Sebum and stratum corneum lipids increase human skin surface free energy as determined from contact angle measurements: A study on two anatomical sites

Abstract

Skin surface wettability is an important factor of the skin protective function, e.g. ecosystem preservation, smoothness and resiliency, barrier to chemicals, but surprisingly it has received little attention. Hence, this work aimed at calculating the surface free energy components of human skin, on the volar forearm, a sebum-poor area, and on the forehead, a sebum-rich area, and at assessing the influence of skin lipids on surface wettability using advancing contact angle measurements of water, glycerol, formamide and diiodomethane. Contact angles on the forearm and the forehead of ten healthy volunteers were measured, using a surgical microscope fitted with a slanted mirror and equipped with a video camera linked to a computer. Both skin areas were examined before and after ether treatment. According to the approach of van Oss et al. to solid-liquid interfacial interactions, the surface free energy components, i.e. Lifshitz-van der Waals, electron acceptor and electron donor components, were calculated. From the contact angle measurements and calculations of surface free energy, the forearm skin surface was found to be an almost apolar surface and the forehead skin surface a monopolar basic surface (electron donor). Ether treatment caused both surfaces to become more apolar, mostly by reducing the electron donor component. These results would suggest that skin surface lipids, mainly sebum, give the skin surface a hydrophilic character. However, no correlation was found between the skin lipid level and the basic component level which raises the question of the mechanism of the obvious wetting-enhancing effect of sebum. This could be due to the free fatty acids, which are especially abundant in the sebum.