Perfecting the Stent
by Will SansomWhen Health Science Center radiology researcher Julio C. Palmaz, M.D., patented the world’s first stent in 1988, he envisioned a wire-mesh, spring-loaded tube to be threaded with a balloon catheter through clogged blood vessels. At the site of built-up plaque, the balloon would inflate and the compacted stent would spring open, restoring normal blood flow. After balloon deflation and removal, the stent would remain in the vessel as a permanent scaffold to keep the vessel open.
The stent quickly proved to be a spectacular success, eliminating the need for open-heart surgery and other vascular bypasses in thousands of very ill patients. It gained U.S. Food and Drug Administration approval for peripheral vascular procedures in 1990 and for coronary vascular procedures in 1994. Today, 2 million people a year have balloon angioplasty with stents to open diseased vessels in the heart and legs. The invention has returned more licensing revenue $2 million a year -- to the Health Science Center than any other in the university’s history.
Health Science Center researchers are writing new chapters of the stent success story. One key finding indicates that future generations of stents will be, to borrow a ’60s phrase, groovier. Research reveals that endothelial cells -- the cells that line our blood vessels - more readily migrate to stents that have tiny grooves etched into them. This migration is crucial for good clinical outcome. Other research shows that endothelial cells tend to migrate in only one direction (with blood flow) at the site of the stent implant. This helps researchers understand the behavior of the blood vessels and how to improve current stents. "We are trying to change the surface of stents to make them more compatible with the surfaces of blood vessels," said Dr. Palmaz, professor of radiology. "We hope to eliminate restenosis, the closing of vessels after balloon angioplasty, which occurs in one-fifth of patients despite improving stent designs."
Scientists use a technique called photo etching to precisely engrave grooves into stents. Endothelial cells, which are the cells that line the blood vessels, migrate more rapidly to the grooved stents than to their smooth counterparts. The faster the cells cover the stent, the less likely a patient will experience blood clots and excess tissue scarring. The grooved stent could eventually prevent the closing of vessels in thousands of patients.
Restenosis occurs when cells, responding to the implant, proliferate as part of the body’s natural inflammatory response. In ongoing research, endothelial cells are placed on a layer of thick gel similar to the arterial wall, said Eugene A. Sprague, Ph.D., who holds the Julio C. Palmaz, M.D., Professorship in Radiology at the Health Science Center. Stent test metals are implanted on the gel sheet and the sheet is placed inside a flow chamber that simulates blood circulation, including its pulses. "For seven to 10 days we observe how rapidly the endothelial cells cover each metal alloy -- how quickly they migrate from the edges of the material. This is a key indication of how good a metal is likely to be as a stent in a real blood vessel. We want a stent to cover with endothelial cells quickly, because the stent or graft will stay open longer."
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