Bone is a hierarchically structured material with remarkable mechanical performance which may serve as a model for the development of biomimetic materials. Understanding its properties is essential for the assessment of diseases such as osteoporosis. This will lead to a critical evaluation of current therapies and aid in their more targeted development. While the full hierarchical structure of bone is extremely complex and variable, its basic building block, the mineralized collagen fibril, is rather universal. Due to the progress in experimental methods to characterize materials at the nanoscale, new insights have been gained into the structure/mechanical function relation in this nanocomposite. The amount of mineral is usually thought to determine the stiffness of the material, but recent results suggest that the properties of the organic matrix as well as the geometrical arrangement of the two components might have a much larger influence on the properties than traditionally assumed. Some recent results from experiment and numerical modeling leading to these ideas are reviewed.

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.