Significance Aiming at the demand of high-energy-absorbing and shock-resistant components for aerospace and transportation, metamaterials with energy absorption have been extensively studied, including truss-lattices, plateslattices, triply periodic minimal surfaces (TPMS), and bionic metamaterials. The truss-lattice metamaterials are the spatial structures formed by the multiple connecting rods with lattice or lattice-like arrangement, which possess high mechanical properties and energy absorption. The plate lattice metamaterials are the spatial structures in which the plate vertices replace the lattice nodes and have special plate arrangements. The plates can also generate multiple cavities through specific combinations, thereby achieving the functional effects of sound absorption and noise reduction. The TPMS metamaterials are the spatial structures that possess infinite periodic, continuous, and smooth surfaces in three independent directions. The surfaces have two disjoint regions in space. There are no sharp protrusions and depressions, which can decrease stress concentration. It is the best choice for manufacturing energy-bearing structures. Besides, the researchers have also found that its spatial configuration is similar to the structure of human bone, so it can be used to fabricate bone implants. The bionic metamaterials for energy absorption were first applied in 2000. The structures that exist in living organisms are the result of natural selection and evolution. They are known for their high specific energy absorption efficiency with small mass, which can be used for fabricating energyabsorbing components with impact resistance and energy absorption. Traditional manufacturing technologies are difficulte to fabricate these metamaterials for their complex structures. The additive manufacturing (AM) technology is based on the principle of discrete slicing and layer-by-layer stacking to rapidly fabricate components, which possesses high manufacturing freedom. The above-mentioned technical characteristics make it an effective way to manufacture energy-absorbing metamaterials with complex structures. Thus, researching and developing metamaterials with energy absorption mean a lot. Progress The design of truss-lattice metamaterials has first changed from the ordinary regular truss arrangement to the gradient arrangement, and then the solid parts inside the unit cell are reasonably distributed through topological optimization of the computational models to maximize the mechanical properties and energy absorption. However, the design of truss-lattice metamaterials has complex geometric models, diverse mechanical properties, and multidisciplinary. Therefore, the main research direction is to develope efficient and specific mathematical models for the truss-lattice metamaterial design. The plate-lattice metamaterial evolves based on the truss-lattice metamaterials, whose mechanical properties and energy absorption can be improved through some optimization strategies such as topological optimization. The combined method can realize optimization again, and the effect is remarkable. However, there are process constraints in the AM of plate-lattice metamaterials. The research direction of plate-lattice metamaterials is to optimize the metamaterial and develop a topology suitable for AM for maintaining excellent mechanical properties and energy absorption. The TPMS metamaterials with heterogeneous and gradient structures are developed. The homogeneous structures is first developed to improve energy absorption, and then the combined TPMS metamaterials to efficiently control the mechanical properties and energy absorption appear. However, diversified TPMS metamaterials have various boundary distributions, so the combination method cannot be simply pieced together. Developing a corresponding mathematical model to achieve the smooth transition of multiple structures needs to be solved urgently. Compared with the traditional metamaterials, the bionic metamaterials improve the energy absorption, and the bionic metamaterials also change from homogeneous forms to gradient forms to achieve high energy absorption. Later, the design of bionic metamaterials also changes from simply imitating their special macrostructures and microstructures to the combined design of biomimetic and lattice. Conclusions and Prospects At present, with the continuous progress of material design and AM technologies, additive manufacturing of intelligent metamaterials has become a new research direction to ensure that the bionic metamaterials do not break during the process of impact resistance and energy absorption, and the designability and repeatability of bionic metamaterials are greatly improved. Bionic smart metamaterials are developing towards imitating shapes, imitating properties, and imitating functions. However, the development of ultra-high recoverable and deformable smart materials and the design of bionic metamaterials are the current study barriers to the metamaterials with energy absorption, and also the main development direction. © 2022 Science Press. All rights reserved.
