The mosaic electrical characteristics of the skin

The goals of this study are: 1) to characterize the structure of the skin by measuring impedance variations with a suction microelectrode; 2) to correlate the observed impedance variations with dc current pathways through the skin; 3) to characterize the breakdown phenomenon. We constructed a suction microelectrode with a 200-microns internal diameter and performed several tests on two male subjects. Skin impedance measured from different locations on the forearm and palm varied considerably. We found that the average skin impedance on the forearm was larger than the average impedance on the palm and that the ratio between the maximal and minimal skin impedance is larger for the forearm than for the palm. For both the forearm and the palm the magnitude and variance of skin impedance decrease with increasing stimulus frequency. The density of low impedance points observed on the forearm and palm are consistent with the density of dc current pathways through the skin as indicated by traces left on 1-cm2 Ag electrodes when we passed dc current through the skin. The ratio between the highest and lowest impedances decreased as temperature decreased--at low temperatures the skin displayed mostly high impedances. We were not able to break down the skin using the suction microelectrode. The tests with dry and wet electrodes suggest that breakdown is of thermal nature, and that the thermal capacitance of the saline in the suction microelectrode prevents the temperature of the underlying skin from increasing very rapidly. In conjunction with the larger impedance values, this would tend to increase the breakdown voltage.