2018 Volume 7 Pages 63-71
Evaluation of the impeller radial stability is important from the bioengineering point of view in the development of mechanical circulatory support devices (MCSDs) for safer use as bridge for several months or destination therapy for years. In this study, radial stability of a magnetically levitated impeller in a centrifugal blood pump with an axially magnetic suspension system was evaluated by investigating the effects of the eccentric impeller position on passive stability, aiming to propose a pump design guide for the development and safer clinical use of MCSDs. First, impeller displacements in the prototype pump were measured using a mock loop together with laser displacement sensors. Then, the radial hydraulic forces exerted on an eccentric impeller were calculated using computational fluid dynamics (CFD) analysis for four volute-casing geometries. In addition, hemocompatibility was assessed using CFD calculations of scalar shear stress exerted on blood. Measurement of impeller displacement showed that the displacement varied from 0.56 to 0.27 mm at a rotational speed of 1800 rpm as the flow rate increased from 0 to 6.5 L/min. In the CFD calculation, the radial hydraulic force increased linearly from 0.2 to 1.7 N as the impeller displacement increased from 0 to 0.5 mm for all the double volute geometries, under conditions of a rotational speed of 1800 rpm and flow rates of 3, 5 and 7 L/min. These results indicate that the impeller stability in the prototype pump is acceptable at the operation conditions of ventricular assist devices, because the magnetic bearing stiffness of radial component was 4.1 N/mm. In the pressure recovery analysis of eccentric impellers, a double volute was not effective because of the unbalanced pressure field generated by the unbalanced pressure recovery. Thus, the increase in radial hydraulic force associated with an eccentric impeller could not be avoided by changing the conventional double volute design. The CFD analysis of geometrical variation indicated that widening of the radial clearance is an effective approach to improve the radial stability as well as hemocompatibility, although the radial clearance should be designed based on trade-offs among impeller stability, hemocompatibility and pump performance.