In selecting a material to form the housing for its innovative medical device for heart attack treatment, TherOx Inc. looked for a material that was clear, easily moldable and met FDA regulations. The material also needed to be from a dependable supplier that could deliver a consistent product on a long-term basis. The company filled this prescription with Makrolon polycarbonate resin from Bayer Polymers LLC.

The TherOx1 AO system hyperoxygenates a patient's blood with aqueous oxygen (AO) and then delivers the hyperoxygenated blood to oxygen-deprived areas of the heart following heart attack. The innovative system received silver status recognition at the 2002 Medical Design Excellence Awards competition.

The TherOx System produces sterile AO solution on demand from hospital-supplied, medical-grade oxygen and sterile, physician-prescribed saline solution. It then mixes the AO solution with the patient's blood from an arterial puncture, and delivers the resulting hyperoxemic blood back to the patient's coronary artery via a sub-selective AO delivery catheter. According to TherOx, this type of treatment has never before been achieved outside a hyperbaric chamber.

The System utilizes two disposable components for each 90-minute AO therapeutic treatment: the TherOx AO cartridge and TherOx AO catheter. The three-chambered TherOx AO cartridge consists of 13 components molded of Bayer's Makrolon 2558 polycarbonate resin. The diameter of the center chamber is 1-1/2 inches; the diameter of each side chamber is 1-1/4-inches. The complete TherOx AO cartridge is approximately 4-1/2 inches tall and weighs 156 grams.

In addition to providing outstanding impact strength, excellent dimensional stability and glass-like transparency, Makrolon 2558 polycarbonate resin meets the requirements of FDA-modified ISO 10993, Part 1 "Biological Evaluation of Medical Devices" tests with human tissue contact time of 30 days or less. This is key since only products that meet these requirements may be considered for applications - like the TherOx System - requiring biocompatibility.

According to molder APEC Plastics, successfully molding the cartridge was challenging. The first challenge was molding large parts with heavy wall thicknesses (the center cylinder and manifold has a .140-inch wall thickness) yet maintaining tight tolerances. The heavy wall thicknesses were required to withstand the high working pressures required by this application - in fact, these components were tested to pressures above 1,000 psi.

The second was the extreme tooling challenge of molding in a .0004-inch hole - just slightly larger than a human hair - in the nozzle body. APEC's sister company, Magor Mold, was responsible for designing and manufacturing the molds and building the tooling.

In addition to supplying the resin, Bayer provided valuable technical expertise, according to APEC and TherOx. For example, Bayer technicians conducted mold flow analysis prior to tool construction to determine gate location, sizing and venting. "Their models were extremely accurate," said Mark Bonifacio, vice president of APEC Plastics.

Greg Watson, vice president for TherOx, has utilized Bayer engineering resins for projects over the last 10 years. He explained that Bayer brought a number of things to the table for this application. "We were confident in their material, they offered true design assistance, and we knew they'd be there in the future, ensuring a dependable resin supply," he said.