Nanomaterials, growth factors and stem cells: putting the package together for bone regeneration

Current bone replacement strategies are largely inadequate for repairing segmental skeletal gap defects and providing long lasting hip replacements. However the capacity to engineer tissue re-growth through application of synthetic biodegradable scaffolds and stem-cell therapy, offers great promise. The research in this thesis is directed toward developing new strategies to deliver biomimetic signalling for stimulating the growth of stem cells toward osteogenic repair. Electrospinning provides a 3-dimensional (3D) platform for engineering implantable nanofibrous devices similar to the extracellular matrix (ECM), which can stimulate cell adhesion, functionality and tissue integration. Electrospraying of bioactive nanoparticles, surface functionalisation and delivery of growth factors are new tools for promoting bone re-growth, in particular bone morphogenetic protein-2 (BMP-2). However these have not yet been effectively co-utilised. In this thesis, BMP-2 was successfully integrated within biomimetic composites, via electrospinning-electrospraying nanotechnologies in conjunction with layer-by-layer (LbL) self-assemblies. This was the first demonstration of a scaffold-system capable of simultaneous surface presentation of any growth factor with electrosprayed nanoparticle coatings. Hydroxyapatite nanoparticles (nano-HAp) and BMP-2 were co-functionalised onto both 2-dimensional (2D) surfaces and electrospun scaffolds, the latter used to mimic the nanofibrous architecture of mineralised collagen fibrils that make up bone at the nanoscale. Research into whether structural and/or biochemical cues are most effective for control over cellular development is very important in determining appropriate design elements for cell-delivery devices. The interaction that nanofibrous scaffolds, nano-HAp, LbL coatings and BMP-2 had on the growth and osteogenic activity of human bone marrow derived mesenchymal stem cells (hBM-MSCs) in vitro was incrementally investigated. It was found that both 2D and 3D scaffolds with electrosprayed nano-HAp, LbL functionalisation and/or BMP-2 attachment could support attachment and expansion of live hBM-MSC populations. Nanofibrous scaffolds were shown to provide important signalling for enhancing cellular adhesion, organisation, expansion and osteogenic mineralisation compared to 2D controls. Genomic expression of early osteogenic markers was specifically enhanced with low-density nano-HAp coatings on 3D scaffolds, indicating that both biochemical and nanostructural stimuli are required for enhancing cellular behaviour. The addition of BMP-2 improved cell adhesion, expansion and osteogenic activity of MSCs on 2D nanocomposites. However the underlying nanofibrous architecture was found to be the most important factor in cell growth, where no additional additive or synergistic effects on osteogenic activity were identified upon the addition of nano-HAp and BMP-2, potentially as a result of limitations in the in vitro culture system. Ultimately, this new combinatorial strategy to functionalise 2D and 3D devices with nano-particles and simultaneous long-term delivery of growth factors provides engineers, biologists and surgeons with a new platform for improving bone regeneration and better recreating the endogenous microenvironment.