A NONLINEAR FINITE-ELEMENT MODEL OF THE ELECTRODE-ELECTROLYTE-SKIN SYSTEM

This study presents a two dimensional finite element model of the elec trode-electrolyte-skin system which takes into account the nonlinear b ehavior of the skin with respect to the amplitude of the voltage. The nonlinear modeling approach has practical value for studies related to transcutaneous stimulation (e.g. maximizing the dynamic range of sens ory substitution systems, optimization of TENS, optimization of transc utaneous cardiac pacing, etc.). The model has three main regions: 1) t he electrolyte; 2) the skin; and 3) the body. The model consists of 36 4 nodes, 690 elements and was generated on a Macintosh II using a vers ion of FEHT (Finite Element for Heat Transfer) adapted for electromagn etics. The electrodes are equipotential lines and the electrolyte is m odeled as a pure resistive region with constant conductivity. Although the electrode-electrolyte interface can introduce nonlinearities, we did not take them into account because the skin displays a much higher impedance. The skin is modeled as a nonlinear material with the condu ctivity dependent on the applied voltage. To account for the mosaic st ructure of the skin, we used ten different nonlinear subregions of fiv e different values of breakdown voltage. The region designated ''body' ' models the effects of the resistance associated with the dermis and the tissues underneath the skin, and has a constant high conductivity. We studied the effects of two different electrolytes on the comfort o f stimulation and found that there was less potential pain delivered w hen high-resistivity electrolytes were used. This was due to the large r nonuniformities in the current density distribution which appeared f or low-resistivity electrolytes. Moreover, increasing the skin tempera ture made the current density even more nonuniformly distributed for l ow-resistivity electrolytes. Experiments performed on the skin of the left arm, using 1-cm(2) Ag-AgCI electrodes, showed that the skin broke down at spots of lowest breakdown voltage. This is consistent with re ports of previous experimental studies and has practical value for the design of optimal electrodes.