Introduction: Two classes of biodegradable metallic materials have been widely investigated: Mg- and Fe- based alloys. Mg degrades too fast, releasing large amounts of hydrogen gas, while degradation rate of Fe is too slow. Newly, zinc has been proposed as an alternative candidate for biodegradable applications. According to the standard electrode potential Zn has a corrosion rate faster than Fe but slower than Mg. Moreover, Zn is an essential element for human beings, and it is involved in various aspects of cellular metabolism. Thus, Zn is believed to be a promising candidate for degradable stent.
Materials and Methods: In the present work, Zn-Mg and Zn-Al binary alloys were investigated as potential biodegradable materials for stent applications. The alloys were developed by melting-casting process. Tube extrusion was performed at 300°C aimed at producing small tubes as precursors for biodegradable stents. Mechanical properties were assessed by tensile test. Corrosion behavior of the alloys was characterized by performing potentiodynamic polarization and static immersion tests in Hanks’ modified solution. The solution temperature and pH were adjusted to 37 ±1◦C and 7.4, respectively.
Results and Discussion: Microstructural observation showed significant grain refinement occurrence in both Zn-based binary alloys after extrusion. Mechanical characterization indicated that by increasing the Mg content tensile strength of the Zn-Mg alloys increased owing to the increasing volume fraction of the hard Mg2Zn11 intermetallic phase. Moreover, Al seemed to be less effective in improving the mechanical properties of Zn-based alloys which is attributed to its higher solubility in Zn. EBSD investigation on the extruded Zn-Mg tubes showed nearly similar grain size and texture orientation to their billet extruded counterparts, resulting in comparable mechanical behavior. The corrosion rates the investigated samples in both cast and extrusion conditions were in the same order of magnitude. Nevertheless, extruded alloys exhibited slightly superior corrosion resistance and a slower degradation ratio than their cast counterparts. A thorough EBSD analysis in the vicinity of the laser cut Zn-Mg tubes featured no occurrence of grain coarsening and texture modification, confirming that after laser cutting the grain size and texture orientation of the final stent remains unchanged. Therefore, the cut stent might have reasonably comparable mechanical properties to that of uncut material.
Tensile properties for all the investigated samples.
EBSD orientation map of Zn-0.5Mg alloy.
SEM view of the laser cut Zn-0.5Mg mini-tube and EBSD orientation map next to the laser cut edge.
Conclusion: Zn-Mg alloy minitubes with different concentrations have been proposed for biodegradable stent applications. Hot tube extrusion led to a considerable grain refinement in all samples. All produced Zn-Mg mini-tubes featured equiaxed grain structure. With increasing Mg content mechanical properties of the alloys improved due to the increasing volume fraction of the hard Mg2Zn11 intermetallic phase. Among all the investigated alloys, Zn-0.5Mg owing to having combination of reasonable strength, ductility, strain hardening exponent value as well as a proper rate of loss of mechanical integrity provided a promising potential for stent application.