Roger I. Martin

Mechanical properties of hydroxyapatite formed at physiological temperature

The mechanical properties of monoliths of calcium-deficient and carbonated hydroxyapatite formed by dissolution-precipitation reactions at 38°C have been determined. Particulate solid reactants were mixed at liquid-to-solid weight ratios of 0.11 and 0.2 and pressed into various configurations on which mechanical tests were carried out. Testing was performed on wet had formed. Calcium-deficient hydroxyapatite produced at a liquid-to-solids ratio of 0.11 exhibited a tensile strength as high as 18 MPa, an average compressive strength of 174 MPa and a Youngs modulus of 6 GPa. These values were lower when a larger proportion of water (liquid-to-solid 0.2) was used in sample preparation. However, the compressive strengths of calcium-deficient hydroxyapatite prepared at 38°C are comparable to the compressive strengths of sintered hydroxyapatite containing an equivalent total porosity. Carbonated hydroxyapatite showed mechanical properties inferior to those exhibited by calcium-deficient material. These differences appear to be related to the microstructural variations between these compositions.

Aqueous formation of hydroxyapatite

The kinetics of stoichiometric (Ca/P = 1.67) and calcium-deficient (Ca/P = 1.5) hydroxyapatite formed in aqueous solution by acid-base reactions involving CaHPO4 and Ca4(PO4)2O were determined. Complete reaction occurs within 6 h at 37.4 degrees C regardless of composition with stoichiometric hydroxyapatite forming more rapidly. Stoichiometric hydroxyapatite formed more rapidly because the particle sizes of its precursors were smaller. Hydroxyapatite formation is characterized by an initial period of surface hydration of the precursors, an induction period, and a period during which the bulk of the conversion to hydroxyapatite occurs. During the first 3 h of reaction at 37.4 degrees C, the pH is about 8.25 and 7.6, respectively, as the stoichiometric and calcium-deficient hydroxyapatite are being formed. Subsequently the pH values move toward those of the related invariant points: Ca10(PO4)6(OH)2-Ca(OH)2 solution, and CaHPO4-Ca9HPO4(PO4)5OH solution. The concentrations of calcium and phosphate in solution never exceed those in serum. Seeding with 5 wt% hydroxyapatite eliminates induction regardless of composition. The kinetics are first-order and follow the Arrhenius relationship regardless of composition. The total heats of reaction (delta Hr) were determined at constant temperatures between 25 degrees C and 70 degrees C. delta Hr values of 261.3 and 320 kJ/mol were determined for the formation of calcium-deficient hydroxyapatite and stoichiometric hydroxyapatite, respectively. Activation energies of 84.5 and 87.4 kJ/mol were calculated for the formation of calcium-deficient and stoichiometric hydroxyapatite, respectively. Heats of formation for Ca4(PO4)2O and Ca9HPO4(PO4)5OH were calculated to be -4764.1 and -12707.7 kJ/mol, respectively.

Formation of hydroxyapatite in serum

The kinetics of the formation of calcium-deficient and carbonated hydroxyapatite at 38°C were investigated by isothermal calorimetry. Hydroxyapatite (HAp) was formed by reaction of the particulate calcium phosphates CaHPO4 and Ca4 (PO4)2O. Compared with its rate of formation in DI water, the formation of calcium-deficient HAp is significantly inhibited in serum. When serum is diluted with DI water, the extent of inhibition varies with the extent of dilution. When collagen or HAp seeds are present the extent of inhibition in serum is reduced. The kinetics of HAp formation were also examined in various concentrations of albumin to establish the extent to which inhibition is associated with the presence of this plasma protein. While HAp formation is inhibited in albumin, the extent of inhibition is not as great as in serum. The formation of carbonated HAp is also inhibited in serum and albumin. However, the extent of inhibition is significantly reduced. The variations in sodium and carbonate in solution during HAp formation indicate that these species are incorporated at different rates, with carbonate incorporation being more rapid. Elevated sodium concentrations in solution result in solution pH values near 12. The reduction in the inhibition of HAp formation may be associated with the reaction to carbonated HAp occurring at elevated pH or with the influence of pH on protein adsorption.

Hydrolysis of CaHPO4 in Sodium Fluoride Solutions at 37.4

A possible chemical process occurring during caries reversal is conversion of acidic calcium phosphates to apatite. The role of fluoride in this process is of particular interest. The effects of fluoride on the rate of CaHPO4 hydrolysis at 37.4 °C were studied by a multimethod analysis involving X-ray diffraction, analyses of variations in solution chemistry, and observation of microstructural evolution. Hydrolysis in low NaF concentrations results in the formation of discrete fluorapatite crystals on the surfaces of the CaHPO4 crystallites. At CaHPO4/NaF molar ratios from approximately 9 :1 to 10 : 2, fluorapatite formed in approximately 3 h as the only crystalline product and complete hydrolysis of CaHPO4 occurred; a pH value as low as 2.4 was attained with solution species being predominantly sodium phosphate. At NaF concentrations beyond those which result in pH minima, fluorapatite and CaF2 are the crystalline products. At 0.6 M NaF, pseudomorphs composed of fluorapatite and CaF2 crystals form without developing morphologies characteristic of individual fluorapatite and CaF2 crystals. CaHPO2 can hydrolyze completely to fluorapatite and CaF2 within a few hours depending on NaF concentration and liquid-to-solids ratio.

Mechanical properties of apatite formed by acid-base reactions at 37.4/spl deg/C

The mechanical properties of monolithic hydroxyapatite were related to porosity, microstructure and composition. The ultimate strength has a logarithmic relationship with the porosity between 4 and 70 vol% pores. The modulus of elasticity is approximately 6-7 GPa for hydroxyapatite and 4-5 GPa for 3.66 wt% carbonate hydroxyapatite.