Adaptive camouflage skin spells out UOI, for University of Illinois. Credit: Yu et al. 2014. PNAS.
Adaptive camouflage skin spells out UOI, for University of Illinois. Credit: Yu et al. 2014. PNAS.

Adaptive Colour-Changing Sheet Inspired By Octopus Skin

The most amazing skins in the world can be found in the sea, stretched over the bodies of octopuses, squid and cuttlefish. These animals, collectively known as cephalopods, can change the colour, shape and texture of their skin at a whim—just watch the ‘rock’ in the video above suddenly reveal its true nature. Their camouflage is also adaptive. Unlike, say, a stick insect or stonefish, which are limited to one disguise, an octopus’s shifting skin allows it to mimic a multitude of backgrounds. It sees, it becomes.

No man-made technology comes close. But one, at least, is nudging in the right direction.

A team of scientists led by Cunjiang Yu at the University of Houston and John Rogers at the University of Illinois at Urbana–Champaign have developed a flexible pixellated sheet that can detect light falling upon it and change its pattern to match. So far, its large pixels can change from black to white and back again. It’s a far cry from an octopus’s skin, but it does share some of the same qualities. For example, it changes colour automatically and relatively quickly—not cephalopod-quick, but within a second or so.

“This is by no means a deployable camouflage system but it’s a pretty good starting point,” says Rogers. Eventually, his team are working towards adaptive sheets that can wrap around solid objects and alter their appearance. These could be used to make military vehicles that automatically camouflage themselves, or clothes that change colour depending on lighting conditions.

Cephalopod skins have three layers. The top one consists of cells called chromatophores, which are sacs of coloured pigment, controlled by a ring of muscles. If the sac expands, it produces a pixel of colour; if it contracts, the pixel hides. These cells are responsible for hues like red, orange, yellow and black. The middle layer contains iridophores, cells that reflect the colours of the animal’s environment—they’re responsible for cooler colours like blues and greens. The bottom layer consists of leucophores, passive cells that diffuse white light in all directions, and act as a backdrop for the other colours.

The skin also contains light-sensitive molecules called opsins, much like those found in your retina. It’s still unclear what these do, but a reasonable guess is that they help cephalopods to “see” with their skin, and adapt their patterns very quickly without needing instructions from their brains.

The team drew inspiration from these skins when designing their own material. It consists of a 16 by 16 grid of squares, each of which consists of several layers.

  • The top one contains a heat-sensitive dye that reversibly changes colour from black at room temperature to colourless at 47 degrees Celsius, and back again. This is the equivalent of an octopus’s chromatophores.
  • The next layer is a thin piece of silver, which creates a bright white background, like the leucophores.
  • Below that, there’s a diode that heats the overlying dye and controls its colour. This is the equivalent of the muscles that control the chromatophores.
  • Finally, there’s a layer with a light-detector in one corner, a bit like a cephalopod’s skin opsins. All the top-most layers—the dye and the silver—have little notches missing from their corners so that the light-detector always gets a unimpeded view of its surroundings.
  • And the whole thing sits on a flexible base so it can bend and flex without breaking.

So, the light-detectors sense any incoming light, and tell the diodes in the illuminated panels to heat up. This turns the overlying dye from black to transparent. These pixels now reflects light from their silver layer, making them look white. You can see this happening in the videos below.  Here, different patches of light are shining onto the material from below, and it’s responding very quickly.

“There are analogies between layers of our system and those in the cephalopod skin, but all the actual function is achieved in radically different ways,” says Rogers. “The multi-layer architecture works really well, though. Evolution reached the same conclusion.”

“The most exciting thing about this is that it’s all automatic, without any external user input,” he adds.

There are obvious military applications for the device and the work was funded by the Office of Naval Research. But Rogers notes that the sheets are designed to sense and adapt—they don’t necessarily have to blend in. “There are a lot of applications in fashion and interior design,” he says. “You could apply these flexible sheets to any surface and create something that’s visually responsive to ambient lighting conditions. But our goal is not to make adaptable wallpaper; it’s on the fundamentals.”

Obviously, the material will have to be improved. Since it relies on heat to change colour, it’s relatively slow, consumes a lot of power, and only works in a narrow range of temperatures. But the team used a heat-sensitive dye because it was easy; it gave them time to focus on the rest of the system.

Now that this framework is in place, they think they could improve it very easily. Rather than heating diodes, they could use components that use changing electric fields. They could replace the dyes with other substances that offer a full range of colours, beyond just black and white. And they should be able to scale the sheet up easily—Rogers, after all, has a lot of experience in building flexible electronics using commonly used substances like silicon, rather than fancy (and expensive) new materials.

But he doubts he’ll ever make something that truly matches a cephalopod’s skin. “As an engineer looking at movies of squid, octopuses, and cuttlefish, you just realise that you’re not going to get close to that level of sophistication,” he says. “We tried to abstract the same principles and do the best we can with what we’ve got.”

Does their artificial skin have any advantages over what an octopus or squid can do?

“Well, it works on dry land!” says Rogers.

Reference: Yu, Li, Zhang, Huang, Malyrchuk, Wang, Shi, Gao, Su, Zhang, Xu, Hanlon, Huang & Rogers. 2014. Adaptive optoelectronic camouflage systems with designs inspired by cephalopod skins. PNAS