Schematic diagrams of 3D chiral structures and unit cells. (a) 3D mechanical metamaterial with a twist [89]. (b) 3D auxetic structure with circular loops [90]. (c) 3D auxetic structure with square loops [90]. (d) 3D tetrachiral, anti-tetrachiral, and hybrid-tetrachiral structures [91]. (e) 3D cross-chiral structure [92]. (f) 3D cross-chiral structure with isotropic auxetic behavior [92]. (g) 3D isotropic anti-tetrachiral metastructure [93]. (h) 3D alternating anti-tetrachiral structures [94]. (i) Node-enhanced 3D chiral NPR structure based on missing-rib architecture [21].