Dielectrophoresis: An assessment of its potential to aid the research and practice of drug discovery and delivery

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Abstract

Dielectrophoresis (DEP) is an electrokinetic technique with proven ability to discriminate and selectively manipulate cells based on their phenotype and physiological state, without the need for biological tags and markers. The DEP response of a cell is predominantly determined by the physico-chemical properties of the plasma membrane, subtle changes of which can be detected from two so-called ‘cross-over’ frequencies, fxo1 and fxo2. Membrane capacitance and structural changes can be monitored by measurement of fxo1 at sub-megahertz frequencies, and current indications suggest that fxo2, located above 100 MHz, is sensitive to changes of trans-membrane ion fluxes. DEP lends itself to integration in microfluidic devices and can also operate at the nanoscale to manipulate nanoparticles. Apart from measurements of fxo1 and fxo2, other examples where DEP could contribute to drug discovery and delivery include its ability to: enrich stem cells according to their differentiation potential, and to engineer artificial cell structures and nano-structures.

Introduction

The word dielectrophoresis (DEP) was first adopted by Herbert Pohl [1], [2] to imply, from the Greek phorein, an effect where a particle is carried as a result of its intrinsic dielectric properties. It is specifically used to describe the motion of particles when they are exposed to an electric field gradient. DEP differs significantly from electrophoresis because the particle need not carry a net electric charge for motion to be induced, and alternating current (AC) as well as direct current (DC) electrical signals can be employed to impose an electric field on a particle.

A summary is given in Table 1 of the forces and methods currently used to manipulate and separate cells in aqueous suspension. The content of this table is restricted to methods that can be scaled down to operate in lab-on-a-chip devices, and so the common technique of centrifugation to separate cells on the basis of size and density is not included. Furthermore, the term manipulate is used here to describe the relocation of a cell with respect to its position within a fluidic device or to that of neighbouring cells. Cell separation implies the physical isolation, either within or outside of a device, of target cells from a mixture of cell types or from cells exhibiting different physiological states. Although Table 1 is not intended to be comprehensive, sufficient details and cited references are presented to place DEP into context. An important aspect to note is that DEP is a label-free technique. Fluorescence-activated cell sorting (FACS) and magnetic-activated cell sorting (MACS) require the use of cell-specific labels. In the case of FACS this requires fluorophore-conjugated antibodies, whilst magnetic beads conjugated with antibodies are used in MACS. Both procedures can be costly in terms of reagents and operation, and are restricted to where cell-specific antibodies are available. Many of the other methods in Table 1 rely on differences in cell size and density for their specificity. This on its own may not lead to efficient separations. For example, relatively high purity of separation of erythrocytes (diameter 6–9 μm; density 1.09–1.10 g/cm3) from leukocytes (diameter 7–9.5 μm; density 1.055–1.085 g/cm3) may be achievable, but separation of the leukocytes into pure fractions of neutrophils, eosinophils and basophils may not.

As indicated in Table 1, the DEP response of a cell is determined by its intrinsic dielectric property. It can therefore be used as a label-free technique to discriminate, enrich or manipulate target cells in heterogeneous cell populations. Demonstrated examples of this include: enrichment of haematopoietic stem cells from bone marrow or blood [27], [28]; mesenchymal stem cells from adipose tissue [29]; and cancer cells from blood [30], [31], [32]. In their recent review of advances made in label-free approaches to the isolation of circulating tumour cells, Cima et al. [33] make the general comment that mechanical-based techniques (filtration and inertial separation) can provide high-throughput, whilst electrical methods (electrical impedance [34] and DEP) can offer high purification. The neurogenic potential of human neural stem/progenitor cell populations can be predicted by their DEP properties [35], [36], and discrimination can be made between stages of differentiation of myoblasts [37], human embryonic stem cells [38] and cancer cell development [39]. DEP has also been used to monitor changes in cell states associated with activation and clonal expansion, apoptosis, necrosis and responses to chemical and physical agents [40], [41], [42]. Importantly, it is found that the AC fields typically used in DEP manipulation and sorting of cells do not damage the cells (e.g., [28], [43], [44], [45]). All of this suggests that DEP can play an important role in cell-based drug discovery. The artificial engineering of different sizes and shapes of cellular aggregates has also been accomplished using DEP forces [44], [45], [46], [47]. Although it is common in the laboratory to study isolated cells, this is an artificial situation. The ability, using DEP, to produce co-cultures and cellular communities could aid studies of the extent to which cell–cell interactions and intercellular signalling enhance drug resistance, and also lead to applications directed towards the production of scaffolds to guide tissue development and controlled drug release. DEP can also be applied at the nanoscale for the manipulation and characterization of nanoparticles [48], [49], [50], [51], [52], [53], [54], [55], [56], [57], and this is of relevance to research directed towards developing nanocapsules and other nano-structures for drug delivery.

Several hundreds of reports describing new developments and applications of DEP are published each year, and recent reviews of this literature are available [58], [59], [60]. Most of these publications are written by and for researchers having biophysical or engineering backgrounds, and so may not readily address the ‘so what, who cares?’ questions raised by some interested researchers with biomedical backgrounds. The intention in this present article is not only to maintain accuracy in terms of the theory and technical aspects of DEP, but also to present this information in as descriptive a manner as possible and to emphasise biological applications of relevance to drug discovery and delivery.

Section snippets

The basic principles of dielectrophoresis

In Fig. 1 an illustrative example is given of how DEP might be applied for testing the effectiveness of a drug designed to induce apoptosis of cancer cells. Although this is an imagined experiment, the effects described will be consistent with DEP results obtained for HL-60 and Jurkat cells undergoing induced apoptosis [40], [41], [42], [43]. The system in Fig. 1 consists of microelectrodes fabricated into the bottom of two fluidic chambers that contain equal number concentrations of model

Dielectric properties of cells

By definition the DEP behaviour of a cell reflects its dielectric, or in broader terms its electrical, properties. The electrical properties of tissues and cells have been studied for a long time, and the purpose of the following brief review of this huge body of work is to establish two important facts, namely that:

  • The dielectro part of the term dielectrophoresis rests on very solid and well established ground.

  • The most important contributions to the dielectric properties of a cell are

Modelling of the dielectric and DEP characteristics of cells

The dielectric characteristics of a mammalian cell can be electrically modelled using the equivalent circuit shown in Fig. 5. The outer membrane is represented as a parallel combination of the plasma membrane resistance and capacitance. With increasing frequency the AC resistance (known as reactance) of the capacitor element Cmem decreases (according to the inverse relationship 1/ω) so that the membrane capacitance increasingly acts to short-circuit the large resistance to passive current

Drug discovery

Under the correct experimental conditions the DEP response of a cell is characterised by two ‘cross-over’ frequencies, fxo1 and fxo2, and measurement of which provides a method to monitor even quite subtle changes of the physiological state of the cell. The most important cell parameters controlling fxo1 are the size and shape of the cell, and in particular any changes which may occur in the extent of membrane folding or microvillus content of the plasma membrane. The value for fxo1 is also

Summary

DEP is an electrokinetic technique capable of selectively sorting cells and monitoring their physiological state, without the need for biomarkers such as fluorescent labels or antibody-coated magnetic beads. The AC electrical fields and protocols typically used in DEP experiments are not known to damage the cells. The DEP response of a cell is characterised by two ‘cross-over’ frequencies, fxo1 and fxo2, measurement of which provides a method to monitor even quite subtle changes of the

Acknowledgement

The author thanks Professor Christopher Gregory for helpful discussions.

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