Biomimetic UV photo-protection of skin surface by structured epicuticular wax films

The realms of biomimicry encourage us to explore and replicate the remarkable functionalities found in the big variety of living organisms such as plants, birds, animals etc. Inspired by the ultraviolet (UV) reflective characteristics of self-assembled epicuticular wax of plant leaves, in this article, we present a biomimetic plant-inspired approach to pattern the surface of skin with wax coatings and enhance its UV resistance. Through a physico-chemical approach, we coat chemically homogenous (as well as heterogenous) chemical composition of waxes from its solution on quartz substrate. By controlling the self-assembly conditions, diverse surface morphologies are obtained with Euphorbia cerifera (commonly known as Candelilla, chemically heterogenous wax) and Myristyl Palmitate (present in Phytolacca Acinosa, chemically homogenous alkyl esters wax). Optical measurements show increased reflectance in visible spectra for Candelilla wax coatings exhibiting globules, plate-like crystalline structures at the surface which contributes to higher roughness parameters. With homogenous wax, maximum reflectance is obtained for dual scale morphology which includes self-assembled 3D plate-like structures at an optimum length-scale. Our experiments reveal that the combined effect of vertically and horizontally placed stacks of crystal plates at micron and sub-micron scales induces maximum scattering effect. This geometrical organisation effectively decreases transmission of incident radiations to the underlying surface leading to enhanced photo-protection. Further, to showcase the feasibility of such approach for potential cosmetic applications, we replicate best performing structures on a commonly used model skin surface for UV absorption evaluation (polymethyl methacrylate plates) and on real ex-vivo Stratum Corneum, the outermost layer of skin. For realistic substrates, scattering effect is additionally dependent on nature of intrinsic substrate patterns and roughness in which case the feature height (h_F ) of 3D structures should be greater than substrate patterns to achieve maximum reflectance. This study highlights the formation of physical structuration with biomaterials and presents insights on scattering induced by such plant-based structures with potential for dermatological or cosmetic applications.