Biomedical application of magnesium and its alloys is expected to realize biodegradable metallic devices since they are easily corroded by reacting with water in the body fluid. For commercialization of biodegradable Mg alloy devices, it is mandatory to perform biological safety evaluation, following to the guidelines such as ISO 10993 series. However, most of these guidelines are established before the commercialization of degradable metallic devices; they suppose only degradable polymeric/ceramic materials or non-degradable metallic materials. Therefore, application of these guidelines to Mg alloys may be inappropriate because their degradation behavior and mechanism are totally different from those of conventional degradable materials. In this study, the degradation behavior of a Mg alloy under the extract condition recommended by ISO 10993 series was examined focusing on the kind and amount of the extract solution.
Coupons with dimensions of 5 × 10 × 0.5 mm were cut from commercially available AZ31 sheet for extract preparation. Every surface of the coupons was polished with #600 (14 µm) SiC paper in ethanol, followed by ultrasonication with acetone for 5 min 3 times. These coupons were immersed into the extract solution [artificial plasma (AP) or Eagle’s minimum essential medium supplemented with 10% fetal bovine serum (E-MEM+FBS)] for 72 h at 37 ˚C under 5% CO2 at the solution to surface area ratio of 0.17, 0.33, 1, 10, and 100 mL/cm2.
Fig. 1 shows the weight loss, concentration of released Mg2+, and pH of the solution after extraction. The pH clearly increased with decrease of solution to surface area ratio. The pH of AP or E-MEM+FBS reached to 8.90 or 8.73, respectively, at 0.17 mL/cm2 whereas those were 7.66-7.52 at 100 mL/cm2. The degradation rate increased along the increase of solution to surface area ratio, but this trend was irregular around 0.33 and 1 mL/cm2. The degradation behavior was also different between AP and E-MEM+FBS.
Electrochemical measurement was also performed in 0.9% NaCl, AP, and E-MEM+10%FBS at 37˚C in air for 0.9% NaCl or under 5%CO2 for AP and E-MEM+FBS using coupons with dimensions of 15 mm square. A standard three-electrode system was employed; an alloy sample (exposed area of 0.950 cm2) as a working electrode, Ag/AgCl (3M NaCl) as a reference electrode, and a platinum mesh as a counter electrode. Electrochemical impedance spectroscopy (EIS) was performed under following conditions; at open circuit potential (OCP), 5mV, 0.01~105 Hz after 2h of OCP measurement. Potentiodynamic measurement is carried out after 48h of incubation at the sweep rate of 0.5mV/s and OCP-0.25V to -1.2V. Both EIS and potentiodynamic measurements revealed that the degradation rate in E-MEM+FBS is lower than those in AP or 0.9%NaCl.
In the human body, blood flow is maintained in every tissue, which assists the diffusion of OH- and Mg2+ generated by the Mg alloy corrosion and contributes to maintain physiological pH of the body fluid. Therefore, the increase of pH over physiological range may not be occurred inside the tissue. Therefore, the small solution to surface area ratio, which results in the extreme increase of pH, is inappropriate for the evaluation of the biological safety of Mg alloys.The smallest degradation rate in E-MEM+FBS, which has a similar composition to the human blood plasma, suggests the importance to use appropriate solution for the evaluation of biological safety and degradation behavior of Mg alloys.
This work was supported by JSPS KAKENHI Grant number 26282151.