New material can switch between hard and soft

Scientists have designed a novel metamaterial that can switch between being hard and soft without damaging or altering itself, and may be used to build cars or rocket launch systems in the future.
The material could be used in bicycle tires that self-adjust to ride more easily on soft surfaces such as sand, or to make damage-resistant, reusable rockets, researchers said.
Researchers at University of Michigan (UM) in the US discovered a way 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.

Since these properties are "topologically protected," meaning that the material's properties come from its total structure, they are easily maintained even as the material shifts repeatedly between its hard and soft states.
"The novel aspect of this metamaterial is that its surface can change between hard and soft," said Xiaoming Mao, assistant professor of physics at UM.
"Usually, it's hard to change the stiffness of a traditional material. It's either hard or soft after the material is made," said Mao.

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.

A guitar string cannot be tightened without putting stress on the string itself, according to Mao.
Mao said the way an object comes in 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.
However, metamaterial's topological protection allows the inside of the material to remain damage free.
The material could one day be used to build cars or rocket launch systems. In cars, 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," Mao said.

"During a collision, you want components to become softer to absorb the energy from the collision and protect the passenger in the car," said Mao.
The study was published in the journal Nature Communications.