Idson B
Curr Probl Dermatol. 1978;7:132-41.
The state of hydration of the normal stratum corneum ranks next to the nature of the penetrating molecule as the most important factor in the rate of percutaneous passage of any substance. If the stratum corneum is removed, the barrier to gain or loss of water vapor is lost. Diseased skin in which the process of keratinization is disturbed becomes more susceptible to transepidermal water loss. Diffusion through the stratum corneum is a passive process, affected only by physical factors, chiefly the water vapor pressure gradient. The low diffusion constant and high activation energy suggest that extensive hydration does not drastically affect the 'barrier' function of the stratum corneum. Below 100% ambient relative humidity, water transfers outward and a gradient in water concentration exists within the stratum corneum. The horny layer swells continuously on immersion in water, absorbing as much as ten times the dry weight. The water is bound within the intracellular keratin. Permeability increases rapidly initially and then slows down to a steady state diffusion. Regardless of the increase in permeability, the highly hydrated stratum corneum remains quite water impermeable, with a diffusional resistance ca. 10(4) times greater than an equivalent layer of water. The diffusion process is determined mainly by the intrinsically low diffusivity and the ultrastructure of the intracellular keratin.
正常角质层的水合状态是任何物质经皮渗透速率的第二重要因素，仅次于渗透分子的性质。如果去除角质层，水蒸气得失的屏障就会丧失。角质化过程受到干扰的患病皮肤更容易发生经表皮水分流失。通过角质层的扩散是一个被动过程，仅受物理因素影响，主要是水蒸气压力梯度。低扩散常数和高活化能表明，大量水合作用不会显著影响角质层的“屏障”功能。在环境相对湿度低于100%时，水分向外转移，角质层内存在水浓度梯度。角质层在水中浸泡时会持续肿胀，吸收量可达干重的十倍。水分结合在细胞内角蛋白中。渗透率最初迅速增加，然后减缓至稳态扩散。尽管渗透率增加，但高度水合的角质层仍然相当不透水，其扩散阻力约比等量水层大10(4)倍。扩散过程主要由细胞内角蛋白固有的低扩散率和超微结构决定。
