The sun protection factor (SPF) depends on the selected ultraviolet (UV) filters. The present study evaluated and compared the rheological behavior and texture profile of two sunscreen formulations and correlated these data with the obtained SPF values. It developed two formulations (F1 and F2) with the same type and amount of UV filters, where one of them also contained ethoxylated lanolin as an additional film former (F2). The study then analyzed their rheological behavior, texture profile and in vitro and in vivo SPF. It evaluated the film-forming properties by skin profilometry and diffuse reflectance spectroscopy, examined the structures of the formulations by two-photon tomography combined with fluorescence lifetime imaging and investigated the penetration profile into the stratum corneum by tape stripping. 

• The formulation with lanolin showed lower and constant values for physical-mechanical parameters, with a higher and better reproducible SPF.
• Both formulations did not infiltrate the viable epidermis.

This study shows that formulations with better surface deposition on the skin surface can influence film formation and thus improve the SPF. These findings are essential to improve the efficacy of sunscreen formulations and reduce the addition of UV filters.

Source: Infante VHP, Maia Campos PMBG, Calixto LS, et al. Influence of physical-mechanical properties on SPF in sunscreen formulations on ex vivo and in vivo skin. Int J Pharma. 2021;598:120262