Engineers developed a material inspired by chain mail that can transform from a foldable, fluid-like state into specific solid shapes under pressure. Materials that change properties in similar ways already exist; for example, coffee in a vacuum-sealed bag. When still packed, it is solid via a process called jamming. As soon as the package is opened, the coffee grounds are no longer jammed against each other and can be poured as though they were a fluid.
Individual coffee grounds and sand particles have complex but disconnected shapes and can only jam when compressed. Sheets of linked rings, however, can jam together under both compression and tension (when pushed together or pulled apart). The team tested a number of particles to see which ones offered both flexibility and tunable stiffness; the ones that only jam under one type of stress tended to perform poorly.
They designed a number of configurations of linked particles, from linking rings to linking cubes to linking octahedrons (which resemble two pyramids connected at the base). The materials were 3D printed out of polymers and even metals. Granular materials are an example of complex systems, where simple interactions at a grain scale can lead to complex behavior structurally. In the chain mail application, the ability to carry tensile loads at the grain scale is similar to having a string that can carry compressive loads.
The engineers applied an outside stress, compressing the fabrics using a vacuum chamber or by dropping a weight to control the jamming of the material. In one experiment, a vacuum-locked chain mail fabric was able to support a load of 1.5 kilograms — more than 50 times the fabric’s weight. The fabrics that showed the largest variations in mechanical properties (from flexible to stiff) were those with a larger average number of contacts between particles, such as linked rings and squares, akin to medieval chain mail.
These fabrics have potential applications in smart wearable equipment. 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.