FORMATION OF CALCIUM DEFICIENT HYDROXYAPATITE AT NEAR-PHYSIOLOGICAL TEMPERATURE

The formation of calcium deficient hydroxyapatite Ca9(HPO4) (PO4)5OH (HAp) at 25-degrees-C by reaction of tetracalcium phosphate (TetCP), monocalcium phosphate monohydrate (MCPM), and calcium hydroxide was investigated. Phase pure, calcium deficient hydroxyapatite was prepared in approximately 3.25 h at 25-degrees-C by the reaction of dilute suspensions. Variations in the solution pH, calcium, and phosphorus concentration were determined during the reactions. The solution conditions for all reactions investigated show similar behaviour. Isothermal calorimetry was carried out to determine the rates of heat evolution during reaction. Results indicate that significant reaction occurs in the first 15 min. During this period the solution pH ranges from 4.5 to 7.5 during three distinct stages of reaction. Early reaction occurs at low pH due to rapid MCPM dissolution. The presence of Ca(OH)2 effects the pH in the first 2 min of reaction in association with the proportion of MCPM. Brushite (CaHPO4.2H2O) is the predominant product formed until MCPM is consumed. X-ray diffraction analysis confirmed that brushite formed as an intermediate in the first few minutes. Brushite precipitated as thin plates having varying geometric shapes. A second stage is the reaction of brushite, tetracalcium phosphate, and Ca(OH)2 (if present) as the solution adjusts to invariant conditions while forming hydroxyapatite. A rise in pH occurs during this period. A third stage of reaction proceeds at relatively constant near-physiological pH conditions for the remainder of the reaction as brushite is consumed along with tetracalcium phosphate in forming hydroxyapatite.