纳米纤维介导的骨抗断裂作用
Nanofibril-mediated Fracture Resistance of Bone
摘要：
天然硬质复合材料（如人骨）具有超过其成分和许多工程复合材料的强度和韧性。这种增强归因于它们复杂的层次结构，跨越多个长度尺度;在骨中，特征尺寸范围从纳米级原纤维到微米级骨片到中尺度骨单位和宏观尺度器官。已经研究了骨的机械性能，理解到与整个组织相比，微米和纳米尺度的孤立微结构产生了更好的强度，并且组织相对于其纳米级成分具有放大的韧性。由于在小尺寸样品上进行断裂实验的挑战，在允许隔离显著微观结构特征的样品尺寸下，骨的纳米级增韧机制尚未得到充分理解。我们开发了一种原位三点弯曲实验方法，该方法可以探测硬质材料的微米级试样的特定部位断裂行为。利用这一点，我们量化裂纹萌生和增长韧性的人骨小梁尖锐的疲劳预裂纹和钝缺口。我们的研究结果表明，具有疲劳裂纹的骨的韧性是具有钝性裂纹的骨的两倍。原位数据相关的电子显微镜视频显示，这种行为是由纳米纤维结构的裂纹桥接。结果表明，纤维桥接（~1 $μ$ m）和裂纹偏转/扭转（~500 $μ$ m）之间的过渡作为长度尺度的函数，并定量地证明了复杂材料中的层次诱导增韧。这种通用的方法能够量化各种复杂材料系统中韧性和微观结构之间的关系，并为设计仿生复合材料提供直接的见解。
英文摘要：
Natural hard composites like human bone possess a combination of strength and toughness that exceeds that of their constituents and of many engineered composites. This augmentation is attributed to their complex hierarchical structure, spanning multiple length scales; in bone, characteristic dimensions range from nanoscale fibrils to microscale lamellae to mesoscale osteons and macroscale organs. The mechanical properties of bone have been studied, with the understanding that the isolated microstructure at micro- and nano-scales gives rise to superior strength compared to that of whole tissue, and the tissue possesses an amplified toughness relative to that of its nanoscale constituents. Nanoscale toughening mechanisms of bone are not adequately understood at sample dimensions that allow for isolating salient microstructural features, because of the challenge of performing fracture experiments on small-sized samples. We developed an in-situ three-point bend experimental methodology that probes site-specific fracture behavior of micron-sized specimens of hard material. Using this, we quantify crack initiation and growth toughness of human trabecular bone with sharp fatigue pre-cracks and blunt notches. Our findings indicate that bone with fatigue cracks is two times tougher than that with blunt cracks. In-situ data-correlated electron microscopy videos reveal this behavior arises from crack-bridging by nanoscale fibril structure. The results reveal a transition between fibril-bridging (~1 $μ$m) and crack deflection/twist (~500 $μ$m) as a function of length-scale, and quantitatively demonstrate hierarchy-induced toughening in a complex material. This versatile approach enables quantifying the relationship between toughness and microstructure in various complex material systems and provides direct insight for designing biomimetic composites.
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