The objective of this dissertation is to provide insight into the mechanisms that are responsible for the nonlinearities and asymmetries of the voltage current characteristic of the human skin. Furthermore to provide an explanation for the partially reversible breakdown of the electrical resistance of the skin that results in a rapid decrease of the skin resistance and occurs when the skin is stimulated with a dry electrode. The skin resistance and impedance was investigated with low frequency constant voltage pulses and with sinusoidal stimulation over a range of amplitudes (10 - 20 V) and frequencies (3 - 30 Hz), using a dry 79 mm2 Ag/AgCI electrode. Evidence is presented that electroporation of the lipid bilayer membranes occurs in the dry skin over in the voltage range 10-20 V, a wider range than previously thought; it is further shown that experimental results are predicted by electroporation theory, if it is assumed that a small percentage of the total surface area of the dry skin consists of 15 lipid bilayers in series, rather than the generally accepted estimate of 70-100 layers. By modeling the dry skin as 15 lipid bilayers in series undergoing electroporation, the non-linearity of voltage-current characteristic of the skin is accurately predicted. Evidence is presented in support of a new hypothesis that the asymmetry of the skin's voltage-current characteristic can be attributed to electro-osmosis occurring within the lipid bilayers of the stratum corneum. It is further suggested that the existing mathematical description of electro-osmosis would not accurately describe this situation and equations were introduced to model the effect of electro-osmosis on the voltage-current characteristic of the skin. An electrical model of the skin is presented based on the hypothesis that the VOltage-current characteristic of the skin is due to the combined effect of electroporation and electro-osmosis on the lipid bilayers in the stratum corneum. In addition the model accounts for the effect of trans epidermal water loss.
Dissertation (M Eng (Bio-Engineering))--University of Pretoria, 2006.