Three-phase soft mechanical metamaterials for tunable negative expansion deformation and bandgaps

As the new artificial composite structures combing the smart materials and extensible topologies, the mechanical negative-expansion metamaterials deformation are promising in the fields of aerospace, robotics, medical devices, and flexible electronics. Based on the swelling characteristics of hydrogels, this work develops 2D and 3D soft mechanical metamaterials suitable for super large negative expansion deformations. The 2D and 3D metamaterials are fabricated by the multi-material 4D printing, and characterized by the experiments of self-driven negative expansion deformation and recovery. To accurately and quickly predict the deformation behavior of negative expansion response, a large-deformation theoretical model is developed based on finite element method and verified by the experimental characterizations. The influence of lattice parameters on the negative expansion response deformation and the corresponding adjustability are characterized and demonstrated through the theory, simulations and experiments. The 2D and 3D soft mechanical metamaterials show that their elastic wave bandgaps can be regulated actively and flexibly using the evolution of the lattice during the negative expansion deformation process. The present metamaterials can be used to design and manufacture instruments with adjustable negative expansion deformation and acoustic functionalities, showing the potential applications in the fields of smart soft robots and active metamaterial devices.