New fiber is strong on the inside and tough on the outside

Researchers at North Carolina (NC) State University have developed a fiber that combines the elasticity of rubber with the strength of a metal, resulting in a tougher material that could be incorporated into soft robotics, packaging materials or next-generation textiles.

"A good way of explaining the material is to think of rubber bands and metal wires," says Michael Dickey, professor of chemical and biomolecular engineering at NC State and corresponding author of a paper on the work in Science Advances. "A rubber band can stretch very far, but it doesn't take much force to stretch it. A metal wire requires a lot of force to stretch it, but it can't take much strain – it breaks before you can stretch it very far. Our fibers have the best of both worlds."

The researchers created fibers consisting of a gallium metal core surrounded by an elastic polymer sheath. When placed under stress, the fiber has the strength of the metal core. But when the metal breaks, the fiber doesn't fail – the polymer sheath absorbs the strain between the breaks in the metal and transfers the stress back to the metal core. This response is similar to the way human tissue holds broken bones together.

"Every time the metal core breaks, it dissipates energy, allowing the fiber to continue to absorb energy as it elongates," Dickey explains. "Instead of snapping in two when stretched, it can stretch up to seven times its original length before failure, while causing many additional breaks in the wire along the way.

"To think of it another way, the fiber won't snap and drop a heavy weight. Instead, by releasing energy repeatedly through internal breaks, the fiber lowers the weight slowly and steadily."

In materials, toughness is a material's ability to absorb energy and deform without breaking; it represents the amount of force a material can absorb as it is deformed over a distance. The new fiber is far tougher than either the metal wire or the polymer sheath on its own.

"There's a lot of interest in engineering materials to mimic the toughness of skin – and we have developed a fiber that has surpassed the toughness of skin yet is still elastic like skin," Dickey says.

What is more, the gallium core is conductive – though it loses its conductivity when the internal core breaks. The fibers can also be reused by melting the metal cores back together.

"We used gallium for this proof of concept work, but the fibers could be tuned to alter their mechanical properties, or to retain functionality at higher temperatures, by using different materials in the core and shell," Dickey says. "This is only a proof of concept, but it holds a lot of potential. We are interested to see how these fibers could be used in soft robotics or when woven into textiles for various applications."

This story is adapted from material from North Carolina 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.