While small bone defects heal spontaneously, critical size defects may exceed the body’s
regenerative capabilities, and require the use of bone substitutes and implants. To
date, in vitro and in vivo testing of implants remains the gold standard for rigorous
mechanical stability and biological safety checks. Current 2D in vitro testing is limited
by a lack of dynamic environment and an inability to investigate mechanical strength
of the attachment between the bone-matrix and implant surface. 3D in vivo tests are
also limited by differences in the behaviour and structure of human and animal cells,
high costs and difficulty of replicating human ageing effects. The aim of this thesis is
to develop biocompatible and osteoconductive polyurethane-based scaffolds with optimal
mechanical and biological properties that can be used as 3D in vitro bone models for
bone regeneration and implant testing.
17 Plain-PU and PU-HA scaffolds were fabricated from three different medical grade
polyether-urethanes, namely, Z1A1, Z3A1 and Z9A1. The polymer’s ability to dissolve
in graded concentrations of DMF/THF solvents was assessed as part of this study.
Composite scaffolds containing nano or micro HA particles were fabricated in a ratio
of 3 PU: 1 HA by doping PU solutions with HA particles. Electrospinning, freeze drying,
freeze extraction and particulate leaching were the main fabrication techniques explored
for creating scaffolds. Electrospun scaffolds with non-aligned fibres were spun at 300 rpm
whilst those with aligned fibres were spun at 1300 rpm. Particulate leaching using NaCl
particles optimized 3 novel fabrication protocols namely, the layer-by-layer, homogenized
or physical-mixing techniques for creating highly porous PU-based constructs.
Investigation of non-aligned electrospun scaffolds showed that the choice of solvents,
on their own or in combination, strongly influences the final properties of solution,
hence the fibre morphology of scaffolds. Reducing the amount of DMF contained in
the solution, increased fibre diameter, eliminated beads in fibres and led to scaffolds
with a more uniform morphology. Moreover, reducing the DMF solvent content led
to lower tensile properties of electrospun scaffolds, whilst incorporation of nano and
micro HA particles reinforced the mechanical properties of both aligned and non-aligned
electrospun composites. RAMAN and FTIR spectroscopy confirmed the presence of
HA in all composites. Xylenol orange staining showed that composite mHA scaffolds
supported a higher percentage of mineral area coverage compared to plain-PU scaffolds.
SHG imaging identified that collagen deposition appeared to be guided by the alignment
of the scaffold fibres in the matrix deposited near to the fibres, but changed orientation
with an increase in distance from the originally deposited layers.
Layer-by-Layer particulate leached scaffolds made from all the three types of PU had
a highly porous 3D structure. 3:1 PU:nano-HA composites had the highest Young’s
Modulus and yield strength in the Layer-by-Layer group and there was no significant
difference between the mechanical properties of 3:1 micro-HA composites and 2:1
micro-HA composites. A novel physical mixing fabrication protocol shortened fabrication
time by about 90% and was used to mass produce particulate leached scaffolds in a shorter
period of time. Physically mixed particulate leached scaffolds had an interesting and
contrasting mechanical profile compared to previously fabricated scaffolds. Physically
mixed PU scaffolds without HA had the highest mechanical properties in this group and
the inclusion of neither nano, micro nor combined micro and nano-HA particles enhanced
their mechanical properties. Similar to the Layer-by-Layer particulate leached scaffolds,
the inclusion of HA particles in physically mixed PU-only scaffolds did not support a
higher cell viability. Osteoid bone formation was present in only nHA composites by
D7 of the in vivo studies, but present in all scaffolds after D45. Collagenous matrix
deposition increased over the 56 day period in all scaffold types, however, this increase
was more pronounced in PU-only scaffolds. Finally, mimicking push-out and pull-out tests
by inserting titanium screws into particulate leached scaffolds, showed that inserting the
screws during cell seeding is a better method than inserting them after a D28 culture
period. PU-based scaffolds that serve as a novel biomimetic in vitro 3D bone model for
testing of small orthopaedic implants have been developed.
