Emulsion versus nanoemulsion : how much is the formulative shift critical for a cosmetic product?

The use of nanoemulsions in cosmetic products has been enlarged in the last decades because of several formulative advantages (e.g., the improved self-life stability, better texture properties). In addition, nanoemulsions seemed to improve the penetration of active ingredients through the human skin, comparing to conventional emulsion. In this contest, the risk of a higher systemic exposure of consumer to active ingredients, due to the ability of nanoemulsion to enhance permeation, results a critical attribute that should be evaluated for assuring the consumer safety. The aim of this work was the evaluation of how an oil-in-water (O/W) nanoemulsion can influence the in vitro skin permeation profiles of two model active ingredients with different polarity (i.e., caffeine and ethyl ximenynate). Preliminarily, since both selected molecules influenced the physical stability of nanoemulsion, formulative studies were carried out to identify the most stable formulation to perform in vitro permeation studies. The overall results demonstrated that nanoemulsions could significantly influence the permeation profiles of molecules as a function of their physicochemical properties. In particular, O/W nanoemulsions significantly improved the permeation profiles of apolar active ingredients in comparison to conventional emulsions, whereas no differences were observable for polar molecules. Considering such findings, it is worth observing that there is room for reconsidering the risk assessment of nanoemulsion-based cosmetic products.