EFFECT OF AcHA ON STRATUM CORNEUM 177 1.2 II 1.0 0.8 0.6 0.4 0.2 0.0 2.5 3.0 3.5 4.0 Degree of substitution (DS) Figure 3. Relationship between the ratio of the dynamic elastic modulus (E'•/E'•= o) at 100 min and the degree of substitution (DS) of AcHA. There is an optimal DS value around 3.3. Since the DS value of AcHA reflects its hydrophobicity/hydrophilicity balance, there may be found a relationship between the skin- softening effect and the affinity of AcHA to stratum corneum. EFFECT OF A LOTION CONTAINING 0.2% AcHA ON SKIN PHYSIOLOGY As shown in Table I, the lotion containing 0.2 w/w% AcHA or HA was applied to ventral forearm skin in vivo. Figure 6 shows the effect of lotion containing HA or AcHA on skin softening. The skin-softening effect of AcHA was significantly greater than that of HA. The result suggests that AcHA could show a strong skin-softening effect even in a lotion. Figure 7 shows the effect of the lotion on the ratio of skin surface conduc- tance. Treatment with AcHA lotion considerably increased the water content of the skin as compared with the nontreated control, and the lotion was found to be more effective than HA lotion. Figure 8 shows the effect of the lotion containing HA or AcHA on transepidermal water loss (TEWL), representing the barrier function of the skin. The reduction in TEWL by AcHA was slightly greater than that of HA. This means that the amount of water loss was reduced by AcHA. The result indicates that AcHA had high water-holding capacity and prevented water loss from the skin even in a lotion. Figure 9 shows the photographs of silicon negative replica of the treated region with the lotion containing HA or AcHA. The in vivo one-week treatment with the AcHA lotion remarkably improved the stratum corneum of dry skin as compared with the treatment with the nontreated control or the HA lotion. The nontreated control shows no im- provement, and the HA-treated skin also shows only moderate improvement. However,

178 JOURNAL OF COSMETIC SCIENCE 1.2 0.8 0.6 0.4 0.:2 0.0 0 i i i i i i i i i i i . 50 1 oo Time / min Figure 4. Time course of the changes in the dynamic elastic modulus of guinea pig stratum corneum treated with a combination of various humectants. The y-axis shows the ratio of the dynamic elastic modulus, E'•/E'•_o, where E'• is the dynamic elastic modulus of stratum corneum treated with a test solution at the specific time after the application, and E't_ o is that of non-treated stratum corneum. The stratum corneum was treated with 2 pl of water or AcHA solution. The x marks represent the modulus of the stratum corneum treated with water. The open circle (¸), 0.2% HA the closed upside-down triangle (•'), 5% glycerol the open square (•]), 0.1% HA with 5% glycerol the closed triangle ('), 0.1% AcHA with 5% glycerol the closed circle (0), 0.2% AcHA. the AcHA-treated skin shows a remarkable improvement because skin grooves and skin ridges can be clearly observed as compared with the other test pieces. DISCUSSION To evaluate the skin-softening effect of AcHA, the dynamic elastic modulus of stratum corneum was measured. The method reported by Takahashi eta/. can evaluate quanti- tatively the skin-softening effect in vitro (19). They reported a clear change in the dynamic elastic modulus of the stratum corneum after the application of 10% aqueous glycerol solution. Although stratum corneum was plasticized to the same extent as pure water, the plasticization of stratum corneum treated with the glycerol solution lasted for a longer time than that obtained with water. This effect is due to its low evaporation rate. Upon the use of AcHA with the DS of hydroxyl groups from 2.6 to 3.8, we found that a value between 3.0 and 3.5 maintained the highest skin-softening effect for a long period of time. The important point to notice is that AcHA shows singly an excellent skin-softening effect. The skin-softening effect of AcHA was significantly more effective than that of glycerol. It can be believed that this strong skin-softening effect can be attributed to its amphiphilic properties following the introduction of acetyl groups to