In Vitro Evaluation of Mg-li-zn Alloys for Biomedical Application

INTRODUCTION Heart disease is the leading cause of death in the United States, with approximately 610,000 deaths every year [1]. Heart disease is often related to atherosclerosis, or plaque buildup in the artery walls. This buildup hardens and narrows the arteries, thereby obstructing blood flow and increasing risk for heart attack, stroke, or peripheral vascular disease. The common treatment for atherosclerosis is the insertion of a bare metal stent, which is inserted to support the artery and facilitate continued blood flow. The common treatment for atherosclerosis is the insertion of a bare metal stent, which is inserted A huge problem associated with bare metal stents are stent thromboses; stent thrombosis is a blockage of a stented artery due to blood clot (thrombus) formation. After implantation for approximately one year, 4.1% of bare metal stents result in stent thrombosis. The clinical consequences of stent thrombosis are often catastrophic and lead to death in 20-48% or major myocardial infarction in 60-70% of cases [2, 3]. Magnesium alloy stents would function similarly to traditional bare metal stents but with significant advantages. Two key advantages are that magnesium alloy stents inhibit tissue growth around the stent, and dissolve in 6-12 months. These qualities would eliminate the need for secondary surgeries to remove the stents in the event of stent thrombosis or recurrent artery narrowing. Magnesium alloys are not without problems though. Most magnesium alloys are not ductile enough for stent applications due to magnesium’s HCP crystal structure. For medical devices that require high plastic deformation, such as stents, the low ductility of Mg alloys result in various limitations particularly in the design and manufacture of these devices. Furthermore, some magnesium alloys degrade too quickly. In order to address the problem of low ductility and fast degradation rates, Mg-Li alloys were investigated. The microstructure of Mg-Li alloys depends on the Li content. With less than 5.7 wt.% of Li, the binary alloy is comprised of single α phase (HCP structure); with more than 10.3 wt.% of Li, the Mg-Li alloy is composed of single β phase (BCC structure). Both α phase and β phase co-exist when Li content is between 5.7% and 10.3%. With a mixture of α phase and β phase, the dual-phase Mg-Li alloys exhibit ultra-high ductility and maintain relative high strength, and therefore are more suitable for load-bearing biomedical application. A study by Leefang et al. demonstrated the mechanical superiority of dual-phase Mg-Li alloys for stent application.