Architected Spiral Microstructures Enable Isotropic Fracture Toughness Beyond Nacre

Abstract Nacre‐inspired materials offer excellent fracture toughness through hierarchical architectures, yet their mechanical performance remains limited by pronounced anisotropy. Here, two novel bioinspired microstructures—beetle‐like and hexagonal spiral—are presented that enable isotropic toughening in microstructural composites. Using multi‐material 3D printing, architected specimens are fabricated and fracture tests are performed along orthogonal loading directions. The beetle‐like structure yields critical fracture toughness J 0 = 5.2 and 2 kJ m − 2 along the x ‐ and y ‐ directions, respectively, whereas the hexagonal spiral structure yields J 0 = 5 and 5.5 kJ m − 2 , indicating a nearly orientation‐independent response. In the x ‐direction, compared with the nacre‐like structure, J 0 increases by 61% and 56% for the beetle‐like and hexagonal spiral designs, respectively. While the beetle‐like design improves directional toughness, only the hexagonal spiral architecture achieves both enhanced and nearly isotropic fracture resistance. Phase‐field fracture simulations reveal how spiral geometry facilitates crack deflection and mixed‐mode fracture behavior across multiple orientations. These findings establish a generalizable strategy for overcoming anisotropy in bioinspired composites and provide a robust microstructural platform for high‐performance structural materials under complex loading conditions.