ABSTRACT
In the previous couple of chapters, we have discussed the nanomechanical properties of the hardest known biomaterials of the human body: tooth enamel. This structure is composed of a calcium phosphate compound or HAp-based composite materials. Further, it possesses a hierarchical structure ranging from the millimeter-to the angstrom-length scale. In this chapter, we will be discussing another biomaterial-human cortical bone-which also offers a hierarchical structure (Figure 44.1) like the tooth enamel. Natural bone is a composite material composed of organic compounds (mainly collagen) reinforced with inorganic compounds (HAp and other minerals). The most prominent structures seen at the nanoscale are the collagen fibers, surrounded and infiltrated by minerals. Bone builds its hierarchical architecture from these nanostructured building blocks. The detailed composition of bone differs, depending on species, age, dietary history, health status, and anatomical location. In general, however, the inorganic phase accounts for about 70% of the dry weight of bone and the organic matrix makes up the remainder. A critical survey of literature data [1-14] establishes clearly that we do not yet know with sufficient detail and accuracy the scientific reasons as to why and in which way the different compositional materials of bone are responsible for their varying nanomechanical properties. Moreover, to the best of our knowledge, the anisotropy in nanomechanical properties on cross section and plan section of the cortical bone has not yet been explored to a significant detail. The directional anisotropic properties of bone at the macrostructural length scale are well known. But we still need to know what happens at the microstructural length scale. This is what we want to explore in the present chapter. Eventually, the results will show us that the elastic properties of bone at local microstructural length scale do exhibit directional anisotropy that has not yet been explored to a significant detail.
