Perspectives for multiphase mechanical metamaterials

Mechanical metamaterials have been attracting increasing attention for a wide range of industrial applications, such as energy storage, biomedical, sensors, robotics, etc., owing to their artificially-controllable unique mechanical properties beyond those of naturally existing materials. Recently, multimaterial additive manufacturing (AM) techniques, coupled with significant progress made in design methodologies, have accelerated the developments of multiphase mechanical metamaterials advancing the concept to "make the best use of different materials". To this end, we first review the up-to-date progress of multiphase mechanical metamaterials regarding their design, fabrication processes, unique mechanical properties and applications. Then, we articulate and conclude the major challenges, e.g., interface and connection, uncertainty and inconsistency, among others, for multiphase mechanical metamaterials. Finally, we outline the perspectives of these metamaterials for future key opportunities.