Co-Encapsulation of Retinol and 3-O-Ethyl Ascorbic Acid in Dual-Loaded Ethosomes Produced by High-Pressure Microfluidization: Characterization, Stability and In Vitro Release

Combining retinol and vitamin C (Vit C) in one topical formulation is challenging: retinol is lipophilic and easily oxidised, whereas Vit C is hydrophilic, and both are chemically unstable. This study aimed to co-encapsulate retinol and a stable Vit C derivative, 3-O-ethyl ascorbic acid, in ethosomes produced by high-pressure microfluidization. The ethosomes were characterised by particle size, polydispersity index, zeta potential, pH and transmission electron microscopy, and their physical stability was followed for 60 days at 2–8 °C, 25 °C, 30 °C and 40 °C. In vitro release through a cellulose acetate membrane and permeation across the synthetic skin-mimetic Strat-M® membrane were studied in Franz diffusion cells and compared with a solution and a conventional gel. The loaded ethosomes were spherical, with sizes of approximately 140–160 nm that remained essentially unchanged for 60 days at 2–8 °C, whereas samples stored at 30 °C and 40 °C had markedly larger vesicles; refrigerated storage is required. Encapsulation efficiency was 98.9% for retinol and 33.1% for Vit C. Expressed as a percentage of the applied dose, the cumulative amounts of both actives released or permeated from the ethosomal product over 2 h were significantly lower than those from the solution and the gel (28–50% of the solution values at 120 min; p < 0.001). Microfluidization is a simple and suitable method for producing ethosomes that co-encapsulate hydrophilic and lipophilic anti-ageing actives with a release-retarding effect. Because synthetic membranes were used, ex vivo and in vivo studies are needed to confirm skin delivery.