HOMOGENEOUS TRANSPORT IN A HETEROGENEOUS MEMBRANE - WATER DIFFUSION ACROSS HUMAN STRATUM-CORNEUM IN-VIVO

The objective of this study was to determine whether a structurally he terogeneous biomembrane, human stratum corneum (SC), behaved as a homo geneous barrier to water transport. The question is relevant because t he principal function of the SC in vivo is to provide a barrier to the insensible loss of tissue water across the skin. Impendance spectra ( IS) of the skin and measurements of the rate of transepidermal water l oss (TEWL) were recorded sequentially in vivo in human subjects as lay ers of the Sc were progressively removed by the serial application of adhesive tape strips. The low-frequency (less than or equal to 100 rad s(-1)) impedance of skin was much more significantly affected by tape stripping than the higher frequency values; removal of the outermost SC layer had the largest effect. In contrast, TEWL changed little as t he outer SC layers were stripped off, but increased dramatically when 6-8 mu m of the tissue had been removed. It follows that the two nonin vasive techniques probe SC barrier integrity in somewhat different way s. After SC removal, recovery of barrier function, as assessed by incr easing values of the low-frequency impedance, apparently proceeded fas ter than TEWL decreased to the prestripping control. The variation of TEWL as a function of SC removal behaved in a manner entirely consiste nt with a homogeneous barrier, thereby permitting the apparent SC diff usivity of water to be found, Skin impedance (low frequency) was corre lated with the relative concentration of water within the SC, thus pro viding an in vivo probe for skin hydration. Finally, the SC permeabili ty coefficient to water, as a function of SC thickness, was calculated and correlated with the corresponding values of skin admittance deriv ed from IS.