Structural and mechanical aspects of hypoeutectic Zn–Mg binary alloys for biodegradable vascular stent applications

Abstract

The study is concerned with the mechanical properties of Zn and three Zn–Mg double alloys with Mg concentrations: 0.5%, 1.0% and 1.5% in the form of rods with a diameter of 5 mm as potential materials for use in biodegradable medical implants, such as vascular stents. The materials were cast, next conventionally hot extruded at 250 °C and finally, hydrostatically extruded (HE) at ambient temperature. Occasionally HE process was carried at liquid nitrogen temperature or in combination with the ECAP process. After HE, the microstructure of the alloys was made up of fine-grained αZn of mean grain size ~1 μm in a 2-phase coat of 50–200 nm nano-grains of the fine αZn + Mg 2 Zn 11 eutectic. The 3 to 4-fold reduction of grain size as a result of HE allowed an increase in yield strength from 100% to over 200%, elongation to fracture from 100% to thirty fold and hardness over 50% compared to the best literature results for similar alloys. Exceptions accounted for elongation to fracture in case of Zn-0.5 Mg alloy and hardness in case of Zn-1.5 Mg alloy, both of which fell by 20%. For the Zn-0.5 Mg and Zn–1Mg alloys, after immersion tests, no corrosive degradation of plasticity was observed. Achieving these properties was the result of generating large plastic deformations at ambient temperature due to the application of high pressure forming with the cumulative HE method. The results showed that Zn–Mg binary alloys after HE have mechanical and corrosive characteristics, qualifying them for applications in biodegradable implants, including vascular stents.

High-Efficient Vacuum Ultraviolet-Ozone Assist-Deposited Polydopamine for Poly(lactic-co-glycolic acid)-Coated Pure Zn toward Biodegradable Cardiovascular Stent Applications

“…Considering that, the standard electrode potential of Zn (−0.76 V SHE ) is between that of Mg (−2.36 V SHE ) and Fe (−0.44 V SHE ), and the mechanical strength of Zn-based metals is superior to that of Mg-based metals . Thus, Zn is considered as a more appropriate implant for cardiovascular stent applications than Mg and Fe in the aspects of preferable corrosion behavior, superior mechanical strength, and acceptable biocompatibility in blood contacting engineering. − The corrosion rate and mechanical strength of Zn are very close to the expected value for biodegradable stents; however, the excessive Zn ion release and fatigue fracture failure during degradation remain huge challenges in clinical practice. − As compared to Zn-based alloys, the in vitro degradation rate of pure Zn decreased to 0.033 mm year –1 as evaluated by the immersion test in Hank’s solution, which is nearly close to the ideal value for biodegradable metals in vascular applications (0.02 mm year –1 ). , Moreover, the median lethal dose values of Zn 2+ have been reported, that is, 50 μM for human dermal fibroblasts (hDF), 70 μM for human aortic smooth muscle cells (AoSMC), and 265 μM for human endothelial cells (HAEC) . Rapid released Zn 2+ can hinder both cell adhesion and mobility, and even quickly cause cell death. , In addition, the stents are subjected to nonuniform corrosion and stress concentration in the complex in vivo environment, eventually resulting in the loss of mechanical support.…”

In Vitro Corrosion Behavior of Zn3Mg0.7Y Biodegradable Alloy in Simulated Body Fluid (SBF)

“…Xivei Liu obtain the following values of microhardness: 93.71 HV for the Zn-1Mg-0.1Sr alloy, and 109.34 HV for the Zn-1Mg-0.5Sr alloy, which are higher than those of pure Zn, suggesting the effectiveness of alloying in improving its mechanical property [29]. Pachla et al present Vickers hardness values for samples Zn0.5Mg, Zn1Mg and Zn1.5Mg after a hot extrusion process: 75 HV, 95HV and 115 HV, respectively [30,31].…”

Texture-Based Optimization of Crystal Plasticity Parameters: Application to Zinc and Its Alloy

“…The self-hardening moduli H r ðabÞ present in Eqs. [9] and [10] are defined in the following way for hardening of slip ða ¼ sÞ or twin ða ¼ tÞ systems due to activity of slip (s):…”

“…It should be also noted that good biocompatibility and optimal corrosion rate have made zinc and its alloys promising candidates for producing bioabsorbable implants e.g., stents. [8,9] Such implants should dissolve gradually in the physiological environment after fulfilling their mission to support damaged tissue during its recovery process. Furthermore, generated corrosion products need to be safely metabolized by human body without any harmful response.…”