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Under normal conditions the stratum corneum in vivo is very much less hydrated than the excised tissue routinely used in laboratory permeability experiments. As a consequence, certain quantitative predictions of skin permeability based on laboratory experiments are not applicable to many cases of clinical and practical interest. In this study a modified in vitro permeability technique was used which permitted the hydration of the stratum corneum to be maintained very close to its natural state. Penetrant molecules were applied by adding a few microliters of acetone solution to the stratum corneum surface. The acetone quickly evaporated leaving a residue of solid penetrant. The time course of penetration of the solid material through the epidermis was observed as a function of applied dose, mild hydration, and solvent contact time. Remarkably steady, uniform penetration persisting over several weeks was observed. Generally, the penetration rates were similar to those observed in vivo and increased in a similar manner upon occlusion. Diffusion through the stratum corneum rather than sorption of the solid at the surface appears to be rate limiting. [ABSTRACT FROM AUTHOR]

Most quantitative permeability data on human skin are based on studies in which water was used as the vehicle. Very little is known of the permeability of normal stratum corneum as it exists in vivo where it is considerably less hydrated. Similarly very little is known of the effect of nonaqueous solvents on the skin although they are common vehicles in topical therapy. In this study the permeation rates of a homologous series of primary alcohols (C1-C10) through skin were measured. The alcohols were applied from aqueous solutions and also as the pure liquids. The permeation behavior in the two cases is compared in terms of (1) the transport rates of the alcohols, (2) the distribution equilibrium between the tissue and vehicle, and (3) the damage to the tissue produced by the vehicle. Fick's law was found to hold as an approximation for both the aqueous and the liquid alcohol systems. Using the measured data, which include permeability and diffusion constants and tissue:vehicle partition coefficients, the diffusional resistance offered by epidermis and dermis is accurately apportioned. The overwhelming role of the stratum corneum in both the aqueous and the pure liquid systems is quantitatively confirmed. The effect of the solubility characteristics of different vehicles on skin permeability is discussed. It is shown that with the aid of a few simple principles and a knowledge of the solubility character of the vehicle, the permeation of different substances can be qualitatively predicted. [ABSTRACT FROM AUTHOR]