THE COOL SEAL SYSTEM - A PRACTICAL SOLUTION TO THE SHAFT SEAL PROBLEMAND HEAT RELATED COMPLICATIONS WITH IMPLANTABLE ROTARY BLOOD PUMPS

A critical issue facing the development of an implantable, rotary bloo d pump is the maintenance of an effective seal at the rotating shaft. Mechanical seals are the most versatile type of seal in wide industria l applications. However, in a rotary blood pump, typical seal life is much shorter than required for chronic support. Seal failure is relate d to adhesion and aggregation of heat denatured blood proteins that di ffuse into the lubricating film between seal faces. Among the blood pr oteins, fibrinogen plays an important role due to its strong propensit y for adhesion and low transition temperature (approximately 50 degree s C). Once exposed to temperature exceeding 50 degrees C, fibrinogen m olecules fuse together by multi-attachment between heat denatured D-do mains. This quasi-polymerized fibrin increases the frictional heat, wh ich proliferates the process into seal failure. If the temperature of the seal faces is maintained well below 50 degrees C, a mechanical sea l would not fail in blood. Based on this ''Cool-Seal'' concept, we dev eloped a miniature mechanical seal made of highly thermally conductive material (SiC), combined with a recirculating purge system. A large s upply of purge fluid is recirculated behind the seal face to augment c onvective heat transfer to maintain the seal temperature below 40 degr ees C. It also cools all heat generating pump parts (motor coil, beari ng, seal). The purge consumption has been optimized to virtually nil ( <0.5 cc/day). An ultrafiltration unit integrated in the recirculating purge system continuously purifies and sterilizes the purge fluid for more than 5 months without filter change. The seal system has now been incorporated into our intraventricular axial flow blood pump (IVAP) a nd newly designed centrifugal pump. Ongoing in vivo evaluation of thes e systems has demonstrated good seal integrity for more than 160 days. The Cool-Seal system can be applied to any type of rotary blood pump (axial, diagonal, centrifugal, etc.) and offers a practical solution t o the shaft seal problem and heat related complications, which current ly limit the use of implantable rotary blood pumps.