COMPOSITION – PROCESSING – MICROSTRUCTURE – PROPERTY RELATIONSHIPS OF THE ZINC-MAGNESIUM SYSTEM FOR ABSORBABLE BIOMEDICAL IMPLANT APPLICATIONS

Absorbable metals have the potential to serve as the next generation of temporary medical implants by safely dissolving in the human body upon vascular tissue healing and bone regeneration. Their incorporation in the market could greatly reduce the need and risks associated with additional surgeries and other complications often related to permanent devices. Despite the extensive research on magnesium (Mg) and iron (Fe) based alloys over the last two decades, they have not exhibited an optimal combination of mechanical properties, biocompatibility, and controlled degradation rate for absorbable implant applications. Zinc (Zn) and Zn-based alloys have recently emerged as an alternative, as they have demonstrated an attractive combination of in vivo biocompatibility and degradation behavior. However, their mechanical properties are generally insufficient for load-bearing implant applications.In this dissertation, Zn-xMg (x = 3, 10, 30 wt.%) hybrid samples were synthesized for the first time using high-pressure torsion (HPT) to mechanically bond pure Zn and Mg disks. These samples were characterized to investigate their processing-microstructure-property relationships. The effects of HPT on the microstructural and hardness evolution were systematically studied in all the materials, with special emphasis on the regions of highest plastic deformation. The effects of post-deformation annealing (PDA) on the Zn-3Mg hybrid were also investigated. Also, an as-homogenized Zn-3Mg (wt.%) alloy was HPT-processed and subjected to PDA, and the microstructure and hardness evolution was compared with that of the Zn-3Mg hybrid. In addition, four different biodegradable coatings (based on zinc phosphate (ZnP), collagen (Col), and Ag-doped bioactive glass nanoparticles (AgBGN)) were synthesized by chemical conversion, spin-coating, or a combination of both, on the as-homogenized Zn-3Mg alloy substrates. The effects of the coatings on the in vitro degradation behavior, cytocompatibility, and antibacterial activity were evaluated.A significant microstructural refinement was obtained after HPT processing for 30 turns, reaching equiaxed grains of ~ 200 nm in both the Zn-3Mg hybrid and alloy. Unlike in the alloy, HPT induced the nucleation of Mg2Zn11 and MgZn2 intermetallic compounds, as well as the formation of supersaturated solid solutions in the hybrid, both of which led to maximum hardness values in the range of ~ 220-230 HV. PDA resulted in both an increased hardness (up to ~ 250 HV) and strain rate sensitivity. A plastic deformation model is proposed to explain the strain-hardening behavior of the hybrid after PDA, which suggested an enhanced strength-ductility relationship. This was consistent with the strain rate sensitivity results from nanoindentation.In vitro degradation of ZnP coated samples for 21 days immersion showed a controlled weight loss over time associated with a decreased corrosion rate, while maintaining a physiological pH range of 7.5-7.6. The uncoated, ZnP coated, and Col-AgBGN coated sample extracts led to higher cell viability over 6 days culture, which generally increased with extract concentration. In addition, the Col-AgBGN coated samples led to ~ 31 % bacterial viability, compared to the ~ 65-75 % of the other samples, hence, indicating a strong antibacterial effect.Overall, the insights gained from this dissertation enabled a better understanding of the composition-processing-microstructure-property relationships of the Zn-Mg system and served as a roadmap for the design of hybrid materials with enhanced mechanical properties. Moreover, the synthesis of biodegradable coatings has potential to tailor the Zn-Mg materials for specific biomedical implant applications.