Structural Characteristics and In Vitro Biodegradation of a Novel Zn-Li Alloy Prepared by Induction Melting and Hot Roll
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INTRODUCTION
BIOABSORBABLE stents are envisaged to support the arterial wall during remodeling after stent deployment and to degrade harmlessly thereafter. This new generation of endovascular stents may eliminate the potential for chronic inflammation[1] and thrombosis risks[2] of permanent stents. They could also alleviate the repetition of invasive procedures when stenting at the same site in the event of restenosis.[3] Over the past decade, polymeric and metallic materials have been widely investigated for endovascular stent applications with very limited success.[4–7] The previous reports demonstrated that neither polymers nor metallic candidates such as iron and magnesium are ideal for biodegradable stenting applications due to either poor mechanical properties,[7–10] incomplete bioabsorption of corrosion products,[11] or premature degradation.[12] Consequently, the search for new bioabsorbable materials continues.[7] Zinc is one of the most abundant nutritionally essential elements in the human body[13] and studies have been initiated examining zinc as a bioabsorbable material in recent years.[14] One major concern regarding SHAN ZHAO, CAMERON T. MCNAMARA, PATRICK K. BOWEN, NICHOLAS VERHUN, JACOB P. BRAYKOVICH and JAROSLAW W. DRELICH are with the Department of Materials Science and Engineering, Michigan Technological University, Houghton, MI 49931. Contact e-mail: [email protected] JEREMY GOLDMAN is with the Department of Biomedical Engineering, Michigan Technological University, Houghton, MI 49931. Manuscript submitted June 23, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
the use of a pure zinc stent is the material’s low intrinsic ultimate tensile strength (UTS) of about 120 MPa or less, where the cardiovascular stent application requires a material with UTS closer to 300 MPa.[15] Improvements in mechanical properties of zinc can be approached through alloying[16,17] and manipulation of metal micro/nanostructure.[18,19] Lithium is one of the few elements with significant solubility in zinc, and Zn-Li is therefore among a few potentially age-hardenable systems. Hypoeutectic 2 at. pct Li (0.2 wt pct), eutectic 4 at. pct Li (0.4 wt pct), and hypereutectic 6 at. pct Li (0.7 wt pct) compositions were chosen for this study based on the phase diagram produced by Pelton[20] (Figure 1). The eutectic reaction under the casting conditions used herein is expected to result in prolific lamellar formation during cooling, so that much of the available Li is consumed and the only Li available for post-solidification strengthening is what is left in supersaturation upon cooling below the eutectic temperature in (Zn)Li regions. Therefore, precipitation hardening is expected to play a major role in the 2 pct, a minor role in the 4 pct, and a negligible role in the 6 pct alloy. However, the LiZn4 formed as part of the lamellar structure does have an impact on yield and ultimate strength as the shear moduli between this and the Zn(Li) phase is different, and the strength increment is proportional to 1/k1/2, where k is the lamella
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