A proof of concept composite material based on gallium combines the strength of metals with the elasticity of rubber, according to research by a team from North Carolina State University published in the journal Science Advances [Cooper, C.B., et al. Sci. Adv. (2019) DOI: 10.1126/sciadv.aat4600]. Tough, new materials based on this approach could be used in soft robotics, packaging materials, and next-generation textiles.

Michael Dickey, who is Alcoa Professor of Chemical and Biomolecular Engineering at NC State and his colleagues made fibers comprising a gallium metal core surrounded by an elastic polymer sheath. Under tension, the fiber displays the strength of the metal core. However, when the metal ultimately breaks under the strain, the fiber as a whole does not snap. The polymer sheath absorbs some of the strain between the breaks in the metal and transfers the stress back to the metal core. One might think of it as being akin to how human tissue holds together a broken bone despite the snap.

"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. So, if such a fiber is being used to bear a heavy load it won't simply snap when the strain gets too much it will continue to support the weight, which might have engineering or other applications in sustaining a structure and delaying its failure considerably.

"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," Dickey adds. This composite system 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," he adds.

One other advantage of such fibers is that the gallium core is electrically conductive, but electrical conductivity is obviously lost when the metal core breaks, so such a fiber would not only delay failure of a structure but might also be to be designed to have an inbuilt early warning system for imminent failure. The team adds that the fibers could 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 adds.