U.S. patent application number 11/116749 was filed with the patent office on 2005-11-17 for blood pump with constant blood pumping area.
Invention is credited to Camarero, Ricardo, Carrier, Michel, Garon, Andre, Obeid, Victor, Pelletier, Conrad.
Application Number | 20050254976 11/116749 |
Document ID | / |
Family ID | 33426242 |
Filed Date | 2005-11-17 |
United States Patent
Application |
20050254976 |
Kind Code |
A1 |
Carrier, Michel ; et
al. |
November 17, 2005 |
Blood pump with constant blood pumping area
Abstract
The blood pump comprises a stationary housing structure and a
rotative impeller. The stationary housing structure has a proximal
end provided with an inlet, a distal end provided with an outlet,
and an impeller housing. The rotative impeller is mounted within
the stationary housing structure to circulate blood in a blood flow
direction generally extending from the proximal end to the distal
end. The rotative impeller also includes an impeller blade
structure mounted in the impeller housing, and having a first
annular face tapered in a direction opposite the blood flow
direction and a second annular face downstream from the first
annular face and tapered in the direction of the blood flow. The
impeller housing includes a first annular face tapered in a
direction opposite the blood flow direction and facing the first
annular face of the impeller blade structure; and a second annular
face tapered in the direction of the blood flow and facing the
second annular face of the impeller blade structure, the tapered,
annular faces being so shaped and dimensioned as to keep the area
through which blood flows constant.
Inventors: |
Carrier, Michel; (Montreal,
CA) ; Garon, Andre; (Ville d'Anjou, CA) ;
Camarero, Ricardo; (Verdun, CA) ; Pelletier,
Conrad; (Montreal, CA) ; Obeid, Victor;
(Collegeville, PA) |
Correspondence
Address: |
QUARLES & BRADY LLP
411 E. WISCONSIN AVENUE
SUITE 2040
MILWAUKEE
WI
53202-4497
US
|
Family ID: |
33426242 |
Appl. No.: |
11/116749 |
Filed: |
April 28, 2005 |
Current U.S.
Class: |
417/423.14 ;
417/352; 417/353; 417/355 |
Current CPC
Class: |
A61M 60/824 20210101;
A61M 60/135 20210101; A61M 60/422 20210101; A61M 60/871 20210101;
F04D 29/047 20130101; A61M 60/205 20210101; F04D 3/00 20130101;
A61M 60/148 20210101; A61M 60/857 20210101; F04D 13/0633
20130101 |
Class at
Publication: |
417/423.14 ;
417/352; 417/353; 417/355 |
International
Class: |
F04B 017/00; F04B
035/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 13, 2003 |
CA |
2,428,741 |
Claims
What is claimed is:
1. A blood pump comprising: a stationary housing structure having a
proximal end provided with an inlet and a distal end provided with
an outlet; said stationary housing structure including an impeller
housing; a rotative impeller mounted within the stationary housing
structure to circulate blood in a blood flow direction generally
extending from said proximal end to said distal end; said rotative
impeller including an impeller blade structure mounted in said
impeller housing; said impeller blade structure having a first
annular face tapered in a direction opposite the blood flow
direction and a second annular face downstream from said first
annular face and tapered in the direction of the blood flow;
wherein a) said impeller housing includes a first annular face
tapered in a direction opposite the blood flow direction and facing
the first annular face of said impeller blade structure; and a
second annular face tapered in the direction of the blood flow and
facing said second annular face of said impeller blade structure;
and b) said tapered, annular faces being so shaped and dimensioned
as to keep the area through which blood flows constant.
2. The blood pump as defined in claim 1, wherein the tapering,
annular faces are frusto-conical faces.
3. The blood pump as defined in claim 1, wherein said impeller
blade structure includes an annular member having an inner
cylindrical surface, said annular member being mounted on an
impeller drive shaft of said rotative impeller, said drive shaft
having an outer cylindrical surface configured and sized to receive
said annular member.
4. The blood pump as defined in claim 2, wherein said impeller
blade structure includes at least two impeller blades mounted to
said annular member, wherein said impeller blades are positioned
around said first annular face of said impeller blade
structure.
5. The blood pump as defined in claim 4, wherein said at least two
impeller blades have a width gradually decreasing in the direction
of the blood flow.
6. The blood pump as defined in claim 1, wherein said stationary
housing structure includes at least two impeller blades mounted to
said impeller housing structure, wherein said impeller blades are
positioned around said second annular face of said impeller
housing.
7. The blood pump as defined in claim 6, wherein said at least two
impeller blades have as width gradually increasing in the direction
of the blood flow.
8. The blood pump as defined in claim 2, wherein said impeller
blade structure comprises a frusto-conical bushing mounted to said
second frusto-conical face of said impeller blade structure in the
vicinity of said outlet.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional of international
application PCT/CA2004/0074 filed May 13, 2004, which claims
priority to Canadian application 2,428,741 filed May 13, 2003.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
FIELD OF THE INVENTION
[0002] The present invention relates to a blood pump with a
constant blood pumping area.
BACKGROUND OF THE INVENTION
[0003] In North America, heart related diseases are still the
leading cause of death. Among the causes of heart mortality are
congestive heart failure, cardiomyopathy and cardiogenic shock.
[0004] The incidence of congestive heart failure increases
dramatically for people over 45 years of age. In addition, a large
part of the population in North America is now entering this age
group. Thus, patients who will need treatment for these types of
diseases comprise a larger segment of the population. Many
complications related to congestive heart failure, including death,
could be avoided and many years added to these patients' lives if
proper treatments were available.
[0005] The types of treatments available for patients experiencing
heart failure depend on the extent and severity of the illness.
Many patients can be cured with rest and drug therapy but there are
still severe cases that require various heart surgeries, including
heart transplantation. Actually, the mortality rate for patients
with cardiomyopathy who receive drug therapy is about 25% within
two years and there still is some form of these diseases that
cannot be treated medically. One of the last options that remain
for these patients is heart transplantation. Unfortunately,
according to the procurement agency UNOS (United Network for Organ
Sharing in the United States), the waiting list for heart
transplantation grows at a rate of more than twice the number of
heart donors.
[0006] Considering the above facts, it appears imperative to offer
alternative treatments to heart transplantation. The treatment
should not only add to a patient's longevity but also improve his
quality of life. In this context, mechanical circulatory support
through Ventricular Assist Devices (VAD) is a worthwhile
alternative given the large deficiency in the number of available
organ donors. It is estimated that eight thousand (8,000) patients
per year in Canada and seventy-six thousand patients (76,000) per
year in the United States could benefit from VADs.
[0007] In 1980, the National Heart, Lung and Blood Institute
(NHLBI) of the United States defined the characteristics for an
implantable VAD (Altieri, F. O. and Watson, J. T, 1987,
"Implantable Ventricular Assist Systems", Artif Organs, Vol. 11,
pp. 237-246). These characteristics include medical requirements
including restoration of hemodynamic function (pressure and cardiac
index), avoidance of hemolysis, prevention of clot formation,
infection and bleeding, and minimisation of the anti-coagulation
requirement. Further technical requirements include: small size,
control mode, long life span (>2 years), low heating, noise and
vibration.
[0008] Several VADs have been developed to enhance blood
circulation and reduce the load on the heart of patients having
poor hemodynamic functions (low cardiac output, low ejection
fraction, low systolic pressure). These VADs include pulsatile and
non-pulsatile VADs.
[0009] A first example of non-pulsative VADs are radial-flow blood
pumps. In radial-flow blood pumps, the rotation of the impeller
produces a centrifugal force that drags blood from the inlet port
to the outlet port. A problem related to radial-flow blood pumps is
that although they are much smaller than pulsatile VADs, they are
still too large to be totally implanted in a human thorax thus
eliminating any intra-ventricular implantation.
[0010] A second example of non-pulsative VADs are axial-flow blood
pumps. These axial-flow blood pumps decrease the hemolysis rate by
decreasing the time of exposure of the blood to friction forces and
by reducing the intensity of these forces. Another interesting
advantage is that axial-flow blood pumps are generally much smaller
than radial-flow blood pumps, and can be much more easily implanted
in the human body, even in the left ventricle of the heart, for
medium and long term mechanical cardiac support.
[0011] Although the above-described VADs can achieve the goals of
restoring the hemodynamic functions and improving end organ
perfusion, both power and pumping efficiency of these VADs can
still be improved. Also, hemolysis and thrombus formation are still
important problems requiring investigation.
SUMMARY OF THE INVENTION
[0012] In order to improve VADs, the present invention is concerned
with a blood pump comprising a stationary housing structure and a
rotative impeller. The stationary housing structure includes a
proximal end provided with an inlet, a distal end provided with an
outlet, and an impeller housing. The rotative impeller is mounted
within the stationary housing structure to circulate blood in a
blood flow direction generally extending from the proximal end to
the distal end. The rotative impeller also includes an impeller
blade structure mounted in the impeller housing, and having a first
annular face tapered in a direction opposite the blood flow
direction and a second annular face downstream from the first
annular face and tapered in the direction of the blood flow. The
impeller housing includes a first annular face tapered in a
direction opposite the blood flow direction and facing the first
annular face of the impeller blade structure, and a second annular
face tapered in the direction of the blood flow and facing the
second annular face of the impeller blade structure, the tapered,
annular faces being so shaped and dimensioned as to keep the area
through which blood flows constant.
[0013] The foregoing and other objects, advantages and features of
the present invention will become more apparent upon reading of the
following non-restrictive description of illustrative embodiments
thereof, given by way of example only with reference to the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] In the appended drawings:
[0015] FIG. 1 is a cross sectional view of a human heart in which a
non-restrictive, illustrative intra-ventricular embodiment of a
mixed-flow blood pump according to the present invention is
implanted;
[0016] FIG. 2 is a graph showing, for different types of pumps, a
curve relating a specific pump rotation speed N.sub.s to a specific
pump diameter D.sub.s at the points where the pump is operating at
maximum hydraulic efficiency;
[0017] FIG. 3 is a side elevational, cross sectional view of the
intra-ventricular mixed-flow blood pump of FIG. 1;
[0018] FIG. 4 is a perspective view of radial blades of an outflow
stator of the intra-ventricular mixed-flow blood pump of FIG. 3,
showing an example of configuration of these radial blades;
[0019] FIG. 5 is a rear perspective view of a frusto-conical inflow
bushing of the intra-ventricular mixed-flow blood pump of FIG. 3,
showing the configuration of the inner face of this bushing;
[0020] FIG. 6 is a side perspective view of the frusto-conical
inflow bushing of FIG. 5;
[0021] FIG. 7 is a front perspective view of the frusto-conical
inflow bushing of FIGS. 5 and 6;
[0022] FIG. 8 is a front perspective view of a frusto-conical
outflow bushing of the intra-ventricular mixed-flow blood pump of
FIG. 3, showing the configuration of the inner face of that outflow
bushing;
[0023] FIG. 9 is a perspective view of an impeller blade structure
of the intra-ventricular mixed-flow blood pump of FIG. 3,
comprising an annular member on which a set of impeller blades are
mounted;
[0024] FIG. 10 is a side elevational view of a portion of an
impeller drive shaft of the intra-ventricular mixed flow blood pump
of FIG. 3 and of the annular member of the impeller blade structure
of FIG. 9, showing how the latter annular member is mounted on the
impeller drive shaft;
[0025] FIG. 11 is side elevational view of an impeller blade of the
impeller blade structure of FIG. 9, showing details of structure of
this impeller blade;
[0026] FIG. 12 is a side elevational and cross sectional view of an
illustrative extra-ventricular embodiment of the mixed-flow blood
pump according to the present invention, incorporating the elements
and/or structure shown in FIGS. 4-11; and
[0027] FIG. 13 is a schematic view of an illustrative embodiment of
a VAD system implanted in a human being and comprising the
mixed-flow blood pump of FIG. 3.
DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS
[0028] The non-restrictive illustrative embodiments of the present
invention will be described in connection with a mixed-flow blood
pump that can be used as part of:
[0029] an intra-ventricular VAD;
[0030] an extra-ventricular VAD, for example a VAD located in a
patient's abdomen or thorax; or
[0031] an extra-corporal VAD, for example in a bridge to heart
transplantation.
[0032] It should also be understood that the mixed-flow blood pump
can be used either in temporary VADs, or medium and long term
VADs.
[0033] FIG. 1 illustrates a possible position for an illustrative
intra-ventricular embodiment 10 of the mixed-flow blood pump in the
left ventricle 11 of a patient's heart 12.
[0034] The intra-ventricular mixed-flow blood pump 10 has been
designed and dimensioned to fit in small adults and in teens.
Feigenbaum, Harvey, "Echocardiography", 5th Edition, 1994, Lea
& Febiger, Philadelphia, has determined that for 95% of the
population the internal diameter of the left ventricle 11 ranges
from 37 to 46 mm in diastole and between 22 to 31 mm in systole.
This diameter is determined at the centre of the ventricular length
(segment AB of FIG. 1). The diameter near the apex at the first
third of the ventricular length is about 15 mm (segment CD of FIG.
1). The internal length of the ventricle from the apex to the
aortic valve ranges from 55 to 70 mm. Finally, the other important
parameter is the surface of the aortic valve opening, which ranges
from 2.5 to 4 cm.sup.2.
[0035] Obviously, the external design (shape and size) of the
intra-ventricular mixed-flow blood pump 10 (FIG. 1) will depend on
the above anatomic dimensions of the left ventricle 11. FIG. 3
shows the external outline of the intra-ventricular mixed-flow
blood pump 10. The diameter of the intra-ventricular mixed-flow
blood pump 10 constitutes a compromise between pumping requirements
and minimal interference with heart contraction. In the
intra-ventricular mixed-flow blood pump 10, the maximum allowable
diameter 13 (FIG. 3) is about 22 mm, which is the diameter of the
left ventricle 11 in systole. This dimension is reasonable since
people with heart failure generally have dilated ventricles.
[0036] The maximum length of the intra-ventricular mixed-flow blood
pump 10, as illustrated in FIG. 3, is set in regard of the average
distance between the apex 14 and the aortic valve 15 of the heart
12. The length 16 (FIG. 3) of the mixed-flow blood pump 10 is about
65 mm.
[0037] It should be understood that the size and shape of the
intra-ventricular mixed-flow blood pump 10 could also be adapted to
meet the anatomical dimensions of individuals falling outside the
above described 95% of the population. Similarly, the size and
shape could be adapted to specific and particular individuals and
heart conditions.
[0038] Since the intra-ventricular mixed-flow blood pump 10 will be
totally immersed inside the left ventricle 11, blood will circulate
around the pump 10. As a consequence, the external surfaces of the
intra-ventricular mixed-flow blood pump 10 will be as smooth as
possible and avoid as much as possible abrupt deviations to thereby
minimise recirculation and stagnation zones that can be at the
origin of clot formation. To overcome this problem, the
intra-ventricular mixed-flow blood pump 10 may be machined, for
example, from surgical quality titanium.
[0039] From a surgical point of view, a non-limitative illustrative
procedure for inserting the intra-ventricular mixed-flow blood pump
10 is to use the same approach as with cardiac valve replacement.
According to this procedure, an incision is made at the root of the
aorta 18 (FIG. 1) and the pump 10 is inserted though the aortic
valve and then into the left ventricle 11. The mixed-flow blood
pump 10 is then pushed until its base reaches the myocardium at the
apex 14 and then fixed in place.
[0040] To prevent motion thereof, the intra-ventricular mixed-flow
blood pump 10 is finally fixed by means of a fixation mechanism 19
(FIGS. 1 and 3) provided at the inflow end of the pump 10. As a
non-limitative example, the fixation mechanism 19 (FIG. 3)
comprises:
[0041] an elongated hollow needle 20 projecting axially from the
inflow end of the pump 10, this needle 20 being driven from the
inside of the left ventricle 11 through the myocardium and the
epicardium at the apex 14 of the heart 12; and
[0042] a fixation disk 21 fastened to the free end of the needle 20
on the outside of the heart 12 to firmly fasten the mixed-flow
blood pump 10 within the left ventricle 11.
[0043] As a non-limitative example, both the free end of the needle
20 and the fixation disk 21 will be threaded to allow the fixation
disk 21 to be screwed on the free end of the elongated hollow
needle 20. Further rotation of the fixation disk 21 on the needle
20 will then be prevented by any suitable means.
[0044] Of course, it is within the scope of the present invention
to use any suitable fixation mechanism other than the needle 20 and
disk 21.
[0045] Since one of the main functions of the intra-ventricular
mixed-flow blood pump 10 is to restore the hemodynamic function in
patients with cardiac failure, and depending on the severity of the
failure and the BSA (Body Surface Area), the intra-ventricular
mixed-flow blood pump 10 is susceptible to work at flow rates
between 2 to 6 litres per minute (l/min) against a pressure as high
as 120 mmHg and, more commonly, at a flow rate between 3 to 5 l/min
against a pressure of 80 mmHg. A high efficiency pump design is
therefore required.
[0046] When designing turbine pumps, dimensionless characteristic
values are used to compare different pump configurations.
Dimensionless characteristic values provide useful indications to
pump designers of expected performance regardless of the size of
the pump, a comparison which would otherwise prove difficult given
a virtually infinite number of operating parameters that depend on
infinite variations of internal pump geometry. These dimensionless
characteristic values, therefore, can be used to provide an
objective starting point for the selection of a general pump
configuration.
[0047] Two of these dimensionless characteristic values are the
specific rotation speed N.sub.s of the pump and the specific pump
diameter D.sub.s. They are defined as follows: 1 N s = Q 1 / 2 H 3
/ 4 ( 1 ) D s = D H 1 / 4 Q 1 / 2 ( 2 )
[0048] where .OMEGA. is the speed of rotation of the pump 10 in
radians/second, Q is the flow rate in m.sup.3/second, H is the head
(i.e. the gain in pressure) of the pump 10 and D the diameter of
the pump, both in meters. N.sub.s remains the same regardless of
the size of the pump and therefore provides an accurate measure of
the performance of a given pump design. D.sub.s relates the pump
diameter to the pump head H and flow rate Q.
[0049] Referring to FIG. 2, the design curve relates the specific
speed N.sub.s with the specific diameter D.sub.s to yield the
optimal pump configuration. Specifically, if the configuration of
N.sub.s and D.sub.s falls on the curve, the maximum hydraulic
efficiency of the design is greater than if it falls away from the
curve. In this regard, hydraulic efficiency is expressed as the
percentage of the power input to the pump which is converted to
energy of movement of the fluid within the pump. From the curve of
FIG. 2 and equation (1) above, it follows that optimally efficient
pumps having a higher specific speed also have a smaller size.
[0050] In order to determine an optimised choice for a pump, it is
necessary to evaluate the specific speed N.sub.s in light of the
characteristics in terms of head H and flow rate Q projected for
the pump. As discussed above, the pump will typically be operated
with a flow rate of 5 litres/minutes and a head of approximately
100 mmHg. Additionally, current motor technology provides small yet
efficient motors operating at a speed of 7,500 RPM. This gives a
specific speed N.sub.s of 1.12 and a specific diameter Ds of 2.45
for a maximum internal diameter of 12 mm.
[0051] Still referring to FIG. 2, an indication is given to the
ranges of N.sub.s and D.sub.s within which a given pump
configuration will provide efficient operation. The specific speed
N.sub.s of 1.12 falls within a transition region of the curve
between axial-flow and radial-flow pumps. In this transition
region, a mixed-flow pump topology would yield a higher efficiency
than purely radial-flow or axial-flow pumps. Additionally, the
specific diameter D.sub.s is around 2.45 which, by applying
Equation (2) above, yields a maximum impeller drive shaft diameter
of 12 mm, i.e. a very small pump. For these reasons, a mixed-flow
pump design was selected for the intra-ventricular pump 10.
[0052] The structure and operation of the non-restrictive
illustrative embodiment 10 of the intra-ventricular mixed-flow
blood pump will now be described.
[0053] Referring to FIG. 1, the intra-ventricular mixed-flow blood
pump 10 rests on the bottom of the left ventricle 11, in the region
of the apex 14 of the heart 12.
[0054] As shown in FIGS. 1 and 3, in order to prevent the inner
walls of the left ventricle 11 from completely obstructing blood
intake, the intra-ventricular mixed-flow blood pump 10 comprises a
stationary housing structure 100 (FIG. 3) including two axially
spaced apart, annular radial-flow inlets 22 and 23. Additionally,
the inflow end of the stationary housing structure 100 presents a
surface 24 presenting the general configuration of a circular
portion of a hemisphere. The diameter of the circular hemisphere
portion is set to approximately 20 mm, which is smaller than the
segment CD (see FIG. 1) and suitable to reduce the level of
pressure on the walls of the left ventricle 11 near the apex
14.
[0055] The stationary housing structure 100 of the
intra-ventricular mixed-flow blood pump 10 comprises a hollow
cylindrical member 25 containing the stator windings such as 26 and
the associated magnetic cores such as 27. The hollow cylindrical
member 25 is made of two mutually mating annular pieces 28 and 29
to enable insertion of the stator windings 26 and cores 27 within
the hollow cylindrical member 25. For example, both the annular
pieces 28 and 29 will be threaded to allow said annular pieces 28
and 29 to be screwed on each other. Further rotation of the annular
pieces 28 and 29 on each other will then be prevented by any
suitable means. Alternatively, the annular pieces 28 and 29 can be
laser welded to each other.
[0056] The stationary housing structure 100 further comprises an
inflow bushing mount 30 mounted on a proximal end of the
cylindrical member 25. More specifically, the inflow bushing mount
30 comprises an annular portion 31 profiled to fit on the proximal
end of the hollow cylindrical member 25 while defining with this
cylindrical member 25 a smooth surface of the annular radial-flow
inlet 22. The inflow bushing mount 30 also comprises a wall 32
presenting the general configuration of a circular portion of a
hemisphere; the outer face of the hemispheric wall 32 defines the
above-mentioned hemispheric surface 24. The inner face of the
hemispheric wall 32 is connected to the annular portion 31 through
a series of radial blades such as 33 and 34 spread out evenly
around a longitudinal axis 41 of the intra-ventricular mixed-flow
blood pump 10, within the radial-flow inlet 22. Another function of
the radial blades such as 33 and 34 is to straighten out the flow
of blood through the radial-flow inlet 22.
[0057] An inflow bushing 35 having the general configuration of a
frustum of cone is mounted inside the annular portion 31 of the
bushing mount 30 and is centered on the longitudinal axis 41 of the
blood pump 10. More specifically, the frusto-conical bushing 35 is
mounted to the annular portion 31 through a series of radial blades
such as 36 and 37 spread out evenly around the axis of the
intra-ventricular mixed-flow blood pump 10, more specifically
around the frusto-conical bushing 35. As illustrated in FIG. 3, the
frusto-conical bushing 35 has an end of larger diameter facing
toward the inflow end of the intra-ventricular mixed-flow blood
pump 10. Again, another function of the blades such as 36 and 37 is
to straighten out the flow of blood passing between the
frusto-conical bushing 35 and the annular portion 31 of the inflow
bushing mount 30.
[0058] The stationary housing structure 100 of the
intra-ventricular mixed-flow blood pump 10 further comprises an
impeller housing 38 and an outflow cannula 42.
[0059] The proximal end of the impeller housing 38 is connected to
the distal end of the cylindrical member 25 through a series of
radial blades such as 39 and 40 spread out evenly around the
longitudinal axis 41 to define the second annular radial-flow inlet
23 between the distal end of the cylindrical member 25 and the
proximal end of the impeller housing 38. Another function of the
radial blades such as 39 and 40 is to straighten out the flow of
blood through the annular radial-flow inlet 23.
[0060] As mentioned hereinabove, the first annular radial-flow
inlet 22 is axially spaced apart from the second annular
radial-flow inlet 23 to reduce as much as possible the effect
occlusion of one of the inlets 22 or 23 may have on normal
operation of the blood pump 10.
[0061] Referring to FIGS. 1 and 3, the diameter of the outflow
cannula 42 reduces from the impeller housing 38 to the free end of
the outflow cannula to reduce as much as possible the obstruction
caused by the intra-ventricular mixed-flow blood pump 10 to the
operation of the aortic valve (not shown); since the function of
the intra-ventricular mixed-flow blood pump 10 is to assist blood
circulation, blood flow contribution from the natural contraction
of the heart 12 should be maintained. In the intra-ventricular
mixed-flow blood pump 10, the area of the outflow cannula 42,
corresponding to diameter 43, is 0.463 cm.sup.2.
[0062] As illustrated in FIG. 3, the impeller housing 38 and
outflow cannula 42 are respectively made of two separate, mutually
mating pieces in order to enable insertion of the impeller within
the impeller housing 38. For example, both the impeller housing 38
and the outflow cannula 42 will be threaded to allow said housing
38 and cannula 42 to be screwed on each other. Further rotation of
the impeller housing 38 and outflow cannula 42 on each other will
then be prevented by any suitable means. Alternatively, the
impeller housing 38 and the outflow cannula 42 can be laser welded
to each other.
[0063] A blood diffuser (not shown) can be mounted on the free end
of the outflow cannula 42 (outflow end of the stationary housing
structure 100). The function of the blood diffuser would be to
reduce the shear stress on blood cells. Without diffuser, the
velocity of blood ejected from the intra-ventricular mixed-flow
blood pump 10 is higher than the velocity of blood ejected through
the aortic valve 15 of the heart 12. The difference of velocity
between the two blood flows would result in shear stresses
proportional to this difference. Since the velocity is inversely
proportional to the cross-sectional area, a solution for reducing
the relative velocity of the blood flows from the pump 10 and from
the heart 12 is (a) to increase the area of the orifice 44 of the
outflow cannula 42 to thereby reduce the velocity of the flow of
blood from the pump 10, and (b) to decrease the area occupied by
the blood flow from the heart 12 to increase the velocity of the
latter blood flow. This would be exactly the role of the blood
diffuser. Of course, parameters such as the angle of opening and
the length of the blood diffuser could be adjusted at will to fit
the mechanical characteristics of the intra-ventricular mixed-flow
blood pump 10 in view of minimising the shear stress on the blood
cells.
[0064] The outflow cannula 42 comprises an outflow stator 45 formed
of a series of inner radial blades spread out evenly around the
longitudinal axis 41 and connected to the inner face of the outflow
stator 45 by diffusion bound. FIG. 4 illustrates an example of
configuration of the radial blades such as 450 of the outflow
stator 45. More specifically, the radial blades 450 are configured
to straighten out the flow of blood exiting the cannula 42 and are
fixedly secured to the inner face of the outflow cannula 42.
[0065] An outflow bushing 46 having the general configuration of a
frustum of cone is mounted inside the outflow stator 45 and is
centered on the longitudinal axis 41. More specifically, the
frusto-conical bushing 46 is secured to the radial blades 450 of
the outflow stator 45. Finally, the end of larger diameter of the
frusto-conical bushing 46 is facing toward the inflow end of the
intra-ventricular mixed-flow blood pump 10.
[0066] The intra-ventricular mixed-flow blood pump 10 also
comprises a rotative impeller 56 provided with an impeller drive
shaft 55 centered on the longitudinal axis 41. The impeller drive
shaft 55 comprises an inflow end portion 57 formed with an inflow
frusto-conical face 58 structured to snugly fit into the inflow
frusto-conical bushing 35.
[0067] FIG. 5 illustrates the inner face 47 of the inflow bushing
35. Inner face 47 comprises at least three axial grooves 48, 49 and
50 generally rectangular in cross section and evenly spread out
around the longitudinal axis 41 (FIG. 3).
[0068] As a non-limitative example, from groove 48 to groove 49,
the inner face 47 of the inflow bushing 35 successively defines in
the direction of rotation of the rotative impeller 56 a taper 51
and a land 52. Taper 51 has a diameter that gradually decreases
from groove 48 to land 52, and land 52 has a constant diameter. The
land 52 spans an angular sector of approximately 17.5.degree. about
the longitudinal axis 41 and presents a clearance of approximately
0.0116 mm to the frusto-conical face 58 (FIG. 3) of the inflow end
portion 57 of the impeller drive shaft 55. The taper 51 spans an
angular sector of approximately 82.5.degree. about the longitudinal
axis 41 and creates, from the groove 48 to the land 52, a gradual
0.030 mm clearance increase. That means that, at the edge 480
separating the taper 51 from the groove 48, there is a clearance of
approximately 0.0416 mm. The groove 48 spans an angular sector of
approximately 20.degree. about the longitudinal axis 41 computed
from the edge 481 joining the land 61 to the groove 48 up to the
edge 480 joining the same groove 48 to the taper 51. This
20.degree. angular sector includes the round of edge 481 blending
the land 61 with the groove 48 and the round of edge 480 blending
the taper 51 with the groove 48.
[0069] As a non-limitative example, from groove 49 to groove 50,
the inner face 47 of the inflow bushing 35 successively defines in
the direction of rotation of the rotative impeller 56 a taper 53
and a land 59. Taper 53 has a diameter that gradually decreases
from groove 49 to land 59, and land 59 has the same constant
diameter as land 52. The land 59 spans an angular sector of
approximately 17.5.degree. about the longitudinal axis 41 and
presents a clearance of approximately 0.0116 mm to the
frusto-conical face 58 (FIG. 3) of the inflow end portion 57 of the
impeller drive shaft 55. The taper 53 spans an angular sector of
approximately 82.5.degree. about the longitudinal axis 41 and
creates, from the groove 49 to the land 59, a gradual 0.030 mm
clearance increase. That means that, at the edge 490 separating the
taper 53 from the groove 49, there is a clearance of approximately
0.0416 mm. The groove 49 spans an angular sector of approximately
20.degree. about the longitudinal axis 41 computed from the edge
491 joining the land 52 to the groove 49 up to the edge 490 joining
the same groove 49 to the taper 53. This 20.degree. angular sector
includes the round of edge 491 blending the land 52 with the groove
49 and the round of edge 490 blending the taper 53 with the groove
49.
[0070] As a non-limitative example, from groove 50 to groove 48,
the inner face 47 of the inflow bushing 35 successively defines in
the direction of rotation of the rotative impeller 56 a taper 60
and a land 61. Taper 60 has a diameter that gradually decreases
from groove 50 to land 61, and land 61 has the same constant
diameter as the lands 52 and 59. The land 61 spans an angular
sector of approximately 17.5.degree. about the longitudinal axis 41
and presents a clearance of approximately 0.0116 mm to the
frusto-conical face 58 (FIG. 3) of the inflow end portion 57 of the
impeller drive shaft 55. The taper 60 spans an angular sector of
approximately 82.5.degree. about the longitudinal axis 41 and
creates, from the groove 50 to the land 61, a gradual 0.030 mm
clearance increase. That means that, at the edge 500 separating the
taper 60 from the groove 50, there is a clearance of approximately
0.0416 mm. The groove 50 spans an angular sector of approximately
20.degree. about the longitudinal axis 41 computed from the edge
501 joining the land 59 to the groove 50 up to the edge 500 joining
the same groove 50 to the taper 60. This 20.degree. angular sector
includes the round of edge 501 blending the land 59 with the groove
50 and the round of edge 500 blending the taper 60 with the groove
50.
[0071] In an example of construction, there are three blades such
as 36 and 37 (FIG. 3) aligned with the three grooves 48, 49 and 50,
respectively, to minimize blood flow perturbation. A number of
blades and grooves different from 3 could obviously be used. Also,
the blades such as 36 and 37 are connected to the grooves 48, 49
and 50, respectively, by diffusion bound.
[0072] Referring to FIG. 6, the geometry of the curvature of the
three trailing edges 350 of the inflow bushing 35 is constant for
the three taper/land zones 51/52, 53/59 and 60/61. More
specifically, the trailing edges 350 each correspond to an arc of
circle of given diameter.
[0073] To keep the geometry of the curvature constant in the taper
zones 51, 53 and 60, a portion of the outer surface of the inflow
bushing 35 has been modified. More specifically, the portion 351 of
the outer surface of the inflow bushing 35, shown in meshed area in
FIG. 6, runs all around the inflow bushing 35 and is a perfect
revolved surface portion. The "perfect circle" 352 outlined in FIG.
6 represents the last section of the inflow bushing 35 where the
outer surface is obtained by a revolved feature.
[0074] The outer surface of the inflow bushing 35 defines a blended
surface portion 356 blending the perfect revolved surface portion
with the growth of the taper diameter, to allow the geometry of the
curvature of the trailing edges 350 to remain constant while the
radial position of each trailing edge 350 changes at the same rate
as the taper diameter.
[0075] At the leading end 353 of the inflow bushing 35, the annular
edge 354 that is created at the intersection between the
cylindrical inner surface portion 355 and the inner surface 47 of
the inflow bushing 35 including the grooves 48, 49 and 50, the
tapers 51, 53 and 60, and the lands 52, 59 and 61 is polished to
smoothen that edge 354 (see FIG. 7).
[0076] In operation, the lands 52, 59 and 61 form a seat for the
inflow frusto-conical face 58 of the inflow end portion 57 of the
impeller drive shaft 55. The grooves 48, 49 and 50 enable flow of
blood between the faces 47 and 58. The hydrodynamic forces produced
by rotation of the impeller drive shaft 55 will produce a thicker
film of blood flowing between the frusto-conical face 58 of the
inflow end portion 57 of the impeller drive shaft 55 and the tapers
51, 53 and 60, and a thinner film of blood flowing between face 58
and the lands 52, 59 and 61 to thereby lubricate the resulting
bearing (frusto-conical face 58 and inner face of the
frusto-conical bushing 35). Since blood flows through the gap
between the frusto-conical faces 47 and 58, minimal hemolysis,
thrombus and clot formation will be produced.
[0077] As a non-limitative example, the resulting inflow bearing
(frusto-conical face 47 and frusto-conical bushing 35) present the
following approximate dimensions and characteristics:
[0078] Larger diameter: 8 mm
[0079] Smaller diameter: 3 mm
[0080] Cone angle: 20.degree.
[0081] Axial length: 6.87 mm
[0082] Cone length: 7.31 mm
[0083] Number of pads (taper and land): 3
[0084] Pad angle about the longitudinal axis: 100.degree.
[0085] Groove angle about the longitudinal axis: 20.degree.
[0086] Taper angle about the longitudinal axis: 82.5.degree.
[0087] Taper gradual clearance increase: 0.030 mm
[0088] Land angle about the longitudinal axis: 17.5.degree.
[0089] Land clearance: 0.0116 mm.
1 INFLOW 6807 RPM C = Radial Axial Axial Minimum Power Maximum
Clearance Gap Gap Load Film Loss Pressure (mm) (mm) (mm) (N) (mm)
(Watt) (Pa) 0.01116 0.0123 0.0339 1.01 0.0116 0.1199 74871
[0090] With a 0.8 N magnetic pull axially at 5 l/min against a
pressure of 80 mmHg.
[0091] Referring back to FIG. 3, the impeller drive shaft 55 also
comprises an outflow portion 63 which, when assembled to the inflow
end portion 57 defines a cavity in which a cylindrical permanent
magnet 64 is inserted. The windings 26 and the permanent magnet 64
form an electric motor structure operative to set the impeller 56
into rotation; the magnetic field produced by the windings 26 is
applied to the magnetic field produced by the permanent magnet 64
to produce a reaction that will set the impeller 56 into
rotation.
[0092] An axial magnetic pull is produced by slightly, axially
offsetting the permanent magnet 64 toward the inflow bushing 35
with respect of the magnetic windings 26. This will produce an
axial magnetic pull of the order of, for example, 0.8 N toward the
outflow bushing 46, i.e. in a direction opposite to an axial force
produced on the impeller drive shaft 55 upon pumping blood.
[0093] An axial screw 65 passes through the magnet 64 and screws
into both the inflow 57 and outflow 63 portions of the impeller
drive shaft 55 to firmly secure these two drive shaft portions 57
and 63 together.
[0094] The outflow portion 63 of the impeller drive shaft 55
comprises an outflow end 66 formed with an outflow frusto-conical
face 67 structured to snugly fit into the frusto-conical bushing
46.
[0095] FIG. 8 illustrates the inner face 68 of the outflow bushing
46. Inner face 68 comprises at least three axial grooves 69, 70 and
71 generally rectangular in cross section and evenly spread out
around the longitudinal axis 41 (FIG. 3).
[0096] As a non limitative example, from groove 69 to groove 70,
the inner face 68 successively defines in the direction of rotation
of the rotative impleller 133 a taper 72 and a land 73. Taper 72
has a diameter that gradually decreases from groove 69 to land 73,
and land 73 has a constant diameter. The land 73 spans an angular
sector of approximately 20.degree. about the longitudinal axis 107
and presents a clearance of approximately 0.0226 mm to the
frusto-conical face 67 of the outflow portion 63 of the impeller
drive shaft 55. The taper 72 spans an angular sector of
approximately 80.degree. about the longitudinal axis 107 and
creates, from groove 69 to land 73 a gradual 0.025 mm clearance
increase. That means that at the edge 690 separating the taper 72
from the groove 69, there is a clearance of approximately 0.0476 mm
clearance between this edge 690 and the frusto-conical face 67 of
the outflow portion 63 of the impeller drive shaft 55. The groove
69 spans an angular sector of approximately 20.degree. about the
longitudinal axis 107 computed from the edge 690 joining the taper
68 to the groove 69 up to the edge 691 joining the same groove 69
to the land 77. This angular sector of 20.degree. includes the
round of the edge 690 blending the groove 69 with the taper 68 and
the round of the edge 691 blending the groove 69 with the land 77.
The edges 692 that form the boundary between the groove 69 and the
leading, annular rounded edge surface 460 are polished to create a
round having a radius of at least 0.100 mm.
[0097] As a non-limitative example, from groove 70 to groove 71,
the inner face 68 defines a taper 74 and a land 75. Taper 74 has a
diameter that gradually decreases from groove 70 to land 75, and
land 75 has a constant diameter. The land 75 spans an angular
sector of approximately 20.degree. about the longitudinal axis 107
and presents a clearance of approximately 0.0226 mm to the
frusto-conical face 67 of the outflow portion 63 of the impeller
drive shaft 55. The taper 74 spans an angular sector of
approximately 80.degree. about the longitudinal axis 107 and
creates, from groove 70 to land 75 a gradual 0.025 mm clearance
increase. That means that at the edge 700 separating the taper 74
from the groove 70, there is a clearance of approximately 0.0476 mm
between this edge 700 and the frusto-conical face 67 of the outflow
portion 63 of the impeller drive shaft 55. The groove 70 spans an
angular sector of approximately 20.degree. about the longitudinal
axis 107 computed from the edge 700 joining the taper 74 to the
groove 70 up to the edge 701 joining the same groove 70 to the land
73. This angular sector of 20.degree. includes the round of the
edge 700 blending the groove 70 with the taper 74 and the round of
the edge 701 blending the groove 70 with the land 73. The edges 702
that form the boundary between the groove 70 and the leading,
annular rounded edge surface 460 are polished to create a round
having a radius of at least 0.100 mm.
[0098] As a non-limitative example, from groove 71 to groove 69,
the inner face 68 defines a taper 76 and a land 77. Taper 76 has a
diameter that gradually decreases from groove 71 to land 77, and
land 77 has a constant diameter. The land 77 spans an angular
sector of approximately 20.degree. about the longitudinal axis 107
and presents a clearance of approximately 0.0226 mm to the
frusto-conical face 67 of the outflow portion 63 of the impeller
drive shaft 55. The taper 76 spans an angular sector of
approximately 80.degree. about the longitudinal axis 107 and
creates, from groove 71 to land 77 a gradual 0.025 mm clearance
increase. That means that at the edge 710 separating the taper 76
from the groove 71, there is a clearance of approximately 0.0476 mm
clearance between this edge 710 and the frusto-conical face 67 of
the outflow portion 63 of the impeller drive shaft 55. The groove
71 spans an angular sector of approximately 20.degree. about the
longitudinal axis 107 computed from the edge 710 joining the taper
76 to the groove 71 up to the edge 711 joining the same groove 71
to the land 75. This angular sector of 20.degree. includes the
round of the edge 710 blending the groove 71 with the taper 76 and
the round of the edge 711 blending the groove 71 with the land 75.
The edges 702 that form the boundary between the groove 71 and the
leading, annular rounded edge surface 460 is polished to create a
round having a radius of at least 0.100 mm.
[0099] In an example of construction, there are three blades such
as 78 and 79 (FIG. 3) aligned with the three grooves 69, 70 and 71,
respectively, to minimize blood flow perturbation. A number of
blades and grooves different from 3 could obviously be used. Also,
the blades such as 78 and 79 are connected to the grooves 69, 70
and 71, respectively, by diffusion bound.
[0100] In operation, the lands 73, 75 and 77 form a seat for the
outflow frusto-conical face 67 of the outflow portion 63 of the
impeller drive shaft 55. The grooves 69, 70 and 71 enable flow of
blood between the faces 67 and 68. The hydrodynamic forces produced
by rotation of the impeller drive shaft 55 will produce a thicker
film of blood flowing between the frusto-conical face 67 of the
outflow portion 63 of the impeller drive shaft 55 and the tapers
72, 74 and 76, and a thinner film of blood flowing between face 67
and the lands 73, 75 and 77 to thereby lubricate the resulting
bearing (frusto-conical face 67 and frusto-conical bushing 46). The
leading, annular rounded edge surface 460 at the edge of larger
diameter will produce smooth flow of blood. Since blood flows
through the gap between the frusto-conical faces 67 and 68, minimal
hemolysis, thrombus and clot formation will be produced.
[0101] As a non-limitative example, the resulting outflow bearing
(frusto-conical face 68 and frusto-conical bushing 46) present the
following approximate dimensions and characteristics:
[0102] Larger diameter: 6 mm
[0103] Smaller diameter: 3 mm
[0104] Cone angle: 19.111.degree.
[0105] Axial length: 4.1212 mm
[0106] Cone length: 4.3603 mm
[0107] Number of pads (taper and land): 3
[0108] Pad angle about the longitudinal axis: 100.degree.
[0109] Groove angle about the longitudinal axis: 20.degree.
[0110] Taper angle about the longitudinal axis: 80.degree.
[0111] Taper gradual clearance increase: 0.025 mm
[0112] Land angle about the longitudinal axis: 20.degree.
[0113] Land clearance: 0.0226 mm.
2 OUTFLOW 6807 RPM Radial Axial Axial Minimum Power Maximum
Clearance Gap Gap Load Film Loss Pressure (mm) (mm) (mm) (N) (mm)
(Watt) (Pa) 0.0226 0.0241 0.0661 -0.07 0.0226 0.0197 9643
[0114] with the above mentioned 0.8 N magnetic pull axially at 5
l/min against a pressure of 80 mmHg.
[0115] Referring to FIGS. 3 and 9, the impeller 56 comprises an
annular, impeller blade structure 80.
[0116] Impeller blade structure 80 comprises an annular member 800
with an inner cylindrical surface 801 mounted on the outer
cylindrical surface 630 of the outflow portion 63 of the impeller
drive shaft 55 between an annular shoulder 631 of the outflow
portion 63 and the frusto-conical face 67 of this outflow portion
63 of the impeller drive shaft 55. For example, the impeller blade
structure 80 can be laser welded to the outflow portion 63 of the
impeller drive shaft 55. As illustrated in FIG. 10, the annular
junction 670 between the annular member 800 and the frusto-conical
face 67 is positioned at the point of maximum slope.
[0117] The impeller blade structure 80 further comprises a set of
impeller blades such as 78 and 79 evenly distributed inside the
impeller housing 38 around a tapered frusto-conical face 802 of the
annular member 800 of the impeller blade structure 80. It should be
noted here that the annular radial-flow inlet 23 leads to the
proximal end of the impeller blades such as 78 and 79 through a
radial-flow inlet passage 231.
[0118] The shape (curvature and angulation) of the impeller blades
such as 78 and 79 should be optimally designed in relation to
pumping performance and other hydrodynamic considerations. In
particular, the influence of the blade angulation on the level of
shearing stresses, turbulence and cavitation responsible for red
blood cell damage and increase of hemolysis rate must be carefully
taken into consideration. To reduce the influence of blade
angulation, FIG. 11 illustrates that each blade 78,79 has a full
round radius at the top edge 780,790. Also, the radius that blends
the top edge 781,791 with the trailing edge 782,792 forms a smooth
surface to allow a better toolpath generation.
[0119] In the approach proposed by the illustrative
intra-ventricular embodiment of the present invention, the
mixed-flow blood pump 10 presents an enclosed-impeller mixed-flow
configuration. The frusto-conical face 802 of the annular member
800 of the impeller blade structure 80, bearing the impeller blades
such as 78 and 79 is tapered in the direction opposite to the
direction of blood flow. This contributes to create the mixed-flow
operation of the intra-ventricular mixed-flow blood pump 10. More
specifically, this taper imparts to the blood flow both axial and
radial components.
[0120] For housing the impeller blades, the impeller housing 38
comprises an inner frusto-conical surface 81 slightly less tapered
in the direction opposite to the direction of blood flow than the
face 802. To fit in the annular space defined between
frusto-conical face 802 and the tapered surface 81, the width of
the impeller blades such as 78 and 79 slightly and gradually
decreases in the direction of blood flow. This also contributes to
impart to the blood flow both axial and radial components.
[0121] The annular member 800 of the impeller blade structure also
comprises an outer frusto-conical face 803 that is tapered in the
direction of blood flow to fit within the outflow stator 45. The
inner surface 83 of the outflow cannula 42 surrounding the outflow
stator 45 is slightly less tapered in the direction of blood flow
than the outer frusto-conical face 803. To fit in the annular space
between the inner tapered surface 83 and the outer frusto-conical
face 803, the radial blades such as 45 has a height that slightly
and gradually increases in the direction of blood flow.
[0122] More specifically, the outer frusto-conical faces 802 and
803 and the inner surfaces 81 and 83 are shaped and dimensioned to
keep the area through which blood flows constant from the leading
end to the trailing end of the impeller blade structure 80.
[0123] The required electrical supply for the stator windings 26 is
made through electrical wires extending through a conduit 84 itself
extending from the cavity in which the stator windings 26 are
installed through the annular portion 31, the radial blade 33, the
hemispheric wall 32, and the hollow needle 20 to reach a controller
and an energy source (both to be described hereinafter). Of course,
this conduit 84 is sealed prior to implantation of the pump 10
within a human body.
[0124] Electric supply of the stator windings 26 will cause
rotation of the impeller drive shaft 55 and therefore rotation of
the set of impeller blades such as 78 and 79. More specifically, in
the illustrative embodiment of FIG. 3, the mixed-flow blood pump 10
is actuated by means of a brushless DC (direct current) motor
formed by the stator windings 26 housed in the cylindrical member
25 and the permanent magnet 64 embedded or housed in the impeller
drive shaft 55. This brushless configuration presents the advantage
of minimal wear. Two other interesting characteristics of brushless
DC motors are high rotational speed and high torque.
[0125] As discussed in the following description, the cylindrical
gap 86 between the outer surface of the impeller drive shaft 55 and
the inner surface of the cylindrical member 25 must be sufficiently
thick to produce sufficient blood flow in order to increase washout
and prevent clot formation. However, increasing the thickness of
the gap 86 decreases the efficiency of the magnetic coupling
between the permanent magnet 64 and the stator windings 26. This
requires an increase in current through the stator windings 26 to
compensate for the decreased efficiency and to maintain the same
characteristics in terms of impeller blade speed and blood volume
throughput. Of course, increase in current leads to an increase in
thermal loss from the stator windings 26; this thermal loss
increases as the square of the current through the stator windings
26. As the temperature of the surface of the stator windings must
remain at or below 40.degree. C., the gap 86 must be sufficiently
small to provide efficient magnetic coupling between the permanent
magnet 64 and the stator windings 26.
[0126] Thermal performance is also improved given the proximate
position of the stator windings 26 to the external surface 92 of
the cylindrical member 25. Blood flow over the external surface 92
efficiently cools the stator windings 26. The flow of blood within
the gap 86 also contributes in efficiently cooling the stator
windings 26.
[0127] Axial spacing between the impeller blades such as 78 and 79
and the permanent magnet 64 along the impeller shaft 55 enables
separate design of the motor and the impeller blades to obtain
simultaneously both efficient coupling between the permanent magnet
and the stator windings and sufficient pumping volume.
[0128] Rotation of the impeller blades such as 78 and 79 will
impart pumping energy to the blood within the annular space between
the outer frusto-conical face 802 and the inner frusto-conical
surface 81. This will cause sucking of blood both through the
annular radial-flow inlets 22 and 23. More specifically:
[0129] Blood flow enters the annular radial-flow inlet 22, is
straightened out by the radial blades such as 33 and 34, fills the
inflow chamber 85 with blood, is again straightened out by the
radial blades such as 36 and 37 and is finally conducted toward the
impeller blades such as 78 and 79 through the axial-flow inlet
passage formed by the cylindrical gap 86 between the impeller drive
shaft 55 and the inner surface of the cylindrical member 25;
and
[0130] Blood flow enters the annular radial-flow inlet 23 and is
conducted through the radial-flow inlet passage 231 where it is
straightened out by the radial blades such as 39 and 40 to finally
reach the impeller blades such as 78 and 79.
[0131] Blood flow then passes through the impeller blades such as
78 and 79, is straightened out by the outflow stator 45 and finally
exits through the outflow cannula 42. As indicated in the foregoing
description, the radial blades of the stationary outflow stator 45
are shaped and disposed to transform the rotational motion of the
blood flow about the longitudinal axis 41 into a translational
motion. Therefore, the stationary outflow stator 45 constitutes a
blood flow straightener.
[0132] Still referring to FIG. 3, the cylindrical gap 86 separating
the impeller shaft 55 and the inner surface of the cylindrical
member 25 should be sufficiently thick to produce sufficient blood
flow in order to increase washout and prevent clot formation. On
the other hand, too large a gap 86 may either reduce the pump
efficiency (by reducing the electromagnetic coupling) or result in
higher hemolysis.
[0133] In the illustrative intra-ventricular embodiment of FIGS. 1
and 3, the volume of blood pumped through the second annular inlet
23 and the annular radial-flow inlet passage 231 is typically 3
liters/minute. This is higher than the volume of blood pumped
through the first annular radial-flow inlet 22 and the axial-flow
inlet passage formed by the cylindrical gap 86 which is typically 1
liter/minute. A number of benefits are associated with the higher
volume of blood pumped through the second annular inlet 23. For
example, installation of the mixed-flow blood pump 2 in the left
ventricle 11 of a patient with the cannula 42 extending through the
aortic valve generally interferes with proper operation of the
aortic valve 15. Optimally, the aortic valve 15 should continue to
function normally; however, in some cases, it has been observed
that the aortic valve 15 ceases to function further until it
remains closed around the cannula 42. Typically, blood would have
the tendency to collect in the region close to the aortic valve and
the cannula 42 which might lead to thrombus formation and other
adverse effects. The increased volume of blood pumped through the
second inlet 23 has the effect of creating blood flow in the region
within the ventricle 11 delimited by the aortic valve 15 and the
cannula 42, thus providing improved washout of this region and
thereby reducing the negative effects of the malfunctioning aortic
valve 15.
[0134] On the one hand, the volume of blood pumped through the
second annular inlet 23 contributes to the radial-flow operation of
the mixed-flow blood pump 10. On the other hand, the volume of
blood pumped through the first annular inlet 22 and the cylindrical
gap 86 (axial-flow inlet passage) contributes to the axial-flow
operation of the mixed-flow blood pump 10.
[0135] The choice of materials for an implantable device such as
the mixed-flow blood pump 10 is crucial and several properties of
the available materials should be considered: strength, durability,
hardness, elasticity, wear resistance, surface finish and
biocompatibility. Biocompatibility is very important to minimise
irritation, rejection and thrombogenesis. The interaction between
the surface of the material and the biological tissues is very
complex. In several cases, treatment of the surface with human
proteins, certain drugs like heparin or other biocompatible
material may considerably increase the biocompatibility and
minimise thrombus formation (CBAS process, Carmeda AB).
[0136] FIG. 12 illustrates an alternative, illustrative
extra-ventricular embodiment 101 of mixed-flow blood pump according
to the present invention. This illustrative embodiment 101 is
adapted for use externally of the heart as a ventricle
bypass/assist. This extra-ventricular blood pump 101 would
typically be implanted above the diaphragm in the thorax and would
be connected to the circulation system using standard vascular
grafts, a first graft (not shown) being attached to the inflow end
102 of the pump and a second graft (not shown) being attached to
the outflow end 103 of the pump.
[0137] Similar to the intra-ventricular mixed-flow blood pump 10 of
FIG. 3, the extra-ventricular mixed-flow blood pump 101 as
illustrated in FIG. 12 comprises a stationary housing structure 105
provided with an impeller housing 104.
[0138] The stationary housing structure 105 further comprises an
outer cylindrical wall 106 centered on the longitudinal axis 107 of
the extra-ventricular blood pump 101. The outer cylindrical wall
106 has a proximal end with a reduction of diameter 108 to receive
the first graft and for connection to the patient's circulation
system. The outer cylindrical wall 106 further comprises a distal
end connected to the impeller housing 104. For example, the distal
end of the cylindrical wall 106 and the proximal end of the
impeller housing will have mutually mating cylindrical threaded
portions at 1060 by means of which their are screwed on each other.
A removable thread locking compound or a small radial hole drilled
in the male threaded surface to include a plastic insert will then
act as a thread locking device to prevent further rotation between
the cylindrical wall 106 and the impeller housing 104. Finally, an
annular groove 1041 will also be formed in the impeller housing 104
adjacent the threaded portions 1060 to receive and O-ring and form
a tight seal between the cylindrical wall 106 and the impeller
housing 104.
[0139] The stationary housing structure 105 of the
intra-ventricular mixed-flow blood pump 10 comprises a hollow
cylindrical member 109 containing the stator windings such as 110
and the associated magnetic cores such as 111. The hollow
cylindrical member 109 is made of two mutually mating annular
pieces 115 and 116 to enable insertion of the stator windings 110
and cores 111 within the hollow cylindrical member 109. For
example, both the annular pieces 115 and 116 will be threaded to
allow said annular pieces 115 and 116 to be screwed on each other.
Further rotation of the annular pieces 115 and 116 on each other
will then be prevented by any suitable means. Alternatively, the
annular pieces 115 and 116 can be laser welded to each other.
[0140] As illustrated in FIG. 12, the hollow cylindrical member 109
is mounted within the outer cylindrical wall 106 coaxially
therewith to form an annular axial-flow inlet passage 112 between
the inner surface 113 of the outer cylindrical wall 106 and the
outer surface 114 of the hollow cylindrical member 109.
[0141] The stationary housing structure 105 further comprises an
inflow bushing mount 117 mounted on a proximal end of the
cylindrical member 109. More specifically, the inflow bushing mount
117 comprises an annular portion 118 profiled to fit on the
proximal end of the hollow cylindrical member 109 while defining
with this cylindrical member 109 a smooth surface of the annular
axial-flow inlet passage 112.
[0142] An inflow bushing 119 having the general configuration of a
frustum of cone is mounted inside the annular portion 118 of the
bushing mount 117 and is centered on the longitudinal axis 107 of
the extra-ventricular blood pump 101. More specifically, the
frusto-conical bushing 119 is mounted to the annular portion 118
through a series of radial blades such as 120 and 121 spread out
evenly around the axis 107 of the extra-ventricular blood pump 101,
more specifically around the frusto-conical bushing 119. As
illustrated in FIG. 12, the frusto-conical bushing 119 has an end
of larger diameter facing toward the inflow end of the
extra-ventricular blood pump 101. Another function of the blades
such as 120 and 121 is to straighten out the flow of blood passing
between the frusto-conical bushing 119 and the annular portion 118
of the inflow bushing mount 117.
[0143] The distal end of the cylindrical member 109 is connected to
the proximal end of the impeller housing 104 through a series of
radial blades such as 122 and 123 spread out evenly around the
longitudinal axis 107 to define an annular radial-flow inlet
passage 1120 between the distal end of the annular flow passage 112
and the proximal end of the impeller blades such as 124 and 125.
Another function of the radial blades such as 122 and 123 is to
straighten out the flow of blood from the annular axial-flow inlet
passage 112 and the annular radial-flow inlet passage 1220.
[0144] An outflow stator 126 comprises an annular member 127
mounted on the distal end of the impeller housing 104 through a
flange 1270 and screws (not shown). An annular groove 1271 is
provided on the outer cylindrical face 1272 of the annular member
127 to receive and O-ring (not shown) to ensure a tight seal
between the outer cylindrical face 1272 of the annular member 127
and an inner cylindrical face 1040 of the impeller housing 104.
[0145] The outflow stator 126 also comprises a series of inner
radial blades such as 128 and 129 spread out evenly around the
longitudinal axis 107 and secured to the outflow stator 126 by
diffusion bound. The radial blades such as 128 and 129 are
configured to straighten out the flow of blood exiting the outflow
stator 126. FIG. 4 illustrates an example of configuration of the
radial blades such as 128 and 129 of the outflow stator 126. More
specifically, the radial blades 128 and 129 are configured to
straighten out the flow of blood exiting the outflow stator 126 and
are fixedly secured to the inner face of the annular member
127.
[0146] An outflow bushing 130 having the general configuration of a
frustum of cone is mounted inside the outflow stator 126 and is
centered on the longitudinal axis 107. More specifically, the
frusto-conical bushing 130 is mounted to the radial blades such as
128 and 129 of the outflow stator 126. Finally, the end of larger
diameter of the frusto-conical bushing 130 is facing toward the
inflow end of the extra-ventricular blood pump 101.
[0147] A blood diffuser 131 is mounted to the distal end of the
outflow stator 126 through a flange 1310 through the same screws
(not shown) that secure the flange 1270 of the outflow stator 126
to the impeller housing 104. The diameter of the flange 1310 may be
extended (see for example 1311) to provide sewing holes therein for
sewing the extra-ventricular mixed-flow blood pump 101 to the
heart. nother annular groove 1273 of the annular member 127
receives an O-ring (not shown) to form a tight seal between the
blood diffuser 131 and the outflow stator 126.
[0148] The function of the blood diffuser 131 is to increase the
cross-sectional area of the pump outlet to the diameter of the
second graft while minimising the shear stress on the blood cells.
Since the velocity of the blood is inversely proportional to the
cross-sectional area, the diffuser 131 will also reduce the
velocity of the blood ejected from the extra-ventricular blood pump
101 to a velocity close to that of blood in the patient's
circulation system. Of course, parameters such as the angle of
opening and the length of the blood diffuser 131 could be adjusted
at will to fit the mechanical characteristics of the
extra-ventricular blood pump 101 in view of minimising the shear
stress on the blood cells.
[0149] The extra-ventricular blood pump 101 also comprises a
rotative impeller 133 provided with an impeller drive shaft 132
centered on the longitudinal axis 107. The impeller drive shaft 132
comprises an inflow end portion 134 formed with a frusto-conical
face 135 structured to snugly fit into the frusto-conical bushing
119.
[0150] The inner face of the frusto-conical bushing 119 has the
same structure and operation as the inner face 47 of the inflow
bushing 35 of FIG. 3 described in detail with reference to FIG.
5.
[0151] Still referring to FIG. 12, the impeller drive shaft 132
also comprises an outflow portion 136 which, when assembled to the
inflow end portion 134 defines a cavity in which a cylindrical
permanent magnet 137 is inserted. The windings 110 and the
permanent magnet 137 form an electric motor structure operative to
set the impeller 133 into rotation; the magnetic field produced by
the windings 110 is applied to the magnetic field produced by the
permanent magnet 137 to produce a reaction that will set the
impeller 133 into rotation.
[0152] An axial magnetic pull is produced by slightly, axially
offsetting the permanent magnet 137 toward the inflow bushing 119
with respect of the magnetic windings 110. This will produce an
axial magnetic pull of the order of, for example, 0.8 N toward the
outflow bushing 130, i.e. in a direction opposite to an axial force
produced on the impeller drive shaft 132 upon pumping blood.
[0153] An axial screw 138 passes through the magnet 137 and screws
into both the inflow 134 and outflow 136 portions of the impeller
drive shaft 132 to firmly secure these two drive shaft portions
together.
[0154] The outflow portion 136 of the impeller drive shaft 132
comprises an outflow end 139 formed with a frusto-conical face 140
structured to snugly fit into the frusto-conical bushing 130.
[0155] Again, the inner face of the frusto-conical bushing 130 has
the same structure and operation as the inner face 68 of the
outflow bushing 46 of FIG. 3 described in detail with reference to
FIG. 8.
[0156] Referring to FIG. 12, the impeller 133 comprises an annular,
impeller blade structure 1330.
[0157] Impeller blade structure 1330 comprises an annular member
1331 with an inner cylindrical surface 1332 mounted on the outer
cylindrical surface 1360 of the outflow portion 136 of the impeller
drive shaft 132 between an annular shoulder 1361 of the outflow
portion 136 and the frusto-conical face 140 of this outflow portion
136 of the impeller drive shaft 132. For example, the impeller
blade structure 1330 can be laser welded to the outflow portion 136
of the impeller drive shaft 132.
[0158] Referring to FIG. 10, the annular junction 1400 between the
annular member 1331 and the frusto-conical face 140 is positioned
at the point of maximum slope.
[0159] The impeller blade structure 1330 further comprises a set of
impeller blades such as 124 and 125 evenly distributed inside the
impeller housing 104 around a tapered frusto-conical face 141 of
the annular member 1331 of the impeller blade structure 1330. It
should be noted here that the annular axial-flow 112 and
radial-flow 1220 inlet passages lead to the proximal end of the
impeller blades such as 124 and 125.
[0160] The shape (curvature and angulation) of the impeller blades
such as 124 and 125 should be optimally designed in relation to
pumping performance and other hydrodynamic considerations. In
particular, the influence of the blade angulation on the level of
shearing stresses, turbulence and cavitation responsible for red
blood cell damage and increase of hemolysis rate must be carefully
taken into consideration. To reduce the influence of blade
angulation, FIG. 11 illustrates that each blade 124,125 has a full
round radius at the top edge 1240,1250. Also, the radius that
blends the top edge 1241,1251 with the trailing edge 1242,1252
forms a smooth surface to allow a better toolpath generation.
[0161] In the approach proposed by the illustrative
extra-ventricular embodiment of the present invention, the
mixed-flow blood pump 101 presents an enclosed-impeller mixed-flow
configuration. The frusto-conical face 141 of the annular member
1331 of the impeller blade structure 1330, bearing the impeller
blades such as 124 and 125 is tapered in the direction opposite to
the direction of blood flow. This contributes to create the
mixed-flow operation of the intra-ventricular mixed-flow blood pump
101. More specifically, this taper imparts to the blood flow both
axial and radial components.
[0162] For housing the impeller blades, the impeller housing 104
comprises an inner frusto-conical surface 142 slightly less tapered
in the direction opposite to the direction of blood flow than the
face 141. To fit in the annular space defined between
frusto-conical face 141 and the tapered surface 142, the width of
the impeller blades such as 124 and 125 slightly and gradually
decreases in the direction of blood flow. This also contributes to
impart to the blood flow both axial and radial components.
[0163] The annular member 1331 of the impeller blade structure 1330
also comprises an outer frusto-conical face 143 that is tapered in
the direction of blood flow to fit within the outflow stator 126.
The inner surface 144 of the annular member 127 is slightly less
tapered in the direction of blood flow than the frusto-conical face
143. To fit in the annular space between the inner tapered surface
144 and the frusto-conical face 143, the radial blades such as 128
and 129 have a height that slightly and gradually increases in the
direction of blood flow.
[0164] The required electrical supply for the stator windings 110
is made through electrical wires extending through a conduit 145
itself extending from the cavity in which the stator windings 110
are installed through the annular piece 116, the radial blade 122,
and the impeller housing 104 to reach a controller and an energy
source (both to be described hereinafter). Of course, this conduit
145 is sealed prior to implantation of the pump 101 within a human
body.
[0165] Electric supply of the stator windings 110 will cause
rotation of the impeller drive shaft 132 and therefore rotation of
the set of impeller blades such as 124 and 125. More specifically,
in the illustrative embodiment of FIG. 12, the mixed-flow blood
pump 101 is actuated by means of a brushless DC (direct current)
motor formed by the stator windings 110 housed in the cylindrical
member 109 and the cylindrical permanent magnet 137 embedded or
housed in the impeller drive shaft 55. This brushless configuration
presents the advantage of minimal wear. Two other interesting
characteristics of brushless DC motors are high rotational speed
and high torque.
[0166] Both the annular axial-flow passage 112 and the cylindrical
gap 146 between the outer surface of the impeller drive shaft 132
and the inner surface of the cylindrical member 109 must be both
sufficiently thick to produce sufficient blood flow in order to
increase washout and prevent clot formation. However, increasing
the thickness of the gap 146 decreases the efficiency of the
magnetic coupling between the permanent magnet 137 and the stator
windings 110. This requires an increase in current through the
stator windings 110 to compensate for the decreased efficiency and
to maintain the same characteristics in terms of impeller blade
speed and blood volume throughput. Of course, increase in current
leads to an increase in thermal loss from the stator windings 110;
this thermal loss increases as the square of the current through
the stator windings 110. As the temperature of the surface of the
stator windings must remain at or below 40.degree. C., the gap 146
must be sufficiently small to provide efficient magnetic coupling
between the permanent magnet 137 and the stator windings 110.
[0167] Thermal performance is also improved given the proximate
position of the stator windings 110 to the annular axial-flow
passage 112 and the annular axial-flow inlet passage formed by the
cylindrical gap 146. Blood flow through these annular axial-flow
passages efficiently cools the stator windings 110.
[0168] Axial spacing between the impeller blades such as 124 and
125 and the permanent magnet 137 along the impeller shaft enables
separate design of the motor and the impeller blades to obtain
simultaneously both efficient coupling between the permanent magnet
and the stator windings and sufficient pumping volume.
[0169] Rotation of the impeller blades such as 124 and 125 will
impart pumping energy to the blood within the annular space between
the frusto-conical annular face 141 and the tapered inner surface
142 of the impeller housing 104. This will cause sucking of blood
both through the annular axial-flow inlet passage 112 and the
annular axial-flow inlet passage formed by the cylindrical gap 146.
More specifically:
[0170] Blood flows through the annular axial-flow inlet passage 112
and the radial-flow inlet passage 1220 and is straightened out by
the radial blades such as 122 and 123 to reach the impeller blades
such as 124 and 125; and
[0171] Blood flow is straightened out by the radial blades such as
120 and 121 to reach the impeller blades such as 124 and 125
through the axial-flow inlet passage formed by the cylindrical gap
146.
[0172] Blood flow then passes through the impeller blades such as
124 and 125, is straightened out by the outflow stator blades such
as 128 and 129 and finally exits through the blood diffuser 131.
The radial blades such as 128 and 129 of the stationary outflow
stator 126 are shaped and disposed to transform the rotational
motion of the blood flow about the longitudinal axis 107 into a
translational motion. Therefore, the stationary outflow stator 126
constitutes a blood flow straightener.
[0173] Again, the choice of materials for an implantable device
such as the mixed-flow blood pump 101 is crucial and several
properties of the available materials should be considered:
strength, durability, hardness, elasticity, wear resistance,
surface finish and biocompatibility. Biocompatibility is very
important to minimise irritation, rejection and thrombogenesis. The
interaction between the surface of the material and the biological
tissues is very complex. In several cases, treatment of the surface
with human proteins, certain drugs like heparin or other
biocompatible material may considerably increase the
biocompatibility and minimise thrombus formation.
[0174] The following features of the mixed-flow blood pump 10, 101
have been designed to (a) minimize flow energy losses and (b)
eliminate stagnation zones leading to thrombus formation:
[0175] The outer 231,112 and inner 86,146 inlet passages of the
mixed-flow blood pump 10,101 divide the blood flow according to a
ratio of 3:1, respectively, which corresponds to the same ratio
between the areas between these two inlet passages. This presents
the advantage of reducing by 75% the shear stress induced by the
rotation of the impeller shaft 56,136 to the blood flowing through
the inner inlet passage 86,146, while also reducing the
corresponding energy losses (see above point (a)). Contrary to a
pump having only one inner inlet passage, the mixed-flow blood pump
is therefore less traumatic.
[0176] The outer inlet passage 112 meets with the radial inlet
passage 231,1220 to supply the impeller with blood. The cross
section of the radial-flow inlet passage 231,1220 is designed equal
to the cross section of the outer inlet passage to prevent an
acceleration of the flow (see above point (a)) or a sudden
reduction of the speed of blood flow to provoke pump's free
wheeling (see above point (b)).
[0177] The inner axial-flow inlet passage 86,146 is the confluent
of the radial-flow inlet passage 231,1220 and completes the supply
of blood to the impeller. Since the ratio of these two flows are
equal to the ratio of the areas of the axial-flow and radial-flow
inlet passages at the junction of these inlet passages, the blood
flows join each other at equal speeds to avoid production of blood
shearing zones (see above point (a)) or interruption of the blood
flow (see above point (b)). Indeed, when two jets join each other
at unequal speeds, a whirling motion flow results at the junction
to increase the losses (see above point (a)).
[0178] The cross sectional area of the annular passage at the inlet
of the set of impeller blades (78,79),(124,125) is made by purpose
slightly smaller than the cross sectional area of the annular
passage at the junction between the outer radial-flow 231,1020 and
inner axial-flow 86,146 inlet passages. In this small zone, a
slight acceleration of the blood flow is produced which slightly
reduces the pressure to improve sucking of blood within the
impeller blades (78,79),(124,125) over the entire operating range
of the pump. However, too high an acceleration would provoke an
interruption in the blood flow and therefore would be detrimental
to the performance of the mixed-flow blood pump 10,101. In the
present illustrative design, a beneficial trade-off was reached to
improve the performance of the mixed-flow blood pump.
[0179] The cross sectional area of the annular passage from the
inlet to the outlet of the set of impeller blades (78,79),(124,125)
is constant. In this manner, the flow is not axially accelerated
within this passage whereby the energy losses are minimized.
Geometrically, the design of radial blades is allowed while the
transfer of pressure energy and whirling kinetic energy is
favoured.
[0180] The cross sectional area of the annular passage from the
inlet to the outlet of the set of outflow stator blades 450, 128,
129 is constant and equal to the cross sectional area of the
annular passage from the inlet to the outlet of the set of impeller
blades (78,79),(124,125). This will prevent pump free-wheeling (see
above point (b)) and minimize losses (see above point (a)) for a
better efficiency in this important portion of the mixed-flow blood
pump 10, 101 whose role is to convert the whirling kinetic energy
into pressure energy. However, it should be noted that the cross
sectional area of the annular passage at the outlet of the set of
outflow stator blades is slightly smaller than the cross sectional
area of this annular passage at the inlet of the set of outflow
stator blades due to the presence of the outflow bearing 46, 130.
By provoking in this manner a slight acceleration of the blood
flow, we obtain good cleaning of the inner space of the outflow
bearing. This approach is used for the inflow bearing 35,119 as
well.
[0181] Examples of radial dimensions and cross sectional areas for
the extra-ventricular mixed-flow blood pump 101 of FIG. 12 are
given in the following Table. Intra-ventricular mixed-flow blood
pump 10 of FIG. 3 has similar dimensions.
3 Location (see FIG. 12) Radius (mm) Cross-sectional area
(mm.sup.2) A (50 microns) 0 64 B 8 C 1.31 19.27 D approximately
4.581 54.74 E approximately 8.702 a 2.048 10.96 b 3.893 c 3.850
7.30 d 4.703 F 4 9.00 G 5 H 10.5 27.81 I 11.75 J 5 27.81 K 7.267 L
4 30.24 M 6.8 N 6.62 30.24 O 8.606 P 1.475 28.83 Q 5.5685 R (50
microns) 0 64 S 8
[0182] FIG. 13 schematically illustrates an embodiment of
implantable VAD system including an axial-flow blood pump 10. The
VAD system is composed of four main parts:
[0183] the axial-flow blood pump 10 implanted in the left ventricle
11 of the patient 87;
[0184] an internal controller 88;
[0185] two energy sources, namely an internal rechargeable battery
89 and an external rechargeable battery 90; and
[0186] a Transcutaneous Energy and Information Transmission (TEIT)
system 91.
[0187] VAD and TEIT Systems are well known in the art and will not
be further discussed in the present specification.
[0188] To conclude, ventricular assist devices (VADs) are now being
used worldwide and their utilisation is becoming more and more
accepted as a solution to treat end stage heart failure. It is
generally accepted that VADs extend life of patients while
improving quality of life of these patients. A poll, made with
patients who received VADs, concerning their quality of life
revealed that these patients would have preferred a heart
transplant but prefer their situation than having to be on
dialyses.
[0189] It is also now being accepted that VAD is becoming a cost
effective solution considering the fact that patients are
discharged from the hospitals more rapidly and may return to normal
life occupations. In the United States, several insurance companies
are now reimbursing the implantation of VADs.
[0190] Finally, the mixed-flow blood pump 10 according to the
illustrative embodiments of the present invention provides an
excellent bridge to heart transplant and aims at long term implant.
The new proposed mixed-flow blood pump 10 should answer most of the
remaining problems and limitations of the prior axial-flow blood
pumps, especially those related to hemolysis. Hemolysis is the
tearing of red blood cells, which empties the content of the cells
in the blood stream resulting in free haemoglobin; the normal level
of plasma free haemoglobin is around 10 mg/dl. A blood pump with a
normalised index of hemolysis (NIH) of 0.005 g/100 litres and lower
is considered to be almost athromatic for red blood cells. A NIH of
about 0.05 g/100 litres could be tolerated. A NIH situated between
0.005 g/100 litres to 0.05 g/100 litres can therefore be envisaged
for a VAD. Of course, a NIH as close to 0.005 g/100 litres as
possible is desirable.
[0191] Although the present invention has been described
hereinabove by way of illustrative embodiments thereof, these
embodiments can be modified at will, within the scope of the
appended claims, without departing from the spirit and nature of
the present invention.
* * * * *