Design of Mechanical Bearings in Cardiac Assist Devices
The goal of this article is to provide an overview of basic principles that are important for design and evaluation of bearings used in blood pumps. While the primary focus is on mechanical bearings used in the second and third generation ventricular assist devices (VADs), bearings used in the first generation pulsatile devices as well as those used in percutaneous and extracorporeal applications are also described. The new generation of continuous flow VADs uses rotating impellers to accelerate the blood and increase the fluid pressure while delivering the required flow rate. These rotary pumps are classified as either axial or centrifugal flow devices. In the axial flow pumps, the fluid, in this case blood, enters at one end of the cylindrical pump housing, follows the axial direction of the housing, and exits at the other end. In contrast with this flow geometry, the impellers used in centrifugal pumps push the fluid out of the housing in the radial direction. In either case, the rotating impeller requires load support in the axial and the radial directions. This support mechanism is provided either by magnetic bearings or by blood-lubricated mechanical bearings.