REVIEW

DC insulator dielectrophoretic applications in microdevicetechnology: a review

Soumya K. Srivastava & Aytug Gencoglu &

Adrienne R. Minerick

Received: 1 July 2010 /Revised: 10 September 2010 /Accepted: 13 September 2010 /Published online: 22 October 2010# Springer-Verlag 2010

Abstract Dielectrophoresis is a noninvasive, nondestructive,inexpensive, and fast technique for the manipulation ofbioparticles. Recent advances in the field of dielectrophoresis(DEP) have resulted in new approaches for characterizing thebehavior of particles and cells using direct current (DC)electric fields. In such approaches, spatial nonuniformities arecreated in the channel by embedding insulating obstacles inthe channel or flow field in order to perform separation ortrapping. This emerging field of dielectrophoresis is com-monly termed DC insulator dielectrophoresis (DC-iDEP),insulator-based dielectrophoresis (iDEP), or electrodelessdielectrophoresis (eDEP). In many microdevices, this formof dielectrophoresis has advantages over traditional AC-DEP,including single material microfabrication, remotely posi-tioned electrodes, and reduced fouling of the test region. DC-iDEP applications have included disease detection, separationof cancerous cells from normal cells, and separation of livefrom dead bacteria. However, there is a need for a critical report

to integrate these important research findings. The aim of thisreview is to provide an overview of the current state-of-arttechnology in the field of DC-iDEP for the separation andtrapping of inert particles and cells. In this article, a review ofthe concepts and theory leading to the manipulation of particlesvia DC-iDEP is given, and insulating obstacle geometrydesigns and the characterization of device performance arediscussed. This review compiles and compares the significantfindings obtained by researchers in handling and manipulatingparticles.

Keywords DC dielectrophoresis . Insulator-baseddielectrophoresis . Electrodeless dielectrophoresis .

Microfluidics . Electrokinetic separations . Bioparticles

Introduction

Microfabrication for the miniaturization of analyticallaboratory procedures has relied heavily on the knowledgegained from the manufacture of integrated circuits by theelectronics industry during the computer revolution [1].Micro total analysis systems (μTAS) have the capacity toperform diverse tasks, ranging from DNA analysis toprotein recognition, and can also be tailored into point-of-caremedical diagnostic tools [2], a term first coined by Manz et al.[3]. The first analytical laboratory miniaturization wasachieved at Stanford University in 1979 by Terry et al., whofabricated a gas chromatograph system on a silicon chip [1].Since this initial breakthrough, a plethora of lab-on-a-chip(LOC) devices integrating reactions, separations, mixing,detections, and other laboratory functions have been developedwith robust and innovative technological strategies in order tohandle extremely small fluid volumes, down to picoliters [4–10]. Compared to traditional or macro-sized instruments,LOCs provide a better platform for handling bioparticles sincethey do not require expensive, specialized instruments or a

S. K. SrivastavaDave C. Swalm School of Chemical Engineering,Mississippi State University,Starkville, MS 39762, USA

A. Gencoglu :A. R. Minerick (*)Department of Chemical Engineering,Michigan Technological University,1400 Townsend Drive,Houghton, MI 49931, USAe-mail: minerick@mtu.edu

A. Gencoglue-mail: agencogl@mtu.edu

Present Address:S. K. SrivastavaGene and Linda Voiland School of Chemical Engineeringand Bioengineering, Washington State University,Pullman, WA 99164, USAe-mail: soumya.srivastava@wsu.edu

Anal Bioanal Chem (2011) 399:301–321DOI 10.1007/s00216-010-4222-6

laboratory environment, and they perform better, yieldinghigher efficiencies, reproducibility and affordability [11].

Fluids are moved through channels and chambers on thischip via applied pressure differences, capillary driving forcesdue to the wetting of surfaces by fluids, and free-surface flowsdriven by gradients [12–14]. In addition to these forces,electrokinetics are commonly utilized to move analytes,because electric fields are versatile and can be preciselycontrolled, allowing for specific, quantifiable analyteresponses. Furthermore, devices employing electric fields caneventually be simplified such that they only require a batteryfor power—a key characteristic for truly portable diagnosticdevices. Dielectrophoresis, the use of alternating current (AC)to create spatially nonuniform electric fields in order to polarizeparticles or cells, is key to the development of such analyticaland diagnostic devices, as it allows operational simplicity andrequires only a low voltage and small sample volumes—all ofwhich enable device portability [8]. Separation and trappingcan be achieved with a high degree of selectivity andsensitivity, which makes dielectrophoresis a powerful tool fordiagnostic applications. An emerging technology in the field ofdielectrophoresis is DC dielectrophoresis, in which insulatingobjects are embedded in the microchannel to create spatialnonuniformities in the electric field [5, 8]. Remotely appliedDC currents are utilized to create electric field gradients aroundthese insulating obstacles. DC dielectrophoresis differs fromAC dielectrophoresis in that particle or cell polarization isaccomplished only via spatial variations in the electric field;the (frequency-dependent) transient polarization that occurswhen using alternating current does not occur in DCimplementations of this technique.

Due to the relatively new nature of the field of DC-iDEP,a review of this field has not yet been conducted. Therefore,the objective of this review is to depict the state of the art inthe use of DC insulator based dielectrophoretic devices tomanipulate inert objects and bioparticles. Below, we discussthe dielectrophoretic theory behind the use of DC electricfields to create spatial nonuniformities in the channel, andthe fabrication of insulator-based microdevices to accomplishDC insulator based dielectrophoresis. In order to fully explorethis novel technique, it is important to understand deviceperformance. Below, we describe some of the mediumconditions that are suitable for trapping or sorting particles viaDC-iDEP in glass and PDMS microdevices. A comprehensivereview of the manipulation of different particles and biologicalcells, leading to applications in analytical tests and medicaldiagnostics, is presented and arranged by bioparticle size. Thus,the review provides a thorough overview of DC-iDEPtechnology for researchers aiming to achieve the sorting ortrapping of particles. Finally, please note that this article uses theterm “particle” to mean an object with any geometry andcomposition, including cells, inert objects and large biologicalmolecules.

Dielectrophoresis

The field of dielectrophoresis envisioned by H.A. Pohl inhis seminal text Dielectrophoresis: The Behavior of NeutralMatter in Nonuniform Electric Fields 32 years ago hasbroadened considerably in the last decade [15]. Dielectro-phoresis (or DEP) is the phenomenon that occurs due to theforce exerted on a dielectric particle when it is subjected toa nonuniform electric field. This is traditionally an ACfield, although DC fields can achieve similar particleresponses. Initially the term DEP only referred to effectson particle translation, but recently several other effectshave also been included in this area, such as traveling wave,quadrapole, and rotational effects. The DEP force experiencedby a particle depends on the permittivity of the mediumsurrounding it (εm), the electric field E

!� �, the radius of the

particle (r), and the dielectric properties of the medium andparticle, which are traditionally described by the Clausius–Mossotti (CM) factor, α [4, 16, 17]:

F!� �

DEP¼ 2pr3"ma rE

!2� �ð1Þ

Spatial field gradients are captured within the term rE!2

,whereas the frequency dependence of the AC field iscaptured within the complex permittivities of the Clausius–Mossotti factor:

a ¼ e"p � e"me"p þ 2 e"m ð2Þ

where e" is the complex effective permittivity of the particle(p) and the medium (m). The CM factor determines theparticle polarization that is observed as positive andnegative dielectrophoresis. Positive α values due to largerparticle permittivities yield positive DEP forces andparticle movement towards high field density regions (i.e.,up the field gradient); negative α values yield particle motiondown the field gradient [15, 17]. While the CM factordescribed above is valid for a perfectly spherical particle, it isfrequently used to predict the behaviors of nonsphericalparticles too.

Conventional AC dielectrophoresis is achieved by either (i)placing nonparallel metal electrodes in direct contact with themedium, thus creating an electric field gradient which isdirectly controlled by an AC power supply, or (ii) placingelectrically free-floating metal electrodes in direct contactwith the solution [18]. However, these embedded electrodesystems incur issues with Joule heating, electrode fouling[19], fabrication complexities due to metal microfabricationin the microchambers, and electrochemical reactions on theelectrode surface [20]. An alternative to AC dielectrophoresisis insulator-based DC dielectrophoresis (DC-iDEP), whichuses insulating obstacles, hurdles or protrusions in thechannel to create spatial field nonuniformities.

302 S.K. Srivastava et al.

DC dielectrophoresis: theory

Direct current insulator-based dielectrophoresis (DC-iDEP)is a relatively new field. Masuda et al. [21] first developedthis concept for a biological application involving cellfusion, which was later summarized by Lee et al. in 1994[22, 23]. DC-iDEP utilizes the spatially nonuniform electricfield component but not the frequency-dependent compo-nent of dielectrophoresis, which has been shown to beadequate for the manipulation of cells [24]. Spatialnonuniformities in the electric field are generated byplacing insulating obstacles in the channels of the micro-device, and passing a remotely applied DC current acrossthese channels. The insulating obstacles are mechanicallyrobust and chemically inert, and are less prone to foulingthan embedded electrodes. Electrolysis—the unwanted sideeffect of DC fields—still occurs at the electrode surface, butremote positioning of the electrodes can prevent bubblesinside the microchannel [18, 25, 26].

Cummings and Singh observed two flow regimes in DC-iDEP systems [26], as a result of the competition betweenelectrokinetic flow (electrophoresis and electro-osmosiscombined) and dielectrophoresis. When DEP overcomesdiffusion but electrokinetic flow dominates, streaming DEPoccurs, but when DEP forces dominate over both diffusionand electrokinetic flow, trapping DEP (into electric fieldminima) occurs [26]. Streaming DEP is the DC equivalentof weak negative DEP, while trapping DEP is the DCequivalent of strong negative DEP. The literature is filledwith reports of negative DEP [20, 23–34]. Observations ofpositive DEP have only been reported by Zhang et al. inDC-iDEP, where a circular channel geometry wasemployed to sort 5 and 10 μm particles [35].

DC dielectrophoresis differs from AC dielectrophoresisin relation to the Clausius–Mossotti factor α in Eq. 2. Eachcomplex permittivity is a function of the frequency ω andconductivity σ:

e" ¼ "� isw

ð3Þ

When the frequency is zero (for strict DC-iDEP), thedielectrophoretic force is estimated from the residual CMfactor [6, 7]:

a ¼ sp � sm

sp þ 2smð4Þ

A positive α results in positive DEP, where particlesconcentrate at field maxima, and vice versa. However, theDC-iDEP literature is dominated by reports of negativeDEP, probably because of the particles explored and theimpact of low-conductivity media on the magnitude of thelinear electrokinetic contribution. The conductivity of a

particle (σp) can be expressed as a function of the surfaceconductivity κs, the bulk conductivity within the particleσb, and the particle radius [36]:

sp ¼ sb þ 2ks

rð5Þ

Homogeneous particles made of a single material, likepolystyrene particles, have electrical conductivities in therange of 10–16 S/m, while many buffers are on the order of10−2 S/m [28, 36, 37]. Cell’s interstitial fluid has higherconductivities on the order of σb=1 S/m, but the conduc-tivity of the cell membrane (i.e., κs) is substantially lower at10−7 S/m [5]. As a result, most cells and particles yieldnegative α values, and thus negative DEP behaviors.

The conductivity of the medium also impacts theelectrokinetic contribution, which is usually treated asbeing the sum of electro-osmosis and electrophoresis. Theionic concentrations in the medium impact the surface zetapotentials within the device, and thus the electro-osmoticflow. Low medium conductivities favor positive DEPbehaviors; in addition, the highest electro-osmotic flowvelocities are achieved under these conditions. Unless thetotal DEP force exceeds the electrokinetic forces fromelectro-osmosis and electrophoresis, streaming DEP will beobserved. Only under conditions involving highly conduc-tive particles and high medium conductivity will positiveDEP be observed in a DC system. As stated, DC-iDEPparticle motion is governed by both linear electrokineticand nonlinear dielectrophoretic forces:

j!/ u!EK þ u!DEP

� � ð6Þwhere j

!is the particle flux, u!EK is the electrokinetic

velocity, and u!DEP , the dielectrophoretic velocity, is theproduct of the dielectrophoretic mobility μDEP and theelectric field gradient. For DC-iDEP, a spatially dense

nonuniform field rE!2� �

is created as the DC field linesdiverge around the insulating obstacle. High electric fielddensities occur within narrow constrictive regions. Fluidflow drives the particle through this narrow constriction,while the shape of the field gradient aids in particle motion.DC-iDEP forces push the particle along the field gradienteither along specific flow streamlines or into trappingregions, depending on the particle’s polarizability. DC-iDEP is increasingly utilized in LOC devices because thegeometrical configurations and operating conditions involvedare well suited to bulk manufacturing techniques like injectionmolding, and precise particle manipulations are possible usingDC or hybrid electric field conditions [38]. For example, aninsulating DEP device can be operated using simultaneousAC and DC fields, with the AC usually in the low frequencyrange, as detailed in the “Electric field characteristics”section.

DC insulator dielectrophoretic applications 303

Insulating obstacle geometry

Researchers have created spatial nonuniformities usingsingle or multiple insulating obstacles, and in some casesmodified the channel geometry to achieve particle sortingor trapping. Single obstacles embedded in the channel, likea rectangular obstacle [7, 25, 34, 39, 40], a triangular hurdle[34], a constriction in the depth of the channel rather thanthe width [41], a single microchannel constriction [42], anoil droplet [43], and oil meniscus used as field-shapingbarriers [33]; multiple obstacles like metallic traps [18, 44],insulating posts [20, 24, 26, 29, 30, 32, 45, 46], tetragonalstructures along the channel [47, 48], and multiplerectangular blocks [49]; as well as other geometries thatmodify the channels, like nonconverging sawtooth channels[50], a set of aligned teeth [51], sawtooth channels [50],open-top microstructures [52, 53], serpentine microchannels[54, 55], a series of iterative curves [54], arrays of circularchannels [35] and a honeycomb membrane design [56] arediscussed below in various sections. Each obstacle shapeyields different spatial variations in the electric field. Suchdevices were fabricated by standard lithography techniques,and the external metal electrodes were connected to a highvoltage source.

Single-obstacle geometry

The microchannel can contain a single insulating obstacleto achieve the manipulation of bioparticles. This insulatingobstacle may be made of PDMS, glass, quartz, or oil.Rectangular and triangular obstacles have been extensivelyapplied to achieve the sorting of particles and bioparticles[25, 34, 39, 40]. An example of how electric field linesdiverge around an insulating rectangular obstacle is shownin Fig. 1, which is adapted from our research [7].

The magnitude of the DEP force is proportional to theparticle size, as governed by Eq. 1. Larger particles tend todeflect away from the corner of the obstacle due to thelarger DEP forces they experience [34], thus pushing theparticle away from the insulating obstacle region. The linesin Fig. 1 represent electric field pathlines, so the dark regionof overlapping lines indicates the high field intensity regionwhere particles experience dielectrophoretic forces thattranslate the particles into the flow streamlines of therespective channels, depending on the polarizability of theparticle. This is consistent with results observed in ourgroup, where smaller-sized (6.2 μm) fluorescent polystyreneparticles were minimally deflected from the insulatingobstacle, while larger-sized (10 μm) particles were translatedaway from the insulating obstacle geometry.

Furthermore, a single curved constriction in the depth ofthe channel has been shown to allow linear and nonlinearelectrokinetic effects to be controlled when employing both

DC and AC fields [41]. Linear effects (electro-osmosis andelectrophoresis) control the particle motion, whereas non-linear effects (dielectrophoresis) are applied when theparticle approaches the constriction. DEP is appliedperpendicular to the channel constriction, and the ratiobetween the linear and nonlinear effects determines thetrapping or sorting of particles. The sorting of 2 and 3 μmpolystyrene particles was accomplished by employing 3Dshaped electric fields in a continuous throughput system[41]. Another group employed a microchannel constrictionalong the length of the channel [42], and used low-frequency AC and DC fields to improve the focusingefficiency for 5 and 10 μm microparticles. Two electricfield gradients were created in the channel due to theconstriction and singularities around the corners. Theelectric field lines at these points are parallel and normalto the electric field applied, respectively.

Another class of single insulating obstacles uses oil tocreate an obstacle, thus achieving spatial nonuniformities inthe electric field, as shown in Fig. 2. The greatest advantageof the oil droplet and oil meniscus design is the controllabledroplet size, which provides direct and dynamic controlover the field gradient, thus enabling various separationconfigurations [33, 43]. The gap width between the oilobstacle and the wall can be easily controlled, and Li et al.reported that this gap width was an important parameter forachieving good separation efficiency [43]. The electric fieldgradient depends on this gap width, and even if lowvoltages are applied, the electric field gradient should behigh enough to achieve the separation of particles. Thismethod can be applied to manipulate biological particlesthat are sensitive to high voltages, thus avoiding cell lysisand significant Joule heating of the medium [43]. The useof low voltages slows the cell separation, decreasingparticle velocity. To achieve the continuous separation ofparticles and to prevent trapping, electro-osmotic flow(EOF) of the medium is required [43]; this can be

Fig. 1 Electric field lines around a rectangular obstacle, where thedarker region represents the high field density region. Figure obtainedfrom COMSOL simulations performed during our research

304 S.K. Srivastava et al.

controlled through the judicious choice of wall materials orcoatings. Higher DEP forces can be realized by increasingthe oil droplet size, which increases the constriction in thechannel and thus compacts the electric field lines andincreases the field gradient, as seen in Eq. 1. An oilmeniscus obstacle used by Thwar et al. succeeded intrapping particles by controlling the position and extent ofthe location of the oil meniscus [33]. Oil-filled syringeswere used to create this hurdle, which allowed for theefficient trapping of conducting biomolecules like DNA, asthe conductivity of these biomolecules is much greater thanthat of the medium. The main disadvantage of this approachis the reduced reusability of the microchips due to thefouling of channels caused by the use of oil.

Multiple-obstacle geometry

Chou et al. was the first to demonstrate the application ofDC-iDEP phenomena to sort single- and double-strandedDNA by employing dielectrophoretic traps in the channel[18, 44]. Figure 3 shows a schematic representation of adielectrophoretic trap [18]. These DEP traps are insulatingconstrictions in the channel.

Another form of multiple obstacle geometry that isextensively used in the DC-iDEP manipulation of particlesis an array of insulating posts [20, 23, 24, 26, 29–32, 36,45, 46, 57]. Such an array made of glass was utilized tocreate field nonuniformities in the channel. These diamond-shaped or circular insulating posts were adjusted to achieveboth streaming and trapping DEP. In the diamond-shapedinsulating posts, high electric field intensity regionsoccurred at the right and left vertices of the insulating post,while low field density regions occurred at the tops andbottoms of the insulating posts [26], as seen in Fig. 4.

Changing the angle of the array of insulating posts affectedthe whole DEP behavior of the particles. With circularinsulating posts, the region of high electric field densitywas between two insulating posts [45]. Lapizco et al.simulated trapping zones near these insulating posts andwere able to successfully predict the locations andmagnitudes of the trapping zones [31]. Sabounchi et al.developed a novel technique combining DC-iDEP andimpedance measurements for trapping and analyzingbacteria, which employed DC and AC signals, respectively[46]. Insulating tetragonal structures in the microchannelswere used to trap the particles. Maximum electric fielddensity regions occurred at the corners of the tetragonalstructures. The spacing between the two tetragonal struc-tures could be adjusted to obtain maximum trappingefficiency [48]. Multiple rectangular insulating blocks wereemployed to concentrate and separate the particles. This useof multiple blocks enhanced the dielectrophoretic effect onthe particles, thus confining the particles to a small regionfor further analysis [49].

Channel variations

Apart from embedding insulating obstacles in the micro-channel, various channel geometry modifications have alsobeen used to achieve the trapping or sorting of particles.Chen et al. explored nonconverging sawtooth channeldesigns for the separation of small particles. Sharpeningthe corners of the teeth increased the dielectrophoreticeffect on the particles due to the large gradients present inthe high electric field density region. The separation ofthese particles depended on electrophoretic and dielectro-phoretic forces [50]. Trapping depended on the propertiesof the particles and the channel geometry. When the

Fig. 3 Schematic of a micro-fluidic DEP trap. (A) A metallicDEP trap made of microfabri-cated wire(s) on a substrate. Thewire(s) may be either free-floating or connected to a volt-age source. (B) An electrodelessDEP trap made of dielectricconstrictions. The solid lines areelectric field lines. (C) A scan-ning electron micrograph of anelectrodeless DEP device con-sisting of a constriction arrayetched in quartz. The constric-tions are 1 μm wide and1.25 μm deep. The whole chipmeasures 1×1 cm. The double-headed arrows show the appliedelectric field direction z(reproduced from [18] with thepermission of Elsevier)

Fig. 2 Electric field lines and field strength contours near the oil droplet.A particle in the high-field region is exposed to a negative DEP force. Thedarker region has a stronger electrical field (reproduced from [43] withthe permission of The Royal Society of Chemistry)

DC insulator dielectrophoretic applications 305

channel geometry was fixed, the trapping depended only onthe particle mobility ratio, the electric field, and the spacingbetween the teeth. Selective trapping of particles could beachieved by varying the channel geometry [50]. Similar tothe nonconverging sawtooth channel design, a convergingsawtooth channel geometry and a geometry with a set ofaligned teeth were also explored, with similar results.

An open-top microchannel design that was developed totrap cells also enabled direct contact with the cells fortreatments or sampling. In this work, an open-top micro-channel adapted the insulating obstacle geometry designused by Chou et al. in order to create DEP traps, as shownin Fig. 3. Here, sample loading into the device and cleaningafter analysis were much easier and faster than in the closedchannel design. Joule heating effects were lower due to thedissipation of heat to the open air. There was also nointerference during the fluorescence detection of emittedlight. However, this design does have certain limitations,including sample evaporation [52, 53] and convectivecurrents. However, the open-channel concept was effective,and constrictions in the channels created the nonuniformityneeded to trap human carcinoma (HeLa) cells. From thenumerical results presented, the DEP force was stillconsidered to be strong within 70 μm of the constriction’sheight [53].

In a serpentine channel design (one left U-turn followedby right U-turn), the DEP force was stronger in the innercorners compared to the outer corners [55]. The particleswere uniformly distributed at the straight sections of thechannel and were focused into streams along the channelcenterline when exiting the serpentine channel. Thisbehavior is consistent with streaming DEP indicative ofnegative DEP (nDEP), where the dielectrophoretic force actsto push the particles down the electric field gradient while theparticle simultaneously experiences large translational forces

due to EOF. The width of the focused stream is also dependenton the channel width and the length of the serpentine section[55]. The same concept holds for an iterative curve channelgeometry, which will not be explained in detail here.

Another channel geometry that has been explored is thecircular channel design used by different research groups,as shown in Figs. 4 and 5. Here, the separation of particlesdepends on their dielectric properties, and they are driventhrough the channel by electro-osmosis [35]. The electricfield gradient directs the particles to the center of thechannel (high field density region). However, due to nDEP,the particles move from the center towards a different

Fig. 4 Common insulating ob-stacle geometries reported in theliterature. Red zones indicatewhere the particles experiencethe maximum dielectrophoreticeffect under DC electric fieldconditions

Fig. 5 Schematic diagram of dielectrophoretic separation in a circularmicrochannel. Reproduced with permission from [37]

306 S.K. Srivastava et al.

location (depending on the dielectrophoretic effect onthem). Another novel design was presented by Cho et al.,in which microfabricated plastic membranes were used toconcentrate bioparticles [56]. The membrane hadhoneycomb-type pores that were positioned between twoelectrodes made of indium tin oxide. A high electric fielddensity occurred at the corners of the pores, and thebioparticles were trapped due to a positive DEP effect.Trapping was achieved experimentally, and COMSOLsimulations allowed the regions of the membrane wheretrapping was concentrated to be characterized.

Some of the insulating obstacle geometries discussedabove are compared in Table 1. This table provides aconcise review of DC-iDEP research, including the mediaand particles used in related studies (potassium-bufferedsaline solution and 10 μm diameter polystyrene particleswere considered for simulations), the obstacle geometriesemployed, and the electric fields applied. Estimates of theDEP force obtained from COMSOL simulations andmanual calculations, along with the COMSOL-estimatedrelative DEP force (i.e., the order-of-magnitude differencebetween the highest and lowest DEP forces within thesystem) are also listed. The latter value is a measure of howeffective a given obstacle is at increasing the DEP force.The geometry of the insulating obstacle is responsible forproducing field nonuniformities, which affect the gradientof the electric field and impact on the dielectrophoreticforce experienced by the particle. Negative DEP causesstreaming or sorting of particles into streamlines, whereaspositive DEP causes the particles to become trapped orattracted towards the insulating obstacle.

The dielectrophoretic force was estimated using COM-SOL Multiphysics 3.3 simulation software. The “conduc-tive media DC physics” mode was used for the simulation.This mode is a time-independent model that can calculateelectric field strength within a conductive medium, such asa microchannel filled with an aqueous solution. Thegoverning equation for the system is

�r:d srV � J eð Þ ¼ dQj ð7Þ

where σ is the electrical conductivity of the medium, V isthe electric potential and Je is an externally generatedcurrent density. A static mesh was used for the simulations.The material properties of the system were kept constant sothat the differences in the estimated DEP forces would onlybe due to the different insulating obstacle geometries andapplied field strengths. A medium conductivity of 9 S/m (atypical value for potassium-buffered saline solution) wasused, and the CM factor was equal to −0.5, since thepermittivity of polystyrene particles is very small comparedto that of potassium-buffered saline solution. The permit-tivity of the medium was 7.08×10−10 F/m. The DEP force

is greatest at the corners of the insulating obstacle, so theDEP force was estimated at this point. COMSOL returnsthe value of the electric field strength in the direction offlow as a variable, and the electric field gradient wascalculated from this value; the width of the channel at thepoint where the DEP force was estimated using Eq. 8. TheDEP force was estimated using Eq. 1 for a spherical particlegeometry.

r ~E�� ��2 ¼

~E�� ��2w

ð8Þ

where ~E�� �� is the electric field strength in the direction of the

fluid flow calculated by COMSOL, and w is the width ofthe channel or the distance between two obstacles at thepoint where the DEP force is estimated. For the values usedin the simulation, the DEP force reduces to:

~FDEP ¼ 2:78� 10�15r ~E�� ��2 ð9Þ

In the case of DC-biased AC fields, only the DCcomponent was taken into account. In the case of AC-iDEP, the DEP force was calculated for both the maximumfield strength and the root mean square (RMS) fieldstrength. In addition to the COMSOL simulations, theDEP forces in each system were also estimated by manualcalculations based on Eq. 1. Finally, the maximum andminimum DEP forces present in the system were comparedto find out which geometries created a greater relativeincrease in the DEP force. The lowest DEP force present inthe system is the “ambient” DEP force, which is theuniform value found away from the obstacles. Themaximum DEP force was typically 3–10 orders ofmagnitude greater than the lowest value.

As the previous discussion demonstrates (and Table 1shows), a complete picture of how insulator shape, insulatormaterial, medium, and voltage operating conditions affectthe DEP force has not yet been attained. A number ofmodels are notable, such as those for a rectangle, triangle,oil droplet, saw tooth, serpentine, circular, open-topmicrostructure, and a model involving four outlet channelsand a rectangular insulating obstacle geometry from ourgroup, but none of these give perfect agreement acrossexperimental operating ranges.

Aside from DC-iDEP and the traditional DEP technique,there are other DEP techniques that allow separation ortrapping to be achieved, as discussed in the next section.These alternative DEP techniques differ from DC-iDEP andAC-DEP in the electrode geometries they employ to createnonuniformities. Not all dielectrophoretic systems useembedded metal electrodes or insulating obstacle geometriesto manipulate particles; other mechanisms do exist, and a briefoverview is presented below. Note, however, that theobjective of this review is to discuss different electrode

DC insulator dielectrophoretic applications 307

Tab

le1

Review

ofDC-iDEPresearch,includ

ingthemedia

andparticlesused

inthosestud

ies,insulatin

gob

stacle

geom

etries

employ

ed,andelectric

fields

applied.

Estim

ates

oftheDEPforce

obtained

from

COMSOLsimulations

andmanualcalculations,along

with

theCOMSOL-estim

ated

relativ

eDEPforce(i.e.,theorder-of-m

agnitude

difference

betweenthehigh

estandlowestDEP

forces

with

inthesystem

)arealso

listed.

The

lattervalueisameasure

ofho

weffectiveagivenob

stacle

isat

increasing

theDEPforce

Reference

Devicematerial

properties

Coatin

g/sealing

method

Particle

ofinterest

(radiusof

particle

inμm)

Electricfield

Insulatin

gobstacle

shapeandgeom

etry

Propertiesof

medium

Com

ments

Max.force(N

)(COMSOL)

Max.force

(N)*

(manual

calculation)

Differencein

orderof

magnitude

Chouet

al.[18]

Quartz

Oxygenplasma/

POP-6

(polyacrylam

ide)

Single-

and

double-stranded

DNA

(0.001)

0–1kV

pp/cm,

200–1000

Hz

DEPtrap:1μm

wide

and1.25

μm

deep

Trisborate

EDTA

,pH

8Trappingand

concentration

viaACDEP

707V/cm

(RMS):

5.109×10

−11

707V/cm

(RMS):

1.39

×10

−9

RMS:11

1kV

/cm

(max):

1.022×10

−8

1kV

/cm

(max):

2.78

×10

−9

Max:10

Cum

mings

andSingh

[26]

Glass

Therm

albonding

Latex

particles(0.1)

250–1000

V/cm

Diamond,

square

and

circular

posts:

distance

between

twopostsis63

μm

1mM

PBS,

pH7.7

Streamingand

trapping

DEP

Square:

1000

V/cm:

1.732×10

−10

4.42

×10

−9Square:

5

Diamond:

1000

V/cm:

5.502×10

−10

Diamond:

3

Circle:

1000

V/cm:

1.608×10

−11

Circle:

6

Lapizco-Encinas

etal.[20]

Glass

NA

T4virus(0.045)

400V/cm

Array

ofmicrobumps

BU

buffer

Firstarticle

ontrapping

ofavirus

2.480×10

−10

4.94

×10

−98

Lapizco-Encinas

etal.[24]

Glass

Therm

albonding

E.coli(0.25)

0–2000

V/cm

Square,

triangle

and

circular

insulatin

gposts:200μm

indiam

eter

and

250μm

between

twoposts

DIwater,

2.25

μS/m

mFirstseparatio

nandconcentration

ofliv

e/dead

bacteria

2000

V/cm:

3.901×10

−10

2000

V/cm:

4.450×10

−11

6

Lapizco-Encinas

etal.[23]

Glass

NA

E.coli,B.subtilis,

tobaccomosaic

virus(0.009)

0–2000

V/cm

Circularinsulatin

gposts:200μm

indiam

eter

and250μm

betweentwoposts

DIwater–N

aOH–

KCl,pH

7.5–

8.0,

10–

20μS/cm

2000

V/cm:

2.560×10

−15

2000

V/cm:

4.450×10

−11

10V:2

2000

V:3

Barbulovic-Nad

etal.[43]

PDMS

Plasm

asealed

Fluorescent

polystyrene

particles(0.5;

2.85;7.85)

80–240

V/cm

Oildroplet:gap

betweenoildroplet

andchannelwall

is46

μm

DIwater

Separationof

microparticles

240V/cm:

6.260×10

−12

240V/cm:

3.480×10

−12

7

Kanget

al.

[25]

PDMS(−80

mV)

NA

Carboxylate-m

odified

polystyreneparticles

(2.85;

5.125;

7.85)

500–900V/cm

Rectangular

(240

×130μm)

obstacle;60

μm

betweenwalland

obstacle

1mM

sodium

carbonate

buffer

Separationof

microparticles

bysize

360V/cm:

9.743×10

−11

360V/cm:

6.010×10

−12

4

Zhang

etal.

[35]

Sim

ulation-based,

discussion

ofparticle

trajectories

Microparticles(5;10)

Circularchannel:

50μm

innercircle,

100μm

outercircle,

20μm

height

Separationof

particlesin

acircular

channel

6.006×10

−12

2.500×10

−16

3

Haw

kins

etal.[41]

Zeonor1020R

Hot

pressbonding

Microparticles

(0.875;1;

1.5)

DC-biasedACfield

(25or

50V/cm;

1kH

zfrequency)

Insulatin

gconstrictio

nNA

Firstreportof

3Dinsulatin

gtechnique

10μm

wide

ridge:

50V/cm:

1.195×10

−10

10μm

wide

ridge:

50V/cm:

6.95

×10

−13

10μm

wide

ridge:

12

100μm

wide

ridge:

50V/cm:

1.013×10

−10

100μm

wide

ridge:

50V/cm:

6.95

×10

−14

100μm

wide

ridge:

8

Thw

aret

al.

[33]

PDMS

UV/ozone

Fluorescent

polystyrene

microparticles

DCvoltage

−500

and+500V

Field-shaping

oil

barriers

0.05

S/m

Sim

ulationand

experiments

6.080×10

−13

2.5×10

−12

6

308 S.K. Srivastava et al.

Tab

le1

(con

tinued)

Reference

Devicematerial

properties

Coatin

g/sealing

method

Particle

ofinterest

(radiusof

particle

inμm)

Electricfield

Insulatin

gobstacle

shapeandgeom

etry

Propertiesof

medium

Com

ments

Max.force(N

)(COMSOL)

Max.force

(N)*

(manual

calculation)

Differencein

orderof

magnitude

Pysheret

al.

[51]

Glass

andPDMS

devices

Plasm

asealed

Bacillus

subtilis,

E.coli,and

Staphylococcus

epidermidis(0.009)

DCfield200V/cm

and300V/cm

Saw

tooth:

varying

toothgeom

etry;max

width

is500μm

NA

Separationof

complex

biological

particles

Height150μm,

base

150μm,

300V/cm:

5.726×10

−12

Height

150μm,

300V/cm:

7.150×10

−13

H150,

B150:

3

H240,

B240,

300V/cm:

1.600×10

−11

H240,

300V/cm:

9.630×10

−13

H240,

B240:

3

H100,

B200,

300V/cm:

4.983×10

−12

H100,

300V/cm:

2.500×10

−12

H100,

B200:

2

H240,

B480,

300V/cm:

1.410×10

−11

H240,

B480:

2

Zhang

etal.

[54]

Silicon–SU8–

glasswafer

assembly

Adhesivebonding

Carboxylate-m

odified

FluoS

pheres

(1)

DC-biasedACfield;

DC50–90V/cm

and1MHz

Iterativecurves:inner

radius

is100μm

and

outerradius

is200μm

NA

Separatingsubm

icron

particlesat

low

voltages

90V/cm:

1.327×10

−13

90V/cm:

2.250×

10−11

8

Sabounchi

etal.[46]

Zeonor

1060

Pressureand

temperature

bonding

B.subtilisspores

andcarboxylate-

modified

microspheres

(1.5;1)

DCforconcentrating

andACfor

flow

ingto

impedance

detectionport

Insulatin

gposts

1–2μS/cm

CouplingiDEP

with

impedance

detection

1.010×10

−10

3.130×10

−11

6

Sabounchi

etal.[58]

Zeonor1060

NA

E.colicells

andB.

subtilis(1.5)

DC500–1500

V/cm

Insulatin

gposts

60–150

μS/cm

Experim

entaland

numerical

study

ofJouleheating

effectsin

apolymeric

iDEP

device

1500

V/cm:

1.925×10

−15

1500

V/cm:

1.000×10

−10

2

Ozuna-Chacon

etal.[36]

Glass

NA

Carboxylated

polystyrene

microspheres(0.5)

DCfield200–

800V/cm

Cylindrical

posts:

440μm

diam

eter,

10μm

high

and

520μm

center-to-

center

separatio

nof

posts

25–100

μS/cm

Higherfields,high

conductiv

ityand

low

pH–stronger

negativ

eDEPforce

fortrapping

800V/cm

1.759×10

−10

800V/cm

2.230×10

−11

5

Lapizco-Encinas

etal.[29]

Glass

NA

Protein

DCfield700–

1600

V/cm

Cylindrical

posts:

440μm

diam

eter,

10μm

high

and

520μm

center-to-

center

separatio

nof

posts

25–100

μS/cm

Trappingin

low-

conductiv

itymedium

conditions

1600

V/cm

6.231×10

−10

1600

V/cm

8.900×10

−11

5

Kanget

al.

[34]

PDMS

NA

Fixed

HIV-infected

white

bloodcells

andliv

emam

malian

breastcancer

cells

(4–7;10–30)

DCvoltage

0–138V/cm

Rectangular

and

triangular

(240

×130μm)obstacles;

60μm

betweenwall

andobstacle

NA

Separationby

size

Rectangle:

138V/cm

2.939×10

−12

138V/cm:

8.830×10

−13

Rectangle:10

Triangle:

138V/cm

2.930×10

−12

Triangle:

2

Zhu

etal.

[55]

PDMS(5.5×

10−8m

2/Vs)

Plasm

asealed

Polystyrene

particles

(2.5;5)

DC1000,2000,

5000

V/cm

Serpentinechannel:

200μm

high,

200μm

between

twocorners,and

50μm

widechannels

160μS/cm

DC-biasedACelectric

fieldin

further

experiments

5000

V/cm:

1.725×10

−95000

V/cm:

1.390×10

−99

Martin

ez-Lopez

etal.[57]

Glass

(1.76–

2.55

×10

−8m

2/Vs)

NA

Carboxylated

polystyrene

microspheres(0.5)

DC800V

Cylindrical

posts

25,50

and

100μS/cm;

pHrange6–9

IncreasedEOFby

increasing

pHor

decreasing

conductiv

ity

1.030×10

−15

3.560×10

−11

2

Zhu

etal.

[42]

PDMS(6×

10−8m

2/Vs)

Plasm

asealed

Polystyrene

particles(2.5;5)

PureDC100kV

/m;

DC-biasedAC

field10

kV/m

with

1kH

zfrequency

Microchannel

constrictio

n:56

μm

high

and

188μm

wide

160μS/cm

PureDCandDC-

biased

ACfield

compared

PureDC:

2.580×10

−9PureDC:

4.970×10

−11

11

Jenet

al.[53]

PDMS

Plasm

asealed

Hum

ancarcinom

a(H

eLa)

cells

(5)

DCfield18

V/cm

and

frequency10

Hz

to1MHz

Opentopmicrostructures:

500μm

between

insulators,anddistance

betweenconstrictio

nsis20

μm

1.76

mS/m

Crossover

frequency

between3and4kH

z1.765×10

−9

4.510×10

−12

5

DC insulator dielectrophoretic applications 309

geometries used for dielectrophoresis, focusing particularlyon insulating obstacle geometries.

Alternative dielectrophoretic schemes

Another approach to creating nonuniformity is to use afloating electrode. In this case, long, coplanar, uncoupledelectrodes are used to generate DEP [38, 61, 62]. Figure 6shows a comparison between classic DEP configurationsand floating electrode DEP (feDEP) configuration devices.Only two electrodes are excited by a signal, while theremaining electrodes are allowed to float as shown inFig. 6b.

These feDEP devices do not require an external signalsource, making them easier to fabricate and miniaturize[38]. Some of the advantages of feDEP devices include (i) adecreased number of connections to the external signalsource, (ii) reduced Joule heating, (iii) easier fabrication,(iv) an ability to manipulate nanoparticles, and (v) reducednanoparticle Brownian motion [38, 62]. The DEP forceexperienced by the particle can be increased by using muchsmaller electrode features and reducing the gaps betweenthem. However, if the applied electric potential is increasedto create a strong DEP effect, Joule heating can occur,which is sometimes hazardous for bioparticles [38].

Another novel technique recently developed by Shafieeet al. is the contactless DEP (cDEP) technique, which issimilar to DC-iDEP technique [63, 64]. Here, the electrodesand the sample are not in contact, and they are separated bythin, insulating microbarriers. The absence of contactbetween the sample fluid and the electrodes preventscontamination, electrochemical effects, gas bubble genera-tion, and Joule heating compared to the conventionaldielectrophoresis. The electrodes are placed in a highlyconductive medium isolated from the main channel by thin,insulating barriers made of PDMS [63]. The main channelcontaining the sample has a funnel-shaped design and thePDMS barriers are placed at the intersection where themain channel tapers into the small channel. Due to thegeometry of the electrode channel, spatial nonuniformitiesare caused when a high-frequency AC signal is applied.High electric field density regions at the corners in the mainchannel are caused by the field nonuniformities in the sidechannel, thus exerting a DEP effect on the particles to befocused and separated. According to this group, thetechnique is robust, simple and inexpensive like DC-iDEP,which overcomes some of the disadvantages of DC-iDEPdevices. The technique is applicable to drug screening,disease detection and treatment, and homeland securitybiomedical applications [64].

All of these insulating obstacle geometries need to beintegrated into the microfluidic platform during fabrication. Inthe next section, a brief overview of the fabrication techniqueT

able

1(con

tinued)

Reference

Devicematerial

properties

Coatin

g/sealing

method

Particle

ofinterest

(radiusof

particle

inμm)

Electricfield

Insulatin

gobstacle

shapeandgeom

etry

Propertiesof

medium

Com

ments

Max.force(N

)(COMSOL)

Max.force

(N)*

(manual

calculation)

Differencein

orderof

magnitude

Gallo-Villanueva

etal.[30]

PDMS(2×

10−4

cm2/Vs)

NA

LinearDNA

particles

(pET28b)

(0.001)

DCfield500–

2000

V/cm

Rectangularinsulating

posts:470μm

indiam

eterandcenter-

to-centerdistance

betweenpostsis510μm

pH10.8–11.15;

100–120μS/cm

500–1500

V/cm

cells

immobilized,

2000

V/cm

cells

concentrated

2000

V/cm:

8.850×10

−11

2000

V/cm:

2.780×10

−10

4

Cho

etal.

[56]

Silicone

Plasm

asealed

E.colicells

(1.5)

ACfield(1280V/cm

and300kH

z)Honeycomb-type

(hexagonal)pores

madeof

SU-8

2100:

side

length

of50

μm,

distance

between

poresis112.6μm

0.5mS/m

Concentratin

gE.colicells

4.05

×10

−11

Chenet

al.

[59]

PDMS

NA

Micro-and

nanoparticles

(0.465)

DC50–1000V

(20–400V/cm)

Insulatin

gmicrostructures

1–10

mS/m

Microfluidic

concentrator—

potentialapplication

forconcentratingtarget

cells

ofinterest

400V/cm:

2.553×10

−920

V/cm:9

400V/cm:10

Aiet

al.[60]

Num

erical

investigation

usingatransientALE

finite

elem

entmodel

Microparticles

NA

Converging/diverging

microchannel

Largerfields

yield

trapping;different

particle

trajectories

due

tochannelgeom

etry

171.9V/cm:

2.786×10

−15

22

310 S.K. Srivastava et al.

most commonly used to develop DC-iDEP devices ispresented (detailed procedure is explained elsewhere [65]).Different sealing techniques and surface modifications areavailable to manipulate the electrokinetic forces in thechannel as briefly described in the next section.

Fabrication of insulator-based microdevices

Different techniques have been employed by researchers tofabricate insulator-based microdevices [66, 67]. Some com-mon methods include soft lithography [65, 68, 69], wetetching, injection molding [70], and hot embossing. Themethod of rapid prototyping developed by Whitesides et al.has been used extensively to fabricate PDMS devices [65, 71].Not all polymers can be used to fabricate devices, and thephysical and chemical properties of these polymers need to beconsidered before fabrication. Zeonor and PDMS [72] are themost commonly used plastics [73]. Once the PDMS deviceshave been developed, they can be sealed onto the samesubstrate or onto glass to form channels. There are differentsealing methods, such as UV/ozone [74, 75], solvent sealing[76], temperature sealing [77], pressure sealing, and a plasmaoxidation process [65, 78, 79]. In order to achieve dielec-trophoretic trapping or separation of particles, it is importantto consider the surface characteristics of the channel in orderto avoid particles being adsorbed onto the channel. Some of

these surface modifications include plasma oxidationtreatment, the use of surfactants [80–82], and thedynamic coating of channels [70, 82–84].

The insulating obstacle geometry can have a substantialimpact on the shape of the electric field and thus the electricfield gradient that each particle is exposed to, as discussedabove. However, the dielectrophoretic force is also depen-dent on the electric field applied to achieve separation ortrapping. The electric field generated by the insulatingobstacles is easy and convenient to control, facilitating particletrapping or separation. In the next section, we present adiscussion of the use of combined AC and DC fields toachieve DC-iDEP trapping with decreased applied potential.

Electric field characteristics

Traditionally, DC insulator based dielectrophoresis employsDC electric fields to achieve particle manipulation, as in ourown research work [7]. The research in our lab employed avery low DC field strength (17.12 V/cm) to achieve theseparation of fluorescent microparticles in a microdevicewith a rectangular insulating obstacle geometry. In this device,the inlet channel branches out into four outlet channels. TwoDC electric fields (6.85 and 17.12 V/cm) were comparedunder different medium conductivity conditions (50–850 mS/cm); higher electric fields were found to yield bettermicroparticle separation. This approach is novel as a verylow DC electric field is employed to sort the microparticlesinto a four-channel outlet system. Previously, researchers haveapplied two channel systems to achieve separation using DC-iDEP, but our lab is the first to apply a four-channel system tosort particles under DC electric field conditions. A highelectric field gradient is caused by the insulating obstacle, andthis is achieved bymanipulating the distance between the walland the region of the insulating obstacle where the highestDEP effect is exerted on the particles. We were successfullyable to distinguish 6.2 μm and 10 μm particles using a voltageof 25Vover a 1.46 cm long device (Srivastava S. K., MinerickA. R., Baylon J., Lapizco-Encinas B. H.; “DC-insulatorDielectrophoretic characterization of fluorescent polystyreneparticles”, under review, Journal of Chromatography A,2010).

On the other hand, separations have recently beencustomized for certain systems by employing DC-biasedAC electric fields [39, 41, 42, 54]. Independent control overlinear and nonlinear electrokinetic effects can be achievedby employing a DC-offset AC electric field. In thisscenario, electric field magnitudes can be reduced but thesame efficiency of separation achieved [54].

Hawkins et al. aimed to overcome the restrictions oftraditional DC-iDEP by applying a DC-offset to an ACelectric field in addition to intelligently designing the

Classic DEP Configuration

(a)

feDEP Configuration

(b)

Φ=1

Φ=-1

∂ ∂Φ/ n=0d

Φ=Φ2 Φ=Φ3

Floating Electrodes

Φ=Φ1 Φ=Φ4

Φ=-1

Φ=1

∂ ∂Φ/ n=0

Fig. 6 Models and boundary conditions used for finite elementcalculations: a the classic DEP configuration; b the feDEP configu-ration (reproduced from [38] with permission from Wiley-VCH)

DC insulator dielectrophoretic applications 311

insulating obstacle to activate nonlinear forces [41]. Theyexplained the phenomena behind DC-biased AC electricfields mathematically as

~E ¼ ~EDC þ~EAC ¼ ~EDC 1þ bð Þ ð10Þwhere E

!DC is the DC field strength and E

!AC is the AC

field strength; β is the ratio of E!

AC to E!

DC . β=0 is thecondition for a pure DC field. When β is applicable, thedielectrophoretic force on a spherical particle from Eq. 1takes the form:

F!

DEP ¼ 1

22b2 þ 1� �

usp � sm

sp þ 2smrE!2 ð11Þ

The mean particle mobility is the sum of the mobilities dueto dielectrophoresis (AC and DC fields), electrophoresis(DC field) and electro-osmosis (DC field) [41]:

u!¼ u!DEP þ u!EP þ u!EO

¼ mDEPr E!� E!

� �þ mEO þ mEPð ÞE! ð12Þ

Equation 12 contains both linear and nonlinear electricfield terms. The first-order dependencies on electro-osmosisand electrophoresis are termed linear electromigratoryeffects, while the second-order dependence on dielectro-phoresis leads to nonlinear electromigratory effects [41].These effects play important roles in separation when DC-biased AC electric fields are applied. Further, by employinga DC offset, pressure-driven flow can be eliminated in thesystem.

If a very high DC voltage or a DC-biased AC field with asmall DC component is applied, the dielectrophoretic motionreaches a threshold where it becomes dominant over theelectrokinetic motion. As a consequence, fluorescent polysty-rene beads were focused faster in a serpentine microchannelgeometry [55]. Using simulations of the serpentine channelgeometry, the authors of [54] confirmed that a 500 kV/m DCfield is comparable to a 20 kV/m DC-biased AC field whenthe DC to AC field ratio (β) is set to 0.04.

The CM factor (Eq. 2) has a direct impact on thedielectrophoretic effect on the particle, as it determines whetherthe particles are trapped or sorted. Some research groups havestudied this property of themedium and how it can bemodifiedto maximize the efficiency of particle manipulation. In the nextsection, these different characteristics—such as pH, conduc-tivity of the medium, bioparticle viability, wall effects, andJoule heating effects—are presented.

Performance characterization

Performance measures are not uniformly studied for eachcustom DC-iDEP device. For example, systematic studies

of the effects of pH, conductivity and electric field strengthoffer insight into insulator-based DEP device performance,as is apparent from Eqs. 1 and 4 in the “DC dielectropho-resis: theory” section. The parameters of the medium playan important role in the separation and trapping of particleswhenever electro-osmotic forces are significant in themicrochannel [36]. Some research groups have attemptedto study these effects on the trapping of particles and cells,as discussed in detail below [36, 41, 58, 70], but universalcorrelations describing the effects of these factors on theseparation of particles and cells in DC-iDEP devices havenot been reported.

Effect of medium pH

Lapizco-Encinas et al. explored the effects of the pH of themedium on the dielectrophoretic effect. Glass microdeviceswith glass insulating posts were used to study the influence ofoperating conditions on the DEP behavior of polystyreneparticles. pH, conductivity and the applied DC field werevaried in the ranges of 8–9, 25–100μS/cm and 200–850 V/cm,respectively. Decreasing the pH of themedium led to increasedprotonization of the silanol groups on the glass surfaces. Theoverall negative charge on glass decreases due to protoniza-tion, which decreases the zeta potential and in turn decreasesthe electro-osmotic mobility and velocity [16, 36]. To suppressthe electro-osmotic flow (EOF), it was determined that amedium with a low pH should be used to decrease wall zetapotentials. As pH increases, a greater dielectrophoretic force isneeded to overcome the strengthening EOF and still achievetrapping in such devices. To create higher DEP forces forparticle trapping, higher electric fields are required [36].Higher electric fields are not always compatible withbiological samples, and may need to be avoided. In anotherexperiment conducted by the same group, the optimalconditions for concentrating 1 μm polystyrene particles werestudied, and the pH of the suspending medium was variedbetween 6 and 9 [57]. Lower pH values (in this case 6) werefound to be optimal for trapping due to the lowerelectrokinetic flow at such pH values.

Effect of the conductivity of the medium

The effects of conductivity were also studied to determinetheir impact on trapping [36]. The conductivity of themedium was varied between 25 and 100 μS/cm and thedielectrophoretic effect on 1 μm polystyrene particles wasstudied. Increasing the medium conductivity increased theDEP force required for trapping in glass microdevices withcylindrical insulating posts to create nonuniformities in thefield [36]. This is also supported and explained by theClausius–Mossotti factor (Eq. 4), which depends on themedium and particle conductivity. The greater the conduc-

312 S.K. Srivastava et al.

tivity, the greater the CM factor, which then increases theDEP force experienced by a particle. This is the onlyresearch publication that reports the effect of mediumproperties on glass devices for trapping 1 μm polystyreneparticles. As reported by this group, a combination of lowpH (6) and high conductivity (100 μS/cm) of the medium isoptimal for achieving the trapping of polystyrene particlesin glass devices [57]. By safely controlling the properties ofthe suspending medium, the energy supplied to the systemcan be minimized, thus minimizing operational costs forDC-iDEP devices [36]. Insulator DEP is still a nascenttechnique. The findings of Lapizco-Encinas et al. could befurther optimized to achieve higher degrees of particleseparation by controlling the properties of the suspendingmedium. It should be noted that, due to the complex natureof wall–fluid interactions, these findings are limited to glassmicrodevices, and the field remains open for the explora-tion of insulator–medium interactions for plastic (PDMS,injection-molded PMMA or PC, or cyclo-olefin), quartz,ceramic, and oil meniscus microdevices.

Cell viability in a DC electric field

Dielectrophoresis depends on the application of an electricfield, which can affect the properties and viability of cells.Articles on DC dielectrophoresis which study the effects of theDC electric field on cells (human or bacterial) are scarce. Cellviability has been discussed in relation to AC dielectrophoresisat different frequencies by Voldman et al. [85]. The DEP forcedepends strongly on cell viability. The impermeable andhighly insulating cell membrane becomes permeable after celldeath [64]. Longer exposure times and higher DC electricfield strengths decreased cell viability, causing cell lysis andcell membrane rupture [32, 53]. The DC field strengthrequired for mammalian cell lysis is reported to be 106 V/mwith 33 ms exposure time [53]. Some research groups havesuccessfully separated viable and nonviable bacterial cellsusing the DC-iDEP insulating post geometry [24]. A completeinvestigation of DC electric field strengths at which cellsremain viable is not provided in the literature.

Wall effects

The electrophoretic motion of particles is affected by thepresence of channel walls in three ways: (a) electro-osmoticflow occurs due to the surface charge on the channel walls;(b) the electric fields around particles and cells are alteredby these insulating channel walls, and; (c) as the cells andparticles move along or against the fluid, viscous retarda-tion is enhanced by the presence of channel walls [86]. Walleffects were studied experimentally by Xuan et al. in 2006on a converging–straight–diverging PDMS channel using10, 20 and 40 μm diameter fluorescent microspheres. The

estimated zeta potential of the PDMS device was −82±2 mV at room temperature. Results show that largerparticles are more viscously retarded than smaller sizedparticles by the side walls of the channels [86].

Joule heating effects

Joule heating effects were researched by Sabounchi et al. onglass and cyclo-olefin copolymer DC-iDEP devices that usedcylindrical insulating posts to create nonuniformity in theelectric field [58]. They observed that Joule heating had asignificant impact on the fluid and particle motion in DC-iDEPdevices. Joule heating creates an asymmetric temperaturedistribution that disturbs the distribution of electric field linesand electrokinetic forces by producing a complex 3D electricfield [58]. The dielectrophoretic mobility and electric fieldchange with conductivity and viscosity when an electricvoltage is applied. Joule heating can be reduced by minimizingthe current in the device, which can be achieved by eitherreducing the applied field or reducing the conductivity of themedium. The electric fields required to achieve trapping orseparation can be minimized by carefully adjusting theconductivity and pH of the medium. Experiments wereconducted by Sabounchi et al. based on optical thermometryusing the fluorescent dye rhodamine B to measure temperaturesensitivity. At higher flow rates of >40 μL/min (optimizedusing a syringe pump), Joule heating was minimized. Anotherstrategy to reduce Joule heating was to use surfactants toinfluence the wall’s zeta potential and thus reduce the EOF andthe electric field required to trap particles [70]. Davalos et al.demonstrated this technique by dynamically coating PluronicF127 nonionic block copolymer surfactant onto microchannelsof Zeonor 1060R devices containing arrays of cylindricalinsulating posts. The surfactant reacts with the hydrophobicsurface of the polymer, making it hydrophilic and thusdisrupting the EOF by suppressing the surface chargemigration that occurs under electro-osmotic flow conditions.The hydrophobicity can be tailored by selecting among avariety of surface modifications for microdevices [81, 87, 88]that suppress or enhance the EOF. Such surface modificationshave been explored with capillary microdevices, but to theauthors’ knowledge this is the first group to have reported theuse of dynamic coating surface modifications in DC-iDEPdevices. Judiciously controlling the medium and wall charac-teristics as well as tuning the applied electric field can facilitatediverse particle manipulations.

Separation and trapping of biomolecules, bacteria,microparticles, and cells

Avariety of particles and cells have been manipulated usingDC-iDEP technology. It is noninvasive and nondestructive,

DC insulator dielectrophoretic applications 313

especially for living cells. The electric field gradient can beadjusted according to the manipulated particle by varyingthe insulating obstacle geometry. In this section, theseparation and trapping of particles of various sizes(biomolecules like DNA and proteins, bacteria, micro-particles of different sizes and mammalian cells) using theDC-iDEP technique is discussed.

DNA and proteins

Initial work on trapping single- and double-stranded DNAwith DC-iDEP was conducted by Chou et al. in 2002 [18].DNA trapping was demonstrated using insulating constric-tions at relatively low frequencies (see Fig. 3). Inconventional AC-DEP, metallic electrodes embedded inchannels fabricated by thin-film deposition decrease thefield gradient and thus reduce trapping efficiency. When theelectric field is too high, complex electrochemical reactionsoccur at the microelectrodes, modifying local ion compo-sitions in the medium and degrading the electrodes [18, 19].Chou et al. also showed that, for a given trapping voltage,the DEP force increased with increasing DNA moleculelength. In 2003, Chou et al. published an article on theconstruction of dielectrophoretic traps using dielectricconstrictions for a sample-to-answer, lab-on-a-chip chemicalanalysis system [44]. In this article, the differences betweenelectrodeless trapping via insulating constrictions in thechannel and conventional trapping via metal electrodes in thechannel, along with the principles of electrodeless dielec-trophoresis, were discussed. They showed several examplesof the application of DC-iDEP, including DNA trapping, celllysing, and cell sorting [44]. Protein purification andrecovery has been a very challenging task for the pharma-ceutical industry. Very little has been reported on themanipulation of protein in DC-iDEP devices. Traditionalmethods of purification are expensive, and cheaper alter-natives are being explored. One such method is the use ofDC-iDEP to manipulate bovine serum albumin (BSA)proteins, as first explored by Lapizco-Encinas et al. in2008 on a glass microdevice containing an array ofinsulating posts [29]. The motions of the protein particleswere affected by the dielectrophoretic force, electro-osmoticflow and electrophoretic flow. The effect of the electric fieldon the dielectrophoretic response of the protein and the effectof the properties of the suspending medium on the trapping ofBSA particles were studied. Strong negative dielectrophoretictrapping occurs at high electric field strengths; by increasingthe conductivity and lowering the pH of the medium,dielectrophoretic trapping can be achieved at lowerelectric field strengths. Trapping of protein particlesoccurred between electric field strengths of 700 and1600 V/cm, depending on the properties of the suspendingmedium [29].

Bacteria and viruses

As discussed in the “DC dielectrophoresis: theory” section,in 2003 Cummings and Singh studied DC-iDEP utilizing anarray of diamond insulating posts, where streaming andtrapping DEP operating regimes were observed [26].Simulations focusing on the fluid velocity and electric fieldwere also performed to predict particle transport around theinsulating posts. Typically, the Poisson and Navier–Stokesequations most be solved in direct numerical simulations ofelectrokinetic flow, but they simplified the simulations bytaking advantage of the fact that the fluid velocity isproportional to the electric field, which allows both theelectric potential and the fluid flow to be computed bysolving only the Laplace equation for electric potential [26].Subsequently, Lapizco-Encinas et al. used cylindricalinsulating posts to concentrate and sort live and dead E.coli cells; live cells exhibited trapping DEP and dead cellsexhibited streaming DEP [24]. In this case, cell size wasconstant while the chemical properties of the cells werevaried, suggesting that cell composition is sufficient toimpact cell polarizability and thus susceptibility to DEPforces. In the device, narrow regions between the insulatingposts generated the highest electric fields (Fig. 4). NegativeDEP into low field gradient regions was observed with liveE. coli, whereas dead cells experienced less negative DEPand were carried by flow streamlines through the array.This was the first report of the separation of live and deadbacteria in a DC-iDEP device.

In 2003, Lapizco-Encinas et al. demonstrated thetrapping and concentration of viruses. They performedexperiments on a glass chip, but their ultimate goal was tocreate a high-throughput polymeric device [23]. For this,they created an array of circular microbumps (100 μm indiameter and 40 μm in height) as insulating posts to createlocally concentrated electric fields. They tested severalpolymeric substrates for this purpose, and observed thatpolyethylene and Zeonor had excellent fabrication charac-teristics and yielded the streaming and trapping of twoforms of T4 viruses at 400 V/cm using this insulating DEPapproach. In 2004 they also worked on the concentration oflive bacteria in water by DC-iDEP [45]. Here, four types ofbacteria were studied, all of which exhibited negativedielectrophoresis. Among them, E. coli was Gram negative,while the other three, Bacillus subtilis, B. cereus, and B.megaterium, were Gram-positive species. In this study, arelation for dielectrophoretic trapping was established:

mDEPr ~E �~E� � � r~E

mEK~E �~E� � > 1 ð13Þ

It was demonstrated that membrane conductivity did notaccount for changes in the DEP behavior of cells. Single bacteria

314 S.K. Srivastava et al.

and mixed-species experiments were performed on glass deviceswith circular insulating posts 150 μm in diameter under electricfield strengths ranging from 250 to 900 V/cm. DEP trapping wasachieved with lower electric field strengths (in this range) for E.coli species than needed for other Gram-positive species. Atrend was established based on the size of the bacteria and theproperties of the cells: E. coli < B. megaterium < B. subtilis <B. cereus. Selective trapping experiments were conducted usingtwo different types of bacteria, where the aim was toconcentrate one species at high electric field and selectivelyelute a species by incrementally lowering the DC voltage. Thetrapping of E. coli bacteria grown in brain-heart infusion brothat 37 °C was achieved in a membrane-type DC-iDEP device[56]. The side length of each hexagonal pore was 50 mm, andthe thickness of the wall between pores was 112.6 mm. Thediameter of the membrane area was 5 mm, and 517 pores weregrouped into a honeycomb pattern. The void fraction in themembrane was 18.7%. Trapping efficiency varied between 61and 73% for a dilute bacteria solution flowing at a rate of100 μL/min under a DC electric field strength of 1280 V/cmand an AC frequency of 300 kHz. The trapped bacteria werereleased with efficiencies between 86 and 100% when the ACfield was turned off. Viable bacteria were quantified using thestandard colony forming unit (CFU) counting method. The cap-ture efficiency of the bacteria was >80% at flow velocitieslower than 10 mm/s, but increasing the flow velocity(>50 mm/s) of the bacterial suspension decreased the captureefficiency by 30% [56].

Sabounchi et al. demonstrated a novel platform forconcentrating Bacillus subtilis spores that combined iDEP,pressure-driven flow and impedance measurements for thefurther analysis of the concentrated sample [46]. This was thefirst report of a detection system built on the same platformthat was used to manipulate particles. The electrodes used forthe impedance measurement were coplanar and rectangular;they were passivated with SiO2 to maintain the stability of thesystem for long time periods. Sample concentration involvedapplying DC electric fields of strength 500 V/cm to aninsulating post geometry, whereas the impedance measure-ments utilized an AC signal with an amplitude of 100 mVanda sine wave frequency of 100 Hz. The lower detection limitestablished was the injection of a sample containing10 spores/mL at 40 μL/min [46]. Once the spores weretrapped at 500 V/cm, the DC voltage was lowered, allowingthe spores to enter the side channel for impedance measure-ments. The aim is to use this platform as a decision-makingcomponent to determine if downstream identification assaysare required [46].

Polystyrene particles

Many researchers have worked on the separation and trappingof polystyrene particles of various sizes (1–15 μm) using a

variety of insulating geometries and field strengths (DC andDC-biased AC fields). Mela et al. demonstrated the trappingof polystyrene particles in an injection-molded hot-embossedcyclo-olefin DC-iDEP device. The trapping of carboxylatedmicrospheres was observed at 340 V/cm in serpentinemicrochannels that were 32 cm long, 160 μm wide and70 μm deep [55]. In 2006, Barbulovic-Nad et al. used an oildroplet as an insulating obstacle to separate 1, 5.7, and15.7 μm fluorescent carboxylate-modified microspheresunder electric fields ranging between 80 and 240 V/cm[43]. The particles were exposed to a negative DEP force.Since that force is linearly dependent on particle volume,particle separation was achieved by size. The greatestadvantage of this oil droplet obstacle was dynamic dropletsize control, which is adaptable to the separation require-ments of various particles [43]. PDMS microdevices on aglass substrate were fabricated by a standard photolithogra-phy technique containing four 40 μm deep microchannels[43]. Effects based on droplet size, particle size and electricfield were explored in this study. They were able todemonstrate that separation was effective at a very low fieldstrength for particles as small as 1 and 5.7 μm. Employinglow voltages for separation lowers the separation efficiencyand the velocity of the particles, which are the maindrawbacks [43]. Zhang et al. simulated and modeled theseparation of positive and negative DEP particles of radii 5and 10 μm in a circular channel driven by electro-osmoticflow [35]. The particles with different dielectric propertiesmove towards different locations across the channel. Thecircular channel had an inner diameter of 50 μm, an outerdiameter of 100 μm, and a height of 20 μm as shown inFig. 5. For small particles, the DEP force is balanced by thefluid drag force which slows particle motion in the channel.However, the higher mobilities of larger particles mean thatthey travel faster towards the inner or outer circle (dependingon whether a positive or negative DEP effect is experiencedby the particles). In this system, the dielectrophoreticmobility of a particle moving in an electric field gradient isdependent on the surface area of the particle. The directionof movement of the particle depends on the sign of the CMfactor (α), as discussed in Eq. 4. The particles following theinner path of the circular channel move faster than thosefollowing the outer path. When the electric field is reduced,the particles move slower due to the slower electro-osmoticflow. Under higher electric field conditions, the particlesmove with the faster electro-osmotic flow, so they arrive atthe end of the channel quicker [54]. The particle separationefficiency was improved by employing higher electric fieldstrengths, as the electro-osmotic force increases at higher DCfield strength. Therefore, the particles travel faster towards theend of channel, but they also tend to be overseparated due tothe large DEP effect exerted on them. In a serpentine channelsystem, Zhu et al. studied the focusing of 5 and 10 μm

DC insulator dielectrophoretic applications 315

polystyrene particles using pure DC fields and DC-biased ACfields [55]. The ratio β from Eq. 10 was varied, and the totalmagnitude of the DC and AC fields was maintained at 100 V/cm. Upon increasing β or increasing AC amplitude withrespect to DC amplitude, the width of the focusing streamnarrowed, thus achieving a higher degree of focusing in thechannel. Zhu et al. also achieved the continuous separation ofmixtures of 5 and 10 μm particles and 3 and 5 μm particles inan asymmetric double-spiral microchannel by applying a DCvoltage [89, 90]. Total separation was achieved with the 5 and10 μm particles at 600 V DC (~265 V/cm in the high electricfield density region), whereas total separation could not beachieved for the 3 and 5 μm particles, even with an appliedvoltage of 1500 V DC (~465 V/cm in the high electric fielddensity region) [89]. Another study by the same groupinvolved focusing the particles using a microchannel constric-tion and DC-biased AC electric fields [42]. The microchannelconstriction was 56 μm high and 188 μm long and located inthe middle of a 2 cm long, 310 μm wide channel. Pure DCvoltage experiments involved an electric field strength of100 V/cm, whereas the total electric field strength was100 Vpp/cm with a frequency of 1 kHz in the DC-biasedAC case. Separation efficiency was the same in both cases,with the added advantage of using a decreased voltage in thecase of DC-biased AC experiments [42].

A recent development in DC-iDEP was a nanoparticleconcentrator developed by Chen et al. [59]. The concen-trator was composed of a series of microchannelsconstructed with PDMS, as shown in Fig. 7. The regionsshown in Fig. 7 were designed such that the field increasesgradually from region 4 up to region 1. The deviceperformance was evaluated using 930 nm green fluores-cent polystyrene microspheres at fields of between 20 and400 V/cm. Both streaming and trapping DEP occur in thedevice of Chen et al., depending on the electric fieldapplied. Streaming aids the transport of the trapped

nanoparticles and trapping DEP aids the concentrationand filtering of the particles. In this concentrator, thestructure of the channel is designed to highly focus andthus amplify the electric field. At 200 V/cm, completetrapping of particles occurred in the device. To collect thetrapped particles, the electric field was turned off and apressure source was applied at the inlet with a syringe.The concentration of 930 nm polystyrene particles wasrapid and efficient at this stage. Further experiments werealso conducted for 500 nm nanoparticles, which required ahigher electric field to completely trap all of the nano-particles. One notable aspect of this implementation ofDC-iDEP is that it has achieved the manipulation ofsmaller particles than those manipulated in other studies.This technique could be implemented to concentratebiomarkers for disease diagnostic applications, as well asfor sample preparations and concentrations for biologicalassays. The design could be further improved by changingthe trapping regions according to the application [59].

To improve the DEP focusing of small particles, thenumber of constrictions or insulating obstacles in thechannel can be increased. This is apparent when comparedwith the results obtained using just one constriction [42].Particle focusing is more effective when the particle size iscomparable with the constriction width or the applied fieldis sufficiently high. If the width of the constriction isreduced, the chances of particle clogging and chip foulingincrease. This issue is usually countered by using higherelectric fields. The choking phenomenon was recentlymodeled by Ai et al. using a finite element model in aconverging–diverging microchannel [60, 91]. This chokingphenomenon was observed when the DEP force increasedwith the electric field at a much faster rate than electro-osmosis and electrophoresis, thus preventing the particlefrom moving into the converging channel. A high nonuni-formity in the electric field, a high particle size ratio and ahigh zeta potential ratio are predicted to be the main causesof this choking phenomenon in DC-iDEP converging–diverging channel geometries. The results presented in thisstudy are not applicable to nanoparticles due to theassumption made by the authors that the particle size ismuch larger than the electric double layer (EDL) presentbetween the charged wall and the particle [91].

Yeast cells and mammalian cells

Kang et al. reported a technique combining AC and DCdielectrophoresis for the continuous separation of a mixtureof 5 and 10 μm microparticles and yeast cells, which wasachieved by employing a PDMS obstacle and a pair ofembedded electrodes to generate localized AC electricfields [39]. Embedded electrodes were viewed as beingadvantageous in this system because low voltages were

Fig. 7 Photograph of the PDMS-insulating microstructure used forthe concentration of particles. The electric field strength increasesgradually from regions 4 to 1. DEP trapping begins at region 2 [59].Reproduced from [59]

316 S.K. Srivastava et al.

applied; this avoided electrokinetic flow and yielded aconfined area for the manipulation of particles. It was foundthat the pressure-driven flow adopted in the rectangularhurdle microchannel for separation was advantageous, as itminimized Joule heating effects. However, the particlestreamlines were close to each other, which affected theseparation efficiency. The crossover frequency was estimatedto be 200 kHz; above this crossover frequency the efficiencyof separation decreased, and belowAC amplitudes of 7 Vpp noseparation occurred. The authors concluded that the optimumflow rates for the highest throughput to improve thistechnique of separation are yet to be determined [39].However, cell viability decreased as the electric fieldincreased due to Joule heating [42, 85, 92]. Previousresearchers have reported that the high electric field strengthcould cause an increase in temperature at the constrictionsdue to Joule heating, and thus damage cells or causedisturbances to particle motions [58].

DEP is an excellent technique for manipulating mam-malian cells, and many researchers have accomplished cellmanipulation and single-cell analysis by fine-tuning theelectric field gradient via traditional AC dielectrophoresis inorder to achieve sorting, focusing, concentrating or trappingin microdevices [4, 10, 93–103]. However, few researchershave investigated DC-iDEP behavior with mammaliancells, aside from our group [7], Kang et al. [34] and Jenet al. [52, 53]. Our group separated red blood cells via theDC-iDEP technique using a rectangular insulating geome-try [7]. The red blood cells (RBC) were separated accordingto human ABO type into one of the four channels,depending on the polarization capacity of the RBCs. Ourprevious work involved using AC-DEP to sort RBCs basedon blood type using an AC electric signal. We coulddistinguish O+ from all the other blood types with >95%confidence [4]. From our previous research, we concludedthat dielectrophoresis depends on antigen expression on theRBC membrane, which aided the sorting of RBCs by type[4, 7]. Kang et al. studied the separation of human whiteblood cells and breast cancer cell groups (MCF7) individ-ually in rectangular and triangular insulating obstaclegeometries, respectively, by applying a DC electric field[34]. The white blood cell suspension was composed ofmonocytes, granulocytes, lymphocytes and other bloodcomponents (red blood cells, platelets, and other debris).Breast cancer cell lines had varying cell sizes in the rangeof 20–60 μm. White blood cells were separated in therectangular design, and a triangular obstacle was used tomanipulate live cells. They claim that because the rectan-gular obstacle is so long, cells may die (due to extra stresson the membrane and the strong electric field in theinsulating obstacle region) in-between, so the triangularobstacle geometry would be better for handling live cells[34]. The applied voltage in the rectangular geometry was

345 V (138 V/cm), whereas lower voltages of about 180 V(72 V/cm) were applied in the triangular geometry toachieve separation. Jen et al. studied the trapping ofhuman carcinoma cells (HeLa) using open-top micro-structures containing DEP insulating traps similar to thoseused by Chou et al. [18, 44]. HeLa cells displayed bothpositive and negative DEP under frequencies of >100 kHzand <1 kHz, respectively, at an applied electric field of60 Vpp/cm. The crossover frequency where the shiftoccurs from negative to positive DEP was estimated tobe ~3.5 kHz. HeLa cells were trapped completely at180 Vpp/cm and a frequency of 1 MHz [53].

In summary, many investigators have researched theseparation or trapping of different types of cells andmicroparticles by employing low and high DC or DC-biased AC fields. DC-iDEP is a novel technique that iscapable of manipulating any type of polarizable particlewhile maintaining the viability of bioparticles. Jouleheating is a concern for certain device geometries andoperating conditions; careful selection of the electric fieldrange and surface modification technique can help toovercome this problem.

Concluding remarks

In this review article, we have provided an extensive reviewof the major contributions in the field of insulator-baseddielectrophoresis. This is a nascent field which has yielded >50publications in the last decade. DC-iDEP devices have variousadvantages compared to traditional AC dielectrophoreticdevices wherein metal electrodes are embedded inside the fluidchannels. Insulator-based dielectrophoretic devices are lessprone to fouling and gas generation, and are easier to fabricatebecause the metal electrodes are remotely positioned in samplewells. Insulating obstacles in microchannels create electric fieldnonuniformities that yield DEP forces on polarizable particlesand can be harnessed to achieve the separation or trapping ofsuch particles of interest. DC-iDEP devices can also beoperated as continuous flow devices or employ DC and ACelectric fields simultaneously for optimal particle manipulation.This review has outlined the standard and novel insulatingobstacles explored thus far. Most obstacles are robust, inert andcan be fabricated by soft lithography in diverse geometries,including a single obstacle (a rectangular obstacle, a triangularhurdle, a constriction in the depth of the channel rather than thewidth, a single microchannel constriction, an oil droplet, and oilmenisci as field-shaping barriers) embedded in the channel;multiple obstacles (metallic traps, insulating posts, tetragonalstructures along the channel, and multiple rectangular blocks);and other channel geometry modifications (nonconvergingsawtooth channels, a set of aligned teeth, sawtooth channels,open-top microstructures, serpentine microchannels, a series of

DC insulator dielectrophoretic applications 317

iterative curves, arrays of circular channels, and honeycombmembrane design). The most extensively used channelgeometries are insulating posts (cylindrical or diamond shaped)made of glass, and a single rectangular obstacle geometry madeof PDMS.

Trapping or streaming separation of particles is depen-dent on various factors, and there is wide scope for thefuture improvement of this technique. The efficiency of thetrapping, focusing, separation or concentration of particlescan be improved by carefully selecting the properties of thesuspending medium and the electric field strength for theparticle of interest. The DC-iDEP literature is dominated byreports of negative DEP, due to the particle and cellconductivities as well as the contributions from theelectro-osmosis and electrophoresis of the particles. However,specific dependencies on the medium pH and conductivityand the judicious selection of the electric field are areas thatneed further exploration in DC-iDEP devices.

Traditional DC-iDEP devices have yielded the separa-tion and trapping of DNA, viruses, bacteria and fluorescentpolystyrene microparticles, the isolation of live and deadcells, and protein manipulation by employing suitable DCfields and various insulating obstacle geometries, such as arectangle, triangle, oil droplet, array of insulating posts,multiple rectangular blocks, serpentine channels, a series ofiterative curves, and circular channels. To achieve separation,high voltages are often necessary, which are typically notconsidered to be conducive to cell viability. However, a limitednumber of reports in the literature comment on cell viability,and they claim that cells remain viable after DC-iDEPseparations [24, 33, 53]. Larger electric fields in high-conductivity buffers promote Joule heating along with slowerparticle velocities and fluid flow. To overcome thesedifficulties, a hybrid approach involving the simultaneousapplication of a lower DC field and an overlaid AC frequencyhas been employed to achieve more efficient particleseparations and trapping. Some examples that have beendiscussed include a nanoparticle concentrator [59], a bacterialconcentrator [46, 56], an open-top microstructure for humancarcinoma cell trapping [53], and a microparticle focuser withlow-power utilization and a higher efficiency than traditionalDEP approaches [39, 41, 42, 51, 54]. Mammalian cells havenot been explored much by investigators in the literature, withthe exceptions of three research groups [7, 34, 53]: Kang et al.studied the separation of breast cancer cell lines (MCF7) andwhite blood cells individually by employing DC electric fields[34], and Jen et al. studied the trapping of human carcinomacells (HeLa) via DC-biased AC fields [53].

In summary, DC-iDEP remains an underexplored yetpowerful particle manipulation technique, and much workis needed to fully realize its potential. The plethora ofapplications that have utilized traditional dielectrophoresisare likely to be adaptable to this more user-friendly

platform, enabling mobile lab-on-a-chip diagnostic appli-cations. Some of these prospective applications include themanipulation of bioparticles; bioseparations and purifica-tions in the pharmaceutical industry; the concentration ofbiomarkers from body fluids for disease detection; envi-ronmental purification or remote monitoring systems thatconcentrate and quantify bacteria, viruses, or hazardouswaste; and drug delivery, biodefense, food applications, andother diverse applications in medicine and biology.

Acknowledgements The authors would like to thank the financialsupport provided by NSF CAREER 0644538 and stimulatingconversations with Blanca Lapizco-Encinas.

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