Raphia hookeri seed-inspired weakly anisotropic nanocomposites

The composites inspired by nacre, bone and teeth etc, often display anisotropic mechanical properties due to the high aspect ratio of their structural units, resulting in sudden failures under specific loading conditions. It still remains great challenge for fabricating the nanocomposites with weakly anisotropic mechanical properties. Herein, we discover the unique structure of the Raphia hookeri seed with a weakly anisotropic crack resistance in multiple directions. The remarkable mechanical properties stem from its tightly bonded, isodiametric cells and interlocking pits. Drawing inspiration from Raphia hookeri seed, we demonstrate an artificial Raphia hookeri seed nanocomposites by assembling layered polymethyl methacrylate-graphene microspheres with pits, which were then interwoven with polymethyl methacrylate to mimic the essential hierarchical structure. The artificial Raphia hookeri seed exhibits effective toughening performance across all crack orientations, and achieves a fracture toughness of up to 4.84 MPa m^1/2 and specific toughness of 4.28 MPa m^1/2/(Mg m^-3), which are higher than both natural Raphia hookeri seed and previously fabricated weakly anisotropic polymer nanocomposites. The optimal distribution of pits and enhanced interfacial interactions between microspheres effectively inhibit crack propagation, contributing to the artificial Raphia hookeri seed's superior toughness in all directions. This pit-enabled granular microsphere architecture provides an avenue for fabrication high-performance weakly anisotropic nanocomposites.