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Table 2. Biomechanical properties of healthy bone

Table 2. Biomechanical properties of healthy bone

Bone formation and remodelation is controlled by many factors: physical, chemical, hormonal, growth factors, and anti-mineralization agents (Karsenty 2000; Karsenty et al. 2009; Wallach et al. 1989). The bone tissue piezoelectricity is the key element which assists the bone formation, remodelation and growth, especially in the early stages of the bone formation and. The piezoelectricity of the bone is due, among the rests, due to the bone tissue anisotropy. The main elements which induce the anisotropy of the collagen based structures are: the anisotropy of the collagen molecules themselves and the anisotropy of collagen fibrils and fibres as well as the oriented morphology of the fibrils and fibres disposed in the isotropic extrafibrillar space (Hellmich et al. 2004).

Trying to obtain such composite with same composition and structure with the natural bone, the researchers elaborated many synthesis methods (Wahl and Czernuszka 2006) such as: in vitro collagen mineralization (Lawson and Czernuszka 1998), thermally - triggered assembly of hydroxy apatite/collagen gels (Pederson et al. 2003), vacuum infiltration of collagen into a ceramic matrix (Werner et al. 2002), enzymatic mineralization of collagen sheets (Yamauchi et al. 2004), freeze drying and supercritical point drying (Pompe et al. 2001), biomimetic synthesis (Li and Chang 2008).

One of the most promising bone graft material seems to be the collagen/nanohydroxyapatite composite (Li et al. 2006) due to its very good compositional and structural similarity with natural bone (Yunoki et al. 2007; Yunoki et al. 2006). The role of each component is not very well known. It is generally accepted that mineral phase, mainly containing hydroxyapatite, provide toughness and rigidity while the organic matrix provide tensile strength and flexibility of bone. In composite materials, collagen and hydroxyapatite play the same roles as they play in natural bone.

The mineral phase is deposited on the organic phase through electrostatic interactions between the carboxyl groups from collagen and Ca2+ from hydroxyapatite, but there is no unison about the mineralization sites. Many researchers assert that hydroxyapatite is deposited only onto the non-collagenous proteins and citrate anions (Rhee and Tanaka 1998) while others assert that mineralization occurs also on the pure collagen (Lin et al. 2004; Zhang et al. 2003). It is well known that materials properties are influenced not only by composition but also by morphology. This is the reason because even for similar composition of many bones their properties differ very much. Starting from these hypothesis, scientists tried to improve or induce new properties of the COLL/HA composite materials by addition of third component (or even more components) or by inducing different morphology. The influence of the morphology can be easily marked out by comparing the mechanical properties of compact and cancellous bone (Table 2) (Hench and Wilson 1993).

Collagen mineralization occurs due to the interactions which appear between collagenous structures and hydroxyapatite nanocrystals. In fact, these interactions occur between carboxylate groups and Ca2+ cations and can be illustrated as presented in Fig. 2. This hypothesis is supported by FTIR data and was explained based on the spectral shifts of C-O and C=O bands in pure collagen and COLL/HA composites (Ficai et al. 2009a).

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