Infrared spectroscopy and microscopic imaging of stratum corneum models and skin

The highly ordered ceramide-containing phases that constitute the barrier t o permeability in the stratum corneum are well-suited for examination by IR spectroscopy, and the chain vibrations have been widely investigated in th is context. The current study focuses on the much less studied Amide I and II vibrations from the polar headgroup region of two major ceramide classes , ceramide 2 (nonhydroxy fatty acid spingosine) and ceramide 5 (alpha-hydro xy fatty acid sphingosine) alone and in three-component stratum corneum mod els (ceramide-cholesterol-hexadecanoic acid 1:1:1). The slight differences in the chemical structures between these species produce remarkable differe nces in the H-bonding interactions and propensity for water penetration. Th e H-bonds in ceramide 2 are interlamellar in origin and are accessible to s olvent at lower temperatures. Ceramide 5 H-bonds are also strong, but diffe rent in nature from those of ceramide 2; the evidence suggests an intralame llar orientation. Ceramide 5 also contrasts to ceramide 2 in that it is muc h more miscible with the other stratum corneum components. Distinct roles f or each ceramide class in the stratum are suggested based on these observat ions. These studies have provided evidence that is consistent with the doma in mosaic model of the skin lipid barrier structure proposed by Forslind (A cta Derm. Venereo., 1994, 74, 1). The application of array-detector based I R imaging to skin is described. The potential of this approach for monitori ng the distribution of lipid and protein constituents in tissues at a spati al resolution of similar to 6 mu m in intact skin sections is demonstrated. In addition, the feasibility for mapping the distribution of topical appli cations on skin is demonstrated through IR images of the nitrile moiety in a sunscreen formulation containing 2-ethylhexyl-2-cyano-3,3-diphenyl acryla te. These images reveal a non-homogenous film. Understanding and visualizin g the coherence, integrity, and homogeneity of such topical sunscreen films is critical to improving the function of these films for successful UV pro tection.