In the past decade, mechanical metamaterials have garnered increasing attention owing to its novel design principles which combine the concept of hierarchical architecture with material size effects at micro/nanoscale. This strategy is demonstrated to exhibit superior mechanical performance that allows us to colonize unexplored regions in the material property space, including ultrahigh strength-to-density ratios, extraordinary resilience, and energy absorption capabilities with brittle constituents. In the recent years, metamaterials with unprecedented mechanical behaviors such as negative Poisson's ratio, twisting under uniaxial forces, and negative thermal expansion are also realized. This paves a new pathway for a wide variety of multifunctional applications, for example, in energy storage, biomedical, acoustics, photonics, and thermal management. Herein, the fundamental scientific theories behind this class of novel metamaterials, along with their fabrication techniques and potential engineering applications beyond mechanics are reviewed. Explored examples include the recent progresses for both mechanical and functional applications. Finally, the current challenges and future developments in this emerging field is discussed as well.

The sp 2 nature of graphene endows the hexagonal lattice with very high theoretical stiffness, strength and resilience, all well-documented. However, the ultimate stretchability of graphene has not yet been demonstrated due to the difficulties in experimental design. Here, directly performing in situ tensile tests in a scanning electron microscope after developing a protocol for sample transfer, shaping and straining, we report the elastic properties and stretchability of free-standing single-crystalline monolayer graphene grown by chemical vapor deposition. The measured Young's modulus is close to 1 TPa, aligning well with the theoretical value, while the representative engineering tensile strength reaches~50-60 GPa with sample-wide elastic strain up to~6%. Our findings demonstrate that single-crystalline monolayer graphene can indeed display near ideal mechanical performance, even in a large area with edge defects, as well as resilience and mechanical robustness that allows for flexible electronics and mechatronics applications.

Flexible fiber-shaped supercapacitors (FSSCs) are recently of extensive interest for portable and wearable electronic gadgets. Yet the lack of industrial-scale flexible fibers with high conductivity and capacitance and low cost greatly limits its practical engineering applications. To this end, we here present pristine twisted carbon fibers (CFs) coated with a thin metallic layer via electroless deposition route, which exhibits exceptional conductivity with ∼300% enhancement and superior mechanical strength (∼1.8 GPa). Subsequently, the commercially available conductive pen ink modified high conductive composite fibers, on which uniformly covered ultrathin nickel-cobalt double hydroxides (Ni-Co DHs) were introduced to fabricate flexible FSSCs. The synthesized functionalized hierarchical flexible fibers exhibit high specific capacitance up to 1.39 F·cm in KOH aqueous electrolyte. The asymmetric solid-state FSSCs show maximum specific capacitance of 28.67 mF·cm and energy density of 9.57 μWh·cm at corresponding power density as high as 492.17 μW·cm in PVA/KOH gel electrolyte, with demonstrated high flexibility during stretching, demonstrating their potential in flexible electronic devices and wearable energy systems.