Electrochemical actuator with a short response time: A new actuation regime

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ContentslistsavailableatScienceDirect

Sensors

and

Actuators

A:

Physical

jo u r n al hom e p 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

Electrochemical

actuator

with

a

short

response

time:

A

new

actuation

regime

Vitaly

B. Svetovoy

a,b,∗

_,

_Ilia

_V.

_Uvarov

a

_,

_Alexander

_V.

_Postnikov

a

_,

_Remco

_G.P.

_Sanders

b

_,

Gijs

Krijnen

b

a_Yaroslavl_Branch_of_the_Institute_of_Physics_and_Technology,_Russian_Academy_of_Sciences,₁₅₀₀₀₇_Yaroslavl,_Russia b_MESA+_Institute_for_{Nanotechnology,}_University_of_Twente,_PO_217,₇₅₀₀_AE_Enschede,_The_Netherlands

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received21December2015

Receivedinrevisedform26February2016 Accepted1March2016

Availableonline10March2016 Keywords: Actuators Electrolysis Bubbles Microsystems

a

b

s

t

r

a

c

t

Thelackoffastandstrongmicroactuatorsisawell-recognizedproblemintheMEMScommunity. Electro-chemicalactuatorscandevelophighpressurebuttheyarenotoriouslyslow.Waterelectrolysisproduced byshortvoltagepulsesofalternatingpolaritycanovercometheproblemofslowgasterminationdue tospontaneousignitionofthereactionbetweenhydrogenandoxygeninnanobubbles.Anactuation regimewiththeterminationtimeasshortas100␮swasdemonstratedpreviously.Herewedescribea newactuationregime,forwhichthegaspressureisrelaxedjustin10␮sandaminimaldegradationof theelectrodesisobserved.Theactuatorconsistsofamicrochamberfilledwithanelectrolyteand cov-eredwithaflexiblesiliconnitridemembrane.Themembranebendsoutwardwhenthepressureinthe chamberincreases.Thenewregimeischaracterizedbytheappearanceofshort-livedmicrobubblesin betweentheelectrodes.Fastterminationofgasandhighpressuredevelopedinthechamberarerelated toahighdensityofnanobubblesinthechamber.Thephysicalprocesseshappeninginthechamberare discussedaswellasproblemsthathavetoberesolvedforpracticalapplicationsofthisactuationregime. Theactuatorcanbeusedasadrivingengineformicrofluidics.

1. Introduction

Thelastdecenniumhaswitnessedanimpressivetrendto minia-turizesystemsofvirtuallyanykind.Thistrendhasmanyreasons: smallsystemsareoftencheapertoproduce,theycanhave proper-tieslargesystemshavenot,andtheymayfacilitateuseoflarge systems (cars,for example). Animportant and generic compo-nentinmicrosystemsistheactuator.Itplaystheroleofamotor transforming electricity or other kindof energy into mechani-calmotion.Incontrastwithlargescalesystems,whereeffective engines are available (internal combustion or electromagnetic motors),microsystemssufferfromthelackofstrongandfast actu-ators[1,2].Smallelectromagneticmotorscannotgenerateforces ofusefulmagnitudeduetounfavorablyscalingofcoils,and inter-nalcombustionengines[3]performpoorlyduetoincreasedheat lossesviathevolumeboundary[4,5]whenthevolumedecreases. Existingmicroactuatorsareusingmostlytwo typesofforces [1,2,6,7]:electrostaticforces,whichareweak,andthosegenerated

∗ Correspondingauthorat:MESA+_Institute_for_{Nanotechnology,}_University_of

Twente,PO217,7500AEEnschede,TheNetherlands. E-mailaddress:v.svetovoy@utwente.nl(V.B.Svetovoy).

bythermalexpansion,whichareslow.Fastandstrongpiezoelectric elementsaredifﬁculttocombinewithmicrotechnology,theyneed ahighvoltageforactuation,andhaveasmallstroke.Electroactive polymersarepromisingintheﬁeldofrobotics[8–11]buttheyare notwellsuitedforanumberofapplicationsinmicrosystems. Actu-atorsbasedontheelectrochemicaldecompositionofwaterwere alsodiscussedinmanypapers[12–19]buttheyarenotoriously slow.Onecanproducealargeamountofgasinashorttimebutit isimpossibletogetridofthisgasfastaswell.

Electrochemicaldecompositionofwaterisawell-known pro-cessbutelectrolysisperformedinmicrosystemsonashort-time scalebroughtunexpectedsurprises[20] (asarecent reviewsee [21]).Itwasfoundthatthelocalcurrentdensitycanbethreeorders ofmagnitudelargerthanthatforthenormallong-time(>1ms) elec-trolysis.Thelocalconcentrationofgasin theshort-timeregime (1–100␮s)wasmorethan1000timeslargerthanthesaturated concentrationofgasatnormalconditions(therelative supersatu-rationS>1000).Undertheseconditionsnucleationofbubblesmust happenhomogeneously,whichwasindeedobserved[22].Applying potentialwithfastchangesofpolarity(>20kHz)visibleproduction ofgasdisappearedbutthecurrentviatheelectrolytepractically didnotchange.Anumberofeffectsindicatedthatthegas disap-pearanceisrelatedtothereactionbetweenhydrogenandoxygen http://dx.doi.org/10.1016/j.sna.2016.03.002

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[20]althoughlargeenergyproducedintheprocessmanifestsitself inanumberofmeasurableeffects.Combustionreactionsinsideof smallvolumesisanadditionalmysteryofnanobubbles.Theother well-knownproblemistheobservedunexpectedlylongstability ofsurface[25]andbulk[26]nanobubblesﬁlledinwiththegases, whicharenotabletoreact.

Inspiteofpoorunderstandingofthereactionmechanismthe discoveredphenomenoncanbeusedtobuildafastandstrong actu-ator[27].Thisactuatorconsistsofamicrochambercoveredwitha ﬂexiblemembraneandisﬁlledwithanelectrolyte.The electroly-sisisobtainedbyshortvoltagepulsesofalternatingpolarity.The pressureinthechamberincreasesandbulgesthemembrane,but novisiblebubblesareformed.Thisisbecausemostofthegasis packedinnanobubbles,whichdonotscatterlight.Whenthepulses areswitchedoff,pressurerelaxationtakesplaceinlessthan100␮s orso.Theactuatorcannotbecomparedtotheelectrochemical actu-atorsusingwaterelectrolysis,forwhichtherelaxationtimescale isminutes[19].

Inthispaperwedescribeanewactuationregime,forwhicha highpressurecanbereachedwhilepressurerelaxationtakesplace inlessthan10␮s.Moreover,incontrastwiththeregimedescribed in[27]thedegradationofelectrodesismuchreduced.

2. Experimental

We didexperiments with devices (see Fig. 1) fabricatedon Siwafers coveredwithalayerofsilicon nitride(530nmthick). Platinumelectrodesweredepositedontopofthislayer. Under-neathoftheelectrodesthereisaheatsensormadeofpolysilicon. Within the chamber area the nitride was released by etching theSiwaferfromthebacksidesothatthenitridelayerplayed theroleofamembrane. Thechamberand fillingchannelswere isotropicallyetchedinborofloatglass.Thesiliconandglasswafers wereanodicallybonded.Nominaldimensionsofthechamberare 100_␮m_×100_␮m_×5_␮m.Thedetailsofthedesignandfabrication werereportedearlier[27].Thechamberwasfilledviathe chan-nelwith1MsolutionofNa2SO4indeionizedwater.Thein/outlet openingsofthechannelweresealedafterthefilling.

Squarevoltagepulsesofalternatingpolaritywereappliedto theelectrodesatfrequenciesf∼100kHz.Togethighcurrentswe usetheelectrochemicalcellintheohmicregimeapplyingvoltage

nelsoftheinstrument.Themembranedeﬂectiondwascalibrated byapplyingastaticgaspressure,givingP=2.03d+0.27d3 _[27]_, wherePistheoverpressureinbarsanddisin␮m.Notethat theresonancefrequencyofthemembraneisestimatedashighas 0.7MHzsothatthisexpressioncanbeappliedforthefrequencies usedinourexperiments.

3. Results

Normalactuationofthedevicewasdescribedinref.[27].Itwas demonstratedthatthepressureinthechambercanbeashighas P=4.6barandthetimeforpressurerelaxationcanbeasshortas 100␮s.Theactuatorworkswellatfrequenciesf>20kHz.Athigh frequenciesverylittlegasisvisibleinthechamberasonecanseein Fig.2(a)and(b),whichcorrespondtof=150and200kHz, respec-tively.Bothimagesweremadeatthetimemomentt=400␮sand theprocessranatavoltageamplitudeU=8V.Atlowerfrequencies theamountofvisiblegasincreasesandbelow20kHzthechamber becomescompletelyﬁlledwithgasandactuationbecomes impos-sible.Asmallamountofvisiblegasinthechamberexistsintheform ofmicrobubbleslocatedabovetheelectrodes.Thenewactuation regimemanifestsitselfwhentheprocessisrunlonger.Inthiscase afaintcontrastappearsinbetweentheelectrodes.Forexample att=600␮sitcanbeseeninFig.2(c)and(d),whichalso corre-spondtof=150and200kHz,respectively.Thiscontrastresembles ratherlargemicrobubbles(10–20␮mindiameter),whichappear outoffocusduetomotionblur.Thesebubblesappearinthe cham-berjustforafewmicrosecondsandareaccompaniedbysigniﬁcant pressurejumpsinthechamber(seebelow,Fig.3(b)).

Atypicalresponseofthemembraneontheelectricalpulseswith amplitudeU=9Vatfrequencyf=100kHzisshowninFig.3(a).Well visibleoscillationsaresuperimposedonthemonotonously increas-ingmembranedeﬂection.Theseoscillationsareinphasewiththe drivingpulses.Aswasexplainedin[27]theyarerelatedtothe reac-tionhappeninginnanobubblescontainingastoichiometricmixture ofH2andO2gases.Thepressurerespondstobothpositiveand neg-ativehalvesofthepulsesbuttheresponseisasymmetricingeneral. Themonotonousdeﬂectionofthemembraneisduetounburned gas.Forexample,ifabubblecontainsonlyhydrogenoronlyoxygen thereactiondoesnothappenandsuchabubblewillcontributeto thepressureincreaseinthechamber.Thisgasisalsocollectedin

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Fig.2. (a)and(b)Stroboscopicsnapshotsofthechamberatt=400␮sforafrequencyofdrivingpulsesf=150and200kHz,respectively.Thearrowshowsapinnedbubble, whichexistedbeforetheprocessstarted.Theseimagesshowthevisiblesituationinthechamberduringnormalactuation.(c)and(d)Thesameparametersofdrivingpulses buttheimagesweremadeatt=600␮s.Someofthebubblesthatappearinthechamberforaveryshorttimearezoomedintheinsetsandﬁttedwithcirclesforbetter visibility.Appearanceoftheshort-livedbubblesisanindicationofthenewactuationregime.

nanobubbles,whichdonotscatterlightandcannotbeobservedin visiblelight.Thegasvolumethatisvisibleinthechamberas,for example,inFig.2(a)and(b)cannotberesponsiblefortheobserved significantpressureincrease.Whenthedrivingpulsesareswitched offthepressuredropsdownevenfasterthanitwasbuildup.Itis explainedbymergingofnon-stoichiometricnanobubbles[29,21]. Thebubbleformedinthiswayonaveragecontainsthe stoichio-metricmixtureofgases.Thisbubbleisignitedspontaneouslyand disappearsdecreasingthepressure.Thetimescaleforthemerging isdefinedbytheconcentrationofnon-stoichiometricnanobubbles. Forverylargesupersaturations(S>1000)existinginthesystem thenanobubblesareformednearlyhomogeneously[20]. Accord-ingtoclassicalnucleationtheory[30] ahighenergybarrier for bubblenucleationisstronglyreducedatlargeSbutisstillan activa-tionprocess.Forthisreasonthenucleationrateisverysensitiveto theexternalorinternaltemperatureincrease.Thenucleationrate explainsalsothesensitivityofpressureinthechambertothe tem-perature.Particularly,thechangeoftheslopeofthemembrane deflectioninFig.3(a)isexplainedbytheinternalheatingofthe electrolytebytheheatproducedinthereactionofwater forma-tion.Sensitivitytotheexternaltemperaturewasdemonstratedin [27].

Ifthesameprocessisrunathigherfrequency,forexample,at f=150kHzasinFig.3(b),thepressureﬁrstincreasesslowerbut thensuddenlyjumpstohighvaluesforaveryshorttime.These pressurejumpsmarkatransitiontothenewregime.Aswasalready explainedthesejumpsarerelatedtotheshort-livedmicrobubbles

appearinginthechamber.Thejumpsneverappearimmediately afterswitchingonthecurrent.Theremustbeanincubationperiod beforethejumpscanappear.Theincubationtimedecreaseswith increasingvoltageamplitudeanddecreasesstronglywith temper-aturerise,evenforsmallchanges(∼10◦_C)._We_did_not_observe_the pressurejumpsatfrequenciesbelow100kHz.Ontheotherhand, theaverageamplitudeofthejumpsincreaseswithfrequencyabove 100kHz.Athighfrequenciesthejumpscanbesoviolentthatthey breakthemembrane.Fig.4showsoneoftheseevents.Theprocess ranatextremeparametersU=10Vandf=500kHz.Inthemoment t≈560␮sthemembraneburstasthevibrometersignalinpanel(c) demonstrates.Inthesamemomentthecurrentinthesystemstops asonecanseeinpanel(d).

Iftheprocessrunsforasufﬁcientlylongtime,thepressurein thechamberreachesasaturationpointonaveragebutcontinues toﬂuctuatearoundthispoint. Thissituationisdemonstratedin Fig.5(a)fortheamplitudeU=8Vandfrequencyf=300kHz.A typ-icalpressurejumpisshowninFig.5(b).Onecanseethatthejumps areveryshortandhighlyenergeticevents.Thewidthofaseparate peakisestimatedas3␮s.Theenergyreleasedinthepressurejump isıE=ıPVch≈3.5nJ,whereıP≈0.7bar(asfollowsfromFig.5(b)) istheamplitudeofthejumpandVch=5×104␮m3isthevolume ofthechamber.Sincethejumpisrelatedtotheappearanceand termination ofoneshort-livedbubble witha sizeofD∼10␮m, the pressure change inside of this bubble can beestimated as Pb=ıE/Vb∼100bar, where Vb=D2h/4 is bubble volume and h=5␮misthechamberheight.Theonlyprocesswhichisableto

Fig.3. (a)Normalactuationregime.Membranedeflectionasafunctionoftimeispresentedasmeasuredwiththevibrometer.Thefrequencyofthedrivingpulsesisf=100kHz. Theinsetshowsadetailedviewaroundthepointofmaximaldeflection.(b)Transitiontothenewactuationregimeathigherfrequencyf=150kHz.Highandshortpeaksare acharacteristicfeatureofthisregime.ThedeflectionwasmeasuredatroomtemperatureT=21◦C.

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Fig.4. Breakingofthemembrane.(a)Chamberbeforetheprocess.Thedirtiscollectedduringtheprevioususeofthesampleinthenormalactuationregime.(b)Chamber aftertheprocess.(c)Rawsignalofthevibrometer(velocity)showsthemomentwhenthemembranewentoff.(d)Thecurrentinthesystemaroundthemomentofthe breaking. 0 200 400 600 800 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 time (µs) deflection ( µ m) 445 449 453 0.3 0.4 0.5 0.6 time (µs) deflection ( µ m) a b

Fig.5.(a)Pressureﬂuctuationsinthesaturatedregime.Theprocessisdrivenat U=8Vandf=300kHz.(b)Zoomedviewofoneseparatepeak.

providethisenergyscaleisthecombustionbetweenhydrogenand oxygeninsideoftheshort-livedbubbles.Weexpectthatthese bub-blesappearinthechamberduetomergingofnon-stoichiometric nanobubbleswhen thedensityofnanobubbles becomes critical [29,21].Theformedmicrobubblecontainsthestoichiometric mix-tureofgases,whichareabletoreact.Thereasonfortheignitionof thereactionisstillnotknown.However,therearenodoubtsthat

thereactionhappens[27]becauseahugeamountofgasproduced bytheFaradaycurrentdoesnotshowupinthechamber.

Themost prominentfeature ofthe newactuationregime is thetimeforthepressurerelaxationwhenthedrivingpulsesare switched off. As one cansee in Fig. 5(a)it happens extremely fastjustin10␮sorso.Duetoaveryhighconcentrationof non-stoichiometricnanobubbles they merge in shorter time than it happens in the regime without pressure fluctuations. Actually, thepressurerises muchslowerthanit isgoingdown.Thisis a veryimportantpropertyforthefastactuation.Fig.6demonstrates actuationofthemembranebyseriesofpulsesperformedintwo differentregimesofactuation.Panel(a)showsthedeflectionofthe membraneinthenormalregime[27](withoutfluctuations).First seriesofpulseslastsfor200␮sandthesecond100␮s-longseries followsaftera100␮spause.Onecanseethattherelaxationtimeis oftheorderof100␮sandthatthesecondseriesgiveslarger deflec-tionofthemembranethanthefirstone.Ithappensbecausethefirst seriesheatsupthesystem.Panel(b)showstheactuationinthe regimewiththepressurefluctuations.Thefirstseriesofpulsesis 400␮slong.Itislongerthantheothertwoseries,whichare200␮s longeachandfollowaftera200␮spause,becauseoftheincubation period.Inthiscasethepressurerelaxesforatimeof∼10␮s.This isthefirstexampleofactuationinthenewregime.Forquiteastiff SiNmembranethestrokeisaround0.5␮mandoverpressureinthe

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Fig.6. Actuationofthemembranebyseriesofpulsesseparatedbytimegaps.(a)Actuationinthenormalregime.Thedeﬂectioninducedbythesecondseriesislargerdue tointernalheating.TheprocessisdrivenatU=9Vandf=100kHz.(b)Actuationintheﬂuctuatingregime.Threeseriesofpulsesseparatedby200␮spausesarepresented. ThedrivingpulseshavetheamplitudeU=8Vandfrequencyf=150kHz.

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chamberisaround1bar.Theresponsetimeislimitedbythe pro-ductionofgasbutthegasterminationinthecombustionreaction happensmuchfaster.

4. Discussion

Longincubationperiodsgivea restrictiononthefast perfor-manceoftheactuator.Thephysicalreasonforthisdelaytimeis thatonehastoproduceasigniﬁcantconcentrationofnanobubbles beforetheystarttomerge.Thisincubationtimeisverysensitiveto thetemperatureandcanbeeasilyreducedafewtimesby increas-ingtemperatureby10–20◦C.Itcanbedone,forexample,witha heatingelement,whichkeepstheactuatorataslightlyelevated temperature.

Fluctuationsofpressureinthechamberarenotfavorablefor smoothactuation.Thisproblemcanbesolvedbytheproperdesign of the electrodes. The size of the short-lived microbubbles is relatedtothedistancebetweentheelectrodes.Makingthis dis-tancesmalleranddistributingelectrodesmorehomogeneouslyin thechamberwecansmoothouttheﬂuctuations.Thechoiceof workingfrequencyisalsoanimportantparameterinthisrespect.

Combustionofhydrogenandoxygenisahighlyexothermic pro-cess.Forthisreasonthereactionbetweengasesinnanobubbles, whichareformedincloseproximitytotheelectrodes,has signifi-cantinfluenceontheelectrodesresultingintheirdegradationwith time.Theeffectisespeciallystrongatrelativelylowfrequencies 20–50kHz[20]andcanbeseeninFig.4(a).Whenthe combus-tionhappensinmicrobubblesonecouldexpectfasterdegradation oftheelectrodes.However,wedidnotobservethiseffectinthe newactuationregime.Thisisbecausemostof thegasreactsin microbubbleslocatedinbetweentheelectrodes.Inthiscasehighly energeticeventsofbubbleterminationdonotharmtheelectrodes. Also,inthisregimemuchhigherfrequenciesareusedtogeta sim-ilarstrokeofthemembrane.Thehigherthefrequencythesmaller isthesizeofstoichiometricnanobubblesandtheweakeristhe influenceontheelectrodes.Nevertheless,long-timestabilityofthe electrodeswasnotanalyzedyet.Materialfortheelectrodesisalso animportantissue.Itwasestablishedthatelectrodewear corre-lateswiththematerial’syieldstrength[20].Theweakesteffectwas observedfortungstenandthestrongestoneforgold.Uptonowwe testedtheactuatorsonlywithplatinumelectrodes.

Oneofthemostimportantactuationparametersisthestroke. Thecurrentversionoftheactuatorusesaratherstiffsiliconnitride membranes.Forthismembrane,withasizeof100␮m×100␮m thestrokethathasbeenreachedis∼1␮m.Itcanbeincreased fur-therusingsoftermaterialforthemembrane.Aconvenientmaterial inthisrespectisSU8.IthasaYoung’smodulus100timessmaller thanthatforSiNanditsthicknesscanbeeasilychangedinawide rangetoprogramadesiredstroke.Thestrokecanalsobeincreased duetohigher pressurein thechamber.It ispossibletodo this byincreasingtheFaradaycurrent,forexample,bydecreasingthe distancebetweentheelectrodesandincreasingtheirarea.

Duringoperationoftheactuatoroneorafewpinnedbubbles oftenappearinthechamberasonecanseeinallfourimagesin Fig.2.Inmostcasespinninghappensattheedgeofthechamberat structuralinhomogeneities.Sometimesapinnedbubblecangrow duringoperation.In thiscaseit becomesa realproblem, which preventsnormalfunctioningofthedevice.Forpracticalapplication oftheactuatoronehastocontrolthepinning.

Summarizing, the fast electrochemical microactuator has a numberofveryattractivefeatures.Ithasarelativelylargestroke even for a stiff SiN membrane, it needs rather low actuation voltage,ithasashortresponsetimeandcandevelopahigh pres-sure.Finally,theactuatoriscompletelycompatiblewithstandard microtechnological processes. The main disadvantage is a low

efficiencyoftheactuator.Onlypartofthegasproduced electro-chemicallydoesausefulwork;asignificantpartofthegasisburned instoichiometricnanobubbleswithoutproductionofthe mechan-icalwork.Forexample,thetotalpowerconsumptioninFig.6(b) correspondsto25mWwhilethepoweravailablefromthe actua-torisjust1␮W.Ofcourse,thereisplentyofroomforoptimization ofthedevice.Significantimprovementisexpectedfromthe reduc-tionoftheelectroderesistanceandfromaslightincreaseofthe workingtemperature.

5. Conclusions

Wepresentedhereanewactuationregimeofthe electrochem-icalactuatorthatis drivenbyshortalternatingpolarity voltage pulses.Extremelyshort(∼10␮s)terminationtimeoftheproduced gascanbereachedinthisregime.Thisfastterminationofgasis possibleduetoaveryhighdensityofnanobubblesinthe cham-berof theactuatorasexplainedin [29,21].Thehighdensityof nanobubblesisalsoresponsibleforthehighpressuredeveloped bytheactuator.Theresponsetimeislimitedbytherateofthegas productionbutnotbythegasterminationtimeasonecouldexpect foranelectrochemicalactuator.Theotherattractivefeatureofthe newactuationregimeisaminordegradationoftheelectrodes.We discussedproblemsandpossiblewaystoimproveperformanceof theactuator.

Acknowledgements

ThisworkissupportedbytheRussianScienceFoundation(grant 15-19-20003) and by the Dutch Technology Foundation (grant 13595).

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1985to2003hehasbeenwiththeInstituteof Micro-electronics,RussianAcademyofSciencesworkingonthe physicalbasesofmicrotechnologies.Hereceivedhis doc-toraldegreein1985intheoreticalandmathematicalphysics.

IliaV.UvarovwasborninYaroslavl(Russia)in1988.He receivedamaster’sdegreewithhonorsinradiophysics andelectronicsin2010fromYaroslavlStateUniversity. Since2009heisworkingontheYaroslavlBranchofthe InstituteofPhysicsandTechnology,RASintheLaboratory ofMicro-andNanosystemTechnologyasaresearcher.He receivedhisdoctoraldegreein2013inmicro-and nano-electronicsfromInstituteofPhysicsandTechnology,RAS. HisresearchinterestsareinMEMSsensorsandactuators andinmicroﬂuidics.

AlexanderV.PostnikovisaseniorresearcherinYaroslavl BranchoftheInstituteofPhysicsandTechnology, Rus-sianAcademyofSciences,whereheisworkingsince1989 aftergraduationinMoscowStateUniversity.Hisscientiﬁc interestsareinthelaserscanningmicroscopy, microsys-tems,andinmeasurementtechniquesformicrodevices.

GijsKrijnen,prof.dr.ir.,headstheTransducersScience &Technology(TST)ChairintheElectricalEngineering departmentoftheUniversityofTwente(since2011).His currentinterestsareinLifelike(MEMS)transducersin general,bio-mimeticﬂow-sensorsinparticular, paramet-ricsensingschemesandadditivemanufacturing.In2005 hewasawardedaVICIgrantbytheNetherlands Orga-nization forScientiﬁcResearchfora5-years program onLifelikeMEMS-sensors(BioEARS).Since1998hehas beenintheTSTgroupoftheMESA+_research_institute

fornanotechnologyandresponsibleformicro-actuator research.From1995to1997heworkedonintegrated opticdevicessimultaneouslyattheUniversityofTwente andtheDelftUniversityofTechnology.From1992to1995hewasafellowofthe RoyalNetherlandsAcademyofArtsandSciencesandstudiedsecond-and third-ordernon-linearintegratedopticsdevicesafterhereceivedthedoctoratedegree withhonorsfromUniversityofTwentein1992onthesubjectofnonlinear inte-gratedopticsdevices.Prof.Krijnenhas(co-)authoredover100refereedjournal papers,10bookchaptersand230conferencecontributions.