Control of Hydroxyapatite Mineralization in an Orthogonal Diffusion System

Mineralization of hydroxyapatite has been under intense investigation to obtain a better understanding of the formation of vertebrate bones and teeth. Indeed, the precise mechanism underlying the in vitro chemical synthesis of hydroxyapatite in neutral or acidic solutions remains poorly understood. Here, we report the precipitation of hydroxyapatite under physiological conditions, introducing a hydrogel-based diffusion method. In this system, the fluxes of calcium and hydroxyl ions occur perpendicular to the agarose hydrogel containing hydrogen phosphate ions. This results in a 2-dimensional variation of the concentrations of the constituent ions in the hydrogel. Notably, flake-like hydroxyapatite is only observed in cases where the supply of ions is balanced at slightly acidic pH. Hydroxyapatite can be formed via two pathways depending on the local pH at an early stage of mineralization, that is, via an acidic or basic route. In the former route, the mineralization of hydroxyapatite starts with dicalcium phosphate dihydrate over transient amorphous calcium phosphate. Growth of hydroxyapatite crystals on the surface of amorphous calcium phosphate is also possible when accompanied by an additional calcium ion supply in the basic route. Our observations provide novel insights into the potential roles of ion supply in hydroxyapatite biomineralization, strongly suggesting that acidic and basic amorphous calcium phosphate occur on separate crystallization pathways.