1. THEORY

Dielectrophoresis is defined as the lateral motion imparted on uncharged particles as a result of polarization induced by non-uniform electric fields.

The electric fields shown in Figure 2 of the Introduction to the Basic Science are generated using parallel plate electrodes, so that in the absence of the particles the fields would be uniform (neglecting fringing effects at the ends of the plates). Because of the symmetry of the system no net electrical force is exerted on the particle. This is not the case when, as shown in Figure 1 below, particles are subjected to a non-uniform field.

Figure 1 Two different particles in a non-uniform electric field. The particle on the left is more polarisable than the surrounding medium and is attracted towards the strong field at the pin electrode, whilst the particle of low polarisability on the right is directed away from the strong field region.

The total electric force acting on a particle of net charge in a non-uniform field E is:

(1)

where is the Del vector operator. If the particle is uncharged (i.e. Q = 0) or for frequencies above around 1 kHz where electrophoretic effects are negligible, the term on the right-hand side of equation (1) involving the dipole moment and field gradient will dominate, so that (using well known theorems of vector calculus) the time-averaged force is given by:

(2)

where Re denotes that the real component of the dipole moment is involved. To distinguish this from electrophoresis, Pohl [1],[2] adopted the term dielectrophoresis.

As described in Introduction to the Basic Science the magnitude of the dipole moment m arising from the induced electrical polarization is given by:

(3)

From equations (2) and (3) the dielectrophoretic force depends on the particle size (and shape [3]) and on the magnitude and degree of non-uniformity of the applied electric field. Furthermore, the polarity of this force depends on the polarity of the induced dipole moment, which in turn is determined by the conductivity and permittivities of the particle and its suspending medium.

For example, bacteria are often classified according to the results of the Gram-staining procedure. Gram-positive bacteria typically have cell walls composed of open networks of teichoic acid in a peptidoglycan matrix, whereas Gram-negative bacteria have a more complicated cell wall structure which includes lipids and proteins. Thus, by and large (but not in every case) the outer walls of Gram-positive bacteria are more conducting than Gram-negative bacteria, and this allows for their mixtures to be separated by dielectrophoresis [4]. Another simple situation arises for mixtures of viable and non-viable cells. Non-viable cells often have membranes that are degraded to the extent that ions can readily diffuse across them, unlike membranes for viable cells whose resistance to non-specific ion diffusion is very large. This again leads to readily achievable dielectrophoretic separation protocols [5]. In all of these protocols the design and geometry of the micro-electrodes used to generate the non-uniform fields is an important factor.

An important finding has been that particles collected near electrodes under the action of negative dielectrophoresis are less immobilized than those drawn to the electrode edges by positive dielectrophoresis [6,7]. Selective separation can thus be achieved by applying an additional force such as gravity or fluid flow. This capability of selective particle manipulation offers the opportunity for a wide range of possible biomedical and biotechnological applications [3], some of which we have explored in collaboration with Drs. Frederick F Becker and Peter Gascoyne at the University of Texas M D Anderson Cancer Center, Houston [8-10], and with Prof Alan Burnett and Dr. Ken Mills, Department of Haematology, University of Wales College of Medicine, Cardiff [11].

The manipulation of cells and micro-organisms can readily be observed and photographed using conventional microscopes, but much smaller particles such as viruses, proteins and DNA can also be manipulated using dielectrophoretic forces and monitored by fluorescent labeling of the particles. The electric field manipulation of such small particles is facilitated by using electrode structures of much smaller scale than those we have employed to date, and to this end collaborative work has commenced with the group led by Professor Chris Wilkinson, Department of Electronics, University of Glasgow, who have expertise in fabricating sub-micron scale electrode structures for studies on cells and biomaterials.

2. REFERENCES

1. Pohl, H. A. (1958) Some Effects of Nonuniform Fields on Dielectrics, J. Appl. Phys. 29, 1182-1188
2. Pohl, H. A. (1978) Dielectrophoresis, Cambridge University Press, Cambridge
3. Pethig, R. (1991) Application of A.C. electrical fields to the manipulation and characterization of cells, In Automation in Biotechnology, (ed. I. Karube) Elsevier, 159-185
4. G H Markx, Y Huang, X-F Zhou and R Pethig, Dielectrophoretic characterization and separation of micro-organisms, Microbiology, 140, 585-591 (1994)
5. G H Markx, M S Talary and R Pethig, Separation of viable and non-viable yeast using dielectrophoresis, J. Biotechnology, 32, 29-37 (1994).
6. X-B Wang, Y Huang, J P H Burt, G H Markx and R Pethig, Selective Dielectrophoretic Confinement of Bioparticles in Potential Energy Wells, J.Phys.D: Appl.Phys. 26, 1278-1285 (1993)
7. G. H. Markx and R. Pethig, Dielectrophoretic Separation of Cells: Continuous Separation. Biotechnol. Bioeng. 45, 337-343 (1995).
8. P R C Gascoyne, J P H Burt F F Becker and R Pethig, Membrane changes accompanying the induced differentiation of Friend erythroleukemia cells studied by dielectrophoresis, Biochim.Biophys.Acta 1149, 119-126 (1993).
9. F F Becker, X-B Wang, Y Huang, J Vykoukal, P R C Gascoyne and R Pethig, The Removal of Human Leukaemia Cells from Blood Using Interdigitated Microelectrodes. J. Phys.D.: Appl. Phys. 27, 2659-2662 (1994).
10. F F Becker, X-B Wang, Y Huang, J Vykoukal, P R C Gascoyne and R Pethig, Separation of Human Breast Cancer Cells from Blood by Differential Dielectric Affinity. Proc. Nat. Acad. Sci. (USA) 92, 860-864 (1995).
11. M S Talary, K I Mills, T Hoy, A K Burnett and R Pethig, Dielectrophoretic Separation and Enrichment of CD34 Cell Subpopulation from Bone Marrow and Peripheral Blood Stem Cells. Med. & Biol. Eng. & Comp. 33, 235-237 (1995).