# Bone: An Outstanding Composite Material
## 2 Nanostructure  
Bone is composed of a weight fraction range of 65-70% inorganic component, 18-25% organic, and 10-12% water, whereas on a volume fraction range, the proportions for each component are between 33 and 43%, 32 and 44%, and 15 and 25%, respectively [9]. It should be noted that this ratio can vary depending on the age and health conditions of the human being [10]. The organic phase of bone composite consists primarily of type I collagen (90% by weight), some other minor collagen types (III and VI), and a variety of non-collagenous proteins such as laminin, fibronectin, vitronectin, osteoclast, osteonectin, osteopontin, and bone sialoprotein [11]. The basic structural unit of collagen is called tropocollagen (see Figure 1), which is a very rigid linear molecule with a diameter and length of 1.1 nm and 300 nm, respectively [12]. This molecule possesses a rearranged, right-handed, triple-helix structure formed by polypeptide chains with a highly repetitive amino acid sequence, glycine-X-Y, where glycine is being used in every third residue and X is often proline and Y is frequently hydroxyproline [13; 14]. This frequent occurrence of proline and hydroxyproline stabilizes the polypeptide strands (e.g., through interchain hydrogen bonds) and limits rotations, leading to its tightly packed triple-helical form [15]. The triple-helix structures are associated with microfibrils, where five tropocollagens are assembled in parallel (D-period) [13]. At the next level of the hierarchy, the tropocollagen molecules congregate into fibrils by being covalently cross-linked at their tips and their axial staggering, forming overlap zones and gaps with a specific periodicity of approximately 67 nm [12; 15]. The inorganic phase of bone is a ceramic crystalline-type mineral, as an impure form of naturally occurring calcium phosphate, most often referred to as biological non-stoichiometric hydroxyapatite. Bone hydroxyapatite is not pure hydroxyapatite, because the tiny apatite crystals contain impurities such as potassium, magnesium, strontium,  
Figure 1: Representation of the collagen fibers’ structural organization. The type I collagen major amino acid components Gly-Pro-HyPro constituting the \(\alpha\) chains are self-assembled in a triple-helix, tropocollagen structure. Stacks of mineral crystals lie between the congregated tropocollagen molecules to form mineralized collagen fibrils with a characteristic D-period. These fibril molecules are then packaged to form collagen fibers [16; 17; 18; 19]. sodium (in place of the calcium ions), carbonate (4 to 6% of the phosphate groups are replaced by carbonate groups), and chloride or fluoride (in place of the hydroxyl ions). These impurities reduce the crystallinity of the apatite and consequently alter some properties, such as solubility, which is crucial for mineral homeostasis and bone adaptation. There is a long ongoing debate about the nature of the mineral particle shape--needles versus platelets [20; 21; 22]. More recently, the existence of mineral particles with a fractal aggregate shape was also discovered [23]. At the submicron scale, bone is composed of building blocks of mainly type I collagen, mineral crystals, water, and non-collagenous proteins [24]. The organic and inorganic components of bone blend together to generate mineralized collagen fibrils, as demonstrated in Figure 1. The mineral component in bone occurs as stacks of thin polycrystalline sheets, which surround and lie between the collagen fibrils [12]. The collagen-mineral system is arranged in parallel arrays within thin sheets called lamellae. As stated previously, the organization of the lamellae differs between cortical and trabecular bone. As identified by [PERSON] et al. [25], the lamellae are assembled in a \"rotated plywood-like fashion\" in cortical bone. In cancellous bone and according to [PERSON] et al. [26], these structures are strongly fused, forming a sheet-like structure.