Improvements Coming in Second Generation Stents

By Hank Russell

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NEW YORK -- Nanotechnology and biotechnology will make significant contributions to the next generation of stents, according to in Dr. Elliot L. Chaikof at the Veith symposium on vascular medicine sponsored by the Cleveland Clinic.

"A number of limitations have been recognized for drug-eluting stents including late-stent thrombosis and rebound stenosis--which have directed the development of second-generation stents to address these problems," said Dr. Chaikof, of the division of vascular surgery at Emory University in Atlanta.

Some design changes he referred to include stents that provide a higher local dose of drug for patients, such as diabetics at increased risk of recurrent lesions, and stents capable of spatially controlled drug delivery with release of antithrombotics into the lumen and antiproliferative and anti-inflammatory agents to the vessel wall.

"Most of these efforts have been directed toward optimizing vessel wall healing after angioplasty, although there is also interesting new stent-based technology in which sensors are incorporated into the stent that provide a noninvasive 'early warning system' for detecting a recurrent lesion prior to a clinically adverse event," he said.

According to Dr. Chaikof, several methods are currently being used to fabricate stents: laser machining and microelectrodischarge machining. Laser machining, the older of these two techniques, uses one of two methods: direct write--in which a laser is pulsed over the metal form, and a mass projection--in which a pulsed laser processes larger regions using a patterned form that controls the region affected by the laser. Laser machining has recently been used to create wells or reservoirs as thin as 50-100 micrometers within the metal struts of the stent.

"These channels create spatially discrete reservoirs that can be filled with drug. Examples include the Janus or Conor CarboStent," he said.

Microelectrodischarge machining uses electrical pulses between the electrode and the work piece to strip away a material. "Any conductive substrate can be machined with features as small as 25 [micrometres]," Dr. Chaikof said. "With electrodes that are appropriately patterned, one can create a variety of different designs of stents."

He cited an example--the "stentenna," an integrated stent and sensor system that was developed by researchers at the University of Michigan, Ann Arbor. A company called MicroEDM has also produced a stent with sensors.

"The stent itself has inductive properties, and it can be used as an antenna for wireless communication," he said. "If there is a change in pressure or flow, one can detect that noninvasively and intervene, if need be ... "

He also cited a number of new polymers that provide enhanced flexibility to control vascular wall healing. "Biodegradable polymers for stent designs have evolved to include amino acid-based biodegradable polymer systems," he explained, citing the REVA stent as an example. The REVA stent is produced using a biodegradable form of tyrosine-based polycarbonate. "It's a relatively rigid, strong material," Dr. Chaikof said, "but in concert with micromachining techniques, it can be fabricated into a balloon-expandable stent that has a self-locking system through latches that are incorporated into the design."

Kathryn Uhrich, Ph.D., at Rutgers University in Piscataway, N.J., has developed a polymer that has been referred to as "polyaspirin." It is a polyanhydride that, upon degradation, releases salicylic acid, which is the active form of aspirin. "It's been fashioned into biodegradable stents in which promising results have been observed in preclinical studies," he added.

"In adversity, there are opportunities, and clinicians, engineers, and chemists are collaborating... to solve some of difficult real-world problems that have arisen with the introduction of first-generation systems," said Dr. Chaikof.

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