# Bone: An Outstanding Composite Material
## 2 Nanostructure
The mechanical properties of the organic and inorganic phases together with their hierarchical arrangement confer bone features that determine its characteristic strength and toughness [27]. The mineral content predominantly contributes to bone stiffness and strength. Therefore, when the organic collagen in bone is removed--by heat or leach--mature bone becomes brittle [25; 26; 28]. Based on a computational study [27] where nanoindentation data were considered, single hydroxyapatite crystals demonstrated stiffness values of 150 GPa and 143 GPa in the [0001] and [1010] direction, respectively. In a more recent study, [PERSON] et al. [29] identified the presence of an organic substance binding the mineral components together, and the authors believe that it contributes to the strength, stiffness, and energy absorption of bone under compression. In a thorough study [24], [PERSON] and [PERSON] demonstrated that the increase in the elastic modulus, ultimate strength, and fracture energy of the mineralized collagen fibrils networks was significantly influenced by both the increased uniformity of mineral distribution and stronger interactions between the components and was modestly influenced by the rise in total volume fraction of minerals. This study also concluded that the mineral distribution had the most drastic influence on the elastic modulus compared to the ultimate strength and fracture energy. However, bone strength does not depend solely on bone mineral distribution, but also on the quality and material properties of the organic component where collagen cross-links play an important role in the reinforcement of bone strength [30]. Significant determinants of the functional integrity of the organic phase are the degree and nature of enzymatic lysyl oxidase-mediated collagen crosslinks and the negative effects of non-enzymatic glycation-induced collagen crosslink (the production of advanced glycation end-products such as pentosidine), which can be considered in a competitive relationship with enzyme-derived cross-linking [31; 32]. Enzymatic cross-linking density has been related to governing post-yield and large-scale mechanics, and cross-link strength governs failure strain, hence improving fracture toughness, bone strength, and stiffness [33; 34; 35]. Meanwhile, non-enzymatic cross-linking is related to the prevention of energy absorption by microdamage formations, reducing bone toughness and ultimate strain [36; 37] and accelerating brittle fracturing [38; 39]. Mineralized collagen fibrils are embedded in an extrafibrillar matrix that consists of water, mineral, and non-collagenous proteins [37]. A recent study [9] used positron annihilation lifetime spectroscopy (PALS), a technique sensible to voids/pores at the nanometer and subnanometer scale, to observe these structures. It was noticed that water mainly occupies the regions in intercollagen molecular spaces, terminal segments (D-spacing) within collagen microfibrils, and interface spacing between collagen and mineral structures. Water was also identified as being loosely bound to the surfaces of collagen fibrils [12]. The bone water content is an essential component since it confers much of bone's unique strength and resilience by stress reduction during dynamic loading [40; 41]. Hence, removing bound water in the bone matrix makes bone stiffer, stronger, but more brittle at different length scales, because dry collagen loses its deformation and energy absorption capacity [42]. As stated by [PERSON] et al. [13], the lack of collagen external water promotes the formation not only of new inter-tropocollagen hydrogen bonds but also of intra-tropocollagen ones, which cause the microfibril gain of compactness and tubularity. These molecular and supramolecular changes are responsible for the generation of very large stresses up to 100 MPa when the collagen undergoes complete dehydration [43].