Hydration measurements of the stratum corneum: comparison between the capacitance method (digital version of the Corneometer CM 825®) and the impedance method (Skicon-200EX®)

Background Measurement of stratum corneum (SC) hydration often involves the use of commercial instruments. The aim of this study was to compare and validate two recent instruments: the Corneometer 825 (R) (digital probe) and the Skicon-200 EX (R). Methods In vitro calibration was carried out on filter pads using different solvents, measurements over different layers of plastic foils, and evaluation of desorption kinetics. In vivo measurements were carried out on skin sites covering a range of very dry to well-hydrated skin areas. Results Conductance measurements are influenced by electrolytes while capacitance measurements are not. Dielectric constant of the solvents influences the values of both instruments (r respectively 0.92 and 0.99). The capacitance method carries information from deeper layers (up to 45 mu m) compared with the conductance instrument (up to 15 mu m). Desorption experiments show a strong relation between the amount of water and respectively the capacitance and the conductance values. The in vivo experiments revealed a strong relation between the two methods (r = 0.97). Sensitivity of the capacitance method is limited for the highest hydration values. Conclusions Both instruments allow a certain calibration and both give good estimates of the SC hydration.