymonitoring.Weused sil-icananoparticleswhichweresynthesizedviasol-gelmethod,as theassayreagent.Silicananoparticlesweremadefluorescentby loadinganaromaticdyecalledpyrenewhichshowssensitive fluo-rescencequenchingtowardsTNT[21].APDMSdevicewasdesigned including a serpentine test channel, a straight control channel (300␮mwidth,100␮mheight)andtworeservoirs(6mm×2mm) forqualitativeobservation(Fig.10a).Plugmicrovalveswereplaced afterthereservoirstocontroltheflowalongthecontrolandtest channels.Flowisgeneratedbyapplyingnegativepressureatthe sharedoutlet using a 0.5ml syringe. Aqueoussolutionof silica nanoparticleswasintroducedtoinlet1andTNTsolution(0.25mM) wasintroducedtoinlet2.Upontheapplicationofnegative pres-sure,thevalveswereturnedONsequentially.Intheserpentinetest channel,TNTsolutionwasmixedwithnanoparticlesusing pas-sivediffusionmixingand filled thetestreservoir whereas bare nanoparticlesfollowedthestraightchannelandfilledthecontrol reservoir.ThetestruncycleisshowninSupplementarymovie3. Then,thevalveswereturnedOFFandthedevicewasimagedunder apalm-sizeUVillumination(366nm)boxforfluorescentdetection. ThecontrolreservoirgaveabrightbluecolorunderUVexcitation, whereasthetestreservoirwaspalebluesincefluorescenceofsilica nanoparticleswasquenchedbyTNTmolecules(Fig.10b)[22].

Asacontrolexperiment,thesameprocedureisrepeatedwith nanoparticle solutionreplaced withwater at inlet2. Mixing of nanoparticleswithwaterindicatednoinfluenceofdilutionon flu-orescencesignalasshown inFig.10c.Quantitativeanalysiscan beperformedusingthecolorimetricsignalsinthereservoirs.To dothis,weconvertedthecolorimetricsignalsobtainedintheUV channelintorelativeintensityvaluesusingImageJ.54.5%ofthe fluorescenceof silicananoparticleswere foundtobequenched by 0.25mM TNT based onthe differencebetween the intensi-tiesofcontrolandtestreservoirs(SupplementaryFig.1).Onthe otherhand, 8.9%intensitydecrease wasobserved upon mixing withwater.Thissimpledevicedemonstratesthepotentialofplug microvalvetobeusedforportablemicrofluidicdetectiondevices whichhavenumerousenvironmentalandhealthcareapplications [23,24].

TheTNTdetectiondevicehighlightsanothercriticalfeatureof theplugmicrovalve.Thevalvesdonotrequireanyoff-chip com-ponents,therefore,aftertheassayisperformedthevalvescanbe turnedOFFandthedevice canbeeasilytransferredtoanother stagesuchasintoachamberfortightlycontrolledincubationor amicroscopeforimaging.TheTNTdetectionassayshowninthis workis a very promising exampledemonstrating thepotential of theplug microvalve tobe extended for various microﬂuidic colorimetricassayswherelowcostandportablesingleusedevices areneeded.

5. Conclusions

In this study,we present a plug microvalvefor microﬂuidic devices.Thevalvecanbeeasilyintegratedtomicroﬂuidicsystems

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Fig.8.(a)PDMSﬂowfocusingdevicewithtwoplugmicrovalves.(b–d)Threedifferentfocusingstatesachievedbydifferingpositionsofthevalves.Theﬂuidsweresupplied at200mbarinletpressure.

Fig.9.(a)PDMSdropletgenerationandsortingdevicewithasingleintegratedplugmicrovalve.Thechannelwidthandheightare400␮mand200␮m,respectively.(b) Schematicdrawingdemonstratingtheoperationofthevalve.Microdropletsarebeingsortedtothe(c)rightor(d)leftattheT-junction.

withoutanyneedforadditionalmolddesign.Ittakeslessthan5min tofabricateamicrovalveandapplyittoachannel.Wehave demon-stratedthatthesevalvescanbeusedforbothelasticPDMSchannels andrigidthermoplasticchannels.Theplugmicrovalveisextremely low-costandsuitedverywellfordevicesthatrequireafewvalves onamicrofluidicdevice.Theplugmicrovalveisalsoreusableand doesnotrequireanyoff-chipcomponents.Thevalveproved suc-cessfulforpressuresupto2barandhavealongcycletime.The plugmicrovalvecannotonlybeusedforflowcontrolacrossasingle microchannel,butcanalsobeutilizedtocontrolfluidflowat multi-channeljunctions.Todemonstratetheversatilityofthevalves,we usedtheplugmicrovalvestocontrolflowfocusing,microdroplet sortingandportablechemicalagentdetectioninthreeexemplary

microﬂuidic systems.Thesimplicityof thesevalvesmakethem goodcandidatesforavarietyofapplications.

Acknowledgments

ThisprojectwaspartiallysupportedbyEuropeanUnionFP7 Marie CurieCareer IntegrationGrant(no. 322019).P.B.is sup-portedbyTUBITAK-BIDEBgraduatefellowship.Theauthorsalso thankMuratSerhatliogluforhishelpontheﬁgures,ZiyaIsiksacan andResulSaritasforcriticalreadingofthemanuscript.

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Fig.10.(a)Plugmicrovalve-integrateddeviceforrapidandportablequalitativeTNTdetection.FluorescentsilicananoparticleandTNTsolutionwereintroducedatthetwo inlets.MixingofTNTandnanoparticlesolutionsleadtoﬂuorescencequenchinginthetestchannnel.(b)DevicetestedwithTNTsolutionunderUVlight(366nm)excitation. (c)ControlassayrunwithwaterinsteadofTNTsolution.

AppendixA. Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound,in theonlineversion,athttp://dx.doi.org/10.1016/j.sna.2017.09.001.

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Biographies

MustafaTahsinGulerreceivedhisB.Sc.degreeinPhysicsfromGaziUniversityand hisPh.D.degreefromKirikkaleUniversityPhysics.Since2015,hehasbeenworking asapost-doctoralfellowatBilkentUniversity,NationalNanotechnologyResearch Center–UNAM.Hisresearchinterestsincluderapidmicrofabricationandintegrated microﬂuidicsystems.

PinarBeyazkilicreceivedherB.Sc.degreeinChemicalEngineeringfromHacettepe University,Turkeyin2011andMScdegreeintheDepartmentofMaterials Sci-enceandNanotechnologyin2013.ShepursuesherPh.D.degreeinDepartmentof MaterialsScienceandNanotechnologyatNationalNanotechnologyResearch Cen-ter–UNAMatBilkentUniversity,Ankara,Turkey.Herresearchinterestsinclude functionalsurfaces,nanomaterialsandbiochemicalassays.

CaglarElbukenreceivedhisB.Sc.degreeinElectricalandElectronicsEngineering fromBilkentUniversity,Turkeyin2004andhisPh.D.degreeinMechanicaland MechatronicsEngineeringfromUniversityofWaterloo,Canadain2008.Heworked asapostdoctoralassociateattheWaterlooMicroﬂuidicsLaboratorybeforejoining toAbbottPoint-of-CareasaseniorR&Dscientist.Later,hejoinedKocUniversity asaresearchassistantprofessor.Since2012,heisworkingatBilkentUniversity, NationalNanotechnologyResearchCenterasanassistantprofessor.Hisresearch interestsincludelab-on-a-chipdevices,microdroplet-basedmicroﬂuidicsystems andsensingtechnologiesforportableapplications.