In vitro degradation behavior of Mg wire/poly(lactic acid) composite rods prepared by hot pressing and hot drawing

In this study, we prepared Mg wire/poly(lactic acid) composite rods by hot pressing (HP) and hot drawing (HD) processes, which show desirable application potential as a biodegradable implant in orthopedics. The influences of the volume content of Mg wires and processing steps on the degradation behavior of composite rods as well as the mutual influence between Mg and poly(lactic acid) (PLA) during degradation were investigated. During degradation, the improved interface bonding between Mg and PLA by micro-arc oxidation can effectively inhibit the diffusion of the medium from the ends to the central part, supported by XR-CT results, which show the degradation cracks in PLA initiated and propagated along the radial direction of the rod rather than the axial direction. When compared with the hot pressed rod, the three passes hot drawn rod had lower degradation rate and better strength retention during long-time immersion. This is ascribed to the higher crystallinity of the PLA matrix by HD. For both hot pressed and hot drawn rods, more Mg content of the wires will result in an increase in the degradation rate of PLA due to alkaline catalytic hydrolysis. Therefore, we can not only predict the degradation period by the present degradation mathematical model but also further control degradation rate by adjusting processing steps and wire volume content. This will be helpful for future material design and indication selection. STATEMENT OF SIGNIFICANCE: We investigated the influence of interface microstructure, self-reinforced PLA matrix, and wire volume content on the degradation behavior and clarified the differences in the degradation mechanism of composite rods. The 3D morphology of cracks in composite rods enlightened us a clear understanding about the emergence and evolution of cracks, which revealed the reason for the failure of composite rods. The degradation model can be used to predict the degradation period and provide valuable information for their future application.