A California company believes it has found a way to alleviate a vexing problem with existing medical stents: the formation of blood clots caused by drug coatings. Conor Medsystems of Menlo Park is currently testing a new design that incorporates tiny holes for drugs and the dissolvable polymer mixtures that hold them in place.

Conor Medsystem's arterial stent, which uses tiny holes to hold and release drugs, is undergoing human trials in the United States and Europe. (Courtesy of Conor Medsystem)

Each year, bare-metal stents are implanted in the coronary arteries of 800,000 Americans, and hundreds of thousands of people worldwide. Stents act as physical supports that help keep an artery open. Most stents used in the United States and Europe are fully coated with a polymer that's pre-impregnated with drugs, often an anti-scarring or anti-inflammatory medication. But the mix of drugs and polymers tends to diffuse into the bloodstream at an unpredictable rate. And arterial areas are sensitive, so that medications need to be released in a controlled fashion.

Researchers at Conor believe they can control drug release by putting the drug and polymer compound into tiny gaps in a stent, instead of coating the whole device with the mix. Along the branches of Conor's metal stents are hundreds of small holes (100 microns long, 125 microns wide, and 100 microns deep). Each tiny gap acts as a reservoir for a drug-and-polymer mix. Furthermore, the mix can be altered from one reservoir to the next, allowing a highly controlled release of different medications, says the company's founder and chief technology officer, John Shanley.

"Instead of having just one polymer coating holding all the drugs being delivered," he says, "you can have different ones that are specifically compatible with the drug you are seeking to have dissolved into the patient. It opens up your ‘tool box' to many other polymers, most with safer properties."

Conor's stents are made from both stainless steel and cobalt chrome alloy, materials that give them the flexibility to be coiled like springs before insertion, usually through an artery in the patient's thigh. And the wells are etched into the stents using a proprietary technology. Finally, robotic devices guide tiny syringes holding various drug and polymer cocktails, which fill the wells with pinpoint accuracy.

Drug-coated stents were first approved in the United States in the late 1990s and now dominate the global market. They became so popular because their ability to deliver drugs directly to a site made them less likely to induce an unpleasant side effect of earlier stents: they'd become clogged by smooth muscle cells and cellular debris. Indeed, in nearly 30 percent of bare-metal coronary stentings conducted as recently as 2000, patients required a repeat balloon angioplasty procedure to clear the blocked area of cellular deposits.