Shapeshifting armor could let humans build bionic exoskeletons

It was the kind of ah-ha moment scientists only dream about when Chiara Daraio, professor of mechanical engineering and applied physics at Caltech, realized that her lab’s work on so-called “granular materials” — like rice or coffee beans — may have an ancient connection as well.

“This project came from the realization that the rings in a chainmail are, from a material perspective, very similar to coffee grounds,” she explains. “They also can be jammed, under the right conditions (like vacuum packaging).”

Instead of developing chainmail the old-fashioned way — via a sweaty blacksmith bent over the forge — Daraio and her team at Caltech used a 3D printer to diligently print out tiny, hollow double-sided pyramid — known by scientists as an octahedron.

But unlike its ancient counterpart, this modern chainmail has an extra secret up its sleeve. This material is designed to shapeshift, Daraio explains, capable of transforming from a lifeless blanket into a stiff bridge that can hold more than 50 times its own weight.

While a shape-shifting material may sound otherwordly, Daraio says this material actually takes advantage of a common phenomenon called “jamming” that we’ve probably all experienced in our day-to-day lives. This happens when granular materials transition from a solid state to a fluid one.

“Granular materials, like sand, rice, and coffee, are composed of many particles packed together, [but] when loose in a cup, [they] behave like a fluid,” explains Daraio. “But when it is vacuum packaged, like what you buy in a store, it is solid, like a brick.”

Daraio and her team published the results of their spark of inspiration earlier this month in the journal Nature.

What’s new — Once the team constructed their chainmail, they set to work studying its properties, using each chain as a sort of “grain.” Daraio says this material feels both like “thick crochet fabric” when it's loose and like a “solid plate, with a somewhat scratchy surface” when it transforms into a solid.

“I like to fidget with the [material] all the time,” says Daraio. “[It] feels great in your hands, very satisfying.”

What sets this apart from other jamming materials, the authors explain in the study, is that their chainmail is not stuck once it transitions. Instead, this material can go back and forth between states with just a few tweaks to its environment, like vacuum pressure.

Why it matters — While it may be fun to play with, this technology is much more than just a party trick, Daraio says. Their study shows how this material could be used in the future as an exoskeleton to support human movement (kind of like a high-tech back brace) or even in the development of new responsive technology — like shape-shifting touch screens.

“These materials are not too far from being ready to use,” Daraio says.

How they did it — The first step in this work was to decide what kind of shape their chainmail would be made from. Daraio says the team tried several shapes, including more traditional ring or square-shaped links and 3D shapes like octahedrons, eight-faced objects with a sort of diamond shape.

“The limit is really our imagination.”

The flexibility of this design comes down to how 3D printing technology has improved in recent years, Daraio says, to produce much more than plastic trinkets.

“We can now make particles of arbitrary shapes — not just rings or squares, like in the ancient times — and fabricate them with many different materials, like nylon, rubber, and even steel and aluminum,” she says.

With their material constructed and weighing in at only 27 grams, the team then exposed it to a vacuum chamber where they could tune its flexibility simply by toggling on or off the pressure. They then draped the material between columns within the chamber and dropped heavy objects — up to 1.5 kilograms, or 1,500 grams — on the materials to see how they’d react.

While the “fluid” sheets bent with the object's weight, the stiffened material was able to support it. This is equivalent to holding more than 50 times its own weight.

In addition to these tests, Daraio also says her team was able to conduct experiments that helped them better understand ancient chainmail as well. In the future, 3D printing such materials may be one way scientists better understand their ancient counterparts.

“The modeling tools we developed could be used to study in more details deformations and impact absorption properties of ancient chainmail,” Daraio explains. “As our study was unfolding, we learned a good deal more about ancient designs. For example, we learned that their designs, commonly fabricated from circular or square linked particles, were really ideal for protection, in addition to being convenient to fabricate at the time.”

What’s next — As for what’s next for modern chainmail, Daraio says the sky’s the limit when it comes to what these materials might be capable of.

“The concept of ‘jamming’ chainmail could be used to look at other types of materials, not just designed to bear loads,” Daraio says. “For example, one could use similar fabrics to control RF/electromagnetic waves (like materials used in antennas), control sound and vibrations, thermal transport, etc. The limit is really our imagination.”

Abstract: Structured fabrics, such as woven sheets or chain mail armours, derive their properties both from the constitutive materials and their geometry. Their design can target desirable characteristics, such as high impact resistance, thermal regulation, or electrical conductivity. Once realized, however, the fabrics’ properties are usually fixed. Here we demonstrate structured fabrics with tunable bending modulus, consisting of three-dimensional particles arranged into layered chain mails. The chain mails conform to complex shapes, but when pressure is exerted at their boundaries, the particles interlock and the chain mails jam. We show that, with small external pressure (about 93 kilopascals), the sheets become more than 25 times stiffer than in their relaxed configuration. This dramatic increase in bending resistance arises because the interlocking particles have high tensile resistance, unlike what is found for loose granular media. We use discrete-element simulations to relate the chain mail’s micro-structure to macroscale properties and to interpret experimental measurements. We find that chain mails, consisting of different non-convex granular particles, undergo a jamming phase transition that is described by a characteristic power-law function akin to the behaviour of conventional convex media. Our work provides routes towards lightweight, tunable and adaptive fabrics, with potential applications in wearable exoskeletons, haptic architectures and reconfigurable medical supports.