Directional assessment of the skin barrier function in vivo

INTRODUCTION: The fundamental function of the epidermis is to provide an inside‐out barrier to water loss and an outside‐in barrier to penetration of external irritants. Transepidermal water loss (TEWL) has been extensively used as a method of estimating the skin barrier quality, typically without any consideration of directionality. The validity of TEWL as an estimate of skin permeability to external substances has been controversial in vitro and in vivo. The aim of this work was to assess the relationship between TEWL and the penetration of a topically applied external marker (caffeine) in healthy skin in vivo before and following a challenge to the barrier. METHODS: The skin barrier was challenged by application of aqueous solutions of mild cleanser products under occlusion for 3 h on the forearms of nine human participants. Skin barrier quality was evaluated before and after the challenge by measuring the TEWL rate and the permeated amount of topically applied caffeine using in vivo confocal Raman microspectroscopy. RESULTS: No skin irritation was observed following the skin barrier challenge. TEWL rates and the caffeine penetrated amount in the stratum corneum after the challenge were not correlated. A weak correlation was observed when the changes were corrected to water‐only treatment. TEWL values can be influenced by environmental conditions as well as the skin temperature and water content. CONCLUSIONS: Measuring TEWL rates is not always representative of the outside‐in barrier. TEWL may be useful in differentiating large changes in skin barrier function (e.g., between healthy and compromised skin) but is less sensitive to small variations following topical application of mild cleansers.