Multi-materiality in metamaterials for enhanced properties and multifunctionalities: progress and opportunities

Metamaterials are highly promising as they possess unique properties that are not available in nature. Single material-based metamaterials often show limited functionality due to constrained design space, and in response multi-materiality has been employed in the design space to address this issue in recent years. In this work, we for the first time comprehensively review the state-of-the-art developments in multi-materiality implementation for the major types of metamaterials, including mechanical, electromagnetic, thermal, acoustic, and hybrid metamaterials. The systematic survey covers the needs, strategies, and applications of multi-materiality in metamaterials. For each major metamaterial type, a corresponding in-depth analysis is provided in terms of multi-materiality innovations, structure designs, fabrication, and performance evaluation. It is highlighted that many unprecedented properties have been obtained based on multi-materiality considerations, such as adjustment of stiffness and Poisson’s ratio with magnet inclusions, simultaneous achievement of electromagnetic and sound waves absorption, realization of wide range tunable coefficient of thermal expansion and mechanical stiffness. More importantly, the review enlightens the challenges that are encountered on employment of multiple materials in metamaterials such as weak joints, low material compatibility, inconsistency, limitations of manufacturing techniques, and design optimization. Accordingly, research opportunities are pointed out. It is hoped that the review will stimulate further developments for this frontier field of metamaterials, and the future technological breakthroughs will redefine the understanding of multi-material metamaterials.