Centrifugal blood pump and magnetic coupling

A centrifugal blood pump having a pumping chamber provided with an axial inlet and a circumferential outlet and provided with a rotatable impeller having a plurality of radially extending blood propelling vanes. The vanes are configured to each have a blade angle which varies monotonically toward the periphery of the chamber. The pumping chamber is delimited by an impeller housing provided with an opening through which the impeller shaft extends into the chamber, the opening having an outline which conforms closely to the outline of the shaft, and the pump further includes a bearing housing secured to the impeller housing and delimiting a bearing chamber into which the shaft extends; bearings disposed in the bearing chamber and rotatably supporting the shaft; and a seal located in the bearing chamber adjacent the opening and surrounding the shaft to form a fluid seal between the blood pumping chamber and the bearing chamber. The impeller is driven by a plurality of magnetizable plates secured to the impeller and spaced apart about the longitudinal axis, and a rotatable magnetic drive assembly disposed outside of the impeller housing and mounted for rotation about the longitudinal axis, the drive assembly producing a magnetic field which passes through each plate for attracting the plates to the drive assembly and rotating the impeller with the drive assembly.

BACKGROUND OF THE INVENTION 
The present invention relates to centrifugal blood pumps intended for 
extracorporeal pumping of blood. 
Known blood pumps of this type have been found to be less than totally 
reliable, due at least in part to their mechanical complexity and to the 
use of configurations which permit blood thrombus formation. 
SUMMARY OF THE INVENTION 
It is a primary object of the present invention to provide a novel 
centrifugal blood pump which reduces the danger of blood hemolysis and 
thrombus formation. 
Another object of the invention is to provide a novel centrifugal blood 
pump which is structurally simpler than existing pumps of this type, and 
hence operates more reliably. 
The above and other objects are achieved, according to the present 
invention, in a centrifugal blood pump composed of an impeller housing 
having a generally circular cross section and a longitudinal axis and 
delimiting a blood pumping chamber having a blood inlet port extending 
along the longitudinal axis and a blood outlet port located at the 
periphery of the chamber, an impeller provided with a plurality of 
radially extending vanes disposed in the chamber, a shaft supporting the 
impeller for rotation about the longitudinal axis of the blood pumping 
chamber, and magnetic drive means for rotating the impeller in a sense to 
cause the vanes to propel blood from the inlet port to the outlet port, by 
the improvement wherein the vanes are configured to each have a blade 
angle which varies monotonically toward the periphery of the chamber. 
The objects of the present invention are further achieved by other novel 
features of the impeller, by a novel magnetic drive system, and by a novel 
sealing arrangement for the impeller shaft, all of which will be described 
in detail below. 
With regard to a primary aspect of the invention, applicants have concluded 
that, in pumps of the type under consideration, blood hemolysis is caused 
by mechanical stresses imposed on the blood by the pumping process and 
have conceived and developed a novel impeller configuration which acts on 
the blood in such a manner as to significantly reduce the occurrence of 
hemolysis. Basically, impellers according to the present invention are 
constructed to subject blood as it enters and flows through the pump to 
smooth velocity transitions and to reduce cavitation in the pump, 
particularly at the inlet. 
Applicants have determined that this objective can be achieved by giving 
the impeller vanes a blade angle which varies from the end associated with 
the pump inlet and to the end associated with the pump outlet such that 
the tangent of the blade angle increases, as a function of radial distance 
from the impeller axis. It is presently believed that an optimum result 
will be achieved if the tangent increases linearly, or at least 
approximately linearly, from the inlet to the outlet.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
One embodiment of a centrifugal blood pump according to the present 
invention is illustrated in FIG. 1, which is a cross-sectional view taken 
along a plane passing through the axis of rotation of the pump impeller. 
The illustrated pump includes a housing composed of a forward housing part 
2 and a rear housing part 4, parts 2 and 4 together enclosing a pump 
chamber 6. The pump housing further includes a bearing housing 8 and a 
bearing cap 10, with the rear end of housing 8 being closed by cap 10 and 
the forward end of housing 8 being closed by rear housing part 4. 
Forward housing part 2 is formed to have an inlet passage 12 which extends 
along the pump axis and an outlet passage 14 which extends in a generally 
tangential direction at the periphery of chamber 6. 
Within chamber 6 there is mounted an impeller which, according to the 
present invention, is formed of a forward impeller part 16 and a rear 
impeller part 18, parts 16 and 18 being joined and bonded together along a 
plane perpendicular to the pump axis. Impeller 16, 18 is mounted on an 
impeller shaft 20 which is rotatably supported by a pair of journal 
bearings 22 secured in bearing housing 8. The region within housing 8 
between bearings 22 is preferably filled with a mass 24 of a suitable 
grease. 
The interior surface of cap 10 is provided with a cylindrical blind bore 
containing a steel ball 26 which constitutes a thrust bearing providing 
axial support for shaft 20 and impeller 16, 18. 
Between rear housing part 4 and the journal bearing 22 adjacent thereto 
there is disposed a shaft seal 36, which will be described in detail 
below. 
Impeller parts 16 and 18 are formed to delimit a plurality of 
circumferentially spaced, arcuate chambers, six such chambers being 
provided in one practical embodiment of the invention. Each chamber holds 
an arcuate drive plate 28 made of a magnetically permeable, but 
unmagnetized, material. Plates 28 can be made relatively thin, a thickness 
of the order of 0.040" having been found to be suitable. 
Forward impeller part 16 carries a plurality of circumferentially spaced 
long vanes 30 and a plurality of circumferentially spaced short vanes 32 
interposed circumferentially between successive long vanes 30. All vanes 
30 and 32 project axially toward inlet passage 12 and the edges of vanes 
30 and 32 which face toward inlet passage 12 conform generally to the 
outline of forward housing part 2. 
Rear impeller part 18 carries a plurality of short vanes 34 each of which 
is aligned with and corresponds in configuration to a respective one of 
short vanes 32 and the outer portions of long vanes 30. The portion of 
each long vane 30 which is radially enclosed by a respective vane 34 
extends axially toward rear housing part 4 to the same level as the 
associated vane 34 so that, at the side facing rear housing part 4, the 
respective vane 34 forms a radial continuation of the associated vane 30. 
Impeller shaft 20 enters chamber 6 via a passage provided in rear housing 
part 4, which passage is dimensioned to provide the minimum permissible 
clearance for shaft 20. Preferably, a radial clearance of no more than 
0.001 to 0.002" is provided. 
Moreover, the edge of the shaft passage which borders chamber 6 is formed 
to be sharp so as to constitute a shear edge. 
The passage for shaft 20 is isolated from the interior of bearing chamber 8 
by shaft seal 36. 
As shown in FIG. 2, shaft seal 36 is composed of an annular flange portion 
38 which will bear against the journal bearing 22 which is adjacent rear 
housing part 4. Shaft seal 36 further includes two concentric, radially 
spaced cylindrical portions 40, the outer one of which bears against the 
surface of a cylindrical relief opening provided in rear housing part 4. 
Inner cylindrical portion 40 is dimensioned to establish a close fit with 
shaft 20. 
Between cylindrical portions 40 there is interposed a pressure member 42 
composed of a spiral spring bent into a toroidal form and made of a 
suitable material, such as stainless spring steel. Member 42 is configured 
to apply a radial pressure to cylindrical portions 40, thereby pressing 
those portions against shaft 20 and the inner surface of the cylindrical 
relief opening provided in rear housing part 4, respectively. Thus, an 
effective seal is provided between chamber 6 and the interior of bearing 
housing 8. 
As a result of the close fit between shaft 20 and the opening in rear 
housing part 4, shaft seal 36 is, in effect, "hidden" from chamber 6. This 
helps to prevent thrombus formation on seal 36. 
The supporting of shaft 20 by journal bearings 22, instead of ball-type 
bearings, and the elimination of air from the bearing chamber by filling 
it with mass 24 of grease, are major contributing factors to the superior 
reliability of pumps according to the present invention. With this 
arrangement, seal 36 is not required to effect a perfect sealing action 
but need only prevent gross migration of blood and grease. 
FIG. 3 is a plan view, looking in the direction of fluid flow into the 
pump, of forward impeller part 16, which is basically composed of an inner 
hub portion 44 and an outer annular portion 46, the two portions being 
secured together by means of long vanes 30. Short vanes 32 are interposed 
between long vanes 30 so that vanes 30 and 32 are equispaced about the 
circumference of upper impeller part 16. 
FIG. 4 is a plan view of rear impeller part 18, looking opposite to the 
direction of fluid flow into the pump, i.e., opposite to the direction of 
the view of FIG. 3. Rear impeller part 18 is composed essentially of an 
annular ring 48 carrying short vanes 34, each of which is aligned with an 
associated portion of a respective one of vanes 30 or 32. 
According to preferred embodiments of the invention, each vane 30, 32, 34 
is given a curvature such that the variation of the tangent of the blade 
angle as a function of impeller radius has a positive value along the 
length of each blade. 
FIGS. 3 and 4 additionally illustrate one of the drive plates 28 which is 
installed between impeller parts 16 and 18 and which are spaced apart 
around the circumference of the impeller. 
As regards the axial spacing between vanes 30, 32, 34 and housing parts 2 
and 4, these selected to be small enough to achieve a satisfactory pumping 
force, and yet large enough to minimize the shear forces imposed on the 
blood. On the basis of these considerations, in one exemplary embodiment 
of the invention, the axial spacing between vanes 30 and 34 and rear 
housing part 4 is of the order of 0.12 inch at the outer diameter of the 
impeller. In this embodiment, which is illustrated in FIG. 1, the surface 
of rear housing part 4 which delimits chamber 6 has a slight upward slope 
toward shaft 20 so that the axial spacing between the vanes and that 
surface of lower housing part 4 exhibits a slight progressive decrease in 
the direction toward shaft 20. This axial spacing dimension was provided 
in a pump whose impeller vanes are configured so that the inner end of 
each vane 30 is at a distance of 0.3 inch from the axis of rotation of 
shaft 20 and the outer end of each vane is at a distance of 1.4 inches 
from the axis of rotation of shaft 20. In fact, FIG. 1 is drawn to scale 
and represents a pump having the above-stated dimensions. 
Vanes 34 and the portions of vanes 30 which project toward rear housing 
part 4 act to subject blood which is present between the impeller and rear 
housing part 4 to a radial outward force, and thereby prevent blood from 
recirculating around the outer edge of the impeller. Thus, the action of 
these vane portions together with the sharp shear edge provided by rear 
housing part 4 around shaft 20 at the side bordering chamber 6 serve to 
sweep blood away from the region where shaft 20 passes through rear 
housing part 4, which is a potential area of stasis, and thus prevent 
thrombus formation at that location. 
As noted earlier herein, vanes 30, 32, 34 are configured with the goal of 
minimizing the acceleration and shock experienced by blood within the 
pump. Preferably, the inlet blade angle of each vane, the blade angle 
being, at any point along a blade, the angle between the blade surface and 
a circle centered on the axis of impeller rotation and passing through the 
point in question, is selected so that, for a selected impeller speed, the 
velocity produced by each vane closely corresponds to the inlet flow 
velocity of the blood. 
According to a preferred embodiment of the invention, the configuration of 
each vane 30, 32, 34 was determined on the basis of the following 
equation: 
EQU R=R1+C1.multidot..THETA.+C2.multidot..THETA..sup.C3 (1) 
where: 
R is the radial distance from each point along the vane to the axis of 
rotation of shaft 20; 
R1 is the radial distance from the end of each long vane 30 closest of the 
axis of shaft rotation to that axis, i.e., at the inlet end of each long 
vane 30; 
.THETA. is the angle, in radians, about the axis of rotation of shaft 20, 
between a line extending between that axis and the inlet end of a long 
vane 30 and a line extending between that axis and the point on the same 
vane whose radial distance from the axis is R; 
C1=R1.multidot.tan.beta..sub.1, where .beta..sub.1 is the blade angle in 
radians, of a vane 30 at its inlet end; 
C2=(R.sub.2 tan.beta..sub.2 -C1)/(C3.multidot..THETA..sub.2 (C3-1)), 
where R2 is the radial distance from the end of each vane furthest from the 
axis of rotation of shaft 20 to that axis, B.sub.2 is the blade angle, in 
radians, of each vane at the end furthest from the axis of rotation of 
shaft 20, and .THETA..sub.2 is the value for .THETA. associated with the 
radial distance R2; and 
C3=(R2.multidot.tan.beta..sub.2 -C1).multidot..THETA..sub.2 /(R.sub.2 
-R.sub.1 -C1 .multidot..THETA..sub.2), 
with the following selected parameters being used: R1=0.3"; R.sub.2 =1.4"; 
.beta..sub.1 =0.1745 Rad=10.degree.; .beta..sub.2 =1.047 Rad=60.degree.; 
and .THETA..sub.2 =2.094 Rad=120.degree.. 
R1, R2, .beta..sub.1, .beta..sub.2 and .THETA..sub.2 are shown in FIG. 3. 
Blade angle, .THETA., is the angle, at a point along a vane, between a 
line tangent to the blade surface and a line tangent to a circle passing 
through that point and centered on the axis of rotation of shaft 20. 
FIG. 5 illustrates the basic components of a magnetic drive according to 
the present invention. This drive is composed of a plurality of permanent 
magnet units 50 mounted on a plate 52 having a central opening 53 for 
connection to the shaft of a drive motor. One-half of the drive is shown 
in FIG. 5. Each magnet unit 50 is composed of two bar magnets 54 each 
having its magnetic axis oriented parallel to the axis of rotation of 
plate 52, with the magnets 54 of each unit 50 being oriented in polarity 
opposition to one another, as shown. Moreover, the magnetic poles of each 
unit 50 are oriented opposite to those of each adjacent unit 50. Each unit 
50 is further composed of an arcuate plate 56 of ferromagnetic material 
completing the magnetic circuit at one end of the associated unit 50. 
The magnetic drive is disposed directly beneath rear housing part 4 so that 
magnet units 50 surround housing 8 and plate 52 is behind cap 10. Thus, 
the end of each magnet unit 50 which is remote from plate 52 faces a 
respective one of plates 28. The spacing between plates 28 and units 50 is 
made as small as possible in order to minimize the air gap between each 
plate 28 and its associated unit 50, and thus maximize the magnetic 
attraction exerted on each plate 28. 
The arrangement of magnetic units 50 is such that the magnetic flux path of 
each unit is completed through a respective one of plates 28 and the 
magnetic paths associated with adjacent ones of plates 28 are maintained 
isolated from each other by the orientations of the magnets associated 
with adjacent units 50. Thus, as plate 52 is rotated, the magnetic 
attraction forces exerted on plates 28 cause impeller 16, 18 to rotate in 
unison therewith. 
In addition, the magnetic attraction exerted by units 50 pulls impeller 16, 
18 downwardly in order to press shaft 20 against ball 26. 
The drive arrangement shown in FIG. 5 produces particularly strong magnetic 
forces, making possible the use of thin, unmagnetized plates 28 and 
permitting a sufficient drive force to be imparted to impeller 16, 18 even 
with a comparatively large air gap between units 50 and plates 28. 
While the description above refers to particular embodiments of the present 
invention, it will be understood that many modifications may be made 
without departing from the spirit thereof. The accompanying claims are 
intended to cover such modifications as would fall within the true scope 
and spirit of the present invention. 
The presently disclosed embodiments are therefore to be considered in all 
respects as illustrative and not restrictive, the scope of the invention 
being indicated by the appended claims, rather than the foregoing 
description, and all changes which come within the meaning and range of 
equivalency of the claims are therefore intended to be embraced therein.