New Advancements in the Nucleation and Growth of Calcium Phosphate Minerals in Biologically Relevant Aqueous Solutions
Halter, Timothy J.
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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 non-equilibrium calcium-deficient clusters. The aggregation of these clusters forms a calcium-deficient amorphous phase I early in the induction period, which slowly transforms to amorphous phase II by dehydration. Pre-critical nuclei form within amorphous phase II later in the induction period, leading to mineral formation. In addition, constant composition kinetic investigations were done to investigate the effectiveness of a novel chemical and heat treatment procedure claimed to increase the bioactivity of Ti surface. The procedure proved to be highly effective at increasing the material which grew on the activated Ti surfaces. The heat treatment is essential to the formation of both the asatase and rutile forms of Ti-O2 both necessary to nucleate calcium phosphate. Using the reported treatment OCP crystallites formed on the disc surface in solutions at the pH and ionic strength of human serum. Using the highly sensitive CC technique control over the rate of reaction was demonstrated by adjusting the driving force. Constant composition kinetic investigations were done to investigate the effect of magnesium, carbonate and citrate ions on Ca-P mineralization and demineralization events. The effect of magnesium on calcium phosphate and calcium carbonate nucleation has been investigated. The most intriguing result was found when calcium phosphate was nucleated in the presence of magnesium at pH values greater than 8.0 and at relatively high carbonate concentration (10.0 mmol); at these conditions magnesium accelerates calcium phosphate nucleation. This is in contrast to the further inhibition of calcium phosphate mineral formation in solutions containing carbonate by magnesium under previously investigated human oral conditions