**10.9. Synthetic apatite analogues in tissue engineering**

Biomaterials in general are based on the materials such as metals, polymers, and ceramics. Typical metallic biomaterials are based on stainless steel, cobalt-based alloys, titanium or titanium alloys and amalgam alloys. Polymeric biomaterial composites from monomers are based on amides, ethylene, propylene, styrene, methacrylates and/or methyl methacrylates. Biomaterials based on ceramics are found within all classical ceramic families (**Table 3**) including traditional ceramics, special ceramics, glasses, glass-ceramics, coatings and chemi‐ cally bonded ceramics (CBCs) [13].


**Table 3** Examples of biomaterials based on ceramics [13].

In the zinc phosphating solution, the addition of Zn2+ supports the formation of crystals of zinc phosphate. Zinc ions combine with phosphate ions to form an insoluble film. The formation

2 4 2

+ -+ - + + + +®

In the phosphate solution of coexisting Zn2+ and Ca2+, zinc calcium phosphate can be formed

It is notable that the phosphating mechanism varies in different phosphating systems and

The formation of conversion coating on zinc-coated samples under cathodic conditions was studied by PERRIN et al [107] in a chromating bath containing phosphate (phosphate-chro‐ mate solution). Thick chromium phosphate (Cr-P) coating has two distinct layers: an outer porous layer and an inner and thinner pore-free adherent one. Both layers contain chromi‐ um phosphate as the main constituent and, to a lower extent, zinc phosphate species, the concentrations of which decrease from the metal-coating interface outwards. Formed zinc

Biomaterials in general are based on the materials such as metals, polymers, and ceramics. Typical metallic biomaterials are based on stainless steel, cobalt-based alloys, titanium or

24 2 24 2 <sup>2</sup> Ca 2 Zn 2 H PO 2 H O CaZn PO 2H O 4 H ++ - <sup>+</sup> + + + ® ×+ (20)

2 2 Ca HPO CaHPO 4 4

( ) 2 2

phosphates have a general formula of *x*CrPO4·*y*Zn3(PO4)2·*z* H2O where *x* > (*y*, *z*).

**10.9. Synthetic apatite analogues in tissue engineering**

× + (15)

Zn 2 e Zn - + ® (16)

2 2 ( )<sup>2</sup> Zn 2 H O Zn OH H +® + (17)

Zn OH ZnO H O ( )<sup>2</sup> ® + <sup>2</sup> (18)

+ - + ® (19)

3 Zn 2 H PO 2 H 4 H O 6 e

In some cases, Zn and ZnO were found in the phosphating process in reactions:

of hopeite is described by the reaction [106]:

by the reactions:

materials [106].

( ) 2

34 2 2 2

Zn PO 4H O 3 H

478 Apatites and their Synthetic Analogues - Synthesis, Structure, Properties and Applications

Whereas many chemists and materials scientists consider the biomaterial to be synthetically produced material, most biologists, geologists and mineralogists consider the materials such as bone and tooth, which are biologically produced, to be the biomaterials. Also a common reference to Ca:P (1.67, **Table 7** in **Chapter 1**) ratio is usually used in the biomaterials litera‐ ture, which disregards the fact that different calcium phosphate phases have different crystalline structures.

There are many phosphate minerals and salts (**Table 4**) that do not have the crystalline structure of apatite [108],[109]. Apatite-based materials have attracted a considerable inter‐ est for orthopedic and dental applications because of their biocompatibility and tight bond‐ ing to bone, resulting in the growth of healthy tissue directly onto their surface. Several combinations of apatite and other phases were proposed in order to improve poor mechani‐ cal properties of apatite [110].



**Table 4** Different apatitic and non-apatitic calcium phosphates [108].

Small organic molecules incorporated in apatite crystals act as porogens that controlthe porous structure of apatite single crystal. The presence of amino acid under the apatite synthesis conditions leads to firm bindings and encapsulation of amino acid within apatite single crystals. The amino acid elimination by heating or electron beam irradiation enhances the pore formation in apatite single crystals. Moreover, the incorporation of acidic amino acid into apatite induces the peapod-like nanotubes in apatite single crystals. That suggests the potential of using small organics for nanostructural control of apatite single crystals, which would be valuable for enhancing thedrug loadings orfor modulating the materialdigestion in vivo [111].
