Squid Inspired Camouflage Coatings
By Kate Wheeling
BU News Service
Camouflage patterned uniforms help soldiers vanish into their surroundings during the day, but at night, under infrared imaging, those same uniforms stand out against the environments they were fashioned to resemble. Now researchers at the University of California-Irvine and Caltech found a novel solution to this problem in the animal kingdom’s master of camouflage – the pencil squid.
Squid use a combination of reflective and pigmented cells to control skin coloration and blend into the background. The reflective cells create red, orange, yellow, green and blue color based on the angle that light hits them – the same process makes soap bubbles appear to change color in sunlight. Brown, red and yellow pigmented cells overlay the reflective cells and filter the light that reaches them. Squid use these two cell types in concert to produce colors than span the entire visible spectrum. Researchers zeroed in on a protein called reflectin, a main component of squid reflective cells and demonstrated its practical applications for stealth technologies in a study published July 30 in Advanced Materials.
The team, led by Alon Gorodetsky, Assistant Professor at UCI, created reflectin-coated thin films with tunable reflectance properties – they could manipulate the films to make them appear and disappear under infrared light. To begin, the team engineered E. coli to express a copy of the reflectin protein – a common strategy in protein engineering to produce and purify large quantities of a desired protein. They integrated purified reflectin onto glass by a process akin to spackling a wall. An adhesive layer between the glass and protein coating ensured the protein stuck to the glass and spread out uniformly. Glass is one of many materials options for this technology. This adhesive layer could be used to integrate the protein with virtually anything – plastic, paper, or even cloth. Once assembled, the thin films appeared orange under visible light, but it was their color under infrared light that interested the researchers.
The team then used a chemical trigger, acetic acid vapor, to tune the reflectance of the films over a large wavelength range – beyond what squid can even do, according to Lydia Maethger, an Assistant Research Scientist at the Marine Biological Laboratory in Woods Hole. Researchers compared the infrared reflectance of the films to leaves, which reflect in the infrared and thus appear red when viewed with infrared cameras. When the researchers looked at the reflectin-coated glass with infrared cameras it appeared black. But when they exposed it to acetic acid vapor it appeared red. These changes were reversible, meaning that Gorodetsky and his team could alter the reflectance to match multiple environments.
This is only the first step towards the application of biological camouflage coatings. “Now we need to modify our approach to develop something that’s a little bit more robust and easier to use,” explains Gorodetsky. Acetic acid vapor triggers a reflectance shift well in the lab, but it’s not the best option for the real world. The group plans to look for other chemical or mechanical approaches to induce the same reflectance changes.
Camouflage coatings also need to stand up to the elements. While Maethger is impressed with the tunable range the researchers achieved, she cautions, “If this kind of thing is to be used in the field, it would have to be able to handle a lot of stress.” This isn’t a hard problem to solve according to Gorodetsky. Reflectin is already fairly tough and the thin films could be strengthened by cross linking the proteins.
But robustness won’t be a problem at all if the technology develops the way Gorodetsky envisions it now, “Really I see it as a disposable coating, maybe even something you could put in an aerosol can and spray yourself with and then once you no longer need camouflage, you just get rid of it” by wiping it off or changing clothes.