Cooperative deformation of mineral and collagen in bone at the nanoscale

TLDR

Bone’s high toughness is attributed to nanoscale anisotropic mineral particles reinforcing collagen, with load transfer proposed to occur via tensile strain transfer between mineral particles through shearing in collagen layers. The study investigates how mineral particles and collagen fibrils deform together during tissue deformation. In situ tensile testing with synchrotron X‑ray diffraction and scattering revealed that tissue, fibrils, and mineral particles experience strain in a 12:5:2 ratio, indicating fibril–matrix decoupling that protects mineral while redistributing strain. Mineral nanoparticles and fibrils deform elastically but differently, with mineral strain up to 0.15–0.20%—about twice bulk apatite’s fracture strain—supporting a staggered load‑transfer model that highlights bone’s hierarchical deformation.

Abstract

In biomineralized tissues such as bone, the recurring structural motif at the supramolecular level is an anisotropic stiff inorganic component reinforcing the soft organic matrix. The high toughness and defect tolerance of natural biomineralized composites is believed to arise from these nanometer scale structural motifs. Specifically, load transfer in bone has been proposed to occur by a transfer of tensile strains between the stiff inorganic (mineral apatite) particles via shearing in the intervening soft organic (collagen) layers. This raises the question as to how and to what extent do the mineral particles and fibrils deform concurrently in response to tissue deformation. Here we show that both mineral nanoparticles and the enclosing mineralized fibril deform initially elastically, but to different degrees. Using in situ tensile testing with combined high brilliance synchrotron X-ray diffraction and scattering on the same sample, we show that tissue, fibrils, and mineral particles take up successively lower levels of strain, in a ratio of 12:5:2. The maximum strain seen in mineral nanoparticles (≈0.15–0.20%) can reach up to twice the fracture strain calculated for bulk apatite. The results are consistent with a staggered model of load transfer in bone matrix, exemplifying the hierarchical nature of bone deformation. We believe this process results in a mechanism of fibril–matrix decoupling for protecting the brittle mineral phase in bone, while effectively redistributing the strain energy within the bone tissue.