Kinetics of Water Sorption and Desorption and Mechanism on Water Homeostasis in the Stratum Corneum.

We focused on the behavior of water in the stratum corneum (SC) at the key water content of about 25 wt%. Isotherm in the SC were observed when the relative humidity (RH) was altered stepwise, and the relaxation time between the step changes was analyzed, where the weight change of the SC was measured by varying the RH by 5 % step. We found that in the sorption/desorption process, almost exponential increase of the water content against the RH appeared, but in the sorption process, a distinct increase in the water content appeared when increasing the water content from around 22-25 wt% to the next by the step increase of the RH, and in the desorption process, a distinct decrease in the water content appeared when decreasing the water content from around 22-25 wt% to the next by the step decrease of the RH. From the analysis of the temporal change of the water content by the step change of the RH, in the desorption process, when the water content was decreased from around 22-25 wt% to the next, the relaxation time of the water content change became significantly long, and furthermore the relaxation time in the water content lower than around 22-25 wt% became approximately twice of that higher than it. We revealed that the former phenomenon is due to the stabilization of the short-period lamellar structure and the latter is due to the removal of the bound water. The above characteristic water content was within the key water content of about 25 wt%, then we newly propose that there is an important mechanism by which the water content of the skin surface is maintained at a homeostatic level of about 25 wt%.