Sweat resistance of sunscreens: Development of a perspiring skin simulator, evaluation of sunscreen failure mechanisms and enhancement of sunscreen substantivity

Sunscreen application is one of the most well-known solutions for the protection of human skin against harmful ultraviolet (UV) radiation from the sun. The substantivity of sunscreen (that is, the ability of the sunscreen to bind to the skin and resist removal), as an indicator of long-lasting protection against UV radiation, can be affected by a variety of activities, such as wearing clothes, swimming, and sweating, that may occur after the application of sunscreen on the skin. Sweating occurs in many situations in which people use sunscreen. However, the interaction of sunscreen and sweat is largely overlooked, likely owing to the inevitable challenges associated with performing in vivo sweat resistance tests and the lack of suitable instrumentation for in vitro studies. Besides active sweating, passive diffusion of water through the skin, known as transepidermal water loss (TEWL) can be affected by the application of topical films, such as sunscreen and polymeric film-forming systems. The aim of this PhD project was to develop specific in vitro setups to mimic human skin perspiration, and then to obtain general information on the impact of sweating on sunscreen substantivity, to evaluate the effect of different parameters on sunscreen failure mechanisms during sweating, and to explore methods to develop sunscreen formulations with greater sweat resistance. Moreover, a systematic evaluation of the breathability of film-forming systems applied on the skin was performed by using an in vitro TEWL simulator. The TEWL simulator was used to characterize the effect of the application of selected topical polymeric film formers on skin occlusion and TEWL. Subsequently, the in vivo TEWL studies were performed on two selected film formers. The comparison between the in vitro and in vivo TEWL studies confirmed that the TEWL simulator was able to predict the breathability of the polymeric filmforming systems. I used the perspiring skin simulator to perform both qualitative and quantitative evaluation of sunscreen film performance in response to sweating. The results showed that sweating negatively affected sunscreen substantivity and UV protection through wash-off and redistribution of the sunscreen film. Further, two approaches for increasing the sweat resistance of sunscreen were investigated: manipulation of the concentration of hydrophobic film-formers and incorporation of water absorbing particles in the sunscreen formulation. The results indicated that the combination of moderate concentrations of a hydrophobic film-former and water-absorbing particles capable of forming a gellike structure in contact with water could successfully increase the film integrity of sunscreen and control the sunscreen wash-off and redistribution by localizing the water pressure.