Double-arrow and star-shaped structures. (a) Illustration of the double-arrow structure before and after uniaxial compression deformation [16]. (b) Double-U auxetic honeycomb structure [38]. (c) Schematic of the planar double-arrow unit cell under load along the Z-axis [37]. (d) Star-shaped structures with n = 1, 3, 4, and 6 [50]. (e) The definition of the auxetic angle, and the effective Young’s modulus, shear modulus, 2D bulk modulus, and Poisson’s ratio of numerical models with different auxetic angles [51].