Hydration of stratum corneum

Abstract

SynopsisA model for the hydration behavior of human stratum corneum has been developed from measurements on in vitro samples isolated in a manner which minimized tissue treatment and trauma. Water sorpt~ion/desorption rate meslsurements as a function of water activity, temperature, and tissue integrity are reported. These data, together with thermodynamic data (infrared and nmr results reported earlier) are consistent with a model in which rapidly sorbed/desorbed water (ca. 0.5 mg water/mg stratum corneum) is associated with ("bound" by) the tissue, while slowly sorbed/desorbed "free" water (up to 12 mg water/mg stratum corneum) is kinetically restricted and probably intracellular in location. Both equilibrium water binding and desorption kinetic data suggest structural changes of this cellular water barrier upon hydration. Evidence for hysteresis in water sorption isotherms (reported by others) could not be reproduced.

Sorption Properties of the Human Stratum Corneum

“…The isotherm shapes illustrated in figure 4 correspond well with others found in earlier sorption studies [2,17,18] . From the figure we noted that our measurements showed very small, if any at all, amounts of hysteresis, being of a size comparable to the uncertainties between the different samples, and may therefore be due to the relatively large inter-individual spread with respect to skin sorption properties.…”

“…From the figure we noted that our measurements showed very small, if any at all, amounts of hysteresis, being of a size comparable to the uncertainties between the different samples, and may therefore be due to the relatively large inter-individual spread with respect to skin sorption properties. Hysteresis is reported, however, much more substantial, in porcine skin [11] , but not found in human SC from the hip [18] . Some hysteresis has been reported to be found in horn keratin [19] , and Hey et al [20] mentioned a low-pressure SC hysteresis.…”

“…The underlying mechanism for this peculiar difference is not clear other than that bound water prefers to remain bound. However, the increasing time constant with decreasing RH was as expected since the diffusion coefficient usually decreases with decreasing water content [15,16,18] . Thus, free and more mobile water will evaporate first and eventually bound water, being more strongly attracted, will desorb.…”

“…giving higher time constants, although bulk water in itself is expected to yield a more rapid diffusion. This part of the RH domain is within the swelling area which has been reported to start at 22-33% wt [17,18,24,26] .…”

“…Our results are similar to those found by Martinsen et al [10] , but in this study the SC samples were allowed sufficient time to reach the equilibrium states after the initial calibration at 0% RH, and a full sorption study was performed, including also desorption data. The observed triphasic behaviour may be related to the 3 different types of water present in the SC (strongly bound, bound and bulk) [18,[22][23][24] and to their corresponding sorption mechanisms. Silva et al [25] reported on 3 regimes for isolated corneocytes where the water contents were !…”