Invisibility with squid skin
Using a protein from colour-changing squids, US scientists have created a coating that reflects infrared and lets objects mimic the infrared signature of their surroundings. The developers of the coating say that it could be used to create camouflage that would make people and vehicles invisible to infrared cameras.
Conventional visual camouflage can help the wearer blend in to the background, but it is notoriously difficult to hide from infrared sensors. 'We're trying to develop something that you could essentially use as reconfigurable infrared reflective paint so that you'd be able to disguise yourself,' says Alon Gorodetsky from the University of California, Irvine, who led the research. 'There's really not much out there in terms of inexpensive, biodegradable non-toxic materials that can be changed on the fly.'
The team turned to some of nature's best camouflage artists - the cephalopods - for inspiration. Pencil squids, for example, have specialised skin cells that can reflect both visible light and infrared, thanks to a protein called reflectin. Layers of reflectin-containing cells act like flexible mirrors which reflect different wavelengths of light. By contracting their muscles, the squids can change the shape of these cells which allows them to change colour.
The team used thin films of reflectin to make a stripped down mimic of squid skin, which they deposited onto glass slides. They then investigated how different substances could chemically alter and 'tune' the film's structure and reflective properties.
'You can swell these types of film and get them to shift their colouration,' explains Gorodetsky, 'so we screened different stimuli to see how far we can actually shift it across the visible and infrared spectrum.'
They found that exposing the film to acetic acid vapour caused it to swell enough to reflect infrared radiation, allowing it to 'disappear' when viewed with an infrared camera. The technology is still at an early-stage, but Gorodetsky thinks it shows promise as a next-generation camouflage coating. 'It hits the range that you would typically use for an infrared imaging camera, so in a proof of concept experiment it works.'
The next challenge, he says, is to find a more user-friendly trigger. 'You wouldn't necessarily want to use acetic acid - it's effectively dousing yourself with very concentrated vinegar!' he says. 'We'd like to find another stimulus - perhaps something mechanical or electrical - to induce the same change in colouration.'