Low temperature synthesis of a self-assembling composite: hydroxyapatite-poly[bis(sodium carboxylatophenoxy)phosphazene

Abstract

The present study was undertaken to investigate the low temperature formation of a hydroxyapatite-polyphosphazene polymer composite likely to be biocompatible. The temperature range studied (25 to 60°C) was selected to bracket physiological temperatures. The composite precursors consisted of CaHPO4·2H2O, Ca4(PO4)2O, and poly[bis(sodium carboxylatophenoxy)phosphazene]. The results indicate that a synergistic relationship exists in the formation of a polyphosphazene network and hydroxyapatite (HAp) matrix phase during composite synthesis. Calcium from the HAp precursors participates in the formation of a Ca crosslinked polymeric network which influences the rate of HAp formation and its morphology. The mechanistic paths taken during composite formation were followed by determining variations in the concentration of species in solution (at physiological temperature), rates of heat evolution, and microstructural development. These analyses indicate that the polymer controls the kinetics of hydroxyapatite formation and the composite microstructure. Low reaction temperatures and a high proportion of polymer facilitate the formation of a highly interconnected composite. The presence of the polyphosphazene allows a metastable calcium phosphate solution to persist for extended periods prior to the formation of hydroxyapatite. The degree of supersaturation and the length of the induction period increase with an increase in polyphosphazene content. The temperature dependence of these induction periods obeyed an Arrhenius relationship.