Characterizations of Micro-particles Trapping and Separation in an ElectrodelessDielectrophoresis

Liang-Ju Chien 1 , Jia-Cheng Pan 2,Chi-Han Chiou 1, and Jr-Lung Lin *21ITRI South, Industrial Technology Research Institute, Tainan Country, Taiwan

2 Department of mechanical and Automation Engineering, I-Shou University, Kaohsiung, Taiwan 840.E-mail: ljl@isu.edu.tw.

Abstract-This work demonstrated the feasibility of a newparticle trapping and separation approach, based on anEDEP-based microfluidic chip. Experimental observationsand numerical simulations were facilitated to investigate thephenomena of EO or DEF depended on the drivingfrequencies at a given constant electric field. The results notonly revealed the fluid cloud be generated aforward/backward EOF by adjusting the driving frequency,but also categorized the optimum trapping/separatingfrequencies for the different particle sizes. Besides, theproposed operating mode can be used to trap/separateparticles and simultaneously convey them through theEDEP-based device, eliminating the need for an additionalmicropump. Certainly, the EDEP-based device can beenvisioned as a single automated device that supports themulti-functions such as transportation, separation, anddetection, facilitating the realization of a Lab-on-a-Chip.

1. INTRODUCTION

In the past decades, cell or microparticle separation hasgained significant attentions in sample preparations forbiological and chemical applications especially in microfluidicsystems. Traditionally, microfluidic separation is typicallyachieved by applying differential forces on the target particles toguide them into different paths. Recently, various cell ormicroparticle separation methods have been successfullydemonstrated, such as density gradient centrifugation [1],electrophoresis [2], magnetic activated cell sorters (MACS) [3]and fluorescent activated cell sorters (FACS) [4]. However,FACS and MACS, which are generally used for cell separation,require immunolabeling to separate specific target cells,exhibited high cost and require relatively large cell populations.Currently, an alternative separation method is dielectrophoresis(DEP). Basically, DEP can be briefly cataloged two types,namely metal DEP (mDEP) [5] or electrodeless DEP (EDEP)[6].MDEP provides many advantages such as flexibility,controllability and ease of application; and it has been proven tobe an efficient non-invasive method for separating various celltypes without any need for labeling. However, mDEP causes fastdecay of the field gradient to damage the bio-sample. Thesecharacteristics significantly yield to reduce the trappingefficiency and throughput. Meanwhile, microparticles areconveyed by using external pressure-driven flows. Therefore, aconcept of insulator-based (electrodeless) DEP (iDEP or EDEP)in which a highly field gradient is generated at a geometricalconstriction of the insulating structures in a conducting solution,avoiding some of the aforementioned problems that areassociated with the use of metallic electrodes. In this study, wesuccessfully demonstrated an EDEP based chip to achieve themicroparticle separation. The different sizes of microparticlescan be easily trapped at the different locations of theEDEP-based chip. Experiments revealed that microparticleseparation is significantly dominated by tuning of the differentdriving frequency. It can be used to continuously separatewithout an external transportation system.

2. THEORY

2.1 Dielectrophoresis

For assuming a spherical particle sounding in a conductivemedium, the DEP force can be expressed [7] as

23 ]Re[2 ECMaF mDEP (1)

where a is m is the permittivity of themedium, and E is the amplitude of the electric field. Re[CM] isthe real part of the complex Clausius Mossotti factor. DEP force,FDEP, is proportional to the gradient of the square of the appliedelectric field and the third power of the particle radius. Thecomplex Clausius Mossotti factor is given by

**

**

2 mp

mpCM (2)

wj* (3)

* * are the complex permittivity and conductivity,respectively. The subscripts p and m refer to the particle and themedium, respectively. w is the angular frequency of the electricfield and j is the imaginary unit. In principle, the real part ofRe[CM] is bounded between 1.0 and -0.5. In this study, thecalculating approximation Re[CM] = -0.49 is employed in thefrequency range from 10 Hz to 5M Hz.

2.2 Electroosmosis

electrical double layer by an applied tangential electric field. Inplanar microelectrode arrays that are used to induce DCelectrokinetics, divergent electric fields are generated such that acomponent of the electric field lies tangential to the EDL, whichis induced on the electrode surface. Accordingly, ions in thediffuse double layer experience a force that, on average overtime, acts from the edge, across the surface of the electrode.In this study, EOF velocity (uEOF) is represented as [8]

ezn

EEzeE

u oyoysDxm

EOF4

)(sinh 1 (4)

where Ex and Ey are the x- and y-directional electric fieldstrength and is also strong functions of the applied voltage aswell as driving frequency. Notably, Ex and Ey are calculated bythe numerical simulations. In this study, a medium with pH 8.0was chosen. The value of Eyo was then calculated to be 1.5x105V/m. Based on the assumption that spherical particles surroundthe viscous medium, the Stoke drag force is given by,

)(6 pEOFst uuaF (5)

where uEOF and up denote the velocities of the medium and aparticle. The Stokes force is proportional to the velocity and theparticle radius.

The aim of the simulation is also to predict the trappingbehavior of particles, i.e. particle separation, under non-uniformAC electric fields. Hence, a three-dimensional (3-D) numericalsimulation was conducted to investigate and to characterize theelectrokinetic phenomena. The simulated results were thenconfirmed by the experimental observations. Currently, the

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Proceedings of the 2011 6th IEEE International Conference on Nano/Micro Engineered and Molecular Systems February 20-23, 2011, Kaohsiung, Taiwan

physics of an EDEP device are numerically calculated usingcommercial computational fluidic dynamics (CFD) software(CFD-ACEU, CFD-RC, USA). In this investigation, flow-,thermal-, spray-, and electric-modules are introduced to makenumerical calculations. Table 1 shows the physical properties ofthe materials that were assumed in the numerical simulations.

3. MATERIALAND METHODS

3.1 Design and Fabrication

This proposed EDEP-based microfluidic device wasdesigned with a 5 -triangular insulating

and a microchannel of width 500

producing a high electric field gradient with a local maximumDEP force for trapping particles. A pair of driving electrodesseparated by 2 mm was adopted to produce the weakest possiblerequired electric field in the microchannel. Figure 1(a)schematically depicts the EDEP-based microfluidic device,comprising a triangular insulating structure, a microchannel, anda pair of driving electrodes.

An EDEP-based chip was fabricated using MEMS(Micro-electromechanical Systems) technology. A silicon waferwas patterned using standard photolithography, and then reactiveion etching (RIE) was empwith the triangular insulating structures and the microchannel.Accordingly, the inverse structures with patterned features werecast using PDMS materials and the Si mold. Finally, the PDMSstructures and the glass substrate with Pt electrodes were bondedeach other by oxygen plasma treatment to yield a completeEDEP-based chip. Figure 1(b) displays the scanning electronmicroscope (SEM) image of the mold. Figure 1(c) shows theassembly of the EDEP-based chip.

3.2 Experiment

A power amplifier (LPA 400, Newtons4th, UK), a signalgenerator (33220A, Agilent, USA) and an oscilloscope (TDS1002B, Tektronix, USA) were connected to a pair of driving Ptelectrodes, generated an electric field with a controlled frequency.All experiments were conducted in a constant electric field of 90Vpp/cm with the continuously driving frequencies ranging from10 to 5 MHz. The fabricated EDEP-based chip was mounted ontop of an inverted microscope (DMI 4000B, Leica, Germany) formicroparticle visualization. The fluorescence images of themicroparticles were captured using a 10 or 20 × objective lens,as well as a cool CCD (Charge-coupled Device) (Cool SNAPHQ2, Photometrics, USA). Four polystyrene microparticles(excitation 580 nm/emission 605 nm, Invitrogen, USA) of 0.2-,0.5-, 1.0- and 2.0- were used to experimentally investigate theelectrophoresis effect in this study. Besides, the viscosity ofsolution was adjusted to be 1 cP. All experiments were conductedin a buffer solution of 10 mM Tris-HCl (pH 8.0) with a fixed

4. RESULTS AND DISCUSSION

4.1 Numerical Characterization

To test the functions of the EDEP device, a simpleinsulating-based cell, defined in a microfluidic channel, as shownin Fig. 1(a), is considered. The boundary conditions of thenumerical simulation are also provided in the figure. An electricfield of 90 Vpp/cm with the range from 10 Hz to 5 MHz wasapplied as a sinusoidal wave across the microchannel through aconstricting gap of 5 m. The 3-D contours of the electric fieldwere non-uniform as presented and the highly non-uniforn fielddensity was exhibited around the 60o-triangular insulatingstructures, as presented in Fig. 2. The electric field in the ionicsolution was easily squeezed in the constricted region. Hence,the electric field gradients were highly non-uniform around thetwo constricted parts. The electrical field (Ex) in the x-direction

was highest at the constriction gap. Conversely, the y-directionalelectric field (Ey) was highest at the wall of the triangularinsulation, from the tips of the constriction. The contours ofEy were symmetrical around the tip of the constriction becausethe conservation of the electric field in the y-direction. Notably,the contours of Ex and Ey did not vary with frequency, but theirmaximum values differed from each other.

4.2 Experimental Characterization

Theoretically, two balanced forces are exerted on the trappedparticles. The first force is the DEP force (FDEP), which acts as aholding force, which stops the particles from moving. The other

Fst), which EO fluid acts on the particlesto transport. Therefore, the net motion of particles is determineby both EO and DEP effect. Figure 3(a) reveals that thefluorescent 0.2- particles passed through the constrictionwhen a field with a frequency from 100 Hz to 5 MHz wascontinuously applied. The microparticles flowed forward throughthe constriction gap by EO conveyance, and was not trapped bythe DEP effect as the driving frequency is below 810KHz.Conversely, the flow direction reversed, i.e., backward flow, asthe driving frequency is beyond 810 kHz. Interestingly, areciprocating motion was experimentally observed at a frequencyof 810 kHz. Fig. 3(b) plotted the relationship of the EOFvelocity and frequencies at an applied voltage of 90 Vpp/cm. It isclearly seen that the EOF velocity initially increases with theoperation frequency. However, the EOF velocity reaches amaximum value and subsequently decreases to a negativeEOF velocity as the operation frequency is increased further.Figures 4 (a) and 4(b) demonstrated the numerically velocitycontours of forward and backward EOF. EOF effect exhibited tooccur within a pair of triangular insulating constructions. It alsoshowed the higher velocities occurred near the constriction walldue to the zeta potential effect. The simulated results can beverified by the experimental observations.

A solution mixed 0.5- with 1.0-into the reservoir, a field of 90 Vpp with 400 Hz was applied ontothe electrodes. The micorparticles were continuously flowed bythe presence of the EOF through the triangular insulators andthen were separated in the DEP-affected area, which was close tothe constricting gap as shown in Fig. 5(a). Clearly, the n-DEPforce acted locally on the 0.5- around 10 fromthe constriction. Similarly, the DEP force acted locally on the1.0- ound 20~30 Notably,the microparticles were trapped dielectrophoretically at one sideof the gap and the particle-trapped contours demonstrated thesemicircle shape. Next, the 0.2-, 0.5- and 1.0-independently operated to evaluate the trapping effect in theproposed EDEP-based chip at a given field of 90 Vpp/cm withthe frequencies of 100~5M Hz. Table 2 showed the differentmicroparticle sizes whether trapped or not. Red color indicted themicroparitlces can be trapped and blue color presented thatcannot be trapped. It was clearly seen that 0.5- particles canbe trapped for the frequency of 400~3k Hz, and 1.0- particlescan be trapped for that of 100~810k Hz. Conversely, 0.2-umparticles cannot be trapped. It was facilitated to easily separatefor the 0.2, 0.5- and 1.0- ng the drivingfrequencies. Figure 5(b) explored the distribution of n-DEP forceat the applied voltage of 90 Vpp/cm and driving frequency of1000Hz. The blue color indicts the n-DEP force, conversely, thered color represents the p-DEP force. Notably, the DEP forcecontours cannot be changed with the different driving frequency.Certainly, these values are only changed by the differentfrequency as well as the particle sizes. The numerical results canbe used to explain why the microparticle trapping contours werelike the semicircular shapes and the trapping location begun inthe constriction wall.

5. CONCLUSIONS

This work have been sucessfully demonstrated thefeasibility of a new particle trapping and separation approach,

844

PDMS

Glass subtractElectrode

Triangular insulator

Constriction gap

Fludic channel

based on an EDEP-based microfluidic chip. The tarpping orseparating effect dominates on whether EO or DEF depended onthe driving frequencies in a given constant electric field of 90Vpp/cm. In this study, the flow produced a forward EOF as thefrequencies were less than 810 kHz, conversely, that did abackward EOF as the driving frequency were larger than 810kHz. Interestingly, an unstable flow, similar to a reciprocatingmotion, was experimetally observed at a frequency of 810 KHz,Experiments reveal that the optimum trapping frequencies forcontinuous flow are the range of 400~3k Hz for 0.5 , of100~810k Hz for 1.0- . Furthermore, this apporach can beused to continousely flow for the separatation of themicroparictles.

REFERENCES

[1] S. L. Friedman and F. J. of hepaticlipocytes, Kupffer cells, and sinusoidal endothelial cells by

Anal.Biochem. vol.161, pp. 207 218, 1987

[2] A. Tulp, D. Verwoerd and J. Pieters, Application of an improveddensity gradient electrophoresis apparatus to the separation of

Electrophoresis, vol.14, pp. 1295 1301, 1993

[3] S. Miltenyi, W. Muller, W. Weichel and A. Radbruch, Highgradient magnetic cell separation with MACS Cytometry, vol.11, pp. 231 238, 1990

[4] P. Malatesta, E. Hartfuss and M. Gotz, Isolation of radial glialcells by fluorescent-activated cell sorting reveals a neuronallineage Development, vol .127 5253 5263, 2000

[5] E. B. Cummings, Streaming dielectrophoresis forcontinuous-flow microfluidic devices, IEEE Eng. Med. Biol.Mag., vol. 23, pp. 75-84, 2003

[6] C. F., Chou and F. Zenhausern, Electrodeless dielectrophoresisfor micro total analysis systems, IEEE Eng. Med. Biol. Mag.,vol. 22, pp. 62-67, 2003

[7] H. A. Pohl, Dielectrophoresis: The behavior of neutral matter innonuniform electricfelds, Cam bridge Cambridge UK, Chapter 4,1978

[8] C. H. Chiou, L. J. Chien, J. T. Tseng and J. L. Lin, Numericaland experimemtal investigation of electrokinese in anelectrodeless dielectrophoresis chi MicrofluidNanofluid, 2010

Table 1. Physic properties of particle and medium

(kg/m3) r (Sm-1)

medium 1000 80 0.07

particle 1009 2.5 0.0009

PDMS 970 2.5

Table 2. Driving frequency verse different microparticelsizes.

Frequency

0.2 0.5 1.0

10

100

400

1 k

4 k

10 k

100 k

810 k

1 M

5 M

(a )

(b) (c)

Figure 1 (a) Schematic EDEP-based microfluidic device,comprising a triangular insulating structure, a microchannel, anda pair of driving electrodes. (b) SEM image of triangularinsulating structures. (c) Photograph of assembly of EDEP-basedchip.

(a)

(b)Figure 2. Electric field contours of x-directional and (b)y-directional at the condition of applied voltage 90 Vpp/cm withthe driving frequency 1000 Hz

0 0.2 0.4 0.6 0.8 1.0

106 (V/m)

-1 .0 -0.8 -0.6 -0 .4 -0.2 0 0 .2 0 .4 0.6 0 .8 1 .0

10 6 (V/m)

845

5.0x10-03

4.5x10-03

4.0x10-03

3.5x10-03

3.0x10-03

2.5x10-03

2.0x10-03

1.5x10-03

1.0x10-03

5.0x10-04

0-50-100 50 100

0

-50

-100

50

100

( m)

U(cm/s)

-6.0x10-04

-1.2x10-03

-1.8x10-03

-2.4x10-03

-3.0x10-03

-3.6x10-03

-4.2x10-03

-4.8x10-03

-5.4x10-03

-6.0x10-03

-50-100 50 100 ( m)

0

-50

-100

50

100U(cm/s)

Frequency (Hz)

EOF(um/s)

100 101 102 103 104 105 106 107-20

-10

0

10

20

30

40

50

(i) (ii)

(iii) (iv)(a)

(a)

(b)

les underelectroosmosis. The red arrow indicated the flow direction. (b)The relation between EOF velocities and different frequencies.Notably, two red dash-line regions referred to Fig. 3(b) indict toexperimentally calculate mean EOF velocity.

(a)

(b)

Figure 4 numerical results of (a) forward EOF at the drivingfrequency of 1000Hz and (b) backward EOF at the drivingfrequency of 1 MHz.

(a)

(b)

Figure 5 (a) A image for the trapping of 0.5- and 1.0-particles at the driving frequency of 400 Hz. (b) Numericaldistribution contour of DEP force. Blue color represents an-DEP force, conversely, red color indicts a p-DEP force.

m

1.0 m

846