Electro-Thermally Controlled Active Mechanical Metamaterials with Programmable Stiffness and Nonreciprocity.

Active mechanical metamaterials have the potential to revolutionize material capabilities, by switching between different properties. The active mechanical metamaterial presented here can be remotely programmed to switch between compressive and shear deformation modes that cause stark changes in stiffness. The considered metamaterial uses controlled instabilities to change the buckling mode of electro-thermally activated beams. The beams form electrical circuits. When selectively charged, they heat (and soften). The effects of manufacturing imperfections are overcome by connecting the beams to a compliant mechanism, allowing reliable control over the compressive buckling modes that cause the stiffness changes. Connection points in the metamaterial resemble a fish-bone structure, known to exhibit static nonreciprocity, which is actively controllable within the considered metamaterial. As such, it is shown (computationally) that this metamaterial is capable of modulating traction and pressure across a surface. Pressure can be doubled between adjacent unit-cells while traction can be shielded (i.e., zero) in selected regions. This concept has potential applications in robotic gripper interfaces, and medical devices.