Infrared spectroscopy and microscopic imaging of stratum corneum models and skin: Invited lecture

The highly ordered ceramide-containing phases that constitute the barrier to permeability in the stratum corneum are well-suited for examination by IR spectroscopy, and the chain vibrations have been widely investigated in this 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 (α-hydroxy fatty acid sphingosine) alone and in three-component stratum corneum models (ceramide-cholesterol-hexadecanoic acid 1 : 1 : 1). The slight differences in the chemical structures between these species produce remarkable differences in the H-bonding interactions and propensity for water penetration. The H-bonds in ceramide 2 are interlamellar in origin and are accessible to solvent at lower temperatures. Ceramide 5 H-bonds are also strong, but different in nature from those of ceramide 2; the evidence suggests an intralamellar orientation. Ceramide 5 also contrasts to ceramide 2 in that it is much more miscible with the other stratum corneum components. Distinct roles for each ceramide class in the stratum are suggested based on these observations. These studies have provided evidence that is consistent with the domain mosaic model of the skin lipid barrier structure proposed by Forslind (Acta Derm. Venereo., 1994, 74, 1). The application of array-detector based IR imaging to skin is described. The potential of this approach for monitoring the distribution of lipid and protein constituents in tissues at a spatial resolution of ~6 μm in intact skin sections is demonstrated. In addition, the feasibility for mapping the distribution of topical applications on skin is demonstrated through IR images of the nitrile moiety in a sunscreen formulation containing 2-ethylhexyl-2-cyano-3,3-diphenyl acrylate. These images reveal a non-homogenous film. Understanding and visualizing the coherence, integrity, and homogeneity of such topical sunscreen films is critical to improving the function of these films for successful UV protection.