The water in stratum corneum is investigated to identify the landmark water contents and characterize it as tightly bound water, loosely bound water, and bulk water. A model is proposed to describe hydrated stratum corneum to characterize the process of adsorption at low water contents by an activity coefficient ( \gamma ) and the process of absorption at higher water contents by the average equivalent molecular weight MW of the binding sites in stratum corneum, which mix with the water according to Raoult's law. A microwave probe system is introduced in which either a microetched surface probe sensitive to surface hydration or a macroscopic depth probe sensitive to deeper hydration may be used. Calibration with in vitro stratum corneum samples demonstrates the range of hydration where the probe response is both sensitive and linear. The probe response is discussed in terms of a proposed parameter, the dielectrochemical potential. Comparison of both the chemical activity of water and the microwave probe response with the mole fraction of water content shows that these two parameters do not behave the same. A protocol for in vivo studies involving a measurement with a 6 µ m Teflon film over the skin, allows application of the in vitro calibration to in vivo probe response data and yields the surface water content. The coefficients of variation for intra-site, inter-site, and inter-subject measurements are evaluated. Pilot clinical studies investigate several issues concerning the hydration of stratum corneum: (1) the short-term and long-term hydration due to exposure to water, (2) the water concentration gradient across the stratum corneum in vivo, (3) characterization of the healing process in damaged skin, (4) evaluation of moisturization by topical applications, (5) characterization of psoriatic lesions, and (6) the relationship between skin friction and surface water content.
S. L. Jacques, "Water content and concentration profile in human stratum corneum," University of California at Berkeley (1984).