Examples of the new smart material (left to right): a flexible strip; a flexible strip that stiffened when twisted; a flexible strip transformed into a hard composite that can hold up a weight. Photo: Christopher Gannon/Iowa State University.A new smart and responsive material can stiffen up like a worked-out muscle, say the engineers at Iowa State University who developed it.
Stress a muscle and it gets stronger. Mechanically stress the rubbery material – say with a twist or a bend – and the material automatically stiffens by up to 300%. In lab tests, mechanical stresses transformed a flexible strip of the material into a hard composite that can support 50 times its own weight.
This new composite material doesn't need outside energy sources such as heat, light or electricity to change its properties. And it could be used in a variety of ways, with potential applications in medicine and industry.
The material is described in a paper in Materials Horizons; the lead authors are Martin Thuo and Michael Bartlett, Iowa State assistant professors of materials science and engineering. First authors are Boyce Chang and Ravi Tutika, Iowa State doctoral students in materials science and engineering. Chang is also a student associate of the US Department of Energy's Ames Laboratory.
Development of the material combined Thuo's expertise in micro-sized, liquid-metal particles with Bartlett's expertise in soft materials such as rubbers, plastics and gels. It proved to be a powerful combination.
The researchers found a simple, low-cost way to produce particles of undercooled metal, which remains liquid even below its melting temperature. The tiny particles (just 1 to 20 millionths of a meter across) are created by exposing droplets of melted metal to oxygen, creating an oxidation layer that coats the droplets and stops the liquid metal inside from turning solid. They also found ways to mix the liquid-metal particles with a rubbery elastomer material without breaking the particles.
When this hybrid material is subject to mechanical stresses – pushing, twisting, bending, squeezing – the liquid-metal particles break open. The liquid metal then flows out of the oxide shell, fuses together and solidifies.
"You can squeeze these particles just like a balloon," Thuo said. "When they pop, that's what makes the metal flow and solidify." The result, Bartlett said, is a "metal mesh that forms inside the material."
Thuo and Bartlett said the popping point can be tuned to make the liquid metal flow after varying amounts of mechanical stress. Tuning could involve changing the metal used, changing the particle sizes or changing the rubbery material.
In this study, the liquid-metal particles are made from Field's metal, an alloy of bismuth, indium and tin. But Thuo said that other metals will work, too. "The idea is that no matter what metal you can get to undercool, you'll get the same behavior," he said.
The engineers say the new material could be used in medicine to support delicate tissues or in industry to protect valuable sensors. It could also find use in soft and bio-inspired robotics or reconfigurable and wearable electronics. The Iowa State University Research Foundation is working to patent the material and it is available for licensing.
"A device with this material can flex up to a certain amount of load," explained Bartlett. "But if you continue stressing it, the elastomer will stiffen and stop or slow down these forces."
This story is adapted from material from Iowa State University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.