Keeping Hearts Pumping

Polycarbonate resins meet FDA requirements for heart-lung bypass system

The CORx compact heart-lung bypass system, manufactured by CardioVention, Inc, Santa Clara, CA, consists of an Integrated Oxygenation System (IOS), which unifies air handling, oxygenation and pumping into a single compact unit. A separate disposable tubing system (CORx A/V Loop) is used to carry blood to and from the heart.

The CORx system is made of two grades of polycarbonate — Makrolon 2558 and DP1-1452 resin, both manufactured by Bayer Plastics, Pittsburgh, PA, and chosen for their compliance with the FDA-modified ISO 10993 Part 1 Biological Evaluation of Medical Devices tests with human tissue contact time of 30 days or less. These resins can also be sterilized using radiation, ethylene oxide, or steam autoclaving, another important consideration when selecting materials for implantable devices.

The core of the CORx system is a 4- x 1.42-in. diameter cylinder is injection-molded by Delta Pacific Products Inc, Santa Clara, CA, in a clean room environment using the Makrolon 2558 resin, which provides good impact strength, dimensional stability and a glass-like transparency. This portion of the system was challenging to mold due to its geometry, which included zero draft, and some thick and thin sections — from 0.125- to 0.028-in. — that create an adhesive groove on the cylinder.

“Because of these design challenges, material melt flow was an important criteria for this part,” says Delta Pacific Products president Fred Betke.

Following molding and degating, the component is double-bagged and shipped to CardioVention for assembly.

Six other CORx system components — VFT cover, VFT top, upper gas manifold, shell, pump housing and pump base — are injection-molded from Makrolon DP1-1452 polycarbonate resin by Pacific Plastics and Engineering, Soquel, CA. The DP1-1452 resin exhibits improved flow and easy release characteristics. It features an internal mold release technology that can, in many cases, eliminate the use of mold release sprays. The resin’s properties also include a balance of high strength, toughness, clarity and processibility. The resin offers increased design flexibility for complicated medical products with difficult-to-fill molds and parts with shallow drafts.

Two of the six components — the VFT top and shell — proved to be particularly challenging from a tooling and molding standpoint. For example, the top, which measures 3-in. tall with a 3-in. diameter, featured complex geometries, including undercuts.

The shell or outer casing of the CORx system measures 4-in. tall with a 3-in. diameter. The part’s complex geometry and minimal draft contributed to the tooling challenge as did the requirement for a full diamond finish. To ensure there are no drag marks on the part, cores and slides were treated with a special PVD chemical.

“It was important to use a coating that wouldn’t inhibit the diamond finish and would, at the same time, have good wear resistance for the components inside the tool,” explains Ray Scherer, engineering manager.

Compared to the blood circuitry of today’s much larger heart-lung bypass machines, the small CORx system is able to function with one-tenth the amount of surface area exposed to blood and with minimal to no priming volume required for activating the system.

The effect of priming, known as hemodilution, thins a patient’s blood, reducing its oxygen carrying capacity and potentially compromising other body functions. By reducing hemodilution and the surface area exposed to blood, CardioVention believes its system will play a critical role in minimizing patient platelet loss, blood damage, blood transfusions and systemic inflamation.

—MP