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Flow rate estimation for ventricular assist devices without additional flow sensors can improve the quality of life of patients. In this article, a novel flow estimation method using the passively stabilized displacement of a magnetically levitated impeller is developed to achieve sufficient accuracy and precision of flow estimation for ventricular assist devices in a simple manner. The magnetically levitated impeller used is axially suspended by a magnetic bearing in a centrifugal blood pump that has been developed by our group. The radial displacement of the impeller, which is restricted by passive stability, can be correlated with the flow rate because the radial hydraulic force on the impeller varies according to the flow rate. To obtain the correlation with various blood viscosities, the relationships between the radial displacements of the magnetically levitated impeller and the pressure head-flow rate characteristics of the pump were determined simultaneously using aqueous solutions of glycerol with a potential blood viscosity range. The measurement results showed that accurate steady flow rates could be estimated with a coefficient of determination of approximately 0.97 and mean absolute error of approximately 0.22 L/min without fluid viscosity measurements by using the relationships between the impeller displacement and the flow rate. Moreover, a precision of approximately 0.01 (L/min)/µm was obtained owing to a strong estimation indicator signal provided by the large displacement of the passively stabilized impeller; thus, the proposed estimation method can help ensure sufficient accuracy and precision for ventricular assist devices in a simple manner, even if the blood viscosity is unknown.

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 uid 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 ow 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 ow 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 eld 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.

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