MESH SPACER FOR HEAT EXCHANGER

Abstract: A blood heat exchange system (10) is disclosed generally comprising a plurality of small polymeric hollow conduits (32) for conveying blood. The hollow conduits (32) are formed in a flat or mat shape that are wrapped in layers around a spindle (16). Layers of the hollow conduits (32) are spaced from each other by a mesh spacer (40). The mesh spacer (40) has holes (44) through it so that a heat transfer fluid such as water may more efficiently flow over and around the outside of the hollow conduits (32). The heat transfer fluid conveys heat from or to the outside surfaces of the hollow conduits (32) which in turn transfer heat from or to the blood passing through the hollow conduits (32). The invention provides a heat exchanger (10) 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 (32) is more efficient.

Description: MESH SPACER FOR HEAT EXCHANGER BACKGROUND OF THE INVENTION Field of the Invention The present invention generally relates to heat exchangers for use in 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 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 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 heat 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 MAXIMA™ by Medtronic, Inc. In the MAXIMA™ blood oxygenator, the heat transfer fluid (water) flows inside relatively large diameter 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 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 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. 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 1 A 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. 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 exchanger apparatus in accordance with the invention. DETAILED DESCRIPTION OF THE 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 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 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 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 exchanger as discussed in more detail below. 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 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 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 opposed layers of wrapping material 14 are spaced apart from each other and that 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 polyolifin 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 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 IA, 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 co-pending U.S. Patent Applications Serial No. filed on January 11, 1996 entitled Surface Treatment for Micro-conduits Employed in Blood Heat Exchange System, U.S. Patent Applications Serial No. filed on January 11, 1996 entitled Compact Membrane-Type Blood Oxygenator With Concentric Heat Exchanger and U.S. Patent Application Serial No. filed on January 11, 1996, entitled Blood Heat Exchange System Employing Micro-Conduit. The entire disclosures of the aforementioned patent applications are specifically incorporated herein by reference. These applications are assigned to Medtronic, Inc. of Minneapolis, Minnesota, U.S.A. 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 co-pending U.S. Patent Application Serial No. filed on January 11, 1996 entitled SURFACE TREATMENT FOR MICRO-CONDUITS EMPLOYED IN BLOOD HEAT EXCHANGE SYSTEM, the teachings of which are incorporated herein by reference. This patent application is assigned to Medtronic, Inc. of Minneapolis, Minnesota who is also the assignee of this U.S. Patent Application. 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 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 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. OPERATION Generally, the heat exchanger 10 serves to regulate the temperature during a medical procedure such as open-heart surgery. Heat exchanger 10 also may be advantageously incorporated into a blood oxygenator such as disclosed in the aforementioned U.S. Patent Application entitled "Compact Membrane-Type Blood Oxygenator With Concentric Heat Exchanger" Serial No. . 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 the 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 flow. 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 exists and flows downwardly. 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 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.

Claims: WHAT IS CLAIMED IS: CLAIMS: 1 A blood heat exchange system comprising: a mesh; a spindle having a center axis; a plurality of hollow conduits 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. 2. The heat exchange system according to Claim 1 wherein the hollow conduits comprise a mat of hollow conduits aligned side-by-side and attached by a woven fiber. 3. The heat exchange system according to Claim 1 wherein the mesh is made of polyethylene. 4. The heat exchanger according to Claim 1 wherein the mesh is made of polypropylene.

Inventors: HIER PETER J, KUBISA PETER Z, PALERMO HENRY W, HIER, PETER, J., KUBISA, PETER, Z., PALERMO, HENRY, W.
Applicants: MEDTRONIC INC, MEDTRONIC, INC.
Application year: 1997
Main classification: A61M1
IPC codes: A61M1, F28F1, F28F9, F28F21