Interactions between UV filters and active substances in emulsion: Effect on microstructure, physicochemical and in-vivo properties.

The objective of the present study was to develop, characterize and evaluate the clinical efficacy of topical formulations containing or not active substances and UV-filters, separate and in combination. To this end a stable formulation was developed to which four aqueous active substances and four lipophilic UV-filters were added. The formulations were then submitted to microscopic characterization by light microscopy, to particle size measurement, and to macroscopic characterization by rheology and texture analysis. Finally, a clinical efficacy study was conducted to determine the effect of the formulations on the skin after application for 1 h. The formulation containing UV-filters showed a high polydisperse microstructure and a large amount of liquid crystals. The formulations containing active substances showed higher resistance to deformation, compression and penetration tests. Regarding spreadability, formulations containing UV filters alone or in combination with active substances showed higher resistance to spreading. This behavior was associated with greater clinical efficacy in terms of stratum corneum water content, protection of the skin barrier function and skin surface brightness. It was demonstrated that the efficacy of the formulation is mainly associated with its structure and the way it interacts with the skin surface. Finally, this study showed that the mixture of these ingredients for the development of multifunctional sunscreens improves the performance of the formulations.