Porosity-Controlled Skin Phantom for Hydration-Dependent Electrical Evaluation of Wearable Electrodes

Reliable evaluation of wearable biopotential sensors requires synthetic skin phantoms that accurately reproduce the hydration-dependent electrical behavior of human skin. However, existing phantoms often fail to maintain stable hydration or lack tunable electrical properties, limiting their effectiveness for electrode benchmarking. This study introduces a porosity-engineered, two-layer skin phantom in which the upper polydimethylsiloxane (PDMS) layer is laser-patterned to control pore density, enabling predictable modulation of hydration, permittivity, and impedance. Measurements performed on the fully assembled phantom demonstrate that increasing porosity produces a coupled increase in permittivity and decrease in impedance, following well-defined power-law relationships. These trends mirror the dielectric behavior of biological skin across dry to-hydrated states and allow porosity to function as a structural proxy for hydration level. Using this relationship, porosity was mapped to operational hydration states ranging from very dry (0% porosity) to moderately hydrated (10% porosity). Lower-porosity phantoms exhibited improved hydration stability and extended electrical lifespan, whereas higher porosity samples showed accelerated dehydration and reduced mechanical robustness. The phantom's tunable electrical properties enabled direct comparison of commercial dry electrodes, revealing distinct impedance responses among Neurotrode, Biopac Flat, JJE SE13, and JJE SE12 electrodes under controlled hydration conditions. Overall, this work establishes porosity as a quantitative design parameter for replicating hydration-dependent skin impedance and presents a scalable, physiologically relevant platform for standardized evaluation of wearable biopotential electrodes.