DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention provides structures and techniques for improving the flow of fluid within a rotary pump so as to reduce or eliminate low flow regions, particularly in areas where rotating portions (such as a rotating impellor) of the rotary pump are proximate to a stationary portion of the rotary pump (such as a housing containing the impellor). These structures and techniques can be particularly important in a blood pump as regions of low flow can give rise to thrombus formation. While the primary embodiment disclosed below is a centrifugal pump having an impellor supported by a shaft and bearing arrangement with a housing, the principles of the invention can readily be applied to axial flow pumps and to rotary pumps not having supporting shafts as well.

[0019] FIGS. 1 and 1A illustrate a rotary pump 10 of the invention. Rotary pump 10 is a centrifugal blood pump having a stationary housing 12 and a rotating impellor or rotor 26. Housing 12 includes a blood inlet conduit 14, a blood outlet conduit 16, a blood flow conduit 18 disposed between the blood inlet conduit 14 and the blood outlet conduit 16, and a bearing receiving bore 20 having a bearing assembly 22 disposed therein.

[0020] Exemplary rotor 26 (further illustrated in FIGS. 2 and 2A) is configured to be disposed within blood flow conduit 18 of housing 12 and to draw blood in through housing inlet 14 and expel blood out through housing outlet 16 upon rotation of rotor 26 (in a counter-clockwise direction in the embodiment illustrated in FIG. 2A). Rotor 26 includes a plurality of impellor blades 28 connected at a central hub 60 and through connecting elements 62. Impellor blades 28 are large enough to carry magnets 52 and are spaced apart by channels that are made small enough to maintain a nearly constant cross-sectional area as measured normal to the flow of blood through pump 10. Gaps between impellor blades 28 and housing 12 may generally be maintained in the range of between about 0.020 and 0.050 inches. Such gap distances are small enough to maintain reasonable efficiency and to keep shear rates large enough at part surfaces to reduce or prevent thrombosis. On the other hand, the distance across such gaps should be large enough so that shear rates do not lead to problematic hemolysis. In addition, the geometric tolerances should not be so tight that a small amount of wear leads to failure. A person of ordinary skill in the art will recognize that the number of impellor blades 28, the shape of impellor blades 28, as well as the connecting features between impellor blades 28 can be varied within the spirit of the present invention.

[0021] Rotor 26 is also connected to and rotates with shaft 30, which interacts with bearing assembly 22 to allow for relative movement between rotor 26 and housing 12. Illustrated bearing assembly 22 comprises toroidal bearing elements 32 and an axial thrust element 34. Bearing assembly 22 is further described and alternative bearing configurations are provided below by reference to FIGS. 4 to 4C, however, a person of ordinary skill in the art will recognize that a variety of bearing assemblies, including those compatible with magnetic and hydrodynamic bearing support, can be used to allow relative motion between rotor 26 and housing 12 within the spirit of the invention.

[0022] An exemplary motor is provided integrally with the housing 12 and rotor 26 to drive the rotation of rotor 26. In the illustrated embodiment, permanent magnets 52 are provided on each of impellor blades 28 while stator windings 54 may be provided integrally with housing 12. As further illustrated in FIGS. 5A and 5B, the motor stator may be a split stator including windings 54 on either side of magnets 52 (illustrated in FIGS. 1 and 5A with stator windings 54 on the inflow 14 and bearing assembly 22 sides of rotor 26). Alternatively, the motor stator may include a flux return path 94 on the inflow 14 side of rotor 26 (FIG. 5B) with stator windings 54 provided on the bearing assembly 22 side. By providing ferromagnetic material on both sides of the rotor 26, the total axial thrust can be minimized, thereby reducing the wear rate on any axial bearings. Heat generated in stator windings 54 can be dissipated across a large area of housing 12 into regions of very high blood flow and high surface shear. In this situation several watts of heat may be dissipated with no measurable heating effect on the blood.

[0023] These motor arrangements are provided for illustrative purposes only as a person of ordinary skill in the art will be able to vary the motor configuration in a variety of ways consistent with the spirit of the invention, including for example the motor configurations provided with the centrifugal pump of Hart et al. as described in U.S. Pat. No. 6,071,093 which is hereby incorporated by reference. Where pump 10 is used as an implanted blood pump, the motor preferably includes stator windings 54 integral with housing 12. Where pump 10 is used as an external blood pump, an external rotating magnet drive (such as that illustrated in Hart et al.) may preferably be employed to drive rotor 26.

[0024] Pump 10 of the invention includes several features configured to address the problem of low flow regions within known pumps. For example, in known rotary pumps, low flow is typically encountered in regions along an axis of rotation 36 of rotor 26, particularly where a portion of rotor 26 along axis 36 is proximate to a stationary portion of the pump such as housing 12. In this region, fluid in the pump is swirling in the same direction in which rotor 26 is spinning and, like the linear velocity of any point on the rotor, the velocity of a typical fluid “particle” will correspond at least in part to the angular velocity of the swirl multiplied by the distance of the particle from the center or axis 36. Approaching axis 36, this component of the velocity of the fluid approaches zero and an area of flow stagnation exists. This phenomenon bears some similarity with “the eye of a storm,” where winds in a storm may swirl at high velocity at a distance apart from a central axis of the storm, but may be quite calm proximate to the axis.

[0025] One such feature illustrated in pump 10 (most clearly shown in FIGS. 1A, 3 and 3A) is a thick central region 38 formed along central axis 36 on a side of rotor 26 proximate to stationary housing 12. Central thick region 38 “fills” the “eye” region present along the central axis, helping to ensure that only the “swirl” portion of the flow remains between rotor 26 and housing 12. The thickness of central thick region 38 can be determined experimentally for a given pump configuration by emperically determining the diameter of the “eye,” and providing the central thick region 38 with a diameter at least as large as that determined for the eye.

[0026] Another feature designed to reduce low flow regions is the definition of a swirl zone 40 proximate to central axis 36. Where a central thick region 38 is present, swirl zone 40 may preferably be formed by providing a surface 42 on central thick region 38 that encourages a flow of pumped fluid that swirls about an axis 43 (axis 43 extends out of the page in FIG. 3) that is transverse to and spaced apart from central axis 36 as indicated by flow arrows 44 (FIG. 3). Viewed in its entirety around the circumference of surface 42, swirl region 40 takes a toroidal shape centered on axis 36. Swirl region 40 is bounded on one side (a superior side) by rotating rotor 26 and on an opposed side (an inferior side) by stationary housing 12. As a result, pumped fluid will be forced outward (away from central axis 36) along a region proximate to the rotor 26, causing the swirl indicated by arrows 44.

[0027] Still another such feature is the configuration of cross-sectional flow areas through blood flow conduit 18 to encourage higher velocity fluid flow. In general, reducing the cross-sectional area through which a fluid flows results in a higher fluid velocity as the same volume throughput (assuming an incompressible or nearly incompressible fluid) of fluid must pass through a smaller area. Referring to FIG. 3B, the cross-sectional flow area through cross-section A is configured (with consideration given to the cross-sectional area taken up by rotor 26) to be smaller than the cross-sectional flow area through the pump inlet 14. This reduction in cross-sectional area results in greater fluid velocity through cross-section A, which in turn promotes a higher velocity swirl in swirl region 40 as well as higher velocity flow in other regions of potentially low flow, such as regions surrounding the junction between impellor blades 28 and rotor hub 62. Preferably, where the pump is used as a blood pump providing physiologically appropriate levels of blood flow, the cross-sectional flow areas are sized so that, at a nominal blood flow of 5 liters per minute, flow velocity through cross-section A is approximately 2 meters per second. This limitation of cross-sectional flow area through cross-section A also provides a down-stream constriction that encourages the formation and continuation of fluid swirl 44 in swirl zone 40.

[0028] Another feature for reducing low flow regions is illustrated in FIG. 3C which shows a cross-section taken along line C-C of FIG. 3B. FIG. 3C illustrates, in particular, the cross-sectional shape of an impellor blade 28 in a region proximate to hub 62. Impellor blade 28 is tilted as it emerges from hub 60 so that it matches the velocity 64 (having lateral 66 and vertical 68 components) of the blood or other fluid as it approaches the impellor vanes 28. In this way, the flow of fluid over the impellor blades 28 near their junction with hub is encouraged, as well as the downward flow of fluid into swirl region 40. This configuration also tends to avoid dead spots or areas of potential stagnation on the back side of impellor blades 28.

[0029] Another characteristic of rotary pumps addressed by the present invention is the location of, and fluid flow across, any gaps that might exist at junctions between rotating and stationary parts. While some rotary pumps will not have such gaps (for example those supported solely by hydrodynamic or magnetic bearings), pumps having shaft 30 and bearing assembly 22 arrangements will have a gap 46 between rotating parts (such as rotor 26 and shaft 30) and stationary parts (such as housing 12 supporting bearing assembly 22). In the embodiment illustrated in FIGS. 3 and 3A, housing 12 provides one surface 48 defining one side of gap 46 while rotor 26 and/or shaft 30 provide an opposed surface 50. Gap 46 terminates in a mouth 70 that is exposed to blood flow in the vicinity of swirl region 40. FIGS. 3 and 3A illustrate two different configurations for exposing mouth 70 to swirl region 40.

[0030] The distance across gap 46 (denominated as A in FIG. 3, and B in FIG. 3A) should be small enough to maintain reasonable efficiency and to keep shear rates large enough at surfaces 48, 50 to prevent thrombosis. The size of gap 46, along with the amount of washing provided over mouth 70, will determine the size of any clots that may be released into the blood flow as a result of any blood trapped in a volume around rotor 26. By providing and enhancing swirl region 40 and exposing mouth 70 to swirl region, the size of any released clots can be minimized for practical gap 46 sizes. Distance A across gap 46 of FIG. 3 may be made as small as the tolerance on radial bearings 32 will allow, typically on the order of 25 microns. An axially configured gap distance (distance B across gap 46 of FIG. 3A) can be as small as the wear expected on axial bearing 34 over the expected life of pump 10. Distance B can be effectively reduced (and under some circumstances made effectively zero) by either allowing the materials lining gap B to wear at the same rate as axial bearing 34, or by forming an axial thrust hydrodynamic bearing across gap B by machining wedges into the surface of one or the other of the surfaces opposed across the gap.

[0031] In the illustrated embodiments, no seal is provided to isolate bearing 32 surfaces from blood, although a person of ordinary skill in the art will recognize that a seal could be put in place if needed. The spaces around bearings 32 may be made small enough so that there is no significant volume for infection. Further, blood products that deposit around the shaft 30 likely will not add to bearing 30 friction or wear.

[0032] Exemplary bearing assembly 22, illustrated in place within a blood pump of the invention in FIGS. 1 and 1A, is illustrated in greater detail in FIG. 4. Bearing assembly 22 supports shaft 30 of rotor 26, and accordingly it supports rotor impellor 28 and provides forces to constrain the position of the impellor axially and radially. In the exemplary bearing assembly, a set of radial bearings 32 having a toroidal shape is held in place by spacers 72 along shaft 30, as shown in FIG. 4. An axial thrust bearing 34 located beneath shaft 30 takes up the thrust generated by the electro-magnetic driver provided by rotor 26 mounted permanent magnets 54 (FIG. 1) and electric coils 52 mounted in housing 12.

[0033] Three possible configurations for axial bearing 72 are shown in FIGS. 4A-4C. One configuration (the “sliding” embodiment of FIG. 4A; also the configuration illustrated in FIG. 4) includes a rotating contact area 80 disposed on the end of shaft 30 which slides on a stationary (i.e., housing affixed) pedestal 82. In the illustrated embodiment, the diameter of shaft 30 is 0.05 inches, however, this diameter may be increased in order to provide a larger contact area 80 and thus improve axial wear rates. In one exemplary pairing of bearing materials, at least contact area 80 of shaft 30 can be formed of zirconia, a ceramic, to slide on a pedestal 82 formed of sapphire. In another potential combination, a zirconia contact area 80 can slide against a pedestal 82 of ultra-high molecular weight polyethylene (UHMWP). This combination of materials is known to have a very low wear rate.

[0034] A second illustrated axial bearing 34 configuration (the “rolling” configuration of FIG. 4B) is one in which a rolling ball 84 is moveably disposed between the shaft 30 and a housing affixed pedestal 86. For typical sizes, speeds and loads as described herein, linear velocities between moving parts in this configuration may be as low as 50 mm/s and axial loads near 1 Newton.

[0035] A third potential axial bearing 34 configuration (the “magnetic” configuration of FIG. 4C) provides an axial bearing thrust bearing 34 using a pair of small permanent magnets 88, 90. In this embodiment, magnet 88 is affixed to the end of shaft 30 and magnet 90 is affixed to the housing. It is an important consideration for this axial bearing configuration that radial bearings 32 hold shaft 30 precisely on center so that magnets 88, 90 provide the desired axial forces.

[0036] Each of the three illustrated axial bearing 34 configurations should provide for absorption of axial shocks. In the sliding (FIG. 4A) and rolling (FIG. 4B) embodiments, axial shocks can be absorbed by providing a relatively soft material to support pedestals 82, 86. In the case of the magnetic axial bearing (FIG. 4C), damping of axial shocks occurs as fluid is pushed in and out of a gap 92 between magnets 88, 90. It is also important to dissipate heat that may be generated by radial 32 and/or axial 34 bearings having contact surfaces that may generate frictional heat by directing that heat to an area of the pump that is well washed by moving blood. This can be accomplished by making sure that the external surfaces of metal parts in thermal contact with such radial bearings reach locations within blood flow conduit 18 (FIG. 1A) having areas of high surface shear with respect to blood flowing over the surface.

[0037] Referring again to FIG. 1, pump 10 can be assembled by inserting rotor 26 into bearing assembly 22 which can be located in a bottom portion 96 of housing 12. A top portion 94 of housing 12 can then be disposed on bottom portion 96 and fastened there by bolts 98. In addition to gap 46, this construction of housing 12 further results in the creation of seam 100. Preferably, this seam is located within 0.5 mm of a surface of impellor blades 28 so that seam 100 is continuously washed at the point where it contacts blood.

[0038] A person of ordinary skill in the art will appreciate further features and advantages of the invention based on the above-described embodiments. Accordingly, the invention is not to be limited by what has been particularly shown and described, except as indicated by the appended claims. All publication and references cited herein are expressly incorporated herein by reference in their entity.