WATER LOSS OF STRATUM CORNEUM 245 13 12 ll l0 1 o lO 20 30 40 50 60 70 80 90 lOO Percent Relative Humidity Figure 3. Elastic modulus of stratum corneum as a function of relative humidity (vertical bars refer to average deviation) the RH increases. The average deviations noted in these experiments are quite high. In fact, the large deviation at 15% RH suggests that relatively dry stratum corneum cannot respond as a homogeneous material. Goodman and Wolf (15) also have noted large standard deviations of in vivo water trans- mission data at ]ow RH's. At higher RH's, the higher moisture content tends to make the stratum corneum act in a more homogeneous way, thus lowering the average deviation. The relationship between modulus and RH has been presented in linear form, although Parks and Baddiel (16) have indicated that this relationship is nonlinear above 60% RH. It is noted in passing that different specimens of stratum corneum yield "lines" having different slopes.

246 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 1.0 0.9 0.8 0.3 0.2 1 2 3 Log t (sec.) Figure 4. Stress relaxation o• stratum eorneum (log plot o• AxE• rs. t). Values o• AxEr at any time are divided by AxEr at time zero ß = 0% RH ,• 76% BH } = 31% RH X= 100% BH The stress relaxation experiments were conducted on samples of stratum comeurn in humidified air. In preliminary experiments, it was found that for strains between 0.01 and 0.10 the relaxation spectra were essentially identical. Therefore, the samples were stressed to a constant stress (4.0 g) rather than a constant strain, since stress is instrumentally more easily controlled. At the start it was necessary to demonstrate that the slopes of the lines re- sulting from a plot of log AxE• vs. log t depend on the humidity of the air. Figure 4 shows that the plots are linear and that their slopes are functions of humidity. It is possible to estimate the errors of the slopes by comparing the slopes resuIting from stress relaxation experiments on adlacent pieces of stra- tum corneum at the same RH. These errors are compared to those obtained by