Crystallographic Aspects Regarding the Insertion of Ag+ Ions into a Hydroxyapatite Structure

The objective of this study was to evaluate how silver can be inserted into hydroxyapatite (HA) via two distinct processes: co-doping with CO(3)(2-)via precipitation in an aqueous medium and immersion of preformed HA crystals into Ag+ solutions. It was concluded that although Ag+ and Ca2+ have different radii, the accommodation of Ag+ ions in the Ca2+ sites of the hydroxyapatite lattice can be explained by the models proposed for inserting monovalent ions such as Na+. In this case, because Ag+ ions are larger than Ca2+ ions and have a different charge, the Ag+ ions are stabilized in the HA structure by co-substitution with CO(3)(2-)ions in both the A- and B-type sites. This simultaneous insertion of Ag+ and CO(3)(2-)appears to thermally stabilize the HA phase because no phase transformation is observed after calcination. In addition, the doping of HA with Ag+ ions can clearly occur via two routes: co-precipitation in the presence of these ions or diffusion in preformed hydroxyapatite crystals. This result appears to indicate the possibility of doping HA with Ag+ using less complex routes at ambient temperature and with prefabricated implants or biomaterials, which reduces the costs of producing devices with antibacterial effects.