Commercially available hydroxyapatite powders exhibit a high degree of chemical variability. The crystal lattice quite readily undergoes substitutions, with ions such as CO32-, HPO42- and F- often found in place of stoichiometric hydroxyapatite components. Also, calcium to phosphorous molar ratios often vary in hydroxyapatites from the stoichiometric ratio of Ca/P = 1.67. For this reason, it is not uncommon for biomedical companies to synthesise their own powders.
Substitutions within the hydroxyapatite lattice can also induce changes in properties. For instance, fluoride substituting for hydroxide, producing fluorapatite, results in a more chemically stable compound. Property changes have inspired researchers to deliberately substitute ions to try and modify the properties and behaviour of apatites, such as bone bonding.
Hydroxyapatite Production Techniques
Hydroxyapatite powders can be synthesised via numerous production routes, using a range of different reactants. Some processing techniques include:
• Wet chemical methods (precipitation)
• Hydrothermal techniques
• Hydrolysis of other calcium phosphates
Of these methods the first two are the most commonly used techniques.
Wet Chemical Production Methods
Calcium Hydroxide and Orthophosphoric Acid
At a pH of greater than 9, orthophosphoric acid solution is added is a drpwise manner to a dilute solution/suspension of calcium hydroxide. The acid is added at a controlled rate, with stirring being maintained throughout the process. The precipitation reaction is slow. Reaction temperatures of between 25 and 90°C are common, the higher temperature producing a higher crystallinity product.
Calcium Nitrate, Diammonium Hydrogen Phosphate and Ammonium Hydroxide
Another precipitation method used for producing hydroxyapatite involves calcium nitrate, diammonium hydrogen phosphate and ammonium hydroxide. This method results in a faster production rate, with ammonium hydroxide being added to maintain a constant pH. Compared to the previous method, this approach requires washing of the precipitate to remove nitrates and ammonium hydroxide. In taking these factors into account, the production rate of these two techniques is similar.
Processing and Variables
Continued stirring and ageing are usually carried out after the reactants have been combined as the calcium is slowly incorporated into the apatitic structure. This process also helps the material to approach stoichiometric Ca/P ratios. A morphological change from needle-like crystals to more “blocky” crystals is associated with the maturation process.
Processing at pH’s lower than 9 can also result in the production of a calcium deficient hydroxyapatite.
After wet chemistry, hydrothermal techniques are the second most popular synthesis techniques for producing hydroxyapatite powders. This method involves reacting a mixture of calcium carbonate (CaCO3) and di-ammonium hydrogen phosphate at high temperatures and pressures such as 275°C and 12000psi.
The resulting hydroxyapatite is carbonate substituted, but commonly well crystallised and chemically homogeneous.