A new material inspired by mussels has demonstrated its capabilities. It can be stretched without breaking, and it repairs its own molecular bonds. So it might be useful in making robotic joints that lift heavy objects, or in packaging to protect delicate goods from accidental falls. The results were recently published in the journal Science.
Mussels and some other mollusks use adhesive proteins and tough fibers like plastic to hold onto solid surfaces. The fibers are extremely tense and can repair themselves when some of the molecular bonds inside are broken. For mussels, these stretchy but still tough fibers come in handy when waves hit.
Megan Valentine and colleagues at the University of California, Santa Barbara, created plastics with the same properties by simulating the chemistry used by mussels. Molecular bonds between iron and an organic compound called catechol make the material difficult to break or tear while remaining stretchable.
The iron-catechol bond dissipates energy from something that strikes or stretches the material. These sacrificed molecular bonds are broken, but the overall structure remains intact. "It's a bit like a bicycle helmet: if you're in an accident on a bike, the foam inside the helmet cushions and dissipates some energy. All the energy that would have caused a skull fracture goes into the helmet," Valentine said. For new materials, we replace foam with sacrificial molecular bonds in order to protect the underlying polymer system."
By sacrificing the iron-catechol bond, the new material can be stretched by 50 percent. Then, once the pressure is removed, the molecular bonds are reformed, allowing them to be used again. Plastics with added molecular bonds can be stretched 770 times and are 58 times tougher than materials without these bonds.
"Usually there's a balance involved: you can make materials that are harder to break but not as stretchable, or materials that break more easily but stretch easily," says Niels Holten-Andersen of MIT. "But by adding these mussel-inspired molecular bonds, the researchers succeeded in combining unbreakability and stretchability."
