Bulk Metallic Glass Applications in Self-Expanding Stent Technology

Diseases such as atherosclerosis in which plaque builds up on the walls of an artery and pathologically narrows the blood vessel. A clogged or narrowed artery increases resistance to blood flow and is a dangerous condition that can cause a heart attack, stroke, or even death. This can be prevented or managed with good dieting and exercise.

Atherosclerosis is characterized by plaque build up on the vessel walls causing a narrowed/clogged artery.

But in the case that a patient becomes aware of a clogged vessel, there are various surgical treatments. Bypass surgery connects unclogged arterties to circumvent the blocked artery. In balloon angioplasty, a device is inserted into the blocked vessel and inflated to physically open up the artery again. Yet another treatment is the stent in which a small tube is inserted to keep the vessel open. Stents are permanent implants and can be designed to elute drugs that prevent further narrowing by blocking cell proliferation. A subset of stents includes self-expanding stents which are inserted in a contracted form and then expand to fit the shape of the vessel, allowing stents to be placed in difficult vessel geometries.

Self-expanding nitinol stents

Bulk metallic glasses (BMGs) are metals with a glass-like structure. As Ayo explained in class, BMGs are amorphous with short-range order instead of the crystalline, long-range order of typical metals. In this paper I found for this week’s blog post, the Cui group considers the application of BMGs in self-expanding stents for the descending aorta. They used computational modeling and simulations to compare the performance of a Zr-based BMG stent and with a traditional nitinol-based stent. Both BMGs and nitinol stents exhibit shape-memory, so after they are formed, they are crimped to reduce their radius, and will reexpanded once deployed in the body. The stents expand until they contact the inner vessel wall and reach a stress equilibrium.

They studied the axial and circumferential stresses produced by the stents in the arterial walls and found that they were comparable to each other, and the stresses were lower than experimental test results on the failure strength of human arteries. This is important because one would not want a deployed stent to expand and cause your blood vessel to burst because the stress was too great! They also compared the stent penetration of the stents defined as the difference between initial artery inner diameter and the deformed stent outer diameter. Whereas nitinol stent penetration was 0.32mm, BMG stent penetration was 0.36mm indicating that the BMG stent can better embed itself into the vessel musculature.

Axial and circumferential stresses produced by stents in the arterial walls were comparable to each other, and the stresses were lower than experimental test results on the failure strength of human arteries

This study suggests the potential of BMGs in stent technology. As we learned in class BMGs have enhanced corrosion resistane, great processability, good biocompatibility, and high strength, all important properties for stents. Previous research BMGs had better biocompability in vitro and in vivo compared to crystalline alloys. Another study showed that under pulsatile fatigue loading, BMG stents were able to withstand the pulsatile pressure loads from blood and vessel walls. This is important because not only is the pressure in the aorta high but also the pressure is pulsatile. A material that can withstand a high pressure static load might fail in the aorta if it can’t handle the dynamic pulsatile stress. This paper highlights BMGs’ strengths and in the future I would like to see the stent move on from computational analysis and be applied in the human body.

Source: https://pubs.acs.org/doi/10.1021/acsbiomaterials.6b00342

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