Percutaneous absorption of urea

Synopsis The effect of several variables on the in vitro permeation of urea through hairless mouse skin has been studied in order to determine the causes of an increasing permeability phenomenon found in studies with a range of hydrophilic compounds. The permeation of urea increased for a period of approximately 100 h after which a steady state permeation pattern was observed for approximately 25 h. Urea did not effect its own permeation in concentrations between 0.01 M and 1.67 M, and the same pattern of increasing permeation was followed in the presence of (N-morpholine)propanesulphonic acid and tris(hydroxyme)amino-methane buffers, as in the presence of normal saline. Urea did not affect the permeation of tritiated water. Methanol and water exhibited the same pattern of increasing permeation as urea. The continuously increasing permeation rate of urea up to 100 h is believed to be due to penetration and extensive association of water with the components of the stratum corneum, altering the ultra-structure of the stratum corneum and leading to the formation of large and extensive hydrophilic diffusion channels which do not exist in fresh, untreated skin. These presumed channels open the stratum corneum to facile permeation of highly polar substances such as urea. The physical events leading up to the ultra structural changes within the tissue at the microscopic level remain obscure and are the subject of ongoing research. L'absorption percutanée de l'urée.