UMass gets energy boost with development of rubber-based metamaterial

AMHERST, Mass.—The snap of a Venus fly trap, the packed punch of a mantis shrimp's dactyl clubs or the leap of a trap-jaw ant are just a few examples of many "lightning quick" responses found in nature.

"A mantis shrimp can accelerate its hammer underwater at 40,000 meters per second squared or faster," said Alfred Crosby, professor of polymer science and engineering at the University of Massachusetts, Amherst. "(It) can shatter clam shells and crab shells in all kinds of impressive motions."

A trap-jaw ant, he said, can launch itself about a meter into the air using the force of closing its jaws at a rate of about 500,000 meters per second squared.

These releases of stored energy are what a team of scientists at UMass are calling latch mediated spring actuation, or LAMSA. And they're what inspired a group of engineers at the university to develop a new rubber-like "metamaterial" that can absorb and release large quantities of energy.

UMass is part of a multi-university team, called a MURI (multidisciplinary university research initiative), Crosby said, noting the university has been part of its current team for the past seven years.

Along with UMass, the MURI includes Duke University; Carnegie Mellon University; the University of California, Irvine; and Harvard University. It's led by Sheila Patek, the Hehmeyer professor of biology at Duke.

The team combines biology with material science and robotics, Crosby said, adding that its focus is "on trying to understand how some of the fastest moving organisms create the power that they can generate repeatably."

The research is supported by the U.S. Army Research Laboratory, the U.S. Army Research Office and the Harbin Institute of Technology of Shenzhen, China (HITSZ).

"This whole team has been trying to understand, how do you store energy, then release it into motion," mimicking the "incredible performances" found in nature and applying it to our everyday lives, Crosby said.

"These metamaterials came out of that research," he said, by combining an elastic field with a magnetic field and taking advantage of phase shifts.

The "metamaterial" combines an elastic, rubber-like substance with tiny, embedded magnets, according to UMass. This new "elasto-magnetic" material uses phase shifts to amplify the amount of energy the material can release or absorb.

The UMass team announced the development of this substance, which is programmable, with the January publication of its research article, "Phase-transforming metamaterial with magnetic interactions," in the Proceedings of the National Academy of Sciences.

"We've created materials that can give us extra boosts of energy when we want them," said Crosby, senior author of the paper.

The metamaterial starts with commercially bought rubber.

Crosby said the team used a carbon black polyurethane-based rubber, but added that the discovery isn't limited to that one type of rubber.

In that rubber, he said, the team laser-cut holes—or voids—into calculated arrangements or patterns and inserted tiny magnets.

"These magnets were positioned such that their poles were in certain orientations to their neighbors," Crosby said. "All of them are specifically laid out in a particular geometric pattern, and that geometric pattern really controls the end properties that you get."

These patterns are what allow the material to be programmable, he said.

"(The math) tells me where I need to place them and what the material properties are to get a certain composite response of being able to get this extra boost of energy or extra dissipation of energy," he said.

As for the use of phase shifts, Crosby said a material must be taken from one solid phase to another to release the energy or harness it as a power source—much like pulling back a bow string to release an arrow or launching a rubber band across a room.

"To amplify (this) energy release or absorption, you have to engineer a new structure at the molecular or even atomic level," Crosby said in a news release. This is a process that is challenging to accomplish—especially in a predictable way—with standard materials.

By using this metamaterial, however, "we have overcome these challenges, and have not only made new materials, but also developed the design algorithms that allow these materials to be programmed with specific responses, making them predictable," Crosby said.

Xudong Liang, lead author of the paper, said in the release that the team has taken the quick responses found in nature "to the next level" with this development.

Liang is currently a professor at HITSZ. He completed this research while a postdoc at UMass.

"By embedding tiny magnets into the elastic material, we can control the phase transitions of this metamaterial. And because the phase shift is predictable and repeatable, we can engineer the metamaterial to do exactly what we want it to do: either absorbing the energy from a large impact, or releasing great quantities of energy for explosive movement," he said.

So where are we likely to see the use of such metamaterials?

While several "entrepreneurs" already have reached out to the colleges about producing them, Crosby said no company is manufacturing products with these metamaterials, nor could he give an estimated timeline as to when they could be put into use.

But the closest use UMass sees, he said, is in robotics.

"The idea here is that you can use springs like this new rubber-based metamaterial, and you can do real-time tuning of how much energy you can get out of (the) material," he said.

"Just with a small change of input energy, you can get a large change of the output energy," he continued. "That's always an attractive kind of feature, I think, when you're trying to manage your payload in terms of how much mass or how much energy do you (need) to do certain tasks."

As technology advances and robots are designed to do more—like jump, crawl or fly—engineers are seeking ways to accomplish storing the extra energy needed to accomplish these tasks without needing to carry extra fuel.

"This spring here gives you that kind of dynamic," he said.

Thinking further out, UMass also sees valuable use of the metamaterial in protective gear, like helmets, due to its ability to rapidly dissipate energy.

There's no prototype yet, Crosby said, but while running experiments, the team realized it could take advantage of the phase shifts in the metamaterial to absorb the energy.

When dropping weight on the end of a spring, the team found that the spring could dampen the impact "extremely quickly," he said.

"The same concepts can extend to three dimensions where you could have a layer of foam made out of these metamaterials and then be tuned to absorb energies and dampen very quickly," he continued.

"In terms of developing protective wear, dissipating energy and turning it into a different form of energy is really attractive," Crosby said, "but that concept is definitely farther off and down the development path."