Batman cloak-like chainmail switches from flexible to tough on command



Researchers at Caltech and JPL have devised a new smart material that can instantly morph from fluid and flexible to tough and rigid. The material’s configuration is inspired by chainmail armors and could potentially prove useful in exoskeletons, casts for broken limbs, and robotics.

This modern chainmail sounds mighty similar to Batman’s cloak, which drapes behind the superhero at rest but stiffens into a glider when he needs to make a fast escape. However, unlike the DC movies, the technology was initially inspired by the physics of vacuum-packed coffee.

Coffee inspiration

 “Think about coffee in a vacuum-sealed bag. When still packed, it is solid, via a process we call ‘jamming’. But as soon as you open the package, the coffee grounds are no longer jammed against each other and you can pour them as though they were a fluid,” Chiara Daraio, a professor of mechanical engineering and applied physics at Caltech, explained.

While individual coffee grounds or sand particles only jam when compressed, sheets of linked rings can jam together under both compression and tension. Starting from this idea, Daraio and colleagues experimented with a number of different configurations of linked particles and tested each using both computer simulations and 3-D printing.

Testing the impact resistance of the material when unjammed (soft). Credit: Caltech.
Testing the impact resistance of the material when jammed (rigid). Credit: Caltech.

Although it doesn’t lead to the stiffest configuration, the researchers settled on an octagonal shape of the chainmail links. The best stiffness effect is achieved with circular rings and squares, which is actually the design used in ancient armors. However, these configurations are also much heavier due to the denser stacking of the links. The octagonal configuration is the most optimal one in terms of both stiffness and lighter weight.

The chainmail is made from linked octahedrons. Credit: Catech.

During one demonstration, 3-D printed polymer chainmail was compressed using a vacuum chamber or by dropping weight to control the jamming of the material. The vacuum-locked chainmail remarkably supported a load more than 50 times its weight.

When stiffened the chainmail can support 40 times its own weight. Credit: Caltech.

“Granular materials are a beautiful example of complex systems, where simple interactions at a grain scale can lead to complex behavior structurally. In this chain mail application, the ability to carry tensile loads at the grain scale is a game changer. It’s like having a string that can carry compressive loads. The ability to simulate such complex behavior opens the door to extraordinary structural design and performance,” says José E. Andrade, the George W. Housner Professor of Civil and Mechanical Engineering and Caltech’s resident expert in the modeling of granular materials.

The modern chainmail fabrics have potential applications in smart wearable clothing. “When unjammed, they are lightweight, compliant, and comfortable to wear; after the jamming transition, they become a supportive and protective layer on the wearer’s body,” says Wang, now an assistant professor at Nanyang Technological University in Singapore.

In parallel, the researchers are working on a new design consisting of strips of polymers that shrink on command when heat is present. These strips could be woven into the chainmail to create objects like bridges that fold down flat when required. The two materials joining together could use prove highly useful when incorporated into robots that can morph into different shapes and configurations.


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