There and back again — the metamaterial that can switch from hard to soft

Metamaterials are man-made materials that get their properties — such as whether the material is hard or soft — from the way the material is constructed rather than the material that constructs it. This allows researchers to manipulate the metamaterial’s structure in order to make the material exhibit a certain property.

Now, scientists from the University of Michigan have discovered how to compose a metamaterial that can be easily manipulated to increase the stiffness of its surface by orders of magnitude — the difference between rubber and steel. And since these properties are ‘topologically protected’ — ie, the material’s properties come from its total structure — they’re easily maintained even as the material shifts from being hard to soft.

“The novel aspect of this metamaterial is that its surface can change between hard and soft,” said Xiaoming Mao, a co-author on the study. This is in contrast to traditional materials, which are either hard or soft after the material is made — for example, a dental filling cannot be changed after the dentist has set the filling without causing stress (either by drilling or grinding) to the original filling.

As explained by Mao, the way an object comes into contact with the edge of the metamaterial changes the geometry of the material’s structure, and therefore how the material responds to stress at the edge. But the metamaterial’s topological protection allows the inside of the metamaterial to remain damage-free.

Writing in the journal Nature Communications, Mao and his co-authors suggest that the material could one day be used in gecko pads, tyres, car components and more. In cars, for example, the material could help absorb impacts from a crash.

“When you’re driving a car, you want the car to be stiff and to support a load,” said Mao. “During a collision, you want components to become softer to absorb the energy from the collision and protect the passenger in the car.”