Analysis of a concentric coplanar capacitor for epidermal hydration sensing

Without conformal contact between hydration sensors and skin, measured signals (either capacitance or impedance) can be susceptible to artifacts associated with motion-induced changes and irreproducibility in contact. Devices with mechanical properties matched to the skin enable conformal contact and adhesion via the action of van der Waals forces alone, thereby facilitating accurate, reproducible measurements. Theoretical investigations of concentric coplanar capacitors in this type of 'epidermal' format yield analytic expressions of the impedance for single- and double-layer capacitors. The calculated dependence of impedance on material and geometric parameters agree well with finite element analysis and experiments, thereby providing important insights into the design of epidermal systems for hydration sensing.