Suspensions of yeast cells (Saccharomyces cerevisiae) were used as model systems to investigate the electrokinetic behaviour of colloidal particles subjected to travelling electric fields generated using microelectrodes. Measurements were made over the frequency range 1 kHz to 10 MHz and for suspending medium conductivities in the range 6-260 mS m-1. A theoretical model is developed to provide a good description of the dependence of the observed translational motion, termed travelling-wave dielectrophoresis (TWD), on the dielectric properties of the particle and suspending medium, on the size of the particle, and on the magnitude and frequency of the applied field. Unlike conventional dielectrophoresis, the TWD effect is found to be related to the imaginary, rather than to the real, component of the induced dipole moment. Dielectrophoresis and electrorotation measurements were made to provide a further understanding of the observed effects and to support the theoretical model.

A multi-layer micro-electrode structure has been developed for the selective manipulation and separation of bioparticles using travelling field dielectrophoresis effects. An important feature is that, in the separation process, the selected particles move in a stationary supporting fluid. Stationary suspensions of viable and non-viable yeast cells were used as a model system to demonstrate the general application of this device for the selective retention or transport of bioparticles in suspended mixtures. The efficiency of this process depends on the dielectric properties of the particles and their suspending medium, and is a sensitive function of the frequency of the travelling field. Apart from their use as particle separators, such micro-electrode devices are also envisaged to form integral components in the development of `biofactory on a chip' technology.

Multilayer microelectrode structures, with 10 µm feature sizes, have been fabricated using excimer laser ablation techniques. These structures will be incorporated into devices for the electro-manipulation and characterization of cells, microorganisms and other particles. The AC electrokinetic phenomena of dielectrophoresis, electrorotation and travelling electric field effects are utilized, all of which are dependent on the dielectric properties of the bioparticles. Examples are presented of a travelling wave junction and selective particle trap to be incorporated into a prototype biofactory-on-a-chip device. A technique for profiling the edges of via-holes has been developed to facilitate robust electrical connections through polyimide films in these multilayer devices.

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