Fluid Force Predictions and Experimental Measurements for a Magnetically Levitated Pediatric Ventricular Assist Device

Abstract

The latest generation of artificial blood pumps incorporates the use of magnetic bearings to levitate the rotating component of the pump, the impeller. A magnetic suspension prevents the rotating impeller from contacting the internal surfaces of the pump and reduces regions of stagnant and high shear flow that surround fluid or mechanical bearings. Applying this third-generation technology, the Virginia Artificial Heart Institute has developed a ventricular assist device (VAD) to support infants and children. In consideration of the suspension design, the axial and radial fluid forces exerted on the rotor of the pediatric VAD were estimated using computational fluid dynamics (CFD) such that fluid perturbations would be counterbalanced. In addition, a prototype was built for experimental measurements of the axial fluid forces and estimations of the radial fluid forces during operation using a blood analog mixture. The axial fluid forces for a centered impeller position were found to range from 0.5 +/- 0.01 to 1 +/- 0.02 N in magnitude for 0.5 +/- 0.095 to 3.5 +/- 0.164 Lpm over rotational speeds of 6110 +/- 0.39 to 8030 +/- 0.57% rpm. The CFD predictions for the axial forces deviated from the experimental data by approximately 8.5% with a maximum difference of 18% at higher flow rates. Similarly for the off-centered impeller conditions, the maximum radial fluid force along the y-axis was found to be -0.57 +/- 0.17 N. The maximum cross-coupling force in the x direction was found to be larger with a maximum value of 0.74 +/- 0.22 N. This resulted in a 25-35% overestimate of the radial fluid force as compared to the CFD predictions; this overestimation will lead to a far more robust magnetic suspension design. The axial and radial forces estimated from the computational results are well within a range over which a compact magnetic suspension can compensate for flow perturbations. This study also serves as an effective and novel design methodology for blood pump developers employing magnetic suspensions. Following a final design evaluation, a magnetically suspended pediatric VAD will be constructed for extensive hydraulic and animal testing as well as additional validation of this design methodology.

The use of computational fluid dynamics in the development of ventricular assist devices

“…The implementations of these CFD packages are either based on the finite volume23 method or finite element method. Fluent 36, 37, 38, 39, 40, 41 and CFX 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 11, 57, 58, 59, both available from ANSYS, Inc. (Canonsburg, PA) and STAR-CD from CD-Adapco 60, 61 (Melville, New York) are the CFD software packages used most often for analysis and modelling of blood flow in VADs. In addition, AcuSolve from ACUSIM Software 62 (Mountain View, CA) and Adina from Adina R&D (Watertown, MA) are also used.…”

“…The standard k-ε model has been used for computing the flow in several pumps including Xi’an Jiaotong University’s pump 40, Nanyang Technological University’s pump 41, the Virginia LEV-VAD 51, the Virginia PVAD (versions 1–4) 53, 11, 56, 55 and the Impella (2001) 59. Of these several investigations compared pressure rises with measurements: the largest error in the Virginia LEV-VAD was around 10 mmHg (10 % over) 51, in the PVAD2 the largest error was around 15 mmHg (13 % under) 11 and in the Impella (2001) the error was up to 24 mmHg (30 % under).…”

“…Hot film anemometry is an invasive technique which can also be used for studying in vitro flow and gives a high temporal resolution 189, 37, 103. Predictions of fluid forces can be validated by comparison with measurements of forces on the impeller 51, 52, 54, 11, 56. Many groups compare their CFD predictions of the pressure head produced by the pump with the measured values to give a bulk flow parameter for validation.…”

Artificial Organs 2007: A Year in Review

“…Amy L. Throckmorton et al. (24) of the University of Virginia (Charlottesville, VA, USA) reported on their magnetically levitated pediatric VAD. The axial and radial fluid forces exerted on the rotor of the pediatric VAD were estimated using computational fluid dynamics (CFD) such that fluid perturbations would be counterbalanced.…”

Echo: Practical Formal Verification by Reverse Synthesis

“…The University of Virginia's LifeFlow Left Ventricular Assist Device (LVAD) [72,73,75] is a prototype artificial heart pump designed for the long-term treatment of Congestive Heart Failure (CHF). LifeFlow has a continuous, axial-flow design that uses magnetic rather than mechanical bearings to avoid damaging blood cells, thus reducing both the potential for the formation of a fatal blood clot and the need to take blood thinning medications.…”