Salem, NH -These days, it's not enough for angioplasty balloons to inflate. Increasingly, it's how they inflate that counts.

While conventional angioplasty balloons open clogged arteries by expanding to a fixed diameter concentric to their catheter shaft, new "offset balloons" from Advanced Polymers Inc. (API) dilate to a single side, carrying the shaft off the balloon's centerline and towards its wall. "Normally, these eccentric dilations are an accident," notes Mark Saab, API's president. If uncontrolled, an offset dilation could, in the worst-case scenario, push the shaft or its tip into an artery wall, Saab explains. Indeed, API and other balloon makers spend a great deal of their testing dollars validating that their conventional angioplasty balloons inflate evenly at a given pressure rating.

The new offset balloons, by contrast, offer a controlled sideways dilation that displaces the shaft from its usual centerline by a predetermined distance. Saab states the offset specifications as a percentage-with 0% representing a centered shaft and 100% indicating a shaft that lies entirely against one of the balloon walls. Whatever the amount of offset, the opposite side of the offset remains a straight wall.

API developed its offset balloons with secondary angioplasty procedures in mind. "Angioplasty's Achilles Heel has traditionally been restenosis, or a reoccurrence of the blockage after the first procedure," he says, explaining that some repeat treatments use lasers, radiation, or microwave energy to ward off restenosis. According to Saab, these emerging "energy-based" therapies, some of which are in clinical trials, are more likely to require the selective deployment provided by the offset balloons. As an example, he cites the delivery of a radioactive "seed," which the angled shaft in the offset balloon can place against one side of the artery wall for an asymmetrical dosage. Or the balloon could serve as a transmission path for a laser, he adds.

Beyond coronary applications, the offset balloons have seen other medical uses. In fact, API's first offset balloon was developed to bring radiation doses to prostate tumors, and some device makers are investigating the offset balloon for carpal tunnel syndrome treatments, Saab reports.

Pumping up balloon performance. Angioplasty balloons have long required a delicate balance between wall thickness and tensile strength. Thin walls make it possible to deliver the balloon into the body, while a high-strength material keeps the balloons from bursting. "Our claim to fame is the high tensile strength we get from very thin walls," says Saab. It's not an idle boast: some of API's balloons often need to withstand pressures up to 350 psi at wall thicknesses as low as 0.00025 inch.

The offset balloons, because they will likely work with the energy-based therapies, take on additional physical requirements that make finding the right balloon material difficult. For one thing, they need better thermal properties than conventional angioplasty balloons. "This hasn't been an issue for conventional angioplasty, but lasers and microwave can generate substantial heat," Saab says. What's more, the balloons must be optically clear and transmit laser-light frequencies, particularly those of the Nd:YAG laser often used in medical applications. The material has to be sterilizable by EtO, electron beam, and other less common methods. And it has to be resistant to solvent-based adhesives and other chemicals found in the manufacturing process.

To get the right balance of properties, Saab turned to a PET film that the company radially orients in a proprietary secondary process. This material exhibits a tensile strength of 30,000 to 60,000 psi at the film thicknesses API uses. "I challenge anyone to find an unreinforced plastic that can match it," says Saab, a plastics engineer. PET also offers temperature performance up to 100C at the wall thicknesses and pressures it sees in a balloon applications, and "it transmits most lasers beautifully," he continues.

API has produced offset balloons for a few years now, but only to meet the experimental needs of a few medical device manufacturers. According to Saab, some of these early efforts remain in clinical trails. Only this year, though, did API go public with the new offset designs. "There will be many applications for this kind of balloon," he predicts.

Other applications

Though designed for a specialized medical task, angioplasty balloons may have a few uses outside the body too. Saab reports that API has supplied balloons for "occluding applications," where a tube must be blocked off for leak or pressure testing. The company has also had its balloons serve as "hydraulic expanders" in a variety of automation applications. Here, the balloon is sandwiched between two metal plates and used to actuate "miniature manipulation devices," that could see use inside nuclear power plants or toxic environments where pull wires would be impractical.

Engineering challenges

Long before API engineers could develop the manufacturing process for a controllable offset angioplasty balloon, they had to find a material with just the right balance of properties. There weren't many choices. According to API president Mark Saab, radially oriented PET turns out to be just about the only unreinforced plastic that met all the following criteria when processed as a film:

Strong and thin. The balloon material had to have enough tensile strength to withstand pressures up to 300 psi with balloon walls as thin as 0.00025 inch thick.

Transmits energy. To be compatible with emerging restenosis therapies and other procedures, the balloon material had to be capable of transmitting energy from a variety of energy sources-including laser, ultrasound, microwave, and radiation.

Works in the body. The material also had to have an established bio-compatibility track record and withstand common sterilization methods.