Engineers Develop Smart Material That Changes Stiffness When Twisted or Bent

Mechanically triggered composite stiffness tuning through thermodynamic relaxation (ST3R)

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. (credit: Christopher Gannon and Iowa State University)

August 27, 2018 | Source: Iowa State University, news.iastate.edu, 14 Feb 2018, University Relations

A new smart and responsive material can stiffen up like a worked-out muscle, say the Iowa State University engineers 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 percent, the engineers said. 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, including applications in medicine and industry.

The researchers found a simple, low-cost way to produce particles of undercooled metal – that’s metal that remains liquid even below its melting temperature. The tiny particles (they’re 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 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 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 soft material.

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 Iowa State University Research Foundation is working to patent the material and it is available for licensing.