Abstract
The development of high purity Mg alloys and new processing techniques have led to the emergence of the first degradable Mg implants on the market. Those commercially available implants are designed for specific bone fixation and atherosclerosis treatments. Nevertheless, the not well-understood degradation mechanisms of Mg-based materials under physiological conditions is one of the reasons hindering the promising use of those materials for other applications as a degradable biomaterial. Due to the current uncertainties in defining/monitoring the complex corrosive in vivo environment, the influence of the different physiological fluid components in the Mg degradation mechanism has to be clarified by in vitro studies. However, most of the in vitro studies monitor the bulk solution environment, defined by the concentrations and pH, as the only relevant corrosive environment. Only a few in vitro studies take into account the mass transfer phenomena at the Mg surface and identify the importance of the local surface environment on Mg degradation mechanisms. However, those studies do not provide insight into the correlation between the solution conditions at the surface and the degradation products layer as a critical degradation rate modifier. Among all the physiological fluids components, the inorganic fraction, and particularly the presence of Ca2+ cations, was revealed in previous studies as a critical modifier of the degradation rate of Mg alloys. For this reason, the effect of Ca2+ cations presence on the pH at the Mg /degradation solution interface (interface pH) were investigated in this work by Scanning Ion-selective Electrode Technique (SIET) on different magnesium materials (HP-Mg, Mg-1.2Ca, Mg-2Ag and E11). The different corrosive solutions based on the commercial HBSS composition were applied under flow conditions to mimic the in vivo homeostasis. Moreover, the influence of the presence of Ca2+ cations in the degradation products layer composition was analysed by optical microscopy (OM), backscattered scanning electron microscopy (BSEM), energy dispersive x-rays (EDX), grazing angle x-ray diffraction (GAXRD), micro x-ray fluorescence (μXRF), and Fourier Transform Infrared Spectroscopy Attenuated Total Reflectance (FTIR-ATR). The results show that the presence of Ca2+ cations generates a decrease in the degradation rate (26% - 65% depending on the Mg material) and the interface pH (around pH 8 for all the alloys). These effects are attributed to the fast development of an amorphous calcium phosphate (CaPs) compact outer layer. The precipitation/dissolution equilibrium of the products forming the degradation layer generates an extra buffering system at the interface. This local buffering system can absorb the differences in the pH promoted by different materials with significant differences in the degradation rate. Therefore, this study contributes to describe the local surface environment of Mg-based materials under simulated physiological conditions and provides relevant information concerning the importance of the interface pH on the development of the degradation products layer.
