Engineering a polymer network to act as active camouflage on demand

The animal kingdom is full of creatures with active camouflage. What looks like a drab pile of sand and rocks might actually be a brightly colored squid, expanding and contracting structures within their skin to reveal shades of brown and gray instead of vibrant blue and yellow. Known as chromatophores, these cells can expand and retract internal reflective plates in response to external stimuli, allowing the animal to match the colors and patterns of their surroundings, and disappear in an instant.

Multicolored, multisized circles forming a patttern to serve as camouflage.
The researchers’ artificial chromatophores consist of membranes stretched over circular cavities attached to pneumatic pumps. Pressurizing the cavity stretches the membrane, changing the pitch of the helix-shaped liquid crystal inside. Correlating the relationship between diameter, pressure, pitch and color, the researchers are able to treat each cavity like pixel, shifting its color to match the surrounding pattern in this demonstration from their recent study.

Now, researchers at the School of Engineering and Applied Science are taking inspiration from this kind of active camouflage. Using thin, flexible membranes made from a polymer network of liquid crystals that are arranged in helical shapes, these researchers have developed a kind of artificial chromatophore that can change colors instantly—from near-infrared to visible to ultraviolet—on command.

These membranes are situated over tiny cavities arranged in a grid, each of which can be pneumatically inflated to a precise pressure. As a cavity inflates, the membrane is stretched, shrinking its thickness and shifting its apparent color.

Critically, these membranes do not need to be stretched much to achieve this effect. Using an amount of pressure equivalent to a gentle touch, their color can be changed to anything within the visible spectrum.

The study, published in Nature Materials, is led by Shu Yang, Joseph Bordogna professor and chair of the Department of Materials Science and Engineering, and Se-Um Kim, then a postdoctoral researcher in her lab. Fellow Yang lab members Young-Joo Lee, Jiaqi Liu, Dae Seok Kim and Haihuan Wang also contributed to the research.

Read more at Penn Engineering Today.