Patent Publication Number: US-10765788-B2

Title: Axial flow rotor with downstream bearing wash flow

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of patent application Ser. No. 15/639,131, filed Jun. 30, 2017 and is a continuation of patent application Ser. No. 14/590,485, filed Jan. 6, 2015, entitled AXIAL FLOW ROTOR WITH DOWNSTREAM BEARING WASH FLOW, the entirety of which is incorporated herein by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     n/a 
     TECHNICAL FIELD 
     The present invention relates to impellers for blood pumps. 
     BACKGROUND 
     Implantable pumps are used for a variety of medical purposes for pumping bodily fluids such as blood. For example, when the output of the heart is insufficient to meet the circulatory needs of a person or animal, a pump can be implanted to boost circulation. 
     The pump can be implanted within the human body to augment the blood flow from the left ventricle of the heart to the body in patients with diminished heart function, such pumps being referred to as left ventricular assist devices (“LVADs”). 
     Referring to  FIG. 1 , a general description of a blood pump within a system for pumping blood can be as found in United States Pre-Grant Publication 2014/0073837 entitled “Blood Flow System with Variable Speed Control,” the disclosure of which is incorporated by reference herein. System  100  includes a housing  110  surrounding a rotational drive assembly including motor  120  and fluid drive element  130  such as impeller  131 . In some embodiments, system  100  comprises a rotational drive assembly similar to that described in U.S. Pat. No. 6,116,862 entitled “Blood Pump”, or U.S. Pat. No. 6,176,848 entitled “Intravascular Blood Pump”, the disclosures of said patents also being incorporated by reference herein. Impeller  131 , is magnetically coupled with and rotated by a spinning drive mechanism  120  having a set of magnetic poles  152  coupled across gap  112  with corresponding magnetic poles  154  of the impeller  131  through force of magnetic attraction. Drive mechanism  120  includes a motor (not shown) which rotates poles  152  around the common axis  137  of the drive motor and impeller. A supporting element such as a shaft  121  extending through a central opening  114  can support the impeller while the impeller is rotating and held in place axially by the magnetic attraction from the drive mechanism  120 . The pump chamber  115  includes the open space between a tubular portion of the housing  110  and other components of the pump such as impeller  131 , shaft  121  and motor  120 . In some embodiments, the chamber  115  comprises a volume less than 100 mL, for example less than 50 mL. In some embodiments, chamber  115  comprises a volume less than 10 mL, for example less than 5 mL, such as less than 2.5 mL or less than 1.2 mL. 
     Housing  110  comprises two ports, inlet port  116  and outlet port  117 . When impeller  131  is rotated, fluid propulsion forces are generated such that fluid flows from inlet port  116  to outlet port  117  through chamber  115 . A hollow tube, inlet cannula  160  includes proximal end  163 , distal end  164  and lumen  161  therebetween. Inlet cannula  160  is attached and/or is attachable to inlet port  116  at its distal end  164 , such as via a compression fitting  162 . In some embodiments, proximal end  163  of inlet cannula  160  is configured to be fluidly attached to a source of blood, such as a source of oxygenated blood, such as at the left ventricle of a patient. In some embodiments, inlet cannula  160  can be configured as described in U.S. patent application Ser. No. 12/392,623, entitled “Devices, Methods and Systems for Establishing Supplemental Blood Flow in the Circulatory System”, published as U.S. Pre-Grant Publication No. 2009/0182188, the disclosure of which is incorporated herein by reference. 
     A second hollow tube, outlet cannula  170  includes proximal end  173 , distal end  174  and lumen  171  therebetween. Outlet cannula  170  is attached and/or is attachable to outlet port  117 , such as via a compression fitting  172 . In embodiments wherein inlet cannula  160  is attached to a source of arterial blood, distal end  174  of outlet cannula  170  can be configured to be fluidly attached to a blood vessel, such as an artery, such as via an anastomosis. In some embodiments, outlet cannula  170  can comprise an anastomotic connector on its distal end  174 , such as is described in U.S. Pat. No. 8,333,727, entitled “Two Piece Endovascular Anastomotic Connector”, the disclosure of which is incorporated herein by reference. 
     Housing  110 , inlet cannula  160  and outlet cannula  170  are typically implanted in the patient, while other components such as control module  150  can be implanted in the patient, or can be coupled with motor  120  via a percutaneous cable  151 . In some embodiments, impeller  131  and motor  120  are constructed and arranged to achieve a flow rate of blood of at least 0.3 L/min. In some embodiments, the system is configured to provide a flow rate of blood between 2.0 and 6.0 L/min. In some embodiments, the fluid flow system allows the speed to be set (e.g., automatically or manually) to a level between a minimum speed and a maximum speed. A typical speed of the impeller is several tens of thousands of revolutions per minute (rpm). 
     Areas of insufficient flow, such as low-flow areas within or proximate to the pump can result in circulated blood undesirably transitioning to solid matter. With blood pumping systems, blood in a stasis or near-stasis condition can transition to thrombus. Creation of a thrombus or other solid matter can result in reduced flow of blood through the pump or release of solid matter into the patient as an embolus. 
     For these and other reasons, there is a need for devices, systems and methods which reduce the potential for blood to stagnate and which may improve the washing of blood on a bearing surface of the pump, which can decrease the risk that blood will transition to solid matter. 
     SUMMARY 
     Provided herein are blood flow and other fluid flow systems, methods and devices for a human or animal, e.g., a mammal. A blood flow system can be implanted or partially implanted in a human or animal to circulate blood through the cardiovascular system. The systems, methods and devices of the present inventive concepts are constructed and arranged to continuously or intermittently eliminate points of flow stasis or other low-flow areas that may serve as stagnation points that could transition to thrombi and emboli. Systems disclosed herein include rotational drive assemblies, such as motors, and fluid drive elements such as impellers which are configured to pump bodily fluids such as blood. 
     An embodiment of the present invention provides an impeller, e.g., a rotor, for an implantable pump for pumping a fluid such as blood. The impeller is configured for driving a primary flow of the fluid along an exterior of the impeller to a region beyond a peripheral edge at a bottom surface of the impeller at a downstream end of the impeller. The impeller may comprise fluid driving surfaces such as blades or channels or the like. The fluid driving surfaces are configured to drive the primary flow in a downstream direction along the exterior of the impeller to a region beyond the peripheral edge of the impeller. 
     The impeller can be configured to drive a secondary flow of blood in a downstream direction through the central opening of the impeller and then outwardly along a bottom surface of the impeller such that the secondary flow then exits to the region beyond the peripheral edge of the bottom surface of the impeller. At the location beyond the impeller&#39;s peripheral edge, the secondary flow rejoins the primary flow and flows further downstream towards an outlet of the pump. 
     An implantable blood pump may include an impeller rotatable about a rotational axis, having a body with a bottom surface at a downstream end and a central opening extending through the body from an upstream entrance to the bottom surface and centered about the axis. A projecting element, e.g., a shaft extends from below the bottom surface into or through the opening to support the impeller. The body is configured to drive a primary blood flow along an exterior of the body to beyond a peripheral edge of the bottom surface, and to provide a secondary downstream flow through the opening, and then along the bottom surface to beyond the peripheral edge. The secondary downstream flow proximate to a bearing surface at an end of the projecting element may improve washing of the bearing surface. 
     In a particular embodiment, the impeller may comprise a hub aligned with the rotational axis and the central opening. In some embodiments, the hub is supported above a bearing surface which is spherical in form or otherwise in form of a surface of revolution about the axis. A projecting element, e.g., a shaft may extend from below the bottom surface at least partially through the opening and may have a socket configured to receive the bearing surface, such that the impeller is supported for rotation above the bearing surface atop the shaft, wherein the secondary downstream flow can improve washing of the bearing surface. 
     In accordance with an aspect of the invention, an impeller for a blood pump is provided. A body of the impeller may have a bottom surface at a downstream end of the body and a central opening extending at least partially through the body from the bottom surface, the central opening centered about a rotational axis of the body. The body can be configured to rotate about an element projecting from a surface below the bottom surface into the opening to drive a primary blood flow along an exterior of the body to beyond a peripheral edge of the bottom surface, and the body is configured to drive a secondary blood flow through the opening and along the bottom surface beyond the peripheral edge. 
     In accordance with one or more embodiments of the invention, the body may comprise a plurality of ridges protruding in an axial direction below portions of the bottom surface, the ridges defining a plurality of fluid channels for driving the secondary flow between the opening and the peripheral edge. 
     In accordance with one or more embodiments of the invention, the ridges may be blades elongated in a direction from the opening towards the peripheral edge. In some embodiments, the fluid channels may have arcuate shape. In some embodiments, the ridges may have straight walls defining the plurality of fluid channels. In some embodiments, the ridges may taper in a direction from the peripheral edge towards the opening. In some embodiments, the walls of each fluid channel may be parallel. In some embodiments, each fluid channel may extend in a radial direction from the opening to the peripheral edge. 
     In a particular embodiment, the bottom surface may undulate in a direction of a circumference of the bottom surface between each of a plurality of troughs and each of a plurality of respective elevations. In such embodiment, the troughs and elevations of the bottom surface may be configured to drive the secondary flow of the fluid towards the peripheral edge of the bottom surface. 
     In one or more embodiments, the impeller may include a bearing surface disposed within or adjacent the central opening. The bearing surface may be arranged to cooperate with a mating bearing surface to control position of the impeller in at least one direction. 
     In one or more embodiments, the bearing surface of the impeller may be disposed adjacent the upstream end of the central opening. In one or more embodiments, the impeller may be configured to drive the secondary flow to an area proximate the bearing surface to provide washing of the bearing surface. 
     In one or more embodiments, the central opening may extend through the body to an upstream entrance, and the impeller may include a hub aligned with the axis and the opening, the hub supporting the impeller atop the bearing surface. 
     In one or more embodiments, the impeller may comprise blades projecting from the exterior of the body for driving the primary flow. 
     In accordance with an aspect of the invention, an impeller for a blood pump is provided. The impeller may comprise an impeller body having a bottom surface at a downstream end of the body and a central opening extending at least partially through the body from the bottom surface. The central opening can be centered about a rotational axis of the body. The body can be configured to rotate about an element projecting from below the bottom surface into the opening to drive a primary downstream blood flow along an exterior of the body to beyond a peripheral edge of the bottom surface and axially downstream from the impeller, and the body can be configured to drive a secondary downstream blood flow through the opening and along the bottom surface to beyond the peripheral edge. The pump may be configured to rotate the body about the axis. 
     In accordance with one or more embodiments, a bearing surface may be provided for supporting rotation of the impeller body atop a shaft extending along the axis through the opening. The base may comprise a plurality of axially protruding ridges for driving the secondary downstream blood flow from the opening towards the peripheral edge, the ridges defining a plurality of fluid channels each disposed between a pair of the ridges. The ridges may be elongated in a direction from the opening towards the peripheral edge. In a particular embodiment, the fluid channels may have arcuate shape. 
     In one or more embodiments, the ridges may have straight walls defining the plurality of fluid channels. In one or more embodiments, the walls of each fluid channel are parallel. In one or more embodiments, each fluid channel may extend in a radial direction from the opening to the peripheral edge. 
     In one or more embodiments, the impeller may include a bearing surface disposed within or adjacent the central opening, said bearing surface being arranged to cooperate with a mating bearing surface to control position of the impeller in at least one direction. 
     In one or more embodiments, the central opening can extend through the body to an upstream entrance, and the impeller may include a hub aligned with the axis and the opening, the hub supporting the impeller atop the bearing surface. 
     In one or more embodiments, the impeller may comprise blades projecting from the exterior of the body for driving the primary flow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the present invention, and the attendant advantages and features thereof, will be more readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein: 
         FIG. 1  is a schematic illustration of a fluid flow system for pumping a liquid fluid, e.g., blood; 
         FIG. 2  is a detailed view of an impeller in accordance with an embodiment of the invention; 
         FIG. 2A  is a detailed view of an impeller in accordance with a variation of the embodiment seen in  FIG. 2 ; 
         FIG. 3  is a perspective view illustrating a fan structure for driving a secondary flow of a fluid by an impeller in accordance with an embodiment of the invention; 
         FIG. 4A  is a plan view illustrating a fan structure for driving a secondary flow of a fluid by an impeller in accordance with an embodiment of the invention; 
         FIG. 4B  is a corresponding sectional view of the fan structure seen in  FIG. 4A ; 
         FIG. 5A  is a plan view illustrating an alternative fan structure for driving a secondary flow of a fluid by an impeller in accordance with an embodiment of the invention; 
         FIGS. 5B, 5C and 5D  each represent portions of a sectional view of the fan structure of  FIG. 5A  along portions of section lines A-B of  FIG. 5A ; 
         FIG. 5E  illustrates a further sectional view of the fan structure of  FIG. 5A  in accordance with the embodiment seen in  FIG. 5A ; 
         FIG. 6  illustrates a plan view of a fan structure for an impeller in accordance with a variation of the embodiment seen in  FIG. 3 ; 
         FIG. 7  illustrates a plan view of a fan structure for an impeller in accordance with another variation of the embodiment seen in  FIG. 3 ; and 
         FIG. 8  illustrates an elevation view of an alternative impeller in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 2 and 3  depict an impeller  131  in accordance with an embodiment of the invention. As seen therein, the impeller  131  can comprise a body which tapers from a peripheral edge  139  at a bottom surface  138  of the body towards an upstream entrance  111  of a central opening  114  that is closer to an upstream end  142  of the impeller. The opening  114  is centered about a rotational axis  137  of the impeller and may extend entirely through the body between the upstream entrance  111  and the bottom surface  138  of the impeller. 
     As used herein, directions aligned with or parallel with an axis of rotation of the impeller is referred to as axial directions. More generally, the direction of the flow of blood between an inlet of the pump and an outlet of the pump is referred to as a “downstream” direction, and the direction opposite thereto is referred to as an “upstream” direction. In addition, a statement that an element is “downstream from” another means that such element is closer to the outlet of the pump than the other element; conversely, a statement that an element is “upstream from” another means that such element is closer to the inlet of the pump than the other. 
     Impeller  131  is configured to drive an axial flow of blood into a chamber disposed axially downstream of the impeller such as chamber  115  shown in  FIG. 1 . In a particular embodiment as seen in  FIG. 2 , the impeller may have a hub  135  configured to support the impeller atop a shaft  121 , the hub being aligned with the axis of rotation  137  and the central opening  114 . In some cases, the hub  135  may include a bearing surface for supporting the hub atop the shaft, or an element  140  having a bearing surface can be interposed between the hub and the shaft. Thus, in one embodiment, a bearing element  140  is in form of a spherical ball bearing and have a spherical surface. Shaft  121  extending through the central opening  114  may have a socket at an end thereof which is configured to receive a surface of the bearing element  140 . While the pump is rotating, the ball bearing may spin as well, such that blood is drawn into the space between the socket and the ball bearing by the relative lower pressure created by the curved bearing surface. Alternatively, the bearing element  140  can be fixed to hub. 
     In other embodiments, the bearing surface may have a shape such as a surface of revolution about the rotational axis  137  of the impeller. In one example, the bearing surface may have a truncated spherical shape, or in other examples, a truncated spheroidal or ellipsoidal shape. 
     The impeller may have fluid driving surfaces such as blades, fluid channels, etc., for driving a primary flow of the fluid along the exterior of the impeller into a space beyond the peripheral edge of the base. For example, as seen in  FIG. 2 , the impeller may have two fluid propulsion blades, arms  132   a  and  132   b , which extend from the hub  135  to connection points  133  at an exterior surface of the impeller  131  for driving a primary flow of the fluid, e.g., blood, along the exterior surface to a region beyond a peripheral edge  139  of the impeller such as the region of chamber  115  downstream of the impeller. Washout area  113  comprises an opening between the hub  135  and arms  132   a  and  132   b  as shown. 
     As further seen in  FIG. 2 , a gap  112  is provided between a bottom surface  138  at a base of the impeller  131  and a surface  122  juxtaposed therewith, such as a surface of a housing of an impeller drive mechanism  120  ( FIG. 1 ). The impeller  131  can be configured to draw a secondary flow of the fluid from a region exterior to the impeller into washout area  113 , then into the central opening  114  and then downstream along a surface of the shaft  121  within opening, and finally along the bottom surface  138  of the base outwardly through gap  112  into a region disposed beyond the peripheral edge  139 . This downstream secondary flow adds to the overall downstream flow of the pump. The downstream secondary flow passing through the washout area  113  and downwardly into the opening  114  can improve washing of the bearing surface  140  provided between the shaft  121  and hub  135 . In this way, lubrication can be improved at the junction between the bearing surface and the receiving socket. The downstream secondary flow may also aid in preventing stagnation of the blood, decreasing risk of thrombi or emboli. 
     In one example, the downstream secondary flow measured in units of fluid volume per time, may range between two percent and 30 percent of the primary flow driven by the impeller. In another example, the secondary flow rate may range between five and 25 percent of the primary flow rate. In yet another example, the secondary flow rate may range between 10 and 20 percent of the primary flow rate. 
     As further seen in  FIG. 2 , the impeller may have a unitary structure in which a body of the impeller can be constructed of a single monolithic piece of metal, which may include, inter alia, a ferro-magnetic or platinum cobalt alloy where arms  132   a ,  132   b  are shown connecting the hub  135  with integral portions of the base of the impeller. In one embodiment, the impeller  131  can be made entirely of a ferro-magnetic material and may have permanently magnetized areas therein which defines poles  154 . In this case, the impeller may operate as a rotor driven by a rotating magnetic field produced by motor  120 . Alternatively, the impeller can be constructed of a combination of metals and elements such as magnets which can be embedded in the structure. In one example, arms  132   a ,  132   b  can be attached by welding, brazing, or other appropriate attachment technique. As seen in  FIG. 2 , the hub  135  and bearing surface  140  can be displaced upwardly away from an entrance  111  to central opening  114  in a direction of the axis  137  to provide easy entry to washout area  113 . Entrance  111  to the opening defines a nonplanar arcuate edge of the body. For example, the entrance  111  to the opening is heart-shaped in cross-section. As further seen in  FIG. 2 , the entrance  111  to the opening may define troughs between the arms  132   a  and  132   b  of the impeller which extend downwardly towards the downstream end of the impeller. In addition, the entrance  111  may extend upstream and define an edge which extends in a direction of an edge of the arm  132   a  or  132   b  adjacent thereto. 
     Alternatively, in the example impeller shown in  FIG. 2A , the entrance  143  to the opening in the body may have a circular edge which is disposed in a plane orthogonal to the axis  137 . 
     A variety of structures, surfaces and shapes of surfaces can be provided in order for impeller to drive the downstream secondary flow through washout area  113 , through the entrance  111  of central opening  114 , and outwardly through gap  112  beyond the peripheral edge  139  at a downstream end of the impeller. Thus, as further seen in  FIGS. 2 and 3 , a fan structure  144  can be disposed at a bottom surface  138  at the downstream end of the impeller. The fan structure includes a plurality of ridges  145  projecting in a direction parallel to axis  137  from other portions of the bottom surface. Thus, the ridges  145  define a plurality of fluid channels  147  between them for driving the secondary flow through the washout area, through the central opening and outwardly through gap  112 . In one embodiment, the fan structure  144  at the bottom surface  138  can be formed integrally with the body of the impeller. Alternatively, the fan structure can be formed separately and then fused to the impeller such as by welding, biocompatible adhesive, or attached using fasteners or any other biocompatible attachment technique. As used herein, “bottom surface” shall mean a surface at a downstream end of the impeller which faces away from the body of the impeller. As assembled in the pump, the bottom surface  138  can be juxtaposed with a surface, such as surface  122  of the impeller drive mechanism  120 . 
     In one example, as seen in  FIG. 3 , the ridges  145  are blades at the bottom surface which are elongated in a direction extending from the exit  146  of the central opening towards the peripheral edge  139  of the impeller. In the example seen in  FIG. 3 , the blades  145  and fluid channels between the blades have arcuate shape. The fan structure may have four ridges  145  as blades on the bottom surface  138  of the impeller as shown in  FIG. 3 , or there can be a greater number or fewer number of ridges  145 . 
     In a further example shown in  FIGS. 4A and 4B , the fan structure may have only two arcuate ridges  145  or blades, each ridge extending from locations proximate the exit  146  of the opening towards, that is, up to or adjacent to the peripheral edge  139  of the impeller base.  FIG. 4B  represents a projection of the ridges  145  or blades in a vertical direction parallel to axis  137  above the bottom surface  139  of the impeller base. In one example, a height of each blade may decrease with distance along each blade away from the opening  114 . In such case, each blade may have a greater height  149  near an end  152  of such blade proximate to the opening  114  than its height  148  nearer to or closest to the peripheral edge  139 . As further seen in  FIG. 4A , junctions between the edges  155 ,  156  of the blades and the bottom surface  138  can be radiused. Alternatively, in some cases the junctions may not be radiused. The blade edge to bottom surface junction radius can be the same on each edge  155 ,  156  of each blade or can be different depending on whether the edge of the blade curves inward upon itself as seen in the case of edge  155 , or whether the edge of the blade curves outward as seen for edge  156 . 
     In a variation of the embodiment illustrated in  FIGS. 4A and 4B , a height of each blade may increase with distance from the opening  114  such that each blade has a greater height near the end  153  proximate to the peripheral edge  139  than the height of such blade at or near the end  152  closest to the opening  114 . 
       FIGS. 5A through 5E  illustrate contours of a bottom surface  238  of an impeller in accordance with another embodiment of the invention. In this case, the bottom surface  238  can be formed with a shape that undulates up and down in a circumferential direction about the rotational axis  137 . In this case, the fluid flows from the exit  146  of the central opening onto the bottom surface  238  where it then flows into lower spaces, e.g., troughs in the undulating bottom surface  238  which lie between the higher elevations or crests of the bottom surface. The higher elevations of the bottom surface help to confine and move the fluid along the bottom surface to the region beyond the peripheral edge  139  of the impeller base. In effect, the elevations or crests of the undulating surface serve as ridges to impel the flow, whereas the troughs act as flow channels. 
       FIG. 5B  illustrates elevational views of the bottom surface  238  along sections A-A and A-B shown in  FIG. 5A .  FIG. 5C  is an elevational view illustrating a height of the bottom surface along the section line A-A extending from a point on peripheral edge  139  towards the rotational axis  137  of the impeller. As seen in  FIG. 5C  a portion  240  of the bottom surface extending along line A-A is at a lower height than a height of another portion  242  of the bottom surface that is at a different circumferential position on the bottom surface. On the other hand, as seen in  FIG. 5D , a portion  244  of the bottom surface extending along section line A-B between axis  137  and a point  139 ′ at the peripheral edge lies at a greater height than other portions of the bottom surface such as portion  240 , for example.  FIG. 5E  illustrates a further example in which peak height of the bottom surface is reached near the center of the bottom surface, and may decrease in height with proximity to the peripheral edge. 
     In accordance with another embodiment as seen in  FIG. 6 , a plurality of ridges  310  in the bottom surface have straight walls  320  which define a plurality of fluid channels  330  extending away from the downstream exit  146  of the opening towards the peripheral edge  139  of the bottom surface. Four fluid channels  330  can be provided for channeling the fluid along the bottom surface from the opening  114  to locations beyond the peripheral edge  139 . As seen in  FIG. 6 , the walls  320  of each fluid channel  330  can be parallel, and each fluid channel extends in a radial direction from the opening  114  to the peripheral edge  139 . In one example seen in  FIG. 6 , the ridges  310  taper inwardly from the peripheral edge  139  towards the opening  114 . 
       FIG. 7  illustrates a variation of the embodiment seen in  FIG. 6  which provides six fluid channels  340  instead of four fluid channels  330 . Other characteristics and features of the bottom surface can be the same as seen in  FIG. 6 . 
     As seen in  FIG. 8 , in a variation of the above-described embodiments, the body  400  of an alternative impeller is of unitary construction, with fluid-driving surfaces or channels  402  arranged between an upstream end  410  of the impeller and a downstream end  420 . In a particular embodiment, the shape of the impeller can be generally conical, tapering from the downstream end  420  towards the upstream end  410 . In such embodiment, the central opening  437  may not extend entirely through the impeller body  400 , but instead be configured as a cavity into which a supporting element such as a shaft  421  may project. One or more through openings  447  may extend through a portion of the impeller and connect with the central opening  437  to permit blood to flow from outside the upstream end  410  of the impeller into the central opening  437 . Similar to the above-described embodiments, fluid driving elements such as the ridges or blades at the bottom surface drive the blood through the at least one opening  447  into the central opening  437  and then outwardly along the bottom surface  438  to a region beyond the peripheral edge  439  of the bottom surface. 
     As further seen in  FIG. 8 , the impeller can be configured to spin atop a thin film of blood which flows past an extremity  423  of the supporting element  421 , wherein the extremity of the supporting element and the adjacent interior surface of the impeller serve as a bearing of the system. In a variation of the  FIG. 8  embodiment, the supporting element can be provided with an additional bearing surface such as a ball bearing or other surface of revolution at the extremity  423 , such bearing surface configured to provide additional wear resistance during operation. In still other embodiments, the bearing may include portions of the impeller within or near the central opening but near the downstream end of the impeller. For example, the surface of central opening  437  adjacent the bottom surface  438  can be configured as a bearing surface, and the central shaft  421  may have a mating bearing surface near its downstream end. In yet another embodiment, the bearing surfaces can be arranged to control the position of the impeller in directions transverse to the axis, in lieu of or in addition to the axial directions. For example, the central opening  437  can be arranged to form a sleeve bearing and the shaft  421  may form a journal for such a bearing. For example, such an arrangement can be used where other bearings control the axial position of the impeller. Also, the bearing surfaces can be arranged for physical contact lubricated by a film of blood, or else can be arranged as hydrodynamic bearing surfaces, where hydrodynamic action maintains a film of blood between the bearing surfaces and the forces acting between the surfaces can be transmitted between the surfaces though the film. 
     Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications can be made to the illustrative embodiments and that other arrangements can be devised without departing from the spirit and scope of the present invention as defined by the appended claims. 
     It will be appreciated that the various dependent claims and the features set forth therein can be combined in different ways than presented in the initial claims. It will also be appreciated that the features described in connection with individual embodiments can be shared with others of the described embodiments.