A comparative evaluation of equivalent circuit and finite element electrical skin modelling techniques

Abstract

Mathematical models are essential to our understanding of the electrical properties of the skin. In this paper, two types of simulation model, an equivalent circuit and a finite element simulation were investigated and compared to evaluate their accuracy. Impedance spectra were measured, between 100 Hz and 50 MHz, (the limits of the available spectrum analyser) of a pair of electrodes placed on skin and these spectra used to find the parameters of a standard equivalent circuit model. The resulting indicated that the components of the equivalent circuit may represent different parts of the skin physiology that indicated by the literature. A simulation model was constructed in COMSOL, with the dimensions, permittivity and conductivity of each skin layer taken from across the published literature. This model was tested for sensitivity to the thicknesses of tissue layers as well as the shape of the boundary between layers. It was found that changing the layer thicknesses only had a significant effect for the stratum corneum and dermis, and that changing the shape of the boundary between layers created an impedance change of up to two times at certain higher frequencies (>1 kHz). While the impedance curves generated by the two models had the same overall profile, there was a difference of up to 100 times in their DC impedance values. This indicated that the broad understanding of how electrical signals of different frequencies pass through the skin is correct, but that significant insufficiencies exist in the published properties of the skin layers, particularly the stratum corneum and that finding more accurate values for these properties is necessary for the development of better models.