A group of University of Michigan researchers has developed a way to design a "metamaterial" that allows the material to switch between being hard and soft without damaging or altering the material itself.

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

In the group's study, published in the journal Nature Communications, they report having discovered a way to compose a metamaterial that can be manipulated to increase the stiffness of its surface by orders of magnitude—the difference between rubber and steel.

Because these properties are "topologically protected," meaning that the material's properties come from its total structure, they're easily maintained even as the material shifts repeatedly between its hard and soft states.

According to researchers, the novel aspect of this metamaterial is that its surface can change between hard and soft. Usually, it's hard to change the stiffness of a traditional material. It's 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. A guitar string cannot be tightened without putting stress on the string itself.

Researchers say that 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. But metamaterial's topological protection allows the inside of the metamaterial to remain damage free.

The material could one day be used to build cars to help absorb impacts from a crash.

When driving a car, motorists want the car to be stiff and to support a load, researchers say. During a collision, components should become softer to absorb the energy of impact and protect passengers.

The researchers also say the material could be used to make bicycle tires that could self-adjust to ride more easily on soft surfaces such as sand, or to make damage-resistant, reusable rockets.