Analysis of Human and Porcine Skin in vivo/ex vivo for Penetration of Selected Oils by Confocal Raman Microscopy

Background: The subject of oil penetration into the skin is controversially discussed in the scientific literature. Methods: Confocal Raman microscopy was used for analyzing oil penetration into the skin. The following methods were applied in the study: methods based on tracking specific peaks (method 1), the nonrestricted multiple least square fit (method 2), analyzing the lipid-to-keratin peak ratio using the perpendicular drop-down cutoff procedure (method 3), and the Gaussian function-based deconvolution procedure (method 4). Results: The results obtained using methods 1, 2 and 4 show that the investigated oils do not penetrate deeper than 11 µm into human and porcine skin. Petrolatum has a prominent swelling effect on the stratum corneum (32% in vivo, 28% ex vivo), while the other oils exhibit no significant swelling effect. By using method 3, the penetration profile of oils, and especially of petrolatum, into the skin was interpreted incorrectly for various reasons that are addressed herein below. Conclusion: Predominantly remaining in the uppermost corneocyte layers of the stratum corneum, topically applied oils do not reach the viable cells of the stratum spinosum. To exclude any possible mistakes when using the lipid-keratin Raman peak (2,820-3,030 cm -1 ), the penetration analysis should be performed using the Gaussian function-based deconvolution procedure.