Effect of pH on the degradation kinetics of a Mg–0.8Ca alloy for orthopedic implants

Abstract One of the promising applications of magnesium and magnesium alloys is their use as biodegradable implants in biomedical applications. The pH around an orthopedic implant greatly affects the degradation kinetics of biodegradable Mg–Ca alloys. At the location of a fracture, local pH changes, and this has to be considered in the optimization of implant materials. In this study, the effect of the pH of a physiological buffer on degradation of a Mg–0.8Ca alloy was studied. The pH of Hank’s balanced salt solution (HBSS) was adjusted to 1.8, 5.3 and 8.1. Degradation of a Mg–0.8Ca implant was tested using immersion test and electrochemical techniques. Immersion tests revealed an initial weight gain for all samples followed by weight loss at extended immersion time. Weight gain was highest at acidic pH (1.8) and lowest at alkaline pH (8.1). This was in agreement with results from electrochemical polarization tests where the degradation rate was highest (7.29 ± 2.2 mm/year) at pH 1.8 and lowest (0.31 ± 0.06 mm/year) in alkaline medium of pH 8.1. The pH of all HBSS buffers except the most acidic (pH 1.8) reached a steady state of ∼pH 10 at the end of the two-month immersion period, independent of the initial pH of the solution. Corrosion products formed on the sample surfaces were investigated by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDX) and X-ray diffractometry (XRD), revealing the formation of magnesium and calcium phosphates with distinct morphologies that were different for each of the pH conditions. Thus, pH of physiological buffers has a significant effect on the degradation and corrosion of Mg–Ca alloys used for biomedical applications.