Researchers create metamaterial with multiple stable states

A team of researchers at Purdue University has developed a way for a material to store energy in its skin through invertible domes. The team’s findings are published in the journals Extreme Mechanics Letters and Advanced Science.

When the dimpled circles on a soft drink lid are pressed, they become either convex or concave. Materials or structures that have two stable states demonstrate a concept called bistability.

Purdue Assistant Professor of Mechanical Engineering Andres Arrieta explained that — inspired from bistability, an important concept found in nature — his team is working to make programmable structures.

Arrieta’s team began with a flat, one-inch-square sheet with a pop-up dome, 3D printed from thermoplastic polyurethane. By pressing with a finger, the dome would snap to become either convex or concave. When they printed a 3x3 grid of these domes, new behaviours were observed.

“When you invert two domes that are close to another, they start interacting,” Arrieta said. “And when you start making patterns of these domes on a sheet, the sheet itself begins to curve globally. Depending on which domes are inverted, you get different shapes.”

The team experimented with larger grids, pressing more complex patterns. By pressing certain domes in or out, the sheet formed into a cylinder, a star or a saddle shape.

“These individual bi-stable domes are combining to form a new metamaterial, which itself has multiple stable states,” Arrieta said. “We call it hierarchical multistability.”

When considering applications, Arrieta’s team built a simple robotic gripper using two lines of domes. When the domes were concave, the gripper arms stayed open. But by applying a small amount of air pressure to actuate the domes to become convex, the gripper arms closed tight enough to grasp and hold a small weight.

“Grasping something is easy, but maintaining that grasp requires constantly expending energy,” Arrieta said. “This is true for both humans and machines. But what’s interesting about this gripper is that when we invert the domes that make the gripper close, we are actually storing energy in the skin. The gripper arms are using that energy to maintain its grasp, rather than requiring some external energy source. In essence, we are using the structure itself as a mechanical battery.”

Eventually, Arrieta hopes to utilise the technology in flexible robotics.

“It’s difficult to re-create a robotic hand that grips like a human hand because of the sheer number of motors and sensors required,” he said. “But if we made the skin from these sheets and printed individual domes at different heights, then only a specific group of them would actuate at different levels of air pressure. By programming specific bursts of air pressure, we could activate a matrix of domes to create the multistable gripping states we need, with minimal energy.”

In addition to varying the height attributes, Arrieta’s team discovered that actuating the domes in a different order produces radically different shapes. With these three attributes — height, position and order — even small grids of domes can produce a remarkable number of possible outcomes. This leads Arrieta to the next step: mechanical computing.

“When you think about it, these up-and-down domes are a lot like the 1s and 0s of computer data,” Arrieta said. “We can imagine ‘programming’ a sheet like this by pressing the domes in certain locations in a certain order, and then ‘reading’ that data mechanically based on the shape of the sheet. This can be done without power or a central processor of any kind. In this way, future machines will function much more like animals, which use mechanical sensing and processing to react much more quickly.”

Image caption: Purdue University researchers have developed a way for a material to store energy in its skin through invertible domes. Credit: Purdue University photo/Jared Pike.