Sunflower-pith-inspired anisotropic auxetic mechanics from dual-gradient cellular structures

The sunflower stem pith is a natural, ultra-lightweight cellular material with superior mechanical properties that provides the required strong lodging resistance and high energy absorption efficiency. Here, we show that the sunflower pith exploits anisotropic auxeticity to adapt to changing environments. The characteristic dual-gradient structure of sunflower pith is found to be responsible for its anisotropic auxeticity. We demonstrate that the synergy of pore size gradient and the thickness gradient in sunflower pith’s microstructures leads to an alternative buckling deformation pattern at relatively larger strain, accomplishing both high energy absorption and sufficient high stiffness. Furthermore, the dual-gradient structure-based auxetic mechanism is used as inspiration to develop anisotropic auxetic cellular metamaterials. The design strategy for auxetic metamaterials revealed here can be extended into cellular structures with common convex polygons and provide insight into the design of auxetic cellular metamaterials for a wide range of realistic applications.