| Low-temperature cement-type formation of hydroxyapatite [Ca10(PO4)6(OH)2 or HAp) has value in terms of developing synthetic compounds similar in compositions to those formed by natural mineralization of bone. Understanding the in vitro kinetics of formation of the synthetic composition could produce insights into developing hard tissue analogs. The kinetics and chemistry of cement-type formation of HAp by hydrolysis of particulate α-tricalcium phosphate (α-Ca 3(PO4)2 or α-TCP) were examined. In particular, the effects of reaction temperature, synthesis route, inorganic salt additives and presence of biodegradable polymers (poly(α-hydroxyl acids) on the hydrolysis rate and microstructural/mechanical properties of HAp were determined using the following analytical techniques: isothermal calorimetry, x-ray diffraction, scanning electron microscsopy (SEM), fourier transform infrared spectroscopy (FTIR), solution chemistry, diametrical compression and 3-point bending tests.; For the phase-pure α-TCP/water system the complete reaction times and morphologies of the resultant HAp were found to be strongly dependent on reaction temperature over a range of 37°C to 56°C. Isothermal calorimetry analyses revealed a thermally activated hydrolysis mechanism, leading to higher reaction rates with an increase in hydrolysis temperature. The microstructure of the resultant HAp typically had entangled, flake-like morphology, with HAp formed at 37°C having a smaller crystalline size than that formed at 45°C and 56°C. The cement hardening contributed to entanglement at the microstructural level. In all cases the hydrated product was phase pure calcium-deficient hydroxyapatite [Ca10−x(HPO4) x(PO4)6−x(OH)2−x], and no other intermediates or by-products were formed through the complete transformation.; According to the proposed kinetic model, a two-step mechanism was found to control the overall hydrolysis reaction and thereby HAp formation at 37°C. During the first step, the reaction rate was controlled by the surface area of the anhydrous TCP particulates hence controlling their initial dissolution. Subsequently, the reaction rate was controlled by a nucleation and growth mechanism. During the second stage, HAp formation initiates preferentially on α-TCP surfaces. Further growth of HAp continues progressively by dissolution and precipitation of unreacted α-TCP, analogous to natural biomineralization events. (Abstract shortened by UMI.)... |
