Diamagnetic Effects of Blood in a Magnetically Levitated Blood Pump

Magnetic bearings are becoming increasingly popular in ventricular assist devices. In most cases, blood fills a portion of the gap between the magnetic actuators and rotor. Understanding the effects of the operating fluid on magnetic suspension is necessary, particularly when the device geometry features a relatively large gap (typically defined as greater than $150$ um and as large as 2 mm in some pumps). A large gap reduces cell damage and allows unrestricted flow, it but increases the distance and amount of material through which magnetic fields must pass. These net effects of the operating fluid on the magnetic suspension can be characterized in terms of a contribution to the overall damping and stiffness of the system. This contribution may be caused by traditional tribological properties (density and viscosity) as well as the magnetic properties of the medium. This research isolates the effects of fluid diamagnetism on a magnetically levitated blood pump. Experimental transient and frequency responses of the system operating at 3000 to 6000 rpm are presented while pumping different liquid media: Blood, water, and fluids with similar densities and viscosities to each, but different magnetic susceptibilities. In order to calculate the stiffness and damping coefficients, a mathematical model of the system is iteratively updated to match experimental transient response. The resulting coefficients are validated in the frequency domain by means of simulations, which agree with experimental frequency response data. The relationship between diamagnetism, damping and stiffness coefficients of the test fluids with known magnetic susceptibility is also used to obtain a quantitative estimate of the magnetic susceptibility of blood.