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J. Soc. Cosmet. Chem. 26 337-353 (1975) ¸ 1975 Society of Cosmetic Chemists of Great Britain Skin impedance and moisturization E. J. CLAR, C. P. HER and C. G. STURELLE* Presented on 26-30th August 1974 in London at the IFSCC VIIIth International Congress on 'Cosmetics--Quality and Safety' organized by the Society of Cosmetic Chemists of Great Britain Synopsis--The results of dielectric studies which led to the development of a simple method for the in vivo evaluation of the HYDRATION state of STRATUM CORNEUM on human subjects are presented. The DIELECTRIC properties of HUMAN SKIN have been investigated over a wide range of frequencies (2 Hz-15 GHz). From stripping experiments it has been established that the low frequency IMPEDANCE (2 Hz-1 K_Hz) is located almost entirely in the HORNY LAYER. Skin impedance measurements have been carried out in relative humidity (rh)-controlled conditions using matched electrodes. An increase in the rh of the environment (at constant T ø) causes an increase in the moisture content of the stratum corneum which is demonstrated by a decrease in impedance. The effect of MOISTURIZING ingredients can be demonstrated by comparison of im- pedance values before and after treatment with the ingredient in a convenient vehicle. Lasting and long-term effects are also assessed by measuring the time-course of the impedance value during the hours or days following a treatment. In skin not protected against dehydration, a decrease of the rh of the environment provokes an increase in the impedance. A BARRIER CREAM will act against dehydration, thus giving rise to a lesser increase in impedance. A procedure of testing protective creams is drawn from this experiment. A tentative interpretation of the relationship between hydration and skin impedance is proposed on the basis of the experimental results. INTRODUCTION The dielectric properties of human skin depend on various factors which include its physico-chemical characteristics, the composition of intra- and extracellular fluids, psychological and physiological changes occurring in the subject and the condition of health or disease of the skin. * Unilever Research Laboratory, 8, impasse de la Montjoie, 93212-Saint-Denis, France. 337