Water Diffusivity in Porcine Stratum Corneum Measured by a Thermal Gravimetric Analysis Technique

Abstract

Water is a natural constituent of the stratum corneum (sc) affecting its plasticity and modulating its barrier function. Diffusion coefficients(D)were calculated by measuring the desorption rates of water from porcine sc and dermis samples by a thermal gravimetric analysis (TGA) technique at isothermal conditions in the range 30–80 °C. Water-loaded samples were exposed to a flow of dry air, and the change of weight and of temperature were monitored with time. Distinct abrupt decreases in rate of desorption marked three different phases of water in the sc, designated as free, bulk, and bound water. Concomitant with the sharp change in evaporation rate, an increase in temperature was observed, in accordance with the absorption of heat accompanied with the water desorption process. Desorption curves were plotted against time and optimized. Values ofDwere estimated from the ratio of the evaporated water to the initial sc water content, as a function of the square root of time. The “initial slope” and thet1/2(time for whichMt/Mo= 0.5, whereMois the equilibrium amount of water absorbed in the membrane andMtis the amount of water released by the membrane in a time t) methods gave similar results. The waterDvalues of sc at 30 °C calculated by the two methods were 3.3 ± 0.6 × 10−10and 2.7 ± 0.8 × 10−10cm2/s, respectively. These values were about two orders of magnitude lower than the calculatedDvalue for water in the dermis. Arrhenius plots of theDvalues showed a break at the 60–70 °C region, with a good fit to a straight line in the 30–60 °C region. The energies of activation for water diffusivity in the sc were 12.9 and 13.9 kcal/mol as calculated by the slope andt1/2methods, respectively. The TGA technique and the related mathematical analysis are suggested as a simple tool to explore chemical mass transport in the sc and in other relevant layers of the skin.