Hierarchical interconnections in the nano-composite material Bone: Fibrillar Cross-Links resist fracture on several length scales

Bone is a complex and very important multi-constituent bio-composite. In this work, we focus on the arrangement of bone constituents from the nanoscopic to the microscopic scale, and investigate the influence of their arrangements on the fracture mechanisms of the whole composite. We find that bone, on the nanoscopic scale, consists of mineralized collagen fibrils held together by a non-fibrillar organic matrix, which results in a primary failure mode of delamination between mineralized fibrils. In turn, these mineralized fibrils form one of three types of filaments that span microcracks in fractured bone samples, possibly resisting the propagation of these cracks. Bone is one of the most important and most complex biominerals. It has become the focus of intensive study in light of the rising average population age and associated bone diseases. In recent years, it has become increasingly clear that, in addition to the insights achieved in the biological sciences and medicine, information on the nanoscopic architecture of the bone is needed to understand and predict bone fracture, e.g. [1]. Bone consists mainly of mineralized collagen fibrils. The collagen fibrils, being the main organic component in bone, are reinforced with nanoscale hydroxyapatite particles [2-8]. This results in a mineral reinforced protein fibril of approximately 50-100 nm diameter. These fibrils are the elementary building block for the large variety of bones in the body. To facilitate the function of the specific bone, they are arranged in several possible patterns [9].