Stability-limited Ion-exchange of Calcium with Zinc in Biomimetic Hydroxyapatite

The exchange of Ca2+ ions in hydroxyapatite (HAp) with Zn2+ ions into Zn-HAp is of interest for applications ranging from bone tissue engineering to the use as a precursor in subsequent ion-exchange reactions. Previous studies, using direct synthesis, showed that ∼20 mol% Zn2+ ions can be incorporated into HAp, before byproducts are observed. However, this is realized at the cost of a loss in crystallinity and control over crystal size and shape with increasing amounts of Zn2+ ion incorporation. In this work a simple post-synthetic ion-exchange strategy for the formation of Zn-HAp has been investigated. By merely exposing HAp to high concentrations of zinc nitrate in water, up to 22 mol% of the Ca2+ ions can displaced by Zn2+ ions without any measured loss in crystallinity and preservation of crystallite size and shape. It was found that the incorporation of Zn2+ ions destabilizes the HAp crystals resulting in their gradual dissolution and reprecipitation. Consequently, promoting the exchange of Ca2+ with Zn2+ions using increased reaction times, sonication and increased temperature results in an increased dissolution of HAp and precipitation of hopeite crystals, thereby preventing the formation of more zinc rich Zn-HAp.