In mammals, the outer skin layer, the stratum corneum, is the ultimate barrier to water loss. In order to relate barrier function to stratum corneum structure, samples from porcine skin were investigated by using differential scanning calorimetry (DSC), infrared (IR) spectroscopy, and water permeability techniques. Results of DSC and IR studies show that stratum corneum lipids undergo thermal transitions between 60 and 80 degrees C similar to lipid thermotropic transitions seen in a variety of synthetic and biological membranes. Results of water flux experiments performed under conditions similar to those of the DSC and IR studies show an abrupt change in permeability at about 70 degrees C. At low temperatures, water flux values are similar to those obtained for human skin in vivo, yielding an activation energy of 17 kcal/mol, in excellent agreement with values obtained for water flux through a variety of lipid biomembranes. In contrast, at temperatures above about 70 degrees C, water flux is characterized by an activation energy only slightly higher than that of free diffusion, suggesting that the stratum corneum offers little diffusional resistance under these conditions. These combined results suggest that increased disorder in stratum corneum lipid structure, brought about by thermotropic transitions, results in dramatically altered diffusional resistance of this tissue to water flux. Thus, as found for numerous biological membranes, water flux and lipid order in porcine stratum corneum are inversely related.
In order to noninvasively measure water concentration in the stratum corneum, infrared spectra were obtained using an attenuated total-reflectance technique in conjunction with Fourier transform spectroscopy. A weak water-absorbance band near 2,100 cm-1 was detected in both in vivo and in vitro spectra. The significance of this band is that it occurs in a region of the mid-infrared where the stratum corneum and most topically applied substances show no absorbance. In vitro spectra obtained as a function of ambient relative humidity showed an increase in the absorbance near 2,100 cm-1 with increasing water concentration in the stratum corneum. The combined in vivo and in vitro results lead to a quantitative assessment of water concentration in the uppermost layers of the stratum corneum.
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