Tailoring the mechanical and degradation performance of Mg-2.0Zn-0.5Ca-0.4Mn alloy through microstructure design

A novel Mg-2.0Zn-0.5Ca-0.4Mn alloy has been formulated and processed through melt spinning and hot extrusion to enhance its mechanical and degradation properties. Microstructural characterization of rapidly solidified alloy ribbons consolidated by extrusion revealed a fine and fully recrystallized microstructure with average size of 4 µm. The conventionally extruded alloy consisted of several course second-phase strips as coarse as 100 µm, while the extrusion-consolidated ribbons were devoid of any second phases larger than 100 nm. Rapid solidification followed by extrusion processing resulted in significantly randomized texture where the majority of the basal planes were tilted toward transverse and extrusion directions. Such a weak texture resulted in higher activity of basal planes and thereby considerably improved the fracture elongation from 4% to 19%, while retaining relatively high tensile strength of 294 MPa. In addition to high strength and ductility due to the reduced activity of deformation twining during compression, the extrusion-consolidated alloy ribbons showed lower yielding asymmetric ratio than that measured for the conventionally extruded alloy (1.25 versus 1.61). Electrochemical measurements and immersion tests indicated that application of rapid solidification followed by extrusion remarkably reduced the corrosion rate from 2.49 mm/year to 0.37 mm/year due to recrystallization completion and suppression of coarse second-phase formation

Towards revealing key factors in mechanical instability of bioabsorbable Zn-based alloys for intended vascular stenting

Zn-based alloys are recognized as promising bioabsorbable materials for cardiovascular stents, due to their biocompatibility and favorable degradability as compared to Mg. However, both low strength and intrinsic mechanical instability arising from a strong strain rate sensitivity and strain softening behavior make development of Zn alloys challenging for stent applications. In this study, we developed binary Zn-4.0Ag and ternary Zn-4.0Ag-xMn (where x = 0.2–0.6wt%) alloys. An experimental methodology was designed by cold working followed by a thermal treatment on extruded alloys, through which the effects of the grain size and precipitates could be thoroughly investigated. Microstructural observations revealed a significant grain refinement during wire drawing, leading to an ultrafine-grained (UFG) structure with a size of 700 nm and 200 nm for the Zn-4.0Ag and Zn-4.0Ag-0.6Mn, respectively. Mn showed a powerful grain refining effect, as it promoted the dynamic recrystallization. Furthermore, cold working resulted in dynamic precipitation of AgZn3 particles, distributing throughout the Zn matrix. Such precipitates triggered mechanical degradation through an activation of Zn/AgZn3 boundary sliding, reducing the tensile strength by 74% and 57% for Zn-4.0Ag and Zn-4.0Ag-0.6Mn, respectively. The observed precipitation softening caused a strong strain rate sensitivity in cold drawn alloys. Short-time annealing significantly mitigated the mechanical instability by reducing the AgZn3 fraction. The ternary alloy wire showed superior microstructural stability relative to its Mn-free counterpart due to the pinning effect of Mn-rich particles on the grain boundaries. Eventually, a shift of the corrosion regime from localized to more uniform was observed after the heat treatment, mainly due to the dissolution of AgZn3 precipitates. Statement of Significance Owing to its promising biodegradability, zinc has been recognized as a potential biodegradable material for stenting applications. However, Zn's poor strength alongside intrinsic mechanical instability have propelled researchers to search for Zn alloys with improved mechanical properties. Although extensive researches have been conducted to satisfy the mentioned concerns, no Zn-based alloys with stabilized mechanical properties have yet been reported. In this work, the mechanical properties and stability of the Zn-Ag-based alloys were systematically evaluated as a function of microstructural features. We found that the microstructure design in Zn alloys can be used to find an effective strategy to not only improve the strength and suppress the mechanical instability but also to minimize any damage by augmenting the corrosion uniformity

Recent advances and directions in the development of bioresorbable metallic cardiovascular stents: Insights from recent human and in vivo studies

Over the past two decades, significant advancements have been made regarding the material formulation, iterative design, and clinical translation of metallic bioresorbable stents. Currently, magnesium-based (Mg) stent devices have remained at the forefront of bioresorbable stent material development and use. Despite substantial advances, the process of developing novel absorbable stents and their clinical translation is time-consuming, expensive, and challenging. These challenges, coupled with the continuous refinement of alternative bioresorbable metallic bulk materials such as iron (Fe) and zinc (Zn), have intensified the search for an ideal absorbable metallic stent material. Here, we discuss the most recent pre-clinical and clinical evidence for the efficacy of bioresorbable metallic stents and material candidates. From this perspective, strategies to improve the clinical performance of bioresorbable metallic stents are considered and critically discussed, spanning material alloy development, surface manipulations, material processing techniques, and preclinical/biological testing considerations. Statement of significance: Recent efforts in using Mg, Fe, and Zn based materials for bioresorbable stents include elemental profile changes as well as surface modifications to improve each of the three classes of materials. Although a variety of alloys for absorbable metallic stents have been developed, the ideal absorbable stent material has not yet been discovered. This review focuses on the state of the art for bioresorbable metallic stent development. It covers the three bulk materials used for degradable stents (Mg, Fe, and Zn), and discusses their advances from a translational perspective. Strategies to improve the clinical performance of bioresorbable metallic stents are considered and critically discussed, spanning material alloy development, surface manipulations, material processing techniques, and preclinical/biological testing considerations

Fabrication, mechanical properties and in vitro degradation behavior of newly developed ZnAg alloys for degradable implant applications

Zn and Zn-based alloys have been recognized as highly promising biodegradable materials for orthopedic implants and cardiovascular stents, due to their proved biocompatibility and, more importantly, lower corrosion rates compared to Mg alloys. However, pure Zn has poor mechanical properties. In this study, Ag is used as a promising alloying element to improve the mechanical properties of the Zn matrix as well as its biocompatibility and antibacterial properties. Accordingly, we design three ZnAg alloys with Ag content ranging from 2.5 to 7.0wt% and investigate the influence of the Ag content on mechanical and corrosion behavior of the alloys. The alloys are developed by casting process and homogenized at 410°C for 6h and 12h, followed by hot extrusion at 250°C with extrusion ratio of 14:1. Degradation behavior is assessed by electrochemical and static immersion tests in Hank's modified solution. Microstructural analysis reveals that hot extrusion significantly reduces the grain size of the alloys. Zn-7.0%Ag alloy shows a reasonably equiaxed and considerably refined microstructure with mean grain size of 1.5μm. Tensile tests at room temperature suggest that increasing the Ag content steadily enhances the tensile strength, while it does not affect the tensile ductility significantly. Zn-7.0%Ag shows high yield strength and ultimate tensile strength of 236MPa and 287MPa, respectively, which is due to the grain refinement and high volume fraction of fine AgZn3 particles precipitating along the grain boundaries during the extrusion process. Among all these alloys, Zn-7.0%Ag displayed superplasticity over a wide range of strain rates (from 5×10(-4)s(-1) to 1.0×10(-2)s(-1)) providing the possibility of exploiting forming processes at rapid rates and/or even at lower temperatures. In addition, extruded alloys exhibit slightly faster degradation rate than pure Zn. X-ray diffraction results show the presence of ZnO and Zn(OH)2 on the degraded surfaces. Moreover, scanning electron microscopy imaging reveals that micro-galvanic corrosion is more pronounced on the alloys with higher Ag content due to the higher volume fraction of AgZn3 particles. [Abstract copyright: Copyright © 2017 Elsevier B.V. All rights reserved.

Long-term in vitro degradation behaviour of Fe and Fe/Mg2Si composites for biodegradable implant applications

This fundamental study provides a basis for the processes of protective film formation on degradable Fe-based biomaterials

Synthesis, mechanical properties and corrosion behavior of powder metallurgy processed Fe/Mg2Si composites for biodegradable implant applications

Recently, Fe and Fe-based alloys have shown their potential as degradable materials for biomedical applications. Nevertheless, the slow corrosion rate limits their performance in certain situations. The shift to iron matrix composites represents a possible approach, not only to improve the mechanical properties, but also to accelerate and tune the corrosion rate in a physiological environment. In this work, Fe-based composites reinforced by Mg2Si particles were proposed. The initial powders were prepared by different combinations of mixing and milling processes, and finally consolidated by hot rolling. The influence of the microstructure on mechanical properties and corrosion behavior of Fe/Mg2Si was investigated. Scanning electron microscopy and X-ray diffraction were used for the assessment of the composite structure. Tensile and hardness tests were performed to characterize the mechanical properties. Potentiodynamic and static corrosion tests were carried out to investigate the corrosion behavior in a pseudo-physiological environment. Samples with smaller Mg2Si particles showed a more homogenous distribution of the reinforcement. Yield and ultimate tensile strength increased when compared to those of pure Fe (from 400 MPa and 416 MPa to 523 MPa and 630 MPa, respectively). Electrochemical measurements and immersion tests indicated that the addition of Mg2Si could increase the corrosion rate of Fe even twice (from 0.14 to 0.28 mm·yearâ\u88\u92 1). It was found that the preparation method of the initial composite powders played a major role in the corrosion process as well as in the corrosion mechanism of the final composite

Surface modification of L605 by oxygen plasma immersion ion implantation for biomedical applications

Co-Cr alloys, more specifically L605, have superior mechanical properties and high-corrosion resistance, making them suitable materials for cardiovascular application. However, metallic materials for biomedical applications require finely tuned surface properties to improve the material behavior in a physiological environment. Oxygen plasma immersion ion implantation was performed on an L605 alloy, after an electropolishing pre-treatment. The oxidized layer was found to be rich in Co and O, it did not show any trace of Cr, and resulted in nanostructured. The corrosion properties were profoundly changed. Endothelial cells showed high viability after 7 days of contact with some modified surfaces

Analysis of vascular inflammation against bioresorbable Zn–Ag-based alloys

Zinc (Zn) has emerged as a promising bioresorbable stent material because of its satisfactory corrosion behavior and excellent biocompatibility. However, for load-bearing implant applications, alloying is required to boost its mechanical properties as pure Zn exhibits poor strength. Unfortunately, an increase in inflammation relative to pure Zn is a commonly observed side effect of Zn alloys. Consequently, the development of a Zn-based alloy that can simultaneously feature improved mechanical properties and suppress inflammatory responses is a big challenge. Here, a bioresorbable, biocompatible Zn–Ag-based quinary alloy was comprehensively evaluated in vivo, in comparison to reference materials. The inflammatory and smooth muscle cellular response was characterized and correlated to metrics of neointimal (NI) growth. We found that implantation of the quinary alloy was associated with significantly improved inflammatory activities relative to the reference materials. Additionally, we found that inflammation, but not smooth muscle cell hyperplasia, significantly correlates to NI growth for Zn alloys. The results suggest that inflammation is the main driver of NI growth for Zn-based alloys and that the quinary Zn–Ag–Mn–Zr–Cu alloy may impart inflammation-resistance properties to arterial implants