Development of an axial flow blood pump: Characteristics of a magnetic fluid seal

In the development of a rotary blood pump, one of the major problems is thrombus formation near the shaft seal. To solve this problem, a magnetic fluid seal was developed for use in an axial flow pump. A magnetic fluid seal has several advantages, including a high degree of airtightness, no wear or noise, no heat generation, and a simple structure. To determine the reliability of the seal, the sealing pressure, durability, and amount of magnetic fluid mixed with liquids were examined. The seal was composed of two ferromagnetic polepieces and a Nd-Fe-B magnet. The magnetic fluid (LS-40) was injected into the gap between the motor shaft and the polepieces. To minimize leakage of the magnetic fluid, a shield was provided on the polepiece on the liquid side. Sealing pressures at motor speeds up to 10000rpm were measured with the seal immersed in water or bovine blood. The sealing pressures were 195mmHg in water and 173mmHg in blood. The results of magnetic FEM showed that the theoretical sealing pressure was about 230mmHg for motor speeds up to 10000rpm. This result was almost the same as the measured pressure in water. The best result of tests on long-term performance was 691 + days (ongoing) at 8000rpm and 51 days, and 47 + days (ongoing) in a rotating condition. There was only a small amount of magnetic fluid mixed with the liquid. The especially designed magnetic fluid shaft seal is considered to be useful for an axial flow pump.