There’s a famous viral video in which a diver slowly swims up to a clump of rock and seaweed, only for part of that clump to turn white, open its eye, and jet away, squirting ink behind it. Few videos so dramatically illustrate an octopus’s mastery of camouflage. But ignore, if you can, the creature’s color, and focus on its texture. As its skin shifts from mottled brown to spectral white, it also goes from lumpy to smooth. In literally the blink of an eye, all those little bumps, spikes, and protuberances disappear.
The project was entirely funded by the U.S. Army Research Office—and it’s not hard to imagine why. There are obvious benefits to having materials that can adaptively hide the outlines of vehicles and robots by breaking up their outlines. But there are other applications beyond military ones, Shepherd says. It might cut down on shipping costs if you could deliver materials as flat sheets, and then readily transform them into three-dimensional shapes—like flat-pack furniture, but without the frustrating assembly. Or, as the roboticist Cecilia Laschi notes in a related commentary, biologists could use camouflaged robots to better spy on animals in their natural habitats.
“I don’t see this being implemented in any real application for quite some time,” says Shepherd. Instead, he mainly wants to learn more about how octopuses themselves work, by attempting to duplicate their biological feats with synthetic materials. “I’m just a big nerd who likes biology,” he says.
Octopuses change their texture using small regions in their skin known as papillae. In these structures, muscle fibers run in a spiderweb pattern, with both radial spokes and concentric circles. When these fibers contract, they draw the soft tissue in the papillae towards the center. And since that tissue doesn’t compress very well, the only direction it can go is up. By arranging the muscle fibers in different patterns, the octopus can turn flat, two-dimensional skin into all manner of three-dimensional shapes, including round bumps, sharp spikes, and even branching structures.
Shepherd’s team—which includes the postdoc James Pikul and the octopus expert Roger Hanlon, who took the famous video at the start of this piece—designed their material to work in a similar way. In place of the octopus’s soft flesh, they used a stretchy silicone sheet. And in place of the muscles, they used a mesh of synthetic fibers that were laid down in concentric rings. Normally, the silicone membrane would balloon outward into a sphere when inflated. But the rings of fibers constrain it, limiting its ability to expand and forcing it to shoot upward instead.
By changing the layout of the fibers, the team could create structures that would inflate into various shapes, like round bumps and pointy cones. Pikul grabbed a stone from a local riverbed and programmed the material to mimic its contours. He set the material to create hierarchical shapes—lumps on lumps. He even programmed it to duplicate the more complicated contours of a field of stones, and a plant with spiraling leaves.
For the moment, the material can only be programmed to mimic one predetermined shape at a time. Still, “the results are impressive,” writes Laschi, and “represent a first step toward more general camouflage abilities.” Indeed, Shepherd is now adapting the material so it can transform more flexibly—just like a real octopus. For example, the team could replace the fixed mesh of fibers with rubber tubes, parts of which could be inflated or delated at whim. That way, they could change which bits of the surface are flexible, to determine how it will eventually inflate.
Shepherd’s team is just one of many groups who are attempting to build soft robots, which eschew the traditional hard surfaces of most machines in favor of materials that are soft, bouncy, and floppy. Such bots would theoretically be better at navigating tough terrain, resisting shocks and injuries, and even caring for people. Often, these researchers use the octopus as an inspiration. Last year, Harvard researchers 3-D printed a soft, autonomous “octobot” that moved by burning small amounts of onboard fuel, and channeling the resulting gas into its arms. Laschi, meanwhile, has built a robot with soft floppy arms that can wiggle through the water.
The robots are certainly cool, but they’re nowhere near as versatile as the real deal. Shepherd’s material, for example, can change texture about as fast as an actual octopus, but it can only make one rough shape at a time. The animal, meanwhile, can produce far finer undulations in its skin, which are tuned to whatever it sees in its environment. For now, nothing we produce comes anywhere close.