Hydration-stable PVA-based skin phantom for wearable biopotential sensor evaluation

The development of biomimetic skin phantoms is critical for reliable biopotential measurements in wearable sensor applications. A major limitation is hydration instability, as moisture loss alters ionic conductivity and leads to unpredictable impedance behavior. Building on prior work using polyvinyl alcohol (PVA) cryogels as benchtop analogs for electrode evaluation, this study investigates hydrophilic additive integration and freeze-thaw processing as complementary strategies to improve hydration retention and electrical stability. Seven commercial hydrophilic additives were screened for their ability to reduce mass loss and stabilize impedance over time. Aloe Vera proved most effective, reducing mass loss and impedance variability and extending phantom electrical lifespan from approximately 4 days for untreated controls to up to 18 days under optimized conditions. Embedded additive testing identified an optimal concentration of approximately 0.75 g (5.5--6.5% w/w), while higher concentrations produced oversaturation and electrical drift. Freeze-thaw optimization demonstrated that a single cycle preserved structural integrity and minimized dehydration, whereas repeated cycling (≥ 2 cycles) increased mass loss, permittivity decay, and electrical instability. Differential scanning calorimetry quantified the distribution of freezable and bound water within the cryogel matrix. Samples containing the additive exhibited markedly reduced endothermic peak magnitudes during melting, indicating a lower fraction of freezable water and a corresponding increase in tightly bound water. These results establish a combined chemical-physical framework that extends phantom functional lifespan by more than fourfold and provides long-term hydration stability and consistent impedance behavior suitable for repeatable wearable biosensor testing.