Programmable Mechanical Metamaterials with Tailorable Negative Poisson's Ratio and Arbitrary Thermal Expansion in Multiple Thermal Deformation Modes.

Mechanical metamaterials pave a way for designing and optimizing microstructure topology to achieve counterintuitive deformation including negative Poisson's ratio (NPR) and negative thermal expansion (NTE). Previous studies were always limited to single anomalous mechanical or thermal deformation, but current applications for high-precision mechanical or optical equipment always require their combination and customized and anisotropic deformation parameters. This work develops programmable two-dimensional (2D) mechanical metamaterials based on chiral and antichiral structures constructed with curved bimaterial strips to produce tailorable NPR and arbitrary thermal deformation. The coefficient of thermal expansion of the mechanical metamaterials is tunable on a large scale across negative, near-zero, and positive values depending on the bimaterial configurations and geometrical parameters of curved strips, while the value of NPR is mainly determined by the radian. Furthermore, it is programmable by coding the unit cells to exhibit customized and anisotropic thermal deformation combining homogeneous, gradient, and shear modes. The proposed mechanical metamaterials are fabricated by multimaterial three-dimensional (3D) printing, and the unusual deformation modes are verified experimentally, which is well in agreement with the results of finite element analysis. This work demonstrates a feasible approach to achieving customized mechanical and thermal deformation through easy block building for specific engineering applications including eliminating thermal stress, shape morphing, and smart actuators.