Micro-tweezers : design, fabrication, simulation and testing of a pneumatically actuated micro-gripper for micromanipulation and microtactile sensing

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

Micro-tweezers:

Design,

fabrication,

simulation

and

testing

of

a

pneumatically

actuated

micro-gripper

for

micromanipulation

and

microtactile

sensing

夽

A.F.

Alogla,

F.

Amalou,

C.

Balmer,

P.

Scanlan,

W.

Shu

,

R.L.

Reuben

SchoolofEngineeringandPhysicalSciences,Heriot-WattUniversity,Edinburgh,UnitedKingdom

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received1November2014

Receivedinrevisedform25June2015

Accepted25June2015

Availableonline20July2015

Keywords:

Micro-gripper Pneumatic Micromanipulation

Finiteelementanalysis

Tactilesensing

a

b

s

t

r

a

c

t

Thispaperpresentsanovelmicro-gripperdesignwiththedualfunctionsofmanipulationandforce sensing.Thedeviceconsistsoftwoparallelplates,eachmountedontorsionbars,whichcanbemade torotatetowardsorawayfromeachotherbyuseofapneumatically-orhydraulically-actuatedelastic membrane.Theplatescanbeconvenientlyfabricatedusingphoto-etchingandthedesignallowsfora rangeofratiosbetweenactuationpressureandtipopeningdisplacementandforce.Theelasticgripping tipscanbedesignedtoprovidesufficientcompliancethattheirstraincanbeusedtomonitorandcontrol thegrippingforce.Anexemplardevicehasbeenfabricatedanditsbehaviourcharacterisedbyaseriesof mechanicalmeasurementsofforceanddisplacement.Thesemeasurementshavebeenrationalisedusing asimpleanalyticalmodel,backedupwithfiniteelementanalysistoemphasisethedesignvariables andscalability.Thisexemplardevice,withamaximumtipopeningamplitudeof1mmandmaximum forceoutputof50mN,hasalsobeendemonstratedtoperformpick-and-placeoperationswith200␮m micro-beads.

©2015TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).

1. Introduction

Therapid evolution in thebiological sciences hasledto an increasedrequirementformanipulatingentitiesatthemicro-and nano-scale. In general, manipulating biological objects such as singlecells, micro-beadsor evenembryoscanbeclassifiedinto contactandnon-contacttechniques.Thenon-contacttechniques are mainlyoptically-based [1–4], where a highly focused laser beamisusedtotrapandmoveabiologicalmicro-object.Although theperformanceofsuchtechniquesissatisfactory,sophisticated andexpensiveopticalsetupsarerequired [5]and theexposure toopticalradiationmayhavelongtermnegativeeffects onthe manipulatedmicro-objects[6].

Micro-pipettesandgrippersarethemostcommonlyused con-tacttechniquesformanipulatingbiologicalmicro-objects.Pipettes useanegativepressureappliedthroughanozzlebut,despiteits capability to manipulate single cells, this technique requires a highlyskilledusertoachievethemanipulationwithoutdamage tothecellsasitisnotpossibletoveryaccuratelycontroltheforces

夽 _{SelectedpaperspresentedatEUROSENSORS2014,theXXVIIIeditionofthe}

conferenceseries,Brescia,Italy,September7–10,2014.

∗Correspondingauthor.Tel.:+441314518165.

E-mailaddress:[email protected](W.Shu).

actingontheobjectbeingmanipulated.Furthermore,pipetteshave limitedversatilityand,ifcellsaresmallerthantheopeningofthe pipettenozzle,severalcanbedrawnintothepipetteratherthan isolatingjustone[7].Micro-grippersshowmuchgreater versatil-itytomanipulatearangeofobjectsizesandalsothepossibility

of controlling and measuring theforces acting on the gripped

objects.

Micro-grippersarecomplexmicro-electro-mechanical-systems (MEMS)[8]andcanbecategorizedaccordingtotheiractuation mechanism:electrostatic,shapememoryalloy(SMA),magneticor piezoelectric,eachofwhichhasitsadvantagesanddisadvantages. Electrostaticactuationgeneratesasatisfactoryamountofoutput force[9]butisdifficulttooperateinion-richliquids(e.g.bodyfluids

[10]).SMAactuators,whileproducingahighforceand displace-ment,areproblematicinliquidenvironmentsduetotheheatloss associatedwiththehighsurface-to-volumeratioofmicrodevices whichmaydamagethebiologicalsurroundings[10].Furthermore, thedisplacementsofSMAsarehardtocontrolbecauseoftheir ther-momechanical nonlinearities [11]. Piezoelectricactuation offers

highspeed and good motionresolution [12] but actuation

dis-placementsarelimited[13]andtherequiredappliedvoltagescan damagebiologicalsystems.Duetothedisadvantageousscalingof

magneticfields,magneticmicroactuatorshavelow forceoutput

andtheirperformanceisalsolimitedbecauseofthermal dissipa-tionbytheconductivematerialsandthepossibilityofsignificant

http://dx.doi.org/10.1016/j.sna.2015.06.032

A.F.Aloglaetal./SensorsandActuatorsA236(2015)394–404 395

Fig.1. Schematicdrawingsofworkingprincipleofthegripperdevice(a–c),andpictureofassembledmicro-gripper(d).

electricalleakagecurrents.Also,magneticfieldscanposeproblems inthebiologicalenvironments[14].

Fluidicmicroactuators,withtheirhighforceandpower densi-ties,havebeenwidelyadvocatedformanipulatingpreciseamounts

of liquid within microfluidic and micro total analysis systems

(␮TAS)[15,16].Theactuationcanbeeitherpneumaticorhydraulic,

and can therefore operate in liquid environments for

biologi-calcellmanipulation[17–19].Althoughtherearerelativelyfew studies of this type of actuator it has been demonstrated that

pneumaticactuators havethepotentialtoproducesomeofthe

highest force and power densities of actuation options at the

microscale[15].Highperformancepneumaticmicro-grippercan befabricatedusingmicro-stereolithography[20]orlasercutting

[21].Despitetheadvantagesofthistypeofactuator,designs typi-callyrequirecomplex3-dimensionalfabricationor3-dimensional assemblytechniquesandhencelimitedthepossibilityfor large-scaleproduction.Thispaperreportsthedesignofamicro-gripper

with a simple method of manufacture and assembly that has

the potential to scale to the level of manipulating a single

biological cell. The main design challenge is to achieve accu-ratecontrolover theopeningandclosing states ofthegripper,

along with the gripping forces, which has been addressed by

a compliant gripper design offering the opportunity for force

feedback and two-way actuationallowing opening and closing

actions.

1.1. Micro-gripperdesignandfabrication

Themicro-gripperdescribedhereispneumaticallyactuatedand isdesignedtobeoperatedinarangeofenvironments,including airandliquid,atarangeofscales.Itcomprisestwomainparts; theactuationmechanism andtheflexible gripperarms(Fig.1a andb).Theactuatorisessentiallyaflexiblemembranethatapplies forcetothegripperpadwhentheairinletispressurised.The grip-perplatewascutfromstainlesssteelsheet(50␮minthickness, GoodfellowUKLtd)usingaphoto-etchingtechnologytogivean

outerenvelope,whichwassandwichedbetweentwoPMMA(poly

methylmethacrylate)layers.Whenpressureisappliedtothepad, thearmspivotaroundatorsionalspringconsistingofabar-shaped ligamentofthegripperplate.Thearmsthemselvesaredesigned tobeflexiblesothatthegrippingforcecan,inprinciple,be mon-itoredbymeasuringthestrainatthepivotendofthearmorthe slopeatitsfreeend.Thedesigncanbeoperatedwithtwoarms

closingtogetherwhenactuated(asshowninFig.1c)orwithone armbearingontoeitherafixedplateorasamplesurface(asshown inFig.1c).Itcanalsobeactuatedinthereversedirectionsothatthe jawsareopenedbyactuatorsontheoppositesidesofthegripper platesusingthespringforceandcomplianceofthearmstoofferthe grippingforce,limited,ifnecessary,bytheairpressure.Thewhole designisscalablebycontrollingthedimensionsofthecomponents ofthegripper,inparticularthearmsandtorsionbars.

Thismicro-gripperisassembledwithmultilayers(inFig.2)of thinPMMAsheet(200␮minthickness,EMKAYPlasticsLtd),elastic siliconemembrane(50␮minthickness)anddouble-sided adhe-sivesheet(50␮m,3MUKPlc),whicharefabricatedusingaCO2

lasersystem[22].ThisdeviceisassembledbycoatingthePMMA withdoublesidedadhesiveandthenlasercuttingthelayers.These layersarethenmanipulatedbytweezersintothecorrectposition. Thewholedeviceisthencoveredinlaminatefilmandheatedto ensurenochanceofleaks.Theassemblyofthisdeviceisa sim-pleandcosteffectivemethodofmanufactureofamicro-gripper, theuseoflayersalsolendsitselftolargescaleproduction,Fig.3, andadditionallayerscanofferfurtheractuationoptions,suchas channelsforgripperclosing.

2. Performanceofprototypedevice

Tovalidatetheabove-mentioneddesignprinciples,aprototype devicewasrealisedin50␮mthickstainlesssteelsheetwithapad diameterof2mm,armsoflength3mmandbreadth0.45mm,and torsionbarsofbreadth0.3mmandlength0.625mm.The perfor-manceofthedevicewasevaluatedusingtwotests;adynamictest wherethefeedairpressurewaspulsedatarangeoffrequencies andpressuresandthetipdisplacementwithnoloadingwas mea-sured,andastatictestwheretheinputpressureandoutputload

weremeasured.

Forthedynamictest,anopticalsensorwasusedtoobtainthe deflectionatthecantilevertipusingasimilarapproachtoanAFM wherealaserbeamatafixedangleisreflectedfromtheendofthe cantileverandthepositionofthespotonaphotodetectorrecorded,

Fig.4(a).Thedeflectionwascalculatedfromthevoltageofthelaser usingthegeometryoftheexperimentalsetup[24].Deflectiondata wereacquiredusingaLabViewcontrolinterfaceatarateof300 samplespersecondforarecordlengthofaround20cycles(e.g.

Fig.2.Assemblysteps.

theelasticityofthepressurefeeddampenstepressureoscillation andfrequenciesabove50Hzareoflittlemeaning.

Therelationbetweenthedeflectionofthemicro-gripperpad andmicro-grippertipisanimportantdesignconsiderationsinceit informsthetorsionalstiffnessofthepivotbarsaswellasthetip sensitivitytogrippingforce.Thisrelationwasmeasuredusinga modificationofthedynamicset-upshowninFig.4(a),wherethe deflectionofthepadandthetipweremeasuredinseparate exper-imentsforpressurepulsesto0.17,0.35,0.56and0.76bar,allata frequencyof1Hz.Fig.5showsthemeanandstandarddeviationof theamplitudesoftipdisplacementandpaddisplacementforeach ofthepressuresplottedagainsteachother.

Forthestatictests,adigitalbalancewithread-outuptofour dec-imalplaces(ingrammes)wasusedtomeasuretheforcegenerated atthetipofthecantilever.Thetipwasbroughtintocontactwith a1.5mmdiametermetallicballmountedonalightweightconical holderasshownschematicallyinFig.6(a).

Thepressureinthefeedlinewasthenincreasedincrementally to5bar,recordingtheoutputforceatapproximately0.2,0.3,0.4,0.5 and1bar,andthereafterat1barintervals.Theresultsareshownin

Fig.6(b)forforwardandreverseincrementsofpressureupto5bar usingtwodwelltimes(10sand30s)beforetheforcewasrecorded. Thesameprocedurewasusedtodeterminetheinputforcefora

givenpressurebyapplyingthemembranedirectlytothemetal

ballandrecordingtheforceatthesameincrementsofpressure.

Fig.6(c)showstherelationshipbetweentheinputforceasdefined bytheinputpressureandtheoutputforce,determinedfromthe massexertedonthebalance,foreachofthepressureincrements.

3. Analyticalmodel

Inordertoselectappropriatedimensionsforatactilegripperof thetypepresented,itisusefultohaveasimpleanalyticalmodelfor thestiffnessesofitskeyparts.Fig.7showsasimplifiedmechanical modelofthegripperpad-armplate,withtheinputforce,theoutput reactionandthetorsionalsprings(Fig.8).

When the cantilever tip is not in contact with an object

(Routput=0),theinputforcedeterminesthereactiontorque:

T=2.35Finput,

foraninputforceinNandtorqueinNmm.Forsmalldisplacements, theangulardisplacementatthebarscanbewrittenintermsofthe dimensionsoftheplate:

= TL

KG =0.841Finput

A.F.Aloglaetal./SensorsandActuatorsA236(2015)394–404 397

Fig.4.(a)Dynamictestset-upwithopticalsensor,(b)Typicaldynamictestoutputfromopticalsensor,(c)amplitudeoftipdisplacementasfrequencyisvariedfordifferent

pulsedpressures.

whereListhelengthofthecantilever,Gistheshearmodulusand thetorsionconstantofthebars:

K=ab3

16 3 −3.36

b a

1− b4

12a4

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

Ti

p

Amp

.

(mm

)

Pad Amp. (mm)

Fig.5.Relationbetweenmeasuredpadandtipdeflectionsforpressurepulse

ampli-tudesbetween0.1and0.75bar.

and2aand2baretheshortandlongdimensionsofthetorsionbar cross-sections.

The expected tip displacement for a given angle is

ıarm=larmtan=larmtan(0.841Finput)mm and, for a simple

tor-sionpivot(i.e.nobending),thedisplacementsatthepadandat thecantilevertipfor agivenangle(i.e.agivenpressure)might thereforebeexpectedtoberelatedıcantilever,T =lcantileverlpad ıpad,T

Aswellascausingtheabovedisplacements,aninputforcemight

beexpectedtointroducecertainamountofbendinginthepad

arm,dependingonthetorsionalspringstiffness,alsobendingin thecantileverarm,dependingonthereactionforceofthegripped object.Toafirstapproximation,thepaditselfcanbeassumednot

tobendsothatthebendingcomponentofthepaddisplacement

canbetakenastheenddeflectionofacantileveroflengthLwhere theloadplusanadditionalappliedmomentMinput=Finput(L−l)is

appliedadistancelfromthefixedend.Therelevantdisplacements atthepadaregivenby_ıpad=_ıarm+slopearm(L−l),so:

ıpad,b=_0.8411 tan−1

cantilever,T

3.15

1 EI

3+l

2 _l pad−l

+l lpad−l

=_0.8411 tan−1

cantilever,T

3.15

12

193×103_×0.75×0.053

1.23

3 +1.2

2_{(2.2−1.2)+1.2×(2.2−1.2)}2

=0.789tan−1

cantilever,T

3.15

[3.22]=2.536tan−1

cantilever,T

3.15

Fig.6. (a)Schematictestset-upforstaticloadingofcantilever),(b)Measuredcantilevertip(output)forcevs.inputpressure,(c)Measuredinputandoutputforcesatpressures

usedin(b)with30sdwelltime.

Thus,therelationshipbetweenpaddisplacementandtip dis-placementforanunloadedtipcanbeexpressedas:

ıpad=0.746ıcantilever+2.536tan−1

ıcantilever

3.15

(1)

Fig.10showsthemeandatafromFig.6formeasuredpadandtip displacementsforthefourincrementsofpressure.Also,foreachof

thetipdisplacements,thecorrespondingpaddisplacementshave beencalculated,firstassumingthetorsionbarstoactmerelyas piv-ots(i.e.notorsionalstiffness)and,second,takingintoaccountthe stiffnessofthetorsionbars.Ascanbeseen,thesecondcalculation isreasonablyconsistentwiththemeasurementsindicatingthatthe analyticalmodelissufficientforthepurposesofsizingthetorsion bars.Afiniteelementmodeloftheplateincludingtheouterframe wasalsobuiltandthiswasloadedusingthemembranepressure

A.F.Aloglaetal./SensorsandActuatorsA236(2015)394–404 399 0 0.2 0.4 0.6 0.8 1 1.2 1.4

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Pad dis pl ace m ent (m m)

Measuredpdisplacement(mm)

Pad(pivot)

Pad(calc.)

Pad(meas.)

Fig.8.Analyticalmodelofcantileverforassessingpaddisplacmentcomparedwith

measuredvalues(Fig.4).

appliedoverthesurfaceofthepad,butsimulatedasapointforce atthecentreofthepad(Fig.9).Fig.10showstheresultingpadand tipdisplacementsforeachofthepressuresusedinthestatictests, alongsidecalculatedvaluesofthepaddisplacementforeachofthe simulatedtipdisplacementsusingEquation(1),showingthat,at thislimitedrangeofinputpressurestheelasticsimulationandthe elasticanalyticalmodelareinagreement.

Basedonthistentative verification,theanalyticalmodelcan nowbeusedtoassesstheinputforcecorrespondingtothe vari-ouspressureamplitudesseeninFig.4(c).However,Fig.4(c)shows

thatthemaximumtipdisplacementfortheunloadedcantilever

dependsonactuationfrequencyatleastupto50Hzabovewhich theactuatorisprobablylimitedbythesupplyofair.Thefrequency sensitivityisunlikelytobeduetoastructuralresonance(for exam-ple,thenaturalfrequencyoftheleaststiffpartofthesystem,the cantilever,isabout4kHz)norisitlikely,giventhedatainFig.6(a),

tobe due to membrane relaxation or leakage. Figs. 11 and 12

Fig.9. FEmodelofgripperplateforassessingpadandtipdisplacmentsforaforce

appliedatRP.

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

0 20 40 60 80

Pa d o r p disp la ceme nt (mm)

Membrane inputforce(mN)

Pad (sim.)

Tip (sim.)

Pad (calc.)

Fig.10.Simulatedpadandtipdisplacementsforpressureloadingofthepadat

valuesshowninFig.4.Solidlineshowscalculatedpaddisplacement(usingEquation

(1))foreachofthesimulatedtipdisplacements.

showthevariationofmaximumtipdisplacement(asvoltage out-putfromthephotodetector)withinputpressurepulseheightat theactuator(dataessentiallyre-plottedfromFig.4(b)).Thecurves arereasonablylinear(moresoatlowerfrequencies)andthusan overallcompliance(inmmtipdeflectionperbar) canbe deter-minedfortheactuator.ThisisplottedinFig.12forthewholerange

offrequency examined,andit canbeseenthat thecompliance

changeswithfrequencyovertherange0–50Hz,andlittle there-after.Thissuggeststhat thedynamic behaviourof theactuator is affectingthetipdisplacementand sothemostreliable mea-surementofthesystemcomplianceisgivenbyextrapolationto zerofrequency(Fig.12),i.e. avalueof0.223mm/bar.Sincethis is only around1% differenttothevalue measuredusing a

fre-quencyof1Hz,thepad and tipmeasurementsshown inFig.5

areunlikelytohavebeenmuchaffectedbytheactuatordynamic response.

Thecompliancecanfurtherbeusedtodeterminetheinputforce correspondingtoa giveninputpressurewhen theoutputforce,

Routput=0usingtherelationshipbetweeninputforceandcantilever

displacement:

Finput,Routput=0=

1 0.841tan

−1

3.15

(2)

wheretheinputpressure,p,isinbar.Whenthecantileverisloaded,

theforcesystembecomesstaticallyindeterminatewhichmakes

ananalyticalmodelabitmoretediousandso,forthepurposesof thispaper,theanalyticalrelationshipbetweentheinputpressure andinputforce(Equation(2))wasusedinalinearelasticFE sim-ulationofthegripperplatewithaloadedcantilevertocalculate theoutputforce.Theresultsofthissimulationarecomparedwith themeasuredoutputforcesat30sdwelltimefromthestatictests (Fig.6(b)),anditcanbeseenthatagreementisgooduptoa pres-sureof1bar,beyondwhichthesimulatedoutputforceishigher than thatmeasured, thediscrepancy increasingwithincreasing pressure.Thereasonforthiscanbeseeninthefiniteelement elas-ticstressdistributioncalculatedat1barinputpressureshownin

y = 1.3664x

y = 1.4404x

y = 1.4718x

0 1 2 3 4 5 6 7 8

0 1 2 3 4 5 6

Ma ximum p dis p lace me nt (V)

Input pressurepulseheight(bar)

50Hz 100Hz

300Hz

y=2.25x

y=2.21x

y=2.15x

y=2.07x

0 2 4 6 8 10 12

0 1 2 3 4 5 6

Maxim um p di spla ce me nt (V)

Inputpressurepulseheight(bar)

1Hz 2Hz 4Hz 8Hz

(a)

(b)

Fig.11.(a)Effectoffrequencyonunloadedcantileversystemcomplianceathigherfrequencies(datafromFig.4),(b)effectoffrequencyonunloadedcantileversystem

complianceatlowerfrequencies(datafromFig.4).

thatthesimulationwouldbeinerrorintermsoftheinputaswellas intermsoftheresultingdeflections.Atanyrate,Equations(1)and

(2)canstillbeusedfordesignpurposessothatdimensionsofthe grippercanbechosentoavoidyieldusingverysimpleanalytical expressions(Fig.14).

Thenextstepforthisworkisfurtherdevelopmentofthe tac-tilesensingofmicro-objects.Extendingtheconsiderationsabove, andassumingnobending,thestaticstiffnessofagrippedlinear elasticobject,k,canbeobtained(forconceptualdesignpurposes)

by considering the relationship between the angular

displace-ments1 and 2 withand without,respectively, theobject in

place:

Withouttheobject:

l1cos1×Fin=1_LJG

Withtheobjectgripped:

l1cos2×Fin=2JG

L +l2cos2×kıobj

So

Finl1= 1JG

Lcos1 =

2JG

Lcos2 +l2kıobj

y

= -0.0

025

x

+

0.22

R²

=

0.98

0.2

0.20

5

0.21

0.21

5

0.22

0.22

5

0

2

4

6

8

10

T

ip

displa

ceme

nt

pe

r

un

it

inpu

tp

re

ssu

re

(mm

/bar)

Input pressure pulse height (bar)

0.1

0.15

0.2

0.25

0

100

200

300

Ti

p

dis

pla

ce

me

nt

per

unit

inpu

tp

re

ssu

re

(m

m/b

a

r)

Actuaon fre

quency

(Hz)

(a)

(b)

A.F.Aloglaetal./SensorsandActuatorsA236(2015)394–404 401

Fig.13.Elasticstressanalysisforloadedgripperplatewithappliedpressureof1bar.

Fig.15.Experimentalseutpforpick-and-placedemonstration.

or

k= JG

Lıobjl2

1

cos1 − 2

cos2

(3)

wherelistheperpendicular distancebetweenthecantilever

tip and torsion springs and L is the total length of the

tor-sion springs.Acknowledging the assumptions of its derivation,

Equation (3) couldbeused tomeasure thestatic stiffness of a grippedobjectprovidedthattheamountofitscompression(ıobj)

canbemeasuredandthattheangles1and2canbemeasured.

Equallythestiffnessofaviscoelasticobjectcouldbeassessedby recordingtheamplitudeofthecantilevertipwhilstgraspingan objectatdifferentfrequencies.

Oncethetechniquehasbeenvalidatedwithgrippedelasticand viscoelasticobjectsusingsimilarmeasurementstothesereported

here, the next stage is to develop a generic design approach

which allows the necessary measurements to be made at the

input side of the gripper where intervention is easier in real applications.

4. Devicedemonstration

Afully-assembledversionofthedevicewasusedtodemonstrate theactuatorinpick-and-placemode,inairandunderwater.The

demonstrationconsistedofmanipulatingacidwashedzirconium

micro-beadsof200␮mdiameter (OPSDiagnostics,LebanonNJ,

USA),forwhichtheexemplardevicereportedabovewasmounted onanXYZstageandthemicro-beadsplacedinapetri-dishwitha monitoringcameraunderneath,asshowninFig.15.

Theabove-mentionedarrangementdemonstratedavery satis-factoryperformancebothinairandunderwater.Fig.16showsthe precisionofplacementthatcouldbeachievedusingtheproposed

manipulation technique, where 200␮mzirconium micro-beads

canbepreciselypositionedtoformeitheradottedarray(onthe left)orashapeofasmileyface(ontheright).

Fig.16. Variouspick-and-placearrangementsof200␮mzirconiummicro-beadsthatareachievablewiththisdesign.

A.F.Aloglaetal./SensorsandActuatorsA236(2015)394–404 403

Onewayinwhichthestiffnessof grippedmicro-objectscan beassessedistovibratethecantileveratvariousfrequenciesand recordtheamplitudeofthedeflectionofthecantilever.Ifthisis doneforthegripperinairandsubsequentlywiththeobjectgripped, thequotientofthesedeflectionsdeterminesthestiffnessofthe object:

AR=Aobj Aair

(4)

Thisprocedurewascarriedoutwitha1baractuatorpressure, gripping400␮mhydrogelbeadsandusingfrequenciesof1Hz(A1), 2Hz(A2),4Hz(A3)and10Hz(A4).Theresults,showninFig.17

clearlyindicatethatthereisareductionintipamplitudeforthe grippedbeadateachfrequency.Theresultsfurthershowthatthe amplitudechangeisdifferentateachfrequencyand,furthermore, thattherearechangesinamplitudewithtimeatagivenfrequency. Thisisprobablyduetoacombinationofdynamiceffectswithin theactuator-membranesystem,aswellasintheresponseofthe hydrogelbead.Thesemattersarenotpursuedinthecurrentpaper, butwillinformfutureworkusingdevicesofthistype.

5. Conclusions

This work demonstrates a pneumatically actuated

micro-gripperthathastheabilitytopickandplacemicron-sizedobjects

with the potential to provide tactile feedback. The pneumatic

micro-actuatorcanhaveitspressure-forcerelationshipmeasured andcontrolledinbothstaticanddynamicmodeswithasimpleset up.Thetwostainlesssteelgripperarmscanbefabricatedusing low-cost,scalablephoto-etchingmanufacturingtechniquewitha rangeofdimensions,allowingthisdesign tobescaledtofitthe chosenapplication.Asthephoto-etchingmanufacturingisa photo-lithographytechniquewithhighprecision,itispossibletofabricate stainlesssteelbasedgripperwithdimensionssmallerthan100␮m forsinglecellmanipulation.Thestiffnessofthetwotorsionbars canaidintailoringtheuseofthegripperwiththebarsbehaving almostasapivotorusedtodown-regulatetheinputforce(formore delicatetasks).Besidescontrolling theinputand outputforces, thedesignofthegripperarmscanbetunedinordertoprovide adeflectionofthearms,whosemeasurementcanbeusedto mea-surethegrippingforceinoperationandcanpotentiallybefedback tocontroltheforceorusedtoassessthegrippedobject.Besides thepotentialuseofgripperformanipulatingembryosforcloning

applications,themechanicalmicro-tweezersmayprovideanew

waytopick-and-placecellaggregatesforbiofabricatingsynthetic tissues[25,26].Futureworkwillfocusonfurtherminiaturizationof thegripperformanipulatingsmallerobjects(e.g.singlecells)and treatmentofthegrippersurfacetoovercomepotentialstriction issues.Furtherimprovementofthegripper’stactilesensingmay alsoleadtomechanicalassessmentofbiologicaltissuesinvivo.

Acknowledgements

ThisresearchworkissupportedbytheEngineeringandPhysical SciencesResearchCouncil(EPSRC)(GrantCode:EP/I019472/1).A. AloglaisgratefultotheSaudiGovernmentforaPhDscholarship. C.BalmerandP.ScanlanaregratefultoEPSRCforPhDscholarship funding.

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AgeelAloglareceivedhisB.Eng.inMechanicalEngineeringfromtheKingFahad

UniversityofPetroleumandMinerals,KingdomofSaudiArabiain2002,andaPh.D.

degreefromtheuniversityofHeriot-Watton2014.HeiscurrentlyanAssistant

ProfessorintheSchoolofEngineeringatTaifUniversity,KingdomofSaudiArabia.

Hisresearchinterestsarefocusedonmicrosystemsandmechanicalgrippers.

FaridAmaloureceivedhisPh.D.inMicroEngineeringandMicroMagnetismatthe

SwissFederalInstituteofTechnologyinLausanne(EPFL)in2003.Followinghis

postdoctoralpositionsatHeriot-WattUniversityandSouthamptonUniversity,heis

currentlyanAssistantProfessorinPhysicsatAlfaisalUniversity,KingdomofSaudi

Arabia.

ChrisBalmergraduatedwithanM.Eng.degreeinMechanicalEngineeringfrom

Heriot-WattUniversity,Edinburghin2014.HeiscurrentlyworkingonhisPh.D.on

researchinterestsarefocusedonmicrogrippersandtactilefeedbackforbiological tissues.

PaulScanlangraduatedwithaB.Eng.degreeinRoboticsandCybertronicsfrom

HeriotWattUniversity,Edinburghin2011.HeiscurrentlystudyingforhisPh.D.in

theMechanicalProbingofHumanTissue.Hisresearchinterestsinclude

microfab-ricationtechniquesandengineeringforbiomedicalapplications.

WenmiaoShureceivedhisB.Eng.degreeinPolymerMaterialsfromtheDalian

Uni-versityofTechnology,China,in1998,andaPh.D.degreefromtheUniversityof

Cambridgein2006.HeiscurrentlyReaderofMicroengineeringintheSchoolof

EngineeringandPhysicalSciencesatHeriot-WattUniversity.Hisresearchinterests

coverarangeofbiomedicalmicroengineeringtopicsincludingmicromechanical

sensorsandactuators,biosensors,lab-on-a-chipsystemsand3Dbiofabrication.

RobertL.ReubenisaProfessorofMaterialsEngineeringintheSchoolof

Engineer-ingandPhysicalSciencesatHeriot-WattUniversity.HegraduatedwithaFirstClass

HonoursinMetallurgyfromtheUniversityofStrathclydein1974,afterwhichhe

workedwiththeUnitedKingdomAtomicEnergyAuthority.In1977hejoinedthe

OpenUniversity’sOxfordResearchUnitresearchingthepermeation

characteris-ticstohydrogenanditsisotopesofcandidatefusionreactorcontainmentmaterials,

gainingaPh.D.inthatareain1980.Hisresearchfieldincludesexperimental

mechan-icsandmicrosystemsengineering(oftenformedicalapplications),andsensor-based