RAPID TEST FOR SKIN MOISTURIZERS 237 These were petrolatum (hydrophobic and occlusive), hydrophilic ointment (oil-in- water emulsion containing 70% water), and 10% urea cream containing 65% water in its base (Urepearl ©, Otsuka Pharmaceutical, Japan). The change in conductance under environments of 33%, 60%, and 90% RH was mon- itored 0, 30, and 60 minutes after application of the test agents. The probe was inserted through a tiny hole in the chamber, allowing the RH to remain undisturbed. The test agents were swiftly applied to the model, which was then instantly put into the chamber. In order to study any "buffer effect," the same specimen was used for the change of RHs in both directions, from low to high and high to low. For in vivo comparisons, we made measurements on the flexor forearms of ten normals, 18-24 years of age, at ambient conditions of 18øC and 27% RH, as described previously (11). The water sorption-desorption test (16) was also performed. This was carried out before and 60 minutes after application of the test agents, similar to measurements made in vivo (16). Briefly, a drop of distilled water was spread over the surface for 10 seconds and quickly blotted with tissue paper. Conductance was measured before and immedi- ately after blotting, and then 30, 60, 90, and 120 seconds after. STATISTICS The statistical significance of the results was evaluated by Student's t-test. RESULTS EFFECT OF OCCLUSION Conductance increased linearly, reaching a value nine times greater than the original after 30 minutes of occlusion with a polyethylene film (Figure 2). HYDRATION AFTER APPLICATION OF TEST AGENTS Figure 3 shows the values obtained in vivo at ambient conditions of 18øC and 27% RH. Application of 10% urea cream and hydrophilic ointment resulted in an immediate increase in hydration followed by a slow decrease. With urea cream, the values 120 minutes after application were still three times higher than those at pretreatment skin. In contrast, although petrolatum did not induce any initial increase, a slow but steady increase up to 60 minutes took place, reaching a plateau between 60 and 120 minutes, showing a fourfold increase in conductance. Figure 4 shows the results obtained with the in vitro models. At 33% RH, the curves were similar to those obtained in vivo at 27% RH. More dynamic changes occurred at higher humidities, particularly at 90% RH. With 10% urea cream, high conductance values, like those noted just after application, were maintained even after 60 minutes. On the other hand, such sustained high levels of conductance at higher RHs were not demonstrated with hydrophilic ointment in fact, values were even lower than the con- trols. With petrolatum, at high humidities conductance decreased greatly just after application, followed by a gradual recovery.

238 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS GX I I I I 1 0 e 20 ao MINUTES OF OCCLUSION Figure 2. A totally occlusive effect of a polyethylene film placed on a sheet of stratum corneum. Measured at 2 IøC and 27% RH. WATER SORPTION AND DESORPTION TEST The curves before and 60 minutes after application were entirely comparable to those previously obtained on forearm skin in vivo (16). Figure 5 shows that 10% urea cream was the most effective for increasing both the hygroscopicity and the water-holding capacity of the stratum corneum. With petrolatum, the hygroscopicity decreased with a reciprocal increase of the water-holding capacity. BUFFER ACTION OF TEST AGENTS Figure 6 shows the hydration changes when the RH was suddenly changed. "Desorp- tion curves" represent the changes that took place when the models that had been already equilibrated with 90% RH for more than three hours were transferred to a 33% RH environment. "Sorption curves" show the changes when the model was moved in the opposite direction. The changes obtained in both directions corresponded well, and a buffering effect was demonstrated most prominently with petrolatum, then with hy- drophilic ointment. DISCUSSION The design of modern moisturizers has depended on selecting various hygroscopic com- ponents that have effects similar to natural moisturizing factors (NMFs) and effective