176 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS From these results it was confirmed that both polar and non-polar oils which are widely used in cosmetic products did not soften the stratum corneum at all as Blank had reported (1). SURFACTANTS Figure 5 shows the effects of aqueous surface active agent solutions (100 mM/1, i.e., above the c.m.c.) on the mechanical properties of the stratum comeurn. The stratum 1.50 1.00 0.50 oo SLS Polyoxyethylene (15) Laurylether 0 I I I I I I I I I I I I I I I I I I I I I 0 30 60 90 120 Time (min) Figure 5. Time dependence of dynamic elastic modulus of the stratum corneum after application of aqueous surfactant solution (100 mM/1) at 25øC, 50% RH. corneum was plasticized by aqueous surfactant solutions more than by water immedi- ately after application. The reason is that the aqueous surfactant solutions have lower surface tensions than distilled water, spreading more readily and penetrating the stratum corneum (9,10). However, the recovery of the elastic modulus started sooner for surfactant-treated stratum corneum since water evaporated more rapidly. Finally, the elastic modulus became markedly higher than the pretreatment levels. Thus, the effects of application of aqueous surfactant solutions on the stratum corneum were characterized by increased elasticity after water evaporation. The extent of elasticity increase depended on the characteristics of the surfactant. Sodium lauryl sulfate (SLS) was more effective than polyoxyethylene (15) lauryl ether. The mechanism of stratum corneum hardening by application of surfactant solution is not clear, but this result may be related to the taut feeling that occurs after washing the face with soap.

EVALUATION OF SKIN SOFTENING 177 DMSO AND LACTIC ACID Figure 6 shows the effects of dimethyl sulfoxide (DMSO) and lactic acid on the me- chanical properties of the stratum corneum. It is well known that DMSO is capable of 1.0 0.5 Lactic Acid DMSO o o i i ! i ! i i i i i i i i i i i i i i i 0 30 60 90 120 Time (min) Figure 6. Time dependence of dynamic elastic modulus of the stratum corneum after application of protein-denaturing agents at 25øC, 50% RH. swelling and unfolding several soluble proteins (11). The stratum corneum was swollen and its elastic modulus was markedly decreased by the application of DMSO. Since DMSO is not volatile, it penetrates gradually into the stratum corneum and results in swelling with the lapse of time hence the plasticization progresses. On the contrary, as water is very volatile, its plasticizing effect diminishes quickly after evaporation. The change of elasticity with lactic acid was similar to that obtained with DMSO. Lactic acid is also known to be caustic and capable of swelling proteins in concentrated solution. It seems to plasticize the stratum corneum in the same manner as DMSO. FORMIC ACID Figure 7 shows the effect of 90% aqueous formic acid solution on the mechanical properties of the stratum corneum. The swelling action of formic acid on keratin fibers has been reported several times (12, 13,14). Epidermal membrane is considerably swollen with rapid and extensive changes in length and thickness by formic acid (10). Our experimental result also agrees with these results, showing that formic acid is more