It’s hard to stump a cuttlefish.
The eight-armed relative of the squid and octopus is a genius with disguise, displaying an ability to blend into nearly any background. While swimming along the sandy seafloor, for instance, its skin turns a light tan, a perfect match for the ground beneath it. Nestled into a bed of gravel, the animal’s skin turns a mottled black and white. As it approaches a mass of algae, this black and white pattern slowly fades out and is replaced by a brownish green hue.
One scientist even tested a cuttlefish’s camouflage skills by putting it in a tank with a black-and-white checkerboard pattern. The cuttlefish responded by displaying crude white squares on its back.
Soon, we may be able to perform similar feats, thanks to the work of Jonathan Rossiter and Andrew Conn, engineers at the University of Bristol in the UK. The pair have now engineered soft materials that that mimic a cuttlefish’s colour-changing skin, paving the way for “smart clothing” that might help us take camouflage to a whole new level.
The cuttlefish owes its talents to millions of specialised cells packed under its skin called chromatophores, which contain miniature sacs full of black, brown, yellow or other coloured pigment. When the cuttlefish’s brain sends the signal to change colour, muscles surrounding the sacs quickly contract, stretching the sacs and allowing the pigment inside to cover a larger surface area – with the result that the colour and pattern of the animal’s skin appears to change to the outside observer. Because of the tight coupling between the brain and the nerves surrounding the chromatophores, this remarkable transformation happens at lightning speed, allowing wild cuttlefish to quickly change colour to evade predators, sneak up on prey, or attract mates.
Rossiter and Conn set out to replicate this process by engineering artificial chromatophores out of small devices known as dielectric elastomer actuators. A DEA, as it’s known, consists of a thin, rubberised membrane sandwiched between two flexible electrodes. The electrodes are connected to a power supply; flip the switch and electricity goes surging into the system. The electrodes take on opposite charges and attract each other, squeezing the rubber membrane between them.
To create a colour-changing effect, Rossiter and Conn built a clear “artificial skin” studded with circular DEAs, each of which contained a thin disc of black rubber sitting between two circular electrodes. When the researchers turned on the power supply, the electrodes squashed the black membranes, which expanded in surface area as they were flattened. “You have a small spot, and you apply electricity and it becomes a big spot,” Rossiter says. Expand all these black spots at the same time, and it looks like the skin is suddenly darkening. The colour change happens fast, within “tens of milliseconds,” says Rossiter.
While they were at it, the researchers also decided to model a second colour-changing organism: the zebrafish, a black-and-white-striped swimmer than can make itself appear darker in response to environmental stimuli. In zebrafish, cells known as melanophores contain reservoirs of black fluid; muscle contractions pump this fluid to the skin’s surface, where the liquid spreads out, darkening the fish. (This process is triggered by hormones – rather than directly by the brain – so the colour change in zebrafish takes longer than it does in cuttlefish. In zebrafish, “there’s a reasonably slow diffusion of chemicals through the body,” says Rossiter, “but they produce a really striking visual effect.”)
To mimic the zebrafish, Rossiter and Conn constructed an artificial ink cell by mounting a flat, oval-shaped silicone display chamber between two glass microscope slides. They used a thin tube to connect a well of black ink to the display chamber; when the researchers gave the command, a small pump pushed the black fluid into the transparent chamber, turning it from clear to an opaque black. To reverse the effect, the researchers turned on a second pump, which sent a supply of clear liquid into the chamber and pushed the black fluid back into its reservoir. (The organic clear fluid and the black water-based ink do not mix.)
Just like in nature, the zebrafish-inspired chromataphores work more slowly than those modelled on the cuttlefish, but the final effect is dramatic. In just a few seconds, the transparent display chamber turns a rich, dark black.
The next step, Rossiter says, is to refine the bio-inspired chromataphores and to network more and more of them together. “We’ll put these things together to produce larger arrays and at the same time we’ll miniaturise them,” he says. By layering different chromatophores and pigment cells, engineers could create a soft material capable of a variety of sophisticated effects, including multiple colour changes and moving patterns that appear to ripple across the fabric.
Such colour-changing materials have obvious military applications, allowing soldiers to blend into different environments with the flick of a switch. But, Rossiter points out, we could also use such clothing to make ourselves more conspicuous. “In modern society,” he says, “you may want to display yourself.” Imagine, for instance, a shirt that turns bright orange when you’re walking or biking along a heavily trafficked road. Or a jacket that can flash bright colours if you get lost in the woods, making it easier for search parties to spot you.
For his part, Rossiter imagines teens coveting cuttlefish-inspired clothing simply for fun and fashion. They could even go out to a nightclub and put on colour-changing displays to impress potential mates, just like their cephalopod cousins.