Cephalopods – marine invertebrates such as the octopus, squid and cuttlefish – are unique for a number of reasons, not least of which is their ability to quickly change their color and shape.
A group led by Rob Shepherd, assistant professor in the Sibley School of Mechanical and Aerospace Engineering, is using the cephalopod as inspiration for a method to morph flat surfaces into three-dimensional ones on demand.
Shepherd has teamed up with Itai Cohen, professor of physics in the College of Arts and Sciences, and former postdoctoral researcher James Pikul, to devise a method for precisely transforming stretchable 2-D objects into 3-D shapes. Their method involves the use of rigid mesh, laser cut in a way that, when attached to a stretchable material, would constrain the material to form targeted shapes when inflated.
The group’s paper, “Stretchable Surfaces with Programmable Texture Morphing for Synthetic Camouflaging Skins,” published Oct. 13 in Science. Pikul, who did his research at Cornell under Shepherd and Cohen, is an assistant professor at the University of Pennsylvania.
Shepherd is using his fascination with the octopus – he recently put a 300-gallon aquarium in his lab, and plans to put an octopus in it to study its color- and texture-morphing abilities – to inform his research into precision shape-changing of objects. It has implications for another of his research areas – robotics.
“There are many complicated ways to create a texture change in a robot, but we wanted a very simple way to do it,” he said.
The challenge: precisely inflating something like a balloon into the exact shape you want.
“If you wanted to take a round balloon and shape it so it looked like a box, it would be pretty difficult – unless you just put it in a box,” Pikul said with a laugh. “That’s the materials and engineering challenge: How can you take these soft materials and control the shape of them?”
Shepherd, Cohen and Pikul are doing it with bio-inspiration and a healthy dose of mathematics. They call their method CCOARSE – Circumferentially Constrained and Radially Stretched Elastomer, for which they’ve submitted a patent application – and, according to Shepherd, “It’s a very simple concept.”
A cephalopod is able to change its shape to blend into its surroundings through activation of its papillae – protuberances that extend from the body by contraction of the erector muscles within. The group’s work mimics that by combining two materials – a fiber mesh embedded in a silicone elastomer – to act as synthetic tissue groupings.
The fabrication process starts with silicone poured into a 3-D printed mold of a desired shape and thickness. Nonwoven, inextensible mesh, patterned with a laser cutter, is placed onto the uncured silicone. The pattern is set using a simple algorithm that ultimately determines the final 3-D shape.
The silicone-mesh is cured at room temperature, then a final coating of silicone is poured on top. The combination of extensible silicone with inextensible mesh forces the silicone to inflate in the direction and shape desired.
“To design a particular shape,” Cohen said, “you figure out what its slope is at every point, then you design the amount of strain by including more or less mesh in the region.”
The group found that its CCOARSE approximations were remarkably accurate, the inflated shapes coming within 10 percent of algorithm-based projections.
This work was funded by a grant from the Army Research Office, which is interested in the technology for camouflage purposes. That is one focus of the research, Shepherd said, adding that there are others.
“You could imagine shipping sheets of material that you would inflate later to be objects,” he said. “One idea we’ve had is to take a sheet of rubber, send it along with a two-part polyurethane foam that becomes stiff, and people can inflate the rubber with the foam so that it becomes a stiff structure – like furniture.”
Cohen sees an application in robotics, in which an arm is inflated to perform a particular task.
“You basically have a sophisticated air hose with little attachments,” he said. “You take your inflatable robot, attach it to your air hose, and it inflates into the structure you want. It does the repetitive task, and when you’re done with that particular task, you throw it away and replace it with one that has a totally different shape that does a different task.”
Future work will focus on the ability to change color as well as texture, and to create more precise shapes with higher-resolution laser patterning.
Also contributing were graduate students Shuo Li and Lillia Bai, from the Shepherd lab. Roger T. Hanlon, senior scientist at the Marine Biological Laboratory in Woods Hole, Massachusetts, provided guidance on replication of octopus skin.