Gregory Burgreen

A Computational and Experimental Comparison of Two Outlet Stators for the Nimbus LVAD

Two designs of an outlet stalor for the Nimbus axial flow left ventricular assist device (LVAD) are analyzed at nominal operating conditions. The original stator assembly (Design 1) has significant flow separation and reversal. A second stator assembly (Design 2) replaces the original tubular outer housing with a converging-diverging throat section with the intention of locally improving the fluid dynamics. Both stator designs are analyzed using computational fluid dynamics (CFD) analysis and experimental particle imaging flow visualization (PIFV). The computational and experimental methods indicate: 1) persistent regions of flow separation in Design 1 and improved fluid dynamics in Design 2; 2) blade-to-blade velocity fields that are well organized at the blade tip yet chaotic at the blade hub for both designs; and 3) a moderate decrease in pressure recovery for Design 2 as compared with Design 1. The CFD analysis provides the necessary insight to identify a subtle, localized flow acceleration responsible for the decreased hydraulic efficiency of Design 2. In addition, the curiously low thrombogenicity of Design 1 is explained by the existence of a three-dimensional unsteady vortical flow structure that enhances boundary advection. ASAIO Journal 1999; 45:328–333.

Progress on development of the Nimbus-University of Pittsburgh axial flow left ventricular assist system

Nimbus Inc. (Rancho Cordova, CA) and the University of Pittsburgh have completed the second year of development of a totally implanted axial flow blood pump under the National Institutes of Health Innovative Ventricular Assist System Program. The focus this year has been on completing pump hydraulic development and addressing the development of the other key system components. Having demonstrated satisfactory pump hydraulic and biocompatibility performance, pump development has focused on design features that improve pump manufacturability. A controller featuring full redundancy has been designed and is in the breadboard test phase. Initial printed circuit layout of this circuit has shown it to be appropriately sized at 5 x 6 cm to be compatible with implantation. A completely implantable system requires the use of a transcutaneous energy transformer system (TETS) and a diagnostic telemetry system. The TETS power circuitry has been redesigned incorporating an improved, more reliable operating topography. A telemetry circuit is undergoing characterization testing. Closed loop speed control algorithms are being tested in vitro and in vivo with good success. Eleven in vivo tests were conducted with durations from 1 to 195 days. Endurance pumps have passed the 6 month interval with minimal bearing wear. All aspects of the program continue to function under formal quality assurance.

Simulation of platelet deposition in disturbed flow

We have developed a two-dimensional computational model of platelet-mediated biomaterial thrombosis. When applied to plane Poiseuille flow over collagen, it showed very good agreement with experimental results. Here, we attempt to simulate platelet deposition onto collagen in the presence of disturbed flow in a tubular expansion. At 0% hematocrit, good agreement with experiments is obtained for the recirculation zone, but not for the fully-developed downstream zone At 20% hematocrit, poor agreement is observed unless the platelet-surface reaction coefficients are altered. In both cases, it appears the deficiencies are due to incomplete modeling of platelet transport mechanisms.

On the mechanical blood trauma: effect of turbulence

Mechanical blood trauma is still one of the major obstacles in the development of cardiovascular devices. The mechanisms of this blood damage are heterogeneous and are not completely identified. Experimental and computational studies were performed to elucidate the role of turbulent stresses in hemolysis. For the experimental study suspensions of bovine red blood cells (RBCs) in saline or in dextran solution were driven through a closed circulating loop by a centrifugal pump. A small capillary tube with the inner diameter of 1 mm and the length of 50 mm was incorporated into loop with tapered connectors. It was shown that, at the same wall shear stress, the level of hemolysis is significantly higher under turbulent flow conditions than laminar flow conditions. This demonstrated that turbulent stresses contribute strongly to blood trauma. These results concurred with predicted hemolysis by computational fluid dynamics (CFD) modeling of the same blood flow conditions.