Recommendation for biological evaluations on biodegradable magnesium-based materials: Based on the coupling impact of pH value and Mg²⁺ on cells and bacteria

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.