Heterogeneous mechanical metamaterials: design paradigms, additive manufacturing, and emergent multifunctionality

Heterogeneous metamaterials constitute a new frontier in architected materials, combining multiphase compositions, hierarchical architectures, and coupled multi physics interactions to achieve unprecedented mechanical and functional performance. By harnessing synergistic interactions among diverse material phases, unit cell topologies, and physical mechanisms, these metamaterials enable programmable, adaptive, and multifunctional responses far beyond those of homogeneous systems. This review provides a systematic synthesis of six major design strategies, multiphase design, hybrid architectures, interpenetrating composites, multi material fusion, multifeature integration, and data driven approaches, emphasising their structure–mechanism–property relationships and nonlinear coupling effects. We critically analyse how structural heterogeneity governs deformation evolution, energy dissipation, and cross field functionality, and highlight their broad potential in impact protection, flexible electronics, acoustic control, adaptive actuation, and thermal management. Finally, we identify key challenges in correlating multiscale structures with emergent functions, improving manufacturing scalability, and developing unified modelling frameworks. By bridging fundamental design principles with engineering applications, this review outlines the roadmap towards intelligent heterogeneous metamaterials, offering fresh perspectives for next generation multifunctional systems.