Hybrid opto-electric manipulation in microfluidics-opportunities and challenges

16 Pages·2017·1.14 MB·English

Purdue University
Purdue e-Pubs
Birck and NCN Publications Birck Nanotechnology Center
2011
Hybrid opto-electric manipulation in microfluidics-
opportunities and challenges
Aloke Kumar
Oak Ridge Natl Lab, Oak Ridge, TN
Stuart J. Williams
University of Louisville
Han-Sheng Chuang
University of Pennsylvania
Nicolas G. Green
University of Southhampton
Steven T. Wereley
Birck Nanotechnology Center - Purdue University, [email protected]
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Part of theNanoscience and Nanotechnology Commons
Kumar, Aloke; Williams, Stuart J.; Chuang, Han-Sheng; Green, Nicolas G.; and Wereley, Steven T., "Hybrid opto-electric manipulation
in microfluidics-opportunities and challenges" (2011).Birck and NCN Publications.Paper 767.
http://dx.doi.org/10.1039/c1lc20208a
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Micro- & nano- fluidic research for chemistry, physics, biology, & bioengineering
www.rsc.org/loc Volume 11 | Number 13 | 7 July 2011 | Pages 2121–2296
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Seok (Sid) Chung at Korea University, Seoul, Korea.
Title: In vitro 3D collective sprouting angiogenesis under
orchestrated ANG-1 and VEGF gradients
A hydrogel incorporating microfl uidic platform is able to generate
precisely orchestrated gradient of multiple angiogenic factors, VEGF
and ANG-1, to induce 3D cooperative endothelial cell sprouting
angiogenesis, which can contribute to exploring cancer treatment,
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See Seok Chung et al.,
Lab Chip, 2011, 11, 2175.
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Registered Charity Number 207890 1 CRITICAL REVIEW
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1 Aloke Kumar et al.
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2 Hybrid opto-electric manipulation in microfl uidics—opportunities
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Cite this: Lab Chip, 2011, 11, 2135
CRITICAL REVIEW
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Hybrid opto-electric manipulation in microfluidics—opportunities and
challenges
*a b c d e
Aloke Kumar, Stuart J. Williams, Han-Sheng Chuang, Nicolas G. Green and Steven T. Wereley
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10 Hybridopto-electricmanipulationinmicrofluidics/nanofluidicsreferstoasetofmethodologies
11
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Aug | strategyand/orthelengthscaleofoperation,haveemerged.Thesetechniquesoffernewopportunities
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sitpu frommillimetre-sizeddropletstonano-particles.Thisreviewarticlediscussestheunderlyingprinciples,
Univerhttp:// applicationsandfutureperspectivesofvarioushybridopto-electrictechniquesthathaveemergedover
due 1 on thelastdecadeunderaunifiedumbrella.
d by PurMay 201 1. Introduction
de0
wnload on 2 EaB-imosacili:enkcuemsaDraiv1is@ioonr,nOl.gaokv;RFidagxe:N+1at8io6n5a-5l7L4a-b5o3r4a5t;oTrye,l:O+a1k8R65id-g5e7,4U-8S66A1. The ability to control matter at all length scales, for various
Dohe bDepartment of Mechanical Engineering, University of Louisville, applications, has been mankind’s relentless goal. The delicate
blis Louisville,USA.E-mail:[email protected];Tel:[phone]- nature of objects involved in lab-on-a-chip devices and micro-
Pu 6340 fluidicplatformstypicallynecessitatenon-invasivemanipulation
cMechanical Engineering and Applied Mechanics, University of
ofmatter.Progressinmicrofluidicsandlab-on-a-chipdevicesin
Pennsylvania, Philadelphia, PA, USA. E-mail: [email protected];
Tel:[phone] the last few decades saw a concurrent, vigorous activity in the
dSchoolofElectronicsandComputerScience,UniversityofSouthampton, fieldofnon-invasivemanipulationofmatterandseveralschemes
Southampton,UK.E-mail:[email protected];Tel:[phone] of manipulation at the micro- and nano-scale have emerged.
eBirckNanotechnologyCenter,PurdueUniversity,WestLafayette,USA. Today tools exist which employ either light waves,1,2 electric
E-mail: [email protected]; Fax: [phone]; Tel: [phone]-
5624 fields,3magneticfields,4oracousticwaves5forthenon-invasive
AlokeKumarreceivedhisbach- Stuart J. Williams received his
elors and masters degrees from BS and MEng degrees in
the Indian Institute of Tech- Mechanical Engineering from
nology, Kharagpur, India in the University of Louisville in
2005 and his doctoral degree 2005andhisPhDinMechanical
from Purdue University, West Engineering from Purdue
Lafayette,USAin2010.During University in 2009 with the
his doctoral career he was support of the NSF Graduate
awarded several fellowships, Research Fellowship. Currently
including the prestigious Jose- he is an Assistant Professor in
phine DeKa(cid:1)rma(cid:1)n fellowship. the Department of Mechanical
Currently,heisaWignerFellow EngineeringattheUniversityof
and staff member at the Oak Louisville where he is the
AlokeKumar Ridge National Laboratory StuartJ:Williams director of the Integrated
(ORNL). At ORNL his Microfluidic Systems Labora-
research focuses on phenomena tory where his research focuses
at the microfluidic length scales, particularly lab-on-a-chip elec- on applications of microfluidics for biology and lab-on-a-chip
trokineticsanddynamicsofmicrobialaggregations. samplepreparation,includingopticalandelectrokineticmethods.
ThisjournalisªThe RoyalSociety ofChemistry 2011 LabChip, 2011,11,2135–2148 | 2135
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manipulationofmatteratthisscale.Forexample,electricfields ahigherdegreeoffreedom.Thisfreedomenablesreconfigurable
generated from patterned electrodes have been employed to and programmable lab-on-a-chip devices driven through illu-
movevariousobjectsfrommillimetre-sizedaqueousdroplets6to mination displays such as a LCD screen or simply with a laser
nanometre-sizedbeads.3 pointer.Avarietyofopticallyinducedelectrokinetictechniques
Innovationsinthelastdecadehaveresultedinthecreationof spanningarangeoflengthscaleshaveemerged.
a new class of non-invasive manipulation techniques—hybrid Astructureddiscussiononthediverseopto-electrictechniques
opto-electric tools. As the name suggests, these techniques is facilitated by a classification based on the operational length
employ simultaneous light waves and electric fields to achieve scale (l) of the technique. Attempting such a classification, one
fluid or particle manipulation at the micro- and nano-scale. finds that the operational length scales can bedivided into two
Unlike, optical tweezing,7 hybrid tools do not rely on optical broadgroups.Oneclasscomprisesoftechniquesthatmanipulate
A pressure for non-invasive manipulation. Instead, these tech- liquid-suspended particulates whose dimensions range from
08 niques use light to activate and control electrokinetic manipu- nanometrestomicrometres i.e.lz10(cid:1)9to 10(cid:1)6m.Techniques
2
20 lation mechanisms. The simultaneous presence of light and suchasoptoelectronictweezers8(OET)andrapidelectrokinetic
C
1L electric fields, and their non-linear interaction can result in the patterning9 (REP) would belong to this class. Another class
C
9/ appearance of physical phenomena unique to opto-electric consists of techniques dealing with object dimensions in the
11103 systemscomparedtotraditionalelectrokineticsystems.Theuse millimetre scale i.e. l z 10(cid:1)4 to 10(cid:1)3 m. Optoelectrowetting10
gust 20doi:10. oflightalsobestowsonthesetechniquesadynamicnatureand ((OOE-OWE)Wa1n1)d aintsddesirnivgaleti-vsiedsedsucchonatsinuoopuesn ooppttooeelleeccttrroowweettttiinngg
Aug | (SCOEW12) comprise this class of hybrid techniques. Unlike
1 or Han-ShengChuangreceivedhis
y on 3bs.rsc. PhD degree in department of RacEtuPatoironOEofTd,rOoEplWetsa.nMduitltsi-dsecrailveamtivaensipduelaaltwiointhutshinegcoanctoromllbeid-
sitpu mechanical engineering from nation of different techniques is also possible and will be
due Univer1 on http:// PLcuuarrfradeyuneettlyte, UINnaivienrs2ipt0yo1,s0td.oHcWteoerasistl deliesSccuturcsihcsetdheychhbenrrieidq.uFteeigsch.t1nhiadqteupheiascvtaesraeemcehenarggretendodfoetrvhienergdtihnffeeewrpeanastpthpdyleibccaraitddioeo.npstoin-
ded by Pur0 May 201 rHPeesaneimnasrychlBveaarnuiaw.aotrkHiUnignsivewrrisetishteyarDcohrf lipanhbve-enosotnimg-aae-tnciohani.pGsaiyvnsedtnemtahspe,parleniccdaentaitoreenmeneoarfgbelfinnucgnednoaofmvtheelnesateapltperceohlaenccithqreuosektsio,nteththiices
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Dh technologies and optical diag-
ublis nostics.In2005,hewasawarded onpiqtuoe-es,lewcthriocsetetcyhpniciqalueosp.erWateiosntaalrtlenwgitthh sacadleisscluiessbioentwoeefntelczh-
P
Han-ShengChuang a competitive fellowship from 10(cid:1)9and10(cid:1)6m,subsequentlywewilldiscusstechniqueswithlz
Ministry of Education in Tai- 10(cid:1)4to10(cid:1)3m.Wewillconcludebydiscussingcurrentchallenges
wan.Heandhisresearchfellows
andopportunitiesforcontinuedresearchinhybridtechniques.
werealsothefinalistsoftheprestigiousBurtonD.MorganBusi-
nessCompetitionin2008and2009,respectively.Inaddition,heis
aco-founderofaUSbasedtechnicalstart-up,MicrofluidicInno-
vations,foundedin2009.
Professor Wereleyhasbachelor
degrees in Physics (Lawrence
NicolasGreenisaReaderinthe University, 1990) and Mechan-
Nano Group and the South- ical Engineering (Washington
amptonNanofabricationCentre University, StLouis,1990)fol-
in ECS at the University of lowed by masters and doctoral
Southampton. He studied for degrees from Northwestern
aPhDfromUniversityofGlas- University(1992and1997).He
gow from 1994 until 1997 and is currently Professor of
then worked as a Marie Curie Mechanical Engineering at
Research Fellow at the Univer- PurdueUniversityandaFellow
sity of Sevilla, Spain and at the Center of Smart Inter-
a Royal Academy of Engi- faces, Technische Universita€t
neering Fellow at the Universi- StevenT:Wereley Darmstadt (Germany).
ties of Glasgow and Professor Wereley served as an
NicolasG:Green Southampton.Hewasappointed AlexandervonHumboldtFellow
toalectureshipattheUniversity at Technische Universita€t Darmstadt (2007) and Universita€t der
of Southampton in 2005. His Bundeswehr (2009). Professor Wereley is the co-author of the
research interests are in the areas of Bioelectronics and Micro- monographs Fundamentals and Applications of Microfluidics
fluidics examining fundamental physics and applications of elec- (ArtechHouse,2002and2006)andParticleImageVelocimetry:
trokineticsinlab-on-a-chipsystems. APracticalGuide(Springer,2007).
2136 | LabChip,2011,11, 2135–2148 Thisjournal isªTheRoyalSociety ofChemistry2011
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Universithttp://pu intheliterature.
due 1 on 2. Hybrid techniques with l z 10(cid:1)9 to 10(cid:1)6 m that the illuminated regions on the ITO surface had a higher
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s
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u
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nucleation be directed with control over crystal annealing and
melting.
2.1 Opto-electrohydrodynamicassembly The basis of the reported colloidal aggregation under the
influenceofUVlightisnotfullyunderstood,butitisbelievedto
One of the first innovations in the field of hybrid opto-electric
be related to planar particle aggregation on electrode surfaces
techniqueswasledbyHaywardetal.13in2000.Theyusedoptical
patterns shone on an electrode surface to assemble suspended
particlesintoopticallytunablecrystals.Theelectrokineticsetup
consisted of a parallel plate configuration with a brass cathode
andanindiumtinoxide(ITO)thinfilmastheanode.Ultra-violet
(UV)lightwasfirstpassedthroughapre-fabricatedmaskandthe
resulting pattern was illuminated on the ITO film. The micro-
channelwasfilledwithadiluteaqueoussolutionofpolystyrene
(PS) beads. Hayward et al.13 observed that over time, the PS
Fig.2 (a)ThefigureshowstheassemblyofPSbeadsonITOsurface
beadsaggregatedontheITOsurface,undertheinfluenceofan
using optical motifs, as outlined in Hayward et al.13 Reprinted with
applied DC field, and the aggregation resulted in dense packed permission from Macmillan Publishers Ltd: Nature, Hayward et al.,13
assembles in patterns similar to the pattern of UV irradiation copyright2000.(b)Non-uniformcurrentsdriveflow,whichinturncan
(Fig.2a).Haywardetal.’s13processofassemblingsuchoptically entrain particles leading to an aggregation. Reprinted with permission
tunablecrystalstookseveralhoursandtheresearchersproposed fromRistenpartetal.33Copyright2008AmericanChemicalSociety.
Table1 Tablelistingthepredominanthybridtechniqueswithlz10(cid:1)9to10(cid:1)6m
Notabledeviations
Technique Originalsetup Distinguishingfeature frominitialsetup
OEA Parallelplate,UVlightsourcewithstaticmask(ref.13) Useofnon-uniformcurrents Ref.14
OET Useofa-Si,DMDwithnear-redlightemittingdiodelaser(ref.8) Useofvirtualelectrodes Ref.40–42
REP Parallelplate,useof1064nmholographiclightsource(ref.9) Useofopticallydrivenelectrothermalflows Ref.72and92
ThisjournalisªThe RoyalSociety ofChemistry 2011 LabChip, 2011,11,2135–2148 | 2137
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due to the action of both DC and AC fields.15–18 The latter todynamicallyreconfigurelightpatterns.Thesedynamicsystems
phenomena first reported by Richetti et al.,16 were modeled by are driven by a digital micromirror device (DMD),8,39 liquid
Trau et al.17,18 as a perturbation of the local electric field by crystaldisplay(LCD),40,41orabeamprojector.42Thesesystems
particlesandthesubsequentactionofthesameperturbationson require the integration of optical components to focus the
the electrode polarization layer. This model proposed by Trau illumination ona microscope stageand providea resolutionof
etal.18isactuallyaninstanceofinduced-chargeelectro-osmosis 1–20 mm. However, an LCD-based system without integrated
(ICEO)flows.19–21Later,Ristenpartetal.22providedscalinglaws opticshasbeendemonstratedthatgrantsmoreportabilityatthe
forsuchflowswithACfrequency((cid:3)1/u)andACfieldstrength cost ofincreased electroderesolution(200mm).40Regardless of
((cid:3)|E|2).However,theinversedependenceonACfrequencyleads the illumination scheme, OET illumination intensity is over
tounrealisticpredictionsinthelimitoflowACfrequencies(u/ 100000timeslessthantraditionalopticaltweezingsystemsand
A 0+). For steady or DC fields, Solomentsev et al.23,24 proposed hasamuchlargerworkingarea.8
8
0 alternatemechanismsbasedonelectro-osmoticslipflows(EOFs) The electrode configuration for a traditional OET device is
2
0
2 ontheparticles’surfacewithnopolarizationchange.Eachmodel similar to of Fig. 3, which illustrates a closed system with the
C
1L hasitsowndrawbacksandapossibilityisthatacombinationof photoconductivematerialononesidewithparticlemanipulation
C
9/ models can satisfactorily explain the diversity of observed occurring on the photoconductive substrate surface. However,
3
1110 phenomena.InfactarecentinvestigationbyRistenpartetal.25 a three-dimensional manipulation scheme has been demon-
gust 20doi:10. hmaosdineldinceaetdedttohabtebointvhoTkreaduientaol.r’ds1e8ratnodeSxoplolaminenatgsegvreegtaatli.o’sn23,i2n4 sbtoratttoemd welietchtrpohdoesto.4c3oSnidnugcleti-vsiedemdaOteEriTalsdeovnicbesothhavtheealtsoopbaenedn
Aug | steadyfields.Acompletereviewofaggregationonelectrodesand developed with a single ITO substrate patterned with interdigi-
y on 31 bs.rsc.or ereffveicetws,obfuvtawrieowusouinldholimkeogteonneiotiteesthisatousetvsiedrealtrheesesacrocpheerosfhtahvies atastiemdpelleecat-rSoid:Hesfianlmdcisoathteedrewsittrhicati-oSni:Hof.4p4aTrhtieclleimmitaantiiopnuloaftiuosnining
sitpu reported significant findings and readers might wish to refer to lowconductivity(<100mSm(cid:1)1)solutions;biologicalmedialike
due Univer1 on http:// oitnhpetriescgoaimlomnesootafifdtshjaiescefboneltllioetowveitdnhgetorielilpnuodmruticsne.a26ht–ii3og5nhHepraryioowmnaipcrdtcinuegrtraeflnlo.’twsd1s3enousfsiettihoeesf pMohveeodrscipuohmmate(eDt-hbMiusfEfiseMsrue)de,hspaahlvioentecoo(tPrnaBdnSus)cistativonirdtsieDwsueflrrbeoemicncco1o’rstpoMo2roaSdteifimde(cid:1)wd1.iE4t5haTgalone
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de0 motif13,33(Fig.2b). though, comes with the added expense of fabricating complex
a2
nloon structuresinsteadofthetraditionala-Si:Hfilm.
wd OETs can generate a variety of electrokinetic particle and
Doublishe 2Il.l2ustrOatpetdoeliencttrhoenicprtewveieozuesrssection, Hayward et al.13 laid the hwyidderloyduysneadmeilcectfroorkceins.e4t8icDmieeclehcatnroispmhoinreOsiEsT(Ds.EDPE)Pirseftehrestmotohset
P
foundations of electric field perturbations by local increase of translation of polarizable neutral particles in the presence of
conductivity of a substrate. Chiou et al.8,36 explored materials a non-uniform electric field, typically an AC field.3 The magni-
whosephotoconductivitylayinadifferentregime.Bytheuseof tude and direction of the DEP force are proportional to the
ahighlyefficientphotoconductivematerialdepositedonaplanar volumeoftheparticle,thesignalvoltageandfrequency,andthe
surfacealongwithaprogrammableilluminationsystem,Chiou gradientoftheappliedfield;it’salsoafunctionofthedielectric
et al.8,36 demonstrated that illuminated areas generate ‘virtual’ properties (permittivity, conductivity) of the fluid and particle.
electrodes, which are dynamically reconfigurable to electro- DEP forces are categorized as positive, being attracted to field
kineticallymanipulateparticlesandfluid.Thistoolcalledopto- gradients, or negative, being repelled from these regions. With
electronic tweezers (OET) made possible the use of standard regards to OETs, particles are attracted to illuminated regions
electrokinetictechniqueswithouttheuseofpatternedelectrodes. under positive DEP but are repelled from them with negative
TheoperationofOETswasdemonstratedbyChiouetal.8and DEP.Fig.4aandbprovideexamplesofparticletrappingusing
is illustrated in Fig. 3. This device consisted of unpatterned bothtypesofDEPforces.
parallel ITO electrodes with one of them being coated with Whenaparticleispolarizedthetwooppositelychargedpoles
hydrogenated amorphous silicon (a-Si:H) serving as the photo- arealignedinthedirectionoftheelectricfield.Ifanon-spherical
conductivematerial.Amorphoussiliconisawidelyusedmaterial particle undergoes polarization, the poles will tend to concen-
in the solar cell industry; when light illuminates a-Si:H its elec- trateattheendsofthelongaxisoftheparticle.Thiswillinduce
trical conductivity increases several orders of magnitude, a torque force on the particle to orient its long axis with the
depending on the illumination intensity.37 The increased directionoftheelectricfield.Thisphenomenoniscalledelectro-
conductivitywillleadtoalocalizedincreaseinfieldstrengthat orientationandcanbeusedtoextractdielectricpropertiesfrom
the substrate surface, enabling dynamic virtual electrode the particle.4 OET devices have applied this technique to align
configurations.OETcanbeactivatedwithawhitelightsource, nanowires49 and bacteria.50 Particle’s charged poles will also
although a-Si:H absorption is most efficient from red to near- interact with neighboring particles. Similarly charged particles
infraredwavelengths.38 willform‘chains’andaligninthedirectionoftheelectricfield.
The types of illumination sources and its delivery to the However,ifadjacentparticlesareperpendiculartothedirection
photoconductivesurfacesvary.Astaticilluminationpatterncan of the electric field they are repelled from each other. These
besimplyappliedbytheuseofasinglelaserspot39oraphoto- particle–particleinteractionshavealsobeenobservedinanOET
mask.13However,OETsystemsarecharacterizedbytheirability device.51
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Fig.3 IllustrationofatypicalOETplatformcharacterizedbyaphotoconductivefilm(a-Si:H)andaprojectionofdynamiclightpatters,inthiscase
fromaDMD.ReprintedwithpermissionfromMacmillanPublishersLtd:Nature,Chiouetal.,8copyright2005.
OETscanalsoinducetwoelectrohydrodynamicmechanisms, electrokineticforcesonlyatilluminationsgreaterthanfortypical
AC electroosmosis (ACEO) and electrothermal (ETH) motion. OETuse.48Jamshidi,etal.55demonstratedaggregationof90nm
ACEOisinducedwhenthetangentialcomponentoftheelectric diameter gold nanoparticles with OET-induced ETH flows.
field moves ions within the substrate’s electric double layer, Fig. 4c provides an example of particle manipulation using
generatingaslipvelocity.52Thisfrequency-dependentbehavioris ACEOandETHelectrohydrodynamicsdrivenbyOETs.
dominant at low frequencies (<10 kHz) and at large Debye OETs in its various forms has used electrokinetic mecha-
lengths(i.e.lowconductivitymedia).ACEOisparticularlyuseful nisms for trapping, patterning, and sorting a variety of parti-
in the trapping of sub-micrometre particles where Brownian cles.Manipulatednon-biologicalparticlesoutsideoftraditional
motion overcomes DEP forces. In OETs, the illuminated area polystyrene/silica beads include quantum dots,39 silver nano-
cangenerateACEOflows,carryingsuspendedparticlestowards rods,49goldnanoparticles,55andcarbonnanotubes.56Biological
these regions. Such flows have captured particles ranging from sorting demonstrations with OET include motile and non-
50nmto2mm.39Selectiveandrapidconcentrationof1mmand motile sperm,57 erythrocytes and leukocytes,58 live and dead
6 mm polystyrene particles was demonstrated utilizing simulta- human B cells,8 and normal and abnormal oocytes.59 Addi-
neouslyappliedACEOandDEP.53ETHhydrodynamicsoccur tional biologicalentities thathavebeenmanipulatedwithOET
when the applied electric field acts upon fluid dielectric inho- include HeLa cells,60 E. coli,61 yeast,42 protozoa,39 DNA,62 and
mogeneities resulting from non-uniform temperature fields.9 HepG2 cells.63 To reduce non-specific adhesion an operational
Such temperature gradients can be generated through Joule OET device coated with poly (ethylene glycol) was demon-
heating or with applied external illumination.54 The photocon- strated.64 For an expanded discussion on the applications of
ductive material in OETs absorb the applied illumination OET, readers are referred to a recent review article by Hwang
resultinginETHflows,butsuchflowsgenerallyovercomeother and Park.65
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Fig.4 (a)PositiveDEPtrappingofE.coliwithOETsisdemonstratedwhentheyareattractedtoanilluminated17mmspot-size.FromChiouetal.61
ReproducedwithpermissionofIEEE,ª2004IEEE.(b)45mmparticlesareaggregatedindarkregionsofthedynamicilluminatedpatternwithnegative
DEP.FromChoietal.40ReproducedwithkindpermissionofSpringerScience+BusinessMedia.(c)Trappingof90nmgoldnanoparticleswith
acombinationofACEOandETHhydrodynamics.ReprintedwithpermissionfromJamshidietal.55Copyright2009AmericanChemicalSociety.
2.3 Rapidelectrokineticpatterning
Rapid electrokinetic patterning (REP) is a more recent hybrid
opto-electrictechniqueintroducedbyWilliamsetal.9in2008as
a real time dynamic particle manipulation technique. The orig-
inal chip configuration utilized by Williams et al.9 consisted of
ITOcoatedparallelelectrodesbiasedwithanACsignal(Fig.5).
Williams et al.9 showed that in a low frequency regime (<200
kHz), micrometre dimension particles could be trapped and
dynamically configured into various shapes on the electrode
surface by using various optical landscapes produced from
alaseroperatingatawavelengthof1064nm.REPisaversatile
techniqueandparticlesasdiverseasgoldparticles(200,250nm)
(unpublished data), silica particles,66 and polystyrene particles
Fig.5 AnexampleoftheREPmanipulationtechnique.Anintenselaser
(50 nm (ref. 67) to 3 mm) have been successfully aggregated. A hologramisshoneonanITOsurfaceandthe690nmredfluorescentPS
video demonstrating REP and highlighting its features is beadstaketheshapeofthehologram.Williamsetal.9Reproducedby
available.68,69 permissionofTheRoyalSocietyofChemistry.
2140 | LabChip,2011,11, 2135–2148 Thisjournal isªTheRoyalSociety ofChemistry2011
View Online
NosingleelectrokineticmechanismdrivesREP-basedparticle aggregations, similar to those demonstrated with an ITO elec-
aggregation and research has shown that it is actually brought trodeusing1064nmlaserbeam.Thisfeatureissignificantasit
about by the concerted action of a variety of electrokinetic allows REP to be replicated at different optical wavelengths,
forces.An understandingofthe REPprocesscanbefacilitated provided that the proper combination of electrode-laser wave-
by seeking answers to these distinct questions: (i) What causes length is chosen to generate the necessary optically induced
rapid particle transport to illuminated regions on the electrode temperaturegradients.
surface? (ii) What keeps the particles on the electrode surface? Oncetransportedtotheilluminatedsite(s),particle–electrode
(iii)Whatisthestructureoftheaggregatedparticles?Whilethese interactionforcesallowtheparticlestobe‘trapped’.Asthedrag
questions are interrelated, the following will approach these from the toroidal EMV would tend to ‘pluck’ the transported
questionsindividually. Firstto beaddressed will bethe driving particles away from the electrodes, the particle–electrode inter-
A forces for particle transport to the illuminated regions. Kumar actionforcesareattractiveintheREPregime,butareafunction
8
0 etal.70demonstratedthatahighlyfocused1064nmlaserbeam of frequency, medium conductivity and possibly even the elec-
2
0
2 shoneonanITOsurfacecancausehighlylocalizedtemperature trolyticspecies,butareexpectedtobeindependentofillumina-
C
1L gradients by the heating of the substrate (Fig. 6a). For a laser tion.Faganetal.’s75–77workcomprisestheveryselectstudiesthat
C
9/ powerof20mWtheabsolutetemperaturechangeitselfwasnot have concentrated their efforts on localized particle–electrode
11103 very significant ((cid:3)5 K), but this change occurred at the length forces. By measuring the temporal variations in a particle’s
gust 20doi:10. s((cid:3)ca1le05oKfmm(cid:1)ic1)r.oSnus,chrehsiuglhtitnegmipnervaetruyrehgigrahdtieenmtpseinratthuereprgersaedniceenotsf mheiicgrhotscaobpoyv,ethaeny ceolencctlruoddeed(thh)a,tutshinegobtosetravledintoesrcnialllatrioeflnesctinionh
Aug | electricfieldscangenerateETHbodyforces3,54,71andthusresult could only be explained through the superposition of different
y on 31 bs.rsc.or idnemdoonmstinraatnetdEthTaHt aflothwrse.e-Idnimaennositohneralweolerckt,roKthuemrmaarlemticarl.o7-2 mhyedcrhoadnyisnmams.icMflooswtssigpnroifidcuacnetlayvtheretyiccaolnfoclrucdeecdomthpaotntheneteloencttrhoe-
sitpu vortex(EMV)formedwithinanREPdevice,whichhasa‘sink- particleforfrequenciesabove500Hz.Apartfromforcesexerted
due Univer1 on http:// ttmhyipacter’otbhneerhesastvoreilounrtgintohenaoprfathtrhteiecellmeeciimtcrraoogdveeovrsteuelroxfcavicmaer.eiUtersysian(smgPtthIweVo)s-qtdhuimeayreesnhosoifowntehadel bvaeyrrottilhceaelinffloutrhicdee,spooatnrhtietchrleem–ceoelecllchotairdnoaidslempisanrtcteiarcnalecsta.iloIsnmo.a7c8g,o7e9nctIhrtiabhruagtseesactlsoaontbpheleaesnye
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de0 dimensional wavefront deformation particle tracking velocim- explainsthesecondquestion,thoughacompletecharacterization
nloaon 2 etry(PTV)developedatUniversit€atderBundeswehrMu€nchen74 of the particle–electrode interaction forces remains elusive.
wd toobtainathree-dimensionalvelocitymapoftheEMV(Fig.6b). However, onecanconceive theuseof the EMVvortexitself to
oe
Dh
blis They also showed good agreement between experimental data measurethesenetattractiveforces.
u andnumericalsimulations,whichwerebasedonETHbodyforce ThestructureofparticleaggregationsinaREPclusterhasnot
P
calculations. Williams et al.9 conjectured that the EMV trans- beenextensivelyinvestigated.Typically,largerparticles(>1mm)
portsparticlesfrombulktotheelectrodesurface,wheretheyare tend to form a monolayer, smaller particles (<300 nm) tend to
‘captured’.Williamsetal.66characterizedcolloidalaccumulation exhibitamulti-layercharacteristic.67Itispossiblethatoscillatory
inREPasafunctionoftimeandtheimposedelectricfield,and behavior of particles near the electrode75–77has an influence on
foundthattherateofcolloidalaccumulationvariedasthesquare theaggregationstructure.
of the electric field strength, in agreement with the hypothesis. Initsveryshortspanofdevelopment,theREPtechniquehas
They went on to show that a 532 nm laser can be used in showntolenditselftoavarietyofdifferentapplications.Kumar
conjunction with a gold electrode to produce particle etal.70proposedanewforcespectroscopytechniquebasedonthe
Fig.6 (a)The1064nmlasercausesintensetemperaturegradientsontheITOsurface.ReprintedwithpermissionfromKumaretal.70Copyright2010
AmericanChemicalSociety.(b)TheEMVasvisualizedusingwavefrontdeformationPTV.TheEMVtransportsanalytesfromthebulktotheelectrode
surface.FromKumaretal.73ReproducedwithkindpermissionfromSpringerScience+BusinessMedia.
ThisjournalisªThe RoyalSociety ofChemistry 2011 LabChip, 2011,11,2135–2148 | 2141
View Online
REP technique. They used REP to capture an aggregation of the applied frequency. The physical basis for such sorting has
1 mm particles, and subsequently deactivated the laser. In the beenexplainedintermsofacriticalACfrequencyphenomenon.
absence of the laser heating, the electrothermal flow ceases, The critical AC frequency is the maximum AC frequency at
allowing the particle–cluster to ‘explode’ due to inter-particle whichparticlescanbetrapped.Kumaretal.70usedfourdifferent
repulsiveforces(Fig.7).UsingDelaunaytriangulation,Kumar particletypesandfoundthatthecriticalACfrequencybehaves
et al.70 defined an ensemble-averaged inter-particle distance, as square of the particle radius ((cid:3)a2) for a given particle type.
whose time rate of increase provides a measure of the average Suchabehaviorisexpectedfromnon-equilibriumdouble-layer
inter-particle repulsive force. Repeating the procedure for polarization theory.85,86,89 The sorting effect demonstrated by
various AC frequencies and voltages, Kumar et al.70 found an Williamsetal.66isthefirstsortingofcolloidsbasedonfrequency
anomalousdecreaseinthedipole-momentoftheparticles.This dependentEDLpolarization.
A anomalousbehaviorwasaccountedforonthebasisofanaddi- Apart from these applications REP has been extended to
8
0 tional dipole moment contribution from a non-equilibrium manipulate live bacteria,90 non-spherical particles91 and even
2
20 EDL.83–86Thisphenomenon arisesfrom the polarization of the nano-rods(unpublisheddata).
C
1L EDL,aroundaparticle,inthepresenceofanelectricfield.79,85–88
C
9/ Kumaretal.70concludedthatthedipolemomentoftheparticle 2.4 Summary
3
1110 in the REP regime is given the combination of the Maxwell–
August 20g | doi:10. pWoaAlagnrnizeaarptipionlnitc.eartfiaocniaolfpRoElaPrizfaotriosno3rtianngdofnocno-leloqiudislibwraiusmdemEDonL- OoHrEaigyAiwn,aaOlrdEimTetpalanelm.d’se1Rn3tEsaePttiuoapnppsaeenaemrddsiinstotinthhcaterifrnreousmsndneeoarnclyh-uinnogitfhoeprrrmibnitociitephsleiisnn.
sity on 31 pubs.rsc.or sbbteertaawdteesdeonbfy3s8Wizaeinslldi0a1.m50,s61ektHaanlz.d6(6F2Wigmi.lml8ia).mbWyseviltaliaraylm.i6n6sgwettehraeel.a6A6bCalelstfooredsqoeumretnoPcnSy- cpfaurborrrneiconauttnioconerdbancshoeandr-geuelneicftdorirosmktriinbeeulteticcitosr.nicR, EfiwPehl,deisnremtahsiemsOiectEkuiTpngemctarpnalodyicteirodenabatyel
Univerhttp:// stratedsortingbetween1mmPSand1mmsilicabeadsbyvarying tWecihllniaimqusese.t aHl.o,9wuevtielirz,esthdeosemidniasntitncEtTioHnsflaorweshuanrldiklye tahbesooltuhteer;
due 1 on the techniques share some common operational principles. All
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Dh research can only be expected to blur the lines between these
s
bli schemes.
u
P
3. Hybrid techniques with l z 10(cid:1)4 to 10(cid:1)3 m
Theprevioussectionmainlydealswithmicron-andsub-micron
sized objects. There exists another class of manipulation tech-
niquesdealingwithdropletsandthe targetlengthscales inthis
caserangefrom10(cid:1)4to10(cid:1)3m.Untilnow,OEW,FEOET,and
some OEW derivatives are the major hybrid opto-electric tech-
niques used for droplet handling. Their operating mechanisms,
configurations, and performances will be discussed in the
followingcontent.Informationregardingthesetechniquesisalso
summarizedinTable2.
3.1 Optoelectrowetting(OEW)
Electrowetting (EW) is a technique well known for aqueous
dropletmanipulationbyelectricallydrivensurfacetensionandis
a cornerstone in the field of digital microfluidics. The phenom-
enon was first explained by Gabriel Lippmann in 1875.93
Accordingtohismodel,thecontactangleofadropofelectrolyte
onanelectrodecanbereducedbyapplyinganelectricpotential
across the electrolyte. The contact angle represents an equilib-
rium state of the surface tensions between the liquid, solid and
gasphases.Theexternalenergytriggerstheredistributionofthe
Fig.7 ForcespectroscopybasedonREP.(a)AREPaggregation.(b)
surfacechargesinbothsolidandliquidphases,whicheventually
The aggregation ‘explodes’ when the laser is deactivated. (c) The time
derivative of the ensemble averaged inter-particle distance provides reshapes the appearance of the aqueous droplet. Later,
a measure of the inter-particle force. Reprinted with permission from researchers6,93,94 came up with a modification, dubbed electro-
Kumaretal.70Copyright2010AmericanChemicalSociety. wettingondielectric(EWOD),byaddingadielectriclayer(e.g.,
2142 | LabChip,2011,11, 2135–2148 Thisjournal isªTheRoyalSociety ofChemistry2011