INHIBITION OF THE AMORPHOUS CALCIUM PHOSPHATE PHASE TRANSFORMATION REACTION BY POLYMERIC AND NON-POLYMERIC INHIBITORS

Abstract

The induction time for the transformation of amorphous calcium phosphate (ACP) to hydroxyapatite (HAP) was monitored at pH7.40, 37°C, and calcium=phosphate=3.20mM, in the presence of polymeric and nonpolymeric inhibitors containing various functional groups. The polymeric inhibitors evaluated include: poly (acrylic acid), molecular weight 850 to 20000; poly (acrylamide), PAm; poly (diallyldimethylammonium chloride), PDa; copolymers of acrylic acid: hydroxylpropyl acrylate, AA: HPA; acrylic acid: 2-acrylamido-2-methylpropane sulfonic acid, AA: SA; acrylic acid: vinyl acetate, AA: VAc; acrylic acid: vinyl alcohol, AA: VOH. The non-polymeric inhibitors investigated include: pyrophosphate, PYP; 1-hydroxyethylidine-1, 1-diphosphonic acid, HEDP; and 2-phosphonobutane-1, 2, 4-tricarboxylic acid, PBTC. It was found that PAc and acrylic acid-based copolymers were effective in delaying ACP transformation reaction. The inhibitors effectiveness was greatly affected by polymer M_w and ionic charge. For homopolymers the order in terms of decreasing effectiveness is: PAc >> PAm> PDa=control (no inhibitor). Among copolymers, a AA: SA copolymer showed the best overall performance in terms of delaying the ACP phase transformation. For non-polymeric inhibitors, it was observed that both polyphosphate and phosphonates are potent inhibitors with HEDP more effective than PYP, as expected. The combination of PAc and PYP or HEDP or PBTC leads to a synergistic delay in the onset of the phase transformation. Reaction solutions containing 30 ppm PAc-2 (M_w 2100) and 17 ppm HEDP resulted in a>400% increase in induction time over what would be anticipated from an additive effect from these inhibitors.