An assessment of Mg-Zn-Ca metallic glasses for use as bioresorbable implants

Abstract

A drive toward biodegradable implant design has emerged in recent decades. Magnesium and its alloys are at the forefront of this paradigm shift, with the discovery of Mg-Zn-Ca metallic glasses a focus of recent studies. The influence of each alloying element and structural relaxation toward the degradation behaviour and mechanical performance of these alloys still remain not well understood.

To improve our understanding of the degradation behaviour of these amorphous alloys a rapid screening technique was developed, which employed electrochemical polarisation and in-situ electrochemical impedance spectroscopy coupled with hydrogen gas collection in simulated body fluid at 37°C. A series of Mg-Zn-Ca metallic glasses were studied to elucidate the impact of each alloying addition towards degradation. Results confirmed these alloys exhibit more noble behaviour than high purity Mg, whilst the complex nature of dissolution requires in-vitro testing beyond utilisation of solely hydrogen collection or polarisation testing to elucidate degradation behaviour.

The relaxation behaviour of Mg66Zn30Ca4 was explored by annealing below the glass transition, to determine its impact upon material properties. The alloys glassy dynamics were shown to be strong whilst flexural strength decreased by 25% and localised dissolution increased, as further order developed in the relaxed structure. Following complete relaxation, dissolution kinetics and hydrogen evolved approached that of high purity Mg.

As no chemical-based surface modifications have been reported on Mg-based metallic glasses, the growth of a phosphate conversion coating was explored as means to improve the degradation behaviour and mechanical performance. The growth mechanism was examined in detail and ideal coating parameters which improved corrosion resistance by an order of magnitude were acquired. These parameters also improved mechanical reliability two fold and increased minimum fracture strength by 100 MPa. Furthermore, the effect of substrates composition on coating growth and degradation behaviour were examined with the presence of Ca shown as detrimental toward coating stability.

Overall, the results within this thesis improve our understanding of the biodegradation behaviour of Mg-Zn-Ca metallic glasses and suitability for use as biodegradable implants. The techniques employed provide enhanced means to characterise materials for biodegradable implants and new avenues of material design were explored.