Influence of physical-mechanical properties on SPF in sunscreen formulations on ex vivo and in vivo skin.

Abstract The sun protection factor (SPF) is related to the selected UV filters. The objective of this study was to evaluate and compare the rheological behavior and texture profile of two sunscreen formulations and to correlate these data with the obtained SPF values. Two formulations (F1 and F2) were developed with the same type and amount of UV filters – whereby one of them also contained ethoxylated lanolin as additional film former (F2). Their rheological behavior, texture profile and in vitro and in vivo SPF were analyzed. The film-forming properties were evaluated by skin profilometry and diffuse reflectance spectroscopy. The structures of the formulations were examined by two-photon tomography combined with fluorescence lifetime imaging, and the penetration profile into the stratum corneum was investigated by tape stripping. The formulation with lanolin presented lower and constant values for physical-mechanical parameters, with a higher and better reproducible SPF. Both formulations did not penetrate the viable epidermis. In conclusion, formulations with better surface deposition on the skin surface can influence the film formation and, consequently, improve the SPF. These findings are important to improve the efficacy of sunscreen formulations and reduce the addition of UV filters.