Organized nanostructures of hydroxyapatite (HAp) were produced using a novel preparation route through the topotactic transition of dicalcium phosphate dihydrate (DCPD) containing gelatin molecules. A nanoscale texture of dicalcium phosphate (DCP) was formed by the dehydration of DCPD prepared in gelatin gel containing phosphate ions. A three-dimensionally oriented framework of HAp consisting of ca. 20 nm grains was prepared by a rapid hydrolysis of the nanotextured DCP with a sodium hydroxide solution. Nanoscale HAp fibers elongated along the c axis with a diameter of ca. 50 nm were also obtained by a moderate hydrolysis with ammonia water. The hierarchical architectures of the nanostructured HAp were successfully achieved by the phase transition associated with the specific interaction of the organic molecules.

A novel type of calcium phosphate-organic polymer composite having a hierarchical structure was prepared by calcification of a poly(acrylic acid) hydrogel. Macroscopically, an organic gel containing phosphate ions was transformed into an opaque solid material by diffusion of calcium ions. We observed the formation of micrometer-scale layered structures consisting of nanoscale crystals of hydroxylapatite (HAp) in the opaque products. The laminated architecture resulting from the periodic precipitation of calcium phosphate varied with the reaction conditions, such as the concentrations of the precursor ions and the density of the gel. The nanoscopic structure of HAp crystals was modified by the addition of gelatin to the polymer matrix. q

Hierarchically laminated calcium phosphate was produced through Liesegang periodic precipitation in a gel matrix of poly(acrylic acid) containing phosphate anions by diffusion of calcium cations.