A new fiber combines the elasticity of rubber with the strength of metal. The hybrid fiber, with a metal core with a polymer sheath, can stretch up to 7 times its original length without breaking. And it is 2.5 times as tough as titin, the giant protein molecule that acts as a spring and imparts elasticity to human muscle tissue. The fiber, reported in the journal Science Advances, consists of a gallium metal core sheathed by the polymer poly(styrene-ethylene butylene-styrene) (SEBS). It could find use in stretchable electronics, soft robots, electronic skin, packaging, and protective gear. 

Rubbers can stretch with very little force but they are not strong. Meanwhile, metals do not stretch easily but are strong. But molecules found in nature, such as collagen and titin, combine the best of both: they are stretchable to sustain deformation but also strong, so they do not fracture or fail. In other words, they are tough.   

Toughness is the area under the well-known stress-strain curve for a material, says Michael Dickey, a professor of chemical and biomolecular engineering at North Carolina State University. A tough material can stretch far but it requires a lot of force, which the material absorbs as it is deformed.   

Tough materials in the past have had complex designs. Researchers have, for example, made polymers containing bonds that can break to dissipate energy and allow the material to stretch. Other materials have complicated structures such as interpenetrating polymer networks.   

Dickey and his colleagues wanted to make a simple, low-cost material. So they made a hollow SEBS fiber with an inner diameter of 0.85 mm, injected molten gallium into the fiber using a plastic needle-tipped syringe, and then let the metal cool. In addition to being simple to make, the fiber should be easy to incorporate into textiles and fiber-reinforced composites. 

“What tough materials have in common is they have some elasticity so they want to go back to their original state, but they also have a mechanism to dissipate energy,” Dickey says. In the core-shell fiber, energy dissipates due to a counterintuitive source: fractures of the gallium core. “When you first start stretching the fiber, it basically acts like a metal wire and doesn’t stretch much,” he says. Then the metal breaks, which would normally be catastrophic, but the rubber absorbs the strain and holds the fiber together.   

As the fiber was pulled, the gallium fractured multiple times at different places along its length, dissipating energy every time and allowing the fiber to continue to absorb energy and stretch further. The fiber goes back to its original shape when released. To repair the fiber, the researchers heated it to 30°C, which melted and resolidified the gallium core. The melting point of the polymer is higher. The researchers could stretch the fiber to seven times its length before the polymer snapped.   

“This study brings a new conceptual idea of how to design materials to fail,” says Frederick Gosselin, a professor of mechanical engineering at Polytechnique Montreal in Canada. “In an engineering application, this kind of gradual failure is highly prized. A component that had been overloaded would look beat up, but would still be holding on, rather than simply breaking catastrophically. The component could then be changed, or better still be warmed up to melt the gallium and repair it, as was demonstrated in the paper.”   

But gallium is not much of a structural material, in addition to being expensive. So the research team is now experimenting with other metal-rubber combinations. “The key to the fiber’s toughness is the combination of material and geometry,” Dickey says. “One of the reasons the fiber works is that gallium is a relatively soft metal so [it is] easier to break relative to other metals. But I suspect that if you had a metal that was stiffer and harder to break, you might be able to use a smaller diameter with bigger diameter rubber shell.”   

Read the article in Science Advances.