Because of the increasing use of dieleetrophoresis in the dielectric characterization and sorting of living cells or their parts, it has become important to establish carefully the theoretical backgrounds for this effect. A comparison with experiment is made of the several versions of the theory for the dielectrophoretic force exerted by nonuniform electric fields upon a neutral object. The three fundamental approaches: the Maxwe11~ tensor) the effective dipole moment, and the 'Helmholtz' energy approach are presented along with the general solution given earlier by Pohl and Crane. These are found to agree closely with experiment in predicting the dieleetrophoretic force upon various rods hung in specially shaped (isomotive) field distributions. On the other hand, an alternative formulation based upon a debatable assignment of fields local to the dipoles gave a good fit to the experimental data only for materials of very low permittivity, and fitted poorly in the ease of highly polarizable materials.An improved derivation of the theory for stable dielectrophoretie levitation is also presented. This phenomenon is of particular interest in that it is based upon an apparent violation of the Earnshaw's theorem, and is useful in the study of the dielectric properties of individual living cells.

The continuous dielectrophoresis of living cells is described.The technique uses stream-centered transport of suspended microorganisms through an especially shaped nonuniform electric field.The cells can be given a positive or negative displacement, i.e., can be pushed into or out of the region of higher field intensity, depending upon the frequency of the applied ac field, and upon the relative permittivities of the cells and the suspending medium.Yeast (Saoo~ron~oe8 oerevisia~) and algal cells (Chlorella vulgaris) were found to provide spectra of dielectrophoretic responses varying with the applied frequency (i0 to 600 kHz) and conductivity. Pohl et al ':~}!L:?'ni{~,~j P~s a~:Z ,~ DEP of Cells. 83 • . , t,. ° .

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Many laboratories studying genetic engineering are using cell fusion to aid their research. The new and highly efficient method of using electrical fields to induce cell-to-cell fusion, or cell-toparticle fusion, or to induce entry of DNA or other compounds into cells opens up rapid ways to accomplish these ends.