Magnetically-suspended centrifugal blood pump

A magnetically levitated blood pump having a single inlet for accepting blood and tangential volute. The pump includes an impeller formed on a hub that is suspended radially by permanent magnet bearings and axially adjusted via and thrust coil. The hub is rotated by an axial gap permanent magnet DC motor having motor magnets mounted on the hub adjacent to the impeller, a stator formed in the area opposite to the motor magnets, and motor coils formed on the stator. The axial air gap of the motor is formed in the impeller blade path. The motor may have two separate stator members. The current in the thrust coil is controlled by feedback of the impeller axial position measured by sensors. The pump components are compactly configured within a housing.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The invention relates to blood pumps suitable for permanent implantation in
 humans. More specifically, it relates to centrifugal pumps with
 magnetically suspended impellers suitable for use as ventricular assist
 devices.
 2. Description of the Prior Art
 Roughly 700,000 patients die from heart disease in the U.S. each year and
 35,000 to 70,000 of these could benefit from mechanical circulatory
 support or a heart transplant. However, only about 2,500 transplant hearts
 become available each year. This translates to a profound need for a
 reliable mechanical blood pump to serve as a cardiac assist device or
 artificial heart.
 Several prior-art devices attempt to solve this problem. Indeed, numerous
 embodiments of blood pumps exist, but are subject to significant
 operational problems. Such prior-art pumps are discussed hereinbelow.
 In U.S. Pat. No. 4,688,998 issued to Olsen et al., a motor stator is
 disclosed that consists of C-shaped rings. The rings substantially
 increase the diameter of the pump contrary to the anatomical requirement
 of small size and weight.
 In U.S. Pat. Nos. 4,763,032, 4,994,748, 5,078,741, 5,326,344, and
 5,385,581, all issued to Bramm et al., a device is disclosed that requires
 two inflow channels, which increase the total blood-wetted surface. Among
 other things, this large contact area between artificial materials and the
 blood increases immune system response to the pump as well as the
 probability of thromboembolism. Further, connecting the two inlets of the
 pump to the heart is complex and requires additional tubing. Thus,
 anatomical interference of such pumps with natural organs and structures
 is increased.
 In U.S. Pat. No. 5,112,202 issued to Oshima et al., a device is disclosed
 in the form of a centrifugal pump that utilizes a magnetic coupling with
 mechanical bearings subject to wear. This pump is not suitable for
 long-term implantation, as the bearings will eventually fail due to wear.
 In U.S. Pat. No. 5,195,877 issued to Kletschka, a device is disclosed that
 requires two inflow channels, which increase the total blood-wetted
 surface. This large contact area between artificial materials and the
 blood increases immune system response to the pump. The large surface area
 also increases the probability of thromboembolism. Further, connecting the
 two inlets of the pump to the heart is complex and requires additional
 tubing. Thus, anatomical interference of the pump with natural organs and
 structures is increased.
 In U.S. Pat. No. 5,443,503 issued to Yamane, a pump device is disclosed
 that has a jewel bearing. Such bearings are subject to wear in a long-term
 implant. Further, the jewel bearing is a point of blood stasis and is
 subject to clotting and may lead to thromboembolism.
 In U.S. Pat. No. 5,470,208 issued to Kletschka, a device is disclosed that
 requires two inflow channels, which increase the total blood-wetted
 surface. This large contact area between artificial materials and the
 blood increases immune system response to the pump. The large surface area
 also increases the probability of thromboembolism. Further, connecting the
 two inlets of the pump to the heart is complex and requires additional
 tubing such that anatomical interference of the pump with natural organs
 and structures is increased. As well, this prior-art device has a point of
 stasis opposite to the inlet, which is a potential site for thrombus
 formation.
 In U.S. Pat. No. 5,507,629 issued to Jarvik, a device is disclosed that
 includes a mechanical bearing in the form of a jewel bearing which is a
 point of blood flow stasis. The blood stasis point is a location of
 thrombus formation and a source of thromboembolism. Other embodiments of
 this invention levitate the rotor using only passive magnetic bearings
 which is inherently unstable especially during the requisite high-speed
 rotor rotation. Unstable rotors can contact the pump housing and
 potentially stop the blood flow.
 In U.S. Pat. Nos. 5,695,471 and 5,840,070, both issued to Wampler, a blood
 pump is disclosed. Wampler '471 is similar to the device of Jarvik '629 in
 that there a stasis point at the jewel bearing. The stasis point is a site
 of thrombus formation and a source of thromboembolism. Further, the jewel
 bearing will eventually wear out and the impeller will cease to rotate.
 Wampler '070 uses a hydrodynamic thrust bearing. Such a bearing is highly
 inappropriate for use within blood processing because such bearings can
 damage the blood via high mechanical shear that is inherent to such
 bearings.
 In U.S. Pat. No. 5,725,357 issued to Nakazeki, a device is disclosed in the
 form of a pump that contains a motor with mechanical bearings subject to
 wear. Such device is not suitable for a long-term implant as the
 mechanical bearings will eventually fail and cause the pump to stop
 working.
 In U.S. Pat. No. 5,928,131 issued to Prem, a device is disclosed that uses
 a radial motor wherein the blood flows through the center of the motor.
 This reduces the allowable permanent magnet material in the motor and
 reduces its efficiency. Further, the elongated structure of the pump
 exposes blood to large regions of foreign material, which increases the
 likelihood of blood damage and thrombus formation. There is also a large
 region of high blood shear. Blood shear causes blood damage and can
 trigger undesirable clotting mechanisms in the body. Finally, the
 cantilevered design, with both bearings on the inlet side of the impeller
 impairs the rotor dynamics stability or requires larger, bulkier magnets.
 From the discussion above, it becomes critically apparent that existing
 devices on the market are overly complex, prone to mechanical failure,
 promote thromboembolism and strokes, and otherwise suffer from
 shortcomings related to their ineffective designs.
 Accordingly, it is desirable to provide for a new and improved, effective
 rotary blood pump suitable for long-term implantation into humans for
 artificial circulatory support. What is needed is such a blood pump that
 is highly reliable. What is also needed is such a blood pump that meets
 anatomical requirements with a compact physical design. What is further
 needed is such a blood pump that minimizes blood-wetted surface area.
 Still, what is needed is such a blood pump that minimizes deleterious
 effects on blood and its circulatory system, the immune system, and other
 related biological functions. What is also needed is such a blood pump
 that is not only resilient to everyday accelerations and bodily movements,
 but also includes stable rotor dynamics, a high motor efficiency, high
 fluid efficiency, low power consumption for levitation, low vibration, low
 manufacturing costs, and increased convenience to the patient. Still
 further, what is needed is a blood pump which overcomes at least some of
 the disadvantages of the prior-art while providing new and useful
 features.
 SUMMARY OF THE INVENTION
 It is an objective of the present invention to provide a highly reliable
 rotary blood pump suitable long-term implantation into humans for
 artificial circulatory support. More specifically, the objects of this
 invention are: to meet anatomical requirements of a compact device design;
 minimize blood-wetted surface area; minimize deleterious effects on the
 blood and the immune system, and other biological functions; be resilient
 to everyday accelerations and bodily movements; have stable rotor
 dynamics; have high motor efficiency; have high fluid efficiency; have low
 power consumption for levitation; have low vibration; have reduced
 manufacture costs; and have increased convenience to the patient.
 The present invention is directed to a magnetically suspended centrifugal
 blood pump, and includes mixed-flow pumps with an axial inlet and radial
 outlet. By nature of the novel component configuration of the pump, it is
 compact and has minimal blood-wetted surfaces. Such compactness reduces
 internal surface area of the pump so that both impact on the immune system
 and probability of the formation of blood clots are significantly
 decreased. Through the use of magnetic levitation, the mechanical shear,
 and hence damage, to the blood is minimized. Further, high mechanical
 reliability is achieved due to there being no moving parts that contact
 one another so as to wear.
 The magnetic suspension takes advantage of high-energy magnet technology.
 Such permanent magnets may be formed of any material such as, but not
 limited to, neodymium-iron-boron magnets so long as such material has a
 high coercivity and remanance. These magnets robustly suspend the pump
 impeller in the blood stream so that it does not impact the housing in
 normal usage. Such high-energy magnets also provide for high efficiency in
 an included direct current (DC) brushless motor as well as enable overall
 compactness of the pump design. Fluid efficiency is achieved by matching
 the specific speed of the pump to the flow conditions resulting in the
 centrifugal (i.e., mixed-flow) design concept of the present invention.
 The pump impeller may be conical or take on any various curved shapes such
 as are commonly found in water pumps and turbochargers so long as blood
 damage is avoided. Finally, magnetic levitation provides for low vibration
 levels in the pump.
 Biocompatibility is assured by the present invention due to three important
 considerations. Such considerations include the operating temperatures,
 the materials used to fabricate the instant device, and the anatomical fit
 inherent to the compact design.
 High efficiency of the pump results in low temperature rise of the motor.
 As contact with bodily tissues is inherent to the invention, the reduction
 in operating temperatures minimizes related damage to surrounding tissues.
 Exterior materials such as titanium or titanium alloys are used for the
 outer housing of the invention. Such material surfaces when pure and
 smooth are relatively inert and aseptic when introduced into the human
 body so occurrences like immune system rejection and bacterial growth are
 substantially eliminated. Although titanium is discussed, any similarly
 suitable material that provides biochemical compatibility with surrounding
 tissues would be appropriate for use in the present invention. The
 invention also includes coating of all blood-wetted surfaces by materials
 such as, but not limited to, diamond-like carbon, titanium nitride, or
 some form of Teflon.RTM. (a non-stick material chemically identified as
 polytetrafluoroethylene, or PTFE). Such materials promote good blood-flow
 and provide for good hemocompatiblity. While only a few materials are
 specifically mentioned hereinabove, it should be understood that equally
 suitable materials could be utilized without straying from the intended
 scope of the present invention.
 A good anatomical fit is assured through the compactness of the present
 invention's design. The existence of a single inlet with extreme placement
 of permanent magnet bearings and motor location within the bladed areas
 results in a compact configuration. Such compactness in design provides
 for an anatomical fit suitable for abdominal placement without crowding of
 body tissues and organs. This is a critical point in that an increase in
 compactness will reduce the stresses placed on the implant patient and
 result in a less obtrusive and more comfortable implant. The ultimate
 result being a decrease in recovery time and an overall increase in device
 effectiveness.
 The invention will be described for the purposes of illustration only in
 connection with certain embodiments; however, it is to be understood that
 other objects and advantages of the present invention will be made
 apparent by the following description of the drawings according to the
 present invention. While a preferred embodiment is disclosed, this is not
 intended to be limiting. Rather, the general principles set forth herein
 are considered to be merely illustrative of the scope of the present
 invention and it is to be further understood that numerous changes may be
 made without straying from the scope of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 The present invention is drawn to a magnetically levitated, axially
 controlled blood pump. Although this preferred design and use is discussed
 in detail herein, it should be noted that the present invention may be
 utilized in a variety of similar manners. Such various settings include,
 without limitation, chemical flow control, gas flow control including
 turbine designs, and any situation where a reliable and efficient movement
 of a fluid substance within a limited space is desired. For purposes of
 illustration, discussion of the present invention will be made in
 reference to its utility as a cardiac assist blood pump. Additionally,
 wiring for power and sensor control has been omitted in the detailed
 description for the sake of clarity. However, it should be readily
 understood that such wiring may be accomplished by any known method
 available to one of ordinary electromechanical skill.
 Referring now to FIG. 1a, there is shown a preferred embodiment of the
 cardiac assist blood pump 100. The pump 100 is intended to be fully
 implanted in an animal or human patient. The pump 100 includes two
 independent subassemblies in the form of a housing 22 and an rotor hub 6.
 The pump also includes an inlet 2 and an outlet in the form of a volute 4.
 For purposes of descriptive clarity, the end of the pump 100 that is
 opposite the inlet 2 will be referred to as the "rear" of the pump 100.
 The rotor hub 6 is inertially balanced in a manner consistent with common
 practice in rotating machinery. The rotor hub 6 is supported magnetically
 and without contact in a radial direction by permanent magnet (PM)
 bearings (8a, 8b) and (10a, 10b) and in an axial direction by thrust coil
 18. Each of the PM bearings (8a, 8b) and (10a, 10b) consists of an outer
 race 8a and 10a and an inner race 8b and 10b with magnetization directions
 shown with arrows. While each of the outer races 8a and 10a and inner
 races 8b and 10b may be formed from single magnet rings, it should be
 understood that any common method might be used so long as a PM bearing is
 formed. For example, magnet ring stacks may be used for the inner and
 outer races, and ferromagnetic rings serving as pole pieces may be
 combined with magnet rings.
 The PM bearing inner race 10b also interacts with thrust coil 18 to form an
 active magnetic thrust bearing that actuates axial movement of the rotor
 hub 6. The thrust coil 18 is disposed toward the outside diameter of one
 of two stators (14a, 16a) and (14b, 16b). The two stators (14a, 16a) and
 (14b, 16b) interact with motor magnets 12 to form a four-pole DC brushless
 motor. The motor magnets 12 are located on the rotor hub 6 alongside
 impeller blades 13. Such placement of the motor allows a small motor fluid
 gap to exist due to the increased velocity of blood therethrough. Because
 of the increased flow of blood in this area, the path need not be wide.
 Reducing the size of this path--i.e., the motor gap--results in a lighter,
 more efficient motor structure.
 The two stators (14a, 16a) and (14b, 16b) consist of ferromagnetic rings
 14a and 14b and motor windings 16a and 16b. The ferromagnetic rings 14a
 and 14b are preferably composed of laminated 3% silicon-iron, 50%
 cobalt-iron, 49% nickel iron for example, or any other ferromagnetic
 materials with high saturation flux density, low bulk conductivity, and
 low hysteresis. Selective energization of the motor windings 16a and 16b
 provides forces that interact with motor magnets 12 in a manner consistent
 with known motor technology so as to rotate the rotor hub 6 and attached
 impeller blades 13. For a three-phase, four-pole motor, motor windings 16a
 and 16b consist of twelve individual coils. By utilizing this axial gap
 motor design having two stators, the overall efficiency is increased while
 physical size is minimized. Control of such an axial gap motor can be by
 any known method in the electrical art. However, the preferred controller
 is the back-EMF type of controller used in most computer disk drives
 manufactured at present.
 With reference to FIG. 1b, the motor magnets 12 and PM radial bearing inner
 race 10b are shown in cross-section where "N" indicates a magnetic North
 pole and "S" indicating a magnetic South pole. While the inner race 10b
 has uniform polarity, the motor magnet 12 as shown includes four motor
 magnets having alternating polarity making a four-pole motor. It should be
 understood that any number of poles as well as Halbach arrays may be for
 the motor permanent magnet structure comprising the magnets 12. Moreover,
 the PM bearings (8a, 8b), (10a, 10b) and motor magnet 12 are designed so
 that magnetic forces do not cause vibrational forces on the rotor during
 operation. This is accomplished by precisely forming the PM bearings (8a,
 8b) and (10a, 10b) and motor magnet 12 from a magnetic material with high
 uniformity in addition to high-energy density. Accordingly, all included
 permanent magnet materials are preferably neodymium-iron-boron or samarium
 cobalt, but may be selected from any other permanent magnet material so
 long as they are have high-energy density exhibiting high coercively and
 remanance.
 With additional reference to FIG. 1c, the volute 4 is shown as taken along
 line 1c--1c FIG. 1a. During operation of the pump 100, blood flowing into
 the inlet 2 is propelled through the pump 100 by the impeller blades 13
 and out of the pump 100 through the volute 4. Through experimentation with
 existing pump designs, computer simulation called computational fluid
 dynamics (CFD), and testing with transparent fluid having dispersed
 particles, the shape of the rotor hub 6, impeller blades 13, and volute 4
 can be optimized. The particles enable the visualization of fluid flow in
 the pump through common means collectively known as "flow visualization."
 Eddy flows and stagnant flow zones are minimized, and overall pump
 efficiency is optimized through such methods. Accordingly, the shapes of
 the rotor hub 6, impeller blades 13, and volute 4 may be adjusted to this
 end and will vary according to nominal pump speed and flow rates.
 With reference to FIG. 2, a second preferred embodiment of the present
 invention is shown. Blood pump 200 differs from that of FIG. 1a in that
 the rear-most PM radial bearings 210a, 210b are relocated and are smaller
 in diameter. Such a diameter in the rear-most PM radial bearings 210a,
 210b has been found to increase overall stability with regard to pitch and
 yaw motion of the rotor hub 206. Further, relative to FIG. 1a this
 embodiment differs in that the impeller blades 213 are moved to the inlet
 side of the rotor hub 206. As well, blood ducts 215 in the rotor hub 206
 are added so as to channel blood quickly and directly therethrough. A
 separate magnet ring 225 is used in conjunction with thrust coil 218 for
 forming the active magnetic thrust bearing that actuates axial movements.
 While such an active magnetic thrust bearing arrangement may be formed
 from as shown by a single coil interacting with a single magnet ring, it
 should be understood that any common method might be used so long as an
 active magnetic thrust bearing is formed.
 Common to both embodiments of the present invention as shown in FIGS. 1a-2,
 the current in thrust coil 18 (218) is controlled as follows. Axial
 position sensors 20 (220) measure the axial position of rotor hub 6 (206).
 An external feedback controller (as depicted in FIG. 3) applies current
 through electrical wiring (not shown) to the thrust coil 18 (218). The
 current is adjusted in order to position the rotor hub 6 (206) axially
 without mechanical contact with the housing 2 (222). While virtual zero
 power (VZP) control is depicted in FIG. 3, it should be understood that
 there are many possible feedback control algorithms for controlling the
 coil current. Such algorithms include proportional-integralderivative
 (PID) or any suitable method well known to those skilled in the art of
 magnetic bearing control. The axial position sensors 20 (220) can be any
 reliable non-contact position sensor suitable for unobtrusive placement
 within the housing 2 (222)--e.g., eddy-current type, variable reluctance
 type, acoustic, infrared reflectance type, and similar sensors.
 With further reference to FIG. 3, one form of a VZP control algorithm is
 shown in a block diagram. Standard control system design notation is used
 with "s" being the Laplace variable. The controller consists of an input
 summing junction 30, an output summing junction 38, integrator block 32,
 differentiator block 34, proportional gain block 36, and a current
 amplifier 40. Together, the blocks 32, 34, and 36 impose a coil current in
 thrust coil 18 (218) in response to the output of the position sensor 20,
 (220) through a computation depicted in the block diagram. The computation
 may be made with analog circuits or with digital circuits. The essential
 features of the VZP controller are that it has low gain at low
 frequencies--e.g., zero gain at DC is commonly used--and it stabilizes the
 position of the rotor hub 6 (206) in the axial direction through choice of
 controller gains K.sub.i, K.sub.p, and K.sub.d.
 The control algorithm embodiment shown in FIG. 3 accomplishes low gain at
 low frequency by way of negative integrator feedback through integrator
 block 32 of the coil current command input to the current amplifier 40. If
 the current command is positive, the integrator block 32 increases its
 output, which is subtracted from the sensor position signal output at 40.
 The net effect is that the coil current is returned to zero in the steady
 state, and the rotor hub 6 (206) is moved to a natural axial equilibrium
 point. The natural equilibrium balances the magnetic negative stiffness
 forces of the PM bearings with the fluid forces pushing on the impeller
 blades 13 (213) and rotor hub 6 (206). It should be realized that there
 are many ways to accomplish VZP control. Accordingly, those skilled in the
 art may use Linear Quadratic Regulator controllers (LQR) or H-infinity
 controllers for example, so long as low gain at low frequencies is
 accomplished and the rotor hub 6 (206) position is stabilized.
 During rotational operation of the instant invention, the gap 3 (203) is
 maintained at a positive value by the magnetic bearings. Blood flows
 through this gap 3 (203) depending on the details of the design. The
 direction of blood flow within the gap 3 (203) may be further controlled
 by adding rifling or small blades on the rotor hub in the gap 3 (203). As
 well, pressure differentials from one end of the gap 3 (203) to the other
 can be used to control the flow direction. In all situations, a nonzero
 flow rate is accomplished and stagnation of blood in gap 3 (203) is
 avoided. The dimension of gap 3 (203) is designed so as to balance
 possible blood damage against PM bearing effectiveness. That is to say,
 blood damage is avoided by keeping the gap sufficiently large, but not so
 large that the PM bearings lose their effectiveness. Further, the gap 3
 (203) can be varied along the length of the pump 100 (200) so that it is
 small near the PM bearings to achieve good bearing stiffness. Likewise,
 the gap 3 (203) can be larger in areas where there are no magnetic
 components and thus reduce average blood shear in the gap 3 (203).
 All blood-contacting surfaces of the pump 100 (200) are preferably formed
 from a blood compatible material such as polished titanium or titanium
 alloy (e.g., Ti.sub.6 Al.sub.4 V).sub.1 diamond-like carbon, titanium
 nitride, or a fluorinated hydrocarbon such as Teflon.RTM. (a non-stick
 material chemically identified as polytetrafluoroethylene, or PTFE). It
 should be clear that any coating having a high blood compatibility as
 adjudged by such tests as defined in the International Standards
 Organization (ISO) document 10993-4 may be used. All included permanent
 magnet materials are preferably neodymium-iron-boron or samarium cobalt,
 or any other permanent magnet material with a high-energy density. The
 ferromagnetic rings 14a and 14b are composed of laminated 3% silicon-iron,
 50% cobalt-iron, 49% nickel iron for example, or any other ferromagnetic
 materials with high saturation flux density, low bulk conductivity, and
 low hysteresis.
 Accordingly, the configuration disclosed above and claimed hereinbelow
 reveals a compact design having a single inlet that provides for superior
 anatomical fit. Combining the motor with the impeller near the impeller
 blades minimizes blood-wetted area. Co-location of the motor with the
 impeller blades also increases efficiency as the blood gap at the impeller
 blades is relatively small compared to other parts of the pump. Such a
 reduced gap results in higher magnetic fields and efficiency in the motor.
 Through the use of high-energy density magnetic materials and feedback
 control, the present invention is robust to everyday accelerations. The
 feedback control of the axial position with the active thrust bearing
 provides for stable rotor dynamics as does inertial balance of the rotor
 hub. High fluid efficiency is achieved by using the mixed-flow design and
 CFD optimization of the pump. Low levitation power is accomplished with
 the preferred VZP control algorithm that adjusts the rotor position to a
 point of axial equilibrium so that that no steady-state current is
 required in the coils during operation. Low vibration is accomplished
 through inertial balancing of the rotor and control of magnet uniformity
 and size in the motor and bearings.
 It should be understood that the preferred embodiments mentioned here are
 merely illustrative of the present invention. Numerous variations in
 design and use of the present invention may be contemplated in view of the
 following claims without straying from the intended scope and field of the
 invention herein disclosed.