3D Bioprinting of Osteochondral Mimic Scaffold for Bone Regeneration

Osteochondral defects remain difficult to repair because the cartilage–bone unit contains soft articular cartilage, a calcified interfacial region and mechanically supportive subchondral bone, each with distinct biological, structural and mechanical requirements. Extrusion-based three-dimensional printing and bioprinting provide promising approaches for fabricating multiphasic scaffolds with controlled architecture and region-specific material composition. However, the development of clinically relevant osteochondral scaffolds is still limited by narrow printing windows, weak integration between hydrogel and thermoplastic phases, and insufficient biofunctionality of bone-mimicking scaffold materials. This thesis aimed to develop and optimise extrusion-printed hydrogel/thermoplastic scaffold strategies as enabling components of a future integrated multiphasic osteochondral construct, with particular emphasis on printability, interfacial bonding and subchondral bone regeneration.

First, the current progress and limitations of extrusion-based hydrogel/thermoplastic osteochondral scaffolds were critically reviewed. This analysis highlighted the importance of combining a hydrated, cell-permissive hydrogel phase with a mechanically stable thermoplastic framework, while also identifying key challenges associated with material compatibility, interlayer adhesion, degradation mismatch, sterilisation, residual solvent control and translational readiness.

Second, extrusion-printing parameters for polyhydroxybutyrate-based thermoplastic scaffolds were systematically optimised using a Design of Experiments approach. The effects of printhead temperature, printing pressure, printing speed, bed temperature and cartridge heating time were evaluated, and an optimised parameter set was identified. This enabled the fabrication of polyhydroxybutyrate scaffolds with improved dimensional fidelity and reproducible architecture, demonstrating that statistical optimisation provides an efficient strategy for tuning thermoplastic extrusion printing.

Third, a mussel-inspired adhesive hydrogel was developed to improve interfacial bonding between the soft hydrogel and rigid thermoplastic phases. Catechol-functionalised chitosan (Cat–CS) and methacrylated hyaluronic acid (MeHA) were combined to form an interpenetrating hydrogel network, with solvent acidity used as a design parameter to regulate polymer dissolution, polyelectrolyte complexation and adhesive performance. Citric acid was identified as the most suitable solvent system, and the resulting hydrogel showed improved network integrity, mechanical reinforcement, reduced swelling, wet adhesion to printed thermoplastic scaffolds and good cytocompatibility after neutralisation. These findings support the use of citric acid-enhanced Cat–CS/MeHA hydrogels as a bioadhesive interfacial phase for a future integrated multiphasic osteochondral construct.

Finally, polyhydroxybutyrate/polycaprolactone/Ti₃C₂Tₓ MXene composite scaffolds were fabricated and evaluated as candidate bone-facing scaffold formulations. Polycaprolactone blending improved the processability of polyhydroxybutyrate. Descriptive initial WCA measurements indicated lower recorded values for the MXene-containing dense-disc specimens than for PHB/PCL (80/20), while the corresponding printed scaffold formulations exhibited more pronounced apatite-forming responses and favourable early cellular responses.

Overall, this thesis establishes a material and process optimisation framework for the component phases of a future integrated multiphasic osteochondral construct. By combining thermoplastic printability optimisation, citric acid-enhanced hydrogel adhesion and the development of MXene-containing bone-facing scaffold formulations, the work provides a foundation for the future fabrication and validation of this integrated construct.