Efficient zinc incorporation in hydroxyapatite through crystallization of an amorphous phase could extend the properties of zinc apatites

Zinc offers many benefits for bone growth and an antimicrobial action for implants. Nearly all studies have synthesized crystals in solution by precipitation, but this produces low crystallinity and poor thermal stability. This work addresses a new approach of incorporating zinc in an amorphous calcium phosphate followed by crystallization at an elevated temperature. A 10 mol% Zn substitution in an amorphous phase only produces 3 mol% incorporation of calcium sites due to zinc removal in an ammonia complex, but 14 mol% Zn inclusion is achieved with hydrothermal processing of an amorphous calcium phosphate in zinc nitrate. Thermal analysis of a 5 mol% Zn enriched apatite tested up to 1000 oC produces a thermally stable apatite, unlike apatites produced by precipitation. This new method provides more complete Zn filling of Ca (II) sites, is more suitable for thermal processing and potentially extends the performance of Zn enriched apatites for use in dentistry and orthopaedics. This also provides direct evidence about crystallization of zinc enriched amorphous calcium phosphate showing that zinc stabilizes the amorphous phase leading to higher crystallization temperatures.