Light-triggered rupture of shellac/polydopamine composite coatings

Triggered release is an elegant approach to achieving “just-in-time” delivery of active substances. Nevertheless, most previous research has focused on hydrogels, leaving nonaqueous organic coatings that are capable of on-demand rupture relatively unexplored. The current investigation examined new light-triggered coatings based on shellac—a natural resin derived from the secretion of insects—modified with Jeffamine® to enhance its mechanical durability and stability. Light sensitivity was appended to the modified shellac by embedding nanoparticles (NP) of polydopamine, a polymer with excellent photothermal capabilities that emits heat when exposed to light. Combining a low percentage of polydopamine (<1 wt%) with the composite films facilitated their rapid rupture upon irradiation with green light. We demonstrate that the NPs are evenly distributed in the shellac matrix, and do not form a heat conduction network. Thus, heat generated by the nanoparticles does not dissipate, but rather increases the shellac's temperature at the shellac–NP interface until the shellac melts or degrades. Thermal degradation mainly occurs in areas directly irradiated with green light, where heat is generated. This mechanism opens possibilities for precisely controlling the degradation site and extent of the film using a focused light source. This capability may pave the way for using such remote releases from encapsulated devices at desired times and locations.