174 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS tioning at 25øC, 50% RH, for 90 min, the weight of the stratum corneum was determined by a Cahn RG Electrobalance and residual water content based on applied humectant weight was calculated. RESULTS AND DISCUSSION WATER Figure 2 shows the time dependence of dynamic elastic modulus for the stratum cor- neum after the application of distilled water. The stratum corneum was plasticized 1.0 0.5 ß ß: 4.0$tl Time (min) Figure 2. Time dependence of dynamic elastic modulus of the stratum corneum after application of distilled water at 25øC, 50% RH. rapidly by water, and then the dynamic elastic modulus gradually recovered concom- itant with water evaporation to finally reach the previous level. The extent of stratum corneum plasticization depended on the quantity of applied water. In the case of a 4 I•l application, the minimum value of the ratio of dynamic elastic modulus was 0.35 and the time to recover to the non-treated level was one and one-half hours. In contrast, in the case of a 0.5 I•l application, the minimum elastic modulus ratio was 0.7 and the recovery time was 30 min. The change of dynamic loss modulus of stratum corneum by the application of distilled water is shown in Figure 3. The loss modulus was about one tenth of the elastic modulus and more unstable, but its behavior was similar to that of the elastic modulus. These results indicate that the change of mechanical properties of the stratum corneum by water is reversible.

EVALUATION OF SKIN SOFTENING 175 1.0 0.5 ß ß o o o o ooo o • -w•.. •, o: o.5,•1 ß: 4.0,•1 I I I I I I I I I I I I I I t •10 I I I I I 0 0 30 60 120 Time (rnin) Figure 3. Time dependence of dynamic loss modulus of the stratum corneum after application of distilled water at 25øC, 50% RH. OILS Figure 4 shows the effect of oils on the softness of the stratum corneum. In addition to mineral oil and jojoba liquid wax, polar oils such as glyceryl diisostearate and trimethyl propane triisostearate were examined. 1.0 mineral oil • 0.5 ß ß ß . i.. - . ß -.. ß ß . . . o . jojoba liquid wax ' ' ' 0 •) 15 3•0 45 0 0 15 0 5 60 Time (min) Time (min) (a) (b) Figure 4. Time dependence of dynamic elastic modulus of the stratum corneum after application of oil at 25øC, 50% RH. (a) mineral oil, (b) jojoba liquid wax.