Rubber and steel are at different ends of the spectrum when it comes to hardness, and wherever an object falls on that scale is typically where it will stay. But researchers at the University of Michigan have now developed a metamaterial that can change the stiffness of its surface, from hard to soft and back, in response to a small amount of stress.

As artificial materials that can be finely tuned for a specific purpose, metamaterials can do some pretty incredible things that you won't find in nature. Interestingly, what they're made of doesn't seem to matter: instead, their attributes stem from their structure, and by manipulating that, engineers can develop metamaterials that could replace optic lenses, make objects effectively invisible, or create vehicle parts that are both very strong and very light.

The University of Michigan team says its new metamaterial specializes in switching its surface between hard and soft states. Applying a small amount of strain allows that stiffness to be changed by several orders of magnitude, without damaging or weakening the material itself.

"The novel aspect of this metamaterial is that its surface can change between hard and soft," says Xiaoming Mao, lead author of the study. "Usually, it's hard to change the stiffness of a traditional material. It's either hard or soft after the material is made."

The new material's geometry is composed of a lattice of tiny struts connected with hinges, and by applying low-level stress in the form of twisting the material, the lattice changes the topological properties of the material, causing it to become either harder or softer as required. Since the hinges absorb the stress, the change can be done over and over without causing damage to the main structure.

The researchers suggest some fairly exciting potential applications for such a material. Reusable rockets could stay rigid for take-off but transition for a softer landing, and bicycle tires could adjust their own hardness to best suit whatever surface you're riding over. In cars, rather than stashing an airbag into the steering wheel, the wheel itself could soften up in the event of an accident.

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

The research is published in the journal Nature Communications.