This bionic butterfly changes color as it flaps

The lesser purple emperor butterfly (Apatura ilia) appears to morph from dull brown to iridescent blue as it flies. The color change is caused by microscopic structures on the butterfly’s wings that reflect light differently depending on the angle of the wings. Inspired by this flap-driven transformation, Mingzhu Li, a chemist at the Chinese Academy of Sciences in Beijing, saw an opportunity to make a color-changing robot from a thermoresponsive hydrogel. (Cell Rep. Phys. Sci. 2022, DOI: 10.1016/j.xcrp.2022.100915). Li and her colleagues started with a photonic crystal film that reflects the full rainbow of the visible light spectrum. Like emperor butterfly wings, the perceived color of the film depends on its orientation to a light source. Next, they made an artificial muscle using a hydrogel that contracts when heated with near-infrared light, then relaxes with a spritz of water. When the researchers joined the materials together, the hydrogel lifts the reflective film up and down with alternating cycles of heat and spritzing. The researchers used this architecture to make a bionic butterfly that changes color from drab to dramatic as it beats its wings. Li and her team found that the hydrogel can contract and relax hundreds of times in a row, which suggests the design has the durability to serve as a useful component for imaging arrays and soft robots.