Bone: A Composite Natural Material

Composite materials are solids with two or more distinct constituents at a larger scale than an atomic one (Hull, 1981). In accordance to this definition, materials or biological tissues such as wood, dentin and bone are regarded as composite natural materials (Cowin, 2001; Lakes, 2003; Park & Lakes, 1992). Bone is regarded as a natural composite material, which appears as it is in nature, rather than the artificial or man made composites. As for other composite materials, the mechanical properties of bone are dependent on its composition and structure, which includes the arrangement of the components and the bond between fibers and matrix. For example, the arrangement of fibers is different in several types of bones, gives rise to distinct properties. An understanding of the mechanical behavior of bone is extremely important for the evaluation of fracture risk. Some types of bones, like vertebrae are constantly submitted to compressive loads and have an increased risk of fracture. Most bones are composed by outer cortical bone and an inner trabecular bone. Fractures on vertebrae, hip and wrist tend to start the global structural failing process in the regions of trabecular bone with decreased bone mass and microarchitecture changes (Fratzl & Weinkamer, 2007; Gibson & Ashby, 1999). The emphasis of this chapter will be on the structure and mechanical properties of bone and how some diseases affect the material’s characteristics. This chapter has two main sections. Firstly, we will describe the structure of bone at different scales, including the two different types of bone, cortical and trabecular. As growth and remodeling are key processes in the continuous renewal of bone, the issue will be also focused in this section. Several techniques can be used to characterize the hierarchical structure of bone. A particular emphasis will be given to transmission electron microscopy, scanning electron microscopy, second-harmonic generation imaging with a two-photon microscope, microcomputed tomography, quantitative backscattered electron imaging, atomic force microscopy, small angle X-ray scattering and neutron scattering. The remaining part of the chapter will be devoted to the mechanical behavior of bone. As bones are often submitted to different types of external loadings, several types of tests will be reviewed namely compression, tension, bending, fatigue, creep, torsion and nanoindentation. Compression tests on femoral epiphysis of patients submitted to total hip