Selective laser melting novel biodegradable iron-based bone scaffolds for load-bearing applications

Autologous bone grafting has long been the “gold standard” for the reconstruction and remodelling of critical maxillofacial bone defects. However, it has several disadvantages, and as such bone scaffolds have become increasingly popular. Ceramic and polymer scaffolds have been extensively researched, but due to their limited mechanical properties, these classes of materials are generally not suited for load bearing applications. Permanent metallic scaffolds have much better mechanical properties, but their permanent nature can result in long term inflammation along and rejection. To overcome these drawbacks biodegradable metallic (BDM) implants are being developed, showing great potential. An ideal BDM would retain its mechanical properties during bone growth and slowly degrade away transferring the stress to the newly formed bone without damaging it; subsequent to full healing, the implant would completely dissolve away leaving behind no residue. Fe has emerged as an enticing candidate for load-bearing scaffold application as it has high mechanical properties, and with the addition of Mn displays excellent MRI compatibility, formability, and an increased corrosion rate compared to pure Fe. Selective laser melting (SLM) can produce highly customisable implants with optimised lattice structures, interconnected porosities and controllable pore sizes, which have been shown to improve bioactivity. Research on the SLM of pure Fe and Fe-based alloys has mostly been focused on maraging steels, tools steels, and other steels used in industry. Until the end of this thesis there had been relatively limited amount of work on the selective laser melting of Fe for biomedical applications. The first part of the thesis aims to understand the SLM processability of Fe-35Mn and how SLM affects the material properties of biodegradable implants by: (i) implementing a design of experiment approach to determine the SLM processing of Fe-35Mn and pure Fe firstly for bulk structures and after for scaffolds; (ii) understanding the effect of SLM processing on the properties of pure Fe by comparing it to other manufacturing methods, specifically for biodegradable implant applications. SLM processing parameters were effectively optimised to achieve a densities over 99.5% for both pure Fe and Fe-35Mn. In terms of process optimisation, most of the quality indicators such as surface roughness, density and apparent density, could be directly correlated to the energy density. Overall, the pure Fe was easier to process than the Fe-35Mn because the lower melting temperature of the latter introduced large pores. For scaffolds the quality was not directly correlated to the density of the struts, rather it was determined by its resemblance to the original CAD model. This required investigation of the designed pore size and porosity as a process defect together. Using a similar process optimization route as the bulk structures, scaffolds with high density struts (over 99 %) and with a close resemblance to the scaffold were manufactured. As opposed to the bulk structures, the lower melting temperature of the Fe-35Mn was advantageous as lower energy densities were used to manufacture the scaffolds. Distinct microstructures were observed for the pure metal and the binary alloy, reflecting also the differences in the processability. Both microstructures observed were finer than those of traditional manufacturing methods because the small melt pools and high cooling rates that typically occur during SLM promote finer grained structures. The microstructure of pure Fe consisted of mostly equiaxed α-ferrite grains, with no indication of the original solidification structure. Through the addition of 35wt% Mn, the γ-austenite phase is stabilised, and thus the microstructure was characterised by columnar γ grains, with each grain containing a network of individual cells.  The final material properties imparted on biodegradable iron implants are significantly affected by the processing route. Laser based additive manufacturing techniques impart smaller grain sizes than cast samples due to the complex heating cycles and high cooling rates typical of these manufacturing methods. As the mechanical properties are considerably affected by the grain size, the smaller grains imparted via SLM resulted in the highest mechanical properties, followed by laser metal deposition (LMD) and casting. When tested in simulated body fluids (SBF), the laser-based AM techniques had a corrosion rate approximately 50% higher than that of their cast counterpart, with the SLM samples having a higher corrosion rate than LMD. The higher cooling rates of SLM compared to LMD impart a higher density of dislocations, residual stresses and internal defects. This can result in local destabilisation of the protective film that typically forms on Fe in an SBF environment. After successfully determining the suitability of SLM as a manufacturing technique and subsequently manufacturing high quality Fe-35Mns scaffolds; the second part of the thesis consisted of determining the feasibility of these scaffolds for biodegradable load-bearing applications. In vitro corrosion testing showed that the scaffold had a vastly improved corrosion rate compared to bulk pure Fe owing to a synergistic combination of the scaffold design, the addition of Mn and the use of SLM as the manufacturing method. However, the mechanical properties were still suitable for load-bearing situations even after 28 days of immersion in SBF. Cytotoxicity assay showed there was no statistically significant difference between the Fe-35Mn scaffold group and control group as proliferation rates were not affected. In fact, the scaffold displayed good cell viability towards mammalian cells along with observed osteoblast filopodia attachment indicating adhesion to the scaffold. After in vivo testing newly formed bone matrix was observed in direct contact with the implant surface after 4 weeks implantation. Osteoid and osteoblasts were also found in the newly formed bone layers, which indicates osteointegration where the bone regeneration process started in the defect area followed by bone apposition on the scaffold. Significant in vitro and in vivo characterisation determined that Fe-35Mn scaffolds manufactured using SLM show great potential for biodegradable bone scaffolds for load-bearing applications.