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Journal of Electrostatics 63 (2005) 823–830

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doi:10.1016/j

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High-gradient electric field system for thedielectrophoretic separation of cells

Zurina Z. Abidin, Gerard H. Markx�

School of Chemical Engineering and Analytical Science, University of Manchester, Sackville Street,

P.O. Box 88, Manchester M60 1QD, UK

Available online 25 March 2005

Abstract

A high-gradient electric field strength system, consisting of a chamber created from two

concentric cylindrical electrodes and filled with glass beads, was constructed for the study of

the dielectrophoretic separation of cells. The glass beads distort the electric field patterns and

generate sites of high electric field gradients, which can capture cells by dielectrophoresis. In

order to experimentally determine the efficacy of the system, yeast cells were injected into a

constant flow through the system, and the number of cells trapped measured. The effects of

changes in voltage, bead size and the size of the inner and outer electrodes on the trapping

efficiency of the dielectrophoretic separation system were investigated. In addition, simulation

and electric field analysis were carried out using FEMLAB. Results indicate that the trapping

of cells occurs by a combination of mechanical trapping and dielectrophoresis. Analysis of the

results allow predictions to be made for the optimum values of the voltage, bead size and size

of inner and outer electrode for any high-gradient system, in order to generate sufficient

electric field gradient for dielectrophoretic cell collection whilst reducing non-specific

mechanical trapping.

r 2005 Elsevier B.V. All rights reserved.

Keywords: High-gradient electric field; Dielectrophoresis; Cell separation; Electric field model

see front matter r 2005 Elsevier B.V. All rights reserved.

.elstat.2005.03.078

nding author. Tel.: +44161 200 64394; fax: +44161 200 64399.

dress: [email protected] (G.H. Markx).

www.elsevier.com/locate/elstat

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Z.Z. Abidin, G.H. Markx / Journal of Electrostatics 63 (2005) 823–830824

1. Introduction

Robust and improved cell separation techniques are important in many areas incell biology, biotechnology and others. The discovery of dielectrophoresis (DEP), theinduced motions of matter suspended in a fluid in a non-uniform electric field [1], hasprovided new techniques for the separation of cells in the basis of their electricalproperties. The theory of dielectrophoresis has been described in a number of booksand reviews [2–4].High-gradient electric field strength separation (HGES) is one of the methods that

utilized the DEP concept. Here materials are placed between electrodes. When theelectrodes are energized, regions of highly non-uniform electric field are inducedaround the materials, and result in a net DEP force being exerted on any particle inthe fluid. This method has shown potential in filtration and separation when Fritsche[5] and Lin et al. [6] both developed a high-gradient dielectric separator and filter. Inlater work, Wakeman et al. [7] used a HGES-DEP system for the filtration ofcontaminants in oil, while Suehiro et al. [8] investigated the ability of a HGES-DEPsystem to recover biological cells from wastewater. However, to date a systematicinvestigation of the effect of column properties has been lacking. In this work we willdescribe the study of some of the characteristics of the HGES system.

2. Materials and methods

2.1. HGES column

The HGES column consisted of coaxial cylindrical electrodes. The diameter of theinner electrode varied between 0.6 and 2mm, while the outer diameter variedbetween 2.7 and 6mm. The column had a length of 50mm. Co-axial connectors wereused as the top and bottom cover to seal the column. The top co-axial connector hada small rod in the middle for attaching and centring the inner electrode (see Fig. 1).

(ii)

Nylon filter tosupport matrix

earthed

Product

Frame/coverfor bottomarea fromplastic

Connected tovoltage

Central electrode

(i)

Syringe forfeed flow

Co axialconnectorfor centering

HGEScolumn

Small rodforelectrodeattachment

Cylindricalouterelectrode

Rubber tubingfor protectivecover

Fig. 1. High gradient electric field strength column for DEP separation of cells. Top section was shown in

part (i) and bottom section in part (ii).

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Z.Z. Abidin, G.H. Markx / Journal of Electrostatics 63 (2005) 823–830 825

A needle was stuck into the top plug for the inlet flow. The bottom cover had a30 mm pore size nylon filter (Millipore, UK) as matrix support, and had a small holein it for the outlet flow. Glass beads were used as the matrix and obtained fromSigmund Lindner GmbH in Germany. The glass beads had size distributions of40–70, 90–150 and 150–250 mm.

2.2. Cells

Saccharomyces cerevisiae yeast (unknown strain) was grown overnight in 100mlMYGP (malt 3 g l�1, yeast extract 3 g l�1, glucose 10 g l�1, mycological peptone5 g l�1) broth at 35 1C in a shaker with a speed of 150 rpm. The cells were centrifugedand washed four times using deionised water to reduce the conductivity. Cellconcentrations were measured using UV spectrophotometer (PYE Unicam, Philips;model 8600) in a cuvette of 1 cm path length.

2.3. Experimental procedure

A standard frequency generator (Thurlby-Thandar, TG120) with a self-built high-frequency amplifier was used to supply voltages up to 60Vpk�pk. One milliliter of theyeast suspension was injected into the HGES-DEP column, and a signal with varyingvoltage and a frequency of 1MHz was applied in order to attract the yeast cells tohigh electric field regions by positive dielectrophoresis. Next, a deionized water flowof 2 ml min�1 was fed into the chamber using a Sage pump syringe (Model 355, USA)to wash away the cells which were not attracted. The outlet suspension was collectedfor 30min, and the cell concentration in the outlet suspension was then measured.

2.4. Electric field calculations

Calculations of electric field strength, E, at dc conditions for the HGES systemwas done using FEMLAB software version 2.3 (Comsol Ltd). The simulations weredone for glass beads size 50, 80, 120 and 200 mm at voltage and frequency of40Vpk�pk and 1MHz, respectively, with the inner and outer electrode diameter being4.7 and 2.0mm.

3. Results and discussion

Initial experiments were performed with a chamber with an inner electrode with adiameter of 2mm, and an outer electrode with a diameter of 4.7mm. The voltagewas varied, while the frequency was fixed at 1MHz. The bead size was 40–70 mm.The results in Fig. 2 showed that a significant number of cells are trapped when noelectric field is applied. However, with increasing voltage an increasing number ofcells are trapped selectively by the electric field. It is likely that the HGES columnacted similar to a deep bed filtration system, and that some of the cells were trappedin the interstices between the beads.

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0.0E+00

5.0E+05

1.0E+06

1.5E+06

2.0E+06

2.5E+06

3.0E+06

3.5E+06

4.0E+06

0 10 20 30 40 50

voltage (V)

no

ofc

ellt

rap

ped

40-70 micron

90-150 micron

150-250 micron

Mechanical trapping at 0 V

- 52%

- 48%

- 44%

Fig. 2. Cell capture in HGES system at different voltages with different bead sizes distributions. Bigger

beads give more mechanical trapping.

0.0

5.0

10.0

15.0

20.0

25.0

0 10 20 30 40 50

voltage (V)

%el

ectr

ical

yiel

d

40-70 micron

90-150 micron

150-250 micron

Fig. 3. The % electrical yield in a HGES system with different bead sizes. The effect of bead size on cell

capture by the electric field is small.


An increase in the bead size is likely to reduce the number of mechanically trappedcells, as the pores between the beads become bigger. However, a change in the beadsize will also change the electric field non-uniformities generated around and betweenthe beads, and hence affect the electrical trapping yield. To investigate this, theprevious experiments were repeated with different bead sizes distributions of 90–150mm and 150–250 mm to give results as in Fig. 2. Increasing the bead sizes result in areduction of the mechanical filtering of cells. Comparison of the electrical trappingyields using different bead sizes (Fig. 3) shows that changing the bead size has littleeffect on the electrical yield. This result was surprising, as it could be expected that,similar to the situation with microelectrodes [9], particles with a size of a smallnumber of multiples of the diameter of the particle to be captured would generatenon-uniform electric field patterns that are most suitable.

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To further investigate the effect of particle size, a two-dimensional reconstructionof the electric field pattern in a column packed with different bead sizes was madeusing FEMLAB. In addition, the electric field around a single bead with noneighbours, at a fixed distance from the inner electrode surface, was calculated. Theresults illustrated in Figs. 4–6 show that the highest electric field strength occurs atthe contact points between the beads. With smaller beads, the region with highelectric field strengths is relatively small. As the bead size increases the highestelectric field strength at the contact point declines, but the electric field strength

2

1

0

-1

-2

-3

-1.7 -1.6 -1.5 -1.4 -1.3 -1.2 -1.1 -1 -0.9 -0.8 0

0.5

1

1.5

2

2.5

Min: 0

Max: 2.6e+00x104

x10-3

Surface: electric field (E)x10-4

Fig. 4. The electric field distributions around beads size 50 mm. Maximum of electric field strength occurs

at the points of contacts of the beads. As the distance from the centre increases, the value of the electric

field strength decreases.

2

1

0

-1

-2

-3

-1.7 -1.6 -1.5 -1.4 -1.3 -1.2 -1.1 -1 -0.9 -0.8 0

0.5

1

1.5

2

2.5

Min: 0

Max: 2.6e+00x104

x10-3

Surface: electric field (E)x10-4

Fig. 5. The electric field distributions around bead size 200mm. The electric field further away from the

points of contacts of the beads for this size is slightly stronger than 50mm beads.

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0.0E+00

5.0E+03

1.0E+04

1.5E+04

2.0E+04

2.5E+04

3.0E+04

0.001 0.0011 0.0012 0.0013

distance from centre of electrode (m)

50

80 micron120 micron200 micron

Electric field calculated alongedge of a bead

bead

electrode

Ele

ctri

c fi

eld

(V

/m)

Fig. 6. Electric field analysis using single bead. The smaller the beads, the greater the electric field strength

at point of contact of beads. The nearly exponential rate of decay of the electric field is also larger for

smaller beads as the distance from the centre increases.

30.0

25.0

20.0

15.0

10.0

5.0

0.00 0.0005 0.001 0.0015 0.002 0.0025

inner electrode diameter (m)

% e

lect

rica

l yie

ld

Fig. 7. The electrical yield decreased with an increase in the dimension of the inner electrode diameter

at 40V.


further away from the contact point declines less strongly. Thus, when the particlesize increases, the local electric field may become less, but the volume in which thecells may be captured increased. This situation is similar to that of microelectrodes ofdifferent sizes, where an increase in electrode size and distance results in less rapidlydeclining electric fields [10].In order to investigate the effect of size of the electrodes used in the HGES, two

sets of experiments were performed: one with a fixed outer electrode of 4.7mm innerdiameter, but the diameter of the inner electrodes was varied from 0.6 to 2mm, andanother one in which the size of the inner electrode was set at 1mm, but the diameterof the outer electrode was varied from 2.7 to 6mm. The columns were filled with

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50.0

45.0

40.0

35.0

30.0

25.0

20.0

15.0

10.0

5.0

0.00 0.002 0.004 0.006 0.008

outer electrode diameter (m)

% e

lect

rica

l yie

ld

Fig. 8. The electrical yield at 40V decreased with the increase in the outer electrode diameter since electric

field became weaker and only persisted near electrode surfaces.


40–70 mm beads, and the trapping efficiencies of the column were measured asdescribed previously. The results in Fig. 7 showed that when the outer electrodediameter is kept constant, the larger the radius of the diameter of the inner electrode,the smaller the yield of the HGES column. Conversely, when the diameter of theinner electrode is kept constant, the column efficiency decreased with the diameter ofthe outer electrode (Fig. 8). These results can readily be explained by the changes inthe electric field strength with changing column geometry [1].

Acknowledgements

We wish to thank Sigmund Lindler for the glass beads and Universiti PutraMalaysia for financial support.

References

[1] H.A. Pohl, Dielectrophoresis, Cambridge University Press, Cambridge, 1978.

[2] T.B. Jones, Electromechanics of Particles, Cambridge University Press, Cambridge, 1995.

[3] M.P. Hughes, Nanoelectromechanics in Engineering and Biology, CRC Press, New York, 2002.

[4] R. Pethig, G.H. Markx, Applications of dielectrophoresis in biotechnology, Trends Biotechnol. 15

(1997) 426–432.

[5] G.R. Fritsche, Electrostatic separator removes FCC catalyst fines from decanted oil, Oil Gas J. 75

(1977) 73–74.

[6] I.J. Lin, L. Benguigui, Dielectrophoretic filtration of non-conductive liquids, Sep. Sci. Technol. 17 (8)

(1982) 1003–1017.

[7] R. Wakeman, G. Butt, An investigation of high-gradient dielectrophoretic separation, Chem. Eng.

Res. Des. 81 (A8) (2003) 924–935.

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[8] J. Suehiro, G. Zhou, M. Imamura, M. Hara, DEP filter for separation and recovery of biological cells

in water, IEEE Trans. Ind. Appl. 39 (5) (2003) 1514–1521.

[9] R. Pethig, Dielectrophoresis: using inhomogeneous AC electric fields to separate and manipulate

cells, Crit. Rev. Biotechnol. 16 (4) (1996) 331–348.

[10] G.H. Markx, R. Pethig, J. Rousselet, The dielectrophoretic levitation of latex beads with reference to

field-flow fractionation, J. Phys. D: Appl. Phys. 30 (1997) 2470–2477.

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