Silicon nitride-based materials for spinal and antipathogenic applications

Abstract: Silicon nitride (Si3N4) is a ceramic material that is well-established in industrial applications due to its stability in demanding environments. The mechanical properties and biocompatibility of the material have led to its approval for clinical use in spinal implants. The unique surface chemistry of Si3N4 has been shown to create a chemical environment that is supportive to bone regeneration while simultaneously reducing bacterial viability, both in vitro and in animal models in vivo. Thus, Si3N4 can be used in the spine to reduce patient recovery times while protecting the implant site from damaging and costly infections. However, results from clinical studies have not shown significant differences between silicon nitride and other spinal implant materials in terms of patient outcomes.   Thus, the first aim of this thesis was to find ways to optimise the biological properties of the material and in turn create spinal implants that would exhibit significantly higher osteointegration while reducing the incidence of infections. To this end, a thermochemical surface modification was developed that changed the surface chemistry and roughness of the material resulting in increased in vitro bioactivity without affecting its antibacterial behaviour. Furthermore, the possibility of creating an osteoconductive, antibacterial bone cement to be used in vertebroplasties in the spine was explored. By adding up to 20%wt of a Si3N4 powder to poly methyl methacrylate (PMMA) cements, a significant (>90%) reduction of bacterial biofilm formation was achieved without affecting the compressive strength or biocompatibility of the modified bone cements in a negative way.A secondary objective of the study was to explore the antipathogenic properties of the material, fulfilling the growing need for a world where the spread of dangerous pathogens will be limited. The efficiency of the material against one of the most resilient DNA-viruses, the human adenovirus, was tested. It was found that contact with Si3N4 in both powder and bulk form rapidly reduced infectivity (>98% and >73%, respectively). Based on these results, a thermal modification of silicon nitride powders was developed, that would enhance their antiviral efficiency against SARS-CoV-2 and thus the applicability of the material. It was found that 10%wt modified-Si3N4 slurries rendered the coronavirus non-infectious after less than a minute of contact. The results of these studies proved that silicon nitride can also be used as an antipathogenic agent in environmental applications.Overall, in this thesis, steps were taken towards the development of Si3N4-based materials that can lead to faster healing, lower infection rates and that can be used to limit the spread of disease.