WATER DIFFUSIVITY IN PORCINE STRATUM-CORNEUM MEASURED BY A THERMAL GRAVIMETRIC ANALYSIS TECHNIQUE

Water is a natural constituent of the stratum corneum (sc) affecting i ts plasticity and modulating its barrier function. Diffusion coefficie nts (D) were calculated by measuring the desorption rates of water fro m-porcine sc and dermis samples by a thermal gravimetric analysis (TGA ) technique at isothermal conditions in the range 30-80 degrees C. Wat er-loaded samples were exposed to a flow of dry air, and the change of weight and of temperature were monitored with time. Distinct abrupt d ecreases 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 t he water desorption process. Desorption curves were plotted against ti me and optimized. Values of D were estimated from the ratio of the eva porated water to the initial sc water content, as a function of the sq uare root of time. The ''initial slope'' and the t(1/2) (time for whic h M(t)/M(o) = 0.5, where M(o) is the equilibrium amount of water absor bed in the membrane and Mt is the amount of water released by the memb rane in a time t) methods gave similar results. The water D values of sc at 30 degrees C calculated by the two methods were 3.3 +/- 0.6 X 10 (-10) and 2.7 +/- 0.8 X 10(-10) cm(2)/s, respectively. These values we re about two orders of magnitude lower than the calculated D value for water in the dermis. Arrhenius plots of the D values showed a break a t the 60-70 degrees C region, with a good fit to a straight line in th e 30-60 degrees C region. The energies of activation for water diffusi vity in the sc were 12.9 and 13.9 kcal/mol as calculated by the slope and t(1/2) methods, respectively. The TGA technique and the related ma thematical analysis are suggested as a simple tool to explore chemical mass transport in the sc and in other relevant layers of the skin.