Proposal for a cost-effective centrifugal rotary blood pump: design of a hybrid magnetic/hydrodynamic bearing

Hilton, Andrew (2010) Proposal for a cost-effective centrifugal rotary blood pump: design of a hybrid magnetic/hydrodynamic bearing. PhD thesis, University of Nottingham.

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The growing worldwide prevalence of cardiovascular diseases (CVD) such as chronic heart failure (CHF) highlights the need for an effective treatment method. Annually, there are an estimated 17.5 million deaths caused by cardiovascular diseases worldwide, representing 30% of all global deaths; of these deaths approximately 50% are due to CHF [1] and about 80% occur in low- and middle-income countries [2]. If current trends are allowed to continue, by 2015 an estimated 20 million people annually will die from CVD [3]. Each year, only about 3,000 people receive a heart transplant as the only current definitive long-term treatment for end-stage CHF. To compound the severity of the situation, organ donations are decreasing (4). Implantable blood pumps offer an effective treatment to CHF, either as a bridge to transplantation / recovery, or as destination therapy i.e. use of long term mechanical circulatory support in patients with end-stage heart failure without the intention of eventual heart transplantation. With the number of sufferers of CHF rising in both the developed and developing world it becomes pertinent to design a cost effective device.

It is the objective of this work to investigate the proposal of a new cost-effective Centrifugal Rotary Blood Pump (CRBP), which employs previously unutilized design methodology. Through the replacement of those complex, custom components seen within existing CRBPs with standard off-the-shelf components, and the implementation of high-throughput manufacturing processes, such as injection moulding, a reduction in component parts allows for a reduced profit margin and hence a reduced total cost of device. It is proposed by the author that the current production cost of LVAD devices may be reduced by up to 95%.

The work presented in this thesis identifies the principal difference between current pump designs; this is their bearing system. It is proposed here to form a new classification of bearing type that combines a passive magnetic bearing and a hydrodynamic bearing such that the relative potential merits of both systems may be exploited. Through the amalgamation of established design techniques with other more modern design practices a rigorous, adaptive design tool has been produced that CRBP designers may use to quickly obtain full impeller and volute geometry from few input parameters. The geometry output provides a platform from which a new conceptual Left Ventricular Assist Device (LVAD) has been envisaged.

Through experimental and computational analysis of the magnetic coupling, this investigation has shown that it is possible to integrate the magnetic bearing and the drive system into one component of design; it is possible to design a magnetic coupling that not only acts as the drive system for a CRBP, but as a bearing system that offers both axial and radial bearing forces.

A spiral groove bearing (5GB) has been implemented as the hydrodynamic bearing as part of the hybrid system. Experimental investigation has shown the spiral groove bearing to be anti traumatic, which may be attributed to the short residence time of blood in the bearing. However, a reduction in the anticipated load capacity shows that the bearings are operating on a reduced viscosity; this is an indication of cell exclusion within the 5GB. Comparisons to aqueous glycerol tests of known viscosities have shown that the blood bearing is operating on a viscosity close to that of plasma. It is suggested that a "shear front differential" is the mechanism behind cell exclusion, in which RBCs migrate away from areas of high shear stress into areas of relatively low shear stress.

This investigation has demonstrated the suitability of the hybrid magnetic / hydrodynamic bearing for use in a new CRBP. It has been shown that the electromagnetic drive system intended for implementation in to this CRBP can be used as an effective passive magnetic bearing. It is intended that the axial and radial bearing forces produced by the drive system are balanced by a conical spiral groove thrust bearing.

The incorporation of the hydrodynamic bearing into the magnetic bearing transforms the previously unstable passive magnetic bearing to a stable hybrid bearing. The stability of the system has been predicted through numerical analysis of the stiffness matrix and through satisfaction of the stability criteria. The natural frequencies of the system have been calculated; these are shown to be sufficiently different from any excitation frequencies identified that may cause the system to behave in an unstable fashion at the operational speed of the pump.

The main point to be realized from the analysis of the hybrid bearing system, however, is that the proposed set-up of the hybrid bearing is not feasible due to the effect of cell exclusion, which causes the SGB to operate on a reduced viscosity. The reduction in viscosity reduces the load capacity of the SGB; the magnetic preload on the impeller cannot be balanced by the chosen SGB geometry. Recommendations have been made as to the design parameters that may be altered such that the design intent of the proposed system may be realised. Future work must concentrate on the realization of that design intent through the manufacture of a prototype, which can provide a proof-of-concept for the proposed system. The work presented here provides a feasibility study for the novel hybrid bearing / drive system and provides sound foundation upon which a prototype may be based.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Garvey, S.
Keywords: Rotary pumps, design and construction, bearings (Machinery), blood, circulation, artificial instruments, heart failure, treatment
Subjects: T Technology > TJ Mechanical engineering and machinery
Faculties/Schools: UK Campuses > Faculty of Engineering > Department of Mechanical, Materials and Manufacturing Engineering
Item ID: 13919
Depositing User: EP, Services
Date Deposited: 15 Jan 2014 09:52
Last Modified: 14 Sep 2016 14:48

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