Core Concept: Mechanical metamaterials bend the rules of everyday physics.

Abstract

Chiara Daraio played for Italy’s junior national basketball team in the 1990s. But when a Swiss running-shoe maker called her up in 2015, it wasn’t to talk about her athletic talents. Mechanical metamaterials, such as this Islamic-art–inspired metamaterial sheet, can pop into new configurations when stretched. Image courtesy of Ahmad Rafsanjani and Damiano Pasini (McGill University, Montreal). The company, called On, was looking for new ways to cushion its clients’ feet by using 3D printing. On had heard about some strange materials Daraio and like-minded researchers had been fabricating in their labs: weird stuff that stretches and squishes in counterintuitive ways. “The sports equipment industry is relatively quick on the uptake,” says Daraio, a professor of mechanical engineering and applied physics at Caltech. “It has been proactive in trying to use these materials.” The soles on Nike’s recent line, Free footwear, offer an illustration. The bottoms of these sneakers don’t look particularly special, just polymer cut into triangles. But the shapes are informed by science. Every time wearers take a step, their foot widens from the impact, stretching the shoes. Nike’s sole responds by both widening and lengthening, to help absorb impact. That shouldn’t happen. Think of a rubber band pulled outward between your fingers; it narrows in the middle. Soft materials stretched wider should shrink in length. But the shoes exhibit bizarre behavior, thanks to materials made of simple patterns of repeating geometric shapes. And shoes are just the beginning, says Daraio. “A new field is emerging that creates unusual materials using simple geometrical architectures,” she says. There’s a name for this stuff: metamaterials, from the Greek word “meta,” meaning “higher” or “beyond.” The term was originally coined to describe things that interact with electromagnetic waves in counterintuitive ways—not because of their composition, per se, but because of their …