Stretchy but tough fibres have potential for robotics and textiles

Combining strength of metal core with resilience of polymer sheath creates fibres that absorb energy of internal breakages

Engineers at North Carolina State University have developed a fibre that combines the elasticity of rubber with the strength of a metal, creating a material that could be incorporated into soft robotics or super-tough textiles. Capable of stretching up to 7 times its length without breaking, the fibre can absorb a great deal of energy as it deforms, becoming tougher than both the materials in its composition.

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When the metal core of the fibre breaks, the material doesn’t fail – the polymer sheath absorbs the strain between the breaks in the metal and transfers the stress back to the metal core. Image: Michael Dickey, NC State University

Developed by a team led by Michael Dickey, Alcoa Prof of chemical and biomolecular engineering at NC State, the fibre consists of a gallium metal core surrounded by a sheath of poly (styrene-ethylene butylene-styrene) (SEBS). In a paper in Science Advances, the team explained that the fibre mimics the behaviour of tough biological materials such as collagen or titin, the giant protein which acts as a molecular spring in muscle. “A good way of explaining the material is to think of rubber bands and metal wires,” said Dickey. “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 fibres have the best of both worlds.”

Paradoxically, the fibre’s properties depend on the metal core breaking. Under stress, gallium fractures, but the SEBS sheath absorbs the strain between the breaks in the metal and transfers the stress back to the metal core. “Every time the metal core breaks it dissipates energy, allowing the fibre to continue to absorb energy as it elongates,” Dickey explained. “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.”

In materials, toughness is defined as the amount of force a material can absorb as it is deformed. The new fibre is tougher than either the metal wire of the polymer sheath. According to the paper, it is 2.5 times tougher than titin and can hold more than 15,000 times their own weight for. 100 times longer than a hollow SEBS fibre. It has the advantage of using relatively simple and low-cost chemistry, in contrast to other attempts to create tough fibres by synthesising polymers which incorporate sacrificial bonds which release “hidden” lengths of material when each bond breaks or have complicated structural architectures such as interpenetrating polymer networks.

The team commented that the core-shell structure is reminiscent of endo-skeletons in nature or the use of steel reinforcement in concrete, but where most materials that can dissipate energy when deformed operate under compression, the structure they describe works best under tension.

“There’s a lot of interest in engineering materials to mimic the toughness of skin – and we have developed a fibre that has surpassed the toughness of skin yet is still elastic like skin,” Dickey said. “We used gallium for this proof of concept work, but the fibres could be tuned to alter their mechanical properties, or to retain functionality at higher temperatures, by using different materials in the core and shell,” he added. “We are interested to see how these fibres could be used in soft robotics or when woven into textiles for various applications.”