GROWTH OF CALCIUM-PHOSPHATE ON ION-EXCHANGE RESINS PRE-SATURATED WITH CALCIUM OR HYDROGENOPHOSPHATE IONS - AN SEM/EDX AND XPS STUDY

Calcium phosphate formed on the surfaces of ion-exchange resins pre-saturated with either Ca-2 + or HPO42- ions has been studied using a combination of scanning electron microscopy (SEM)/energy dispersive X-ray (EDX) analysis and X-ray photoelectron spectroscopy (XPS). Calcium phosphate was formed at a temperature of 36.5 degrees C via two methods. On Ca2+ or HPO42- -saturated resins, 1.5 x SBF (simulated body fluid) solution was used while on Ca2+ -saturated resins only, a novel biomimetic growth medium using the alkaline phosphatase-catalysed hydrolysis reaction of disodium p-nitrophenylphosphate as a source of inorganic phosphate was employed. SEM micrographs showed that the use of 1.5 x SBF growth medium solution led to extensive coverage of the resins with calcium phosphate. In contrast, calcium phosphate coatings formed via the alkaline phosphatase-catalysed reaction were of a more variable quality whose morphology could be influenced by adding albumin and collagen to the growth medium. Average Ca:P ratios determined by EDX for coatings formed from the 1.5 x SBF growth medium were in the range 1.62-1.74 suggesting that hydroxyapatite had formed. In contrast, Ca:P ratios for the calcium phosphate compounds formed on resins from the alkaline phosphatase reaction were lower at 1.50 suggesting that calcium-deficient hydroxyapatite had formed which was confirmed by inductively coupled plasma (ICP) analysis and X-ray diffraction of isolated amorphous and crystallized powder samples, respectively. Evidence from X-ray photoelectron studies supports a mechanism of formation of the coatings which involves diffusion of the ion out of the interior of the resin to create a high local concentration at the surface thus stimulating precipitation of the coating material on the resin beads.