J. Soc. Cosmet. Chem., 34, 191-196 (July 1983) Water diffusivity and water concentration profile in human stratum corneum from transepidermal water loss measurements MAW-SHENG WU, Personal Care Division, The Gillette Company, Gillette Park, Boston, M/I 02106. Received October 21, 1982. Synopsis The diffusivity, D(C), of water in the stratum comeurn of human volar forearms was determined from passive water loss data obtained under various relative humidities by Goodman and Wolf and from the analytical method developed by Wu. The diffusivity is a function of the water concentration (C) in the stratum corneum it was found to be D(C) = 1.6 x 10 -•ø q- 5.1 x 10 -•ø C2'7o The water concentration profile within the stratum comeurn at various relative humidities was determined. It was found that except for the surface and first few cell layers near the surface, the water content in stratum corneum was essentially the same for all humidities. INTRODUCTION Measurement of transepidermal water loss (TEWL), the passive transpiration of water through stratum corneum, has been used as a method to determine the hydration state of the stratum corneum and to evaluate the occlusive effect of a skin moisturizer on the water content in the tissue (1). However, very few efforts have been devoted to a systematic analysis of the TEWL data to obtain quantitative information regarding the water concentration profile in stratum comeurn. Stockdale (2) has derived an equation for the water diffusivity in stratum comeurn as a function of water activity. The equation does not adequately represent the water diffusivity when the water concentra- tion in the tissue approaches saturation. The passive diffusion of water across stratum comeurn is governed by Fick's law. For transport (e.g., of water) in a single direction (e.g., across the thickness of stratum corneum), Fick's law has the form (3) dC F = --D d•' (1) where F is the flux of matter (e.g., the rate of transpiration of water across any place in the stratum comeurn), C is the concentration of the diffusion substance (e.g., water), x 191