U.S. patent application number 13/758887 was filed with the patent office on 2013-08-29 for rotary pump with hydrodynamically suspended impeller.
This patent application is currently assigned to Thoratec Corporation. The applicant listed for this patent is Thoratec Corporation. Invention is credited to Geoffrey Douglas Tansley, Peter Andrew Watterson, John Campbell Woodard.
Application Number | 20130225910 13/758887 |
Document ID | / |
Family ID | 3803312 |
Filed Date | 2013-08-29 |
United States Patent
Application |
20130225910 |
Kind Code |
A1 |
Woodard; John Campbell ; et
al. |
August 29, 2013 |
Rotary Pump with Hydrodynamically Suspended Impeller
Abstract
A pump assembly 1, 33, 200 adapted for continuous flow pumping
of blood. In a particular from the pump 1, 200 is a centrifugal
pump wherein the impeller 100, 204 is entirely sealed within the
pump housing 2, 201 and is exclusively hydrodynamically suspended
therein as the impeller rotates within the fluid 105 urged by
electromagnetic means external to the pump cavity 106, 203.
Hydrodynamic suspension is assisted by the impeller 100, 204 having
deformities therein such as blades 8 with surfaces tapered from the
leading edges 102, 223 to the trailing edges 103, 224 of bottom and
top edges 221, 222 thereof.
Inventors: |
Woodard; John Campbell;
(Turramurra, AU) ; Watterson; Peter Andrew;
(Denistore, AU) ; Tansley; Geoffrey Douglas;
(Kegworth, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Thoratec Corporation; |
|
|
US |
|
|
Assignee: |
Thoratec Corporation
Pleasanton
CA
|
Family ID: |
3803312 |
Appl. No.: |
13/758887 |
Filed: |
February 4, 2013 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
13181452 |
Jul 12, 2011 |
8366381 |
|
|
13758887 |
|
|
|
|
12347263 |
Dec 31, 2008 |
8002518 |
|
|
13181452 |
|
|
|
|
11212227 |
Aug 26, 2005 |
7476077 |
|
|
12347263 |
|
|
|
|
10634211 |
Aug 5, 2003 |
6966748 |
|
|
11212227 |
|
|
|
|
09893319 |
Jun 26, 2001 |
6638011 |
|
|
10634211 |
|
|
|
|
09299038 |
Apr 23, 1999 |
6250880 |
|
|
09893319 |
|
|
|
|
09281608 |
Mar 30, 1999 |
6227797 |
|
|
09299038 |
|
|
|
|
PCT/AU98/00725 |
Sep 7, 1998 |
|
|
|
09281608 |
|
|
|
|
Current U.S.
Class: |
600/16 ;
29/889.7 |
Current CPC
Class: |
A61M 1/1015 20140204;
A61M 1/1012 20140204; A61M 1/1031 20140204; A61M 1/1017 20140204;
F01D 25/22 20130101; F04D 13/00 20130101; F16C 2316/18 20130101;
F16C 32/044 20130101; A61M 1/101 20130101; F16C 2360/44 20130101;
Y10S 415/90 20130101; F04D 29/047 20130101; Y10T 29/49336 20150115;
A61M 1/12 20130101; A61M 1/122 20140204; F04D 13/064 20130101 |
Class at
Publication: |
600/16 ;
29/889.7 |
International
Class: |
A61M 1/10 20060101
A61M001/10; F04D 13/00 20060101 F04D013/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 5, 1997 |
AU |
PO 9027 |
Claims
1-18. (canceled)
19. A heart assist device, comprising: an impeller suspended in use
within a pump housing by at least one of magnetic forces and
hydrodynamic forces generated by relative movement of said impeller
with respect to and within the pump housing; the impeller including
a magnet material for deriving drive torque by interaction with
currents in windings within the pump housing; and a coating
configured to minimize wear on touching surfaces of the impeller
and inner walls of the pump housing during start-up or touch
down.
20. The device of claim 19, the impeller including blades, wherein
the blades are encapsulated in the coating.
21. The device of claim 19, wherein the coating is applied to the
inner walls of the pump housing.
22. The device of claim 19, wherein the coating comprises a thin
coating of biologically compatible material.
23. The device of claim 19, wherein the coating comprises a hard
material.
24. The device of claim 23, wherein the coating comprises diamond
or titanium-nitride.
25. The device of claim 19, wherein the coating is applied to the
inner walls of the pump housing, the coating comprising a hard
material.
26. The device of claim 19, wherein the coating has a thickness of
approximately 1 micron.
27. A method for making a heart assist device, comprising: forming
an impeller including blades; forming a housing for the impeller;
and coating one of the impeller blades and inner walls of the
housing to minimize wear between contacting surfaces during a
start-up or touch down.
28. The method of claim 27, wherein the coating comprises a
biologically compatible material.
29. The method of claim 27, wherein the coating comprises a hard
material.
30. The method of claim 29, wherein the coating comprises
depositing a coating of the hard material using chemical vapor
deposition.
31. The method of claim 29, wherein the coating comprises
depositing a coating of the hard material using physical vapor
deposition.
32. The method of claim 29, wherein the hard material is diamond or
titanium nitride.
Description
[0001] This application is a continuation of U.S. patent
application Ser. No. 13/181,452 filed Jul. 12, 2011(the entire
contents of which are hereby incorporated by reference for all
purposes), which is a continuation of U.S. patent application Ser.
No. 12/347,263 filed Dec. 31, 2008, which is a continuation of U.S.
patent application Ser. No. 11/212,227 filed Aug. 26, 2005, now
U.S. Pat. No. 7,476,077, which is a continuation of U.S. patent
application Ser. No. 10/634,211 filed Aug. 5, 2003, now U.S. Pat.
No. 6,966,748, which is a continuation of U.S. patent application
Ser. No. 09/893,319 filed Jun. 26, 2001, now U.S. Pat. No.
6,638,011, which is a continuation of U.S. patent application Ser.
No. 09/299,038 filed Apr. 23, 1999, now U.S. Pat. No. 6,250,880,
which is a continuation-in-part of U.S. patent application Ser. No.
09/281,608 filed Mar. 30, 1999, now U.S. Pat. No. 6,227,797, which
is a continuation of Application PCT/AU98/00725 filed Sep. 7, 1998,
which claims priority to Australian patent document PO 9027 filed
Sep. 5, 1997.
FIELD OF THE INVENTION
[0002] This invention relates to rotary pumps adapted, but not
exclusively, for use as artificial hearts or ventricular assist
devices and, in particular, discloses in preferred forms a
seal-less shaft-less pump featuring open or closed (shrouded)
impeller blades with at least parts of the impeller used as
hydrodynamic thrust bearings and with electromagnetic torque
provided by the interaction between magnets embedded in the blades
or shroud and a rotating current pattern generated in coils fixed
relative to the pump housing.
BACKGROUND ART
[0003] This invention relates to the art of continuous or pulsatile
flow rotary pumps and, in particular, to electrically driven pumps
suitable for use although not exclusively as an artificial heart or
ventricular assist device. For permanent implantation in a human
patient, such pumps should ideally have the following
characteristics: no leakage of fluids into or from the bloodstream;
parts exposed to minimal or no wear; minimum residence time of
blood in pump to avoid thrombosis (clotting); minimum shear stress
on blood to avoid blood cell damage such as haemolysis; maximum
efficiency to maximise battery duration and minimise blood heating;
and absolute reliability.
[0004] Several of these characteristics are very difficult to meet
in a conventional pump configuration including a seal, i.e. with an
impeller mounted on a shaft which penetrates a wall of the pumping
cavity, as exemplified by the blood pumps referred to in U.S. Pat.
No. 3,957,389 to Rafferty et al., U.S. Pat. No. 4,625,712 to
Wampler, and U.S. Pat. No. 5,275,580 to Yamazaki. Two main
disadvantages of such pumps are firstly that the seal needed on the
shaft may leak, especially after wear, and secondly that the rotor
of the motor providing the shaft torque remains to be supported,
with mechanical bearings such as ball-bearings precluded due to
wear. Some designs, such as U.S. Pat. No. 4,625,712 to Wampler and
U.S. Pat. No. 4,908,012 to Moise et al., have overcome these
problems simultaneously by combining the seal and the bearing into
one hydrodynamic bearing, but in order to prevent long residence
times they have had to introduce means to continuously supply a
blood-compatible bearing purge fluid via a percutaneous tube.
[0005] In seal-less designs, blood is permitted to flow through the
gap in the motor, which is usually of the brushless DC type, i.e.
comprising a rotor including permanent magnets and a stator in
which an electric current pattern is made to rotate synchronously
with the rotor. Such designs can be classified according to the
means by which the rotor is suspended: contact bearings, magnetic
bearings or hydrodynamic bearings, though some designs use two of
these means.
[0006] Contact or pivot bearings, as exemplified by U.S. Pat. No.
5,527,159 to Bozeman et al. and U.S. Pat. No. 5,399,074 to Nose et
al., have potential problems duo to wear, and cause very high
localised heating and shearing of the blood, which can cause
deposition and denaturation of plasma proteins, with the risk of
embolisation and bearing seizure.
[0007] Magnetic bearings, as exemplified by U.S. Pat. No. 5,350,283
to Nakazeki et al., U.S. Pat. No. 5,326,344 to Bramm et al. and
U.S. Pat. No. 4,779,614 to Moise et al., offer contactless
suspension, but require rotor position measurement and active
control of electric current for stabilisation of the position in at
least one direction, according to Earnshaw's theorem. Position
measurement and feedback control introduce significant complexity,
increasing the failure risk. Power use by the control current
implies reduced overall efficiency. Furthermore, size, mass,
component count and cost are all increased.
[0008] U.S. Pat. No. 5,507,629 to Jarvik claims to have found a
configuration circumventing Earnshaw's Theorem and thus requiring
only passive magnetic bearings, but this is doubtful and contact
axial bearings are included in any case. Similarly, passive radial
magnetic bearings and a pivot point are employed in U.S. Pat. No.
5,443,503 to Yamane.
[0009] Prior to the present invention, pumps employing hydrodynamic
suspension, such as U.S. Pat. No. 5,211,546 to Isaacson et al. and
U.S. Pat. No. 5,324,177 to Golding et al., have used journal
bearings, in which radial suspension is provided by the fluid
motion between two cylinders in relative rotation, an inner
cylinder lying within and slightly off axis to a slightly larger
diameter outer cylinder. Axial suspension is provided magnetically
in U.S. Pat. No. 5,324,177 and by either a contact bearing or a
hydrodynamic thrust beating in U.S. Pat. No. 5,211,546.
[0010] A purging flow is needed through the journal bearing, a high
shear region, in order to remove dissipated heat and to prevent
long fluid residence time. It would be inefficient to pass all the
fluid through the bearing gap, of small cross-sectional area, as
this would demand an excessive pressure drop across the bearing.
Instead a leakage path is generally provided from the high pressure
pump outlet, through the bearings and back to the low pressure pump
inlet, implying a small reduction in outflow and pumping
efficiency. U.S. Pat. No. 5,324,177 provides a combination of
additional means to increase the purge flow, namely helical grooves
in one of the bearing surfaces, and a small additional set of
impellers.
[0011] U.S. Pat. No. 5,211,546 provides 10 embodiments with various
locations of cylindrical bearing surfaces. One of these
embodiments, the third, features a single journal bearing and a
contact axial bearing.
[0012] Embodiments of the present invention offer a relatively low
cost and/or relatively low complexity means of suspending the rotor
of a seal-less blood pump, thereby overcoming or ameliorating the
problems of existing devices mentioned above.
SUMMARY OF THE INVENTION
[0013] According to one aspect of the present invention, there is
disclosed a rotary blood pump for use in a heart assist device or
like device, said pump having an impeller suspended in use within a
pump housing exclusively by hydrodynamic thrust forces generated by
relative movement of said impeller with respect to and within said
pump housing.
[0014] Preferably at least one of said impeller or said housing
includes at least one deformed surface which, in use, moves
relative to a facing surface on the other of said impeller or said
housing thereby to cause a restriction in the form of a reducing
distance between the surfaces with respect to the relative line of
movement of said deformed surface thereby to generate relative
hydrodynamic thrust between said impeller and said housing which
includes everywhere a localized thrust component substantially and
everywhere normal to the plane of movement of said deformed surface
with respect to said facing surface.
[0015] Preferably the combined effect of the localized normal
forces generated on the surfaces of said impeller is to produce
resistive forces against movement in three translational and two
rotational degrees of freedom thus supporting the impeller for
rotational movement within said housing exclusively by hydrodynamic
forces.
[0016] Preferably said thrust forces are generated by blades of
said impeller.
[0017] More preferably said thrust forces are generated by edges of
said blades of said impeller.
[0018] Preferably said edges of said blades are tapered or
non-planar so that a thrust is created between the edges and the
adjacent pump casing during relative movement therebetween.
[0019] Preferably said edges of said blades are shaped such that
the gap at the leading edge of the blade is greater than at the
trailing edge and thus the fluid which is drawn through the gap
experiences a wedge shaped restriction which generates a
thrust.
[0020] Preferably the pump is of centrifugal type or mixed flow
type with impeller blades open on both front and back faces of the
pump housing.
[0021] Preferably the front face of the housing is made conical, in
order that the thrust perpendicular to the conical surface has a
radial component, which provides a radial restoring force to a
radial displacement or the impeller axis during use.
[0022] Preferably the driving torque of said impeller derives from
the magnetic interaction between permanent magnets within the
blades of the impeller and oscillating currents in windings
encapsulated in the pump housing.
[0023] Preferably said blades include magnetic material therein,
the magnetic material encapsulated within a biocompatible shell or
coating.
[0024] Preferably said biocompatible shell or coating comprises a
diamond coating or other coating which can be applied at low
temperature.
[0025] Preferably internal walls of said pump which can come into
contact with said blades during use are coated with a hard material
such as titanium nitride or diamond coating.
[0026] Preferably said impeller comprises an upper conical shroud
having said taper or other deformed surface therein and wherein
blades of said impeller are supported below said shroud.
[0027] Preferably said impeller further includes a lower shroud
mounted in opposed relationship to said upper conical shroud and
whereas said blades are supported within said upper and said lower
shroud.
[0028] Preferably said deformed surface is located on said
impeller.
[0029] Preferably said deformed surface is located within said
housing.
[0030] Preferably forces imposed on said impeller in use, other
than hydrodynamic forces, are controlled by design so that, over a
predetermined range of operating parameters, said hydrodynamic
thrust forces provide sufficient thrust to maintain said impeller
suspended in use within said pump housing.
[0031] Preferably at least one face of the housing is made conical,
in order that the thrust perpendicular to it has a radial
component, which provides a radial restoring force to a radial
displacement of the impeller axis. Similarly, an axial displacement
toward either the front or the back face increases the thrust from
that face and reduces the thrust from the other face. Thus the sum
of the forces on the impeller due to inertia (within limits),
gravity and any bulk radial or axial hydrodynamic force on the
impeller can be countered by a restoring force from the thrust
bearings after a small displacement of the impeller within the
housing relative to the housing in either a radial or axial
direction.
[0032] In a preferred embodiment, the impeller driving torque
derives from the magnetic interaction between permanent magnets
within the blades of the impeller and oscillating currents in
windings encapsulated in the pump housing.
[0033] In a further broad form of the invention there is provided a
rotary blood pump having an impeller suspended exclusively
hydrodynamically by thrust forces generated by the impeller during
movement in use of the impeller.
[0034] Preferably said thrust forces are generated by blades of
said impeller or by deformities therein.
[0035] More preferably said thrust forces are generated by edges of
said blades of said impeller.
[0036] Preferably said edges of said blades are tapered.
[0037] In an alternative preferred form said pump is of axial
type.
[0038] Preferably within a uniform cylindrical section of the pump
housing, tapered blade edges form a radial hydrodynamic
beating.
[0039] In a further broad form of the invention there is provided a
rotary blood pump having a housing within which an impeller acts by
rotation about an axis to cause a pressure differential between an
inlet side of a housing of said pump and an outlet side of the
housing of said pump; said impeller suspended exclusively
hydrodynamically by thrust forces generated by the impeller during
movement in use of the impeller.
BRIEF DESCRIPTION OF THE DRAWINGS
[0040] Embodiments of the present invention will now be described,
with reference to the accompanying drawings, wherein:
[0041] FIG. 1 is a longitudinal cross-sectional view of a preferred
embodiment of the invention;
[0042] FIG. 2 is a cross-sectional view taken generally along the
line Z-Z of FIG. 1;
[0043] FIG. 3A is a cross-sectional view of an impeller blade taken
generally along the line A-A of FIG. 2;
[0044] FIG. 3B is an enlargement of the blade-pump housing
interface portion of FIG. 3A;
[0045] FIG. 3C is all alternative impeller blade shape;
[0046] FIGS. 4A, B, C illustrate various possible locations of
magnet material within a blade;
[0047] FIGS. 5A, B and C are left-hand end views of possible
winding geometries taken generally along the line S-S of FIG.
1;
[0048] FIG. 6 is a diagrammatic cross-sectional view of an
alternative embodiment of the invention as an axial pump;
[0049] FIG. 7 is an exploded, perspective view of a centrifugal
pump assembly according to a further embodiment of the
invention;
[0050] FIG. 8 is a perspective view of the impeller of the assembly
of FIG. 7;
[0051] FIG. 9 is a perspective, cut away view of the impeller of
FIG. 8 within the pump assembly of FIG. 7;
[0052] FIG. 10 is a side section indicative view of the impeller of
FIG. 8;
[0053] FIG. 11A is a section of a blade of the impeller of taken
through plane DD as defined in FIG. 10. FIG. 11B illustrates in
cross-section a bottom edge of the blade cut along plane BB of FIG.
10;
[0054] FIG. 12 is a block diagram of an electronic driver circuit
for the pump assembly of FIG. 7;
[0055] FIG. 13 is a graph of head versus flow for the pump assembly
of FIG. 7;
[0056] FIG. 14 is a graph of pump efficiency versus flow for the
pump assembly of FIG. 7;
[0057] FIG. 15 is a graph of electrical power consumption versus
flow for the pump assembly of FIG. 7;
[0058] FIG. 16 is a plan, section view of the pump assembly showing
a volute arrangement according to a preferred embodiment;
[0059] FIG. 17 is a plan, section view of a pump assembly showing
an alternative volute arrangement;
[0060] FIG. 18 is a plan view of an impeller according to a further
embodiment of the invention;
[0061] FIG. 19 is a plan view of an impeller according to a further
embodiment of the invention;
[0062] FIG. 20 is a perspective view or an impeller according to a
further embodiment of the invention;
[0063] FIG. 21 is a perspective view of an impeller according to
yet a further embodiment of the invention;
[0064] FIG. 22 is a perspective, partially cut away view of an
impeller according to yet a further embodiment of the
intention;
[0065] FIG. 23 is a top, perspective view of the impeller of FIG.
22;
[0066] FIG. 24 is a perspective view of the impeller of FIG. 22
with its top shroud removed;
[0067] FIG. 25 illustrates an alternative embodiment wherein the
deformed surface is located on the pump housing; and
[0068] FIG. 26 illustrates a further embodiment wherein deformed
surfaces are located both on the impeller and on the housing.
[0069] FIG. 27 illustrates diagrammatically the basis of operation
of the "deformed surfaces" utilised for hydrodynamic suspension of
embodiments of the invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0070] The pump assemblies according to various preferred
embodiments to be described below all have particular, although not
exclusive, application for implantation in a mammalian body so as
to at least assist, if not take over, the function of the mammalian
heart. In practice this is performed by placing the pump assembly
entirely within the body of the mammal and connecting the pump
between the left ventricle and the aorta so as to assist left side
heart function. It may also be connected to the right ventricle and
pulmonary artery to assist the right side of the heart.
[0071] In this instance the pump assembly includes an impeller
which is fully seared within the pump body and so does not require
a shaft extending through the pump body to support it. The impeller
is suspended, in use, within the pump body by the operation of
hydrodynamic forces imparted as a result of the interaction between
the rotating impeller, the internal pump walls and the fluid which
the impeller causes to be urged from an inlet of the pump assembly
to an outlet thereof.
[0072] A preferred embodiment of the invention is the centrifugal
pump 1, at depicted in FIGS. 1 and 2, intended for implantation
into a human, in which case the fluid referred to below is blood.
The pump housing 2, can be fabricated in two parts, a front part 3
in the form of a housing body and a back part 4 in the form of a
housing cover, with a smooth join therebetween, for example at 5 in
FIG. 1. The pump 1 has an axial inlet 6 and a tangential outlet 7.
The rotating part 100Q is of very simple form, comprising only
blades 8 and a blade support 9 to hold those blades fixed relative
to each other. The blades may be curved as depicted in FIG. 2, or
straight, in which case they can be either radial or back-swept,
i.e. at an angle to the radius. This rotating part 100 will
hereafter be called the impeller 100, but it also serves as a
bearing component and as the rotor of a motor configuration as to
be further described below whereby a torque is applied by
electromagnetic means to the impeller 100. Note that the impeller
has no shaft and that the fluid enters the impeller from the region
of its axis RR. Some of the fluid passes in front of the support
cone 9 and some behind it, so that the pump 1 can be considered of
two-sided open type, as compared to conventional open centrifugal
pumps, which are only open on the front side. Approximate
dimensions found adequate for the pump 1 to perform as a
ventricular assist device, when operating at speeds in the range
1,500 rpm to 4,000 rpm, are outer blade diameter 40 mm, outer
housing average diameter 60 mm, and housing axial length 40 mm.
[0073] As the blades 8 move within the housing, some of the fluid
passes through the gaps, much exaggerated in FIGS. 1 and 3, between
the blade edges 101 and the housing front face 10 and housing back
face 11. In all open centrifugal pumps, the gaps are made small
because this leakage flow lowers the pump hydrodynamic efficiency.
In the pump disclosed in this embodiment, the gaps are made
slightly smaller than is conventional in order that the leakage
flow can be utilized to create a hydrodynamic bearing. For the
hydrodynamic forces to be sufficient, the blades may also be
tapered as depicted in FIGS. 3A and 3B to form a tapered-land type
hydrodynamic bearing, so that the gap 110 is larger at the leading
edge 102 of the blade 8 than at the trailing edge 103 thereby
providing one example of a "deformed surface" as described
elsewhere in this specification. The fluid 105 which passes through
the gap thus experiences a wedge shaped restriction which generates
a thrust, as described in Reynolds' theory of lubrication (see, for
example, "Modern Fluid Dynamics, Vol. 1 Incompressible Flow", by N.
Curie and H. J. Davies, Van Nostrand, 1968). For blades
considerably thinner than their axial length, the thrust is
proportional to the square of the blade thickness at the edge, and
thus in this embodiment thick blades are favored, since if the
proportional of the pump cavity filled by blades is constant, then
the net thrust force will be inversely proportional to the number
of blades. However, the blade edges can be made to extend as tails
from thin blades as depicted in FIG. 3C in order to increase the
blade area adjacent the walls.
[0074] In one particular form, the tails join adjacent blades so as
to form a complete shroud with wedges or tapers incorporated
therein. An example of a shroud design as well as other variations
on the blade structure will be described later in this
specification.
[0075] For manufacturing simplicity, the housing front face 10 can
be made conical, with an angle of around 45.degree. so that it
provides both axial and radial hydrodynamic forces. Other angles
are suitable that achieve the functional requirements of this pump
including the requirements for both axial and radial hydrodynamic
forces.
[0076] Other curved surfaces are possible provided both axial and
radial hydrodynamic forces can be produced as a result of rotation
of the blades relative to the housing surfaces.
[0077] The housing back face 11 can include a roughly conical
extension 12 pointing into the pump cavity 106, to eliminate or
minimise the effect of the flow stagnation point oil the axis of
the back housing.
[0078] Alternatively extension 12 can resemble an impeller eye to
make the flow mixed.
[0079] In this preferred embodiment, for manufacturing simplicity
and for uniformity in the flow axial direction RR the housing back
face 11 is made flat over the bearing surfaces, i.e. under the
blade edges. With this the case, a slacker tolerance on the
alignment between the axes of the front, part 3 and back part 4 of
the housing 2 is permissible. An alternative is to make the back
face 11 conical at the bearing surfaces, with taper in the opposite
direction to the front face 10, so that the hydrodynamic forces
from the back face will also have radial components. Tighter
tolerance on the axes alignment would then be required, and some of
the flow would have to undergo a reversal in its axial direction.
Again a roughly conical extension (like 12) will be needed. There
may be some advantage in making the housing surfaces and blade
edges non-straight, with varying tangent angle, although this will
impose greater manufacturing complexity.
[0080] There are several options for the shape of the taper, but in
the preferred embodiment the amount of material removed simply
varies linearly or approximately linearly across the blade. For the
back face, the resulting blade edges are then planes at a slight
inclination to the back face. For the front face, the initial blade
edges are curved and the taper only removes a relatively small
amount of material so they still appear curved. Alternative taper
shapes can include a step in the blade edge, though the corner in
that step would represent a stagnation line posing a thrombosis
risk.
[0081] For a given minimum yap, at the trailing blade edge, the
hydrodynamic force is maximal if the gap at the leading edge is
approximately double that at the trailing edge. Thus the taper,
which equals the leading edge gap minus the trailing edge gap,
should be chosen to match a nominal minimum gap, once the impeller
has shifted towards that edge. Dimensions which have been found to
give adequate thrust forces are a taper of around 0.05 mm for a
nominal minimum gap of around 0.05 mm, and an average
circumferential blade edge thickness of around 6 mm for 4 blades.
For the front face, the taper is measured within the plane
perpendicular to the axis. The axial length of the housing between
the front and back faces at any position should then be made about
0.2 mm greater than the axial length of the blade, when it is
coaxial with the housing, so that the minimum gaps are both about
0.1 mm axially when the impeller 100 is centrally positioned within
the housing 2. Then, for example, if the impeller shifts axially by
0.05 mm, the minimum gaps will be 0.05 mm at one face and 0.15 mm
at the other face. The thrust increases with decreasing gap and
would be much larger from the 0.05 mm gap than from the 0.15 mm
gap, about 14 times larger for the above dimensions. Thus there is
a net restoring force away from the smaller gap.
[0082] Similarly, for radial shifts of the impeller the radial
component of the thrust from the smaller gap on the conical housing
front face would offer the required restoring radial force. The
axial component of that force and its torque on the impeller would
have to be balanced by an axial force and torque from the housing
back face, and so the impeller will also have to shift axially and
tilt its axis to be no longer parallel with the housing axis. Thus
as the person moves and the pump is accelerated by external forces,
the impeller will continually shift its position and alignment,
varying the gaps in such a way that the total force and torque on
the impeller 100 match that, demanded by inertia. The gaps are so
small, however, that the variation in hydrodynamic efficiency will
be small, and the pumping action of the blades will be
approximately the same as when the impeller is centrally
located.
[0083] While smaller gaps imply greater hydrodynamic efficiency and
greater bearing thrust forces, smaller gaps also demand tighter
manufacturing tolerances, increase frictional drag on the impeller,
and impose greater shear stress an the fluid. Taking these points
in turn, for the above 0.05 mm tapers and gaps, tolerances of
around 0.005 mm are needed, which imposes some cost penalty but is
achievable. A tighter tolerance is difficult, especially if the
housing is made of a plastic, given the changes in dimension caused
by temperature and possible absorption of fluid by plastic
materials which may be in contact with the blood such as Acrylic of
polyurethane. The frictional drag for the above gaps produces much
smaller torque than the typical motor torque. Finally, to estimate
the shear stress, consider a rotation speed of 3,000 rpm and a
typical radius of 15 mm, at which the blade speed is 4.7 ms.sup.-1
and the average velocity shear for an average gap of 0.075 mm is
6.2..times.10.sup.4 s.sup.-1. For blood of dynamic viscosity
3.5.times.10.sup.-3 kgm.sup.-1s.sup.-1, the average shear stress
would be 220 Nm.sup.-2. Other prototype centrifugal blood pumps
with closed blades have found that slightly larger gaps, e.g. 0.15
.mu.m, are acceptable for haemolysis. A major advantage of the open
blades of the present invention is that a fluid element that does
passing through a blade edge gap will have very short residence
time in that gap, around 2.times.10.sup.-3 S, and the fluid element
will most likely be swept though the pump without passing another
blade edge.
[0084] With particular reference to FIGS. 3A and 3B typical working
clearances and working movement for the impeller 8 with respect to
the upper and lower housing surfaces 10, 11 is of the order of 100
microns clearance at the top and at the bottom. In use
gravitational and other forces will bias the impeller 8 closer to
one or other of the housing walls resulting, typically in a
clearance at one interface of the order of 50 microns and a
corresponding larger clearance at the other interface of the order
of 150 microns. In use, likely maximum practical clearances will
range from 300 microns down to 1 micron.
[0085] Typical restoring forces for a 25 gram rotor mass spinning
at 2200 rpm are 1.96 Newtons at a 20 micron clearance extending to
0.1 Newtons at an 80 micron clearance.
[0086] To minimise the net force required of the hydrodynamic
bearings, the net axial and radial hydrodynamic forces on the
impeller from the bulk fluid flow should be minimised, where "bulk"
here means other than from the bearing thrust surfaces.
[0087] The radial force on the impeller depends critically on the
shape of the output flow collector or volute 13. The shape should
be designed to minimise the radial impeller force over the desired
range of pump speeds, without excessively lowering the pump
efficiency. The optimal shape will have a roughly helical perimeter
between the "cutwater" and outlet. The radial force can also be
reduced by the introduction of an internal division in the volute
13 to create a second output flow collector passage, with tongue
approximately diametrically opposite to the tongue of the first
passage.
[0088] An indicative plan view of impeller 100 relative to housing
2 is shown in FIG. 2 having a concentric volute 13.
[0089] FIG. 17 illustrates the alternative volute arrangement
comprising a split volute created by volute barrier 107 which
causes volute 108 in a first hemisphere of the housing 2 to split
into first half volute 109 and second half volute 110 over the
second hemisphere. The hemispheres are defined respectively on each
side of a diameter of the housing 2 which passes through or near
exit point 111 of outlet 7.
[0090] In alternative forms concentric volutes can be utilised,
particularly where specific speed is relatively low.
[0091] In a further particular form a vaneless diffuser may also
reduce the radial force.
[0092] In regard to the bulk hydrodynamic axial force, if the blade
cross-section is made uniform in the axial direction along the
rotational axis, apart from the conical front edge, then the
pressure acting on the blade surface (excluding the bearing edges)
will have no axial component. This also simplifies the blade
manufacture. The blade support cone 9 must then be shaped to
minimise axial thrust on the impeller and minimise disturbance to
the flow over the range of speeds, while maintaining sufficient
strength to prevent relative blade movement. The key design
parameter affecting the axial force is the angle of the cone. The
cone is drawn in FIG. 1 as having the same internal diameter as the
blades, which may aid manufacture. However, the cone could be made
with larger or smaller internal diameter to the blades. There may
be advantage in using a non-axisymmetric support "cone" e.g. with
larger radius on the trailing surface of a blade than the radius at
the leading surface of the next blade. If the blades are made with
non-uniform cross-section to increase hydrodynamic efficiency, then
any bulk hydrodynamic axial force on them can be balanced by
shaping the support cone to produce an opposite bulk hydrodynamic
axial force on it.
[0093] Careful design of the entire pump, employing computational
fluid dynamics, is necessary to determine the optimal shapes of the
blades 8, the volute 13, the support cone 9 and the housing 2, in
order to maximise hydrodynamic efficiency while keeping the bulk
fluid hydrodynamic forces, shear and residence times low. All edges
and the joins between the blades and the support cone should be
smoothed.
[0094] The means or providing the driving torque on the impeller
100 of the preferred embodiment of the invention is to encapsulate
permanent magnets 14 in the blades 8 of the impeller 100 and to
drive them with a rotating magnetic field pattern from oscillating
currents in windings 15 and 16, fixed relative to the housing 2.
Magnets of high remanence such as sintered rare-earth magnets
should be used to maximise motor efficiency. The magnets can be
aligned axially but greater motor efficiency is achieved by tilting
the magnetisation direction to an angle of around 15.degree. to
30.degree. outwards from the inlet axis, with 22.5.degree. tilt
suitable for a body of conical angle 45.degree.. The magnetisation
direction must alternate in polarity for adjacent blades. Thus
there must be an even number of blades. Since low blade number is
preferred for the bearing force, and since two blades would not
have sufficient bearing stiffness to rotation about an axis through
the blades and perpendicular to the pump housing (unless the blades
are very curved), four blades are recommended. A higher number of
blades, for example 6 or 8 will also work.
[0095] Some possible options for locating the magnets 14 within the
blades 8 are shown in FIG. 4. The most preferred which is depicted
in FIG. 4A, is for the blade to be made of magnet material apart
from a biocompatible shell or coating to prevent fluid corroding
the magnets and to prevent magnet material (which may be toxic)
entering the blood stream. The coating should also be sufficiently
durable especially at blade corners to withstand rubbing during
start-up or during inadvertent bearing touch down.
[0096] In one particular form the inside walls of the pump housing
2 are also coated with a biologically compatible and wear resistant
material such as diamond coating or titanium nitride so that wear
on both of the touching surfaces is minimised.
[0097] An acceptable coating thickness is approximately 1
micron.
[0098] A suitable impeller manufacturing method is to die-press the
entire impeller, blades and support cone, as a single axially
aligned magnet. The die-pressing is much simplified if near axially
uniform blades are used (blades with an overhang such as in FIG. 3C
are precluded). During pressing, the crushed rare-earth particles
must be aligned in an axial magnetic field. This method of
die-pressing with parallel alignment direction is cheaper for
rare-earth magnets, although it produces slightly lower remanence
magnets. The tolerance in die-pressing is poor, and grinding of the
tapered blade edges is required. Then the magnet impeller can be
coated, for example by physical vapour deposition, of titanium
nitride for example, or by chemical vapour deposition, of a thin
diamond coating or a teflon coating.
[0099] In an alternative form the magnet material can be potted in
titanium or a polymeric housing which is then, in turn, coated with
a biologically compatible and tough material such as diamond
coating or titanium nitride.
[0100] Finally, to create the alternating blade polarity the
impeller must be placed in a special pulse magnetisation fixture,
with an individual coil surrounding each blade. The support cone of
a die-pressed magnet impeller acquires some magnetisation near the
blades, with negligible influence.
[0101] Alternative magnet locations are sketched in FIG. 4B and
FIG. 4c in which quadrilateral or circular cross-section magnets 14
are inserted into the blades. Sealing and smoothing of the blade
edges over the insertion holes is then required to reinstate the
taper.
[0102] All edges in the pump should be radiused and surfaces
smoothed to avoid possible damage to formed elements of the
blood.
[0103] The windings 15 and 16 of the preferred embodiment are
slotless or air-gap windings with the same pole number as the
impeller, namely four poles in the preferred embodiment. A
ferromagnetic iron yoke 17 of conical form for the front winding
and an iron ferromagnetic yoke 18 of annular form for the back
winding may be placed on the outside of the windings to increase
the magnetic flux densities and hence increase motor efficiency.
The winding thicknesses should be designed for maximum motor
efficiency, with the sum of their axial thicknesses somewhat less
than but comparable to the magnet axial length. The yokes can be
made of solid ferromagnetic material such as iron. To reduce "iron"
losses, the yokes 17 can be laminated, for example in layers or by
helically winding thin strip, or can be made of iron/powder epoxy
composite. The yokes should be positioned such that there is zero
net axial magnetic force on the impeller when it is positioned
centrally in the housing. The magnetic force is unstable and
increases linearly with axial displacement of the impeller away
from the central position, with the gradient being called the
negative stiffness of the magnetic force. This unstable magnetic
force must be countered by the hydrodynamic bearings, and so the
stiffness should be made as small as possible. Choosing the yoke
thickness such that the flux density is at the saturation level
reduces the stiffness and gives minimum mass. An alternative can be
to have no iron yokes, completely eliminating the unstable axial
magnetic force, but the efficiency of such designs may be lower and
the magnetic flux density in the immediate vicinity of the pump may
violate safety standards and produce some tissue heating. In any
case, the stiffness is acceptably small for slotless windings with
the yokes present. Another alternative would be to insert the
windings in slots in laminated iron stators which would increase
motor efficiency and enable use of less magnet material and
potentially lighter impeller blades. However, the unstable magnetic
forces would be significant for such slotted motors. Also, the
necessity for fat blades to generate the required bearing forces in
this embodiment allows room for large magnets, and so slotless
windings are chosen in the preferred embodiment.
[0104] Instead of determining the yoke positions so that the
impeller has zero magnetic axial force in the central position, it
may be possible to provide a bias axial magnetic force on the
impeller, which can counteract other forces such as any average
bulk hydrodynamic axial force. In particulars by ensuring a net
axial force into the conical body, the thrust bearings on the cover
surface can be made superfluous. However, such a bias would demand
greater average thrust forces, smaller gaps and increased blood
damage, and so the recommended goal is to, zero both the magnetic
and bulk hydrodynamic axial forces on the impeller when centrally
positioned.
[0105] The overall design requirement for exclusive hydrodynamic
suspension requires control of the external force balance to make
the relative magnitude of hydrodynamic thrust sufficient to
overcome the external forces. Typical external forces include
gravitational forces and net magnetic forces arising as a result of
the motor drive.
[0106] There are many options for the winding topology and number
of phases. FIG. 5A depicts the preferred topology for the body
winding 15, viewed from the inlet axis.
[0107] The cover winding 16 looks similar but the coils need not
avoid the inlet tube and so they appear more triangular in shape.
The body winding has a more complex three-dimensional shape with
bends at the ends of the body cone section. Each winding consists
of three coils. Each coil is made from a number of turns of an
insulated conductor such as copper with the number of turns chosen
to suit the desired voltage. The coil side mid-lines span an angle
of about 50.degree.-100.degree. at the axis when the coils are in
position. The coils for body and cover are aligned axially and the
axially adjacent coils are connected in either parallel or series
connection to form one phase of the three phase winding. Parallel
connection offers one means of redundancy in that if one coil
fails, the phase can still carry current through the other coil. In
parallel connection each of the coil and body winding has a neutral
point connection as depicted in FIG. 5A, whereas in series
connection, only one of the windings has a neutral point.
[0108] An alternative three phase winding topology, depicted in
FIG. 5B, uses four coils per phase for each of the body and cover
windings, with each coil wrapping around the yoke, a topology
called a "Gramm ring" winding.
[0109] Yet another three phase winding topology, depicted in FIG.
5c, uses two coils per phase for each of the body and cover
windings, and connects the coil sides by azimuthal end-windings as
is standard motor winding practice. The coils are shown tilted to
approximately follow the blade curvature, which can increase motor
efficiency, especially for the phase energising strategy to be
described below in which only one phase is energised at a time. The
winding construction can be simplified by laying the coils around
pins protruding from a temporary former, the pins shown as dots in
2 rings of 6 pins each in FIG. 5C. The coils are labelled
alphabetically in the order in which they would be layed, coils a
and d for phase A, b and e for phase B, and c and f for phase C.
Instead of or as well as pins, the coil locations could be defined
by thin fins, running between the pins in FIG. 5C, along the
boundary between the coils. The coil connections depicted in FIG.
5C are those appropriate for the winding nearest the motor
terminals for the case of series connection, with the optional lead
from the neutral point on the other winding included.
[0110] The winding topologies depicted in FIGS. 5B and C allow the
possibility of higher motor efficiency but only if significantly
higher coil mass is allowed, and since option FIG. 5A is more
compact and simpler to manufacture, it is the preferred option.
Material ribs between the coils of option FIG. 5A can be used to
stiffen the housing.
[0111] Multi-stranded flexible conductors within a suitable
biocompatible cable can be used to connect the rotor windings to a
motor controller. The energisation of the three phases can be
performed by a standard sensorless controller, in which two out of
six semiconducting switches in a three phase bridge are turned on
at any one time. Alternatively, because of the relatively small
fraction of the impeller cross-section occupied by magnets, it may
be slightly more efficient to only activate one of the three phases
at a time, and to return the current by a conductor from the
neutral point in the motor. Careful attention must be paid to
ensure that the integrity of all conductors and connections is
failsafe.
[0112] In the preferred embodiment, the two housing components 3
and 4 are made by injection moulding from non-electrically
conducting plastic materials such as Lexan polycarbonate plastic.
Alternatively the housing components can be made from ceramics. The
windings and yokes are ideally encapsulated within the housing
during fabrication moulding. In this way, the separation between
the winding and the magnets is minimised, increasing the motor
efficiency, and the housing is thick, increasing its mechanical
stiffness. Alternatively, the windings can be positioned outside
the housing, of thickness at least around 2 mm for sufficient
stiffness.
[0113] If the housing material plastic is hygroscopic or if the
windings are outside the housing, it may be necessary to first
enclose the windings and yoke in a very thin impermeable shell.
Ideally the shell should be non-conducting (such as ceramic or
plastic), but titanium of around 0.1 mm to 0.2 mm thickness would
give sufficiently low eddy losses. Encapsulation within such a
shell would be needed to prevent winding movement.
[0114] Alternatively, the housing components 3 and 4 may be made
from a biocompatible metallic material of low electrical
conductivity, such as Ti-6A1-4V. To minimize the eddy current loss,
the material must be as thin as possible, e.g. 0.1 mm to 0.5 mm,
wherever the material experiences high alternating magnetic flux
densities, such as between the coils and the housing inner surfaces
10 and 11.
[0115] The combining of the motor and bearing components into the
impeller in the preferred embodiment provides several key
advantages. The rotor consequently has very simple form, with the
only cost of the bearing being tight manufacturing tolerances. The
rotor mass is very low, minimising the bearing force needed to
overcome weight. Also, with the bearings and the motor in the same
region of the rotor, the bearings forces are smaller than if they
had to provide a torque to support magnets at an extremity of the
rotor.
[0116] A disadvantage of the combination of functions in the
impeller is that its design is a coupled problem. The optimisation
should ideally link the fluid dynamics, magnetics and bearing
thrust calculations. In reality, the blade thickness can be first
roughly sized to give adequate motor efficiency and sufficient
bearing forces with a safety-margin. Fortuitously, both
requirements are met for four blades of approximate average
circumferential thickness 6 mm or more. The housing, blade, and
support cone shapes can then be designed using computational fluid
dynamics, maintaining the above minimum average blade thickness.
Finally the motor stator, i.e. winding and yoke, can be optimised
for maximum motor efficiency.
[0117] FIG. 6 depicts an alternative embodiment of the invention as
an axial pump. The pump housing is made of two parts, a front part
19 and a back part 20, joined for example at 21. The pump has an
axial inlet 22 and axial outlet 23. The impeller comprises only
blades 24 mounted on a support cylinder 25 of reducing radius at
each end. An important feature of this embodiment is that the blade
edges are tapered to generate hydrodynamic thrust forces which
suspend the impeller. These forces could be used for radial
suspension alone from the straight section 26 of the housing, with
some alternative means used for axial suspension, such as stable
axial magnetic forces or a conventional tapered-land type
hydrodynamic thrust bearing. FIG. 6 proposes a design which uses
the tapered blade edges to also provide an axial hydrodynamic
bearing. The housing is made with a reducing radius at its ends to
form a front face 27 and a back face 2a from which the axial
thrusts can suspend the impeller axially. Magnets are embedded in
the blades with blades having alternating polarity and four blades
being recommended. Iron in the outer radius of the support cylinder
25 can be used to increase the magnet flux density. Alternatively,
the magnets could be housed in the support cylinder and iron could
be used in the blades. A slotless helical winding 29 is
recommended, with outward bending end-windings 30 at one end to
enable insertion of the impeller and inward bending windings 31 at
the other end to enable insertion of the winding into a cylindrical
magnetic yoke 32. The winding can be encapsulated in the back
housing part 20.
Third Embodiment
[0118] With reference to FIGS. 7 to 15 inclusive there is shown a
further preferred embodiment of the pump assembly 200.
[0119] With particular reference initially to FIG. 1 the pump
assembly 200 comprises a housing body 201 adapted for bolted
connection to a housing cover 202 and so as to define a centrifugal
pump cavity 203 therewithin.
[0120] The cavity 203 houses an impeller 204 adapted to receive
magnets 205 within cavities 206 defined within blades 207. As for
the first; embodiment the blades 207 are supported from a support
cone 208.
[0121] Exterior to the cavity 203 but forming part of the pump
assembly 2030 there is located a body winding 209 symmetrically
mounted around inlet 210 and housed between the housing body 201
and a body yoke 211.
[0122] Also forming part of the pump assembly 200 and also mounted
external to pump cavity 203 is cover winding 212 located within
winding cavity 213 which, in turn, is located within housing cover
202 and closed by cover yoke 214.
[0123] The windings 212 and 209 are supplied from the electronic
controller of FIG. 12 as for the first embodiment the windings are
arranged to receive a three phase electrical supply and so as to
set up a rotating magnetic field within cavity 203 which exerts a
torque on magnets 205 within the impeller 204 so as to urge the
impeller 204 to rotate substantially about central axis TT of
cavity 203 and in line with the longitudinal axis of inlet 210. The
impeller 204 is caused to rotate so as to urge fluid (in this case
blood) around volute 215 and through outlet 216.
[0124] The assembly is bolted together in the manner indicated by
screws 217. The yokes 211, 214 are held in place by fasteners 218.
Alternatively, press fitting is possible provided sufficient
integrity of seal can be maintained.
[0125] FIG. 8 shows the impeller 204 of this embodiment and clearly
shows the support cone 208 from which the blades 207 extend. The
axial cavity 219 which is arranged, in use, to be aligned with the
longitudinal axis of inlet 210 and through which blood is received
for urging by blades 207 is clearly visible.
[0126] The cutaway view of FIG. 9 shows the axial cavity 219 and
also the magnet cavities 206 located within each blade 207. The
preferred cone structure 220 extending from housing cover 202
aligned with the axis of inlet 210 and axial cavity 219 of impeller
204 is also shown.
[0127] FIG. 10 is a side section, indicative view of the impeller
204 defining the orientations of central axis FE, top taper edge
DL) and bottom taper edge BB, which tapers are illustrated in FIG.
11 in side section view.
[0128] FIG. 11A is a section of a blade 207 of impeller 204 taken
through plane DD as defined in FIG. 10 and shows the top edge 221
to be profiled from a leading edge 223 to a trailing edge 224 as
follows; central portion 227 comprises an ellipse with centre on
the dashed midline having a semi-major axis of radius 113 mm and a
semi-minor axis of radius 80 mm and then followed by leading
conical surface 225 and trailing conical surface 226 on either side
thereof as illustrated in FIG. 11A. The leading surface 225 has
radius 0.05 mm less than the trailing surface 226. This
prescription is for a taper which can be achieved by a grinding
wheel, but many alternative prescriptions could be devised to give
a taper of similar utility.
[0129] The leading edge 223 is radiused as illustrated.
[0130] FIG. 11B illustrates in cross-section the bottom edge 222 of
blade 207 cut along plane BB of FIG. 10.
[0131] The bottom edge includes cap 228 utilised for sealing magnet
205 within cavity 206.
[0132] In this instance substantially the entire edge comprises a
straight taper with a radius of 0.05 mm at leading edge 229 and a
radius of 0.25 mm at trailing edge 230.
[0133] The blade 207 is 6.0 mm in width excluding the radii at
either end.
[0134] FIG. 12 comprises a block diagram of the electrical
controller suitable for driving the pump assembly 200 and comprises
a three phase commutation controller 232 adapted to drive the
windings 209, 212 of the pump assembly. The commutation controller
232 determines relative phase and frequency values for driving the
windings with reference to set point speed input 233 derived from
physiological controller 234 which, in turn, receives control
inputs 235 comprising motor current input and motor speed (derived
from the commutation controller 232), patient blood flow 236, and
venous oxygen saturation 237. The pump blood flow can be
approximately inferred from the motor speed and current via
curve-fitted formulae.
[0135] FIG. 13 is a graph of pressure against flow for the pump
assembly 200 where the fluid pumped is 18% glycerol for impeller
rotation velocity over the range 1500 RPM to 2500 RPM. The 18%
glycerol liquid is believed to be a good analogue for blood under
certain circumstances, for example in the housing gap.
[0136] FIG. 14 graphs pump efficiency against flow for the same
fluid over the same speed ranges as for FIG. 13.
[0137] FIG. 15 is a graph of electrical power consumption against
flow for the same fluid over the same speed ranges as for FIG.
13.
[0138] The common theme running through the first, second and third
embodiments described thus far is the inclusion in the impeller of
a taper or other deformed surface which, in use, moves relative to
the adjacent housing wall thereby to cause a restriction with
respect to the line of movement of the taper or deformity thereby
to generate thrust upon the impeller which includes a component
substantially normal to the line of movement of the surface and
also normal to the adjacent internal pump wall with respect to
which the restriction is defined for fluid located
therebetween.
[0139] In order to provide both radial and axial direction control
qt least one set of surfaces must be angled with respect to the
longitudinal axis of the impeller (preferably at approximately
45.degree. thereto) thereby to generate or resolve opposed radial
forces and an axial force which can be balanced by a corresponding
axial force generated by at least one other tapered or deformed
surface located elsewhere on the impeller.
[0140] In the forms thus far described top surfaces of the blades
8, 207 are angled at approximately 45.degree. with respect to the
longitudinal axis of the impeller 100, 204 and arranged for
rotation with respect to the internal walls of a similarly angled
conical pump housing. The top surfaces of the blades are deformed
so as to create the necessary restriction in the gap between the
top surfaces of the blades and the internal walls of the conical
pump housing thereby to generate a thrust which can be resolved to
both radial and axial components.
[0141] In the examples thus far the bottom faces of the blades 8,
207 comprise surfaces substantially lying in a plane at right
angles to the axis of rotation of the impeller and, with their
deformities define a gap with respect to a lower inside face of the
pump housing against which a substantially only axial thrust is
generated.
[0142] Other arrangements are possible which will also, relying on
these principles, provide the necessary balanced radial and axial
forces. Such arrangements can include a double cone arrangement
where the conical top surface of the blades is mirrored in a
corresponding bottom conical surface. The only concern with this
arrangement is the increased depth of pump which can be a problem
for in vivo applications where size minimisation is an important
criteria.
Fourth Embodiment
[0143] With reference to FIG. 18 a further embodiment of the
invention is illustrated comprising a plan view of the impeller 300
forming part of a "channel" pump. In this embodiment the blades 301
have been widened relative to the blades 207 of the third
embodiment to the point where they are almost sector-shaped and the
flow gaps between adjacent blades 301, as a result, take the form
of a channel 302, all in communication with axial cavity 303.
[0144] A further modification of this arrangement is illustrated in
FIG. 19 wherein impeller 304 includes sector-shaped blades 305
having curved leading and trailing portions 306, 307 respectively
thereby defining channels 308 having fluted exit portions 309
[0145] As with the first and second embodiments the radial and
axial hydrodynamic forces are generated by appropriate profiling of
the top and bottom faces of the blades 301, 305 (not shown in FIGS.
18 and 19).
[0146] FIG. 20 illustrates a perspective view of an impeller 304
which follows the theme of the impeller arrangement of FIGS. 11 and
19 in perspective view and where like parts are numbered as for
FIG. 19. In this case the four blades 305 are joined at
mid-portions thereof by a blade support in the form of a conical
rim 350 and have edge portions which are shaped so as to have an
increased curvature on the trailing edge 351 thereof compared with
the leading edge 352.
Fifth Embodiment
[0147] A fifth embodiment of a pump assembly according to the
invention comprises an impeller 410 as illustrated in FIG. 21
where, conceptually, the upper and lower surfaces of the blades of
previous embodiments are interconnected by a top shroud 411 and a
bottom shroud 412. In this embodiment the blades 413 can be reduced
to a very small width as the hydrodynamic behaviour imparted by
their surfaces in previous embodiments is now given effect by the
profiling of the shrouds 411, 412 which, in this instance,
comprises a series of smooth-edged wedges with the leading surface
of one wedge directly interconnected to the trailing edge of the
next leading wedge 414.
[0148] As for previous embodiments the top shroud 411 is of overall
conical shape thereby to impart both radial and axial thrust forces
whilst the bottom shroud 412 is substantially planar thereby to
impart substantially only axial thrust forces.
[0149] It is to be understood that, whilst the example of FIG. 21
shows the surfaces of the shroud 411 angled at approximately
45.degree. to the vertical, other inclinations are possible
extending to an inclination of 0.degree. to the vertical which is
to say the impeller 410 can take the form of a cylinder with
surface rippling or other deformations which impart the necessary
hydrodynamic lift, in use.
[0150] With reference to FIGS. 22 to 24 a specific example of the
concept embodied in FIG. 21 is illustrated and wherein like
components are numbered as for FIG. 21.
[0151] It will be observed that, with reference to FIG. 24, the
blades 413 are thin compared to previous embodiments and, in this
instance, are arcuate channels 416 therebetween which allow fluid
communication from a centre volume 417 to the periphery 418 of the
impeller 410.
[0152] In this arrangement it will be noted that the wedges 419 are
separated one from the other on each shroud by channels 419. The
channels extend radially down the shroud from the centre volume 417
to the periphery 418.
[0153] In such designs with thin blades, the magnets required for
the driving torque can be contained within the top or bottom volute
or both, along with the optional soft magnetic yokes to increase
rotor efficiency.
[0154] A variation of this embodiment is to have the wedge
profiling cut into the inter surfaces of the housing and have
smooth shroud surfaces.
Sixth Embodiment
[0155] In contrast to the embodiments illustrated with respect to
FIGS. 3A, 3B and 3C an arrangement is shown in FIG. 25 wherein the
"deformed surface" comprises a stepped formation 510 forming part
of an inner wall of the pump housing (not shown). In this instance
the rotor including blade 511 includes a flat working surface 512
(and not having a deformed surface therein) which is adapted for
relative movement in the direction of the arrow shown with respect
to the stepped formation 510 thereby to generate hydrodynamic
thrust therebetween.
Seventh Embodiment
[0156] With reference to FIG. 26 there is shown an arrangement of
rotor blade 610 with respect to stepped formation 611 and wherein
the rotor blade 610 includes a deformed surface 612 at a working
face thereof. In this instance the deformation comprises curved
edges 613, 614. As for the previous embodiment relative movement of
the rotor blade 610 in the direction of the arrow with respect to
deformed surface 611 forming part of the pump housing (not shown)
causes relative hydrodynamic thrust therebetween.
[0157] The foregoing describes principles and examples of the
present invention, and modifications, obvious to those skilled in
the art, can be made thereto without departing from the scope and
spirit of the invention.
Principles of Operation
[0158] With particular reference to FIG. 27 this specification
describes the suspension of an impeller 600 within a pump housing
601 by the use of hydrodynamic forces. In this specification the
suspension of the impeller 600 is performed dominantly which is to
say exclusively by hydrodynamic forces.
[0159] The hydrodynamic forces are forces which are created by
relative movement between two surfaces which have a fluid in the
gap between the two surfaces. In the case of the use of the pump
assembly 602 as a rotary blood pump the fluid is blood.
[0160] The hydrodynamic forces can arise during relative movement
between two surfaces even where those surfaces are substantially
entirely parallel to each other or non-deformed. However, in this
specification, hydrodynamic forces are caused to arise during
relative movement between two surfaces where at least one of the
surfaces includes a "deformed surface".
[0161] In this specification "deformed surface" means a surface
which includes an irregularity relative to a surface which it faces
such that, when the surface moves in a predetermined direction
relative to the surface which it faces the fluid located in the gap
there between experiences a change in relative distance between the
surfaces along the line of movement thereby to cause a hydrodynamic
force to arise therebetween in the form or a thrust force including
at least a component substantially normal to the plane of the gap
defined at any given point between the facing surfaces.
[0162] In the example of FIG. 27 there is a first deformed surface
603 forming at least part of a first face 604 of impeller 600 and a
second deformed surface 605 on a second face 606 of the impeller
600.
[0163] The inset of FIG. 27 illustrates conceptually how the first
deformed surface 603 may form only part of the first face 604.
[0164] The first deformed surface 603 faces first inner surface 607
of the pump housing 601 whilst second deformed surface 605 faces
second inner surface 608 of the pump housing 601.
[0165] In use first gap 609 defined between first deformed surface
603 and first inner surface 607 has a fluid comprising blood
located therein whilst second gap 610 defined between second
deformed surface 605 and second inner surface 608 also has a fluid
comprising blood located therein.
[0166] In use impeller 600 is caused to rotate about impeller axis
611 such that relative movement across first gap 609 between first
deformed surface 603 and first inner face 607 occurs and also
relative movement across second gap 610 between second deformed
surface 605 and second inner surface 608 occurs. The orientation of
the deformities of first deformed surface 603 and second deformed
surface 605 relative to the line of movement of the deformed
surfaces 603, 605 relative to the inner surfaces 607, 608 is such
that the fluid in the gaps 609, 610 experiences a change in height
of the gap 609, 610 as a function of time and with the rate of
change dependant on the shape of the deformities of the deformed
surfaces and also the rate of rotation of the impeller 600 relative
to the housing 601. That is, at any given point on either inner
surface 607 or 608, the height of the gap between the inner surface
607 or 608 and corresponding deformed surface 603 or 605 will vary
with time due to passage of the deformed surface 603 or 605 over
the inner surface.
[0167] Hydrodynamic forces in the form of thrust forces normal to
the line of relative movement of the respective deformed surfaces
603, 605 relative to the inner surfaces 607, 608 thus arise.
[0168] With this configuration it will be noted that the first gap
609 lips substantially in a single plane whilst the second gap 610
is in the form of a cone and angled at an acute angle relative to
the plane of the first gap 609.
[0169] Accordingly, the thrust forces which can be enlisted to
first gap 609 and second gap 610 are substantially normal to and
distributed across both the predominantly flat plane of first
deformed surface 603 and normal to the substantially conical
surface of second deformed surface 605 thereby permitting restoring
forces to be applied between the impeller 600 and the pump housing
601 thereby to resist forces which seek to translate the impeller
600 in space relative to the pump housing 601 and also to rotate
the impeller 600 about any axis (other than about the impeller axis
611) relative to the pump housing 601. This arrangement
substantially resists five degrees of freedom of movement of
impeller 600 with respect to the housing 601 and does so
predominantly without any external intervention to control the
position of the impeller with respect to the housing given that
disturbing forces from other sources, most notably magnetic forces
on the impeller due to its use as rotor of the motor are net zero
when the impeller occupies a suitable equilibrium position. The
balance of all forces on the rotor, effected by manipulation of
magnetic and other external sources, may be adjusted such that the
rotor is predominantly hydrodynamically born.
[0170] It will be observed that these forces increase as the gaps
609, 610 narrow relative to a defined operating position and
decrease as the gaps 609, 610 increase relative to a defined
operating gap. Because of the opposed orientation of first deformed
surface 603 relative to second deformed surface 605 it is possible
to design for an equilibrium position of the impeller 600 within
the pump housing 601 at a defined equilibrium gap distance for gaps
609, 610 at a specified rotor rotational speed about axis 611 and
rotor mass leading to a close approximation to an unconditionally
stable environment for the impeller 600 within the pump housing 601
against a range of disturbing forces.
[0171] Characteristics and advantages which flow from the
arrangement described above and with reference to the embodiments
includes:
[0172] 1. Low haemolysis, hence low running speed and controlled
fluid dynamics (especially shear stress) in the gap between the
casing and impeller. This in turn led to the selection of radial
off-flow and minimal incidence at on-flow to the rotor;
[0173] 2. Radial or near-radial off-flow from the impeller can be
chosen in order to yield a "flat" pump characteristic (HQ)
curve.
INDUSTRIAL APPLICABILITY
[0174] The pump assembly 1, 200 is applicable to pump fluids such
as blood on a continuous basis. With its expected reliability it is
particularly applicable as an in vivo heart assist pump.
[0175] The pump assembly can also be used with advantage for the
pumping of other fluids where damage to the fluid due to high shear
stresses must be avoided or where leakage of the fluid must be
prevented with a very high degree of reliability--for example where
the fluid is a dangerous fluid.
* * * * *