Abstract:
A centrifugal blood pump includes a housing that defines an inlet passage, a chamber, and an outlet passage. The pump includes an impeller rotatably positioned in the chamber to transfer blood from the inlet passage through the chamber and to the outlet passage and magnetic members embedded in the impeller such that the impeller and the magnetic members rotate together within the chamber. The pump includes a motor to control movement of the impeller in the chamber. The motor is adjacent the chamber and separated from the chamber by a partition member. The pump includes an inner annular magnetic member and an outer annular magnetic member embedded in a side of the housing opposite the partition member. A first net magnetic force between the inner annular magnetic member and the magnetic members exhibits greater attraction than a second net magnetic force between the outer annular member and the magnetic members.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application No. 62/115,741, filed Feb. 13, 2015 and entitled “IMPELLER SUSPENSION MECHANISM FOR HEART PUMP,” which is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Conventional heart pumps utilize magnetic elements and/or hydrostatic bearings within a housing of the pump to compensate attractive forces produced by a stator motor to maintain an impeller of the pump in a desired position within a chamber of the pump. Such magnetic attractive forces from the magnetic elements provide negative stiffness. This negative stiffness increases as a distance between the magnetic elements within the housing and magnets on the impeller becomes shorter. Any tilt of the impeller will decrease a gap between the impeller and the wall of the chamber at an outer edge of the impeller. At low impeller speeds, hydrodynamic bearing forces are sufficient to maintain this gap. However, in conventional pump designs, at high speeds the impeller tends to tilt, resulting in a decrease of a size of the gap near the outer edges of the impeller. 
       BRIEF SUMMARY OF THE INVENTION 
       [0003]    In one aspect, a centrifugal blood pump is provided. The pump may include a housing that defines an inlet passage, a chamber, and an outlet passage. The pump may also include an impeller rotatably positioned in the chamber to transfer blood from the inlet passage through the chamber and to the outlet passage. The impeller may include an inner portion and an outer portion. The pump may further include a plurality of impeller magnets embedded in the impeller such that the impeller and the plurality of impeller magnets rotate together within the chamber. The plurality of impeller magnets may include an inner impeller magnet and an outer impeller magnet relative to a central axis of the impeller. The pump may include a motor to control movement of the impeller in the chamber. The motor may be positioned adjacent the chamber and separated from the chamber by a partition member. The pump may also include an inner annular magnetic member embedded in a wall of the housing opposite the partition member and an outer annular magnetic member embedded in the wall of the housing opposite the partition member. A first net magnetic force between the inner annular magnetic member and the inner impeller magnet may exhibit greater attraction than a second net magnetic force between the outer annular member and the outer impeller magnet. 
         [0004]    In another aspect, a centrifugal blood pump may include a housing that defines an inlet passage, a chamber, and an outlet passage. The pump may also include an impeller rotatably positioned in the chamber to transfer blood from the inlet passage through the chamber and to the outlet passage. The pump may further include a plurality of impeller magnets embedded in the impeller such that the impeller and the plurality of impeller magnets rotate together within the chamber. The pump may include a motor to control movement of the impeller in the chamber. The motor may be positioned adjacent the chamber and separated from the chamber by a partition member. The pump may further include at least one annular magnetic member embedded in a wall of the housing opposite the partition member. A first net magnetic force between the at least one annular magnetic member and a proximal portion the plurality of impeller magnets may exhibit greater attraction than a second net magnetic force between the at least one annular magnetic member and a distal portion of the plurality of impeller magnets. The proximal portion and the distal portion may be relative to a central axis of the impeller. 
         [0005]    In another aspect, a centrifugal blood pump may include a housing that defines an inlet passage, a chamber, and an outlet passage. The pump may also include an impeller rotatably positioned in the chamber to transfer blood from the inlet passage through the chamber and to the outlet passage. The impeller may include an inner portion and an outer portion relative to a central axis of the impeller. The pump may further include at least one impeller magnet embedded in the impeller such that the impeller and at least one magnetic member rotate together within the chamber. The pump may include a motor to control movement of the impeller in the chamber. The motor may be positioned adjacent the chamber and separated from the chamber by a partition member. The pump may also include at least one annular magnetic member embedded in a side of the housing opposite the partition member. A first force exhibited on the inner portion may have a greater attraction than a second force exhibited on the outer portion of the impeller. The first force and the second force may each result from interactions between the at least one impeller magnet and the at least one annular magnetic member. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  shows an example centrifugal blood pump according to the disclosure. 
           [0007]      FIG. 2  shows the blood pump of  FIG. 1  in an alternate view. 
           [0008]      FIG. 3  shows a cross-section of the blood pump of  FIG. 1 . 
           [0009]      FIG. 4  shows another cross-section of the blood pump of  FIG. 1 . 
           [0010]      FIG. 5  shows yet another cross-section of the blood pump of  FIG. 1 . 
           [0011]      FIG. 6  shows yet another cross-section of the blood pump of  FIG. 1 . 
           [0012]      FIG. 7  shows yet another cross-section of the blood pump of  FIG. 1 . 
           [0013]      FIG. 8  shows one embodiments of magnetic stabilization features of a blood pump according to embodiments. 
           [0014]      FIG. 9  shows one embodiments of magnetic stabilization features of a blood pump according to embodiments. 
           [0015]      FIG. 10  shows one embodiments of magnetic stabilization features of a blood pump according to embodiments. 
           [0016]      FIG. 11  shows one embodiments of magnetic stabilization features of a blood pump according to embodiments. 
           [0017]      FIG. 12  shows one embodiments of magnetic stabilization features of a blood pump according to embodiments. 
           [0018]      FIG. 13  shows one embodiments of magnetic stabilization features of a blood pump according to embodiments. 
           [0019]      FIG. 14  shows one embodiments of magnetic stabilization features of a blood pump according to embodiments. 
           [0020]      FIG. 15  shows one embodiments of magnetic stabilization features of a blood pump according to embodiments. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    The ensuing description provides exemplary embodiments only, and is not intended to limit the scope, applicability or configuration of the disclosure. Rather, the ensuing description of the exemplary embodiments will provide those skilled in the art with an enabling description for implementing one or more exemplary embodiments. It being understood that various changes may be made in the function and arrangement of elements without departing from the spirit and scope of the invention as set forth herein. 
         [0022]    Specific details are given in the following description to provide a thorough understanding of the embodiments. However, it will be understood by one of ordinary skill in the art that the embodiments may be practiced without these specific details. For example, with regard to any specific embodiment discussed herein, any one or more details may or may not be present in all versions of that embodiment. Likewise, any detail from one embodiment may or may not be present in any particular version of another embodiment discussed herein. Additionally, well-known circuits, systems, processes, algorithms, structures, and techniques may be shown without unnecessary detail in order to avoid obscuring the embodiments. The absence of discussion of any particular element with regard to any embodiment herein shall be construed to be an implicit contemplation by the disclosure of the absence of that element in any particular version of that or any other embodiment discussed herein. 
         [0023]    The present disclosure is directed to, among other things, minimizing or preventing a decrease in gap size at high impeller speeds between the outer edge of the impeller and the inner wall of a chamber of a blood pump. Some aspects of the disclosure are directed to reducing the risk of undesirable tilting of the impeller and/or improving the overall stability of the impeller during operation. Embodiments maintain an appropriately sized gap through all impeller speeds by decreasing the net attractive magnetic force on an outer portion of the impeller, or by having a lower net attractive force on an outer portion of the impeller than an inner portion. Although the feature or aspects of the present disclosure are not limited to a specific type of mechanical blood pump, an example of a blood pump in which embodiments of maintaining an appropriate gap size may be practiced is shown and described in connection with  FIGS. 1-7 . As will be understood by one of skill from the description herein, some of the features described increase a stabilizing force on the impeller over conventional non-contact pump bearings, in various respects, along the tilt axis. 
         [0024]    In  FIGS. 1-7 , an exemplary centrifugal blood pump is shown that includes a pump unit  1  that includes a housing  2  made of a nonmagnetic material. Housing  2  includes a cylindrical body portion  3 , a cylindrical blood inlet port  4  that extends from one end surface of body portion  3 , and a cylindrical blood outlet port  5  that extends from another end surface of body portion  3 . Blood outlet port  5  extends in a tangential direction of the outer circumferential surface of body portion  3 . 
         [0025]    As shown in  FIG. 2 , a position of one or more annular shaped magnetic members is shown. In some embodiments, pump unit  1  may include an inner annular magnetic member  30  and an outer annular magnetic member  32 . Other embodiments may include one annular magnetic member or more than two annular magnetic members. The annular magnetic members  30  and  32  may each be formed from a single ring-shaped magnetic member, or may be formed from a number of magnetic members arranged in an annular pattern. 
         [0026]    As shown in  FIG. 3 , a blood chamber  7  and a motor chamber  8  are partitioned from each other by a dividing wall  6  within housing  2 . Blood chamber  7 , as shown in  FIGS. 3-4 , includes a rotatable disc-shaped impeller  10  having a through hole  10   a  in a center thereof. Impeller  10  includes two shrouds  11 ,  12  in an annular shape, and a plurality (e.g., six) of vanes  13  formed between two shrouds  11  and  12 . Shroud  11  is arranged on the blood inlet port  4  side, and shroud  12  is arranged on the dividing wall  6  side. Shrouds  11 ,  12  and vanes  13  are made of a nonmagnetic material. 
         [0027]    A plurality (six in this case) of blood passages  14  are formed between two shrouds  11  and  12  and are partitioned from one another by the plurality of vanes  13 . As shown in  FIG. 4 , blood passage  14  is in communication with through hole  10   a  at the center of impeller  10 , and extends with through hole  10   a  of impeller  10  as a starting point to an outer circumference such that blood passage  14  gradually increases in width. In other words, each vane  13  is formed between two adjacent blood passages  14 . In the first embodiment, the plurality of vanes  13  are provided at regular angular intervals, and each has the same shape. Thus, the plurality of blood passages  14  are provided at regular angular intervals and has the same shape. 
         [0028]    When impeller  10  is driven to rotate, blood that has flowed in through blood inlet port  4  is delivered by centrifugal force from through hole  10   a  to an outer circumferential portion of impeller  10  via blood passages  14 , and flows out through blood outlet port  5 . It is contemplated that the blood inlet port  4  may be configured and/or arranged to minimize or prevent the formation of thrombosis within (i.e., internal) the blood inlet port  4 , and also to minimize turbulence at a fluid interface between the blood inlet port  4  and the blood chamber  7 . 
         [0029]    A plurality of permanent magnets may be embedded in shroud  11 . For example, an inner magnet  15  and an outer magnet  16  may be included in shroud  11 . One or more annular magnetic members may be embedded in an inner wall of blood chamber  7  facing shroud  11 . For example, inner annular magnetic member  30  and outer annular magnetic member may be embedded in the inner wall. The annular magnetic members  30  and  32  may be permanent magnets or may be electromagnetic elements. Either a soft magnetic element or a hard magnetic element may be used as the annular magnetic members  30  and/or  32 . 
         [0030]    The annular magnetic members  30  and  32  may each be formed as a single permanent magnet or as a plurality of permanent magnets. If a single permanent magnet is provided, the permanent magnet is formed in an annular or ring shape. If a plurality of permanent magnets are provided, the plurality of permanent magnets may be arranged at regular angular intervals along the same circle. While described as annular magnetic members, it will be appreciated that each of the magnetic members described herein may be formed from one or more magnets, and may be in any non-annular arrangement, such as other symmetrical shapes. In some embodiments, the inner annular magnetic member  30  may have a greater net attractive force with the inner magnet  15  than the net attractive force between the outer annular magnetic member  32  and the outer magnet  16 . Such a configuration may decrease the tilt of the impeller, especially at high impeller speeds, thus maintaining a size of the gap between the outer edge of the impeller and the housing wall. 
         [0031]    As shown in  FIG. 4 , a plurality (e.g., nine) of permanent magnets  17  are embedded in shroud  12 . The plurality of permanent magnets  17  are arranged with a gap therebetween at regular angular intervals along the same circle such that magnetic polarities of adjacent permanent magnets  17  are different from each other. In other words, permanent magnet  17  having the N-pole toward motor chamber  8  and permanent magnet  17  having the S-pole toward motor chamber  8  are alternately arranged with a gap therebetween at regular angular intervals along the same circle. 
         [0032]    As shown in  FIG. 3  and  FIG. 7 , a plurality (e.g., nine) of magnetic elements  18  are provided in motor chamber  8 . The plurality of magnetic elements  18  are arranged at regular angular intervals along the same circle to face the plurality of permanent magnets  17  in impeller  10 . A base end of each of the plurality of magnetic elements  18  is joined to one disc-shaped magnetic element  19 . A coil  20  is wound around each magnetic element  18 . In the direction of a central axis of impeller  10 , the length of magnetic element  18  is shorter than that of coil  20 . That is, when an axial length of magnetic element  18  is expressed as x and an axial length of coil  20  is expressed as L relative to the surface of disc-shaped magnetic element  19 , a relationship of 0&lt;x&lt;L is satisfied. 
         [0033]    Referring back to  FIG. 7 , space for winding coil  20  is equally secured around the plurality of magnetic elements  18 , and surfaces facing each other of every two adjacent magnetic elements  18  are provided substantially in parallel to each other. Thus, a large space for coils  20  can be secured and turns of coils  20  can be increased. As a result, large torque for driving impeller  10  to rotate can be generated. Further, copper loss that occurs in coils  20  can be reduced, thereby enhancing energy efficiency when impeller  10  is driven to rotate. The plurality of magnetic elements  18  may be formed in a cylindrical shape. In this case, a circumferential length of coils  20  can be minimized to reduce copper loss that occurs in coils  20 , thereby enhancing energy efficiency when impeller  10  is driven to rotate. 
         [0034]    An outline surface surrounding the plurality of magnetic elements  18  (a circle surrounding the peripheries of the plurality of magnetic elements  18  in  FIG. 7 ) may correspond to an outline surface surrounding the plurality of permanent magnets  17  (a circle surrounding the peripheries of the plurality of magnetic elements  18  in  FIG. 4 ), or the outline surface surrounding the plurality of magnetic elements  18  may be larger than the outline surface surrounding the plurality of permanent magnets  17 . Further, it is preferable that magnetic element  18  be designed not to be magnetically saturated at maximum rating of pump  1  (a condition where torque for driving impeller  10  to rotate becomes maximum). 
         [0035]    Voltages are applied to nine coils  20  in a power distribution system shifted by 120 degrees, for example. That is, nine coils  20  are divided into groups each including three coils. Voltages are applied to first to third coils  20  of each group, respectively. To first coil  20 , a positive voltage is applied during a period of 0 to 120 degrees, 0 V is applied during a period of 120 to 180 degrees, a negative voltage is applied during a period of 180 to 300 degrees, and 0 V is applied during a period of 300 to 360 degrees. Accordingly, a tip surface of magnetic element  18  having first coil  20  wound therearound (end surface on the impeller  10  side) becomes the N-pole during the period of 0 to 120 degrees, and becomes the S-pole during the period of 180 to 300 degrees. A Voltage VV is delayed in phase from a voltage VU by 120 degrees, and a voltage VW is delayed in phase from voltage VV by 120 degrees. Thus, rotating magnetic field can be formed by applying voltages VU, VV, VW to first to third coils  20 , respectively, so that impeller  10  can be rotated by attractive force and repulsion force between the plurality of magnetic elements  18  and the plurality of permanent magnets  17  in impeller  10 . 
         [0036]    When impeller  10  is rotating at a rated rotation speed, attractive force between the magnetic elements  15  and  16  and the annular magnetic members  30  and  32  and attractive force between the plurality of permanent magnets  17  and the plurality of magnetic elements  18  are set to be balanced with each other substantially around a center of a movable range of impeller  10  in blood chamber  7 . Thus, force acting on impeller  10  due to the attractive force is very small throughout the movable range of impeller  10 . Consequently, frictional resistance during relative slide between impeller  10  and housing  2  which occurs when impeller  10  is activated to rotate can be reduced. In addition, a surface of impeller  10  and a surface of an inner wall of housing  2  are not damaged (no projections and recesses in the surfaces) during the relative slide, and moreover, impeller  10  is readily levitated from housing  2  without contacting even when hydrodynamic force is small during low-speed rotation. 
         [0037]    A number of grooves of hydrodynamic bearing  21  are formed in a surface of dividing wall X facing shroud  12  of impeller  10 , and a number of grooves of hydrodynamic bearing  22  are formed in the inner wall of blood chamber  7  facing shroud  11 . When a rotation speed of impeller  10  becomes higher than a prescribed rotation speed, a hydrodynamic bearing effect is produced between each of the grooves of hydrodynamic bearings  21  and  22  and impeller  10 . As a result, drag is generated on impeller  10  from each of the grooves of hydrodynamic bearings  21  and  22 , causing impeller  10  to rotate without contacting in blood chamber  7 . 
         [0038]    Specifically, as shown in  FIG. 5 , each of the grooves of hydrodynamic bearing  21  are formed with a size corresponding to shroud  12  of impeller  10 . Each groove of hydrodynamic bearing  21  is positioned with one end on an edge (circumference) of a circular portion slightly distant from a center of dividing wall  6 . Each groove extends from the edge spirally (in other words, in a curved manner) toward a portion near an outer edge of dividing wall  6  such that the groove of the hydrodynamic bearing  21  gradually increases in width. Each of the grooves of hydrodynamic bearing  21  has substantially the same shape, and the grooves are arranged at substantially regular intervals. Each groove of hydrodynamic bearing  21  includes a concave portion. Each groove may have a depth of between about 0.005 to 0.400 mm. Between about 6 to 36 grooves may form hydrodynamic bearing  21 . 
         [0039]    In  FIG. 5 , ten grooves in an equiangular arrangement with respect to the central axis of impeller  10  form hydrodynamic bearing  21 . Since the grooves of hydrodynamic bearing  21  have a so-called inward spiral groove shape, clockwise rotation of impeller  10  causes an increase in fluid pressure from an outer diameter portion toward an inner diameter portion of the grooves for hydrodynamic bearing  21 . As a result, a repulsive force that acts as a hydrodynamic force is generated between impeller  10  and dividing wall  6 . 
         [0040]    In some embodiments, alternatively, or in addition to, providing grooves for hydrodynamic bearing  21  in dividing wall  6 , grooves for hydrodynamic bearing  21  may be provided in a surface of shroud  12  of impeller  10 . The hydrodynamic bearing effect produced between impeller  10  and the grooves of hydrodynamic bearing  21 , causes impeller  10  to move away from dividing wall  6  and to rotate without contacting the dividing wall  6 . Accordingly, a blood flow path is secured between impeller  10  and dividing wall  6 , thus preventing occurrence of blood accumulation therebetween and the resultant thrombus. Further, in a normal state, the grooves of hydrodynamic bearing  21  perform a stirring function between impeller  10  and dividing wall  6 , thus preventing occurrence of partial blood accumulation therebetween. 
         [0041]    It is preferable that a corner portion of each of grooves for hydrodynamic bearing  21  be rounded to have R of at least 0.05 mm. As a result, occurrence of hemolysis can further be reduced. 
         [0042]    As with the grooves of hydrodynamic bearing  21 , as shown in  FIG. 6 , the grooves of hydrodynamic bearing  22  are each formed with a size corresponding to shroud  11  of impeller  10 . Each groove of hydrodynamic bearing  22  has one end on an edge (circumference) of a circular portion slightly distant from a center of the inner wall of blood chamber  7 . The groove extends spirally (in other words, in a curved manner) toward a portion near an outer edge of the inner wall of blood chamber  7  such that the groove gradually increases in width. Each of the grooves has substantially the same shape. The grooves are arranged at substantially regular intervals. Each groove of hydrodynamic bearing  22  includes a concave portion. Each groove may have a depth of between about 0.005 to 0.4 mm. It is preferable that about 6 to 36 grooves form hydrodynamic bearing  22 . In  FIG. 6 , ten grooves forming hydrodynamic bearing  22  are equiangularly arranged with respect to the central axis of impeller  10 . 
         [0043]    Alternatively, or in addition to, providing the grooves of hydrodynamic bearing  22  in the inner wall of blood chamber  7 , the grooves of hydrodynamic bearing  22  may be provided in a surface of shroud  11  of impeller  10 . It is preferable that a corner portion of each of grooves of hydrodynamic bearing  22  be rounded to have R of at least 0.05 mm. As a result, occurrence of hemolysis can further be reduced 
         [0044]    The hydrodynamic bearing effect produced between impeller  10  and the grooves for hydrodynamic bearing  22  causes impeller  10  to move away from the inner wall of blood chamber  7  and rotates without contacting the inner wall. In addition, when pump unit  1  is subjected to external impact or when the hydrodynamic force generated by hydrodynamic bearing  21  becomes excessive, impeller  10  can be prevented from being in close contact with the inner wall of blood chamber  7 . The hydrodynamic force generated by hydrodynamic bearing  21  may be different from the hydrodynamic force generated by hydrodynamic bearing  22 . 
         [0045]    It is preferable that impeller  10  rotate in a state where a gap between shroud  12  of impeller  10  and dividing wall  6  is substantially equal to a gap between shroud  11  of impeller  10  and the inner wall of blood chamber  7 . If one of the gaps becomes narrower due to serious disturbance such as fluid force acting on impeller  10 , it is preferable that grooves of hydrodynamic bearing  21  and  22  have different shapes so that the hydrodynamic force generated by the hydrodynamic bearing on the narrower side becomes higher than the hydrodynamic force generated by the other hydrodynamic bearing to make the gaps substantially equal to each other. 
         [0046]    While each groove of hydrodynamic bearings  21  and  22  has the inward spiral groove shape shown in  FIGS. 5-6 , grooves having another shape may be used. Nevertheless, for blood circulation, it is preferable to employ grooves having the inward spiral groove shape, which allows for a smooth flow of blood. 
         [0047]    As mentioned above, it is contemplated that the blood inlet port  4  may be configured and/or arranged to minimize or prevent the formation of thrombosis within (i.e., internal) the blood inlet port  4 , and also to minimize turbulence at a fluid interface between the blood inlet port  4  and the blood chamber  7 . In general, it is contemplated that thrombosis formation may occur due to a vortex forming in or within one or both of blood inlet port  4  and the blood chamber  7  in a location near or adjacent blood inlet port  4 , and/or due to stress or forces imparted on blood as it transitions into a spinning motion once it reaches the impeller  10 . 
         [0048]      FIGS. 8-15  depict embodiments of pumps using one or more annular magnetic members of a magnetic suspension system to maintain a size of a gap between an impeller and a chamber wall of a housing at high impeller speeds. Theses pumps may be configured as those described in  FIGS. 1-7  above. The annular magnetic members may correspond to the annular magnetic members  30  and  32  described herein. The magnetic suspension systems are often made up of magnetic elements within the impeller, annular magnetic members embedded within a housing of the pump, a stator motor, and/or hydrostatic bearings formed in the housing. Embodiments maintain this gap size by producing a lower net attractive force at an outer or distal portion of the impeller than at an inner or proximal portion of the impeller. In some embodiments this net attractive force relationship is achieved by decreasing the attractive force at the outer portion or by producing a repulsive force at the outer portion. Other embodiments achieve the greater inner attractive force by increasing the attractive force at the inner portion of the impeller. In other embodiments, the gap may be maintained by using electromagnets as the annular magnetic members and utilizing active magnetic control to adjust the magnetic forces as impeller speeds and/or gap size change. Alternative methods of increasing and/or maintaining the gap size at high impeller speeds may also include increasing the gap between the motor stator and the motor magnet, although his may decrease the efficiency of the motor. It will be appreciated that combinations of the techniques described herein may be used to further adjust and/or maintain the gap size. 
         [0049]      FIGS. 8 and 9  depict systems that decrease the outer annular magnetic member&#39;s magnetic force to create a greater net magnetic attraction at the inner annular magnetic member. In  FIG. 8 , a pump  800  having a housing  802  is shown. An impeller  804  is shown having a plurality of magnetic elements embedded therein. The impeller is configured to rotate within the housing  802 . Here, an inner magnetic element  806  and an outer magnetic element  808  are embedded within impeller  804 . One or more annular magnetic members may be embedded within housing  802 . For example, an inner annular magnetic member  810  and an outer annular magnetic member  812  are embedded within a side wall of the housing  802 . As shown here, by making a distance between the inner annular magnetic member  810  and inner magnetic element  806  less than a distance between outer annular magnetic member  812  and outer magnetic element  808 , the net attractive force along the outer edge of impeller  802  may be decreased and/or made lower than the net attractive force at an inner portion of the impeller  802 . This lessened attractive force results in a reduction of negative stiffness at the outer annular magnetic member  812  and an increase in the gap between the impeller  802  and the inner wall of the housing  804 . Making the distance between the inner magnets smaller than the distance between the outer magnets can be achieved by moving the inner annular magnetic member  810  closer to the impeller, by moving the outer annular magnetic member  812  away from the impeller, and/or by a combination of both. 
         [0050]    In some embodiments, making the distance between the inner magnets smaller than the distance between the outer magnets can be achieved by changing a position of the inner magnet and/or the outer magnet relative to the impeller as shown in  FIG. 9 . For example, a pump  900  may have a housing  904  and an impeller  906  such as described in  FIG. 8 . An inner magnet  906  of the impeller  902  may be moved closer to an inner annular magnetic member  910  within the housing  904  and/or an outer magnet  908  may be moved away from an outer annular magnetic member  912 . In some embodiments, one or more of both the housing magnetic members  810  and  812  and the impeller magnetic elements  806  and  808  may be positioned to create the larger distance between the outer magnets than the inner magnets. In some embodiments, the net force difference between the inner and outer portions of the impeller may be attained by decreasing the attractive force at the outer annular magnetic member, such as by reducing the magnet size and/or otherwise reducing the strength of the outer annular magnetic member  912 . 
         [0051]      FIGS. 10 and 11  depict embodiments of pumps that increase a magnetic force of an inner annular magnetic member and decrease or eliminate a magnetic force of an outer annular magnetic member. For example, in  FIG. 10 , a pump  1000  is shown having an impeller  1002 , housing  1004 , an inner magnetic element  1006 , and an outer magnetic element  1008  as described above with regard to  FIG. 8 . Pump  1000  may include an inner annular magnetic member  1010  having a net magnetic attraction with the inner magnetic element  1006  that is greater than a net magnetic attraction between an outer annular magnetic member  1012  and the outer magnetic element  1008 . In pump  1000 , this is done by increasing the magnetic force of the inner annular magnetic member  1010  in combination with reducing the magnetic force of the outer annular magnetic member  1012  and/or by increasing the distance between the outer annular magnetic member  1012  and the outer magnetic element  1008 . The increase in magnetic force of the inner annular magnetic member may be realized by increasing the magnet volume and/or by using a stronger magnet. In some embodiments, the magnetic force of the inner annular magnetic member  1010  may be increased sufficiently such that the outer annular magnetic member  1012  and/or outer magnetic element  1008  may be eliminated. For example,  FIG. 11  shows a pump  1100  having only a strong inner annular magnetic member  1110  and an inner magnetic element  1106 . 
         [0052]    In some embodiments, the gap between the impeller and the housing may be maintained by increasing the attractive force of the inner annular magnetic member while using an opposite polarity magnet as the outer annular magnetic member to create a repulsive force on the outer edge of the impeller and to increase the impeller suspension stiffness. For example,  FIG. 12  shows a pump  1200  having an inner annular magnetic member  1210  having a sufficiently high attractive magnetic force and an outer annular magnetic member  1212  that has a polarity relative to an outer magnetic element  1208  to create a repulsive force that serves to maintain the gap size, even at high impeller speeds. 
         [0053]      FIG. 13  shows one embodiment of a pump  1300  where a diameter of an outer annular magnetic member  1312  is increased to be larger than a diameter of an outer magnetic element  1308  on an impeller  1302  and/or to extend radially beyond at least a portion of the outer magnetic element  1308 . The outer annular magnetic member  1312  also has a polarity selected to create a net repulsive force with the outer magnetic element  1308 . When the outer annular magnetic member  1312  extends beyond at least a portion of the outer magnetic element  1308 , the repulsive force has a force component toward the rotational axis of the impeller  1302 , which increases the radial stiffness. The impeller rotation center shifts toward an outlet side of pump  1300  when the flow rate is high. The repulsive force of the rotational axis direction increases as the impeller is pushed toward the outlet side, compensating against pump pressure distribution due to the high flow rate. Thus, the repulsive force produce by the outer annular magnetic member  1312  helps maintain the impeller position, and thus gap size, as impeller speeds increase. 
         [0054]    In some embodiments, reduction of a diameter of an inner annular magnetic member may be used in conjunction with increasing a diameter of an outer annular magnetic member, resulting in an increase in the radial stiffness of the magnetic suspension system of the pump. For example,  FIG. 14  shows a pump  1400  having an outer annular magnetic member  1412  of increased diameter and an inner magnetic member  1410  having a reduced diameter. By reducing the diameter of the inner annular magnetic member  1410  such that the inner annular magnetic member  1410  is positioned at least partially inward of the inner magnetic element  1406 , a thickness of a pump housing  1404  may be reduced by putting an inner annular magnetic member  1410  of increased size within dead space of a pump inflow conduit  1414 . This positioning results in an increase in the negative stiffness of the magnetic suspension system. The radial component of the repulsive force of the outer annular magnetic member maintains the impeller radial stiffness as the position of the inner annular magnetic member  1410  increases the negative stiffness. Additionally, the inward position of the inner annular magnetic member  1410  increases the magnetic resistance while reducing the magnetic flux, and thus, the net attractive force acting on the impeller  1402 . The repulsive force of the outer annular magnetic member  1412  helps compensate for the reduction of attractive force of the inner annular magnetic member  1410  to maintain the gap size between the housing  1404  and the impeller  1402 . 
         [0055]    In some embodiments, a ferromagnetic ring, such as a steel ring, may be positioned between an inner annular magnetic member and an inner magnetic element when the inner annular magnetic member has a diameter positioned inward of an inner magnetic element on an impeller.  FIG. 15  shows a pump  1500  having an outer annular magnetic member  1512  extending radially beyond an outer magnetic element  1508  and an inner annular magnetic member  1510  positioned inward of an inner magnetic element  1506 . Pump  1500  also includes a ferromagnetic ring  1516  positioned between inner annular magnetic member  1510  and inner magnetic element  1506 . The ring  1516  skews the magnetic flux such that the attractive force from the inner annular magnetic member  1510  is better directed to act upon the inner magnetic element  1506 . 
         [0056]    The invention has now been described in detail for the purposes of clarity and understanding. However, it will be appreciated that certain changes and modifications may be practiced within the scope of the disclosure.