Corrosion behaviour and cytotoxicity of Mg alloys for biomedical applications

Magnesium (Mg) and its alloys are emerging biomaterials for orthopaedic and vascular stent applications owing to their desirable mechanical and biological features. However, their clinical applications are significantly restricted by the rapid and uncontrollable degradation, corresponding hydrogen gas evolution, deterioration of mechanical strength and dramatic changes in local pH in physiological environments. Furthermore, a comprehensive understanding of toxicity for Mg alloys remains essential for the engineering of biomedical Mg implants. Particularly, little is known regarding the biocompatibility of corrosion surface films that have direct contact with tissues/cells. In this work, alloying Mg with micro additions of strontium (Sr) with or without Zn was selected as a promising strategy to utilise the biological effect of Sr in accelerating bone tissue growth, and the modification role of Zn in mitigating corrosion rate of Mg-Sr alloys. The effect of alloying upon degradation rate was investigated via electrochemical measurements and immersion tests in minimum essential medium (MEM), a widely employed simulated body fluid. The results reveal a comparative degradation rate of the alloys tested herein, indicating no detrimental effect of Sr on degradation of the resulting alloys. Cytotoxicity experiments on primary mouse osteoblasts exhibit good biocompatibility and enhanced proliferation of osteoblasts for all the tested Mg alloys. The findings could provide a new insight to design and prepare Mg alloys as biomaterials with desired functionalities. The present work also attempted to elucidate how the Mg surface evolves during dissolution in cell culture medium, and to assess the influence of corrosion surface films on cell viability, attachment and morphology by means of Methylthiazolyldiphenyl-carboxymethoxyphenyl-sulfophenyl tetrazolium (MTS), live/dead staining and scanning electron microscopy (SEM) characterisation. Such cytotoxicity experiments demonstrate an enhanced biocompatibility of corroded surface relative to bare metal, and good spreading of cells on all tested surfaces. The work herein provides a preliminary understanding of the influence of surface films that act as the interface between implants and tissues/cells on cell behaviours, which is crucial for the surgical success of Mg-based implants.