Engineers at the Massachusetts Institute of Technology have demonstrated a fast, scalable, and cost-effective method to produce films that change color when stretched (Nat. Mater. 2022, DOI: 10.1038/s41563-022-01318-x). Using a digital projector and commercially available light-sensitive polymers, they were able to create large sheets of the material, which could find use in fields such as medicine and robotics.
Study lead author Mathias Kolle of MIT’s mechanical engineering department designs photonic materials using bio-inspired techniques. His team had been looking for ways to replicate structural color found in nature. Such colors result from nanoscale structures that create wave interference effects when light travels through them. The team developed a simple technique to create a synthetic version of these natural structures at scale and added a twist: they made the material elastic. They envisage applications such as color-changing pressure bandages, touch screens, and wearable devices.
Serendipity played a role in the invention of the new material, says study coauthor Benjamin Miller, a graduate student in Kolle’s lab. He happened upon a 19th-century photographic technique developed by Nobel-winning physicist Gabriel Lippmann, who had produced color photographs by putting a mirror of mercury behind a transparent, light-sensitive emulsion and exposing it to light. The interference of the light waves reflected from the object being photographed and back from the mirror reconfigured the grains in the emulsion to create the image.
The MIT team adapted Lippmann’s technique with a digital projector and a commercially available holographic film (used for passport and credit card holograms) that contains two kinds of polymers. Light changes the densities of the polymers wherever the waves interfere, creating nanostructures to form a photonic crystal.
“Each individual pixel projected will create a little point of structural color in the material,” he says. The recorded image remains when the light source is removed. Because the material is elastic, these embedded nanostructures are deformed when stretched, changing the color of the reflected light.
Using their process, the researchers produced meters-long, 30 cm wide rolls of the film and are now working on how to bond the printed elastomers to textiles and other materials. They also want to tweak the optical properties of the material—to reduce iridescence, for example.
Lauren Zarzar, a chemist at Penn State University, says that while structural color materials aren’t new, fabrication methods have been cumbersome. The easy, accessible method described by Kolle’s team opens doors for others to explore new applications for these materials and fine-tune them. Working in some sort of nonreversible color change could also allow the material to be used in sensors that track mechanical force history, she adds.