When people think of high-powered machines, they'd likely think of muscle cars before their own muscles. But muscles and other living tissues can do energetic things very quickly—they twitch, snap and beat—which is how physics defines power.
Scientists and engineers have long drawn inspiration from biology to create new soft, elastic and lightweight materials for a variety of applications, including engines, robots and other devices. But these synthetic materials have not yet been able to match the active, powerful features of living tissue. Now, researchers at the University of Michigan have published a model, or a theoretical framework, that shows a pathway to achieve just that in the journal Physical Review Letters.
"Our interest was to think about fast movements," said Suraj Shankar , U-M assistant professor of physics. "If we want to make soft engines and soft machines that pack a punch and can drive extremely fast motions, that's a really difficult task."
Shankar and colleagues showed that that challenge could be overcome by coupling a material's internal mechanics and chemistry so that its innate resistance to motion becomes an enabling ingredient. The research was supported by the U.S. National Science Foundation, U.S. Army Research Office and Office of Naval Research.
"Think of a passive soft material, like a piece of rubber, if you stretch it, it'll slowly go back and relax to its original shape. The language we'd use to describe that is to say the energy gets dissipated through the damping of the material," said Xiaoming Mao , U-M professor of physics and senior author of the study. "If we want to give rise to a beating heart kind of behavior in a real material, we need something to fight that dissipation. Here, the mechanism we make use of is chemical reactions."
A key feature of this framework is that the material contains reactive chemical ingredients that can provide energy to the system through its reactions. But those reactions also need to be sensitive to the forces at work when the material is stressed or strained from being deformed.
This coupling between the reactions and mechanical force creates a sort of positive feedback loop that counteracts the natural damping behavior and makes the material's motion more complex. Normally, that damping would overwhelm a material's inertia—its natural resistance to switching from rest to motion and vice versa—but it becomes a key player when the feedback is active.
"This property that is usually neglected—the inertia of the system—is actually important," said Biswarup Ash , a research fellow and co-author of the study. "It actually generates this interesting behavior."
The team showed that If the feedback loop is strong enough, the material's motion becomes chaotic, mathematically speaking.
"Imagine you have a gel that's shivering or twitching," Shankar said. "That's physically what this sort of chaotic behavior would look like for an actual material."
Although such active materials have not been realized yet, researchers have demonstrated individual components of the feedback cycle in other experiments, Mao said. For example, there have been materials that change color when squished because a chemical reaction is activated. Or others have engineered chemical reactions that cause a material to change shape or move.
"As far as we know, though, these components have not been combined together," Mao said. "But it's plausible that in the near future they could be, with some smart chemistry."
The study's co-authors also include Siddhartha Sarkar, who contributed as a U-M postdoctoral scholar; Nicholas Boechler, professor of engineering at the University of California, San Diego; and Yueyang Wu, who contributed as a U-M undergraduate researcher.
