Tracking Amorphous Precursor Formation and Transformation during Induction Stages of Nucleation

Hydroxyapatite (HAP) participates in vertebral bone and tooth formation by a nonclassical hitherto unknown nucleation mechanism, in which amorphous precursors form and transform during long induction periods. Elucidation of the mechanism by which amorphous precursors assemble and transform is essential to understanding how hard tissues form in vivo and will advance the design and fabrication of new biomaterials. The combination of conductance and potentiometric techniques to monitor Ca-P mineral formation has given new insight into the mechanism of nucleation. Differences detected in the dehydration rates of calcium and phosphate ions indicate the formation of nonequilibrium calcium-deficient clusters. The aggregation of these clusters forms a calcium-deficient amorphous phase I [Ca-(HPO₄)₁₊ ·nH₂O]²) early in the induction period, which slowly transforms to amorphous phase II [Ca-(HPO₄)·mH₂O] by dehydration. Precritical nuclei form within amorphous phase II later in the induction period, leading to mineral formation.