A nonlinear finite element model of the electrode-electrolyte-skin system

Presents a 2-dimensional finite element model of the electrode-electrolyte-skin system which takes into account the nonlinear behavior of the skin with respect to the amplitude of the voltage. The nonlinear modeling approach has practical value for studies related to transcutaneous stimulation. The model has 3 main regions: (1) the electrolyte; (2) the skin; and (3) the body. The model consists of 364 nodes, 690 elements and was generated on a MacIntosh II using a version of FEHT (Finite Element for Heat Transfer) adapted for electromagnetics. The electrodes are equipotential lines and the electrolyte is modeled as a pure resistive region with constant conductivity. Although the electrode-electrolyte interface can introduce nonlinearities, the authors did not take them into account because the skin displays a much higher impedance. The skin is modeled as a nonlinear material with the conductivity dependent on the applied voltage. To account for the mosaic structure of the skin, the authors used 10 different nonlinear subregions of 5 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. The authors studied the effects of 2 different electrolytes on the comfort of stimulation and found that there was less potential pain delivered when high-resistivity electrolytes were used. This was due to the larger nonuniformities in the current density distribution which appeared for low-resistivity electrolytes. Moreover, increasing the skin temperature made the current density even more nonuniformly distributed for low-resistivity electrolytes. Experiments performed on the skin of the left arm, using 1-cm ² Ag-AgCl electrodes, showed that the skin broke down at spots of lowest breakdown voltage. This is consistent with reports of previous experimental studies and has practical value for the design of optimal electrodes.<>