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Research Papers

# Particle Manipulation in Insulator Based Dielectrophoretic Devices1OPEN ACCESS

[+] Author and Article Information
Blanca H. Lapizco-Encinas

Associate Professor
e-mail: bhlbme@rit.edu
Microscale Bioseparations Laboratory,
Chemical and Biomedical
Engineering Department,
Rochester Institute of Technology,
Rochester, NY 14623

The present work is based on the IMECE2013-66439 paper accepted to the 2013 IMECE proceedings.

2Corresponding author.

3Present address: Department of Chemical and Biomedical Engineering, Rochester Institute of Technology, Institute Hall (Bldg. 73), Room 3103, 160 Lomb Memorial Drive, Rochester, NY 14623-5604.

Manuscript received August 20, 2013; final manuscript received September 4, 2013; published online October 3, 2013. Assoc. Editor: Sushanta K Mitra.

J. Nanotechnol. Eng. Med 4(2), 021002 (Oct 03, 2013) (7 pages) Paper No: NANO-13-1060; doi: 10.1115/1.4025368 History: Received August 20, 2013; Revised September 04, 2013

## Abstract

Microfluidic devices can make a significant impact in many fields where obtaining a rapid response is critical, particularly in analyses involving biological particles, from deoxyribonucleic acid (DNA) and proteins, to cells. Microfluidics has revolutionized the manner in which many different assessments/processes are carried out, since it offers attractive advantages over traditional bench-scale techniques. Some of the advantages are: small sample and reagent amounts, higher resolution and sensitivity, improved level of integration and automation, lower cost and much shorter processing times. There is a growing interest on the development of techniques that can be used in microfluidics devices. Among these, electrokinetic techniques have shown great potential due to their flexibility. Dielectrophoresis (DEP) is an electrokinetic mechanism that refers to the interaction of a dielectric particle with a spatially non-uniform electric field; this leads to particle movement due to polarization effects. DEP offers great potential since it can be carried out employing DC and AC electric fields, and neutral and charged particles can be manipulated. This work is focused on the use of insulator based dielectrophoresis (iDEP), a novel dielectrophoretic mode that employs arrays of insulating structures to generate dielectrophoretic forces. Successful micro and nanoparticles manipulation can be achieved employing iDEP, due to its unique characteristics that allow for great flexibility. In this work, microchannels containing arrays of cylindrical insulating posts were employed to concentrate, sort and separate microparticles. Mathematical modeling with COMSOL® was performed to identify optimal device configuration. Different sets of experiments were carried out employing DC and AC potentials. The results demonstrated that effective and fast particle manipulation is possible by fine tuning dielectrophoretic force and electroosmotic flow.

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## Introduction

Food and water safety, environmental monitoring, and clinical screening are examples of fields where microfluidic devices or lab-on-a-chip systems can make a significant impact, since in these areas obtaining a rapid response is critical, particularly in analyses involving biological cells, from DNA and proteins, to cells. Microfluidics has revolutionized the manner in which many different assessments/processes are carried out. Miniaturization offers attractive advantages over traditional bench-scale techniques: small sample and reagent amounts, higher resolution and sensitivity, improved level of integration and automation, lower cost and much shorter processing times [1,2]. Miniaturization allows for the possibility of portable devices, opening the opportunity for on-field applications and online-process monitoring; i.e., taking the laboratory where it is needed, to the field or to the production line. Biotechnology is one of the fields that has most benefited from the marriage between microfluidics and separation/analytical sciences.

There is a growing interest on the development of microscale techniques. Among these, electrokinetic techniques have shown great potential due to their flexibility. DEP is an electrokinetic mechanism that refers to the interaction of a dielectric particle with a spatially nonuniform electric field; this leads to particle movement due to polarization effects. DEP offers great potential since it can be carried out employing DC and AC electric fields, and neutral and charged particles can be manipulated. EK can also be used effectively with nano and microparticles. The majority of the studies on DEP have employed arrays of microelectrodes; however, employing microelectrodes has some drawbacks such as higher cost and loss of functionality due to fouling. iDEP is an attractive alternative, since it employs arrays of insulating structures, instead of electrodes, to create nonuniform electric fields; resulting in inexpensive and robust devices that do not lose functionality despite fouling effects. Additionally, iDEP systems allow for the use of electroosmotic flow to pump liquid and particles, making the devices simpler and more portable [3,4].

This work is focused on the use of iDEP in microfluidic devices to achieve dynamic particle manipulation, sorting, and separation. Mathematical modeling with COMSOL was performed to identify optimal device configuration. Different sets of experiments were carried out employing DC and AC potentials. Videos and pictures demonstrating successful particle manipulation with iDEP are presented and discussed, with special focus on the effects of insulator geometry and characteristics of the applied electrical signals. The results demonstrated that effective and fast particle manipulation is possible by fine tuning dielectrophoretic force and electroosmotic flow.

## Theory

Dielectrophoresis is the transport or movement of particles due to polarization effects when particles are exposed to a nonuniform electric field. The dielectrophoretic force exerted on a spherical particle under a DC field can be expressed asDisplay Formula

(1)$F→DEP=2πɛmrp3Re(fCM)∇E2$

where εm is the permittivity of the suspending medium, rp is the particle radius, ∇E2 is the gradient of the squared electric field, and Re(fCM) is the real part of the Clausius-Mossotti (fCM) factor, which accounts for polarization of the particle. The fCM factor can be positive or negative and it is defined in terms of the complex permittivities asDisplay Formula

(2)$fCM=ɛp*-ɛm*ɛp*+2ɛm*$

Usually, below 100 kHz is considered the “conductivity regime” and the fCM can be approximated in terms of the real conductivities as [5,6]Display Formula

(3)$fCM=[σp-σmσp+2σm]$

where subscripts p and m refer to particle and suspending medium, respectively; $ɛp*$ and $ɛm*$ are the complex permittivities of the particle and the medium, respectively; and σp and σm are the real conductivities of the particle and suspending medium, respectively. The complex permittivity is related to the real permittivity $ɛ$ by $ɛ*=ɛ-(iσ/ω)$, where σ and ω are the real conductivity and angular frequency of the applied electric field, respectively, and $i=-1$ [7].

Dielectrophoretic forces can be positive or negative. Positive DEP behavior is obtained when the particle is more polarizable than the suspending medium, where the particle will be attracted to the regions of higher electric field gradient. Negative DEP behavior, i.e., repulsion from the areas of higher field gradient, is observed for particles with lower polarizability than that of the suspending medium [7].

Important studies have focused on the modeling of iDEP systems and results obtained from mathematical simulations have allowed for better designs to be developed. Cummings and Singh [8,9] reported the first demonstration and modeling work of trapping and streaming iDEP. Chen et al. [10] modeled particle pathlines in an iDEP sawtooth channel; particle pathlines in a curved microchannel have also been studied by Xuan's research group employing curvature-iDEP [11,12]. Particle dielectrophoretic mobilities were determined by Weiss et al. [13] using a tapered section of a microchannel. Ros' research group [14-16] studied concentration profiles of nano-bioparticles (proteins and DNA) under streaming dielectrophoresis. Li's research group [17,18] modeled particle trajectories under different electric fields, analyzing trajectory shifts, also estimated total electrical force on a particle. Our group has modeled the dielectrophoretic trapping of inert particles [19,20] and biological cells [21]. Kwon et al. [22] studied how the distribution of the insulating posts could be used to enhance dielectrophoretic manipulation of particles in microchannels.

## Mathematical Model

COMSOL Multiphysics® (COMSOL, Inc., Burlington, MA) software was employed to mathematically model the electric field distribution, electric field gradient and the forces acting on the particles inside the microchannel. The software solves the Laplace equation within the microchannel using finite elements approximations. Briefly, the electric potential (ϕ) on the device can be represented by the Laplace equationDisplay Formula

(4)$∇2φ=0$

with the following boundary conditionsDisplay Formula

(5)$n→·J→=0 at the channel boundaries$
Display Formula
(6)$φ=φin at the inlet of the microchannel$
Display Formula
(7)$φ=0 at the outlet of the microchannel$

where $n→$ is the vector normal to the surface, $J→$ is the electrical current density, and ϕin is the electrical potential applied across a microchannel containing an array of cylindrical insulating posts (see Fig. 1Fig. 1

Schematic representation of one of the microchannels employed for experimentation

). The microchannel walls and surface of the insulating posts are considered the boundaries, where the electric field does not penetrate. The objective of the insulating structures is to “pinch” the electric field in order to generate regions of higher field intensity, i.e., and electric field gradient. Figure 2(a)Fig. 2

(a) Distribution of the electric field (V/m) and (b) distribution of the gradient of the squared electric field (V2/m3) for the microchannel shown in Fig. 1 when a potential of 500 V is applied

shows a representation of the electric field distribution and Fig. 2(b) shows a representation of the gradient of the square of the electric field for one of these channels when a potential of 500 V is applied. As shown in the images, the distribution of the electric field is significantly distorted due to the presence of the insulating structures. In the case of ∇E2, the maxima values when using cylindrical posts are reached in four regions next to the post boundary at the constriction (see Fig. 3Fig. 3

Representation of ∇E2 and the forces acting on the particles, showing the locations of the regions of dielectrophoretic immobilization of particles

).

There are other forces acting on the particles in this type of devices. These forces are electroosmotic flow (EOF) and electrophoresis (EP). For particles around 1 μm in or larger, EP effects can be neglected because the charge to mass ratio is low, since these particles have little surface charge [19]. For smaller particles, in the nanometer size range, EP effects are important. In the present study, where microparticles were used, it was assumed that the particle movement inside the microchannel is a balance between EOF and DEP. The main objective in iDEP applications is to achieve particle enrichment, sorting, and separation. In order to do this, the DEP velocity of the particles has to overcome the EOF velocity. The EOF and DEP mobilities and velocities can be expressed as follows:Display Formula

(8)$μEO=-ɛmζsη$
Display Formula
(9)$v→EO=μEOE→$
Display Formula
(10)$μDEP=rp2ɛmfCM3η$
Display Formula
(11)$v→DEP=μDEP∇E2$

where μEO, μDEP,$v→EO$, and $v→DEP$ are the EOF and DEP mobilities and velocities, respectively. ζs is the zeta potential of the microchannel substrate (PDMS) and η is the suspending medium viscosity, respectively. The superposition of EOF and EP is called electrokinetics, since in these systems EP effects are negligible, the EK mobility and velocity can be approximated asDisplay Formula

(12)$μEK=μEO+μEP≈μEO$
Display Formula
(13)$v→EK=μEKE→$

In these systems, since low frequency and DC electric potentials are applied, the particles exhibit negative dielectrophoretic behavior. The direction of the DEP force exerted on the particles does not depend on the direction of the local electric field. DEP force is directly proportional to the magnitude of ∇E2. Figure 3 shows the representation of the forces acting on the particles and the distribution of ∇E2 at one of the constrictions between the cylindrical posts. As it can be seen, there are four regions of ∇E2 maxima (shown in dark color). The negative DEP force is the highest at the four regions of maxima of ∇E2.

As mentioned, in order to immobilize particles, DEP must overcome the other forces present in the system, such as pressure-driven flow, diffusion, and EK motion. It was assumed that only DEP and EK were contributing to the flux of particles $j→$, thus [9,19,22]Display Formula

(14)$j→=C(v→EK+v→DEP)$
Display Formula
(15)$v→particle=v→EK+v→DEP=ɛmζsηE→+rp2ɛmfCM6η∇E2$

where C is the particle concentration. Particle immobilization was achieved in the regions where the following “trapping” condition was satisfied. This condition can be written in terms of EK and DEP mobilities as [19,20,22,23]Display Formula

(16)$j→·E→=0$
Display Formula
(17)$(μEKE→+μDEP∇E2)·E→=0$
Display Formula
(18)$-μDEP∇E2μEKE2·E→>1$

For AC-iDEP systems, where the applied potential depends on time E = Esf(t), the expression for dielectrophoretic trapping is [20]Display Formula

(19)$(-μDEP∇Es2μEKEs2·E→s)f(t)>1$

## Materials and Methods

###### Microdevices.

Experimental work was carried out in microdevices made from PDMS using conventional soft lithography techniques [24]. Channels and insulating structures were defined in SU-8 3050 (MicroChem, Newton, MA) on a Si wafer (Silicon Inc, Boise, ID). PDMS layers (Dow Corning, Midland, MI) that were 3 mm thick were cast onto this mold to produce channels. The PDMS channels were sealed onto 3 in. × 2 in. glass slides (Ted Pella, Redding, CA) that were spin-coated with PDMS. Thus, all channel surfaces in the device were made of PDMS. Both the PDMS channel layer and the PDMS-coated glass substrate were activated using a corona wand (Electro Technic Products, Chicago, IL) to promote sealing. A schematic representation of one of the microchannels is shown in Fig. 1. All microchannels contained an inlet and an outlet reservoir, and an array of insulating structures embedded at the center of the channel. Two shapes of insulating structures were utilized: cylinders and diamonds that had the “same” dimensions. The channels were 10.16 mm long, 2 mm wide, 40 μm deep, and had an array of 8 × 4 insulating posts that were 450 μm wide and spaced 500 μm center-to-center. The dimensions were verified with scanning electron microscopy. The posts transverse the entire depth of the microchannel, creating a true three-dimensional effect. In order to prevent particles from crashing against the posts and clogging the system, a “dovetail” geometry was used in the first row of posts on either side of array of cylindrical posts (see Fig. 1).

###### Equipment.

Visualization of the experiments was achieved employing two different microscopes: Black and white images were taken with an epifluorescence video microscope designed for microfluidics, model SVM340 (LabSmith, Livermore, CA) and color images were taken with an inverted phase contrast microscope with UV lamp for fluorescence, model Axiovert 50 CFL (Carl Zeiss, Göttingen, Germany). A filter set with dual excitation and emission filters was used with the Axiovert 50 CFL to visualize fluorescent particles with different excitation/emission spectra simultaneously. Electrical stimulation was achieved using a high voltage sequencer, model HVS448, (LabSmith, Livermore, CA) with platinum-wire electrodes. A personal computer was required to control and use the microscopes and voltage sequencer.

###### Experimental Procedure.

Suspending mediums used in DC-iDEP experiments (Fig. 5) and AC-iDEP experiments with offset steps and two particle sizes (Fig. 7) were 0.2 mM K2HPO4 buffer with pH = 8 and σm = 70 μS/cm. Suspending mediums used in AC-iDEP experiments with asymmetrical signals and a single particle size (Fig. 6) were 0.1 mM K2HPO4 buffer with pH = 8 and σm = 50 μS/cm. For the experiments in Fig. 8 were carried out with de-ionized water with a pH of 8 and a conductivity of 20 μS/cm. Fluorescent polystyrene microparticles 1 μm in diameter, color yellow-green, ex/em 505/515 nm (Invitrogen, Carlsbad, CA), were resuspended in the buffer solution to concentrations of 7.3 × 108 spheres/mL. In AC-iDEP experiments with offset steps, 0.5 μm in diameter color red ex/em 538/584 nm (Magsphere, Pasadena, CA) and 2 μm diameter fluorescent polystyrene particles, color red, ex/em 580/605 nm (Invitrogen) were resuspended in the buffer solution to concentrations of 7.3 × 108 and 7.07 × 107 spheres/mL, respectively.

Before each experiment, the microchannels were soaked in KOH for 2 h, then rinsed with DI water and soaked with the buffer solution for at least 2 h prior to the experimental session. To start an experiment, the selected microchannel was observed under the microscope, 30 μL of the microparticle suspension was introduced at the inlet, and electrodes were placed at the reservoirs, as shown in Fig. 1. Care was taken to eliminate pressure driven flow. Low frequency AC or DC electric potentials were then applied and particle response was observed employing the microscopes and recorded.

## Results and Discussion

Different sets of experiments and simulations were carried out to study the effect of the specific post geometry/shape and the effect of the characteristics of the applied electric signal (shape, frequency, and offset for AC signals) on the dielectrophoretic forces exerted on the particles.

###### Effect of the Shape of the Insulating Structures.

There has been great progress on iDEP systems reported in the literature [14,15,19,20]. However, no reported study has focused on the effects of different post geometries on the distribution of ∇E2, which is directly related to the magnitude of the dielectrophoretic force exerted on a particle (Eq. (1)). Figure 4 shows how the “sharpness” of DEP force distribution at one constriction changes as a function of the geometry of the insulating structures. Figure 4 shows the predictions of DEP force for 1 -μm polystyrene particlesin channels with cylindrical and diamond-shaped insulating posts with the same size constriction of 50 μm and “essentially” same effective width. For the cylindrical posts, the diameter is 450 μm spaced 500 μm center-to-center; diamond shaped posts are 450 μm wide and long, with the same center-to-center spacing, the channel size is the same in both cases.

As observed from Fig. 4, even though the opening is essentially the “same” and the posts are essentially the “same size,” the distribution of the DEP force varies considerably. The gradient is softer and weaker for the cylindrical posts as it has a much more gradual geometry for the constriction. Meanwhile, the gradient is much sharper and higher over a larger area for the diamond shaped posts since the opening has an abrupt geometry; as depicted by the dark red color that covers entirely the 50 μm opening region. There is an area of nearly 200 μm2 near the tip of each diamond post where the magnitude of DEP force is an order of magnitude larger than the maximum force observed in the case of cylindrical posts.

Experiments have shown that the differences in field gradient distribution have an effect on the iDEP behavior observed in these two geometries. When 1 μm particles were introduced into these microfluidic channels and DC signals of 100–800 V were applied, particle trapping started at 300 V for the circle geometry, while it started at 200 V for the diamond geometry. At a specific applied voltage, the effectiveness of particle trapping was always higher with the diamond geometry compared to the circle geometry. Typical particle trapping behavior for both geometries at 300 V, 500 V, and 700 V are shown in Fig. 5. Streaming iDEP was also observed more clearly with diamond posts, showing that the particle trajectories are shaped differently with different geometries.

It must be stressed that in both geometries, the opening decreased from 450 μm to 50 μm over a lateral distance of 225 μm. Only the shape or angle of the constriction was changed. The entire process of device design, fabrication, and experiments was identical, yet a more effective trapping could be achieved merely by choosing one CAD design over another. This shows that rational design of post geometries can make iDEP devices much more effective.

###### Effect of the Characteristics of an AC Signal.

The effect of the signal characteristics was also studied for a variety of low frequency AC potentials. The objective here was to study how by varying parameters such as the frequency, amplitude and offset of an applied AC potential, different schemes for particles manipulation can be obtained. Figure 6 has a sample of the results obtained, where a full sinusoidal signal of 800 V was applied at two different frequencies: 1 Hz and 10 Hz.

Figure 6(a) shows a representation of the signal. Figures 6(b)6(e) show the results obtained at 1 Hz and Figs. 6(f)6(i) show the results obtained at 10 Hz. As expected, due to negative DEP, the particles were depleted from the constrictions between the posts. At 1 Hz, significant EOF was obtained and the particles flowed back and forth as the signal changed from positive to negative. At the signal maxima (ϕ = ±800 V) the particles were immobilized due to DEP far from the center of the constriction, and this immobilization occurred to the left (Fig. 6(c), t = 0.5 π) and to the right (Fig. 6(e), t = 1.5 π) of the constriction, depending on the magnitude and sign of the applied potential. The group of particles labeled “B” in Figs. 6(b)6(e) migrated between adjacent constrictions, but never went past them and stayed in the area shown in these figures, due to nDEP. Also, DEP weakened and particles became closer to the center of constrictions when potential was 0 V, and they were repulsed back when potential increased again.

The depletion zones were much smaller at a frequency of 10 Hz (Figs. 6(f)6(i)), and the constriction was filled with particles when potential was 0 V. Figure 5(f) shows no DEP response when ϕ = 0 V, while Fig. 6(g) shows a depletion region obtained when the signal amplitude is 800 V (t = 0.5π). The same was observed as time progressed, no depletion when ϕ = 0 V (Fig. 6(h)) and particle depletion at the constriction when the signal reached ϕ = −800 V at t = 1.5 π (Fig. 6(i)).This was because the particles did not have enough time to fully polarize at this higher frequency. DEP effects at 20 Hz were similar to 10 Hz, but DEP was weakened further at 20 Hz.

Offset AC signals are also useful for particle manipulation, since net EOF is nonzero in that case. However, this net EOF is small, so a smaller DEP force, requiring smaller applied potentials, may be enough for particle manipulation. This was demonstrated with 1 and 2 μm diameter polystyrene particles. Microfluidic channels with cylindrical insulating posts were filled with a mixture of 1 and 2 μm diameter particles, whose concentrations were 1.21 × 108 and 6.06 × 107 particles/ml, respectively. A sinusoidal, 20 Hz AC signal was applied across the channel. Figure 7(a) shows the representation of the applied signal. Initially, the amplitude oscillated between −500 and +500 V, followed by changes of 100 V steps to produce increasing offset, until the signal oscillated between 0 and +500 V. Figure 7 shows the behavior of 1 μm (green) and 2 μm (red) particles under these conditions. With a symmetrical signal (Fig. 7(b)), particles oscillated in place with EOF. With a −300/+500 V signal (Fig. 7(c)), particles were retained at constrictions thanks to iDEP. The offset signal caused net EOF that brought the particles to the constrictions. In Fig. 7(c), particles are seen separated into two bands in the trapping regions because of the difference in DEP force they experience due to their size difference. Also, some green 1 μm particles are not trapped but pass through the constrictions, while virtually all red 2 μm particles are trapped. Figure 7(d) shows the particles under a more significantly offset signal of −100/+500 V, where more particles escape the trapping regions and bands are less clear. It was observed that 1 μm particles flowed through the channel at a high rate, and some 2 μm particles also escaped. This sequence of offset steps facilitates size based separation consisting of these stages: (i) All particles are trapped in the constrictions, (ii) A relatively small offset releases the smaller particles from the constrictions, (iii) A larger offset releases the larger particles from the constrictions, preferably once the smaller particles have all been eluted. This separation scheme has the potential to be applied to complex mixtures of particles, including particles in the sub-micron size range.

To assess the potential of low frequency AC-iDEP systems for handling particles in the sub-micron range, an additional experiment was performed, using a microchannel with diamond shaped posts (same as in Fig. 5(b)) and a low frequency AC signal at 20 Hz with an offset. Diamond shaped posts were used since they produced a sharper electric field gradient that leads to stronger trapping, as demonstrated in Fig. 5. These results shown in Fig. 8 demonstrate trapping of 0.5 μm particles. Fig. 8(a), obtained at 800 V/+900 V, shows clear trapping of three different types of particles: red 0.5 μm, green 1.0 μm, and red 2.0 μm diameter particles. As it can be seen in the image, three distinct bands of dielectrophoretically immobilized particles were obtained at the constrictions between the posts. The smallest particles (0.5 μm red) are trapped much closer to the constriction, since they exhibit the lowest negative dielectrophoretic force (particles are red, but the band looks yellow due to color oversaturation). The medium-sized particles (1 μm green) are trapped in a nice well-formed green band between the other two particle types. The largest particles (2 μm red) are trapped much further away from the constrictions (to the left) since they are strongly repelled by negative DEP. Figures 8(b)-8(d) show the predicted dielectrophoretic force for these three particles, these COMSOL simulations were plotted with a maximum value of 1.236 × 10−13 N for the dielectrophoretic force, where the dark red color indicates the maximum (refer to color legend in Fig. 4). It is very clear from these images that a much lower force is exerted on the 0.5 μm particles; however, this force is enough to achieve the clear dielectrophoretic immobilization shown in Fig. 8(a).

## Conclusions

Microfluidics is a dynamic field that is continuously growing. Electrokinetic mechanisms are one of the main techniques employed in miniaturized devices. iDEP is an attractive method that combines dielectrophoretic forces and electroosmosis to effectively manipulate particles. Presented here is the study of the effect of the shape of the insulating structures on the magnitude and characteristics of the DEP forces exerted on the particles. Insulating structures of essentially the “same” size can produce very different distribution of the gradient of the squared electric field, which directly affect the magnitude and distribution of the DEP forces exerted on the particles. Additionally, the characteristics of low frequency AC signals were also studied, in order to analyze how different parameters affect DEP forces. It was found that low frequency signals allow for more flexible dielectrophoretic effects than DC signals by simultaneously manipulating the magnitude of electroosmotic flow. Particles in the submicron range were also manipulated with these devices. This type of systems has great potential for the manipulation of a wide array of nano and microbioparticles.

## Acknowledgements

The authors would like to acknowledge the financial support provided by the Kate Gleason College of Engineering at Rochester Institute of Technology through a start-up package to BHLE, Summer Research Internships for Undergraduates, Seed Funding and Faculty Education and Development Grant Program (FEAD).

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Gallo-Villanueva, R., Pérez-González, V. H., Davalos, R., and Lapizco-Encinas, B. H., 2011, “Separation of Mixtures of Particles in a Multipart Microdevice Employing Insulator-Based Dielectrophoresis,” Electrophoresis, 32(18), pp. 2456–2465. [PubMed]
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Chávez-Santoscoy, A. V., Baylon-Cardiel, J. L., Moncada-Hernández, H., and Lapizco-Encinas, B. H., 2011, “On the Selectivity of an Insulator-Based Dielectrophoretic Microdevice,” Sep. Sci. Technol., 46(3), pp. 384–394.
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## Figures

Fig. 1

Schematic representation of one of the microchannels employed for experimentation

Fig. 2

(a) Distribution of the electric field (V/m) and (b) distribution of the gradient of the squared electric field (V2/m3) for the microchannel shown in Fig. 1 when a potential of 500 V is applied

Fig. 3

Representation of ∇E2 and the forces acting on the particles, showing the locations of the regions of dielectrophoretic immobilization of particles

Fig. 4

Distribution of DEP force for 1 μm particles in N obtained at one of the constrictions by applying a potential of 500 V. Dark color represents the maxima. (a) Microchannel with cylindrical posts, post diameter 450 μm spaced 500 μm center-to-center. (b) Microchannel with diamond-shaped posts, 450 μm wide and long, spaced 500 μm center-to-center.

Fig. 5

Particle trapping in iDEP devices with circle (Left) and diamond (Right) shaped insulating posts. Both post geometries had an effective diameter of 450 μm and the opening was 50 μm at its narrowest point. ((a) and (b)) 300 V: Trapping was observed in both geometries, but only effective with diamond posts; ((c) and (d)) 500 V: Fewer particles flowed through diamond constrictions, and they were more tightly focused into streams. ((e) and (f)) 700 V: Greater particle concentration could be achieved with diamond posts.

Fig. 6

Particle behavior observed by applying and AC sinusoidal signal of 800 V with frequency of 1 Hz and 10 Hz. (a) Applied AC signal; ((b)–(e)) results obtained at 1 Hz; ((f)–(i)) results obtained at 10 Hz. Stronger negative DEP effects are observed at 1 Hz, and groups of particles (Labeled (A)–(C)) move between adjacent constrictions over 1 period of the 1 Hz signal.

Fig. 7

(a) Representation of the applied signal with offset steps at 20 Hz; ((b)–(d)) 1 and 2 μm particles during various offset steps. (b) Particles did not have a net migration through channel with no offset; (c) particles were trapped in separate bands but some 1 μm particles were not trapped; (d) 1 μm particles were effectively eluted, along with a smaller amount of 2 μm particles.

Fig. 8

(a) Dielectrophoretic trapping of a mixture of red 0.5 μm, green 1 μm, and red 2 μm diameter particles, trapped with a 20 Hz asymmetrical AC signal of −800 V/+900 V. Estimation of the dielectrophoretic force exerted on the particles for (b) 0.5 μm, (c) 1 μm particles, and (d) 2 μm particles.

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