Ordered Fibril Arrays in Osteons Promote the Multidirectional Nanodeflection of Cracks: In Situ AFM Imaging.

The high fracture resistance of cortical bone is not completely understood across its complex hierarchical structure, especially on micro- and nanolevels. Here, a novel in situ bending test combined with atomic force microscopy (AFM) is utilized to assess the micro-/nanoscale failure behavior of cortical bone under the external load. Unlike the smoother crack path in the transverse direction, the multilevel composite material model endows the longitudinal direction to show multilevel Y-shaped cracks with more failure interfaces for enhancing the fracture resistance. In the lamellae, the nanocracks originating from the interfibrillar nanointerface deflect multidirectionally at certain angles related to the periodic ordered arrangement of the mineralized collagen fibril (MCF) arrays. The ordered MCF arrays in the lamellae may use the nanodeflection of the dendritic nanocracks to adjust the direction of the crack tip, which subsequently reaches the interlamellae to sharply deflect and finally form a zigzag path. This work provides an insight into the relationship between the structure and the function of bone at a multilevel under load, specifically the role of the ordered MCF arrays in the lamellar structure.