Trade-offs are present in every facet of our lives, and neither are the materials that we use today exempted from this issue. Material properties are often interconnected, where increasing a certain property will degrade another. Mechanical metamaterials represent a new paradigm of materials where the intrinsic property of a material is combined with the concept of architecture to produce lightweight materials with unprecedented properties that are otherwise unattainable with conventional bulk materials. Recent developments in additive manufacturing (AM) have enabled the creation of mechanical metamaterials with controllable architectures spanning across multiple orders of magnitude, from the nano and up to the macro-scale. By employing these additive manufacturing technologies in conjunction with a myriad of post-processing techniques such as thin film deposition, thermal annealing, and chemical modification, we designed, fabricated, and characterized a wide range of multiscale mechanical metamaterials with exceptional mechanical and/or functional properties. In this thesis, via multiscale mechanical metamaterials, we aim to abate or overcome the intrinsic coupling between several material properties, which represent some of the most elusive and long-standing challenges in the materials selection for engineering applications.

We first conducted a thorough review and summarized the developments and recent progresses made in the field of mechanical metamaterials with critical feature sizes spanning multiple length scales (from nano to macro). Particularly, the design concepts, fabrication methods, and engineering applications that have produced the most breakthroughs within the past decade are emphasized, laying the foundation for creating high performance mechanical metamaterials.

We then to developed lightweight, high-strength, and fracture-resistant microlattice metamaterials with core-shell architecture, a strategy widely found in nature, such as human bones, plant stems, and turtle shells to provide exceptional structural integrity at low densities. By harnessing size-induced ductility through the combination of ultrahigh resolution AM and multi-component alloys, we created metallic nanolattices that can exhibit a unique wrinkling deformation behavior, completely suppressing brittle fracture of the nanolattices even at large compressive strains of up to 50%. The nanolattices could exhibit an exceptional balance between strength, density, and ductility that outperforms previously reported micro/nanolattice metamaterials.

Although size-induced ductility is a highly effective phenomenon that can be exploited to manufacture mechanically robust micro-scale materials and devices, its efficacy diminishes at the macro-scale. Therefore, we proposed alternative approaches to create light, strong, and ductile mechanical metamaterials which mechanisms could be employed at the centimeter scale. One of the approaches involves fabricating partially carbonized lattices by using low-temperature pyrolysis that could convert a low-strength and brittle polymer lattice into a high-strength and ductile lattice. The strength is mainly derived from the formation of interconnected graphene-like and diamond-like carbon flakes, while the ductility comes from the presence of polymer chains that inhibits the shear fracture of the graphene sheets. Another method involves the fabrication of chemically engineered hydrogel-based lattices, where various energy dissipation mechanisms are incorporated into a soft hydrogel such as ionic crosslinking and hydrogen bonding to enhance its mechanical properties. Owing the flexibility of the fabrication process, it is possible to tune the mechanical properties of these hydrogel-based metamaterials across more than 3 orders of magnitude, which is useful for dynamic applications such as impact attenuation in personal protective equipment.

Finally, we aim to decouple material property trade-offs beyond mechanics and create multifunctional metamaterial-based devices. Specifically, we proposed a novel strategy to overcome 2 of the biggest problems in thermoelectric generators (TEGs): (1) inherent brittleness of its constituents, and (2) heat stagnation in its legs, arising from the coupling between electrical and thermal conductivity, which limits power conversion efficiency (PCE). We developed 3D architected thermoelectric generators (A-TEGs) that are not only strong and ductile, but also exhibits high electrical conductivity and thermal insulation properties, all desirable for high performance TEGs. Our A-TEG can exhibit ultrahigh toughness and power conversion efficiency, which is unlike any other TEGs developed thus far.

In summary, this thesis systematically investigated several approaches to create high performance mechanical metamaterials at multiple length scales which could be used to overcome material property trade-offs and colonize unexplored regions in the material property space. The methods employed could potentially create a paradigm shift for the design and manufacture of next-generation mechanical metamaterials for both structural and functional applications.