Comparative behavior between sunscreens based on free or encapsulated UV filters in term of skin penetration, retention and photo-stability

Background: The growing incidence of photodamaging effects caused by UV radiation (e.g. sunburn, skin cancer) has increased the attention from health authorities which recommend the topical application of sunscreens to prevent these skin damages. The economic stakes for those companies involved in this international market are to develop new UV filters and innovative technologies to provide the most efficient, flexible and robust sunscreen products. Today the development of innovative and competitive sunscreen products is a complex formulation challenge. Indeed, the current sunscreens must protect against skin damages, while also being safe for the skin and being sensory and visually pleasant for the customers when applied on the skin. Organic UV filters, while proposing great advantages, also present the risk to penetrate the stratum corneum and diffuse into underlying structures with unknown consequences; moreover, their photo-stability are noted thorny outcomes in sunscreen development and subsequent performance. In recent years, the evaluation of the interaction between skin and sunscreen in terms of penetration after topical application has been considered from European authority but still its testing as their photo-stability assessment are not mandatory in most countries. Objective: This study, based on in-vitro approaches, was performed to evaluate and compare the retention and the penetration of organic UV filters in free or encapsulated form inside the skin as well as their respective photostability. Methods: Sunscreen formulation with a combination of Avobenzone and Octocrylene in free form and a formulation using the same UV filters but encapsulated in a sol-gel silica capsule, were analyzed and compared by FTIR Imaging Spectroscopy. Tape stripping method was used to investigate the penetration of these UV filters inside the stratum corneum. Their photo-stabilities were evaluated by spectroscopic measurements (FTIR, UV/Vis) and standard measurements were calculated: AUC (Area Under the Curve) and SPF (Sun Protection Factor). Result: With traditional formulation, the organic UV filters penetrated significantly into the stratum corneum while the same UV filters combined with encapsulation technology remained on the skin surface. The encapsulation technology also improved significantly their stability. Conclusion: Encapsulation technology is a promising strategy to improve the efficacy of sunscreen product using organic UV filters and to reduce safety problem. On the other hand, this study highlighted the pertinence of the FTIR Spectroscopy to test, compare and investigate sunscreen formulations.