# Biodegradable and Biocompatible Systems Based on Hydroxapatite Nanoparticles
## 2 Preparation of HAp Nanoparticles  
CaPs can be found in different individual phases (e.g., amorphous calcium phosphate (ACP), tricalcium phosphate (TCP), tetracalcium phosphate (TTCP), brushite (B), monetite (M), octacalcium phosphate (OCP) and hydroxyapatite (HAp)) that differ on crystalline structure, composition and Ca/P ratio (e.g., calcium-deficient HAp), degree of substitution (e.g., carbonate-substituted HAp) and properties (e.g., solubility and bioresorbability). Nevertheless, none of these phases can be considered as an ideal material and consequently the development of multiphasic systems is gaining attention [8]. In any case, HAp is the most stable and less soluble calcium orthophosphate, being its structure defined by a monoclinic \(P2_{1}/b\) space group (\(a=0.984\) nm, \(b=2a\), \(c=0.688\) nm and \(\gamma=120^{\circ}\)), although at high temperatures (ca. 250 \({}^{\circ}\)C) converts to a hexagonal \(P6_{3}/\)m space group a (\(a=b=0.943\) nm, \(c=0.689\) nm and \(\gamma=120^{\circ}\)) [9; 10]. Porous HAp particles can be prepared according to a great diversity of methods [11] such as self-assembly [12; 13; 14; 15], spray drying [16], double emulsion [17], solvothermal [18], sol-gel [19] and hydrothermal [20; 21; 22; 23]. In general, the most widely used methods are based on chemical precipitation routes according to any of the following chemical reactions:  
\[10\;\mathrm{Ca(OH)}_{2}+6\;\mathrm{H}_{3}\mathrm{PO}_{4}\rightarrow\mathrm{Ca }_{10}(\mathrm{PO}_{4})_{6}(\mathrm{OH})_{2}+18\;\mathrm{H}_{2}\mathrm{O} \tag{1}\]  
\[10\;\mathrm{Ca(NO}_{3})_{2}+6\;\mathrm{(NH}_{4})_{2}\mathrm{HPO}_{4}+2\; \mathrm{H}_{2}\mathrm{O}\rightarrow\mathrm{Ca}_{10}(\mathrm{PO}_{4})_{6}( \mathrm{OH})_{2}+12\;\mathrm{NH}_{4}\mathrm{NO}_{3}+8\;\mathrm{HNO}_{3} \tag{2}\]  
Despite the high simplicity of nanoprecipitation, a great control over morphology and size can be achieved by modification of experimental conditions [24; 25; 26] and the addition of surfactants [27] (e.g., stearic acid, monosaccharides and cetyltrimethylammonium bromide (CTAB)) and chelating agents [28; 29] (e.g., trisodium citrate and potassium sodium tartrate). Thus, the way reactants are mixed, the solvents that are employed or the pH conditions play a fundamental role on the final structure and morphology. For example, a rapid mixing of reactant solutions firstly leads to amorphous calcium phosphate [24], which after thermal treatment converts to a crystalline phase [25]. In addition, it is well-known that high and low pHs tend to form isotropic and anisotropic particles, respectively [26]. Self-assembly using a soft template approach is another simple method that is receiving great attention. In this case, the control of pore size can be undertaken by selection of co-surfactants and pore expanders. Specifically, large pore hydroxyapatite particles have been synthesized using 1-dodecanethiol (RSH) as a pore expander and CTAB as cationic surfactant [11]. Swelling of particles was reported to be a consequence of electrostatic interactions between the dissociated1-dodecanethiol molecule (RS\({}^{-}\)) and the cationic molecules of the surfactant (CTA\({}^{+}\)). A decrease in particle length/diameter ratio with the increase in the synthesis temperature and the RSH/CTAB ratio (R) was observed (Figure 1). The common methods applied to produce HAp usually require relative long times (e.g., 24 h) and need elevated processing conditions such as high pH, temperature or ultra-sonication, which usually rendered HAp with properties deviating from biological apatite. Moreover, HAp is usually produced according to the stoichiometric Ca/P ratio being absent other ions (e.g., Na\({}^{+}\), K\({}^{+}\), Mg\({}^{2+}\), Cl\({}^{-}\), HCO\({}_{3}\)\({}^{-}\), and SO\({}_{4}\)\({}^{2-}\)) found in bones.