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Redesigned blood pump could save lives and dollars

Jun 1, 2009 12:00 PM, Andrew Hilton School of Mechanical, Materials and Manufacturing Engineering The University of Nottin

Design changes made to centrifugal blood pumps could reduce the total cost of these life-saving devices by as much as 95%. Currently pumps cost about $68,000.

Research by Geoff Tansley at Aston Univeristy (Birmingham, UK) and myself shows that the cost reduction becomes possible by replacing complex, custom components such as electromagnets with standard stock parts available from engineering suppliers, and by implementing high-throughput manufacturing processes such as injection molding.

There are some technical challenges facing blood pump designers. The pump must operate continuously, pumping at 6 liters (1.6 gal.) per minute, and cycling at a rate between 2,000 and 20,000 rpm. The pump must have an operating temperature under 37°C (98.6°F) and provide sufficient energy on a beat-by-beat basis to replicate the pressures seen within the heart. The system must be nontoxic to the human body, and damage to cellular components and elements such as platelets must be prevented. The system must not produce blood clots (thrombi) that can lead to a malfunction of the pump and ultimately the demise of the patient.

In designing the new pump, we optimized their model by using Amperes, a CAE 3D software package from Integrated Engineering Software, Winnipeg, Manitoba, Canada. The geometry was designed for a pump operating at 2,000 rpm. Full specifications included inlet and outlet diameters, blade heights at each inlet and outlet and the appropriate casing size that enabled the designer to maintain a constant velocity of the fluid. The new drive system employed the use of electromagnetic coils to drive the pump. There was only one moving part in the whole system (the impeller), no mechanical connections or seals around rotating components, and a full non-contact bearing that minimizes heat generation and maximizes the reliability of the design.

In the design of the bearing-drive system, various arrangements of magnets were considered. Primarily, the magnets had to be small enough to fit inside the impeller. This has direct consequence on the attractive force seen between the two components. Secondly, the angular orientation (0° to 60° included cone angle) of the magnets produces a significant radial force maintaining the central axis of rotation of the impeller. By using off-the-shelf components, such as pre-sized permanent magnets and stock electromagnets, it is possible to construct a bearing- drive system similar to that seen in current LVAD designs.

The drive system was designed by evaluating the forces that occur within a blood pump, such as shock forces due to patient movement. The appropriate magnet configuration was selected, and has been modeled using computational analysis. It was necessary to measure how the force of attraction between the parts varied at angles of misalignment; this was modeled using Boundary Element Method (BEM) analysis using Amperes. The program provides analysis options, including the ability to create contour plots and graphs of field quantities. To perform a simulation, a geometric model of the physical system was constructed, by using the built-in Amperes geometric modeler.

Once the geometric model was built, physical properties such as material and magnet flux density vector orientation were assigned. The magnets are Grade N42 NdFeB rare-earth magnets with axial magnetization through the thickness of the magnet. The BEM analysis yielded results for the axial force and the torque between the components at angles of misalignment, and showed how the coupling may be used as a radial bearing by examining how the radial forces vary with an increasing cone angle.

The research also concluded that the total cost of the pump could be reduced by implementing a drive system that acts as a magnetic bearing. Permanent magnets are housed within the impeller and electromagnets with ferromagnetic core are housed within the pump casing. The permanent magnets and ferromagnetic cores of the electromagnets form the magnetic bearing. The employment of ferromagnetic cored electromagnets as part of the magnetic bearing allows for the possibility of driving the rotation of the impeller using the electromagnets and the impeller permanent magnets.

That is, the magnetic bearing used to support the impeller also provides drive to the pump. Using stock components available from standard engineering suppliers, the cost of manufacture may be reduced up to 95%. An upcoming prototype incorporates a hybrid electromagnetic drive/bearing system, together with a hydrodynamic bearing.

LIFE-SAVING DESIGN CHANGES TARGET CHF

The work described one these pages is a matter of life and death when you consider that 9 million persons throughout the world die annually from congestive heart failure (CHF). And the annual growth rate for CHF-related death is 10%. While today's blood pumps serve as a bridge to transplants (about 3,000 a year) or recovery, their availability is limited and their cost - approximately $68,000 per pump - is high.