An electrical analog with a single time-constant, but with one frequency-dependent resistance, has been devised to account for the variation with frequency of the skin impedance vector. The dielectric behavior of biological systems is traditionally simulated by a distribution of time constants for simple R-C units. Unlike the electrical model, a mathematical distribution of time constants cannot provide a one-to-one correspondence between psychological measures and physiological units of the epidermal layer.
Computer on-line measurements of the skin impedance were made in the frequency range lO-1OOO Hz. The locus of the impedance vector described a semicircular arc plot with a depressed center in the Argand diagram. A simple parallel-to-series transformation of the proposed electrical model yields the same behavioral pattern as the experimentally observed one, when the equivalent series reactance is plotted as a function of the corresponding resistance.
The model permits quantitative evaluation of the skin membrane parallel resistance, capacitance, phase-angle and relaxation-time. In addition, the electrical resistance of the extracellular fluid as well as that of an unusual biological resistor, which changes value inversely with frequency, were evaluated.