A team of engineers has developed stretchable surfaces with programmable 3D texture morphing, a synthetic "camouflaging skin" inspired by studying and modeling the real thing in octopus and cuttlefish. The engineers, along with collaborator and cephalopod biologist Roger Hanlon of the Marine Biological Laboratory (MBL), report on their controllable soft actuator in Science.
The team's pneumatically-activated material takes a cue from the 3D bumps, or papillae, that cephalopods can express in one-fifth of a second for dynamic camouflage, and then retract to swim away without the papillae imposing hydrodynamic drag.
"Lots of animals have papillae, but they can't extend and retract them instantaneously as octopus and cuttlefish do," said Hanlon, who is the leading expert on cephalopod dynamic camouflage. "These are soft-bodied molluscs without a shell; their primary defense is their morphing skin."
Papillae are examples of a muscular hydrostat, biological structures that consist of muscle with no skeletal support (such as the human tongue). Hanlon and members of his laboratory, including Justine Allen, now at Brown University, were the first to describe the structure, function, and biomechanics of these morphing 3D papillae in detail.
"The degrees of freedom in the papillae system are really beautiful," Hanlon said. "In the European cuttlefish, there are at least nine sets of papillae that are independently controlled by the brain. And each papilla goes from a flat, 2D surface through a continuum of shapes until it reaches its final shape, which can be conical or like trilobes or one of a dozen possible shapes. It depends on how the muscles in the hydrostat are arranged."
The engineers' breakthrough was to develop synthetic tissue groupings that allow programmable, 2D stretchable materials to both extend and retract a range of target 3D shapes. The materials consist of a fiber mesh embedded in a silicone rubber. The fiber mesh is layered in rings that constrain the material, preventing localized expansion while the rest of the material stretches during inflation. The mesh acts in a similar way to the muscles of an octopus, while the flexible rubber mimics its skin.
"Engineers have developed a lot of sophisticated ways to control the shape of soft, stretchable materials, but we wanted to do it in a simple way that was fast, strong, and easy to control," said James Pikul, currently an assistant professor of mechanical engineering at the University of Pennsylvania. "We were drawn by how successful cephalopods are at changing their skin texture, so we studied and drew inspiration from the muscles that allow cephalopods to control their texture, and implemented these ideas into a method for controlling the shape of soft, stretchable materials."
Octopus and cuttlefish only express papillae for camouflage purposes, Hanlon says, and not for locomotion, sexual signaling, or aggression. "For fast swimming, the animal would benefit from smooth skin. For sexual signaling, it wouldn't want to look like a big old wart; it wants to look attractive, like a cool-looking mate. Or if it wanted to conduct a fight, the papillae would not be a good visual to put into the fight. Signaling, by definition, has to be highly conspicuous, unambiguous signals. The papillae would only make it the opposite!"
For an in-depth look at shape-shifting materials, check out our feature on The Promise and Peril of Programmable Matter.