Fig. 2. Mechanical metamaterial based on Taiji gears.
a, Metamaterial architecture. The blue and green colours show different orientations of gears. b, A pair of meshing gears for P+(3°). The x, y and z denote the global coordinates; the origin of the local cylindrical coordinates for spin rotation θ is set at the gear centre. ta, the thickness of the arm. c, Typical Mises stress contour of a meshing pair for θ = 0 and compressive strain ε = 0.18%. Deformation is enlarged 20 times. d, Numerical value of log10(Ey(θ, β)) for P+(β). e, Measured loading–unloading stress–strain curves for metamaterials with different rotation angles θ. f,g, Young’s modulus as a function of the gear rotation angle θ for positive P+(3°) and negative P−(15°) polarities, respectively. Inset: the meshing pair for P−(15°) h, Directly printed micro metamaterial consisting of P+(0°) 5 × 6 Taiji gears, with close-up images showing dimensions. i, Ey(θ) of the P+(0°) micro specimen. The error bars and the average values in f,g and i are evaluated by choosing different intervals along the curve in e.