Effects of grain boundaries on the biocompatibility of the pure magnesium

The influences of the grain size on the biocompatibility and biodegradation behavior of an extruded and post-annealed pure Mg were investigated. The immersion test, electrochemical analysis, and surface characterization were carried out to study the degradation behavior, and the Bromodeoxyuridine (BrdU) and cell adhesion assays were applied to investigate the cells interaction with the substrate. The results depicted a striking influence of the grain size variations on the degradation behavior, cell viability, and cell adhesion characteristics of the pure magnesium. Here, the grain size was increased from 17.05 ± 2.64 μm for exruded sample to 30.62 ± 3.18 μm for the sample annealed at 400 °C and the degradation rate measured by hydrogen evolution in simulated body fluid (SBF) method increased from 2.42 ± 0.65 to 9.59 ± 1.33 mm/y, respectively. Moreover viability calculated by BrdU assey showed a decrease from 68 % (75 % extract) and 64.5 % (100 % extract) for extruded sample to 57 % (75 % extract) and 58 % (100 % extract) for the sample annealed at 400 °C. Higher annealing temperatures and consequent greater grain size were found to negatively influence the cytocompatibility of the pure Mg. In fact, annealing at the optimum temperature of 300 °C reduced the biodegradation rate by providing a uniform and stable MgO/Ca–P containing surface layer and hence resulted in higher cell adhesion and less cell toxicity.