WO2010029296A2

WO2010029296A2 - Rotary blood pump

Info

Publication number: WO2010029296A2
Application number: PCT/GB2009/002163
Authority: WO
Inventors: Geoffrey Douglas Tansley; Andrew Steven Hilton
Original assignee: Aston University
Priority date: 2008-09-15
Filing date: 2009-09-09
Publication date: 2010-03-18
Also published as: WO2010029296A3; GB0816883D0

Abstract

There is described a rotary blood pump (10) comprising a rotor (14) and a stator (12) which houses the rotor. The rotor defines an axis of rotation (AA). The rotor has a rotor hydrodynamic bearing surface (26) and rotor bearing magnets (24a, 24b). The stator has a stator hydrodynamic bearing surface (28) and stator bearing magnets (22a, 22b). The rotor hydrodynamic bearing surface and the stator hydrodynamic bearing surface are arranged to form an axial hydrodynamic bearing with a bearing gap (30) therebetween for the passage of blood. The hydrodynamic bearing is arranged to provide a hydrodynamic force which acts to force the hydrodynamic bearing surfaces axially apart during rotation of the rotor. The stator bearing magnets and the rotor bearing magnets are arranged to form an axial magnetic bearing for preloading the hydrodynamic bearing by providing a magnetic force which acts to force the hydrodynamic bearing surfaces axially together.

Description

ROTARY BLOOD PUMP

FIELD OF THE INVENTION

The present invention relates to a rotary blood pump. In particular, the present invention concerns a rotary blood pump with a hybrid hydrodynamic/magnetic bearing system.

BACKGROUND OF THE INVENTION

Heart failure is a major cause of death in the developed and developing world; it is estimated that there are currently 901,500 sufferers in the United Kingdom with 65,000 new cases added annually. The British Heart Foundation estimates the annual cost of heart failure is £625 million in the United Kingdom alone. In the United States, these statistics are 5,000,000 sufferers 550,000 new cases annually and an annual cost to the United States economy of $296,000 million. The World Health Organisation estimates that Cardiovascular Disease, around 7% of which is heart failure, contributed to 1/3 of all deaths worldwide and will be the major cause of death by 2010.

The prognosis for heart failure sufferers is poor with just less than 40% dying within the first year and with 5% of all deaths in the United Kingdom, approximately 24,000 per annum, being attributable to heart failure. Around 40% of these patients, suffer from impaired left ventricular systolic function and could benefit from mechanical support e.g. with a left ventricular assist device. The best therapy for many of these patients would be heart transplantation; however the demand for donor hearts in the United States alone is around 100,000 per annum and far exceeds the 2,200 donor hearts available per annum. The other main therapies commonly available are medical, such as inotropes, ACE inhibitors, Beta blockers, diuretics and nitrates, or are mechanical support therapies, such as the use of total artificial hearts or ventricular assist devices, of which rotary blood pumps are a subset. There is a continuum of treatment modalities for patients suffering chronic heart failure. As the disease progresses patients will receive increasingly aggressive medical therapies, but most patients become refractory to medical therapies at some point and their health will decline. Patients eligible for cardiac transplantation would typically receive medical therapies whilst awaiting transplantation; if their condition deteriorates then mechanical "bridge to transplantation" may be adopted in the form of a mechanical support device such as an intra-aortic balloon pump, extra-aortic balloon pump, or other left ventricular assist device. Patients who are supported in bridge to transplantation whilst awaiting transplantation are in a better state of health at the time of transplantation, and are more likely to survive transplant surgery and have a better long-term prognosis. Patients ineligible for transplantation typically follow a medical therapy-only path, though with the most aggressive healthcare providers may receive mechanical support, e.g. a ventricular assist device or a total artificial heart in "destination therapy".

Several mechanical devices are currently available or are in development which support cardiac function in heart failure. Rotary blood pumps take blood typically from the ventricle of the native heart, energise it through the action of a rotating impeller, and deliver the blood to the ascending aorta. These devices allow the patient to be ambulatory, to leave hospital, and to have a reasonably normal lifestyle. Bearing systems used to support the rotor of rotary blood pumps fall in to three main categories : mechanical contact bearings, magnetic bearings and hydrodynamic bearings. Mechanical contact bearings suffer from heat generation which can lead to thrombosis and even seizure. Mechanical contact bearings also suffer from mechanical wear which limits their longevity. Magnetic bearings can be either passive, using permanent magnets only, or active using permanent and electromagnets and sensors to determine the position of the rotor. Following Earnshaw's theorem, magnetic bearings require at least one degree of freedom to be actively controlled for stability, which can lead to complex sensing and control requirements and additional power draw. Hydrodynamic bearings typically require large surface areas on their bearing pads and have a large solidity on the rotor which reduces stage efficiency and increases power draw (see, for example, the device in US 6,250,880) . Additionally rotary blood pumps tend to be expensive due to the use of exotic materials such as titanium, because of the need for manufacture to very tight tolerances or positional sensing elements brought about primarily by the requirements of the system used to bear the rotor of the blood pump.

The present invention seeks to provide an alternative rotary blood pump which provides various advantages over those of the prior art . SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided a rotary blood pump comprising a rotor and a stator. In this context, the term "rotary blood pump" means a rotary pump suitable for pumping blood. Such a pump could be used, for example, as part of a ventricular assist device. Although the rotary blood pump of the first aspect of the present invention is suitable for pumping blood, it could also be used to pump other fluids if desired. According to the first aspect of the present invention, the rotor of the rotary blood pump defines an axis of rotation, and the rotor has a rotor hydrodynamic bearing surface and rotor bearing magnets. The stator houses the rotor, and the stator has a stator hydrodynamic bearing surface and stator bearing magnets. The rotor hydrodynamic bearing surface and the stator hydrodynamic bearing surface are arranged to form an axial hydrodynamic bearing with a bearing gap between the two hydrodynamic bearing surfaces for the passage of blood. The hydrodynamic bearing is arranged to provide a hydrodynamic force which acts to force the hydrodynamic bearing surfaces axially apart during rotation of the rotor. The stator bearing magnets and the rotor bearing magnets are arranged to form an axial magnetic bearing for preloading the hydrodynamic bearing by providing a magnetic force which acts to force the hydrodynamic bearing surfaces axially together. Note that the stator hydrodynamic bearing surface and the rotor hydrodynamic bearing surface are together referred to as "the hydrodynamic bearing surfaces" for ease of reference. A key feature of the rotary blood pump of the present invention is the axial magnetic preload of the axial hydrodynamic bearings. The use of a hydrodynamic bearing only on one end of the rotor, e.g. within a shroud which forms part of the rotor, allows for the possibility of a semi-open rotor design with low rotor solidity on the blades of the rotor which do not incorporate any hydrodynamic bearing. This allows for much improved fluid flow efficiencies on the rotor blades which leads to extended battery life, or to less cumbersome batteries for the patient to carry. There are low energy losses in the bearings such that high pump- stage efficiencies are achieved.

Another advantage of this design is that, since the hydrodynamic bearing is preloaded by a magnetic bearing, rather than by another hydrodynamic bearing, the rotary blood pump is tolerant of wider manufacturing tolerances and may be manufactured using high-throughput manufacturing processes including injection moulding. Therefore, according to a second aspect of the present invention, there is provided a method of manufacturing the rotary blood pump of the first aspect, wherein the rotor and the stator are formed by injection moulding.

The rotary blood pump of the present invention is a versatile mechanical cardiac prosthesis. It is suitable for implantation in human patients for the longer-term or shorter-term treatment of heart failure in bridge to transplant, bridge to recovery or bridge to alternative therapy, or as a disposable pump used in the very short- term.

In one embodiment, the hydrodynamic bearing is the principal bearing component which acts to force the hydrodynamic bearing surfaces apart in an axial direction. In other words, there are no significant magnetic bearing forces acting to push the hydrodynamic bearing surfaces apart. In a preferred embodiment, the hydrodynamic bearing is the only bearing component arranged to force the rotor hydrodynamic bearing surface away from the stator hydrodynamic bearing surface in an axial direction. In other words, there are no magnetic forces acting to push the hydrodynamic bearing surfaces apart .

In one embodiment, the hydrodynamic bearing provides hydrodynamic forces on the rotor in one axial direction only. For example, the hydrodynamic bearing may provide an upward axial hydrodynamic bearing force which opposes a downward magnetic axial bearing force, but there is no downward hydrodynamic bearing force .

In one embodiment, the hydrodynamic bearing surfaces are substantially planar. In an alternative embodiment, the hydrodynamic bearing surfaces are substantially conical. This embodiment advantageously allows hydrodynamic forces to develop in all three translational degrees of freedom and in two rotational degrees of freedom. Thus, this embodiment of the rotary blood pump has increased stability.

The hydrodynamic bearing may be a spiral groove bearing, or a pad bearing, for example. However, other forms of hydrodynamic bearing surfaces are envisaged within the scope of the invention.

In one embodiment, all of the stator bearing magnets act to preload the hydrodynamic bearing by forcing the hydrodynamic bearing surfaces axially together. In other words, a unidirectional axial magnetic preload is applied to the hydrodynamic bearing by the stator bearing magnets. So, there are no stator bearing magnets which provide an axial magnetic force component in the opposing direction which would act to force the hydrodynamic bearing surfaces axially apart (i.e. which would act to increase the bearing gap) . Thus, a magnetic force is imposed on the rotor only in one axial direction (e.g. downwards) and all upward force on the rotor is hydrodynamic (in the absence of any shock forces, etc. ) .

Advantageously, the axial magnetic bearing is passive. In other words, the stator bearing magnets and the rotor bearing magnets are all passive such that there is no active control of the magnetic bearing forces based on feedback from one or more sensors. Thus, the rotary blood pump provides completely passive suspension of the rotor within the stator.

In one embodiment, the stator bearing magnets include an array of electromagnets. Advantageously, the stator bearing electromagnets and the rotor bearing magnets are arranged to drive rotation of the rotor about the axis of rotation. Thus, the use of electromagnets as some/all of the stator bearing magnets allows for the possibility of driving rotation of the rotor about the axis of rotation using the stator bearing magnets and the rotor bearing magnets. In other words, the magnets used to form the axial magnetic bearing are also used for the purpose of driving the rotation of the rotor about the axis of rotation. This is essentially achieved by applying time varying currents to the various stator electromagnets at their circumferential locations around the rotor, whilst at the same time ensuring that the same overall axial magnetic force is applied to the bearing. Alternatively, the stator may further comprise an array of driving electromagnets that are distinct from the stator bearing magnets, such that the array of driving electromagnets is used to drive rotation of the rotor about the axis of rotation, whereas the stator bearing magnets are used to form part of the axial magnetic bearing. Thus, in this embodiment, the driving and bearing magnetic functions are performed by different sets of magnets. In one embodiment, the stator bearing magnets array include an array of permanent magnets. Thus, the stator bearing magnets may in fact include permanent and/or electromagnets.

In one embodiment, the stator comprises a substantially conical surface at an end of the stator remote from the hydrodynamic bearing surface, and the rotor comprises a substantially conical profile at an end of the rotor remote from the hydrodynamic bearing surface . Other known non- conical embodiments are also envisaged within the scope of the claims. For example, a spline-fitted or linear curve configuration may be used for the end of the rotor remote from the hydrodynamic bearing surface (i.e. the end of the rotor including the blades) . See, for example, Figure 5 and the associated description later in the application.

In one embodiment, the magnetic axes of the stator bearing magnets and the magnetic axes of the rotor bearing magnets are parallel to the axis of rotation. The magnetic axes of the stator bearing magnets may be radially offset from the magnetic axes of the rotor bearing magnets. In addition to the axial magnetic forces, this embodiment provides radial magnetic forces which gives stability against radial translations of the rotor position within the stator housing. In an alternative embodiment, the magnetic axes of the stator bearing magnets and the magnetic axes of the rotor bearing magnets are angled with respect to the axis of - S -

rotation. The angle may be between 0° and 180°, with the limiting cases of 0° and 180° giving parallel axes as described above. This is another way of providing radial magnetic bearing forces for giving magnetic control in additional magnetic degrees of freedom (as well as the axial direction) .

In one embodiment, the magnetic axes of the stator bearing magnets are coaxial with the magnetic axes of the rotor bearing magnets. In one embodiment, the axial magnetic bearing is attractive. In this case, the hydrodynamic bearing surfaces are disposed between the stator bearing magnets and the rotor bearing magnets, and there is an attractive magnetic force between the stator bearing magnets and the rotor bearing magnets so as to force the hydrodynamic bearing surfaces axially together. This is the preferred embodiment. Alternatively, the axial magnetic bearing is repulsive. In this case, the rotor bearing magnets are disposed between the stator bearing magnets and the hydrodynamic bearing surfaces, and there is a repulsive magnetic force between the stator bearing magnets and the rotor bearing magnets so as to force the hydrodynamic bearing surfaces axially together. This alternative "repulsive" configuration is less stable than the preferred "attractive" configuration, and would therefore ideally be used in conjunction with stator bearing magnets including an array of actively-controlled electromagnets.

Advantageously, the magnitude of the axial force generated by the magnetic bearing is approximately half the magnitude of the maximum opposing axial force able to be generated by the hydrodynamic bearing. The maximum axial force able to be generated by the hydrodynamic bearing is generated as the hydrodynamic bearing surfaces are moved very close together and the bearing gap approaches zero. In this embodiment, there is equal resistance to bearing touchdown provided in both axial directions (i.e. both the positive and negative axial directions) under shock loading conditions. Thus, gravity and shock loading on the rotor in the downward direction may be accommodated by hydrodynamic forces, whereas upward disturbing forces (shock or patient standing on their head) may be accommodated by magnetic forces only. The magnitude of the axial force generated by the magnetic bearing need not be exactly 50% of the magnitude of the maximum opposing axial force able to be generated by the hydrodynamic bearing; axial magnetic forces having a magnitude of, say, between 10 and 90% of the maximum opposing axial force able to be generated by the hydrodynamic bearing are also envisaged within the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings in which:

Figure 1 is a schematic cross-sectional view of a rotary blood pump in accordance with one embodiment the present invention;

Figures 2A and 2B are schematic representations of two alternative embodiments of the rotor hydrodynamic bearing surface of the rotary blood pump of Figure 1; Figure 3 is a schematic partial cross-section of an alternative hydrodynamic bearing of the rotary blood pump of Figure 1 ; Figure 4 shows two alternative embodiments of the rotary blood pump of Figure 1 with differing magnet configurations; and

Figure 5 is a perspective view of an alternative shape of rotor which could be used in the rotary blood pump of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Figure 1 is a schematic illustration of a cross- section through a rotary blood pump 10 according to one embodiment of the present invention. The rotary blood pump 10 of Figure 1 has many features in common with a standard configuration of a radial (i.e. centrifugal) pump. The rotary blood pump 10 has a stator 12 which houses a rotor 14. The rotor 14 rotates within the stator 12 about an axis of rotation AA' . In Figure 1, the axis of rotation AA' is shown aligned with the Z-axis for simplicity. Very roughly, the rotary blood pump 10 approximates a conical shape with the axis of rotation AA¹ extending along the axis of the cone .

The stator has a first end 12a at the narrow end of the cone shape and a second end 12b at the wide planar end of the cone. The size of the stator 12 increases from the first end 12a towards the second end 12b. The stator has two openings which respectively form an inlet 16 and an outlet 18 of the rotary blood pump 10 for the passage of blood.

The inlet 16 is located at the first end 12a of the stator 12 and extends axially (i.e. along the axis of rotation AA¹ of the rotary blood pump 10) . The second end

12b of the stator 12 is substantially formed as a disc which extends perpendicular to the axis of rotation AA' . In Figure 1, the direction of rotation of the rotor 14 within the stator 12 is shown by arrow R and is clockwise when viewed from above through the inlet 16.

The outlet 18 is located near the disc-shaped second end 12b of the stator 12 at an edge of the disc (i.e. at a distance from the axis of rotation AA¹) . A volute 29 collects blood as it flows off the rotor 14. In the embodiment of Figure 1, the cross-sectional area of the volute 29 increases as a function of circumferential position around the Z-axis in the direction of rotation indicated by the arrow R. For example, as shown in Figure 1 the volute 29 has a relatively small cross-sectional area on the left hand side 29a increasing to a relatively large cross-sectional area on the right hand side 29b. Then the volute 29 has its largest cross-sectional area on the right hand side 29b in Figure 1, at which point the volute 29 extends outwards in the Y-direction to form the outlet 18 of the rotary blood pump 10. Thus the second end 12b of the stator 12 is asymmetric due to the increasing size of the volute 29. The outlet 18 extends substantially perpendicular to a plane including the axis of rotation AA' ; in Figure 1, this plane is the X-Z plane such that the outlet 18 extends in a direction parallel to the Y-axis (i.e. into or out of the page) . It is also envisaged within the scope of this invention that other volute arrangements may be used for the collection of blood as it flows off the rotor; these arrangements would include a circular essentially concentric volute along with split volutes widely known in radial pump design. The rotor 14 also has a substantially conical outer profile as it increases in size from a first end 14a at the narrow end of the cone to a substantially disc-shaped second end 14b at the planar end of the cone. The rotor 14 has blades 20 which each extend from the first end 14a and increase in size as they extend to the second end 14b. The rotor has a bore 21 which extends axially from the first end 14a to the second end 14b. In alternative embodiments, the bore 21 may be omitted.

As an alternative to the substantially conical outer profile of the rotor 14 shown in Figure 1, other non-conical shapes of outer profile are also envisaged within the scope of the invention. See, for example, the alternative shape of rotor 14 shown in Figure 5. In Figure 5, the rotor 14 is essentially discoid but with a slight rise towards its axis of rotation AA' . Thus, each blade 20 comprises an upper surface 19 in the form of a low swept curve. The stator 12 includes stator magnets 22, and the rotor 14 includes rotor magnets 24. In the cross sectional view of Figure 1, two stator magnets 22a and 22b and two rotor magnets 24a and 24b are shown, but there may be more. The rotor magnets 24 are permanent magnets. The stator magnets 22 include stator electromagnets for driving rotation of the rotor 12 about the axis of rotation AA' . The stator magnets 24 additionally include magnets to provide a magnetic axial bearing as described further below. The bearing magnets may be electromagnets and/or permanent magnets (i.e. the use of electromagnets is not essential for the bearing magnets, cf . the drive magnets) . Advantageously, a single set of electromagnets may be used both to drive rotation of rotor 12 and to act as an axial magnetic bearing, although different sets of magnets could alternatively be used for these different purposes.

In use, a time-varying current is applied to the stator electromagnets so as to drive rotation of the rotor 12 by magnetic coupling between the stator electromagnets and the rotor magnets 24. The rotation of the rotor 12 and the associated rotor blades 20 causes blood to flow through the rotary blood pump 10 from the inlet 16 to the outlet 18. In other words, blood is caused to flow through the rotary- blood pump 10 through the action of the blades 20 incorporated into the rotor 12 as it rotates under the influence of the magnet coupling produced by the stator electromagnets and the rotor magnets 24.

The rotary blood pump 10 uses a non-contact hybrid hydrodynamic/magnetic bearing system to allow the rotor 12 to rotate within the stator. The non-contact hybrid hydrodynamic/magnetic bearing system suspends the rotor 14 within the stator 12. Depending on the type of stator magnets 22 used, the rotor 14 may be suspended completely passively within the stator 12, as described below.

The hydrodynamic bearing is formed from two substantially planar hydrodynamic bearing surfaces 26 and 28 with a bearing gap 30 therebetween for the passage of blood. The stator hydrodynamic bearing surface 28 is an interior substantially circular surface of the stator 12 at the second end 12b. The rotor hydrodynamic bearing surface 26 is an exterior surface of the rotor 14 at the second end 14b of the rotor 14; the rotor hydrodynamic bearing surface 26 is substantially circular with a central aperture 27 formed by the axial bore 21. The bearing surfaces 26 and 28 face each other in use. For hydrodynamic forces to be developed within the bearing gap 30 under relative rotation of the rotor 14 and the stator 12 about axis AA¹ , the width W of the bearing gap 30 varies circumferentially in the direction of rotation indicated by the arrow R.

Figure 2 shows two alternative embodiments of the rotor hydrodynamic bearing surface 26 which provide a suitable variation in the width W of the bearing gap 30 when the stator hydrodynamic bearing surface 28 is flat. It is of course feasible in an alternative embodiment that the rotor hydrodynamic bearing surface 26 is flat and variations in the width W of the bearing gap are achieved instead by height variations incorporated into the stator hydrodynamic bearing surface 28.

Figure 2A shows a "spiral groove bearing" configuration of the rotor hydrodynamic bearing surface 26. In particular, the rotor hydrodynamic bearing surface 26 is substantially planar but comprises a series of shallow spiral grooves 32 between raised lands 34. Each groove 32 is arcuate and extends from an edge of the rotor hydrodynamic bearing surface 26 adjacent the central aperture 27 to the circumferential peripheral edge of the rotor hydrodynamic bearing surface 26. All of the grooves 32 are curved in the same sense around the annular rotor hydrodynamic bearing surface 26. In Figure 2A, the grooves curve in a forwards direction relative to the direction of rotation R; however, it is envisaged that backward-curving grooves 32 could be used instead. The difference in height between the grooves 32 and the lands 34 may be in the region of 5μm to lOOμm and it is this height difference that facilitates the development of hydrodynamic bearing forces between the rotor hydrodynamic bearing surface 26 and the stator hydrodynamic bearing surface 28. The difference in height between the grooves 32 and the lands 34 may take the form of a step change or a chamfer or may be curved, for example. As shown in Figure 2A, the direction of rotation R of the rotor 14 is anticlockwise when viewed from below.

Figure 2B shows a "pad bearing" configuration of the rotor hydrodynamic bearing surface 26. In particular, the rotor hydrodynamic bearing surface 26 is substantially planar but comprises a series of much wider raised lands 38 which each vary in height from a minimum at the leading edge 38a of the land 38 to a maximum at the trailing edge 38b of the land 38. The difference in height between the leading 38a and trailing 38b edges of the lands 38 may be in the region of 5μm to 100/xm and it is this height difference that facilitates the development of hydrodynamic bearing forces between the rotor hydrodynamic bearing surface 26 and the stator hydrodynamic bearing surface 28. As shown in Figure 2B, deeper radially disposed grooves 36 may be incorporated into the rotor hydrodynamic bearing surface 26 in addition to the variable height lands 38. The grooves 36 improve the flow of lubrication fluid between the outer periphery and the central aperture 27, thereby preventing starvation of the hydrodynamic bearing.

In the embodiment of Figure 1 which incorporates substantially planar hydrodynamic bearing surfaces 26 and 28, hydrodynamic forces are developed which provide control over one translational degree of freedom, namely the axial Z-direction as shown in Figure 1, and two rotational degrees of freedom, namely rotation about the X and Y axes as shown in Figure 1. It should be noted that the hydrodynamic bearing only provides hydrodynamic forces which act on the rotor 14 in one axial direction (i.e. upwards in the +Z direction in Figure 1) . These hydrodynamic forces must be sufficient to withstand the opposing magnetic preload force (discussed below) and any disturbing forces (shock and gravitational loading) .

The axial bore 21 allows for the passage of blood therethrough in use. This ensures that there is always blood present in the hydrodynamic bearing to thereby prevent bearing starvation.

Rather than the hydrodynamic bearing surfaces 26 and 28 being substantially planar as described above, they may instead be substantially conical as shown in Figure 3, which is a partial cross-section of an alternative hydrodynamic bearing of the rotary blood pump 10. This "conical hydrodynamic bearing" embodiment employs radial as well as axial hydrodynamic forces so as to provide control over the two other translational degrees of freedom, namely translation along the axes X and Y which are perpendicular to the axis of rotation AA' in Figure 1.

In this conical hydrodynamic journal bearing embodiment, the bearing gap 30 between the stator 12 and the rotor 14 is defined by the essentially conical hydrodynamic bearing surfaces 26 and 28.

The rotor hydrodynamic bearing surface 26 is formed as a frustroconical surface due at least to the presence of the central aperture 27 at the end of the axial bore 21. In addition, as shown in Figure 3, there may be an optional annular land 31 on the end of the rotor 14 around the exit of the axial bore 21. In this case, the axial bore 21 has a smaller diameter than the annular land 31. The frustroconical rotor hydrodynamic bearing surface 26 projects outwards from the rotor 14 in Figure 3 such that the rotor hydrodynamic bearing surface 26 is essentially convex.

In Figure 3, the stator hydrodynamic bearing surface 28 is a conical surface which forms a conical aperture in the stator 12 for receiving the outwardly-projecting (convex) frustroconical rotor hydrodynamic bearing surface 26. Thus, in the embodiment of Figure 3, the stator hydrodynamic bearing surface 28 is essentially concave such that the rotor 14 is received within the stator 12 at the hydrodynamic bearing.

In an alternative embodiment, the stator hydrodynamic bearing surface 28 could incorporate a central land such that the stator hydrodynamic bearing surface 28 would be formed as a frustroconical surface. In a further alternative embodiment, the rotor hydrodynamic bearing surface 26 could be essentially concave and the stator hydrodynamic bearing surface 28 could be essentially convex such that the stator 12 would be received within the rotor 14 at the hydrodynamic bearing. It will be appreciated that, rather than being formed as substantially planar or substantially conical surfaces as described above, the hydrodynamic bearing surfaces 26 and 28 could instead be formed substantially as portions of a sphere or other surfaces formed by rotating an arbitrary curve through 360°.

As for the above embodiment which incorporates substantially planar hydrodynamic bearing surfaces 26 and 28, the width W of the bearing gap 30 in the embodiment of Figure 3 varies circumferentially in the direction of rotation indicated by the arrow R. A "spiral groove bearing" or a "pad bearing" configuration may be used on the rotor hydrodynamic bearing surface 26 or the stator hydrodynamic bearing surface 28 in this regard.

If the rotor 14 is pushed downwards (in direction -Z) towards the stator 16, it meets resistance due to the need for blood to be expelled from the bearing gap 30. The width W of the bearing gap 30 between the hydrodynamic bearing surfaces 26 and 28 may also vary radially (i.e. in the direction indicated by the arrow F in Figure 3) . If the bearing gap 30 at larger radius is smaller than the bearing gap 30 closer to the axis, then this resistance will be greater as it will be harder for the blood to exit the bearing gap 30. This effect is known as "squeeze-film damping". In Figure 3, the bearing gap 30 is shown to increase radially (i.e. the bearing gap 30 increases with increasing distance from the axis AA¹) . This results in less resistance, from squeeze-film damping, to any downward force on the rotor 14 which acts to reduce the axial bearing gap 30. In particular, the resistance is less than would be the case if the bearing gap 30 reduced with increased radial distance from axis AA' .

As for the planar embodiment of Figure 1, the hydrodynamic bearing in Figure 3 is arranged to force the rotor hydrodynamic bearing surface 26 away from the stator hydrodynamic bearing surface 28 in an axial direction on average. There will also be some radial components to the hydrodynamic bearing forces in the embodiment of Figure 3. However, when the rotor 14 is completely aligned with the stator 12 such that both hydrodynamic bearing surfaces 26 and 28 are symmetrically disposed about the axis of rotation AA', the radial hydrodynamic forces will cancel out over the whole rotor 14 so that the net force is purely axial . In this configuration, the axis of rotation AA¹ is aligned with central axes of both the stator 12 and the rotor 14. Of course, if the rotor 14 is displaced radially (e.g. in direction X or Y) with respect to the stator 12 such that the central axes of the rotor 14 and the stator 12 are no longer aligned, then the radial hydrodynamic forces will become unbalanced. For example, if the rotor 14 is displaced in direction +Y with respect to the stator 12, then a hydrodynamic force will be developed in direction -Y with the effect of restoring the rotor 14 back to the centre of the stator 12 such that the central axes of the stator 12 and the rotor 14 are aligned once again.

Turning to the magnetic part of the non-contact hybrid hydrodynamic/magnetic bearing system, Figure 4 shows alternative embodiments of the rotary blood pump 10 with differing magnet configurations. In both cases, only the rotor 14 and the stator magnets 22 of the stator 12 are shown. The stator 12 itself is omitted for simplicity. In the main embodiment described below, the same stator magnets 22 are used both to drive rotation of the rotor 14 and to form part of an axial magnetic bearing. Both magnetic embodiments shown in Figure 4 use substantially planar hydrodynamic bearing surfaces 26 and 28, as described above with reference to Figure 1. However, other forms of hydrodynamic bearing surfaces 26 and 28 could alternatively be used, as described above with reference to Figure 3.

In the "axial motor" magnet configuration of Figure 4A, the stator magnets 22 have magnetic axes 23 which are aligned parallel to the axis of rotation AA' of the rotor

14. In addition, the rotor magnets 24 have magnetic axes 25 which are aligned parallel to the axis of rotation AA' of the rotor 14. Both the rotor magnets 24 and the stator magnets 22 are spaced from the axis of rotation AA' in Figure 4A, however this is not essential if separate electromagnets are used to drive rotation of the rotor 14 within the stator 12. The hydrodynamic bearing is located in between the stator magnets 22 and the rotor magnets 24. The interaction between the magnetic fields of the stator magnets 22 and the rotor magnets 24 leads to high axial magnetic forces acting on the rotor 14. In particular, there is a strong attractive axial magnetic force between the stator magnets 22 and the rotor magnets 24 which acts to pull the rotor downwards (i.e. in direction -Z) such that the rotor hydrodynamic bearing surface 26 is forced axially towards the stator hydrodynamic bearing surface 28, thereby acting to reduce the bearing gap 30 of the hydrodynamic bearing.

Thus the interaction between the stator magnets 22 and the rotor magnets 24 acts as a magnetic bearing which provides an unbalanced downward axial magnetic force (in direction -Z in Figure 1) which acts to preload the hydrodynamic bearing formed between the hydrodynamic bearing surfaces 26 and 28. The hydrodynamic bearing is an integral part of the bearing system and relies on the unbalanced downward axial magnetic load to oppose the upward hydrodynamic bearing force.

As it rotates about the axis of rotation AA' in use, the rotor 14 is suspended within the stator 12 by a combination of hydrodynamic and magnetic forces. The magnetic configuration forces the rotor 14 downward (i.e. in direction -Z in Figure 1) onto the hydrodynamic bearing so as to preload the hydrodynamic bearing to make the rotor 14 stable. If a shock is imparted to the rotary blood pump 10 (e.g. through the patient falling over) , the hydrodynamic bearing is sufficiently stiff so as to prevent the rotor 14 coming in to contact with the stator housing 12. Clearly, such "touchdown events" are undesirable for efficient working of the rotary blood pump 10. The magnetic bearing formed by the stator magnets 22 and the rotor magnets 24 incorporates magnetic forces that are sufficiently strong as to prevent the first end 14a of the rotor 14 and the rotor blades 20 from touching the adjacent internal surfaces of the stator 12 if the rotary blood pump 10 is inverted or shocked in a direction which causes upward displacement of the rotor 12 (i.e. displacement of the rotor 12 in direction +Z in Figure 1) . The maximum practical hydrodynamic bearing force might be roughly twice the magnetic preload such that equal resistance to bearing touchdown is provided in both the upward (+Z) and downward (-Z) directions under shock loading conditions. In other words, the downward magnetic force is about 50% of the maximum envisaged hydrodynamic force, which is largest when the hydrodynamic bearing surfaces 26 and 28 come close together such that bearing gap

30 approaches zero. Thus, gravity and shock loading on the rotor 14 in the downward direction are accommodated by hydrodynamic forces, whereas upward disturbing forces (shock or patient standing on their head) are accommodated by magnetic forces only. However it is envisaged that other ratios of magnetic preload vs maximal hydrodynamic force might be used to optimise bearing stability.

As shown in Figure 4A, radial magnetic force components may be introduced by offsetting the axes 23 of the stator magnets 22 by a distance D relative to the axes 25 of the rotor magnets 24. The rotary blood pump 10 of this embodiment might have increased stability as this magnet configuration allows tuning of the translational degrees of freedom of the rotor in the X and Y directions whilst also imparting control over two rotational degrees of freedom (namely rotation about the X and Y axes) whilst allowing rotation of the rotor 14 around the axis of rotation AA¹ (i.e. the Z-axis) . In an alternative embodiment, the axes 23 of the stator magnets 22 are aligned with the axes 25 of the rotor magnets 24. In this case, D=O such that the axes 23 and the axes 25 are located at the same distance from the axis of rotation AA¹. This configuration maximizes the attractive magnetic force between the stator magnets 22 and the rotor magnets 24 at the expense of radial force components .

In the magnet configuration of Figure 4B the magnetic axes 23 of the stator magnets 22 and the magnetic axes 25 of the rotor magnets 24 are angled at an angle a with respect to the axis of rotation AA¹. The angle a between the magnetic axes 23 and 25 and the axis of rotation AA¹ may be anywhere between 0° and 360°. This "angled" embodiment naturally provides radial magnetic bearing components. However, when the rotor 14 is centred with the stator 12, the radial magnetic forces cancel out over the whole rotor 14 so that the net force is purely axial. In some embodiments, there may be a small radial bias on the rotor to yield greater bearing stability. In Figure 4B, the magnetic axes 23 of the stator magnets 22 and the magnetic axes 25 of the rotor magnets 24 are all at the same angle with respect to the axis of rotation AA' . In Figure 4B, the magnetic axes of each pair or stator and rotor magnets 22 and 24 are coaxial; for example, the magnetic axes 23 and 25 of the first pair 22a and 24a of rotor and stator magnets are coaxial, and the magnetic axes 23 and 25 of the second pair 22b and 24b of rotor and stator magnets are coaxial. Thus, the stator magnets 22 are located further from the axis of rotation AA' than the rotor magnets 24 in an axial direction, and the magnetic axes 23 and 25 are angled inwards toward the axis of rotation AA¹ , thus providing a radial magnetic bearing force component along with the main axial magnetic bearing force component . In alternative embodiments, different angles of inclination to the axis of rotation AA' could be used for the magnetic axes 23 and 25 of the stator 12 and the rotor 14 respectively, including outward angling of the axes 23 and 25. In situations where there is a linear translation in direction X and/or Y of the magnetic axes 25 of the rotor magnets relative to the magnetic axes 23 of the stator magnets 22, the "angled" embodiment of Figure 4B also provides advantages over the "axially aligned" embodiment of Figure 4A. With the "axialIy aligned" embodiment of Figure 4A, the axial magnetic bearing force falls off steeply as the translation increases. However, in the "angled" embodiment of Figure

4B, the fall off is very small indeed. Linear translations of this type would occur if there were a linear translation of the rotor 14 with respect to the stator 12.

The magnetic bearing is completely passive in a preferred embodiment. By "passive", we mean that there is no active control of the magnetic bearing forces based on feedback from one or more sensors as to the position of the rotor 14 within the stator 12. In this case, the stator bearing magnets and the rotor bearing magnets are all completely passive (i.e. they are either permanent magnets or passive, non-controlled electromagnets) . Thus, the rotary blood pump 10 can provide completely passive suspension of the rotor 14 within the stator 12.

Nonetheless, the present invention does not preclude the use of active magnetic components in addition to or instead of passive magnetic components. Thus, there may be sensors provided to give some active suspension of the rotor 14 by the stator magnets 22 and the rotor magnets 24 in axial and/or radial directions.

Known rotary blood pumps which use a first hydrodynamic bearing to force the rotor upwards axially, and a second hydrodynamic bearing to force the rotor downwards axially require high levels of precision in the manufacturing process to ensure that the bearing gaps on the opposing hydrodynamic bearings are a suitable size. In contrast, the rotary blood pump 10 described herein has a single hydrodynamic bearing preloaded by a magnetic bearing. This design means that there is no need for a precise width of gap between the housing stator 12 and the end 14a of the rotor 14 axially remote from the hydrodynamic bearing.

Thus, the rotary blood pump 10 is tolerant of wider manufacturing tolerances and may be manufactured using high- throughput manufacturing processes including injection moulding. Thus, advantageously, the rotor 14 and the stator 12 may be manufactured by injection moulding from biocompatible rigid materials such as Polyetheretherketone (PEEK) . A moisture barrier would then be required to prevent the magnets 22 and 24 from rusting in use. Suitable moisture barrier materials include gold or nitride coatings. The use of injection moulding techniques means that the rotary blood pump 10 may be manufactured more cheaply than existing rotary blood pumps, so may be more widely used in the developing world as well as for shorter term applications .

During start-up of the rotary blood pump 10, there is initially no upward hydrodynamic force due to the lack of rotation of the rotor 14 within the stator 12. Furthermore, there are no secondary bearings (e.g. magnetic bearings) to provide an alternative form of upward force during the start-up period before the hydrodynamic forces are developed by high enough rotation speeds. Therefore, at start-up, there will briefly be wear on the hydrodynamic bearing surfaces 26 and 28 during the first half revolution or so of the rotor 14. In cardiac applications, the rotor 14 should, of course, only be started once in the patient and possibly a couple of times during manufacture. Thus, there is negligible damage to the hydrodynamic bearing surfaces 26 and 28. Nonetheless, a coating may be add to the hydrodynamic bearing surfaces 26 and 28 to act as a secondary bearing during these very brief start-up periods if desired.

Although preferred embodiments of the invention have been described, it is to be understood that these are by way of example only and that various modifications may be contemplated.

Claims

CLAIMS :

1. A rotary blood pump comprising: a rotor defining an axis of rotation, the rotor having a rotor hydrodynamic bearing surface and rotor bearing magnets; and a stator which houses the rotor, the stator having a stator hydrodynamic bearing surface and stator bearing magnets; wherein the rotor hydrodynamic bearing surface and the stator hydrodynamic bearing surface are arranged to form an axial hydrodynamic bearing with a bearing gap therebetween for the passage of blood, the hydrodynamic bearing being arranged to provide a hydrodynamic force which acts to force the hydrodynamic bearing surfaces axialIy apart during rotation of the rotor; and wherein the stator bearing magnets and the rotor bearing magnets are arranged to form an axial magnetic bearing for preloading the hydrodynamic bearing by providing a magnetic force which acts to force the hydrodynamic bearing surfaces axially together.

2. The rotary blood pump of claim 1 wherein the hydrodynamic bearing is the principal bearing component which acts to force the rotor hydrodynamic bearing surfaces axially apart in use.

3. The rotary blood pump of any preceding claim wherein the hydrodynamic bearing provides hydrodynamic forces on the rotor in one axial direction only.

4. The rotary blood pump of any of claims 1 to 3 wherein the stator and rotor hydrodynamic bearing surfaces are substantially planar.

5. The rotary blood pump of any of claims 1 to 3 wherein the stator and rotor hydrodynamic bearing surfaces are substantially conical .

6. The rotary blood pump of any preceding claim wherein there are no stator bearing magnets which act to force the hydrodynamic bearing surfaces axially apart .

7. The rotary blood pump of any preceding claim wherein the stator bearing magnets and the rotor bearing magnets are passive.

8. The rotary blood pump of any preceding claim wherein the stator bearing magnets include an array of electromagnets .

9. The rotary blood pump of claim 8 wherein the stator bearing magnets and the rotor bearing magnets are further arranged to drive rotation of the rotor about the axis of rotation.

10. The rotary blood pump of any of claims 1 to 8 wherein the stator further comprises an array of driving electromagnets that are distinct from the stator bearing magnets, the array of driving electromagnets being arranged to drive rotation of the rotor about the axis of rotation.

11. The rotary blood pump of any preceding claim wherein the stator bearing magnets include an array of permanent magnets .

12. The rotary blood pump of any of claims 1 to 11 wherein the magnetic axes of the stator bearing magnets and the magnetic axes of the rotor bearing magnets are parallel to the axis of rotation.

13. The rotary blood pump of claim 12 wherein the magnetic axes of the stator bearing magnets are radially offset from the magnetic axes of the rotor bearing magnets.

14. The rotary blood pump of any of claims 1 to 11 wherein the magnetic axes of the stator bearing magnets and the magnetic axes of the rotor bearing magnets are angled with respect to the axis of rotation.

15. The rotary blood pump of any of claims 1 to 12 or claim 14 wherein the magnetic axes of the stator bearing magnets are coaxial with the magnetic axes of the rotor bearing magnets .

16. The rotary blood pump of any of claims 1 to 15 wherein the hydrodynamic bearing surfaces are disposed between the stator bearing magnets and the rotor bearing magnets, an attractive magnetic force being provided between the stator bearing magnets and the rotor bearing magnets so as to force the hydrodynamic bearing surfaces axially together.

17. The rotary blood pump of any of claims 1 to 15 wherein the rotor bearing magnets are disposed between the stator bearing magnets and the hydrodynamic bearing surfaces, a repulsive magnetic force being provided between the stator bearing magnets and the rotor bearing magnets so as to force the hydrodynamic bearing surfaces axially together.

18. The rotary blood pump of any preceding claim wherein the magnitude of the axial force generated by the magnetic bearing is approximately half the magnitude of the maximum opposing axial force able to be generated by the hydrodynamic bearing.

19. The rotary blood pump of any preceding claim wherein the rotor comprises a semi-open rotor design with low rotor solidity.

20. A method of manufacturing the rotary blood pump of any preceding claim wherein the rotor and the stator are formed by injection moulding.