Finite element analysis of stress and strain on Zr-Al-Fe-Cu BMG under 50 or 150 mmHg pressure loads.An unusual amorphous metal alloy known as a bulk metallic glass (BMG) could increase the longevity of stents, which are used to open up blocked arteries, according to researchers from the University of Tennessee and Dalian University of Technology in China.
The build up of plaques in arteries, narrowing or clogging the blood vessel, leads to atherosclerosis, which can result in heart attacks or strokes. Treatment often takes the form of the surgical insertion of a metallic stent – a mesh-like tube, which expands once in place to open up blocked arteries. Stainless steel and other metals are commonly used for stents, but they have to be able to withstand a heart pulsating at 72 beats per minute for ten years without corroding or producing an inflammatory response.
Wei He believes that bulk metallic glasses have all the right attributes for stents – and could perform better in the body than conventional stainless steel. He and her colleagues have, for the first time, compared the mechanical properties, corrosion resistance, and biocompatibility of Ni-free BMG Zr-Al-Fe-Cu with 316L stainless steel for use as stents [Huang et al., Acta Biomaterialia (2015), http://dx.doi.org/10.1016/j.actbio.2015.07.012].
The lack of a long-range, regular arrangement of atoms in BMGs gives rise to a unique combination of properties including strength, resistance to corrosion, and formability when heated. Computational modeling of the behavior of Zr-Al-Fe-Cu in physiological conditions reveals some potential advantages, says He.
The greater strength of Zr-Al-Fe-Cu compared with stainless steel indicates that stents could be made with thinner struts, which would be easier to implant and could reduce the likelihood of restenosis, where arteries narrow again after surgery. To mimic conditions in blood vessels, the researchers also tested the BMG under cyclic loading conditions, where it also performed well.
The corrosion resistance is much improved with Zr-Al-Fe-Cu too, which the team believes is the result of the formation of a ZrO2-rich surface oxide in the in vivo environment.
BMGs are already well known for their biocompatibility, but the researchers found that endothelial cells, which line the interior walls of blood vessels, grow readily on the surface of Zr-Al-Fe-Cu while muscle cells, that can lead to complications if they proliferate, grew more slowly than on stainless steel.
“Our findings demonstrate several advantages of Zr-BMG for vascular stent applications,” says He, “including excellent mechanical properties, high corrosion resistance, good cytocompatibility, and imaging compatibility (with MRI, for example).”
The researchers now plan to investigate the hemocompatibility of Zr-based BMGs and tweak the composition to bring additional benefits such as antimicrobial activity.