Diffusivity and structural polymorphism in some model stratum corneum lipid systems.

Mixtures of model stratum corneum lipids were prepared in water from cholesterol, six fatty acids and ceramides. The influence of composition on the polymorphism of these mixtures and also on the diffusivity of a model drug within them, Dlip, was determined. The former was obtained from X-ray diffraction and Fourier transform infrared spectrometry, and the latter from a diffusional release model. An L beta structure was formed for the composition approximating that of the extracellular lipids in intact human abdominal stratum corneum. Dlip was independent of water content in the range 20-40% w/w, with the bilayers showing one dimensional swelling without lateral expansion. Although removal of the ceramides did not result in a significant alteration in Dlip, crystalline cholesterol now appeared. The ceramides were, therefore, necessary for solubilization within the fatty acid bilayers of the large proportion of cholesterol present in the lipid fraction of intact SC. They were also responsible for a thermal L alpha-HII transition observed at approx. 68 degrees. At the concentration in which it exists in intact SC, cholesterol also had only a minimal effect on Dlip, but was necessary to suppress HII phase formation within the fatty acids and ensure an L beta structure. All lipid mixtures that had an L beta structure presented a diffusional barrier approx. 1 order of magnitude greater than that of an unstructured, isotropic lipid mixture. HII structures formed at cholesterol/fatty acid proportions less than approx 8:92 mol% and appeared more permeable than L beta ones. All the results indicate that the diffusional barrier within the model lipid mixtures is guaranteed essentially by the presence of an L beta phase. Although the ceramides and cholesterol exert no intrinsic influence on the magnitude of Dlip, their presence in necessary for the existence of an L beta phase at 33 degrees that is free of both crystalline cholesterol and HII character.