Processing and properties of biocompatible metallic glasses

Abstract

Amorphous metals, or bulk metallic glasses (BMGs), based on the Mg-Zn-Ca ternary alloy system, are an emerging class of material that offer many improvements over conventional crystalline magnesium alloys. These materials present a very real possibility of replacing current metallic and polymeric implant materials in a variety of roles. However, there is a general lack of understanding in these alloys, particularly in regard to their degradation processes. The aim of this thesis is to assess the potential of these newly discovered amorphous Mg-Zn-Ca alloys to be used as a bioresorbable biomaterial. The thesis began by focusing on a promising BMG composition, Mg65Zn30Ca5. The evaluation of its degradation and associated effects on cell viability provided a quantitative baseline for the subsequent work. However, its poor thermoplastic formability prompted the exploration of other BMG compositions within the Mg-Zn-Ca ternary system. A range of Mg-rich and Ca-rich BMG compositions were systematically explored. Although Ca-rich BMGs exhibited superior thermoplastic formability compared to the Mg-rich BMGs, they underwent rapid dissolution in biocorrosion environments. By enriching Ca-rich BMGs with Zn, their corrosion potential was enobled, but the corrosion rate remained high. Nevertheless, the amount of hydrogen evolved was low considering the degree of corrosion. Mg-rich BMGs on the other hand contain significantly less Ca, the most reactive element within the alloy. As such, Mg-rich BMGs exhibited superior corrosion resistance, and signs of passive corrosion kinetics. The interaction between the elements Mg, Zn, Ca was found to be very complex. The monotonic increase in Zn content in both Ca-rich and Mg-rich BMGs did not guarantee an improvement in their corrosion resistance. This suggested that the corrosion rate was not solely determined by the Zn content but, rather, the combination of Mg:Zn:Ca ratio. Overall, the results and findings from this research furthered our understanding on the amorphous Mg-Zn-Ca alloys. Quantitative data on the in-vitro performance of a range of amorphous alloy were amongst the first to be reported. This newly attained data and knowledge may be used to support the technological push to use bioresorbable metals in biomedical applications, as well as critically assess their suitability.