EVALUATION OF SKIN SOFTENING 173 ments consist of the application of a fixed amplitude sinusoidal extension (5 I•m am- plitude, 30 Hz) on one end of a stratum corneum strip extended by 1% strain and held in a horizontal position by clamps. A cyclic stress response is measured by a load cell attached to the other end of the stratum corneum. The ratio of stress to strain leads to a complex modulus, E*. This is divided into two components which are the real part (E') or in-phase with the applied cyclic strain and the imaginary part (E") or 90 degrees out-of phase. That is, one can write E* = E' + iE" where i is X/-1, and E' and E" are the dynamic elastic modulus and dynamic loss modulus, respectively. These rheological parameters are calculated by the operation circuit and printed out. This method for the measurement of dynamic mechanical parameters is extensively utilized in the field of polymer science and has been applied to the stratum corneum by Wilkes (8). However, in the case of quite extensible materials such as wet stratum corneum, the dynamic modulus shows the tendency of decreasing with time while taking continuous measurement because of stress relaxation. In order to minimize this influence on me- chanical properties, the measurements were taken at 3-minute intervals as controlled through a servomotor by the measurement time set circuit. The test strip of stratum corneum was slackened to remove strain except during measurement. Dynamic elastic modulus (E') and loss modulus (E") were measured on non-treated stratum corneum pre-conditioned at 25øC, 50% RH. Following this, 2 I•1 of a test solution was applied on the outer side of the stratum corneum by a microsyringe, and then mechanical properties were remeasured as a function of time. This technique has superiority to previous methods in that (1) the mechanical properties can be measured on the same sample of stratum corneum and under the same experi- mental conditions before and after treatment, (2) the softening effects of cosmetic products on stratum corneum can be evaluated under conditions simulating usual product usage, i.e., by topical application rather than by soaking, and (3) the change of mechanical properties over time can be examined after the application of cosmetic ingredients. The softening effect was evaluated from the ratio of dynamic elastic modulus, E't/E't =0, where E't is the dynamic elastic modulus of treated stratum corneum at time t after the application of a test solution, and E't = 0 is for the non-treated stratum corneum. HYGROSCOPICITY OF HUMECTANTS The water uptake of humectants was measured by equilibrating them in an atmosphere of constant relative humidity, weighing, and comparing this with the dry weight. The dry weight was obtained by keeping the humectants over phosphoric oxide. WATER-HOLDING CAPACITY OF HUMECTANTS For the determination of water-holding capacities of humectants, 2 I•1 of 10% aqueous solutions of humectants were applied to preweighed stratum corneum. After condi-

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.