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j. Soc. Cosmet. them., 37, 35-41 (January/February 1986) In vitro percutaneous absorption experiments: A technique for the production of intact epidermal membranes from rat skin R. C. SCOTT, M. WALKER, and P. H. DUGARD, Central Toxicology Laboratory, Imperial Chemical Industries Limited, Alderley Park, Macclesfield, Cheshire, England. Received July 8, 1985. Synopsis When measuring the in vitro percutaneous absorption of chemicals it is preferable to use an epidermal membrane because the dermis in whole skin may act artificially as a barrier to some lipophilic molecules. Techniques are available for preparing epidermal membranes from human skin. We have developed a novel technique for preparing epidermal membranes from rodent (and human) skin. After soaking 28-day-old rat skin (and human cadaver skin) in 2 M sodium bromide for up to 24 hours, sheets of epidermis could be separated from whole skin. Preservation of the diffusion properties of the barrier was assessed by measuring the permeability to the polar molecules [3HI water and [•4C] paraquat and the lipophilic molecule, toluene. Within each species epidermal membranes and full-thickness skin had the same permeability to the polar molecules however, the dermis acted as a barrier to the penetration of the lipophilic molecule toluene, when whole skin was used. For these three chemicals, human skin was less permeable than rat skin the greatest difference was measured with paraquat. INTRODUCTION In vitro percutaneous absorption experiments offer many advantages over in vivo experi- ments (1-3). The in vitro measurement allows the permeability of human and animal skin to be compared using the same experimental technique. Data from such studies can be used, for example, to predict the absorption of topically applied substances, e.g. drugs or cosmetic ingredients, or to investigate variations of the formulation. The in vitro technique facilitates the extrapolation of animal data to man. Scientists investi- gating the permeability of human skin in vitro use epidermal membranes prepared from whole skin. However, there is evidence that the dermis in vitro can act as a significant additional "artificial" barrier to the absorption of lipophilic molecules (4,5). Epidermal membranes mimic the in vivo barrier where a diffusing molecule must pass through _ the stratum corneum and viable epidermis before entering the capillary bed at the dermo-epidermal junction. In order to permit accurate in vitro measurements of the penetration rates of lipophilic molecules through skin, we have been seeking a suitable technique for separating epidermal membranes from whole skin from a variety of species. A number of separation techniques have been proposed, some of which have been suc- 35