| The purpose of this work is to develop, characterize and evaluate the ability of novel vascular endothelial growth factor (VEGF)-loaded three dimensional (3-D) composite poly(ethyl phenylalanato-glycinato)phosphazene-hydroxyapatite (PNPhGly-HAp) microsphere scaffolds to promote neovascularization and perform superiorly to bone treatment systems lacking a vascular network. PNPhGly was synthesized via a thermal ring opening polymerization method and the degradation profile of this polymer revealed minimal fluctuation in pH compared to the conventionally employed biodegradable poly(lactide-co-glycolide) (PLGA), which erodes to harmful acidic byproducts. Once PNPhGly microspheres were formed using emulsion-solvent evaporation, microsphere scaffolds were optimized for bone repair in terms of morphology, pore architecture, and mechanical strength. A 25%THF, 75% hexanes sintering solution generated scaffolds with a porosity measured by mercury intrusion porosimetry to be suitable for replacing trabecular bone tissue; and the addition of HAp improved the scaffold compressive modulus (37.24 +/- 11.99 MPa). Evaluation of protein release patterns from four sintering/VEGF loading solutions revealed that less VEGF was loaded and released from composite PNPhGly-HAp scaffolds compared to noncomposite PNPhGly matrices due to inherent properties of the protein and release media (i.e., isoelectric point and pH). A series of protein elution and extraction procedures also revealed that using this loading method, three physiologically relevant fractions of VEGF exist on the scaffolds - freely diffusible, adsorbed, and physically entrapped. Next, human umbilical vein endothelial cell (HUVEC) tubule formation assays proved that the 'free' fraction of VEGF released from the loaded PNPhGly scaffolds maintains bioactivity when compared to the 50ng/mL VEGF positive control. Though, in vivo, the low concentration of VEGF released from the scaffolds exhibited an angiogenic inhibitory behavior, using a conventional biocompatibility grading system that takes into the number and types of inflammatory cells, the PNPhGly polymer was determined to be suitable for drug delivery and bone tissue repair. |
