Evolution of the in vitro degradation of Zn-Mg alloys under simulated physiological conditions

The primary reactions occurring upon the insertion of Zn-derived materials inside an organism are of the utmost importance as the chemical species resulting from degradation of these new resorbable biomaterials will be crucial for the interaction with the surrounding tissues. In this sense, the degradation of Zn-Mg alloys under physiologically simulated conditions was investigated. The presence of magnesium (1-2%) as an alloying element in Zn alloys affected the composition of the corrosion layer and the associated in vitro degradation behaviour. A detailed physico-chemical characterization of the in vitro built-up of the corrosion layers of pure Zn and two Zn-Mg alloys (Zn-1Mg and Zn-2Mg) was achieved by confocal Raman spectroscopy, scanning electron microscopy and energy-dispersive X-ray spectroscopy. This study revealed that the presence of Mg in Zn-Mg alloys modulated simonkolleite turnover, promoted brucite formation and yielded a calcium phosphate layer containing skorpionite and hydroxyapatite. These last compounds, by being bone analogues may favour the osseointegration of Zn-Mg-based materials over that of pure Zn. When comparing both Zn-Mg alloys, the distinct evolutions observed in these compounds' (skorpionite and hydroxyapatite) formation may present specific advantageous according to the bone-healing process required. A detailed analysis of the corrosion behaviour, achieved by electrochemical techniques, showed that Zn-Mg alloys had a corrosion resistance inferior to that of pure Zn; however, the built-up of potential biocompatible corrosion layers together with superior strength and castability of these Zn-Mg alloys make them special attractive biomaterials for clinical bone implants, particularly adequate in load-bearing applications.