Ana Grzeszczak: Additive manufacturing of resorbable polymer–ceramic composite structures for bone regeneration

Abstract

In cranial reconstruction, patient-specific implants can be used to restore structural integrity and protect underlying tissue. Current materials such as titanium alloys, polymethyl methacrylate, and polyether ether ketone provide reliable mechanical performance but remain permanently in the body, potentially causing complications including infection, limited adaptability in paediatric patients, and interference with medical imaging. Composite systems combining calcium phosphate ceramics with metallic reinforcement have improved biological outcomes, but still rely on non-resorbable components. Replacing these with degradable materials could enable fully resorbable implants, although this requires balancing mechanical performance, degradation behaviour, and biological response, particularly for additively manufactured structures. The aim of this thesis was therefore to develop and evaluate degradable polymer–calcium phosphate composite systems for such bone implant applications.

The work first investigated the feasibility of replacing metallic reinforcement with a degradable polymer, demonstrating that 3D-printed poly(L-lactic acid) (PLLA) can act as a structural backbone within calcium phosphate composite structures. Mechanical testing showed that PLLA reinforcement enabled load redistribution and delayed catastrophic failure, supporting mechanical integrity during early degradation. The early biological response to degradation products from PLLA–calcium phosphate composites was subsequently evaluated in vitro, indicating no detrimental effects on preosteoblastic cell metabolic activity or osteogenic differentiation. The material system was further developed through exploration of polylactic acid/polycaprolactone blends and hydroxyapatite-filled composites, enabling tuning of mechanical properties and degradation kinetics while maintaining processability for additive manufacturing. Degradation rates were adjusted through composition, with moderate increases induced by polycaprolactone and more pronounced acceleration achieved through hydroxyapatite incorporation. Finally, the influence of processing and post-processing on additively manufactured PLLA structures was investigated. Thermal treatments significantly affected mechanical behaviour, enabling transitions between ductile and brittle responses depending on annealing conditions, while inducing anisotropic dimensional changes linked to the relaxation of residual stresses from printing. These results highlight the importance of processing conditions, alongside material composition, in achieving mechanically robust and dimensionally stable geometries.

Overall, this thesis demonstrates the feasibility of designing degradable, additively manufactured composite structures with tuneable properties for bone implant. The findings support the development of material systems adaptable to specific mechanical and biological requirements, advancing patient-specific, resorbable implant strategies.