In vitro evaluation of the permeation through reconstructed human epidermis of essentials oils from cosmetic formulations

The permeation of essential oils through SkinEthic^® reconstructed human epidermis, (RHE), was studied in vitro to establish a convenient tool to monitor the kinetics of release of active principles from cosmetic formulations. Twelve days old human epidermis held on polycarbonate disks was revitalized by addition of growth medium and incubated at 37 °C in 5% CO₂ atmosphere for five days prior to investigation. A system of six custom designed glass Franz-type diffusion cells were used for the permeation studies at 34 °C. The diffusion kinetic for 8 selected terpenes (camphor, carvone, 1,8-cineole, linalool, menthol, α-thujone, menthone, t-anethole), chosen as analytical markers of a mixture of plant essential oils contained in a cosmetic formulation, was probed by HS/SPME–GC–MS analysis and elaborated according to Fick's first law to obtain skin permeability coefficients (P_S = 1.51, 1.47, 1.36, 0.80, 0.62, 0.40 and 0.14 × 10⁻³ cm/h, respectively). The method proved to be sensitive, simple and reproducible, and RHE represents a convenient model for safety/quality assessment of cosmetic formulations.