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J. Soc. Cosmet. Chem., 41, 235-241 (July/August 1990) A rapid in vitro test to assess skin moisturizers MASAAKI OBATA and HACHIRO TAGAMI, Department of Dermatology, Tohoku University School of Medicine, Sena•i 980, Japan. Received April 14, 1990. Synopsis We developed a simulation model using human stratum corneum to quickly evaluate the efficacy of topical "moisturizers" under controlled ambient conditions. We determined high-frequency conductance as a mea- sure of the hygroscopicity of the horny layers. We examined three emollients under varied relative humi- dities and were able to discriminate among them with regard to hydrating capacity. This method is a relevant and quick screening model for moisturizers, yielding reproducible and reliable results. INTRODUCTION Water in the stratum corneum mainly controls its softness and pliability (1,2). De- creased hydration is the common factor in various dry skin conditions such as winter xerosis and xerosis of the aged (3-6). Dry skin is also a classic feature of persons who suffer from atopic dermatitis (7). The factors that control the.moisture content of the normal horny layer are not fully understood. It is assumed that certain water-soluble substances, the so-called natural moisturizing factors (NMFs) (4,8) and intercellular lipids of the stratum corneum (9,10), are crucial for maintaining the softness of the surface. In everyday life the aesthetic properties of the skin surface depend upon an adequate level of hydration. The cosmetic industry makes extensive use of hygroscopic ingredients that are supposed to prevent drying out of the outer horny layer. Estimating the moisturizing capabilities of the many products designed to relieve dry skin is a difficult task. Moreover, the results are often inconsistent and are strongly influenced by ambient weather conditions. Various noninvasive techniques have been utilized to measure changes in the hydration level of the stratum corneum in vivo after application of skin moisturizers (11-15). Determination of skin conductance against 3.5 MHz high-frequency electric current has been found to be valuable for demonstrating the effects of topical agents on the water- holding capacity of the desquamating, outer portion of the stratum corneum (3,10,11). The water sorption-desorption test described by Tagami et al. (16) is the principle underlying such tests. In this report we describe an in vitro method for evaluating the efficacy of moisturizers based on our previously reported human stratum corneum model (17). 235