**2. Biomimetic nano-apatites**

In biology, calcium phosphates are the major inorganic constituents of bones, teeth, fish enameloid, deer antlers, and some species of shells [9]. Human hard tissues are composed principally of calcium phosphates with the exception of small portions of the inner ear. They are poorly crystalline carbonate-substituted nanosized apatites, with the exception of enamel, which has a high degree of crystallinity. Nanocrystalline apatites are nonstoichiometric (Ca/P ratio less than 1.67) and calcium (and OH)-deficient and may incorporate substituted ions in the crystal lattice (Na+ , Mg2+, K+ , Sr2+, Zn2+, etc.), in contrast to HA [Ca10(PO4)6(OH)2], which is the stoichiometric hydroxyapatite phase that is the most stable and least soluble calcium phosphate at physiological conditions. The nanocrystalline apatites exhibited higher solubility compared with HA; the responsible are calcium and hydroxide deficiencies. If they are submitted to humid environment, they are able to mature; as a result, "mature" bone crystals in vertebrates are less soluble and reactive than embryonic (young) bone mineral crystals [10].

The chemical composition of nanocrystalline apatites differs significantly from that of HA. The global chemical composition of biological apatites (or their synthetic analogues) can generally be described as

$$\text{Ca}\_{10-x}\text{(PO}\_4\text{)}\_{6-x}\text{(HPO}\_4\text{ or CO}\_3\text{)}\_x\text{ (OH or }\%\text{CO}\_3\text{)}\_{2-x}\text{ with }0 \le x \le 2\tag{1}$$

**35**

CO3

*Nature-Inspired Processes and Structures: New Paradigms to Develop Highly Bioactive Devices…*

surface layer containing water molecules and relatively weakly bound ions (e.g.,

The hydrated surface layer is responsible for most of the properties of biomimetic apatites. The role of bone mineral in homeostasis in vivo could be explained by the high surface reactivity of biomimetic apatites in relation to surrounding fluids (which is probably directly linked to a high mobility of ionic species contained within this layer). The ions inside the hydrated surface can potentially be exchanged by other ions from the surrounding solution or by small molecules, which may be exploited for couplings with proteins or drugs. It is interesting to remark that during the aging of the nanocrystals, the typical non-apatitic features mentioned above tend to be progressive. This process that has been related to the progressive growth of apatite domains at the expense of the surface hydrated layer is called "matura-

The metastability of such poorly crystallized nonstoichiometric apatites, which steadily evolve in solution toward stoichiometry and better crystallinity, is thought to be linked to the maturation process. This evolution can be, for example, wit-

Synthetic HA exhibits excellent biological properties such as biocompatibility, bioactivity, lack of toxicity, absence of inflammatory and immune responses, and relatively high bioresorbability. Improving their biomimetism, that is, by preparing them with dimensions, morphology, and nanostructure, can significantly enhance these properties and chemical characteristics that are similar to those found in biological apatites [9]. In the recent years, many different strategies have been employed in the preparation of synthetic nanosized HA crystals, with the most common method being stoichiometric titration of calcium hydroxide slurry with

Several methods have been successfully employed in the synthesis of nanocrystalline apatites, including wet chemical precipitation, sol-gel synthesis, coprecipitation, electrodeposition, vapor diffusion, and a number of others [13]. The physicochemical characterizations carried out on several synthesized apatites at low temperatures have shown that they have the typical features of biological apatite, such as the size domain, the low degree of crystallinity, and the existence of surface

The method of ionic substitution has been proposed for improving not only the biomimetic features of apatite but also the biological performance of apatite-based materials. Many attempts have been made to synthesize HA that contains carbonate as a raw material for the manufacture of biomaterials. Carbonate can substitute for

apatites can be distinguished by the different positions of the carbonate infrared absorption bands and by their different lattice constants. In biological apatites,

established to be the most stable arrangement. The incorporation of carbonate usually results in poorly crystalline structures with increased solubility, because it

Divalent ions, such as magnesium and strontium, that replace calcium are particularly active during the first stages of the remodeling and regenerative processes. In particular, magnesium enhances skeletal metabolism and bone growth, so is associated with the first stages of new bone formation. Like carbonate, magnesium decreases with the aging of the bone and with increasing calcification. In synthetic HA, the presence of magnesium increases the chemical-physical mimesis of the mineral bone. In fact, magnesium affects the kinetics of HA nucleation on collagen,

Ca2+ vacancy, together with an H atom that bonds to a neighboring PO4

<sup>3</sup><sup>−</sup> (B-type substitution). A and B carbonated

<sup>3</sup><sup>−</sup>, has been

<sup>3</sup><sup>−</sup> in B-type apatite. Charge compensation by a

<sup>2</sup><sup>−</sup> ions upon aging or

nessed by the decrease of the amount of non-apatitic HPO4

else by the decreased potentialities to undergo ion exchanges [12].

<sup>2</sup><sup>−</sup>, etc.) [11] occupying non-apatitic crystallographic sites.

*DOI: http://dx.doi.org/10.5772/intechopen.82740*

<sup>2</sup><sup>−</sup>, CO3

phosphoric acid up to neutrality.

compositions different from the bulk [14, 15].

OH (A-type substitution) or for PO4

<sup>2</sup><sup>−</sup> substitutes mainly for PO4

inhibits apatite crystal growth [16].

Ca2+, HPO4

tion" [12].

Minor substitutions are also found in biological apatites that involve monovalent cations (especially Na+ and K+ ), for example. In this case, charge compensation mechanisms must be taken into account.

The nanocrystalline apatites (whether biological or their synthetic analogues prepared under close-to-physiological conditions) could be probably described as the composition of an apatitic core (often nonstoichiometric) and a hydrated
