Abstract:
A flow thru mechanical bearing system can be used in an implantable rotary blood pump that includes a pump housing. An impeller housed in the pump housing can be radially supported by a row of at least three low friction contact points located on an inside bore of the impeller. The impeller may be axially supported by a second mechanical bearing. The second mechanical bearing may provide at least three axial contact points on the top surfaces of the impeller. The impeller is driven by a magnetic coupling which functions mainly to transmit torque to the impeller. Further, in some cases, the magnetic coupling may restrain axial motion of the impeller.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to blood pump systems. More particularly, to bearings used in implantable rotary blood pumps. 
       BACKGROUND OF INVENTION 
       [0002]    Implantable blood pumps can be utilized for total artificial heart replacement or ventricular assistance. Implantable blood pumps may be utilized for temporary or long term ventricular assistance or to permanently replace a patient&#39;s damaged heart. Some blood pumps may mimic the pulsatile flow of the heart. However, some blood pumps have progressed to designs that are non-pulsatile. Non-pulsatile blood pumps are typically rotary and propel fluid with impellers that span the spectrum from radial flow, centrifugal type impellers to axial flow, auger type impellers. 
         [0003]    A common issue encountered by blood pumps is blood trauma. The causes of blood trauma can be partially attributed to shear stress and/or heat generated by the bearings supporting the impeller. Shear stress and/or heat may cause hemolysis, thrombosis, and the like. In some blood pumps, the impeller may be driven by a shaft. The shaft may be sealed off with shaft seals to prevent blood from entering undesirable areas, such as a motor driving the shaft. However, shaft seals generate excess heat that may produce blood clots, and shaft seals may fail and allow blood to enter unwanted areas. A great deal of effort has been devoted to reducing or eliminating blood trauma in rotary blood pumps. One solution to minimizing or eliminating blood trauma is to provide hydrodynamic support of the impeller. For example, hydrodynamic support may be provided by ramp, wedge, plain journal, multi-lobe or groove hydrodynamic bearings. Another solution is to provide mechanical support of the impeller using mechanical bearings, such as jewel type bearings in the form of a shaft and sleeve or ball and cup. These mechanical bearings may utilize biocompatible hard ceramic materials. To function properly in blood, a mechanical bearing must generate very little heat and should avoid stagnant or recirculating areas of blood flow to prevent the formation of blood clots. Another solution proposed is the utilization of passive permanent magnetic and active controlled magnetic bearings to provide impeller support in blood pumps. Magnetic bearings, hydrodynamic bearings, and/or mechanical bearings may be combined to provide impeller support in blood pumps. However, magnetic bearing systems may require sensors and complex controls. Hydrodynamic bearings may require small clearances which may cause slow moving or stagnant blood flow between hydrodynamic bearing surfaces. Further, some blood pumps incorporate electric motors into the pumping chamber, rather than providing separate motor and pumping chambers. For example, a stator may be provided in the pump housing and magnets can be incorporated into an impeller to provide a pump impeller that also functions as the rotor of the electric motor. 
         [0004]    The various embodiments discussed herein provide mechanical blood pump bearings that cause minimal blood trauma, generate very little heat from friction, and can be thoroughly washed by blood flow to prevent the formation of blood clots. Further, these bearing systems are simple and robust, without requiring complicated controls and sensors or small clearances. 
       SUMMARY OF THE INVENTION 
       [0005]    The discussion herein provides a description of flow thru mechanical blood pump bearings that are energy efficient, cause minimal blood trauma, and are simple and robust. 
         [0006]    In one embodiment, an impeller for a blood pump is radially supported by a first mechanical bearing that provides at least three contact points located on an inside bore surface of the impeller. In another embodiment, a second mechanical bearing may provide at least three contact points located on the top surfaces of an impeller to support the impeller axially. In yet another embodiment, the first mechanical bearing and the second mechanical bearing may be combined to provide contact points on an internal bore surface of an impeller and a top surface of an impeller to provide radial and axial support of the impeller. 
         [0007]    In some embodiments, contact points may be formed from or coated with a biocompatible low friction material. In some embodiments, an impeller may be magnetically coupled to a driver or motor through a diaphragm of a pump housing. 
         [0008]    The foregoing has broadly outlined various features of the present disclosure in order that the detailed description that follows may be better understood. Additional features and advantages of the disclosure will be described hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    For a more complete understanding of the present disclosure, and the advantages thereof, reference is now made to the following descriptions to be taken in conjunction with the accompanying drawings describing specific embodiments of the disclosure, wherein: 
           [0010]      FIG. 1  is a top view of an illustrative embodiment of a pump; 
           [0011]      FIG. 2  is a cross-sectional side view of an illustrative embodiment of a pump with a radial mechanical bearing; 
           [0012]      FIG. 3  is a cross-sectional top view of an illustrative embodiment of a pump thereof; 
           [0013]      FIG. 4  is a close up cross-sectional view of an area of an illustrative embodiment of a pump thereof; 
           [0014]      FIG. 5  is a cross-sectional view of an illustrative embodiment of an impeller; 
           [0015]      FIG. 6  is a cross-sectional view of an illustrative embodiment of a pump housing; 
           [0016]      FIG. 7  is a cross-sectional view of an illustrative embodiment of a motor housing of a pump; 
           [0017]      FIG. 8  is a top view of an illustrative embodiment of a pump; 
           [0018]      FIG. 9  is a cross-sectional side view of an illustrative embodiment of a pump with a radial and axial mechanical bearing; 
           [0019]      FIG. 10  is a close up cross-sectional view of an area of an illustrative embodiment of a pump thereof; 
           [0020]      FIGS. 11 and 12  are top and isometric views, respectively, of an illustrative embodiment of an impeller with a radial mechanical bearing; and 
           [0021]      FIGS. 13 and 14  are top and isometric views, respectively, of an illustrative embodiment of an impeller with a radial and axial mechanical bearing. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    Refer now to the drawings wherein depicted elements are not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views. 
         [0023]    Referring to the drawings in general, it will be understood that the illustrations are for the purpose of describing particular implementations of the disclosure and are not intended to be limiting thereto. While most of the terms used herein will be recognizable to those of ordinary skill in the art, it should be understood that when not explicitly defined, terms should be interpreted as adopting a meaning presently accepted by those of ordinary skill in the art. 
         [0024]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only, and are not restrictive of the invention, as claimed. In this application, the use of the singular includes the plural, the word “a” or “an” means “at least one”, and the use of “or” means “and/or”, unless specifically stated otherwise. Furthermore, the use of the term “including”, as well as other forms, such as “includes” and “included”, is not limiting. Also, terms such as “element” or “component” encompass both elements or components comprising one unit and elements or components that comprise more than one unit unless specifically stated otherwise. 
         [0025]    Implantable blood pumps utilize a variety of different methods to support impellers, such as mechanical, hydrodynamic, and magnetic bearings. Mechanical bearings can generate heat that can cause blood trauma resulting in blood clots or hemolysis. Hydrodynamic bearings may require small clearances for the bearing to properly operate. Magnetic bearings may utilize sensors and/or complex controls. 
         [0026]    An improved mechanical bearing system for a blood pump is discussed herein. A flow thru mechanical blood pump bearing system may cause minimal blood trauma, generate very little heat from friction, and can be thoroughly washed by blood flow to prevent the formation of blood clots. Further, these bearing systems are simple and robust, without requiring complicated controls and sensors, such as used in active magnetic bearing systems, or small clearances, such as used in some hydrodynamic bearing systems. 
         [0027]    Mechanical bearing systems discussed herein may be utilized in a variety of blood pump systems. A blood pump system may include a housing, an inlet for receiving blood, an impeller for aiding blood flow, a driver that rotates the impeller, and an outlet for outputting blood. Blood pump systems may utilize radial or axial flow impellers. In some embodiments, the driver may be a motor and shaft coupled to the impeller. In other embodiments, the driver may be a magnetic coupling that rotates the impeller. In some embodiments, other mechanical bearings, hydrodynamic bearings, magnetic bearings, or a combination thereof may be utilized in conjunction with the mechanical bearing system discussed herein. In some embodiments, a mechanical blood pump bearing system may be capable of operating for extended periods (e.g. &gt;5 years) with minimal mechanical wear and low friction. 
         [0028]    While the embodiments of mechanical bearing systems discussed herein may refer to specific types of blood pumps, including particular housing, inlet, impeller, driver, or outlet designs, it will be recognized that such discussion is provided for illustrative purposes only. 
         [0029]      FIG. 1  is a top view of an illustrative embodiment of pump  10 . Pump  10  is formed from pump housing  15  providing inlet  20  and outlet  25 . Pump housing  15  may be composed of two or more pieces and may be joined by welding. In other embodiments, pump housing  15  may be joined by fusing, press fit, threading, screw and elastomeric sealing, bonding, fasteners, any other suitable joining method or combinations thereof. Line A-A passing through pump housing  15  indicates the plane from which the cross-sectional view in  FIG. 2  is provided.  FIG. 2  is a cross-sectional side view of an illustrative embodiment of pump  10 . Motor housing  35  may be joined to pump housing  15  by welding, fusing, press fit, threading, screw and elastomeric sealing, bonding, fasteners, any other suitable joining method or combinations thereof. Pump housing  15  provides impeller chamber  30  for impeller  75 . Impeller chamber  30  has inlet  20  for connection to a fluid source and outlet  25  for providing fluid to a desired location. Impeller chamber  30  may be sealed and pressure tight to prevent fluid from entering/exiting impeller chamber  30  from locations other than inlet  20  and outlet  25 . 
         [0030]    Motor housing  35  is attached to pump housing  15  to form a fluid and/or pressure tight chamber for motor  40 . While motor housing  35  is shown as a separate component from pump housing  15 , in other embodiments, pump housing  15  and motor housing  35  may be combined to form a single combined housing. A cross-sectional view of an illustrative embodiment of motor  40  and motor housing  35  of pump  10  is shown in  FIG. 7 . In particular, motor housing  35  is shown separate from pump housing  15 . Motor  40  is entirely contained between pump housing  15  and motor housing  35 . A high efficiency electric motor can be utilized, such as an electric motor with efficiency of about 85% or greater. However, in other embodiments, any other suitable driving means can be utilized. Motor  40  provides shaft  45  with hub  50  mounted to shaft  45 . Hub  50  contains one or more permanent magnets and/or magnetic materials  55 . Motor  40  rotates shaft  45  causing permanent magnets  55  placed in hub  50  to rotate. In some embodiments, a motor with a useful life equal to or greater than 5 years is utilized. 
         [0031]    A cross-sectional view of an illustrative embodiment of pump housing  15  without impeller  75  is shown in  FIG. 6 . Pump housing  15  may provide a non-ferromagnetic and/or non-electrically conductive diaphragm  60  separating impeller chamber  30  from the chamber housing motor  40 . Diaphragm  60  defines cavity  70  providing a region for hub  50  to rotate within. A cross-sectional view of an illustrative embodiment of impeller  75  is shown in  FIG. 5 . Impeller  75  includes one or more permanent magnets and/or magnetic materials  80 . Permanent magnets  80  allow impeller  75  to be magnetically coupled to hub  50 . This magnetic coupling allows motor  40  to cause impeller  75  to rotate when motor  40  rotates hub  50 .  FIG. 4  is a close up cross-sectional view of an area C (see  FIG. 2 ) of an illustrative embodiment of pump  10 . Diaphragm  60  may provide cylindrical bearing surface  65  for impeller  75  to rotate around. An internal bore of impeller  75  provides at least three contact points  76  protruding from the internal bore surface that may contact cylindrical bearing surface  65 . Contact points  76  may be referred to as radial contact points since radial loads are supported by these contact points. While the nonlimiting embodiment shown utilizes four contact points  76 , other embodiments may utilize three or more contact points. Contact points  76  may be semi-spherically shaped, semi-ellipsoid shaped, concave shaped, or the like. Contact points  76  may be nonconforming or conforming. In some embodiments, the curvature of contact points  76  matches the curvature of bearing surface  65 . Contact points  76  may be equidistantly spaced around the internal bore surface of impeller  75  at an angle (a) to provide balanced operation, wherein a represent an angle between two contact points  76  relative to the center of impeller  75 . In the embodiment shown, the three contact points  76  are positioned at an equal height (h) from the bottom of impeller  75 , which is referred to herein as a row. In other embodiments, one or more contact points  76  may be aligned at two or more different heights. In another embodiment, multiple sets of contact points  76  may be utilized, and each set may be arranged in two or more rows. In other embodiments, contact points  76  may be disposed on cylindrical bearing surface  65 , rather than the internal bore of impeller  75 . 
         [0032]    The bearing surface  65  of diaphragm  60  may be coated with a biocompatible low friction coating, such as Diamond Like Carbon (DLC) or the like. Contact points  76  may also be coated with a biocompatible low friction coating, such as Diamond Like Carbon (DLC) or the like. In some embodiments, the contact points  76  may have a sliding Coefficient of Friction (COF) of 0.15 or less. The low friction coatings utilized on the bearing surface  65  and contact points  76  lower the amount of heat generated by friction. In other embodiments, bearing surface  65  and contact points  76  may be made from a hard biocompatible ceramic material, such as aluminum oxide, zirconium dioxide, silicon carbide, or silicon nitride. 
         [0033]      FIGS. 11 and 12  are top and isometric views, respectively, of an embodiment of an impeller. Because impeller  75  is precision balanced and is a high efficiency impeller operating near its Best Efficiency Point (BEP), radial loads acting on impeller  75  are kept to a minimum of less than 0.5 N under conditions suitable for a ventricular assist device (VAD). In the embodiment shown, low radial loads, low coefficient of friction contact points, and low sliding speed results in low heat generation. For example, the radial mechanical contact bearing system may result in heat generation less than 0.5 watts. In addition to low heat generation, the bearing system allows a large radial clearance  78  between the inside bore of impeller  75  and the bearing surface  65  of diaphragm  60 . In some embodiments, the radial clearance is equal to 0.005 inches or greater. A large radial clearance allows for improved continuous flushing of the radial clearance  78  with blood due the pressure differential between the underside of impeller  75  and pump inlet  20 . The continuous flushing provides two benefits: (1) minimizing the formation and/or growth of blood clots; and (2) removing heat generated within the radial mechanical contact bearing, which both minimize blood trauma. 
         [0034]    Line B-B of  FIG. 2  passing through pump housing  15  indicates the plane from which the cross-section view in  FIG. 3  is provided.  FIG. 3  is a cross-sectional top view of an illustrative embodiment of pump  10 . Impeller  75  is composed of an array of arc shaped segments  90  joined by central ring  95 . Pump housing  15  has volute  110  feeding the outlet  25 . In other embodiments, volute  110  could be omitted from pump housing  15  and outlet  25  could have any suitable orientation and shape. Pump housing  15  is designed in a manner where impeller  75 , when rotated, pressures and moves fluid received from inlet  20  to outlet  25 . 
         [0035]    Permanent magnets  55  in hub  50  and permanent magnets  80  in central ring  95  of impeller  75  form a magnetic coupling between the impeller  75  and hub  50 . In contrast to radial magnetic bearings that are arranged to repel each other, permanent magnets  55  and  80  are arranged so that they are attracted to each other. In order to further minimize radial loads, permanent magnets  55  and  80  provide a minimal magnetic coupling or just enough of a magnetic coupling to rotate impeller  75  under load. The attractive force of the magnetic coupling of permanent magnets  55  and  80  also provides axial restraint of impeller  75 . For example, axial movement of impeller  75  may misalign permanent magnets  55  and  80 . The attractive magnetic forces of permanent magnets  55  and  80  would restrain and re-align the magnets. Because of the magnetic forces caused by permanent magnets  55  and  80 , axial movement of impeller  75  may cause axial force to be exerted on shaft  45  and hub  50  of motor  40 , which is then transferred to bearing(s) (not shown) of motor  40 . 
         [0036]    Permanent magnets  80  may be sufficiently small in size that they have no impact on the main fluid flow paths of impeller  75 , thereby allowing the design of impeller  75  to focus on fully optimizing pump efficiency. These benefits can allow pumping efficiencies of greater than 50% to be achieved. 
         [0037]    Impeller  75  may be an open, pressure balanced type impeller to minimize axial thrust. Impeller  75  is considered to be open because there is no endplate on either side of arc shaped segments  90 . Further, clearance  78  relieves pressure under impeller  75  and minimizes axial thrust during the rotation of impeller  75 . However, other types of impellers may be suitable in other embodiments. Impeller  75  could be any other suitable blade shape, rotate in the opposite direction, or non-pressure balanced. For example, other suitable impellers may be semi-open type (i.e. end plate on one side of impeller) or closed type (i.e. end plate on both sides of impeller). 
         [0038]    Referring to  FIG. 4 , the magnetic coupling transmits torque from shaft  45  of the motor  40  to impeller  75 . Impeller radial support is provided by mechanical contact points  76 . In the embodiment shown, permanent magnets  55  and  80  are radially distributed around hub  50  and impeller  75 . The poles of permanent magnets  55  and  80  are arranged to attract to each other. The attractive force of the magnetic coupling of permanent magnets  55  and  80  provides axial restraint of impeller  75 . While permanent magnets  55  and  80  are shown as arc shaped like quadrants of a cylinder in  FIG. 3 , it should be recognized that permanent magnets  55  and  80  may be shaped in a variety of different manners to provide the magnetic coupling. For example, one or more ring shaped magnets polarized with arc shaped magnetic regions, square/rectangular shaped, rod shaped, disc shaped, or the like may be utilized. In the magnetic coupling arrangement shown, permanent magnets  80  are shown in the internal portion of impeller  75 . Internal magnetic couplings, similar to the arrangement shown, can be more efficient than face or external type magnetic couplings that place the magnets in the blades of an impeller or rotor because they have a smaller diameter and less eddy current losses. Diaphragm  60 , intermediate the coupling, is non-ferromagnetic and/or non-electrically conductive to minimize eddy current losses. For example, couplings with non-electrically conducting diaphragms such as bio-compatible ceramic, glass, or the like, would exhibit less eddy current losses than those with electrically conducting diaphragms. 
         [0039]      FIG. 8  is a top view of an illustrative embodiment of pump  120 . Line D-D passing through pump housing  150  indicates the plane from which the cross-sectional view in  FIG. 9  is provided.  FIG. 9  is a cross-sectional side view of an illustrative embodiment of pump  120 . Similar to the embodiment shown in  FIG. 2 , pump  120  provides pump housing  150 , impeller  125 , shaft  130 , hub  132 , permanent magnets  135  and  140 , motor housing  142 , motor  145 , impeller chamber  160 , and cylindrical bearing surface  169  which all provide a similar function to the components discussed previously. These common elements may operate in substantially the same manner as previously described. The substantial differences in the embodiments are discussed below. 
         [0040]      FIG. 10  is a close up cross-sectional view of an area E (see  FIG. 9 ) of an illustrative embodiment of pump  120 . In some embodiments of a bearing system, one or more top surfaces  165  of impeller  125  may include contact points  167  to provide axial restraint of the impeller  125 . Contact points  167  may be referred to as axial contact points since axial loads are supported by these contact points.  FIGS. 13 and 14  are top and isometric views, respectively, of impeller  125 . Top surfaces  165  of impeller  125  provide one or more contact points  167  that may contact a bearing surface  155  of pump housing  150 . While top surfaces  165  may be referred to as a singular “top surface” herein, it will be recognized that a “top surface” is not limited to a single surface and may collectively refer to all or a combination of one or more top surfaces provided by each arc shaped segment of impeller  125 . While the embodiments shown provide three contact points  167 , other embodiments may provide more than three contact points. Contact points  167  may be semi-spherically shaped, semi-ellipsoid shaped, flat shaped, or the like. Contact points  167  may be nonconforming or conforming. In some embodiments, the shape of contact points  167  matches the contour of bearing surface  155  of pump housing  150 . The bearing surface  155  of pump housing  150  may be coated with a biocompatible low friction coating, such as Diamond Like Carbon (DLC) or the like. Contact points  167  may also be coated with a biocompatible low friction coating, such as Diamond Like Carbon (DLC) or the like. In some embodiments, the contact points  167  have a sliding Coefficient of Friction (COF) of 0.15 or less. The low friction coatings utilized on bearing surface  155  and contact points  167  lowers the amount of heat generated by friction. In other embodiments, bearing surface  155  and contact points  167  may be made from a hard biocompatible ceramic material such as aluminum oxide, zirconium dioxide, silicon carbide, or silicon nitride. 
         [0041]    Because radial clearance  178  may be large enough to substantially relieve pressure under impeller  125 , axial loads are kept to a minimum. For example, in the embodiment shown an axial load of less than 0.5 N at each contact point may be achieved. The combination of low axial loads, low coefficient of friction contact points, and low sliding speeds results in low heat generation by the axial mechanical contact bearing system. For example, the axial mechanical contact bearing system may result in heat generation less than 0.5 watts. By using the axial mechanical contact bearing, large axial clearance  179  equal to 0.005 inches or greater may be used between the top surfaces  165  of impeller  125  and the bearing surface  155  of pump housing  150 . This allows for improved continuous flushing of the axial clearance  179  with blood due to the centrifugal action of impeller  125 . The continuous flushing provides two benefits: (1) minimizing the formation and/or growth of blood clots; and (2) removing heat generated within the axial mechanical contact bearing, which both minimize blood trauma. 
         [0042]    Implementations described herein are included to demonstrate particular aspects of the present disclosure. It should be appreciated by those of skill in the art that the implementations described herein merely represent exemplary implementation of the disclosure. Those of ordinary skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific implementations described and still obtain a like or similar result without departing from the spirit and scope of the present disclosure. From the foregoing description, one of ordinary skill in the art can easily ascertain the essential characteristics of this disclosure, and without departing from the spirit and scope thereof, can make various changes and modifications to adapt the disclosure to various usages and conditions. The implementations described hereinabove are meant to be illustrative only and should not be taken as limiting of the scope of the disclosure.