Effect of strain on degradation behaviors of WE43, Fe and Zn wires.

The biodegradable metallic devices undergo stress/strain-induced corrosion when they are used for load-bearing applications. The stress/strain induced-corrosion behavior causes differences in corrosion rate, corrosion morphology, strain distribution and mechanical performance of the devices. One representative example is the biodegradable stent. Biodegradable stents undergo complex inhomogeneous deformation that can cause dramatic non-uniform stent degradation, resulting in stress concentration and stents failure. The degradation of biodegradable devices requires special attention to the mutual effect between the applied strain and degradation. The quantitative relationship between strain and corrosion of the sample alloys (WE43, Fe and Zn), selected from three typical biodegradable metals, is firstly investigated and compared in this study. The in vitro degradation and the strength retention of WE43, Fe and Zn wires were investigated under different elastic and plastic strain levels ranging from 0.1% to 30%. The results indicated that the applied strain could bring down the corrosion potential, increase corrosion current and accelerate the degradation of three biodegradable metals. Specifically, remarkable enhanced localized corrosion was observed for plastic strained WE43 compared with those with elastic strains. This localized corrosion morphology significantly accelerated the strength decline at first, while the differences diminished with longer immersion period. Fe and Zn exhibited increased degradation with plastic strain applications than those under elastic strains. However, the degradation was not further increased with the increasing magnitude of plastic strains. Moreover, the bended wires were subcutaneously implanted in the dorsal aspect of the rats and the effect of bending deformation on in vitro and in vivo degradation of three metallic wires were also compared. The U-bended WE43 wires suffered more severe in vitro degradation at the stress concentrated region. Surprisingly, the early fracture of the undeformed regions was observed in the in vivo test. In conclusion, the corrosion rate, corrosion morphology and mechanical properties of WE43, Fe and Zn was sensitive to magnitude of the applied strains. The quantification results provided new insights into understanding the strain-dependent corrosion of three biodegradable metals both in vitro and in vivo. STATEMENT OF SIGNIFICANCE: Biodegradable implants are subjected to various mechanical environment during the deployment and subsequent physiological activity. It is necessary to have a clear understanding of the effects of the applied stress on degradation. This study addresses the quantitative effects of applied strain/stress on the in vitro and in vivo degradation of three typical biodegradable metals (Mg, Fe and Zn). These quantification results provide new insights into understanding the strain-induced corrosion of three metals.