Formation, characterization and properties of hydroxyapatite-calcium polycarboxylate and calcium polyvinylphosphonate composites for biomedical applications

A hot pressing technique was used to prepare composites anticipated to be biocompatible. Composites were formed by reactions between tetracalcium phosphate (Ca₄(PO₄)₂O, TetCP) and a biomedical polymer. Polymers used in this study were poly(acrylic-co-itaconic), and poly(vinyl phosphonic acid) (PVPA). The processing technique is commonly used in metallurgy where powder mixtures are hot pressed at elevated pressures, and temperatures. Powder mixtures of TetCP with both polymers were compacted at temperatures up to 300°C, pressures up to 690 MPa for up to 60 minutes. The effects of varying these conditions as well as the TetCP:polymer weight ratios on the reaction kinetics were studied using X-ray diffraction (XRD), Fourier-transform-infrared (FT-IR), 13C, and 31P nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC), scanning electron microscope (SEM) and transmission electron microscope (TEM).; Results showed that TetCP was converted to hydroxyapatite (Ca₁₀ (PO₄)₆(OH)₂, HAp) with the formation of a Ca salt of the polymer. The reaction kinetics were found to increase with increasing compaction time, temperature and pressure. Formation of anhydrous calcium phosphate (CaHPO₄, DCPA) was also observed when PVPA was used. The reaction appears to start with the softening of the polymer when it was heated at temperatures equal to or greater than its glass transition temperature (T_g). The molten polymer flows and surrounds the TetCP grains, permitting a direct reaction to take place on the interface between them. The Ca polysalt appear to form first followed by formation of HAp in case of the copolymer and DCPA then HAp in case of PVPA.; Tensile strengths and elastic moduli of the composites increased when the compaction time and temperature were increased. However, when the applied pressure was increased, these properties increased then reduced at higher pressures. The improvement in mechanical properties was related to the increase in densification of the composites with these conditions. These results were correlated to the microstructure of the composites, where HAp crystals are embedded in a network of the calcium salt of the polymer. The reinforcing of the salt by these HAp crystals is considered the main reason of the improvement in mechanical properties.; The effects of bioactive glass (bioglass®) and wollastonite fibers additions on the mechanical properties and in vitro behavior of these composites were also studied. Results showed that the mechanical properties were improved by ∼50% in case of bioglass® and by ∼100% when wollastonite was used. In vitro studies were conducted by immersing these composites in a simulated body fluid (SBF) for up to 14 days, and measuring the changes in the concentrations of Ca, PO₄, and SiO₄ ions in these solutions. Concentrations of these ions increased with the time of immersion in SBF due to the formation of apatite nuclei on the surface of the composites, suggesting increased bioactivity of the reinforced composites.