Fig. 1: Overview of bone’s hierarchical structure and bone’s toughening mechanisms.

a Bone’s hierarchical structure shows features ranging from the macroscale to the nanoscale. b Bone’s extrinsic toughening mechanisms are illustrated using a 3D SRμCT image acquired during a toughness test. Extrinsic toughness mechanisms occur in the wake of the crack (green) and are attributed to microscale features in the bone. c Bone’s intrinsic toughening mechanisms, which are attributed to nanoscale features, occur ahead of the crack and are primarily attributed to aspects of collagen structure and composition. Fibrillar sliding and uncoiling of the collagen structure are mechanisms that contribute to intrinsic toughness in bone. There are additional mechanisms that impact bone’s intrinsic toughness, such as collagen cross-linking and sacrificial bonds. Collagen cross-linking at the nanoscale is known to influence mechanical properties³⁴. Sacrificial bonds form between collagen fibrils creating a glue-like layer that contributes to bone’s resistance to fracture^32,36. Figure adapted from Zimmermann et al.¹¹⁵, with permission from The Proc. Natl. Acad. Sci. USA.