New rubbery material gets stronger the more stressed it becomes

Examples of the new smart material (L-R): 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 credit: Christopher Gannon/Iowa State University

Iowa State University engineers have developed a new smart and responsive material that acts just like a worked-out muscle.

The team says their new rubbery material gets stronger the more stressed it becomes — twisting or bending makes up to 300 per cent stronger.  A flexible strip of the material was tested in a lab, showing it transforming into a hard composite that can support 50 times its own weight. And it does this all through mechanical stress, not needing an outside energy source like heat, light or electricity to change its properties.

Development of the material combined Martin Thuo’s expertise in micro-sized, liquid-metal particles with Michael Bartlett’s expertise in soft materials such as rubbers, plastics and gels, both of whom are Iowa State assistant professors of materials science and engineering.

The team found a way to produce particles of undercooled metal, where metal that remains liquid even below its melting temperature. The tiny particles are created by exposing droplets of melted metal to oxygen, creating an oxidation layer that coats the droplets and stops the liquid metal from turning solid. Researchers 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 the liquid-metal particles break open and the liquid metal flows out of the oxide shell, fuses together and solidifies.

Thou explains that the particles can be squeeze, much like a balloon, that popping is what makes the metal flow and solidify. Bartlett adds that the result is a “metal mesh that forms inside the material.”

The duo explain that 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 soft material.

The liquid-metal particles the team used for its experiment contain Field’s metal, an alloy of bismuth, indium and tin. But Thuo said 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. There could also be uses in soft and bio-inspired robotics or reconfigurable and wearable electronics.

The material is described in a paper recently published online by the scientific journal Materials Horizons. The lead authors are Martin Thuo and Michael Bartlett 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 U.S. Department of Energy’s Ames Laboratory.

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