Bio-inspired two-dimensional mechanical metamaterials with switchable Poisson's ratio

Most of the materials' mechanical performance is fixed after the material is manufactured. Here, inspired by three rotational symmetry leaves in natural plants, we introduced two two-dimensional (2D) mechanical metamaterials, line-to-line connected clover structure (CLC) and point-to-point connected clover structure (CPC), exhibiting switchable deformation modes and Poisson's ratio. Theoretical expressions on the Poisson's ratio of the CLC and CPC metamaterials are established to relate to the angle between the triangles of the clover structure, showing good agreement with numerical simulations as well as experimental results. Mechanical training was performed on the CLC metamaterial to obtain different deformation modes. Experiment results showed that the Poisson's ratio of the CLC metamaterials can be trained to switch between positive and negative. Similar to the rotating rigid triangle structure, the Poisson's ratio of the CLC and CPC metamaterials was governed by the rotation of the triangles in their structures. Poisson's ratio of the CPC metamaterials can reach -1 at two perpendicular directions. The proposed CLC and CPC metamaterials are expected to find applications in smart sensors, energy absorption devices, actuators, and wave convertors, especially in environments of alternating mechanical changes.