Effects of temperature on the formation of hydroxyapatite
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The formation of hydroxyapatite by an acid-base reaction between solid calcium phosphates at temperatures from 5 to 60 °C was examined. The basic reactant used is Ca 4 (PO 4 ) 2 O, while the acidic reactants include CaHPO 4 , CaHPO4 • H 2 O, and Ca(H2 PO 4 ) 2 • H 2 O. Rates of heat evolution during reaction were determined by isothermal calorimetry. The variations in the proportions of reactants and hydroxyapatite and the formation of intermediate products were assessed by x-ray diffraction. Development of microstructure was observed. Generally, hydroxyapatite formation occurs by rapid initial reaction followed by a period during which reaction occurs slowly. Apparent activation energies calculated for the reaction when CaHPO 4 is the acidic reactant show differing values depending on its surface area. When the acidic reactant is Ca(H2 PO 4 ) 2 • H 2 O, intermediate products are formed. At low temperatures the intermediate is CaHPO 4 • 2H 2 O, while at higher temperatures it is CaHPO 4 . Above 38 °C, the rate during the period of slow reaction decreases with increasing temperature. This appears to be related to the retrograde solubilities of the reactants, CaHPO4 • 2H2O and CaHPO4 , and of HAp.
I. INTRODUCTION It is possible to form monolithic hydroxyapatite by low-temperature reactions between acidic and basic calcium phosphates which are paniculate solids. When hydroxyapatite is formed in this manner at 38 °C, the reactions appear to occur under conditions compatible with those in vivo. This is potentially significant in medical and dental applications in which hard tissue replacement is required. If the reactants used to form hydroxyapatite are limited to calcium phosphates, the basic reactant must be tetracalcium phosphate (TetCP): Ca 4 (PO 4 ) 2 O. TetCP is the only calcium phosphate that is more basic than hydroxyapatite. However, a variety of acidic reactants may be used. These include dicalcium phosphate (DCP), CaHPO 4 , dicalcium phosphate dihydrate (DCPD), CaHPO4 • 2H 2 O, and monocalcium phosphate monohydrate (MCPM), Ca(H 2 PO 4 ) 2 • H 2 O. Depending on the proportions of these reactants, either calcium deficient or stoichiometric hydroxyapatite may be formed. The following series of reactions illustrate the formation of hydroxyapatite (HAp) in this manner when different acidic reactants are used. In the first instance, the acidic reactant is DCP:
• 6CaHPO4 -1- 3Ca 4 (PO 4 ) 2 O DCP TetCP 2Ca 9 (HPO 4 )(PO 4 ) 5 OH -1- 2H 2 O
HAp
(1)
J. Mater. Res., Vol. 8, No. 7, Jul 1993
http://journals.cambridge.org
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where calcium deficient HAp (Ca/P = 1.5) forms. When DCP and TetCP are present in equimolar proportions, 6CaHPO 4 + 6Ca 4 (PO 4 ) 2 O
H2O
3Ca 10 (PO 4 ) 6 (OH) 2
(2)
and stoichiometric HAp (Ca/P = 1.67) forms when reaction is complete. These same proportions of reactants are used when hydroxyapatite is formed using DCPD as the acidic reactant. If MCPM is the acidic reactant, HAp formation occurs as follows: -^* Ca(H 2 PO 4 ) 2 • H 2 O + 2Ca 4 (PO 4 ) 2 O Ca 9 (HPO 4 )(
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