Preparation and characterization of calcium hydroxyapatite and balloon-like calcium phosphate particles from forced hydrolysis of Ca(OH)2–triphosphate solution

Calcium phosphate particles were prepared by aging a solution of dissolved Ca(OH)₂ and sodium triphosphate (sodium tripolyphosphate, Natpp: Na₅P₃O₁₀) at 100–150 °C for 18 h in a Teflon-lined screw-capped Pyrex test tube. Large spherical and/or small aggregated spherical particles were precipitated with an extremely fast rate of reaction under 100 °C. The large spherical particles were amorphous and the small aggregated ones were α-CaNa₂P₂O₇^.4H₂O. The former amorphous ones crystallized to β-Ca₂P₂O₇ after being calcined above 600 °C. Calcium hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂, Hap), with rod-like and ellipsoidal or spherical aggregated shapes, was successfully produced using polyphosphates as a source of orthophosphate ions. Time resolved TEM measurement revealed that the crystallization of Hap particles takes place on the surface of tiny amorphous particles precipitated before aging. The tiny particles played the role of nuclei for Hap crystallization. The aging temperature drastically varied the particle shape under conditions for producing uniform amorphous spherical particles; solid spherical particles were produced with an aging temperature of up to 120 °C, whilst transparent balloon-like hollow spheres were precipitated at 125 °C. Finally, fully transparent balloon-like hollow spheres were produced with mere trace amounts of small rod-like particles after aging the solution above 127 °C. The time resolved TEM observation and ICP-AES measurements revealed that the balloon-like hollow spheres were produced by dissolving the interior of solid spherical particles after reinforcing their shell by the adsorption of unhydrolyzed tpp and/or pyrophosphate (pp) ions, which are the hydrolysis product of tpp. The balloon-like hollow spheres of calcium phosphate may have the potential use as drug delivery vehicles and have biocompatibility advantages.