Development of a Load-Bearing Nanocomposite Biodegradable Scaffold for use in Bone Tissue Engineering

The deficiencies of current bone graft substitutes have been a driving force in developing new and more effective bone substitutes. An optimal implant needs to possess the following features: mechanical properties that closely match those of bone, an interconnected porous network (allowing for cell and blood vessel ingrowth), and the capacity to induce a positive interaction with the surrounding tissue (bioactivity). Furthermore, an optimal implant would be absorbed by the body over time and replaced by native bone. To date, no graft alternative has been able to combine all of these aspects. The aim of this thesis was to develop new and improved materials as well as improving scaffold design and manufacturing techniques with the goal of producing a load-bearing bone substitute. In an attempt to mimic the structure of bone the research focussed on a bioabsorbable polymer/mineral composite. A novel process was developed in which poorly crystalline nano-sized hydroxyapatite particles could be distributed throughout a Polyhydroxybutyrate-valerate (PHBV) matrix by taking advantage of particle stabilisation and a dual solvent technique. PHBV was chosen as the polymer because of its mechanical properties, biocompatibility and slow degradation rate, whilst hydroxyapatite is a natural constituent of bone. The process produced a good dispersion of well-bonded, high aspect ratio nanoparticles in the polymer matrix as evident from high resolution TEM. The nanocomposites produced by this process were compared to traditional micronsized hydroxyapatite/PHBV composites. The incorporation of nano-sized hydroxyapatite improved the modulus of the polymer from 700MPa to 2GPa at a maximum concentration of 20% by weight, compared with 1.4GPa for the 20%microcomposite. The compressive yield strength of such a nanocomposite material increased from the 40MPa measured for PHBV and 85MPa for a 20%microcomposite, to 170MPa for the 20%nanocomposite. This figure is equivalent to the compressive strength of cortical bone and is the highest recorded to date for any fully biodegradable polymer composite. In order to produce a load-bearing scaffold it was necessary to improve upon current scaffold design methodologies. Most current scaffold designs rely on open-celled foam configurations, which are unsuitable for load-bearing applications. Since a scaffold requires strength and stiffness as well as interconnected porosity, anisotropic sandwich designs were investigated as possible implants options. By modelling these scaffolds using finite element analysis and adjusting variables such as the width of the porous and solid regions, and overall porosity, it was possible to predict the mechanical properties of these shapes and hence produce a scaffold with improved mechanical properties whilst still retaining interconnected porosity. The scaffolds most suited for this purpose contained 400µm plates and struts 400µm in length, providing a combination of interconnected porosity, 40%, and high axial modulus. This structure retained almost 50% of the stiffness of the solid material and had pore sizes within the biological constraints placed on the design process. To produce the types of structures modelled a novel manufacturing method had to be developed. An effective way to produce highly replicable scaffolds with controlled architectures is through the use of laser processing. By casting a sheet of dye-doped polyvinyl alcohol on polymer substrates it was possible to accurately ablate holes using an argon-ion laser. By controlling the power, dye concentration and exposure time it was possible to accurately and reproducibly create holes from 100µm to 500µm in the dye-doped polymer film. The film, to be used as a reverse mould, was then layered between nanocomposite powder and melt processed. The PVOH is then dissolved out using warm water, leaving a sandwich structure. The scaffolds produced using the nanocomposite materials were considerably stiffer and stronger than those produced using straight PHBV. The values of compressive strength and modulus for the reinforced strut design composite scaffold, 45Mpa and 750Mpa respectively, are the highest recorded values for a biodegradable hydroxyapatite/polymer scaffold. The modulus compares favourably to cancellous bone, with the scaffolds possessing a higher compressive strength. Preliminary in vitro biological testing was also performed on the nanocomposites and the scaffolds. The composite materials improved the osteoblast cell attachment compared with PHBV and tissue culture plastic, but no differences were seen between composite materials. However, the nanocomposite materials enhanced the proliferation of osteoblast cells compared with the microcomposites, indicating that nanomaterials not only improve mechanical properties but may enhance biological activity. The osteoblast cells attached and proliferated on the scaffolds and showed no preference for pore size (size range 200 - 500µm) with all designs performing similarly. Subject to further testing, these fully biodegradable nanocomposite scaffolds have the potential to be used as load-bearing implants