Water-in-water emulsions as a promising platform for UV-protective sunscreen formulations

Sunscreens have primarily been developed in the form of oil-in-water (O/W) or water-in-oil (W/O) emulsions to protect against ultraviolet (UV) radiation, which can cause sunburn and increase the risk of skin cancer. However, the oils used in these formulations have raised concerns due to their environmental impact and potential toxicity. In particular, concerns about the cytotoxicity and ecological effects of lipophilic UV filters have been increasingly highlighted. To address these issues, this study introduces, for the first time, an innovative sunscreen system that completely excludes oil. The proposed system is based on an aqueous biphasic water-in-water (W/W) emulsion, which eliminates oil-related issues while offering an eco-friendly and biocompatible solution. This system can incorporate water-soluble UV filters and inorganic particles, functioning as an effective sunscreen. Compared to colloidal aqueous suspension formulations, it shows excellent UV protection performance and high uniformity in application over the entire surface. In particular, W/W emulsions with 15 w/v% TiO₂ achieved an in vitro SPF of about 40, surpassing the value of about 30 observed for a commercial SPF 50 product. While suspensions became increasingly non-uniform as the TiO₂ concentration increased, reaching a standard deviation of 0.148 at 15 w/v%, the emulsions maintained a much lower deviation of 0.041, demonstrating superior film uniformity even at high particle loadings. This innovative approach significantly expands formulation options for sunscreen development and opens new avenues for aqueous-template-based sunscreen technologies.