Ks. Tenhuisen et Pw. Brown, EFFECTS OF MAGNESIUM ON THE FORMATION OF CALCIUM-DEFICIENT HYDROXYAPATITE FROM CAHPO4-CENTER-DOT-2H(2)O AND CA-4(PO4)(2)O, Journal of biomedical materials research, 36(3), 1997, pp. 306-314
Calcium-deficient hydroxyapatite (KA) with a Ca/P molar ratio of 1.50
was synthesized in various concentrations (0.01-75 mM) of MgCl2 at 37.
4 degrees C by reaction between particulate CaHPO4 . 2H(2)O and Ca-4(P
O4)(2)O. The effects of magnesium on the kinetics of HA formation were
determined using isothermal calorimetry. All reactions completely con
sumed the precursor phases as indicated by X-ray diffraction analysis
and a constant enthalpy of reaction (240 kJ/mol). Magnesium concentrat
ions below 1 mM had no effect on the kinetics of HA formation. Magnesi
um concentrations between 1 and 2.5 mM affected the reaction path but
did not affect the time required for complete reaction. Higher concent
rations extended the times of complete reaction due to magnesium adsor
ption on the precursor phase(s) and HA nuclei, and stabilization of a
noncrystalline calcium phosphate (NCP). HA formation in the presence o
f magnesium resulted in separation of the following two events: initia
l formation of HA nuclei and NCP, and consumption of CaHPO4 . 2H(2)O.
This was indicated by the appearance of an additional calorimetric pea
k. Variations in calcium, magnesium, and phosphate concentrations and
pH with time were determined. Increasing the magnesium concentration r
esulted in elevated calcium concentrations. After an initial decrease
in magnesium owing to its adsorption onto PIA nuclei and precursor(s),
a period of slow reaction al constant magnesium concentration was obs
erved. Both the magnesium concentration in solution and the proportion
s of precursors present decreased prior to any evidence of a crystalli
ne product phase. This is attributed to the formation of NCP capable o
f incorporating magnesium. This noncrystalline phase persisted for mor
e than 1 year for reactions in magnesium concentrations about 2.5 mM.
Its conversion to HA resulted in the release of magnesium to the solut
ion. (C) 1997 John Wiley & Sons, Inc.