3D printed chain mail inspired material transforms from flexible to rigid
As needed. Study published in Nature
Engineers at Caltech and NASA JPL have developed a material inspired by 3D printed chain mail that can transform from a foldable, fluid-like state into specific solid shapes under pressure. The study on the 3D printed tunable structured fabrics, which began in 2018 and was delayed due to the pandemic, was conducted by Prof. Andrade and Prof. Daraio (PI) from Caltech and NASA JPL’s Douglas Hofman (who is responsible for most 3D printing related projects at the lab). The paper describing the new digital material structure was published in Nature.
“The premise—Hofman explained—is that if you have two sheets of 3D printed chainmail and you wish to interlock them, what is the ideal shape of each node to maximize jamming? First authors Yifan Wang and Liuchi Li did some beautiful simulations to arrive at the optimal shape for jamming and then we 3D printed the structures in nylon and metal to demonstrate. If you layer two sheets in a vacuum bag, you can conform them into complex shapes and then rigidize by pulling the vacuum with a small pump. The sheets jam and can hold the arbitrary shape with exceptional strength. One ideal application is integrating the fabrics into clothing for conformal back support, either as a medical device or to limit fatigue. A wearable fabric exoskeleton, if you will. We are interested in developing this concept for commercial applications so please reach out if interested.”
The material has potential applications as a smart fabric for exoskeletons, or as an adaptive cast that adjusts its stiffness as an injury heals, or even as a deployable bridge that could be unrolled and stiffened, according to Chiara Daraio, Caltech’s G. Bradford Jones Professor of Mechanical Engineering and Applied Physics and corresponding author of a study describing the material that was published in Nature on August 11.
“We wanted to make materials that can change stiffness on command,” Daraio said. “We’d like to create a fabric that goes from soft and foldable to rigid and load-bearing in a controllable way.” An example from popular culture would be Batman’s cape from the 2005 movie Batman Begins, which is generally flexible but can be made rigid at will when the Caped Crusader needs it as a gliding surface.
Materials that change properties in similar ways already exist all around us, Daraio notes. “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,” she said.
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). “That’s the key,” Daraio says. “We tested a number of particles to see which ones offered both flexibility and tunable stiffness, and the ones that only jam under one type of stress tended to perform poorly.”
The materials were 3D printed (or 4D printed, since they continue to evolve after the manufacturing process) out of both polymers and even metals, with help from Douglas Hofmann, at JPL, which Caltech manages for NASA. These configurations were then simulated in a computer with a model from the group of José E. Andrade, the George W. Housner Professor of Civil and Mechanical Engineering and Caltech’s resident expert in the modeling of granular materials.
“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,” said Andrade.
The engineers applied 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 fabrics’ own 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,” said Wang, now an assistant professor at Nanyang Technological University in Singapore.
