School of Chemistry, Key Laboratory of Bio-inspired Smart Interfacial Science and Technology of Ministry of Education, Beihang University, Beijing 100191, People's Republic of China.
School of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021, People's Republic of China.
Proc Natl Acad Sci U S A. 2022 Dec 6;119(49):e2211458119. doi: 10.1073/pnas.2211458119. Epub 2022 Nov 28.
Natural structural materials typically feature complex hierarchical anisotropic architectures, resulting in excellent damage tolerance. Such highly anisotropic structures, however, also provide an easy path for crack propagation, often leading to catastrophic fracture as evidenced, for example, by wood splitting. Here, we describe the weakly anisotropic structure of (ginkgo) seed shell, which has excellent crack resistance in different directions. Ginkgo seed shell is composed of tightly packed polygonal sclereids with cell walls in which the cellulose microfibrils are oriented in a helicoidal pattern. We found that the sclereids contain distinct pits, special fine tubes like a "screw fastener," that interlock the helicoidal cell walls together. As a result, ginkgo seed shell demonstrates crack resistance in all directions, exhibiting specific fracture toughness that can rival other highly anisotropic natural materials, such as wood, bone, insect cuticle, and nacre. In situ characterization reveals ginkgo's unique toughening mechanism: pit-guided crack propagation. This mechanism forces the crack to depart from the weak compound middle lamella and enter into the sclereid, where the helicoidal cell wall significantly inhibits crack growth by the cleavage and breakage of the fibril-based cell walls. Ginkgo's toughening mechanism could provide guidelines for a new bioinspired strategy for the design of high-performance bulk materials.
天然结构材料通常具有复杂的多层次各向异性结构，从而具有出色的损伤容限。然而，这种高度各向异性的结构也为裂纹的扩展提供了一个容易的途径，这常常导致灾难性的断裂，例如，银杏种子壳的弱各向异性结构，其在不同方向上具有出色的抗裂性。银杏种子壳由紧密堆积的多边形石细胞组成，细胞壁中的纤维素微纤维呈螺旋状排列。我们发现石细胞中含有明显的凹陷，类似于“螺丝紧固件”的特殊细管，将螺旋细胞壁相互锁定。因此，银杏种子壳在各个方向上都具有抗裂性，表现出特定的断裂韧性，可以与其他高度各向异性的天然材料（如木材、骨骼、昆虫外骨骼和珍珠母）相媲美。原位表征揭示了银杏独特的增韧机制：凹坑引导的裂纹扩展。这种机制迫使裂纹偏离弱的复合中层，并进入石细胞，其中螺旋细胞壁通过基于纤维的细胞壁的劈开和断裂显著抑制裂纹的扩展。银杏的增韧机制为设计高性能块状材料提供了新的仿生策略的指导。
