Page 562 - Handbook of Biomechatronics

Artificial Hearts 555

Fig. 28 Generic axial-flow VAP pump showing the positions of the mechanical pivot
bearings.

Jarvik-2000 which originally used ceramic pin bearings but later changed to
cone bearings, with the latter found to be superior on most counts (Stanfield
and Selzman, 2013a, b). The HeartMate II uses cup-socket ruby bearings.
Centrifugal pumps of this generation were generally supported on a single
ceramic pivot.
From a practical perspective, mechanical bearings have tiny precise com-
ponents which can position the rotor assembly in all six directions and are
stable at any speed and operating condition. However high load concentra-
tions during normal use, and potential shock impacts can result in a limited
life. Additionally, though these bearings are designed to minimize damage to
red blood cells (hemolysis) in the contact area, it does occur with increasing
contact area tending to increase the hemolysis index.

4.2 Hydrodynamic Bearings
Most third-generation VADs used centrifugal pumps with noncontact bear-
ings (either hydrodynamic or electromagnetic). Hydrodynamic bearings use
the pressure distribution of the fluid squeezed into a wedge shaped or
stepped channel to support the rotor. Asymmetrical forces as illustrated in
Fig. 29 will force blood convergence and as a result will increase the hydro-
dynamic pressure on one side of the bearing to counteract any asymmetries
in the rotor position. Analysis of these bearings relies of the Reynolds lubri-
cation equation and is beyond the scope of this chapter.
From a practical perspective these are simple and reliable, and once the
motor has started spinning, it slides on a thin film of blood. However, the
load-bearing film of blood suffers from high shear stress and potentially more
hemolysis can occur than with other bearing types. In addition, as the motor