Glycosaminoglycan-based scaffolds for bone repair

Adding complexity to biomaterials by mimicking extracellular matrix-like components as well as biomechanical cues is a powerful strategy for enhancing the performance of implantable materials for orthopaedic purposes. In recent years, bone morphogenic protein 2 (BMP-2), a potent osteoinductive factor has gained popularity within the orthopaedic community to stimulate bone regeneration. Despite showing strong osteoestimulatory effects, the short half-life of BMP-2 requires supraphysiological doses to achieve efficacious clinical outcomes which have been repeatedly associated with life-threatening side effects. Glycosaminoglycans (GAGs) of the heparin/heparan sulphate (HS) are key components of the extracellular matrix that bind, stabilise and protect factors such BMP-2 from degradation. Despite being available at a clinical grade, the anticoagulant property of heparin coupled with its ability to bind non-selectively to a wide range of stimulatory and inhibitory osteogenic factors is driving strategies to generate heparin structures that overcome these limitations. The actions of heparin/HS GAGs are dependent on their sulphation pattern and overall chain length. Recent findings from our group suggest that heparin fragments of 12 saccharides in length (dp12) optimally bind and stabilise BMP-2 to enhance its osteogenic effects. This suggests a path to increasing the selectivity of heparin for BMP-2 through a sizing strategy to reduce off-target effects. Therefore, this thesis aims to develop heparin-functionalised scaffolds for bone repair. Initially, we have explored adding micropatterns into electrospinning scaffolds or biomaterial biofunctionalisation (i.e., heparin) for increased scaffolds complexity. In search of a more sophisticated and reliable functionalisation method, bioactive copolymers containing nitrous acid generated heparin oligosaccharides were synthetised by ring-opening metathesis polymerisation (ROMP) to sustain BMP-2 signals over a prolonged period and manufactured into electrospinning scaffolds. Bioactive oligosaccharides F6 (dp10), F7 (dp12) and F8 (dp14) were successfully isolated from nitrous acid digested mixtures by size exclusion chromatography (SEC). Data obtained from BMP-2 response C2C12 cells suggested that heparin oligosaccharides have the ability to enhance BMP-2-mediated osteogenic responses determined by quantitative polymerase chain reaction (qPCR) of osteogenic genes, alkaline phosphatase (ALP) assay, mineralisation assays and Smad 1/5/9 phosphorylation assays. Nitrous acid generated oligosaccharides presented low anti-Factor Xa in comparison with native heparin and retained dose and chain length-dependent bioactive properties as a cheap alternative to enzymatic depolymerisation. Bioactive oligosaccharides were then used to synthesise bioactive copolymers for bone repair. ROMP-derived bioactive copolymers were synthesised using polycaprolactone (PCL) macromonomers and oligosaccharide macromonomers with high conversion. The final copolymers presented a ~3-3.5 wt.% incorporation of oligosaccharide macromonomer into the final copolymer. The structural variability of bioactive electrospinning scaffolds was characterised in terms of fibre diameter, hydrophobicity, stiffness and thickness. In conclusion, the presented bioactive copolymers are a relevant platform for the design of advanced bone repair scaffolds able to tackle interdisciplinary tissue engineering challenges.