Abstract:
The blood pump comprises a pump casing ( 10 ) in which an impeller ( 16 ) is installed without any bearing. Said impeller ( 16 ) is rotated via a magnetic coupling ( 32,36 ) by an external magnetic driving means ( 33 ). The impeller is radially centered via the magnetic coupling ( 32,36 ). The lower side ( 30 ) of the blades ( 19 ) of the impeller is configured as supporting surface ( 30 ) sloping towards the trailing end. In this way a hydrodynamical supporting effect is attained during rotation such that the impeller ( 16 ) raises from the bottom surface ( 12 ) of the pump casing ( 10 ). Since no bearings and sealings are provided on the pump casing the danger of thrombosis and the danger of penetration of foreign bodies in the form of abrasive particles into the blood is reduced.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a blood pump without bearing, operating according to the rotary pump principle, for temporary or long-term blood conveyance. 
     For temporary short-term blood conveyance extracorporeal blood pumps are used which comprise a rotationally driven impeller. Said impeller is supported on bearings in the pump casing. Examples of such blood pumps are described in EP 0 451 376 B1 and DE 43 21 260 C1. The impeller is driven via a magnetic coupling by a rotating rotor located outside the pump casing. The bearings supporting the impeller pose a problem in connection with the blood pump since thrombosis may occur at the bearings. Further, there is the danger of abrasive particles of the bearings contaminating the blood. Seals designed to protect the bearings against penetration of blood have also turned out to be unsuitable for the medium-term to long-term use (days to years). Blood pumps with mechanical support of the impeller are not suited for the long-term use for the aforementioned reasons. Pump systems having magnetic bearings (U.S. Pat. No. 5,385,581 A, DE 196 13 388 A1) which contactlessly support the impeller in an electromagnetic bearing means require a considerable controlling effort and a voluminous configuration because of the complex supporting structure where additional energy must be supplied to a large extent due to the active impeller centering. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a blood pump having a rotor rotating in a pump casing where the danger of blood contamination and thrombosis minimized. 
     This object is solved according to the invention with the features stated in claim  1 . 
     The blood pump according to the invention is a blood pump without bearing which is not provided with any mechanical bearings. The impeller is freely movable within a limited clearance in the pump casing. The impeller is rotated by an external magnetic driving means thus being self-centering. At least a front side of the blades comprises supporting surfaces which hydrodynamically lift the impeller during rotation. The static force of attraction of the permanent magnets in the impeller and the driving means tends to press the impeller against the pump casing wall facing the driving means. However, the supporting surfaces in the impeller cause the impeller to be lifted from the bottom surface during rotation such that the impeller slides on a blood cushion thus being kept at a distance from the wall. The impeller without bearing is passively centered in the pump casing via permanent magnets in combination with hydrodynamically acting driving forces. The lateral centering of the impeller is also effected by the magnets cooperating with the driving means. In this way it is possible to create a blood pump without bearing and shaft where the impeller is suspended in the pump casing. 
     The blood pump without bearing according to the invention offers the advantage that due to the fact that no bearings and sliding seals are provided the risk of thrombosis of the blood and penetration of foreign bodies into the blood is reduced. Thus the blood pump according to the invention cannot only be used as an extracorporeal blood pump for short-term application but also as an implantable blood pump for long-term operation. The blood pump is operable with high efficiency due to the low centering-induced losses wherein the required capacity lies in the range of 6 W under physiologically relevant operating conditions such that the pump has a long service life even when configured as a battery-operated portable device. 
     The impeller may comprise a straight continuous passage extending from the inlet to a bottom wall of the pump casing. Thus the impeller is provided with vanes on both sides. 
     Preferably, the impeller blades are arranged such that they protrude to opposite sides from the circumferential wall of a disk-shaped or cone-shaped supporting body. The impeller does not form a disk which would, together with the bottom wall of the pump casing, define a narrow gap. This also reduces the risk of thrombosis. In all areas of the pump casing a blood flow is maintained without there being the danger of dead water areas. 
     As seen from the top the blades are of essentially triangular configuration and comprise the blade-side magnets. The triangular form of the blades allows the blade volume to increase with increasing radius such that the fluid passage area available between the blades can be kept constant on all radii. Thus the conicality of the pump casing, which would be required to ensure that on all circumferential circles approximately the same volume is available, is reduced or eliminated. 
     The blood pump according to the invention is a centrifugal pump where the outlet is arranged essentially tangentially to the outer edge of the pump casing. Since the maximum pressure prevails in the outlet a radial force is produced which tends to press the impeller away from the outlet. To counteract this decentering force a peripheral ring diffusor is provided on the pump casing according to a preferred aspect of the invention, the ring diffusor ending in a tangential outlet. Said ring diffusor is a helical duct which causes the pressure prevailing in the outlet to be distributed over the circumference of the pump casing thus having a centering effect on the impeller. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Hereunder embodiments of the invention are explained in detail with reference to the drawings in which: 
     FIG. 1 shows a schematic longitudinal section across a first embodiment of the blood pump, 
     FIG. 2 shows a perspective view of the pump casing of the blood pump shown in FIG. 1, 
     FIG. 3 shows a view of the impeller of the pump shown in FIG. 1, 
     FIG. 4 shows another perspective view of the impeller of the pump shown in FIG. 1, 
     FIG. 5 shows a second embodiment of the blood pump, 
     FIG. 6 shows a perspective view of the pump casing of the pump shown in FIG. 5, 
     FIG. 7 shows a perspective view of the impeller of the pump shown in FIG. 5, 
     FIG. 8 shows a third embodiment of the blood pump, 
     FIG. 9 shows the pump casing of the blood pump shown in FIG. 8, and 
     FIG. 10 shows the impeller of the blood pump shown in FIG.  8 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The blood pump shown in FIG. 1 comprises a pump casing  10  having a truncated circumferential wall  11 , an essentially flat bottom wall  12  and a peripheral cylindrical wall  13  extending between said bottom wall  12  and said circumferential wall  11 . The blood is supplied via the axial inlet  14  to the pump casing and leaves the latter via the tangential outlet  15  on the outer casing circumference. 
     In the pump casing  10  an impeller  16  is rotatably arranged. Said impeller comprises a truncated supporting body  17  whose slope is approximately half as large as that of the circumferential wall  11 . The supporting body  17  is made of surface material of approximately identical thickness at all locations. On the supporting body  17  blades  18 , 19  protruding to the top and to the bottom are arranged wherein the upper blades  18  and the lower blades  19  are congruent as seen from the top, i.e. they have the same projection surfaces. 
     Said blades  18 , 19  are of triangular configuration as seen from the top and comprise a convex circumferential surface  20  coinciding with the circumferential circle of the supporting body  17 , a convex leading surface  21  leading in the direction of rotation, and a concave inner surface  22 . Said convex leading surface  21  coincides with the concave inner surface  22  at the inner edge  23 . The circle on, which lie the inner edges  23  of the three blade pairs, form the limit of a circular passage  24  arranged in axial extension of the inlet  14 . This means that the impeller  16  is open in its center such that a direct axial passage  24  extends down to the bottom wall  12  wherein a central raised portion  25  extending into said passage  24  is provided in the bottom wall  12 . The cross-section of the passage  24  is at least as large as that of the inlet  14 . 
     When the impeller rotates, the respective inner edge  23  precedes the outer edge  26  of the same leading surface  21 . This means that the leading surface  21  presses the medium radially to the outside by setting said medium into a swirling motion. The trailing edge  27  moves along the same path as the leading edge  26 . 
     The upper side  28  of the upper blades  18  moves in a truncated plane having the same cone angle as the circumferential surface  11  of the pump casing. Between the upper sides  28  of the blades and the conical circumferential surface  11  of the pump casing a gap is formed which provides the play required for axial movement of the impeller. 
     The lower sides of the lower blades  19  form supporting surfaces  30  which lift the impeller from the bottom wall  12  of the pump casing when the impeller rotates in the direction indicated by arrow  31 . Said supporting faces are formed in that on the lower side of the blade the lower edge of the leding surface  21  is positioned at a larger distance to the bottom wall  12  than at the trailing end, namely at the edge  27 . In this way a gap is formed between the supporting surface  30  and the bottom wall  12 , the gap decreasing towards the trailing end such that fluid in the gap tends to lift the impeller. Further, the vertical height of the gap above the bottom wall increases from the inner edge  23  towards the outside whereby the impeller is also radially centered. The inclination angle α of the supporting surface  30  in the circumferential direction is approximately 2 to 40°. 
     The blades  18 , 19  which are of triangular configuration as seen from the top are each provided with a magnet  32  with north pole N and south pole S. Said magnet extends through the two blades  18 , 19 . 
     The blood pump is driven by an external magnetic driving means  33  onto which the pump casing  10  is placed. Said driving means comprises a rotor  34  supported in bearings  35  and being provided with magnets  36  on its circumference. Each of said magnets  36  attracts a magnet  32  located in the pump casing  10 . The rotor  34  is rotated by stationary electromagnets  37 . Each electromagnet  37  comprises a U-shaped yoke through which passes a magnet  38  arranged on the circumference of the rotor  34 . The poles of the electromagnets  37  are cyclically changed such that they generate a rotating magnetic field carrying along the rotor  34 . Via the magnetically coupled magnets  32  and  36  the rotor  34  rotates the impeller  16 . All parts of the impeller  16 , with the exception of the magnets  32 , are made of plastic material or another nonmagnetic material. 
     The type of magnet arrangement of the rotor magnet  32  at the drive magnet  36  results in a radial centering of the impeller  16 . Thus  2  cartesian axes and  3  rotating axes are defined. The last remaining degree of freedom in the direction of magnetic attraction is fixed by the convergent gap formed between the supporting wall  30  and the bottom wall  12  and extending in circumferential direction. Thus the impeller, when rotating, raises from the bottom wall  12  against the magnetic attraction. When a sufficient circumferential velocity of the impeller has been reached, a blood film capable of bearing forms in the convergent gap and the impeller is suspended in the pump casing without mixed friction. 
     In the embodiment shown in FIG. 5 the casing  10   a  comprises a flat bottom wall  11  and a flat upper wall  11   a  extending essentially in parallel to the former. The supporting body  17   a , from which the blades  18 , 19  protrude to the top and to the bottom, is a flat disk. 
     According to FIG. 5 the external driving means  33   a  comprises electromagnets  40  distributed on the circumference of the pump casing  10   a  and generating a peripheral magnetic field. The yokes of the electromagnets  40  directly act upon the magnets  32  of the impeller  16   a . Here, too, the magnets do not only carry out the rotary drive of the impeller but also its radial centering. For axial centering of the impeller the blades are provided with an inclined supporting surface  30  on their lower side and with an inclined supporting surface  41  on their upper side, said supporting surfaces forming, together with the upper wall  11   a  of the pump casing, a convergent centering gap. 
     The blades  18 , 19  have the blade form shown in FIG. 7 deviating from that of the first embodiment in that the vanes are curved in forward direction as seen in the direction of rotation. In all cases the blades extend up to the passage  24  and the blade width (in circumferential direction) increases from the passage  24  towards the outside such that each blade has its maximum width at the edge of the supporting body  17  and  17   a , respectively. According to FIG. 6 the pump casing  10   a  generally has the form of a flat cylinder with a flat upper wall  11   a  and a cylindrical circumferential wall  13 . Since during rotation of the impeller  16   a  the maximum pressure builds up in the outlet  15  it may happen that this pressure presses the impeller against the pump casing side located opposite to the outlet. To compensate for this pressure force a ring diffusor  44  extends around the circumference of the pump casing, said ring diffusor  44  completely enclosing the circumference of the pump casing and being configured as a helical bulge whose cross-section continuously enlarges from the inlet end  44   a  towards the outlet  15 . 
     The embodiment shown in FIGS. 8 to  10  corresponds to a large extent to that shown in FIGS. 5 to  7 . The pump casing  10   b  is essentially configured as a flat cylinder with a flat upper side  11   a  and a flat bottom wall  12 . The lower side of the lower blades  19  forms a hydrodynamical supporting surface  30  which increases, as in the previous embodiments, towards the leading edge. Further, the supporting surface  30  shown in FIG. 8 increases towards the outside. 
     The driving means  33   b  comprises a disk rotor motor  45  supported in bearings  35  and being provided with magnets  36  which cooperate with the magnets  32  of the impeller  16   b.