Turning push into pull: 'unprecedented in nature' metamaterial reverses impact force

Spacecraft, cars and people’s heads could be better protected against dangerous impacts thanks to an origami-inspired ‘metamaterial’ that creates a “counterintuitive” response to forces.

Created by researchers at the University of Washington in the US, the team built a paper model that uses folding creases to soften impact forces – and promote forces that reduce stresses.

“If you were wearing [an American] football helmet made of this material and something hit the helmet, you'd never feel that hit on your head,” said aeronautics associate professor and corresponding author Jinkyu Yang. “By the time the energy reaches you, it's no longer pushing, it's pulling.”

The metamaterial was built of individual ‘cells’ connected together in a chain.

“Metamaterials are like Lego,” said Yang. “You can make all types of structures by repeating a single type of building block, or unit cell as we call it. Depending on how you design your unit cell, you can create a material with unique mechanical properties that are unprecedented in nature.”

The researchers cut dotted lines into paper using a laser cutter. They then folded the paper along the lines to form a cylindrical structure, and glued acrylic caps on either end to connect the cells into a long chain.

The researchers lined up 20 cells and connected one end to a device that pushed and set off a reaction throughout the chain. The team tracked the initial compression wave and the following tension wave as the unit cells returned to normal.

The chain showed a “counterintuitive wave motion”, an announcement said. Even though the compressive pushing force from the device started the whole reaction, that force never made it to the other end of the chain. Instead, it was replaced by the tension force that started as the first unit cells returned to normal and propagated faster and faster down the chain. The unit cells at the end of the chain only felt the tension force pulling them back.

The metamaterial design could be used to reduce impact in a range of applications across many different sectors, the researchers said – shock absorbing legs to protect reusable spacecraft, for example. The team also hope to use it to protect people and vehicles in car accidents, Yang said.

“By changing where we introduce creases into flat materials, we can design materials that exhibit different degrees of stiffness when they fold and unfold,” said co-author Yasuhiro Miyazawa.

The research was published in Science Advances.