Altering Silk Film Surface Properties through Lotus‐Like Mechanisms

The nonwetting and self‐cleaning properties of the lotus depend on microscale and nanoscale roughness provided in part by a covering of epicuticular waxes that crystalize on the surface of its leaves. Wax deposition is driven by the evaporation of water, which carries waxes to the surface as it moves through the epidermis and cuticle. If the wax layer is damaged, repair occurs through the same mechanism. The experiments described herein have exploited this principle to establish a completely biologically derived system based on silk and lotus epicuticular wax, showing that it is possible to coat silk surfaces with waxes and thereby change their wetting characteristics and tensile properties. The robustness of the material is also documented by crystal regrowth after damage to the wax layer through abrasion (scratching and rubbing), resistance to water‐jetting, and UV exposure. To further characterize this system, the diffusion of natural and synthetic waxes through two types of silk films, Bombyx mori fibroin and engineered spider silk are studied, showing that the extent of wax diffusion through silk membranes depends upon wax type and protein structure, which remains unchanged through the process. Making use of the simple passive phenomenon of advection, these studies represent a method of low‐energy fabrication of completely biological, lotus‐inspired membranes with tunable surfaces.