In vitro biological evaluations are important in developing and clinically approving biodegradable magnesium (Mg)-based devices. However, such methods specified in ISO Standards for these in vitro evaluations are flawed. The particular concern is the excessive inhibition of Mg extracts on cells and bacteria, attributed to the increased pH value and Mg²⁺ concentration due to Mg degradation. To figure out this issue, the current investigation detailed the coupling impact of these two factors on the viability of normal cells, tumor cells and bacteria. The results showed that the response of cells and bacteria to Mg²⁺ heavily depended on the medium pH value. Normal human dermal fibroblasts (NHDF) and human non-small-cell lung cancer cells (H23) exhibited a high tolerance to Mg²⁺ under the neutral condition, while human umbilical vein endothelial cells (HUVEC) under the alkaline condition. The alkaline condition coupled with increased Mg²⁺ concentrations could produce antibacterial effects against Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli), while the tolerance of these two bacteria to Mg²⁺ at different pH values was also different. The overlap analysis on the survival predominant regions of these cells and bacteria indicated that Mg degradation could not achieve a win-win situation of antisepsis and the safety of the two normal cells by affecting the medium pH value and Mg²⁺ concentration. However, increasing Mg²⁺ concentration and pH might have anti-H23 effects without inducing cytotoxicity to NHDF and/or HUVEC cells. Based on these results, a recommendation to evaluate the biological effects of magnesium-based materials in terms of their interfacial characteristics during degradation and associated key environmental thresholds influencing the behavior of cells and bacteria was proposed.

Previous work indicated that long-period stacking ordered (LPSO) phase and/or γʹ in rare earth containing Mg biomaterials had contradictory mechanisms responsible for their degradation in less complex or standard salt media, such as 0.9 % NaCl solution. They needed to be further investigated in a more realistic simulated body fluid (SBF). The present work investigated the influence of the amount and types of intermetallics on the degradation behavior of as-cast Mg-xDy-Zn (x = 5, 10, 15 wt.%) alloys using immersion test in Dulbecco’s modified Eagle’s medium (DMEM) + Glutamax together with 10 % Fetal bovine serum (FBS) under cell culture conditions. It was revealed that the existence of intermetallics exhibited different effects on the degradation behavior of alloys. At the early stage of immersion, Mg-10Dy-1.5Zn alloy suffered the most serious degradation among these three alloys, owing to its more severe micro galvanic corrosion. With the immersion proceeding, the degradation rate of Mg-5Dy-1.5Zn alloy consistently increased because of the scattered distribution of few intermetallics. In contrast, the continuous network structure of intermetallics and a compact degradation layer provided protection from further degradation for Mg-10Dy-1.5Zn and Mg-15Dy-1.5Zn alloys. In the as-cast Mg-5Dy-1.5Zn alloy, only small amount of intermetallics composed of W, γʹ and 18R LPSO phases acted as galvanic cathodes, accelerating its degradation. With Dy content increasing to 10 and 15 wt.%, large amounts of intermetallics including 18R LPSO and dense γʹ phases were formed, which on the other hand can serve as a continuous network barrier to retard degradation propagation. Finally, the good adhesion and proliferation of the Human umbilical cord perivascular (HUCPV) on the surface of the Mg-10Dy-1.5Zn and Mg-15Dy-1.5Zn alloy indicated their good biocompatibility.