Other structures of auxetic metamaterials. (a) General 4-vertex origami unit cell structure [106]. (b) Interlocking hexagonal model structure [108]. (c) Assembly-free double-ring connected structure [109]. (d) Egg-box structure [110]. (e) Node-fibril structure [111]. (f) 3D generalized tethered-nodule model [112]. (g) 2D negative Poisson's ratio material designed via Voronoi tessellation and conditional generative adversarial network [113]. (g) Modular chiral origami structure. (i) Programmable NPR metamaterial structure [115].