Note: Descriptions are shown in the official language in which they were submitted.
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WO 97126032 PCTJLTS97/00480
MESIi SPACER FOR HEAT EXCHANGER
BACKGROUND OF THE INVENTION
' Field of the Invention
The present invention generally relates to heat exchangers for use in
r
regulating the temperature of a patient's blood during surgery, and more
particularly
to a micro-conduit heat exchanger with enhanced flow of the heat transfer
fluid
around the micro-conduits.
2. Description of the Related Art
"Heart-lung" machines are known in the medical field. One component of
1 o these machines is a blood oxygenator. Blood oxygenators are typically
disposable
and serve to infuse oxygen into a patient's blood during medical procedures
such as
heart surgery. Most commercially available blood oxygenators employ a
membrane-type oxygenator, which comprises thousands of tiny hollow fibers
having
microscopic pores. Inside the membrane oxygenator blood flows around the
outside
surfaces of these fibers while a controlled oxygen rich gas mixture is pumped
through the fibers. Due to the relatively high concentration of carbon dioxide
in
the blood arriving from the patient, carbon dioxide from the blood diffuses
through
the fibers' microscopic pores and into the gas mixture. Due to the relatively
low
concentration of oxygen in the blood arriving from the patient, oxygen from
the gas
2o mixture diffuses into the blood through the fibers' microscopic pores.
Most blood oxygenators also employ a heat exchanger to precisely regulate
the temperature of a patient's blood. The heat exchanger usually includes one
or
more relatively large conduits housed in a vessel. The patient's blood is
continuously pumped through the conduits while a heat transfer fluid such as
water
flows through the vessel around the conduits, or vice versa. The neat exchange
medium is either heated or cooled to maintain the patient's blood at a desired
temperature.
One example of a commercially successful blood oxygenator is sold under
the designation MAXIMAL by Medtronic, Inc. In the MAXIMAL blood
oxygenator, the heat transfer fluid (water) flows inside relatively large
diameter
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metal tubes while blood flows on the outside of the tubes
within the vessel. The TERUMO* brand oxygenator uses a
different configuration, where blood flows inside relatively
large diameter metal tubes. In the BARD WILLIAM HARVEY HF-5700
blood oxygenator, the blood flows outside plastic tubes that
contain a flow of temperature-regulated water.
Heat exchangers in blood oxygenators are subject to a
number of design constraints. The heat exchangers should be
compact due to physical space limitations in the operating room
environment. Also, small size is important in minimizing the
internal priming volume of the blood oxygenator due to the high
cost and limited supply of blood. However, the heat exchanger
must be large enough to provide an adequate volumetric flow
rate of blood to allow proper temperature control and
oxygenation. On the other hand, blood flow rate or flow
resistance inside the blood oxygenator and heat exchanger must
not be excessive since the cells and platelets in the human
blood are delicate and can be traumatized if subjected to
excessive shear forces resulting from turbulent flow.
One way to meet the above requirements is to provide
a heat exchanger with improved heat exchange efficiency. A
more efficient heat exchanger can provide adequate temperature
control in a compact space with minimal priming volume.
SUMMARY OF THE INVENTION
A blood heat exchange system is disclosed generally
comprising a plurality of small polymeric hollow conduits for
conveying blood. The hollow conduits are formed in a flat or
mat shape that are wrapped in layers around a spindle. Layers
*Trademark
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2a
of the hollow conduits are spaced from each other by a mesh
spacer. The mesh spacer has holes through it so that a heat
transfer fluid such as water may more efficiently flow over and
around the outside of the hollow conduits. The heat transfer
fluid conveys heat from or to the outside surfaces of the
hollow conduits which in turn transfer heat from or to the
blood passing through the hollow conduits.
The invention provides a heat exchanger that is more
efficient than previously known heat exchangers because the
flow of the heat transfer fluid over and around the outer
surface of the hollow conduits is more efficient. More
66742-667 CA 02241844 2000-09-25
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efficient flow of the heat transfer fluid allows for more
efficient heat transfer from or to the walls of the hollow
conduits. More efficient heat transfer from the walls of the
hollow conduits to the heat transfer fluid in turn produces a
more efficient heat transfer from and to the blood passing
through the hollow conduits. Thus, the invention
advantageously provides a blood heat exchanger with markedly
improved heat exchange characteristics.
Another advantage of the invention includes its low
cost, since the mesh can be made from inexpensive materials.
The mesh is also easy to make and assemble with the hollow
conduits.
It is therefore an object of the present invention to
provide an improved blood heat exchanger.
It is another object of the present invention to
provide a blood heat exchanger which has improved heat transfer
characteristics.
It is another object of the present invention to
provide a blood heat exchanger which utilizes small size
polymeric conduits.
These and other objects of the invention will be
clear from the description contained herewith and more
particularly with reference to the attached drawings and
detailed description of the invention. Throughout this
disclosure, like elements wherever discussed, are referred to
with like reference numbers.
In accordance with a broad aspect, the invention
provides a blood heat exchange system comprising: a mesh; a
spindle having a center axis; a plurality of hollow conduits
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3a
for conveying blood therethrough, the plurality of hollow
conduits arranged in a flat configuration, the mesh and the
plurality of hollow conduits wrapped around the spindle so that
alternating layers of hollow conduits and mesh are formed
radially outward from the center axis of the spindle; heat
transfer fluid flow path for conveying a heat transfer fluid
around the outside surfaces of the hollow conduits; an inlet
chamber for directing blood into the hollow conduits and outlet
chamber for receiving blood leaving the hollow conduits, the
hollow conduits being arranged in a bundle, wherein each hollow
conduit has a first end terminating in the inlet chamber and a
second end terminating in the outlet chamber; each end of the
conduit bundle being embedded in one of two sealing members
which seals the inlet and output chambers respectively from the
heat transfer fluid flow path disposed therebetween.
BRIEF DESCRIPTION OF THE DRAWINGS
The nature, objects and advantages of the invention
will become more apparent to those skilled in the art after
considering the following detailed description in connection
with the accompanying drawings, in which like reference
numerals designate like parts throughout, wherein
Figure 1 is a vertical sectional view of a blood heat
exchanger apparatus in accordance with the invention;
Figure lA is an enlarged vertical sectional view of a
portion of the blood heat exchanger apparatus shown in Figure
1;
Figure 2 is a greatly enlarged plan view of a micro-
conduit wrapping material in accordance with the invention.
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WO 97/26032 PCT/US97/00480
' 4
Figure 3 is further enlarged perspective view of a section of micro-conduit
wrapping material of Figure 2;
Figure 4 is a plan view of the mesh of the present invention. '
Figure 5 is a flow chart of a sequence of steps used in fabricating a heat
v
exchanger apparatus in accordance with the invention.
DETAILED DESCRIPTION OF TIIE PREFERRED EMBODIMENTS
The present invention concerns a blood heat exchanger which employs a
polymeric micro-conduit to carry blood while a heat transfer fluid passes
around the
micro-conduits to permit temperature control of the blood. The invention
provides a
1o mesh like spacer that provides for more efficient flow of the heat transfer
fluid
around the micro-conduits.
STRUCTURE
Figure 1 depicts an example of a heat exchanger of the present invention.
The heat exchanger 10 includes a generally cylindrical heat exchange core 12
which
is made from a mat of micro-conduit wrapping material 14 wound about a central
spindle 16. The spindle has first and second ends 18, 20. The individual
fibers of
the wrapping material 14 (shown in more detail in Figures 2 and 3) are cut to
provide substantially flat end surfaces proximate the first and second ends
18, 20 of
the spindle 16. The core 12 may include, for example, about five thousand four-
hundred individual fibers.
Figure 2 depicts the micro-conduit wrapping material 14 prior to wrapping
into the core 12 shown in Figure 1. The micro conduit wrapping material 14
comprises a plurality of small fibers 32. Each fiber 32 is hollow, with a
cross-
sectional shape preferably being rounded, or alternatively triangular,
rectangular or
other appropriate shape. As shown in Figure 3, since the fibers 32 are hollow,
each
fiber 32 has defined therein an inner channel 34 having an inner surface 36.
In a
preferred embodiment, the fibers outer diameter is about five hundred and
seventy- '
five microns, while the inner channel 34 has a diameter of about four-hundred
and
twenty-eight microns. As an example, the fibers 32 may be about ten
centimeters
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Long. However, a wide range of fiber lengths may be used, depending upon the
requirements of a particular application for the blood heat exchanger.
~ The fibers 32 are preferably made from a plastic material such as
polypropylene, polyethylene, a different polymeric substance, or other
material that
is inexpensive, pharmacologically safe, lightweight, easily cut and flexible.
The
material of the fibers 32 should also be easily formed into fibers with
sufficiently
small inner and outer dimensions.
The mat of micro-conduit wrapping material 14 includes a thin flexible
interconnect 38 that maintains the fibers 32 at predetermined spacing in
substantially
1 o parallel alignment with each other. In the illustrated embodiment, the
interconnect
38 comprises substantially parallel, flexible, non-active, multifilament
threads that
are woven or knotted to hold the fibers about 0.5 millimeters apart generally
parallel
to each other to form a flat or mat shape. The wrapping material 14 aides in
positioning the fibers 32 during surface treatment and construction of the
blood heat
,c 1,.,.~.~o,. c ien»ec rl i r d tail be v~:~
1J V.xV.uayv.a ao daov.uo~eu an mo a a 1 . -
The wrapping material 14 is preferably made of a commercially available
product from Mitsubishi Rayon Company Limited sold under the designation
HFE430-1 Hollow Fiber. The fibers of this product are made of polyethylene.
Similar wrapping material is also commercially available from Hoechst Celanese
2o Corp. under the designation Heat Exchanger Fiber Mat, which uses
polypropylene
fibers.
Figure 4 shows a mesh 40 that overlays the wrapping material 14 as
wrapping material 14 is wrapped around spindle 16. Mesh 40 comprises. an open
matrix 42 that produces a variety of holes 44. As wrapping material 14 and
mesh
25 40 are wound around spindle 16, mesh 40 spaces layers of wrapping material
14
from each other.
Mesh 40 preferably has a width W approximately the same as the width W'
of wrapping material 14. In addition, mesh 40 preferably has a thickness of
about
.030 " but could have a thickness considerably larger or smaller; the key
being that
3o opposed layers of wrapping material 14 are spaced apart from each other and
that
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heat exchange fluid may pass through the holes 44 as will be
described hereafter. This spacing combined with the holes 44
provides passages through holes 44 for the heat exchange fluid
to more freely travel around the fibers 32 of wrapping material
14.
Mesh 40 is preferably made of a polyolefin material
such as polyethylene or polypropylene but could be made of
other materials. Whatever the material used for mesh 40, the
material should be flexible enough to allow mesh 40 to be
wrapped around spindle 16 with wrapping material 14 and should
be rigid enough to prevent mesh 40 and holes 44 from collapsing
so that heat exchange fluid can pass through and around mesh
40.
A rigid cylindrical shell 22 encloses the core 12 and
spindle 16. The shell 22 includes an inlet 24 and an outlet
(not shown) to facilitate the flow of a heat transfer fluid
through the shell 22 and around the micro-conduit wrapping
material 14 inside the shell 22. In a preferred embodiment the
heat exchange medium is water which has adequate heat exchange
properties while also being relatively biocompatible as
compared to other commonly used heat exchange mediums.
The core 12 includes an upper seal 26 and a lower
seal 28, the upper and lower seals 26,28 comprise a layer of
potting compound sealingly applied between the individual
fibers of the micro-conduit wrapping material 14 approximate
the spindles first and second ends 18,20. In the preferred
embodiment, the potting compound comprises a urethane material.
However other materials of suitable utility and
biocompatability may be utilized. The upper and lower seals
26,28 are applied in a manner described in more detail below.
Importantly, the seals 26,28 provide a tight and reliable
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isolation between the heat exchange medium entering inlet 24
and the blood passing through the individual fibers of the
micro-conduit wrapping material 14.
Referring now to both Figures 1 and lA, the core 12
is enclosed within the shell 22 by an upper blood transition
manifold 29, forming outlet chamber 33 and a lower blood inlet
manifold 30 forming inlet chamber 31. Further details of the
heat exchanger 10 are described in U.S. Patent No. 5,922,281
which issued on July 13, 1999 entitled Surface Treatment for
Micro-conduits Employed in Blood Heat Exchange System, U.S.
Patent 5,823,987 which issued on October 20, 1998 entitled
Compact Membrane-Type Blood Oxygenator With Concentric Heat
Exchanger and U.S. Patent 5,876,667 which issued on March 2,
1999 entitled Blood Heat Exchange System Employing Micro-
Conduit.
FABRICATION
Referring now to Figure 5, a sequence 78 for
manufacturing a blood heat exchanger in accordance with one
example of the invention is illustrated. First, in task 80,
the surface of the fibers 32 are treated in accordance with one
of the surface treatment techniques such as is described in the
above referenced U.S. Patent 5,922,281.
Next, in task 82, the micro-conduit wrapping material
14 and mesh 40 are simultaneously wrapped around the spindle
16, preferably without any substantial tension on the wrapping
material. After task 82 the shell 22 is installed over the
core 12 in task 84.
In task 86, the upper and lower seals 26, 28 are
formed. In the preferred embodiment, urethane potting compound
is injected between the fibers 32 to substantially seal the
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spaces between the fibers. This is done by putting the ends of
the fibers in potting cups and inserting the core 18 into a
centrifuge and spinning it while urethane from a reservoir
fills the cups. The high G forces of the spinning process
forces the urethane around the exterior of the fibers. The
thickness of the upper and lower seals 26,28 is determined by
the amount of urethane which is used during the potting
process. Before task 80, the ends of the fibers may be sealed
to prevent the potting compound from entering therein.
In the preferred embodiment the potting material is a
bio-compatible urethane commercially available under the name
BIOTHANE from CasChem Corporation of Bayonne, New Jersey,
U.S.A. This is a particular formulation of urethane which has
as its primary components Polycin and Vorite. Other kinds of
urethane may also be suitable in some applications, as well as
non-urethane potting materials such as epoxy and silicone.
Next, the fibers 32 are trimmed proximate the first
and second ends 18,20 of the spindle 16 as shown in task 88.
Preferably, the trimmed fibers 32 form uniform flat upper and
lower surfaces of the core 12. This trimming is preferably a
two-stage process in which a rough cut is initially made with a
rotary blade and then the ends are trimmed with a microtome.
Finally, in task 90, the manifolds such as manifolds 29,30 are
attached to the shell 22. Also, in task 90 hoses and other
plumbing lines are attached to the heat exchanger 10 as needed
for transportation of heat exchange fluid, blood, priming
solution, and other media as appropriate.
!'1DL~DTTTIITT
Generally, the heat exchanger 10 serves to regulate
the temperature during a medical procedure such as open-heart
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8a
surgery. Heat exchanger l0 also may be advantageously
incorporated into a blood oxygenator such as disclosed in the
aforementioned U.S. Patent No. 5,823,987 entitled "Compact
Membrane-Type Blood Oxygenator With Concentric Heat Exchanger"
Referring to Figures 1-2, a heat transfer fluid such as water
flows into the shell 22 through the inlet 24 during the medical
procedure. While in the shell 22, the heat transfer fluid
passes between and around the exterior of the fibers 32 in the
core 12, preferably flowing in a direction opposite to that of
blood. This flow is improved considerably by the presence of
mesh 40 which provides holes 44 through which the heat transfer
fluid more easily flows. This counter-flow is achieved using a
flow channel, (not illustrated) for the water which flows from
inlet 24 to the top of the shell 22 where the water exits and
flows downwardly.
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_ 9
Due to the large number of fibers 32 and their small size and thin walls,
there is substantial area of surface contact and heat exchange between the
heat
exchange fluid and the blood inside the fibers 32. During ongoing operation of
the
heat exchanger 10, a patient's blood which flows into inlet manifold 30 and
chamber
31 through the fibers 32 of the core 12 and exits through the upper end of the
fibers
past seal 26 through a transition manifold 29 and outlet chamber 33. As a
result,
the temperature of the blood flowing through the core may be easily regulated
by a
heat exchange fluid temperature controlling unit (not shown) due to the high
degree
of thermal contact between the blood and the heat exchange medium as well as
the
1o relatively high thermal conductivity of the thin wall fibers.
While there have been shown what are been presently considered to
be the preferred embodiments of the invention, it will be apparent to those
skilled in
the art that various changes and modifications can be made herein without
departing
from the scope of the invention as defined by the appended claims.
