Hydrodynamic thrust bearings for rotary blood pump

Abstract

A rotary blood pump includes a casing defining a pumping chamber. The pumping chamber has a blood inlet and a tangential blood outlet. One or more motor stators are provided outside of the pumping chamber. A rotatable impeller is within the pumping chamber and is adapted to cause blood entering the pumping chamber to move to the blood outlet. The impeller has one or more magnetic regions. The impeller is radially constrained in rotation by magnetic coupling to one or more motor stators and is axially constrained in rotation by one or more hydrodynamic thrust bearing surfaces on the impeller.

Claims

1. A rotary blood pump comprising: a pumping chamber in fluid communication with a primary fluid flow path; an impeller rotatable on an axis within the pumping chamber and having a plurality of upper surface areas circumferentially disposed about the axis, each upper surface area facing an interior wall of the pumping chamber as the impeller rotates, each of at least a diametrically opposed pair of the upper surface areas being configured with an inclined surface area tapered in an upward axial direction and defining a hydrodynamic bearing surface having a lower pressure fluid entrance end and a higher pressure fluid exit end causing an increase in pressure acting axially downwardly on the impeller as the impeller rotates; and each of the plurality of upper surface areas further including a pressure relief surface downstream of the higher pressure fluid exit, the pressure relief surface being configured to lower the hydrodynamic pressure from the high pressure fluid exit to form a lower pressure fluid exit end and to define a secondary fluid flow path to impel fluid into an adjacent upper surface area as the impeller rotates, the higher pressure entrance end of the downstream pressure relief surface being spaced from the higher pressure fluid exit end of the hydrodynamic bearing surface by a flat bridging surface area of the same respective upper surface area therebetween, the lower pressure fluid exit end and the lower pressure fluid entrance end of adjacent ones of the upper surface areas engaging axially directed spaced apart sidewalls having unequal surface areas, the sidewalls defining fluid flow channels therebetween.

2. The rotary blood pump of claim 1 in which an angle of inclination of each hydrodynamic bearing surface is less than one degree relative to the horizontal.

3. The rotary blood pump of claim 1 in which the flat bridging surface is about 0.050 inches wide at its narrowest point with a tolerance of ±0.028 inches.

4. The rotary blood pump of claim 1 in which an angle of taper of each pressure relief surface is more severe than the angle of inclination of each hydrodynamic bearing surface.

5. The rotary blood pump of claim 1 in which each of the upper surface areas defines a hydrodynamic bearing surface and an associated pressure relief surface.

6. The rotary blood pump of claim 1 in which each upper surface area is formed on one of a plurality of raised bodies, a selected portion of one or more of which is configured for magnetization.

7. The rotary blood pump of claim 1 comprising a one-piece ferromagnetic impeller made from an alloy of approximately 77.6% platinum by weight and 22.4% cobalt by weight.

8. The rotary blood pump of claim 6 in which each raised body is configured with two straight sidewalls of unequal length which intersect at approximately 90°.

9. The rotary blood pump of claim 8 in which the longer side wall faces the shorter side wall of an adjacent raised body across a respective secondary fluid flow path therebetween.

10. The rotary blood pump of claim 6 in which each hydrodynamic bearing surface comprises an inwardly facing and downwardly tapered concave inner wall section of a raised body.

11. The rotary blood pump of claim 10 in which an angle drop for each such downwardly tapered concave inner wall section is about 0.012 inches and an angle of downward taper thereof is about 8°.

12. The rotary blood pump of claim 6 in which a cavity in each raised body is fitted with a permanent magnet, the permanent magnets being approximately 90° apart at a periphery of the impeller with solid wall members therebetween.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) For a further understanding of the present invention, reference may be had to the accompanying drawings from which the nature and attendant advantages of the invention will be readily understood, and in which:

(2) FIG. 1 is an exploded view of a rotary blood pump according to an embodiment of the present invention;

(3) FIG. 2 is a perspective view of the outer surface areas of an impeller according to an embodiment of the present invention;

(4) FIG. 3 is a perspective view of a section of the outer surface area of the impeller of FIG. 2 which contains a hydrodynamic bearing surface;

(5) FIG. 4 is a perspective view of the underside of the impeller of FIG. 2;

(6) FIG. 5 is a cross-sectional view of an assembled rotary blood pump according to an embodiment of the present invention;

(7) FIG. 6 is a sectional view of a portion of a passive magnetic bearing structure for an impeller according to an embodiment of the present invention;

(8) FIG. 7 is an exploded view of a magnetic assembly for supporting and driving an impeller according to an embodiment of the present invention;

(9) FIG. 8 is a top plan view of a motor stator according to an embodiment of the present invention; and

(10) FIG. 9 is a system view of an implanted rotary blood pump according to an embodiment of the present invention.

(11) FIG. 10 is a top plan view of a rotary blood pump according to an embodiment of the present invention to which a pliable cylinder carrying power cables is attached.

(12) FIG. 11 is a cross section of the pliable cylinder of the present invention illustrating a plurality of lumen therein for carrying power cables.

DETAILED DESCRIPTION OF THE INVENTION

(13) In describing the embodiments of the present invention illustrated in the drawings, specific terminology is employed for sake of clarity. However, the present disclosure is not intended to be limited to the specific terminology so selected, and it is to be understood that each specific element includes all technical equivalents which operate in a similar manner.

(14) Referring now to FIG. 1 there is shown a rotary blood pump 10 having a pump housing that consists of a substantially circular front or upper pump casing 1 and a substantially circular rear or lower pump casing 2 of equal diameter that interlocks with the upper pump casing 1 to form a closed pumping chamber between them. The configuration of the upper and lower pump casings is such that the assembled pump housing defines a substantially cylindrical pumping chamber 3 therein (FIG. 5). In one embodiment the pumping chamber has a displaced volume of 45 cc. The upper pump casing 1 may have a plurality of peripheral positioning holes 4 for receiving a corresponding plurality of positioning pins 6 projecting from the periphery of the lower pump casing 2. The configuration of positioning holes 4 and positioning pins 6 ensures that the upper pump casing 1 and the lower pump casing 2 interlock in the correct position when the rotary blood pump 10 is assembled. The contact area between the upper pump casing 1 and the lower pump casing 2 may be sealed, for example using screws or a chemical sealant.

(15) In the embodiment shown in FIG. 1 blood is supplied to the pump through an axial inlet cannula 7 adapted for apical insertion into a heart ventricle. The cannula 7 is affixed to or may be integral with the upper pump casing and is in fluid flow communication with the pumping chamber 3 of the pump 10. As shown in the cross sectional view of FIG. 5, an embodiment of the inflow cannula 7 is of two-piece design, consisting of an outer cylindrical section 8 and a coaxial inner cylindrical section 9. The outer cylindrical section 8 of the inflow cannula 7 maybe welded in an appropriate sealable manner to the outer surface of the upper pump casing 1. The inner cylindrical section 9 defines an inlet channel 11 for the blood when the pump is installed and in operation. The sections 8 and 9 may be laser welded together at the outer end 12 of the cannula, shown in FIG. 1. In one embodiment, the outside diameter of the outer section 8 is about 0.81 inches while the inside diameter of the inner section 9 is about 0.50 inches.

(16) In one embodiment, the pumping chamber is in fluid-flow communication with a volute or diffuser section to avoid alteration of the position of the impeller in a radial direction as blood pressure increases during operation of the pump. The upper pump casing 1 and lower pump casing 2 together define the diffuser by a pair of complementary upper and lower half-round sections 14 and 16 formed as part of the upper and lower housing casings, respectively. The sections 14 and 16 together define a short open-ended cylindrical diffuser tube. The diffuser extends completely around the circumference of the pump terminating at a tangential outlet port 13 (FIG. 5). In one embodiment, the cross section of the diffuser section enlarges from an inlet end along its length to a maximum at the outlet 13. Blood exits the pumping chamber 3 through the outlet 13 in a direction substantially perpendicular to the longitudinal axis of the inlet cannula 7, an arrangement that has been found to be anatomically advantageous for locating the pump in the pericardial space. When the pump is installed and in operation, the outlet 13 is adapted to be joined to an outflow graft 17, shown in FIG. 9, which in turn is suitably connected to the aorta 18. In one embodiment, the pump housings or casings and the cannula may be made of titanium, a biocompatible titanium alloy, or a biocompatible ceramic material. The pump structure may be machined from titanium or an alloy thereof. Alternatively, the pump structure, including the cannula, may be formed entirely from ceramic material.

(17) Sealing of the cannula 7 to the heart ventricle may be accomplished with the assistance of a peripheral ring groove 19 (FIG. 5) formed in the outer cylindrical surface of the cannula near the upper pump casing 1. The ring groove is fitted with an annular O-ring to provide a leak proof seal to a sewing ring of a ventricular connector [not shown] of the type described, by way of example, in commonly owned U.S. Pat. No. 6,732,501. According to another embodiment, a peripheral ring groove is unnecessary and an O-ring surrounding the cannula may be incorporated into the sewing ring to ensure a leak proof seal.

(18) With reference to FIG. 1, a motor rotor or pump impeller 22 is located within the pumping chamber 3 between the upper pump casing 1 and the lower pump casing 2. The impeller 22 is circular in cross section and may have a diameter of an inch or an inch and a quarter. The impeller is provided with a central hole 23. A center post or spindle 24 is attached to the lower pump casing 2 and protrudes from the axial, center thereof through the impeller hole 23 when the pump is assembled to support rotation of the impeller in the manner described in detail below. The center post 24 is provided with a peripheral lower flange 26 by which a lower annular ceramic disc 27 is retained to an interior surface of the lower pump casing 2. In one embodiment, the gap between the outer diameter of the center post 24 and the diameter of the impeller hole 23 is in the range of from 0.019 inches to 0.029 inches. The top portion of the center post 24 is formed as a conical surface 28. A substantial portion of the conical, surface 28 of the center post protrudes above the impeller hole 23 during operation of the pump. In one embodiment, the radius of curvature of the cone shape is a relatively constant 0.389 inches. The tip of the cone is not necessarily a sharp point having, in one embodiment, a blending radius of 0.010 inches.

(19) In operation, blood entering the cannula 7 from a heart ventricle passes axially over the conical surface of the center post 24 into the pumping chamber 3 where it is engaged by the rotating impeller. Blood entering the pumping chamber from the cannula 7 is redirected from axial flow exiting the cannula to a radial flow within which the impeller 22 is submerged. The rotating impeller presses the blood radially into a whirling motion as a result of the configuration of the spinning impeller, described in detail below, and moves within the diffuser at the perimeter of the pumping chamber to the outlet 13.

(20) The upper pump casing 1 may contain the upper half 29 of an electrical feed through connector and header for a power and control cable to supply power to the electrical motor of the pump. The lower pump casing 2 may contain a corresponding lower half 31 of the electrical header. When the pump is assembled, the upper and lower halves 29 and 31 interlock to form the header through which feed-through power wires are connected to the electromagnetic motor stators. In one embodiment, the feed-through wires are platinum. A PEEK header may be used to connect the feed through wires to the external drive cables. The header may be made of a material such as PEEK or a suitable plastic such as Tecothan or polysulfone. The header may also be made of a medical grade epoxy. With reference to FIG. 10, the upper half 29 of a PEEK front pump header is shown connected to the power line cable through a strain relief section 81. The strain relief section is, in turn, connected to a pliable elongated cylinder 82 which may be as long as desired to reach a suitable external power source, which may be the output of a controller (not shown). A connector and locking plug device 83 for connection to the power source is affixed to the tubing 82 at its distal end. The pliable cylinder is adapted to carry a plurality of power cables to carry electrical power to the pump. In one embodiment the pliable cylinder is made of silicone. The pliable cylinder may be covered by a thin urethane sheath (not shown) for extra abrasion resistance. It will be understood by those skilled in the art that other bio-compatible materials such as urethane may be used for the pliable cylinder without departing from the scope of the present invention. Referring to FIG. 11, the pliable cylinder 82 contains a plurality of lumens 84 having circular cross sections, through each of which individual power cables are threaded. In one embodiment there are six such lumens spaced around the center of the cylinder adjacent its periphery in a generally circular configuration. The centers of each lumen are approximately 60° apart. In one embodiment, the diameter of the pliable cylinder is about 0.138 inches and the diameter of each lumen is approximately about 0.034 inches. Such lumens can be used to hold power cables having a diameter of about 0.029 inches. The use of individual lumens within the cable tubing 82 has the benefit of extra fatigue resistance because the individual power cables cannot rub together. In addition, when replacement is necessary replacement in situ is enabled since one power cable at a time may be replaced in order to minimize any stop time of the pump. A strain relief mechanism 86 may be used adjacent the distal connector and locking plug device.

(21) Referring now to FIG. 2 the impeller 22 is shown in greater detail. In this embodiment, the impeller is substantially circular in cross section and has a plurality of identical substantially hollow raised bodies 32 circumferentially arranged thereon. Each of the raised impeller bodies 32 has a generally right triangular cross section in a horizontal plane, with a curved hypotenuse defining a portion of the circumference of the impeller. In one embodiment there are four such raised impeller bodies, the mid points of which are approximately 90 degrees apart.

(22) The raised impeller bodies 32 are separated by flow slots or channels 33 adapted to permit the flow of blood from the central portion of the impeller to the surrounding pumping chamber. In one embodiment, the width of each of the slots 33 is about 0.150 inches. The flow slots 33 are defined by vertical planar sidewalls 33a and 33b of unequal length extending parallel to but offset from a diameter of the impeller. In one embodiment, the sidewall closest to the diameter of the impeller, for example the sidewall 33a of FIG. 2, is offset from the diameter by about 0.164 inches. Each of the slots 33 has a downward-sloping bottom surface 33c, which constitutes an inclined ramp forming an angle of about 32 degrees with the horizontal. The exit points of the flow slots 33 at the circumference of the impeller are approximately 90 degrees apart. Each ramp surface 33c is longitudinally at right angles with the corresponding longitudinal axis of the flow slots on either side.

(23) The primary flow path for blood entering the inflow cannula 7 is to strike the conical surface 28 of the center post 24 and pass through the flow slots or channels 33 to fill the pumping chamber. As indicated, the rotating impeller causes the fluid pressure in the pumping chamber to increase resulting in continuous movement of the blood from the inflow 11 to the outflow port 13.

(24) The upper surface of each impeller block 32 is provided with a curved and tapered or inclined ramp 34 defining an axial hydrodynamic bearing surface. In one embodiment, each ramp surface 34 spirals upward in a clockwise direction from a relatively lower fluid pressure entrance region 36 to a relatively higher fluid pressure exit region 37. The angle of inclination of the bearing surface 34 is less than one degree relative to the horizontal. When the impeller 22 is rotating, the sidewalls 33a define leading edges so that blood passing over the hydrodynamic bearing surfaces is compressed with increasing force against the adjacent interior surface of the upper pump casing 1 with result that a net axially downward pressure is exerted on the upper projection surface of each raised impeller body. In operation, the thickness of the blood layer between the bearing surfaces 34 and the adjacent housing surface is a function of the fluid viscosity, the impeller rotational speed and the geometry of the impeller bearing. As the fluid viscosity increases the fluid layer thickness increases. As the rotational speed increases the fluid layer thickness increases and, because of the net axial hydrodynamic pressure on the impeller and the fact that the impeller is suspended within the pumping chamber in part by a magnetic preload described below, the distance from each bearing surface 34 to the adjacent upper casing face can change with rotational speed and fluid viscosity. However, in one embodiment that distance will be within the range of from 0.003 inches to 0.020 inches.

(25) Each raised impeller body 32 may also have wedge-shaped region forming a pressure relief surface 38 downstream of the bearing surface 34. The pressure relief surface 38 ensures a controlled and predictable lowering of the hydrodynamic pressure to minimize the blood shear stress and hemolysis. In addition, each pressure relief surface assists in defining a secondary flow path for blood within the pumping chamber whereby blood exiting a bearing surface 34 is re-entrained across the adjacent pressure relief surface into the next downstream impeller flow slot or channel 33, and from there into a lateral annular space defining the diffuser portion of the pumping chamber.

(26) A relatively flat surface area on the upper surface of each impeller body defines a substantially planar bridging surface 39 between each exit end 37 of a bearing surface 34 and the associated pressure relief surface 38. In one embodiment, the width of each of the bridging surfaces 39 at its narrowest point is about 0.050 with a reasonable tolerance of ±0.028 inches. In such an embodiment, the pressure relief surface 38 may be inclined relative to the horizontal at an angle of from 2 to 4 degrees.

(27) Referring now to FIG. 3 there is shown in perspective one of the hydrodynamic bearing surfaces 34. Each bearing surface is of approximately uniform width from the entrance region 36, which defines a junction edge 41 with the leading substantially vertical sidewall, for example sidewall 33a of the slot 33 (FIG. 2), to the exit region 37. In one embodiment, the junction edge 41 is relatively sharp, having a maximum radius of curvature of less than 0.010 inches, and may be as small as 0.005 inches or smaller. As indicated, each bearing surface 34 is inclined upwardly from the entrance end 36 at an angle of less than 1 degree relative to the horizontal and terminates at approximately the flat bridging surface 39.

(28) In one embodiment, each bearing surface 34 is bounded along its length on opposite sides by inner and outer shrouds 43 and 44, respectively. The outside surface of the outer shroud defines a portion of the peripheral surface of the impeller. In operation, the inner shroud 43 and the outer shroud 44 effectively minimize the fluid leaking out of the sides of the bearing surfaces thereby assisting the retention of blood engaging the bearing surface to maximize the fluid layer thickness and minimize the fluid shear stress. The shrouds also serve to guide the blood toward the exit end 37 of the bearing surface from which it flows over the pressure relief surface 38 and into the next downstream flow slot 33. The top surface of each of the shrouds 43 and 44 is relatively planar or flat and, in one embodiment, each has a width of not less than 0.020 inches. The top surface of each of the shrouds 43 and 44 may be higher than the entrance end 36 of the bearing surface 34 by about 0.230 inches. At the exit end 37 of the bearing surface, the top surface of the shrouds 43 and 44 and the bearing surface may merge into the planar bridging surface 39.

(29) In one embodiment, there is formed on each of the raised impeller bodies 32 an inwardly facing and downwardly tapered curved section 46 inside of the inner shroud 43. An axial drop distance for each section 46 is about 0.012 inches and the angle of taper is about 8°. The section 46 assists in directing blood deflected from the conical surface 28 of the central post 24 to the central portion of the impeller, which then flows from there into the slots 33 formed between the impeller bodies 32.

(30) The inner surface of the upper pump casing 1 is provided with an upper annular ceramic disk (not shown) similar to the lower ceramic disc 27 on the inner surface of the lower pump casing 2. The upper ceramic disk serves to minimize friction on start-up of the pump. An annular flange 40 formed at the inner end of the inner cylindrical section 9 of the cannula 7 (FIG. 5) serves to retain the upper ceramic disc in place. The ceramic disks reduce electrical losses between the motor stators (described below) and the rotor magnets within the impeller, as well as provide very flat surfaces for the hydrodynamic thrust bearings on the impeller top surface. When the impeller is at rest it sits against the surface of the upper ceramic disc. When rotational speed is imparted to the impeller during startup, the impeller lifts off of the upper ceramic disc and becomes fully suspended as described below. The impeller may be coated with titanium nitride to minimize wear during the starting and stopping process of the pump.

(31) The impeller may be a single integral structure made of a magnetically isotropic alloy. The material of a one-piece impeller of the type described above may be biocompatible to avoid having to coat the impeller or sub-assemblies. An example of a suitable magnetically isotropic biocompatible material is an alloy of approximately 77.6% to platinum (by weight) and 22.4% (by weight) cobalt. Such a one-piece impeller may be easier and less expensive to manufacture than impellers formed from multiple parts. Each raised impeller body 32 may have a magnetized portion. Magnetization of such an impeller may be performed by techniques known in the art, such as the exposure to a relatively strong magnetic field. In one embodiment, the raised projection surfaces of each of the impeller bodies may be magnetized to provide magnetic poles. The magnetic poles of the impeller couple magnetically with magnetic poles provided by motor stators 69 (FIG. 5) thereby enabling one or both of the stators to provide both a magnetic drive force to cause the impeller to rotate within the pumping chamber and magnetic axial and radial support. In one embodiment, every other upper projection surface is magnetized to the same magnetic pole while the projection surfaces therebetween are magnetized to have the opposite magnetic pole. For example, where an upper projection surface has a North magnetic pole each projection surface on either side has a South magnetic pole. The particular arrangement of magnetic poles may be determined as desired without departing from the scope of the present invention. It will be understood that the motor stator coils that drive the impeller provide magnetic poles in a pattern complementary to those employed on the impeller.

(32) Referring now to FIG. 4, there is shown in perspective the underside of the impeller 22 in which each raised impeller body 32 is hollowed out to define a plurality of interior cavities or pockets 47. In cross section, each pocket 47 substantially corresponds in size and shape to the raised impeller body which defines its boundaries. The upper projection surface of each such raised impeller body contains a hydrodynamic bearing surface which defines the top of the interior cavity therebeneath. In one embodiment, the outer curved boundary of each of the pockets is concentric with the impeller and subtends an angle relative to the center of the impeller of about 56.5 degrees. The inner radius of each such pocket relative to the center of the impeller is about 0.4 inches and the outer radius is about 0.665 inches. The pockets are located about 90 degrees apart around the periphery of the impeller. As described in detail below, the pockets 47 are adapted to receive rotor magnets forming part of the motor drive system for the impeller. The pockets 47 are separated by a plurality of substantially equally sized inwardly projecting wall members 48 integrally formed with the impeller and defining substantially horizontal flat lower surfaces or shelves 49 terminating radially inwardly at curved edge portions 51. In one embodiment, there are four such wall members, each of which is situated between two pockets. Each wall member and pocket is situated diametrically opposite a corresponding wall member or pocket. The edge portions 51 define a boundary of a substantially vertical inwardly facing curved surface 52, substantially concentric with the circumference of the impeller.

(33) A hollow cylinder 53 projects axially inwardly and defines the central hole 23 of the impeller. In one embodiment, the central hole has a diameter of about 0.437 inches. When the pump is assembled, the center post 24 extends through the cylinder 53 into the pumping chamber. In one embodiment, the radial gap between in the inner diameter of the cylinder 53 and the outer diameter of the center post 24 is about 0.022 inches.

(34) An annular cavity or space 54 is formed between the hollow cylinder 53 and the curve surfaces 52. In this embodiment, the annular cavity 54 has an inside diameter of about 0.437 inches, an outside diameter of about 0.575 inches, and is adapted to receive passive magnetic bearing components, as described in detail below.

(35) Each of the wall members 48 may be provided with one or more balancing holes or bores 56 which are formed to ensure a balanced and even rotation of the impeller during operation of the pump. In one embodiment each wall member is provided with a set of two balancing holes of unequal depth and approximately equal diameters situated side-by-side along a radius of the impeller. In this embodiment, the depth of the balancing hole closest to the center of the impeller is about 0.10 inches, while the depth of the outermost balancing hole is about 0.25 inches. Each set of holes is situated diametrically opposite another set, whereby the diametric distance between the outermost holes of two opposite sets of holes is about 1.22 inches and the diametric distance between the inner most holes of the sets is about 1.02 inches.

(36) With reference to FIG. 5, there is shown a cross-sectional view of an assembled rotary blood pump according to an embodiment of the present invention. The upper casing 1 has affixed thereto the inflow cannula 7 with its inlet channel 11. The outflow port 13 is formed by joinder of the half-round tubular extensions 14 and 16. The center post 24 extends upwardly into the pumping chamber through the bottom of the lower casing 2.

(37) With reference to FIGS. 5 and 6, in one embodiment an impeller suspension system utilizes a passive magnetic bearing to provide radial impeller support with respect to the center post 24. The passive magnetic bearing is adjustable to provide an axially directed magnetic preload adapted to be resisted by the forces generated by the hydrodynamic thrust bearings described above in connection with each of the impeller blades 32. In one embodiment, one portion of the passive magnetic bearing is formed by a stack 56 of permanent bearing magnets 57 enclosed within the center post 24. The stack 56 may consist of three ring-shaped permanent magnets 57 placed one on top of the other and coaxially aligned along the axis of rotation of the pump impeller 22. Each of the ring magnets 57 has an axial height of less than 0.10 inches, and an outer diameter of about 0.34 inches.

(38) In one embodiment, and as seen best in FIG. 6, each of the three center post bearing magnets 57 may provide a magnetic vector oriented in the axial direction, for example either north-on-top, south-on-bottom (N-S) or south-on-top, north-on-bottom (S-N). Thus, the stack of center post bearing magnets 57 may have alternating magnetization such that the polarizations of the magnets within the stack may be N-S, S-N, N-S or S-N, N-S, S-N, as desired, whereby the magnetic forces established by each ring shaped magnet 57 of the stack 56 act to repulse its adjacent magnet in the axial direction.

(39) As there are repulsive forces between each magnet, the magnets may be fixed to or otherwise mechanically held in their coaxial relationship by suitable engagement with an axially positioned center post rod 58. To ensure that the ring magnets are held in place, each magnet may be provided with a thin ring-shaped spacer or washer 59 on the top and the bottom of the magnet, the upper most spacer being engaged beneath a protruding circular flange 61 formed near the top of the center post rod 58 to assist in holding the magnets in their coaxial arrangement. The spacers 59 may also function to minimize demagnetization caused by the proximity of the stacked magnets. In one embodiment each such spacer would have a thickness of less than 0.015 inches. Alternatively, where desired, the spacers may be adapted to act as flux concentrators for re-directing and concentrating in the radial direction the magnetic flux produced by the magnets 57. Alternative embodiments for the magnetic vectors of the permanent magnets forming the stack 56 within the central post 24 may be employed without departing from the scope of the present invention. For example, the N-S orientations may be radial, with North on the left and South on the right.

(40) The other portion of the passive magnet bearing for the impeller is formed by another stack 62 of ring-shaped permanent magnets 63 placed within the impeller and surrounding the cylinder 53. The stack 62 may consist of three ring-shaped permanent magnets 63. As shown in FIG. 6, each impeller bearing magnet 63 has a magnetic vector oriented in the axial direction with, for example, either north-on-top, south-on-bottom (N-S) or south-on-top, north-on-bottom (S-N). In one embodiment, the magnet pole arrangement of the stack of impeller magnets 63 corresponds to the magnetic pole arrangement of the stack of center post bearing magnets 57. Thus if the stack of center post bearing magnets 57 has its magnetic vectors oriented N-S, S-N, N-S then the magnetic vectors of the adjacent stack 62 of impeller magnets 63 may also be N-S, S-N, N-S. Provided there is a sufficient radially oriented magnetic flux concentration, such an arrangement of magnetic vectors, and others, would effect repulsive forces between the corresponding stacks 56 and 62, thereby establishing, in operation, a radially acting magnetic bearing between the rotating impeller and its fixed center post. In one embodiment, the inner and outer diameters of the ring-shaped magnets 63 within the impeller are about 0.44 inches and 0.52 inches, respectively, while the radial distance between the ring-shaped magnets 63 within the impeller and the ring-shaped magnets 57 within the center post 24 is about 0.050 inches.

(41) With reference to FIG. 5, in one embodiment, the axial alignment of the center post magnet stack 56 with respect to the impeller magnet stack 62 may be adjustable so as to provide a selected axial preload force that biases the impeller toward the upper casing 1. In one embodiment, the flange 26 on the center post 24 holds the center post in position relative to the lower pump casing 2. The center post rod 58 extends upwardly through the center post and is axially movable within the center post by an appropriate adjustment screw 66 which threadably engages the lower end of the rod 58. Appropriate thread density could be on the order of 64 threads per inch.

(42) The adjustment screw has a cap 67 engageable from beneath the impeller to adjust the axial position of the center post rod 58 and thereby the alignment of the impeller and center post bearing magnets. Thus, the center post rod 58 may be moved downwardly, for example, thereby moving the center post magnet stack 56 downwardly relative to the impeller magnet stack 62, as shown in FIGS. 5 and 6. When the misalignment between the corresponding magnet stacks 56 and 62 reaches approximately that shown in FIGS. 5 and 6, it will be apparent that repulsive forces between the N-S, S-N, N-S magnetic vectors of the impeller stack and the N-S, S-N, N-S magnetic vectors of the center post stack will provide a preload axial force that biases the impeller toward the upper pump casing 1 and assists in keeping the impeller running near the inner surface of the upper casing. When the desired magnet alignment is established, the cap 67 may be welded to the center post to establish a hermetic seal and to prevent inadvertent movement of the adjustment screw. The adjustment screw is thereby sealed at the outside surface of the lower pump casing 2. Other mechanical arrangements suitable for adjusting the axial position of the stack 56 may be adopted without departing from the scope of the invention.

(43) When the pump is activated, the axial upwardly directed magnetic preload force caused by the offset between corresponding stacks of bearing magnets is balanced against the downward force in the axial direction created by the hydrodynamic thrust bearings on the impeller upper surface. Therefore, the impeller may be suspended in both the axial and radial directions and is submerged within the blood filling the pumping chamber. The inner and outer magnet bearing assemblies 56 and 62 thus work together to provide primary radial and axial stiffness to avoid wear and to ensure the presence of yet another open flow path for the blood being moved through the pump. This flow path is from the housing where the fluid collects after exiting the impeller flow slots 33, underneath the impeller and up through the annular gap between the center post and the impeller that is maintained by the passive magnetic bearing described above, from where the blood is re-entrained through the impeller flow slots 33 into the primary flow path described above. The impeller hydrodynamic thrust bearings described above provide axial stiffness only when the impeller is running near the inner surface of the upper casing 1.

(44) As indicated above, the pump of the present invention may include a three-phase dual stator axial flux gap motor for driving the impeller. An advantage of a dual stator motor is that one of the stators may be used to cause the impeller to rotate should the other stator fail to function. In one embodiment, the lower stator is spaced farther from the impeller 22 than the upper stator so as not to degrade a net axial preloading of the impeller resulting from its magnetic interaction with the upper stator. With reference to FIGS. 5 and 7, the impeller is provided with a set of four drive magnets 68. Each drive magnet 68 is contained within one of the pockets or cavities 47 (FIG. 4) formed within the raised portions 32 of the impeller at the underside of the impeller. The drive magnets 68 are enclosed within the impeller by a suitable annular base plate 70.

(45) As shown in FIG. 5, one stator is located above the impeller on the upper pump casing 1 while the other stator is positioned below the impeller on the lower pump casing 2. Each stator contains a plurality of the motor drive windings 69 and a back iron ring 71. The motor drive windings 69 consist of coils of electrically conductive wire and may be circular in cross section or have other appropriate cross-sectional configurations, as desired. In one embodiment shown in FIG. 8, the coils are circular in cross section and each stator consists of six such coils placed on the outside of the respective back ring. The coils are placed on the back ring such that the coil axis is perpendicular to the surface of the ring. As will be understood by those skilled in the art, the motor drive coils 69 generate electromagnetic fields that interact with the magnetic fields of the impeller drive magnets 68 to cause the impeller to rotate. The back iron ring 71 serves to enhance the magnetic flux produced by the drive magnets. The magnetic forces produced by the motor stator coils also provide secondary radial impeller and axial magnetic preloading support to the impeller. The result is that the impeller is dynamically balanced in both the radial and axial directions during normal operation. It will be understood that only a single stator is needed to operate the pump motor of the present invention. Two stator assemblies are desirable because if one stator assembly should fail, the other will operate the motor, although operating power consumption will be increased.

(46) Each stator is contained within a stator can 72, 73. Each stator can is hermetically sealed to its respective pump casing and, in one embodiment, has a thin wall less than 0.007 inches thick closest to the motor drive magnets 68. The thin wall allows the use of the ceramic discs between the impeller and the stators. Each stator can has a hermetic feed-through arrangement for the electrical connections to the conjoined external headers or connectors 29 and 31.

(47) FIG. 9 illustrates an implanted rotary blood pump according to an embodiment of the present disclosure. The inflow cannula 7 is inserted apically into the left ventricle 74 of the patient's heart 76. A blood transport graft or tube 17 connects the blood outlet of the rotary blood pump to the patient's aorta 18. The power and control cable 77 may be connected to a controller 78 having a power source 79. The controller 78 and the power source 79 may be implanted within the patient's body or worn by the patient. The controller is used to provide clinicians information on how the device is performing, to provide run status and alarm conditions and controls the rotational speed of the impeller, as may be desired. For example, impeller rotational speed may be controlled by using a pulsed drive waveform and measuring the back emf of the rotor when the drive pulse is at zero. Such a technique is set forth in commonly owned International Application No. PCT/US00/40325 having an International Publication Number WO 01/05023 A1, incorporated herein by reference.

(48) The above specific embodiments are illustrative, and many variations can be introduced on these embodiments without departing from the spirit of the disclosure or from the scope of the appended claims. For example, elements and/or features of different illustrative embodiments may be combined with each other and/or substituted for each other within the scope of this disclosure and appended claims.