Fig. 1. Classification of mechanical metamaterials^[2-6]

Fig. 2. Lattice metamaterials. (a) Typical lattice mechanical metamaterials^[2]; (b) bending properties^[30]; (c) fatigue properties^[31]; (d) shearing properties^[32]; (e) compression properties^[34]

Fig. 3. Plate lattice metamaterials and their mechanical properties. (a) Typical single cell structure of plate lattice metamaterials; (b) composite plate lattice metamaterials with different structures fabricated via AM; (c) comparison of elastic modulus and yield strength between plate lattice metamaterials and lattice metamaterials

Fig. 4. TPMS metamaterials. (a) Common TPMS metamaterials^[4]; (b) heterogeneous TPMS metamaterials^[67]; (c) gradient TPMS metamaterials and stress-strain curves^[68]; (d) hybrid controllable anisotropic TPMS metamaterials^[69]; (e) combined TPMS metamaterials^[70]

Fig. 5. Design of metamaterials for bionic bamboos and beetle forewings. (a) Macroscopic morphology and microstructure of bamboo^[81]; (b) tubular metamaterial of bionic bamboo with ribs^[81]; (c) gradient metamaterial of bionic bamboo^[80]; (d) macrostructure of beetle forewing^[82]; (e) microstructure of beetle forewing^[84]; (f) metamaterial of bionic beetle forewing^[85]; (g) tubular metamaterial of bionic beetle forewing^[86]

Fig. 6. Design of cellular metamaterials. (a) Curved rod-shaped cylindrical metamaterial^[5]; (b) fractal self-similar regular hexagonal bionic honeycomb metamaterial^[88]; (c) macrostructure and microstructure of sculptodon and its reconstruction design^[6]; (d) gradient honeycomb metamaterial^[90]; (e) bionic lattice metamaterial^[91]