Ion mobility across human stratum corneum in vivo.

The aim of this study was to develop methods to determine ionic transport parameters, in particular ionic mobilities across human stratum corneum (SC) in vivo. It has been shown previously that the SC, a structurally heterogeneous biomembrane, behaves as a homogeneous barrier to water transport; that is, water diffusivity does not vary as a function of position within the SC; in this work, therefore, the question posed was whether ion motion behaved similarly. Low-frequency impedance measurements (1.61 Hz) reported on the decrease of SC impedance as the barrier was progressively removed by serial adhesive tape stripping. This corresponded to an increase in ion mobility of approximately 2 orders of magnitude across the SC (from the external surface to the interior). Therefore, an algorithm was developed from the absolute impedance data to calculate ion mobility as a function of position within (i.e., depth into) the SC. The mobilities deduced from the algorithm correlated well with water permeability across the SC. The data presented here are thought to be the first measurements of ionic mobility across human skin in vivo.