3D gradient auxetic soft mechanical metamaterials fabricated by additive manufacturing

Applied Physics Letters, Volume 118, Issue 14, April 2021. Materials with a negative Poisson's ratio, also known as auxetics, have attracted a lot attention as they have shown innovation potential in applications for energy damping, modern fasteners, biomedical implants, piezoelectric sensors, and soft actuators. In this study, we introduce different patterns of graded distribution of unit cells with positive and negative Poisson's ratio in tubular configurations. Seven types of patterns are programmed into the fabric of the metamaterial to create desired shape changes upon applying far-field external loads. Two of the patterns demonstrate near-zero Poisson's ratio even at axial strains as high as 44%. Other pattern distributions convert the initially cylindrical shape of the tubes to vase, barrel, hourglass, nonsymmetrical vase, and nonsymmetrical hourglass geometries. The experimental Poisson's ratio values for the linear negative-to-positive-to-negative gradient (resulting in hourglass shape) and linear positive-to-negative-to-positive gradient (resulting in barrel shape) cases are +0.53 and −0.47, respectively. The measured Poisson's ratio values at tube level are in good accordance with the analytical values of +0.5 and −0.5. Benefits of the proposed designs in applications such as action-at-a-distance actuators and wrinkle-free jointless hinges in both 3D and 2D configurations are demonstrated.