Cephalopods like octopuses have long been masters at camouflage, able to blend in with and mimic a wide variety of different objects and textures. This disguising act is made possible by papillae, small round protuberances covering cephalopod skin that can quickly change shape. Researchers at Cornell University and the Woods Hole Marine Biological Laboratory have now teamed up to make a synthetic material that functions in a similar manner. As they describe in an article recently published in Science, they were able to create a silicon-mesh skin that can change conformation to take on many different shapes, creating forms that look like gravel, rocks, leaves, and a succulent plant (Graptoveria amethorum).

Papillae are a type of muscular hydrostat. Hydrostats are biological structures like the tongue that are composed mostly of muscles, and supported by the incompressible liquid therein. Papillae consist of concentric rings of muscle within an elastomeric skin, that can expand and contract. Mimicking this design, the team created circular silicon membranes with concentric rings of isotropic mesh, that is, the mesh deforms equally in all directions. The rings of mesh do not extend much, while the silicon does so elastically. By varying the spacing and width of these rings, the researchers can create a wide variety of shapes once these membranes are pneumatically activated, explains first author James Pikul, assistant professor of mechanical engineering and applied mechanics at the University of Pennsylvania, who was a postdoctoral researcher at Cornell when the article was published.

One big difference between the two is that papillae are powered by muscles, whereas the synthetic skin is powered by bursts of air from pneumatic tubes. But the rings of mesh act similarly to muscles by constraining the shape achieved.  “We’re understanding what happens in biological materials and [are] translating it to our synthetic systems,” Pikul says.

Octopuses also control their textures by having many different small papillae of a certain shape clustered together. Pikul describes these as “pixels” of texture. By recreating this design, the researchers hope to alternate between shapes in the future, although currently they have designed a specific set of textures as a proof of principle.

To manufacture the stretchy skin, the researchers began by pouring liquid silicone into a three-dimensional-printed mold to set the membrane shape and thickness. Next, they put two layers of fiber mesh on top of this wet silicon. After that the team shaped the membranes using a laser cutter, and removed the mesh before curing the silicone, which then hardened. Once dry, the mesh was then glued back onto the surface of the silicon in carefully tailored rings and bands, to achieve desired shapes.

Pikul says it was a challenge to create a material that can be quickly moved, sustain high stresses and forces, have high energy storage, and be flexible. “There are materials out there that can hit one to three of [these], but our system basically hits all [four] of them for the first time.”  

The work could have a number of applications. “Such soft structures able to change their shape could [allow for] developing disruptive technologies, such as completely new haptic interfaces to supply tactile stimulation to people and  innovative displays for human-machine interaction based on touch, instead of visual or auditory stimulation,” says Barbara Mazzolai, director of the Center for Micro-BioRobotics at the Italian Institute of Technology, who was not involved in this study.

This basic technology could also allow engineers to alter the structure of surfaces, say from hydrophobic to hydrophilic. This could have wide applications, for example in batteries or any “technology that depends on energy transport at an interface,” Pikul says. Mazzolai also envisions its use in creating camouflage-capable robots.

Read the abstract in Science.