Mechanical metamaterials have emerged and received increasing interests in recent years due to their unique properties and unprecedented mechanical behaviors—ultrahigh strength-to-density ratios, extraordinary resilience with brittle constituents, tunable stiffness, negative Poisson’s ratio, negative compressibility, and vanishing shear modulus—owing to its novel design principles in utilizing various topological configurations and micro/nanoscale size effects, in addition to the intrinsic properties of its constituents. Besides enabling us to colonize unexplored regions in the material property space, the deformation mechanism of these metamaterials could be rationally controlled to cater specialized engineering applications across a multitude of fields, such as structural materials, energy generation and storage, biomedical, photonics, acoustics, electromagnetics, and thermal management, to name a few.
Researchers from City University of Hong Kong (CityU) and Massachusetts Institute of Technology (MIT) have provided a general insight into the developments and recent progress made in the field of mechanical metamaterials with critical feature sizes spanning multiple length scales (from nano to macro). In particular, they focus on the design concepts, fabrication methods, and engineering applications that have produced the most breakthroughs within the past decade. They identified the core theories behind various classes of mechanical metamaterials and how each could be integrated to alter the properties and behavior of a material. All stages of development, from design and modeling, up to testing and implementation in numerous applications are reviewed.
Overall, it is hoped that this overview can equip researchers in this emerging field with knowledge of the fundamentals and state-of-the-art developments in these various areas, providing possible directions that could be further investigated and could potentially pave a way to the industrial application of mechanical metamaterials across multiple disciplines.