To help with narrowed or obstructed arteries, surgeons typically perform balloon angioplasty, a procedure that first widens the blood vessels with a balloon and then keeps them propped open with a wire mesh called a stent. The metal stents permanently stay in the body, potentially causing issues such as chronic inflammation and local clotting. Scientists have now discovered that stents made from bioabsorbable zinc could be exactly what clinicians and patients need—they could give arteries enough time to heal but not linger in the body long enough to cause additional problems.
“We found that the degradation rate of zinc is perfect,” said Jaroslaw Drelich, a materials researcher at the Michigan Technological University and co-author of the new study, published in the May 14 issue of Advanced Materials (DOI: 10.1002/adma.201300226; p. 2577).
In the last decade, scientists have looked into the efficacy of bioabsorbable stents, focusing on iron- and magnesium-based stents. Iron stents are not ideal because the material produces a large volume of potentially hazardous iron oxide, which does not degrade easily in the human body. And magnesium is innocuous but dissolves in the body much too quickly. The research team decided to start with a different metal as a base material. They chose zinc because previous research has shown that zinc can help slow down the biodegradation process when added to other materials.
To test the biodegradation properties of metallic zinc, graduate student Patrick Bowen crafted 15-mm-long zinc wires. The wires, which were less than half a millimeter in diameter, did not comprise a full stent—they represented a supportive portion of the stent called a strut, Drelich said. Collaborator Jeremy Goldman, a biomedical engineer, placed these tiny wires into the abdominal aorta of rats, and then removed and examined the wires’ corrosion after 1.5, 3, 4.5, and 6 months.
For the first three months, the researchers found that the zinc wires degraded in the rats at a rate that they calculated as just below 20 µm a year, the ideal corrosion rate. At four months, the pure zinc still retained about 70% of its cross-sectional area, and then its degradation rate accelerated rapidly, ensuring that it would not stay in the body for too long. Moreover, the team saw that more healthy arterial tissue stuck to the wires the longer the wires remained in the body, suggesting that the zinc was not damaging the arterial walls. And because zinc is known to fight arterial plaque, a zinc-based stent could further help patients suffering from ischemic problems.
Carlo Di Mario, a clinical cardiologist at Imperial College London who was not involved in the research, said that the work is interesting and it appears that zinc-based stents would last longer in the body than magnesium-based stents. However, he said that the low tensile strength of zinc is an issue: The wires work perfectly in rats, but the material is not strong enough to hold open human arteries. Using zinc alloys is necessary, but “the avenue to alloys seems a bit too long for a clinician to be interested at this stage,” Di Mario said.
However, Drelich said they are already testing promising new zinc alloys to fix the strength issue—the invented alloys meet all of the benchmarks necessary for a bioabsorbable stent material candidate. The team is now continuing in vivo testing of new alloys and expects to complete prototype mini-stents this year. Within a few years, they should be ready to move on to clinical trials, he said.
