Cooperative Interaction of Monoterpenes and Phenylpropanoids on the in vitro Human Skin Permeation of Complex Composed Essential Oils

For essential oils, several biological effects such as antibacterial, anti-inflammatory, expectorant, and blood-circulation-enhancing properties have been described. The method of application depends on the pathophysiology, the desired outcome, safety, and toxicity data. For treating respiratory symptoms and nervous disorders, inhalation may be the best means of application, whereas topical application is the best way for treating skin diseases. For dermal application, percutaneous absorption of essential oil and oil components is of great interest. Essential oils are complex mixtures of different volatile Substances. So, the question is raised whether all components of a complex composed essential oil are equivalent with respect to their human skin permeation. By means of artificial mixtures of different essential oil ingredients, we investigated the cooperative effect of monoterpenes and phenylpropanoids on the permeation through heat-separated human skin epidermis in static Franz-Diffusion Cells. Limonene showed an enhancing effect on the permeation of citronellol and eugenol. Both a-pinene and myrcene increased the apparent permeability (P-app) value of phenylethanol. P-Pinene had an enhancing effect on the permeation behaviour of methyleugenol but not of geraniol. The investigations clearly show that cooperative effects of single essential oil components may influence percutaneous essential oil absorption.