Materials processing and physical characterisation of a hybrid composite structure for bone replacement applications

Abstract

The development of scaffold-based solutions for large bone defect repair has the potential to overcome the limited efficacy of conventional bone grafting strategies. A promising approach that contemplates the complex bone architecture consists in the development of hybrid scaffolds, composed of distinct but integrated layers able to mimic the different bone regions. The aim of the research presented in this thesis was integrated in the MeDe (Medical Devices innovation) research challenges focused on fabricating a hybrid biopolymer-bioceramic composite structure for mimicking the complex bone-tissue organisation repair, and on developing the manufacturing process required to make them. The developed manufacturing process consisted in three steps: 1) pre-fabrication of a macroporous polylactic acid (PLA) structure and a microporous apatite wollastonite glassceramic (AW) structure, 2) assembly of the pre-fabricated parts to obtain a hybrid PLA-AW composite structure and 3) characterisation of obtained composite interface properties. A novel two-step fabrication route: 1) 3D printing of a porous bar and 2) laser cutting smaller scaffolds from the 3D printed bar) was developed for creating PLA porous structures with well-defined and open architectures. No decrease in mechanical properties and mass were observed over a 10 weeks’ immersion study in PBS, indicating suitability as a trabecular bone analogue. Concerning the AW porous structure fabrication, the impact of powder blend formulation on the indirect three-dimensional printing route was investigated. Bimodal mixtures with appropriate AW large-small particle ratios (45% large and 25% small) and average particle size values in the 37-61μm range resulted in the best printability outcome. Sinterability was mainly affected by the AW powder production route and, to a lesser degree, by particle size distribution and choice of sintering protocol. Finally, a hybrid PLA-AW composite structure was fabricated through physical interlocking, created by the PLA locally melting and infiltrating into the AW structure. The high interfacial shear strength values obtained in this study (0.5 to 2.5 MPa) indicated that physical bonding might be considered a promising way to avoid delamination in bi-layered scaffold systems. The novel manufacturing route developed in this work has the potential to offer an alternative route to the fabrication of bioactive bone implants which can provide a match to both cortical and trabecular bone properties.