624 S. Nicholls, C. S. King and R. Marks of replicas and SSBs for assessment. In this way it is possible to compare what is taking place at the surface with changes of some cell layers down in the SC. We hope to employ the techniques described in this study to evaluate the effects of chronic application of topical preparations thought to have a hydrating action on the SC and to correlate these effects with changes in the pattern of epidermal growth and maturation. ACKNOWLEDGMENTS We are-extremely grateful to Vick International Ltd, and Westwood Pharmaceuticals Inc for generous assistance towards these studies. REFERENCES 1 Marks, R. and Dawber, R. P. R. Skin surface biopsy: an improved technique for examination of the horny layer. Brit. J. Dermatol. 84 117 (1971). 2 Nicholls, S., King, C. S. and Marks, R. Morphological and quantitative assessment of physical changes in the horny layer in Ichthyoses. In: Marks, R. and Dykes, P. J. The Ichthyoses. MTP Press, Lancaster, U.K. (1978). 3 Marks, R. and Pearse, A.D. Surfometry: a method of evaluating the internal structure of the stratum corneum. Brit. J. Derrnatol. 92 651 (1975). 4 Garber, C. A. and Nightingale, C. T. Characterizing cosmetic effects and skin morphology by scanning electron microscopy. J. $oc. Cosmet. Chem. 27 509 (1976). 5 Quattrone, A. J. and Laden, K. Physical techniques for assessing skin moisturization. J. $oc. Cosmet. Chem. 27 607 (1976).

J. $oc. Cosmet. Chern. 29 625-639 (1978) Water diffusion coefficients versus water activity in stratum oorneum: a oorrelation and its implioations M. STOCKDALE The Boots Company Limited, Consumer Products Development, Nottingham Received 27 April 1978 Synopsis Literature reports of rates of trans-epidermal water loss through skin exposed to differing external relative humidities have been used to generate water diffusion coefficients by means of Fick's law. These have been related, within a given series, to the diffusion coefficient at a standard average water activity to give relative water diffusion coefficients. These have been combined, together with literature values of water diffusion coefficients obtained by kinetic experiments on stratum comeurn exposed to various humidities, to yield a correlation between the average water activity of stratum corneum and the relative water diffusion coefficient over the full range of water contents. The asymptotic relationship derived shows that the diffusion coefficient increases rapidly as the water activity approaches its maximum. The im- plications of this relationship in terms of water activity profiles of stratum corneum are discussed. A theoretical analysis of the subject is presented and water activity profiles of model systems are derived. INTRODUCTION The importance of the water content of the stratum corneum in determining its ob- servable properties is well documented (1-4). Simple diffusion theory shows that the water content will be lower in the outer layers than in the inner layers under normal ambient conditions. This variation with position will be non-linear, since the water diffusion coefficient will vary with water content (5, 6). A formula correlating the diffusion co- efficient with the water content of the stratum corneum at that point would be useful and would enable a water content profile of stratum corneum under different external conditions to be derived. Although there are several reports of variations in trans-epidermal water loss (TEWL) with external relative humidities, no single set of experiments has been sufficient to derive an empirical relationship between diffusion coefficient and water content. By combining the data from several literature reports, a correlation has been derived between stratum corneum water activity and relative diffusion coefficient. DEFINITIONS Fick's Law has been used extensively and successfully in relation to diffusion processes in stratum corneum (7). This is the basic equation involved in the theoretical treatment of this topic. dC• J= Di• (1) dx 0037-9832/78/1000-O625 $02.00 ¸ 1978 Society of Cosmetic Chemists of Great Britain 625