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A new material inspired by mussels is flexing its muscles. It can stretch without snapping and repair its own molecular bonds, so it could be useful in robot joints that lift heavy objects, or for packaging to protect delicate cargo from accidental falls.
Mussels and some other molluscs hang onto solid surfaces using an adhesive protein and tough, plasticky fibres, which are extremely strong and can repair themselves when a few molecular bonds within them are broken. For a mussel, these stretchy yet strong fibres come in handy when a wave hits.
Megan Valentine at the University of California, Santa Barbara, and her colleagues created a plastic with these same properties by mimicking the chemistry the mussels use. Molecular bonds between iron and an organic compound called catechol make the material difficult to break or tear, while still allowing it to remain stretchy.
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The iron-catechol bonds dissipate energy from something hitting or stretching the material. These “sacrificial bonds” break, but the overall structure stays intact.
“It’s like a bike helmet: if you’re in a bike accident, the foam inside the helmet crushes and dissipates some of the energy. All that energy that would have gone into a skull fracture, instead goes into the helmet,” says Valentine. “In our case, instead of foam we have this sacrificial bonding that protects the underlying polymer system.”
Bond, sacrificial bond
By sacrificing the iron-catechol bonds, the material can stretch by 50 per cent. Then, once the stress is taken away, the bonds reform, making it reusable. Adding these bonds results in the plastic being 770 times stretchier and 58 times stronger than it is without them.
“Typically, there’s a trade-off: you can make a material harder to break but less stretchy, or easier to break and easier to stretch,” says Niels Holten-Andersen at the Massachusetts Institute of Technology. “But by adding these mussel-inspired bonds, they’ve made it so that you don’t have to make that choice.”
Something that is both strong and stretchy could act as a shock absorber in packaging for fragile objects or be used to make body armour that repairs itself on a molecular level after taking a hit, Holten-Andersen says.
Valentine says the material could also find an application in the joints of robotic arms that need to bear heavy weights but still move around. Someday, she says, it could even be used to repair the tendons in our joints.
Journal reference: Science, DOI: 10.1126/science.aao0350
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