Study unveiled why polymer stents failed

Microscopic flaws in material structure can lead to stent deformation after implantation.

Researchers hope that their work will lead to a new approach to designing and evaluating polymer stents and other types of degradable medical devices
Researchers hope that their work will lead to a new approach to designing and evaluating polymer stents and other types of degradable medical devices. Image: Pei-Jiang Wang

Many patients with heart disease who went for surgery have a metal stent implanted to keep their coronary artery open. The metal stent prevents blood clotting that can lead to heart attacks. But the drawback is using them long term leads to artery damage.

Several years ago, a new type of stent made from biodegradable polymers was introduced. The stent was expected to eventually be absorbed by the blood vessel walls, removing the risk of long-term implantation. Initially, the stents were seemed as successful but later these patients experienced more heart attacks than patients with metal stents, and the polymer stents were taken off the market.

Now MIT scientists have discovered why these stents failed. Their study uncovers why the issues were not revealed amid the improvement procedure: The evolution methodology, which depended on those utilized for metal stents, were not appropriate for assessing polymer stents.

Elazer Edelman of Health Sciences and Technology at MIT said, “People have been evaluating polymer materials as if they were metals, but metals and polymers don’t behave the same way. People were looking at the wrong metrics, they were looking at the wrong timescales, and they didn’t have the right tools.”

This work is now expected to lead a new approach to design and evaluating polymer stents and other types of degradable medical devices.

Lead author Pei-Jiang Wang, a Boston University graduate student said, “When we use polymers to make these devices, we need to start thinking about how the fabrication techniques will affect the microstructure, and how the microstructure will affect the device performance.”

The degradable stents are made from a polymer called poly-l-lactic acid (pLLA), which is also used in dissolvable sutures. Preclinical testing (studies were done in the lab and with animal models) did not reveal any cause for concern. In human patients, the stents appeared stable for the first year, but then problems began to arise. After three years, over 10 percent of patients had experienced a heart attack, including fatal heart attacks, or had to go through another medical intervention. That is double the rate seen in patients with metal stents.

Detecting it on a microscopic level, scientists found that polymer stents have a heterogeneous structure that eventually leads to structural collapse. The stent’s outermost layer composed of a smooth crystalline structure made of highly aligned polymers. On the other hand, its inner layer was made of a less ordered structure. When the stent is inflated, these regions are disrupted, potentially causing early loss of integrity in parts of the structure.

Once the stents get deformed, they can block blood flow, leading to clotting and potentially heart attacks. Thus researchers are expecting designers to take an idea from this and fabricate a better approach.

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