In vivo human skin permeability enhancement by oleic acid: transepidermal water loss and fourier-transform infrared spectroscopy studies

Abstract

The skin barrier function can be evaluated by measuring the rate of transepidermal water loss (TEWL). This technique can also be used to elucidate the effect of enhancers on the skin barrier. In this in vivo study the effect of oleic acid in propylene glycol (OA/PG) has been investigated on 10 human volunteers of both sexes (age: 20–40 years). A 3-h and a 24-h occlusion of 0.16 M OA/PG resulted in a 2-fold increase in TEWL, whereas PG alone only increased TEWL with a ratio of 1.1. An optimal enhancement of TEWL has thus been achieved from the synergistic working of OA and PG. Furthermore, the enhancing effect lasted long: for a 3-h and a 24-h application, the TEWL increase could be monitored up to 25 and 72 h, respectively. The results indicate that OA remains inside the stratum corneum and is able to maintain its effect for a relatively long, yet limited, period of time. Another in vivo technique, attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) has been employed to follow the effect at a molecular scale. Following the absorbances at 1710 and 1740 cm −1, representing the carboxyl group of the free acid and of the ester, respectively, it was observed that the absorbance ratio of both wavenumbers is levelled off to that of an untreated condition already 3 h after the removal of OA/PG application system. These results may indicate either the migration of OA into lower cell layers or lateral spreading within the stratum corneum. Comparing the enhancement ratios from TEWL measurements of the OA/PG applied sites to the untreated sites with the ratio of absorbance at 1710 and 1740 cm −1 in time, there is a congruency with the disappearance of free acid in the uppermost layers of SC and the stabilization of the increased TEWL value (`steady-state' water diffusion). This suggests that the barrier perturbation by OA `outlives' its presence in the superficial cell layers in the free acidic form.