Why polymer stents fail

Different materials behave differently, to state the obvious. Consequently, appropriate tools should be used to evaluate the behavior of materials in a potential application and, ideally, deliver insights on their performance over time. By evaluating polymer stents using procedures borrowed from the development of metal stents, medtech engineers were unable to foresee complications that would arise, according to researchers at the Massachusetts Institute of Technology (MIT).

Polymer stent
Image of polymer stent courtesy Pei-
Jiang Wang/MIT.

Metal stents implanted in coronary arteries to prevent blood clotting are life savers, but they do have a drawback: Extended implantation can cause damage to the artery. The development of plastic stents made with bioresorbable polymers several years ago seemed to offer an elegant solution.

The poly-l-lactic acid (pLLA) polymer used in this application degrades over time and is absorbed by the blood vessel walls. This worked well initially, but after three years, more than 10% of patients with a polymer stent experienced a heart attack, some fatal, or had to undergo another medical intervention. That is double the rate seen in patients with metal stents, writes Anne Trafton on the MIT news page. The plastic stents subsequently were taken off the market. MIT researchers in the Institute for Medical Engineering and Science and the Department of Materials Science and Engineering have now discovered why these stents failed. Armed with this knowledge, they hope that scientists will be better able to design and evaluate polymer stents as well as other degradable polymer devices moving forward.

The reason polymer stents failed—and why this problem was not discovered during the development process—stems from the evaluation procedures, “which were based on those used for metal stents [and] were not well-suited to evaluating polymer stents,” writes Trafton. The research is described in a paper published in the Proceedings of the National Academy of Sciences the week of Feb. 26.

“People have been evaluating polymer materials as if they were metals, but metals and polymers don’t behave the same way,” Elazer Edelman, the Thomas D. and Virginia W. Cabot Professor of Health Sciences and Technology at MIT, told Trafton. “People were looking at the wrong metrics, they were looking at the wrong timescales, and they didn’t have the right tools.” Edelman is the senior author of the paper.

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Using Raman spectroscopy to analyze the microstructure of the stents, the MIT researchers found that the polymer stents have a heterogeneous structure that eventually leads to structural collapse. While the outer layers of the stent have a smooth crystalline structure made of highly aligned polymers, the inner core tends to have a less ordered structure. When the stent is inflated, these regions are disrupted, potentially causing early loss of integrity in parts of the structure. When the stents become deformed, they can block blood flow, leading to clotting and potentially heart attacks, according to the article in MIT News.

When the polymer stents were undergoing pre-clinical tests, which are typically six months, they were starting to degrade at the microscopic level, but the flaws couldn’t be detected with the tools that scientists were using at the time, notes the article.

The method used by MIT researchers “provides a tool that allows you to look at a metric that very early on tells you something about what will happen much later,” Edelman said in the article. “If you know about potential issues in advance, you can have a better idea of where to look in animal models and clinical models for safety issues.”

The research was funded by Boston Scientific Corporation and the National Institutes of Health.

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