In this review, we present an overview of the state of the art concerning the fundamental properties of electrode polarization (EP) of interest in the measurement of high conductivity samples and its implications for both dielectric (DS) and impedance spectroscopy (IS). Initially a detailed description of what constitutes EP is provided and the problems that it induces. Then, we review some of the more popular models that have been used to describe the physical phenomena behind the formation of the ionic double layer. Following this we shall enumerate the common strategies used historically to correct its influence on the measured signals in DS or in IS. Finally we also review recent attempts to employ fractal electrodes to bypass the effects of EP and to offer some physical explanation as to the limitations of their use.

No abstract

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.

A dielectrophoresis (DEP) cell profiler, with concurrent FACS measurements, was used to monitor the morphological changes of Jurkat T-cells as they progressed through chemically induced apoptosis using etoposide. The cell 'physiometry' profiling technique measures the radius and the so-called DEP crossover frequency f(xo) of individual cells in a suspension, and this information was used to determine the effective plasma membrane capacitance of each cell. Control cells (n=526) exhibited a dynamic spread of f(xo) values, ranging from 50 to 250 kHz, and as apoptosis progressed over 6 h, the upper value for f(xo) progressively increased and extended beyond 500 kHz. This corresponded to a reduction in plasma membrane capacitance from 13.34 (+/-2.88) to 10.49 (+/-4.00) mF/m(2), and reflected a general smoothing of the membrane through loss of microvilli, for example. This is in broad agreement with previously reported studies of HL-60 cells undergoing apoptosis, but the authors' observation of a dynamic spread of f(xo) values does not agree with the earlier report that the f(xo) values for viable and apoptotic cells fall into two separable, relatively narrow, frequency bands. This has implications when devising protocols for the efficient DEP separation of viable, apoptotic and necrotic cells.

No abstract