alpha-Tricalcium phosphate hydrolysis to hydroxyapatite at and near physiological temperature

The kinetics of hydroxyapatite (HAp) formation by direct hydrolysis of alpha-tricalcium phosphate (alpha-TCP) [alpha-Ca(3)(PO(4))(2)] have been investigated. Transformation kinetics were examined for reactions at 37 degrees C, 45 degrees C and 56 degrees C by isothermal calorimetric analysis. Setting times and morphologies of the resultant HAp were found to be strongly dependent on reaction temperature. XRD analysis accompanied by FTIR confirmed that phase pure calcium-deficient hydroxyapatite (CDHAp) [Ca(10-x)(HPO(4))(x)(PO(4))(6-x)(OH)(2-x)] was formed. Complete reaction occurs within 18, 11, 6.5 h at 37, 45 and 56 degrees C, respectively. The extent of HAp formation differs for particulate slurries and pre-shaped forms of reactant alpha-TCP. Formation of hydroxyapatite in pre-formed pellets was hindered due to limited water penetration, but enhanced with the presence of NaCl as a pore generator. Regardless of the precursor characteristics and temperature, HAp formation is characterized by an initial period of wetting of the alpha-TCP precursor, an induction period and a growth period during which the bulk transformation to HAp occurs. The microstructures of the resultant HAp at all temperatures were generally similar and are characterized by the formation porous flake-like morphology. Microstructural coarsening was observed for the CDHAp formed above the physiological temperature. The hardening generated by the hydrolysis reaction was demonstrated using diametrical compression tests. The original tensile strength of 56% dense alpha-TCP increased from 0.70+/-0.1 MPa to 9.36+/-0.4 MPa after hydrolysis to CDHAp at 37 degrees C, corresponding to a density of 70%.