Resorbable Polymer-Hydroxyapatite Composites for Bone Trauma Treatment: Synthesis and Properties

Contemporary therapies for hard tissue replacement involve allographs, where the donor and recipient are of the same species, or the use of xenogenic transplants. Intraspecies materials are often in short supply and interspecies materials are subject to immunological barriers such as disease and tissue rejection. Synthetic composites are unencumbered by these limitations. Poly lactic acid (PLLA) and its copolymers have been used in medicine due to their ability to be resorbed by the body without adverse effects. PLLA’s are currently most commonly used in resorbable sutures and gauzes, but there is increasing interest in using them in conjunction with hydroxyapatite (HA) as bioceramics for hard tissue replacement. Early attempts to create a viable PLLA/HA involved dispersing HA particles in a PLLA matrix. There are well defined phase transitions in these types of mixtures. These transitions readily become material failures when placed under stresses or strains. Fortunately the surface hydroxyls on HA can serve as an effective initiator for the ring-opening-polymerization of lactide and other lactones of biological interest. I have shown that bioceramics made in this manner produce materials with superior interfacial strength and that mammalian sources of HA can be used to prepare biomimetic materials that further enhance the physical properties of the resulting composites. I have examined the kinetics of the ring-opening polymerization reaction of L-lactide using bovine derived HA under various conditions and found that the sintering temperature and sintering time of the HA plays a major role. Experiments designed to elucidate the influence of these parameters on reaction rate reveal that two competing processes determine the polymerization rates. Firstly, specific surface area decreases with sintering time, and since the polymerization is initiated by surface nucleophiles, decreased surface areas give slower polymerization rates. Secondly, thermal decomposition of carbonate to oxide or hydroxide generates nucleophiles in the HA during sintering, resulting in increased heterogeneous rate constants. I have also performed preliminary tests to examine the basic physical properties that these composites possess