Synchrotron FT-IR microscopic study of chemical enhancers in transdermal drug delivery: example of fatty acids.

This article illustrates the analysis by synchrotron infrared microscopy of skin treated with penetration enhancers. Pig skin was treated with two fatty acids commonly employed as penetration enhancers, palmitic (C16) and myristic (C14) acids, in propylene glycol (PG). The use of perdeuterated fatty acid chains enabled the penetrating molecules to be perfectly distinguished from the endogenous lipids due to the difference between C-D and C-H stretching modes. Palmitic acid was detected in the stratum corneum (SC), a particularly alkyl-rich region, whereas myristic acid penetrates deeper into the epidermis. Similar experiments with lead and calcium soaps were also performed, but no detectable signal was observed, indicating a much weaker penetration. Additionally, the C-D2 stretching frequency provides information about the conformational order of the penetrating molecules inside the skin. The results indicate that fatty chains are in an ordered state. The improved spatial resolution allows the determination of both chemical composition and distribution in the different layers, from the SC to the dermis. Copyright 2004 Elsevier B.V.