Proceedings of the 2nd IEEE InternationalConference on Nano/Micro Engineered and Molecular Systems

January 16 - 19, 2007, Bangkok, Thailand

Numerical Analysis and Optimization of Insulator-basedDielectrophoresis Devices for Cell Sorter Applications

Jinpeng Wang, Huan Hu, Zheyao Wang, and Litian Liu

All author are with the Lab ofMicro/Nano Devices and Systems,Institute ofMicroelectronics, Tsinghua University, Beijing, China

Abstract-Insulator-based dielectrophoresis (iDEP) is aninnovative approach in which the nonuniform electricfield needed to drive Dielectrophoresis (DEP) is produced byinsulators, avoiding problems associated with the use ofelectrodes. The aim of this thesis is to analyse the iDEPdevices, demonstrate the function of it by numericalcalculation, and optimize the parameters of iDEPmicrodevices. In this study, the electrical field generatedby iDEP microdevice is calculated by Matlab softwareusing Finite Element Analysis (FEA) method. Based onthis, the Dielectrophoretic force experienced by cells inthe microdevice are calculated. Furthermore, we optimizethe parameters of iDEP microdevice such as dielectricshape, dielectric size, dielectric intervals, the size ofMicrochannel and etc. At last, we give a set of optimumparameters.

do not allow electrokinetic conveyance, so particles areusually conveyed using highly dispersive advection(pressure-driven flow).

B. Principles ofInsulator-based dielectrophoresis (iDEP)Insulator-based dielectrophoresis (iDEP) is an alternative

way to implement DEP traps using insulators instead ofconductive electrodes[2] [3]. As shown in Fig. 1, theconstriction of the insulators in the microchannel squeezes theelectric field in conductive solutions, creating a high fieldgradient with a local maximum. IDEP avoids the shortcomingsof DEP, and the maximum field and field gradient arepreserved in the entire cross section of the trap, high electricfield may be applied without gas evolution due to electrolysis.

Keywords-uTAS;Dielectrophoresis; Microfluidics; microarrays;iDEP

I. INTRODUCTION

A. Principles ofDielectrophoresisDEP, the translational movement of a polarizable object

toward the direction of the electric field gradient, is capable ofperforming, including sample sorting, trapping, manipulation,and concentration[1] [2], and is suitable for operating nucleicacids, proteins, cells, and virus in iTAS. Previousapplications of DEP include the separation of colloidalparticles, the separation of biological objects such asyeast cells, viruses, and cancer cells, and the trappingand manipulation of DNA molecules. All these studiesused thin-film deposition techniques by making planarmetallic microelectrodes as DEP traps, which are eitherdirectly driven from a voltage source or free-floating inthe presence of an ac field.

Despite the advantage of low voltage required to generatehigh field gradient, there are serious limitations in theapplications of the metallic DEP layout. First, since themicroelectrodes are fabricatedn by thin-film deposition,the field gradient decays fast from above the electrodesand thus the dielectrophoretic force. This dramaticallyreduced the trapping efficiency. Second, complexelectrochemical reactions, such as electrolysis, mayoccur at the microelectrodes when the field is too high,which promotes the degradation of the electrodes.Moreover, virtually all arrangements of such electrodes

Nonu. i form

insulaator

Figure 1 Principles of iDEP

II. CURRENT RESULTS

A. Function Testing

Electric Field

A0.2 -

0.15-

0.1 -

0.05 -

0-

0

-200 -200

Figure 2 10 V was applied across a 400pm long channel with410pm width and lOpm constriction (field unit: V/m).

All author are with the Lab of Micro/Nano Devices and Systems, Instituteof Microelectronics, Tsinghua University, Beijing, China

Contacting Author: Jinpeng Wang is with The Institute ofMicroelectronics, Tsinghua University, Beijing 100084, China (phone: +8613810009974; email: wangjpO2gmails.tsinghua.edu.cn).

1-4244-0610-2/07/$20.00 C)2007 IEEE

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For a spherical object of radius with radius a, the DEPforce can be obtained from[4]:

FDEP= 2zTa £ ReF(£ ) + 2£m VE(

where eV and cm are the complex permittivities of thedielectric object and the medium.

To testify the functions of the iDEP device, we nowconsider a simplest iDEP cell defined in a microfluidicchannel made of glass as in Fig.i. Fig.2 shows thesimulated electrical field distribution of a channel with 40O0mlong, 41Opm width, IOtm constriction, and 10 V appliedacross the channel with, it is clear that the maximum electricalfield occurs at the constriction, in particular, at the tips ofthe constriction.

Electric Potential on y axis

C,(L

.20La,I

-)

.2

LLI

y

w-- 0 -coL-

y(c)

Figure 3 (a) electrical potential (b) electrical field (c) VE2

For more study, The electrical potential V, the electricalfield E, and VE2 along the y-axis, are numerically calculatedand depicted in Fig.3(a)-(c), y is the applied field direction,The line plots are taken through the central line of theconstriction in y-direction. As shown in Fig.3(a) andFig.3(b), the potential changes sharply at the constriction wherethe electric field is highly focused. The DEP force isproportional to VE2, and the ratio is mainly determined by thecomplex permittivities. From Figure 3(c), it can be found thatthe DEP force direction is toward the constriction of the iDEPcell. The maximum electrical field is higher than 18xI04, andoccurs at the tips of the constriction, as in Fig.2.

B. The applications ofiDEP in [TASWe have demonstrated the function of the iDEP device.

The iDEP device uses insulating constrictions to squeeze theelectric field, thereby creating a high field gradient with a localmaximum which is necessary to create DEP force. iDEP can beused for manipulating, fusing, sorting, and lysing biologicalcells and particles by control the parameters of iDEPmicrodevice. For example, cell lysing may be achieved if weadd high electrical potential to the device shown in figure2.after lysing, critical cellular materials such as chromosomalDNA may be selectively extracted and analyzed.

This part we take the iDEP concentrator as an example toexplain the device function in detail. For a cell concentratorwith a 5x5 circular posts array, the macrophages in human'sblood are driven by lOOV, the DEP force can be obtained using(1), and the fluid resistance and the buoyancy are given by

stke Fy f (2)Fstokes = Ux stokes f y

where f =6zrqa. The macrophages experiences a force of1.3091 x I 0-8N in the fluid with velocity of 0. lm/s. The regionswhere the DEP force is higher than the force of fluid are theregions where concentration is implemented. Fig 3 shows theconcentration areas for macrophages in a 5 x 5 iDEP device.The white regions are circular post arrays, and the pink regionsare concentration regions. This calculation demonstratesthat iDEP can be used for concentrating particles.

concentration areas of iDEP

^ 00 -400 -200 0 200 400 600

Figure 4 concentration areas of iDEP

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C. Analysis and Optimization1) The influences ofthe microchannel lengthFor an iDEP device shown in Fig.5(a), the square posts are

1OOtm wide and 5Opm spacing, and tha 'a' in Figure5 is65Opm. Taking macrophages as the polarizable object andusing (1), b is the distance from the utmost posts to the voltageapplied point. To make the fabrication easy, We don notuse thin-film deposition techniques to make planarmetallic microelectrodes, but directly put the probeelectrode at the end of the microchannel. b is animportant parameter. It can not be too small, or it willbe hard to insert the probe electrode. It also can not betoo large, because we guess that the DEP force willdecrease sharply. The dependence of the DEP force on b isobtained and shown in Fig.5(b). The DEP force is nearly anexponential function of b. As b increases, the DEP forcedecrease rapidly.

D E00Vu: C

El* ElC

(a)

post shapes on DEP force. Among these three shapes, thecircular posts generates the largest DEP force.

1.60E-007 -

1.40E-007 -

,-1 .20E-007 -

u0"-1.OOE-007 -

a-0

8.OOE-008 -

6.OOE-008 -

* square

I circlediamond

Figure 6 influence of the shape

3) The influences ofpost intervalsFig.7 (b) shows the effect of post intervals on DEP force.

With the interval increases, DEP force decrease sharply, andapproaches horizontally when the interval is greater than 40pm.

1.6x10 6 -

1.4x10-6 -

1.2x10 -

-1 10 6-

m 8.0x107-u

L 6.0X10 -CLW 4 0X107--100

2.Ox10- -

0.0 -

-2. Ox 10 -

-*-DEP Force0

0

0

0

11%--m-0-0-0-

0.0 2.Ox1l O 4.0x 10b (um)

6.OxlO 8.Oxl03

1.G6xlo-" -

1.4x10-6 -

1.2x10 -

U_

o 8.Ox10-7-Li-

CL 6010-7 -wU 6.Oxl0

4.Ox10-7

2.OxlO7

n n

(b)Figure 5 (a) configuration (b) influences of the b

2) The influences ofthe post shapesDifferent insulator posts generate different electric fields,

and thus the DEP forces. The maximum DEP forces ofmacrophages experienced from square, circular, diamond postsare calculated and shown in Fig.6 to compare the influences of

Figure 7

4) optimum result

(a)

-*- DEP Force

-~

20 40 60 80interval (um)

(b)

1050

100 120

influences of the interval

Il

I

IJ. IJ

0

0

0

0,

_.- _.

*0-

According to these analyses, the distance b is optimized tobe 150Om,and the interval is set to d=1ORm for a 5x5circular iDEP device. Under this condition, macrophagesexperience DEP forces with maximum up to 3.29 x 10-7N.

III. CONCLUTION

In summary, we have shown several examples where amicro total analysis systems ([tTAS) may benefit from iDEPtechnology, such as cell sorting, cell lysing, DNAconcentration, and purification. We make theoretic analysis tothe iDEP device, demonstrate its function, and optimize thedevice parameters. In the end, we give out the optimum result.

REFERENCES

[1] Manz A, Graber N, Widmer H M (1991), "pTAS - A novel conceptforchemical sensors," Sensors and Actuators, BI -1990-pp. 244-248.

[2] E.B. Cummings, S.K. Griffiths, R.H. Nilson, and P.H. Paul, "Conditionsfor similitude between the fluid velocity and electric field inelectroosmoticflow," Anal. Chem., vol. 72, pp. 2526-2532, 2000.

[3] E.B. Cummings and A.K. Singh, "Dielectrophoretic trapping withoutembedded electrodes," in Proc. SPIE Conf. Micromachining andMicrofabrication, vol. 4177, 2000, pp. 164-173.

[4] H.A. Pohl, Dielectrophoresis: The Behavior of Neutral Matter inNonuniform Electric Fields. Cambridge, UK: Cambridge, 1978, ch. 4.

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