A NONLINEAR ELECTRICAL-THERMAL MODEL OF THE SKIN

This work presents a model for the skin which accounts for both the no nlinearities and the asymmetries in its voltage-current characteristic . This model consists of an electrical submodel and a heat transfer su bmodel. The electrical submodel uses nonlinear devices in which some p arameters depend on skin temperature. The heat transfer submodel model s the heat exchange between the skin, the surrounding tissues, and the ambient medium and calculates the temperature of the skin to update t he necessary parameters of the electrical submodel. The model is based on experiments designed to determine: 1) the dry skin voltage-current characteristic; 2) the changes in the skin breakdown voltage with loc ation; 3) the moist skin voltage-current characteristic; 4) the change s in the voltage-current characteristic of the skin with duration afte r the onset of stimulation; and 5) the effect of skin temperature on i ts voltage-current characteristic. During these experiments we used 84 -mm(2) square Ag-AgCl electrodes to apply sinusoidal voltage of 0.2 an d 20 Hz. The simulations were performed using the Advanced Continuous Simulation Language (ACSL), capable of solving differential and integr al equations with variable coefficients. The model predicted the skin behavior satisfactorily for a large range of amplitudes and frequencie s. We found that the breakdown occurred when the energy delivered to t he skin exceeded a threshold. Above this threshold the voltage-current characteristic of the skin became nonlinear and asymmetric and, in a real situation, the subject would experience an uncomfortable sensatio n which could rapidly develop into pain.