Microscopic Auxetic Hierarchical Mechanical Metamaterials exhibiting controllable Mechanical Properties and Shape Morphing

Abstract

Shape morphing [1, 2, 3] and the ability to control mechanical properties of functional materials [4, 5] remain some of the biggest challenges in the field of materials science. This stems from the fact that these properties, with the emphasis on the unusual types of mechanical behavior such as the auxeticity or negative stiffness, are of great significance in the case of numerous applications. Some of these applications include protective equipment, vibration damping and biomedical devices. Particularly important in this regard is a recent progress made in the field of hierarchical mechanical metamaterial [6, 7], i.e. structures composed of components being able to deform irrespectively to the rest of the system. The concept of hierarchy implemented in mechanical metamaterials enables them to exhibit a range of different mechanical properties without being reconstructed. This in turn makes it possible to potentially construct mechanical metamaterials capable of adjusting their mechanical response depending on the external stimulus. Nevertheless, despite several significant achievements reported in this field, studies related to hierarchical mechanical metamaterials are still at the very early stage and hierarchical structures reported in the literature normally share several limitations. First of all, it is worth mentioning that a vast majority of known hierarchical metamaterials are two-dimensional which significantly limits their applicability since they cannot respond to the stimulus applied from an arbitrary direction. However, an even larger limitation is the fact that the behavior of the currently known hierarchical mechanical metamaterials has been primarily studied at the microscale. On the other hand, the possibility of such a system to exhibit the programmable shape morphing would be of great significance at the much lower scales including the microscale where it could be utilized in the design of novel robotics and biomedical devices. In this work, we propose a novel microscopic 2D and 3D hierarchical metamaterial capable of exhibiting a wide range of auxetic behavior depending on the variation in its geometric parameters. We also show that the considered system is capable of shape morphing at the microscale where it can assume an arbitrary predefined shape. Finally, we show that in addition to the auxetic behavior, the considered hierarchical structure can exhibit very different rates of energy absorption and different values of stiffness. All of these results indicate that the proposed concept can be utilized in the design of novel types of robotics as well as protective equipment and vibration damping materials.