POSSIBLE MECHANISMS FOR THE GROWTH OF THE BIOMATERIAL, CALCIUM HYDROXYAPATITE MICROCRYSTALS

Experimental data for the growth of calcium hydroxyapatite crystals, HA, are reported. The rate is shown to be controlled by a surface process. Two models, the spiral growth model and the polynuclear model, may explain the results. From the polynuclear theory, the surface tension is found to be 80 mJ/m2. The deviation of this value from the value expected from theory, 240 mJ/m2. is suggested to be due to imperfections in the surface of HA. If all rates are dependent on activities and not concentrations, the frequency of ion integration is found to be 9 X 10(4) s-1 for the polynuclear model, close to the value 1.6 X 10(5) s-1, expected if the rate-determining step is partial dehydration of calcium ions. If the rate of lateral growth of nuclei is concentration-dependent, the value of the frequency of ion integration depends on assumptions made for the value of the activity coefficient of a monomer in the adsorbed layer, or the adsorption constant. For the spiral mechanism, the frequency of ion integration is also dependent on the value assumed for the surface tension. If similar assumptions to those used for the application of the polynuclear theory are applied to the spiral theory, similar values of the ion integration frequency are obtained, thus making it difficult to distinguish the rate-controlling mechanism.