A mathematical model for the physical properties of electrodes suitable for use in electric current computed tomography is discussed. The model includes the effects of discretization, shunt, and contact impedance. The complete model was validated by experiment. Bath resistivities of 284.0, 139.7, 62.3, and 29.5 Omega -cm were studied. Values of effective contact impedance used in the numerical approximations were 58.0, 35.0, 15.0, and 7.5 Omega -cm/sup 2/, respectively. Agreement between the calculated and experimentally measured values was excellent throughout the range of bath conductivities studied. It is desirable in electrical impedance imaging systems to model the observed voltages to the same precision as they are measured in order to be able to make the highest-resolution reconstructions of the internal conductivity that the measurement precision allows. The complete electrode model, which includes the effects of discretization of the current pattern, the shunt effect due to the highly conductive electrode material, and the effect of an effective contact impedance, allows calculation of the voltages due to any current pattern applied to a homogeneous resistivity field. >

Electrode models for electric current computed tomography

In vivo measurement of cortical impedance spectrum in monkeys: implications for signal propagation.

There are four criteria that must be considered when choosing material for an implanted electrode: (1) tissue response, (2) allergic response, (3) electrode-tissue impedance, and (4) radiographic visibility. This paper discusses these four criteria and identifies the materials that are the best candidates for such electrodes. For electrodes that make ohmic contact with tissues: gold, platinum, platinum-iridium, tungsten, and tantalum are good candidates. The preferred insulating materials are polyimide and glass. The characteristics of stimulator output circuits and the importance of the bidirectional waveform in relation to electrode decomposition are discussed. The paper concludes with an analysis, the design criteria, and the special properties and materials for capacitive recording and stimulating electrodes.

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Criteria for the selection of materials for implanted electrodes.

Electrode polarization is a major nuisance while determining dielectric properties of cell and particle suspensions and tissues, particularly at low frequencies. Understanding of these interfacial phenomena and appropriate modelling are essential in order to correct for its distortion of the dielectric properties of the sample of interest. I survey the following topics, concentrating on contributions from our laboratory: Linear properties of electrode polarization and relevant models. Effects of electrode polarization on sample impedance. Effects of sample on polarization impedance. Techniques of correction. Extension of linear to nonlinear models Harmonics generated in the nonlinear range.

Linear and nonlinear electrode polarization and biological materials.

The linear and non-linear electrical properties of the electrode-electrolyte interface

The resistive and capacitive properties of stainless-steel electrodes in contact with saline solutions of various concentrations were investigated over a frequency range extending from 20 to 10 kHz by using a variable-length conductivity cell. With a low current density, the series-equivalent resistance and capacitance of a single electrode-electrolyte interface were found to vary almost inversely as the square root of frequency. In the frequency range studied, it was found that the reactance was very nearly equal to the resistance and both varied almost inversely as the square root of the frequency, verifying Warburg's postulate. When current density was increased, resistance decreased and capacitance increased at all frequencies studied; the change in each was most apparent in the low frequency region. As a consequence, it can be stated that the impedance of an electrode-electrolyte interface decreases with increasing current density.

G. Wise

Papers

The impedance of stainless-steel electrodes.