180 JOURNAL OF COSMETIC SCIENCE 2.0 0.2% HA-lotion 0.2% AcHA-Iotion Figure 6. Effect of a lotion containing 0.2% HA or 0.2% AcHA on the skin softening of the human ventral forearm. Each column shows the mean of ten samples, and the bars indicate the standard deviation. **p 0.01 statistical significance (Student t-test). the number of AcHA molecules adsorbed on human skin could be remarkably greater than that of HA. The possible increase in affinity to human skin can be attributed to the introduction of acetyl groups to HA. Since, as described in the Experimental section, AcHA can lower its surface tension (55.0 mN'm -•) and dissolve in 90 w/w% aqueous ethanol, it is clear that this enhancement of the affinity to the skin surface can be supported by its amphiphilic properties. To verify the efficacy on skin physiology of AcHA in a cosmetic formulation, the in vivo skin-softening effect using lotion was evaluated. AcHA also showed a strong skin- softening effect even in a cosmetic formulation. Because of its strong moisturizing effect on stratum corneum, AcHA could be expected to produce improvement in dry skin. Andrew and Gregory have reported that the application of a 4% aqueous solution of SDS induced a dry scaly skin in human subjects (25). Therefore, the treatment effect of AcHA for dry skin was investigated using SDS-induced dry skin. To evaluate the treatment effect of AcHA for dry skin condition, a dry skin model, which was induced by 5 w/w% SDS, was used in this study. Actually, in this study, the water content of SDS-induced dry skin decreased to about 75% of normal water content. This model is suitable for evaluating the treatment effect of AcHA. From the results of the skin surface conduc- tance and the TEWL, AcHA improved the water-retaining function of skin, and also enhanced the barrier function of stratum corneum. It is clear that AcHA can show a high efficacy on the maintenance of fresh skin. To explain the strong skin-softening effect of AcHA, the following mechanism is proposed (Fig. 10). It is believed that the humectant sticks to stratum corneum, and it prevents the evaporation of water from the skin surface, which has a natural tendency to

EFFECT OF AcHA ON STRATUM CORNEUM 181 nontreated 0.2%HA-lotion 0.2%AcHA-Iotion Figure 7. Effect of a lotion containing 0.2% HA or 0.2% AcHA on the ratio of skin surface conductance of the human ventral forearm. Each column shows the mean of four samples, and the bars indicate the standard deviation. **p 0.01 statistical significance (Student t-test). become harder with decreasing hydration (13,19). Therefore, AcHA can firmly adhere to the stratum corneum and prevent water loss from the skin. AcHA helps to sufficiently soften the stratum comeurn and allows the skin to have increased hydration by protect- ing it from dryness for a long period of time. Wichrowski et al. reported that the moisturization of the stratum corneum was assessed by a non-invasive and non- destructive in vivo method using Fourier transform infrared spectroscopy (26). It was shown that the intensity ratio of amide 1 band (1645 cm -•) and amide 2 (1545 cm -•) indicated indirectly the water content of the stratum comeurn and was assumed to be a relative measure of stratum comeurn hydration. The strength of interaction between keratin and humectant might be evaluated by this method. Using a solid-state •3C- nuclear magnetic resonance spectroscopy, Jokura et al. showed that structural keratin proteins, mainly consisting of 10-nm filaments, acquire their elasticity with the help of hydrated natural moisturizing factor via the reduction of intermolecular forces, probably through non-helical regions between keratin fibers (27). Further investigation is needed to clarify the mechanism of the skin-softening effect of AcHA. In the near future, we expect to further elucidate this process. CONCLUSION To summarize our results, we can conclude as follows: (a) An amphiphilic polymer, AcHA, was found to show superb moisturizing and excellent skin-softening effects as compared with HA. (b) Due to its hydrophobicity, AcHA could adhere efficiently to the human skin surface and showed a strong skin-softening effect. This adsorption enhance-