Tetrapolar Electrode Geometries for Skin Phantoms Using Electrical Impedance Spectroscopy

This work presents the fabrication and electrical characterization of multilayer gelatin-based phantoms designed to reproduce the passive electrical properties of the epidermis, dermis, and hypodermis, and to evaluate the sensitivity of tetrapolar electrode geometry to subsurface impedance changes. A temperature-resistivity calibration was first performed, comparing agar- and gelatin-based formulations; agar exhibited excessive intrinsic conductivity and a limited resistivity range, whereas gelatin provided both adequate mechanical stability and the target resistivity values needed for multilayer fabrication. Using an empirical regression relating temperature, NaCl concentration, and conductivity, three phantoms (P1, P2, P3) were built with identical epidermis and hypodermis resistivities but distinct dermis resistivities, allowing isolated evaluation of subsurface sensitivity. Electrical Impedance Spectroscopy (200~Hz--100~kHz) was performed on each phantom using five tetrapolar electrode configurations (three linear, two coplanar), and the resulting spectra were analyzed with Relative Difference (RD) and Bland--Altman statistics. Coplanar arrangements showed moderate, stable sensitivity to dermis variations, with impedance differences that gradually converged at higher frequencies. Among the linear configurations, Linear Arrangement 1 was the most sensitive, with relative differences exceeding 100\% at low frequencies for the phantom with the greatest resistivity change, while Linear Arrangements 2 and 3 produced smaller but more consistent responses across the spectrum. Bland--Altman analysis confirmed that the observed differences were systematic rather than random, verifying that electrode geometry governs current distribution and sensing depth. The results establish a reproducible, low-cost experimental platform for optimizing electrode configurations in non-invasive EIS-based skin characterization.