Dielectrophoresis Study of Electroporation Effectson Chinese Hamster Ovary Cells

1Elham Salimi, 2Katrin Braasch, 2Michael Butler, 1Douglas J. Thomson, 1Greg E. Bridges1Department of Electrical and Computer Engineering, 2Department of Microbiology

University of Manitoba, Winnipeg, Manitoba, Canada

Email: Elham.Salimi@umanitoba.ca, Greg.Bridges@umanitoba.ca

Abstract—Electroporation affects the dielectric properties ofcells by creating pores in their membrane which allow transportof ions. Dielectrophoresis (DEP) is a powerful technique toinvestigate electroporation of single biological cells. Here westudy the frequency dependent response of Chinese hamsterovary (CHO) cells and investigate the effect of different cellparameters on the DEP spectra. The results suggest low frequencyDEP application (less than 200 kHz) for investigating the cellmembrane conductivity, mid frequency DEP application (1-30 MH) for investigating the cell cytoplasm conductivity, andhigh frequency DEP application (greater than 100 MHz) forinvestigating cell cytoplasm permittivity.

I. INTRODUCTION

When a cell is exposed to an external electric field thecell membrane is polarized, increasing the membrane potential.This results in the formation of transient conductive poresthat are permeable to ions and water-soluble molecules [1].The phenomenon that is called electroporation (EP) has foundnumerous biological and medical applications such as elec-trogenetherapy [2], electrochemotherapy [3], and irreversibleelectroporation of cancerous cells [4]. Electroporation affectsthe dielectric properties of a cell membrane, as the createdpores are significantly more conductive than the intact partsof the membrane [5], as well as the cell cytoplasm due to theinflux and efflux of ions [6]. Therefore, dielectric characteriza-tion techniques can be employed to study electroporation andits subsequent physiological effects on cells.

Dielectrophoresis (DEP), translation of polarizable cellsin a non-uniform electric field toward or away from regionsof high field intensity, is a technique employed in studiesof biological cells for their characterization, separation, andmanipulation [7]. In this paper we simulate the frequencydependent response of Chinese hamster ovary (CHO) cells andinvestigate the effect of cell parameters on the DEP spectra.The results can be employed in single cell electroporationstudies where the DEP response is used to infer physiologicalchanges.

II. DIELECTROPHORESIS

Dielectrophoresis (DEP) is a field-induced force acting ona polarizable particle exposed to a non-uniform electric field.We previously presented a microfluidic device that implementsDEP and EP and demonstrated its application in single cell’selectroporation studies [8], [9]. A DEP force actuates singlecells in a microfluidic channel and electronically measurestheir lateral displacement due to the force before and afterelectroporating pulse application (Fig. 1). The DEP force

nDEP

pDEPno DEPActuation Region

Differential microwave

interferometer S

Fluid Flow

100 um

VDEP+VEP

Fig. 1: DEP actuation and detection of single cells in amicrofluidic channel for studying electroporation [8], [9].

exerted on a cell is directed along the gradient of the electricfield and is expressed as [10]

�FDEP =3

2εeVcellRe {KCM} �∇(E2

rms) (1)

where εe is the permittivity of the medium, Vcell is the volumeof the cell, and Erms is the rms value of the electric field atthe cell location. Re {KCM} is the real part of the Claussius-Mossotti factor which is a measure of the cell’s polarizabilityin the medium and is given by

KCM =ε̃c − ε̃eε̃c + 2ε̃e

. (2)

In (2) ε̃e and ε̃c are the complex permittivity of the media andthe cell, defined as ε̃ = ε− jσ/ω with ω being the frequencyof the electric field. ε̃c is an effective value incorporating theelectrical properties of the cell complex internal structure. Inthis study we use a double-shell model for cells (Fig. 2) [11].The DEP force is oriented along (pDEP) or against (nDEP) thegradient of the square of the electric field depending on thesign of Re {KCM}. The spectra of Re {KCM} for a typicalviable CHO cell with parameters given in Table I suspendedin a medium of σe = 0.17S/m is presented in Fig. 3 (solidlines). Fig. 3(a),(b), and (c) show Re {KCM} versus frequencyfor three different values of membrane conductivity, cyto-plasm conductivity, and cytoplasm permittivity, respectively.As depicted, the membrane conductivity only impacts the lowfrequency part of the Re {KCM} spectra (less than 200 kHz),whereas the cytoplasm conductivity affects the spectra betweenthe two crossover frequencies (1-30 MHz). The impact of thecytoplasm permittivity is observable at frequencies above 100MHz. Based on these results the frequency of the DEP forcecan be selected appropriately for monitoring specific changesin a cell electrical parameters during physiological changesinduced by electroporation.

978-1-4799-2225-3/14/$31.00 ©2014 IEEE

Fig. 2: Effective permittivity of a cell with a nucleus generatedfrom a double-shell model [11].

TABLE I: Dielectric and geometric parameters of a typicalviable CHO cell.

Parameter Symbol Value

Membrane conductivity σm 3 × 10−6S/m [12]Membrane relative permittivity εrm 6.8 [13]Cytoplasm conductivity σi 0.32S/m [13]Cytoplasm relative permittivity εri 60 [13]

Nuclear envelope conductivity σne 6 × 10−3S/m [13]Nuclear envelope permittivity εrne 28 [13]Nucleus conductivity σn 1.35S/m [13]Nucleus permittivity εrn 52 [13]Medium conductivity σe 0.17S/mMedium permittivity εre 78Nucleus radius Rn 3.25nmNuclear envelope thickness dn 40nm [13]Cell radius R 6μmMembrane thickness d 5nm [12]

III. CONCLUSION

Electroporation affects the dielectric properties of a cell.This can be studied by the DEP technique. We presentedthe effect of different cell parameters on the Re {KCM}.The results suggest distinct frequency ranges for studying thecell membrane conductivity (less than 100 kHz), the cyto-plasm conductivity (1-30 MHz), and the cytoplasm permittivity(greater than 100 MHz).

ACKNOWLEDGMENT

This work was supported by the Natural Sciences andEngineering Research Council of Canada, Western EconomicDiversification Canada, and CMC Microsystems.

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104 105 106 107 108 109 1010−0.5

0

0.5

Frequency (Hz)

Re{

Kcm

}

σmem

=3e−6

σmem

=3e−5

σmem

=3e−4

104 105 106 107 108 109 1010−0.5

0

0.5

Frequency (Hz)

Re{

Kcm

}

σcyt

=0.32

σcyt

=0.25

σcyt

=0.16

104 105 106 107 108 109 1010−0.5

0

0.5

Frequency (Hz)

Re{

Kcm

}

εr−cyt

=60

εr−cyt

=70

εr−cyt

=78

Fig. 3: Re {KCM} vs. frequency for three values of (a)membrane conductivity, (b) cytoplasm conductivity and (c)cytoplasm permittivity. Solid graphs are Re {KCM} for atypical CHO cell (parameters in Table I) in a medium withconductivity of 0.17 S/m.

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[13] K. Asami et al., “Dielectric properties of mouse lymphocytes and ery-throcytes,” Biochimica et Biophysica Acta - Molecular Cell Research,vol. 1010, no. 1, pp. 49–55, 1989.