Stent manufacturers now have a better handle on accurately applying costly anti-rejection coatings to the underlying mesh. They use ultrasonic-assisted coating. Ultrasonic energy shatters the liquid into uniform, tiny droplets, creating a soft, low-velocity spray that makes for a thin, straight, spray pattern. Depending on the nozzle, the pattern can be as small as 0.010-in. wide and repeatable within 3%. Ultrasonic coatings are done in a nitrogen atmosphere to improve flow and reduce surface tension as droplets hit the stent.

Older spray methods are more difficult to control. Such methods, along with dip coatings, vary by up to 5 to 7% and are just 7 to 15% efficient.

Stent manufacturers say transfer efficiencies can now reach 30 to 70%. The finished coating is smooth, has uniform thickness, and is free of webbing in the mesh.

“The gains are important because the active agent's elution rate depends directly on coating thickness,” says Chris Coccio, president of Sono-Tek Milton, N.Y. (sonotek.com). His company supplies ultrasonic coating systems to medical-device manufacturers. “Higher efficiency means less waste of a costly ingredient.”

For typical coatings, a program rotates and axially moves a stent on a mandrel under the ultrasonic-spray nozzle. Injecting liquid into a fine stream of inert gas coming out of the ultrasonic nozzle creates the fine spray. The focused gas stream puts the spray on the stent. Droplet size can be regulated to a few microns that vary in size by ±3%. Gas flow containing the droplets is slower than air atomizers, which means for less overspray and closer control over coating placement and thickness.

Flow rates are set for 20 and 100 µl/min, and line widths range from 0.020 to 0.080 in. It often takes several passes to build up a specified thickness which is usually measured by weight gain.

The ultrasonic method also delivers solvents such as tetrahydrofuran, chloroform and ethanol, THF, acetone, DMAC, and toluene. The polymers delivered include urethanes, polyectides, polycarbonates, silicones and styrenes. Active-agent concentrations range from 0.5 to 3.0%. A production-scale version of the Medi-Coat system in the works will coat up to eight stents at a time.