| Summary: | Osteoarthritis (OA) is a disease that is characterised by the progressive erosion of articular cartilage, synovial inflammation, subchondral bone remodelling, and osteophyte formation. Osteoarthritis is a leading cause of disability and affects over 500 million people world-wide. There is a lack of disease modifying anti-osteoarthritic drugs (DMAODs) which means the disease pathobiology cannot be stopped, resulting in existing treatments having transient efficacy and being focused mainly on symptom management. Pro-inflammatory signalling is a key driver of OA progression at the early stages of the disease and is the result of an imbalance in M1 and M2 macrophage signalling. Regenerative medicine approaches for OA have historically neglected to target inflammation and osteochondral tissue engineering still remains elusive. An early intervention therapy promoting M2 macrophage polarisation may balance immune signalling in the osteoarthritic joint and decelerate progression to retain cartilage tissue that would otherwise be irreversibly lost.
The controlled release of cytokines from an implantable biomaterial to promote M2 macrophage polarisation in-vivo is a possible strategy to achieve this goal. The aims of this thesis were to generate hydrogels that exhibited the controlled release of an M2 promoting cytokine for a clinically relevant period of time, that were a synthetic, 3D printable material with the capacity to be up-scale manufactured.
The work in this thesis was subdivided into three sections. Firstly, affinity hydrogels were cast using ultraviolet (UV) photocrosslinking and protein release was characterised for 70 days using the model protein lysozyme. A linear release trend was observed in hydrogels containing 5% w/v 3- sulfopropyl acrylate (SPAK) and 10% w/v poly (ethylene glycol) diacrylate (PEGDA), with the bioactivity of released lysozyme being over 50% after 2 months of in-vitro release.
Hydrogel size and loading concentration were then scaled down for the in-vitro release of interleukin-4 (IL-4), in which 5% SPAK 10% PEGDA exhibited sustained release in the ng/mL concentration range for 73 days. To assess the bioactivity of released IL-4, conditioned media from IL-4 in-vitro release time points was used to promote M2 polarisation of THP-1 macrophages to model immunomodulation in osteoarthritis. It was determined that IL-4 released from as late as 53 days was capable of inducing an M2-like polarisation state as evidenced by secretion of CCL-18, CCL-22 and increased Mannose receptor surface expression. Media conditioned by 5% SPAK 10% PEGDA hydrogels did not decrease the viability of THP-1 macrophages in comparison to monolayer controls. In the presence of an M1 promoting stimulus, direct incubation of IL-4 releasing SPAK PEGDA hydrogels with THP-1 macrophages enhanced the secretion of TNF-α and IL-6.
Finally, 5% SPAK 10% PEGDA hydrogels were 3D printed using digital light projection additive manufacturing. The release rate of lysozyme from a porous hydrogel design was compared to that of a non-porous design in effort to show proof-of-concept for the fine tuning of protein release rate using 3D printed hydrogel architecture with different surface area to volume ratios.
In summary, SPAK PEGDA hydrogels have been shown to be a promising material for electrostatic controlled protein release. Future studies may further utilise 3D printing for controlled release targeting immunomodulation in osteoarthritis and in other broader regenerative medicine applications such as implantation into large animal models of osteoarthritis.
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