Patent Publication Number: US-2021170141-A1

Title: Catheter shaft with uniform bending stiffness circumferentially

Description:
FIELD OF THE INVENTION 
     The present invention is related to catheters, and is more particularly related to the construction of a catheter shaft. 
     BACKGROUND 
     Among devices commonly used to access vascular and other locations within a body and to perform various functions at those locations are medical catheters, or delivery catheters, adapted to deliver and deploy medical devices such as prosthetic heart valves, stent-grafts, and stents to selected targeted sites in the body. Such medical devices typically are releasably carried within a distal region of the delivery catheter in a radially compressed delivery state or configuration as the catheter is navigated to and positioned at a target treatment/deployment site. In many cases, such as those involving cardiovascular vessels, the route to the treatment/deployment site may be tortuous and may present conflicting design considerations requiring compromises between dimensions, flexibilities, material selection, operational controls and the like. 
     Typically, advancement of a delivery catheter within a patient is monitored fluoroscopically to enable a clinician to manipulate the catheter to steer and guide its distal end through the patient&#39;s vasculature to the target treatment/deployment site. This tracking requires a distal end of the delivery catheter to be able to navigate safely to the target treatment/deployment site through manipulation of a proximal end by the clinician. Such manipulation may encompass pushing, retraction and torque forces or a combination of all three. It is therefore required for the distal end of the delivery catheter to be able to withstand all these forces. 
     A delivery catheter desirably will have a low profile/small outer diameter to facilitate navigation through tortuous vasculature; however, small outer diameter catheters present various design difficulties resulting from competing considerations, resulting in design trade-offs. For instance, such delivery catheters must be flexible enough to navigate the tortuous vasculature or anatomy of a patient. However, typical constructions of delivery catheters must attempt to balance a requisite flexibility, with axial strength/stiffness (the property that permits the delivery catheter to be pushed and pulled) and torsional strength/stiffness (the property that permits the delivery catheter to be rotated about its longitudinal axis). It is especially important to balance these properties in a distal portion of the delivery catheter within which a prosthesis is held in its radially compressed, delivery state. 
     A need in the art still generally exists for improved catheters configured to navigate through or within a patient&#39;s anatomy. 
     SUMMARY 
     Embodiments of the present invention relate generally a catheter shaft including an inner layer defining an innermost circumferential surface of the catheter shaft and defining a lumen of the catheter shaft, and an outer layer defining an outermost circumferential surface of the catheter shaft. The inner layer is formed by a first polymer having a first durometer and a first melting temperature. The outer layer is formed by alternating first and second segments of the first polymer and a second polymer, respectively, that alternate in a circumferential direction. The second polymer has a second durometer softer than the first durometer and a second melting temperature lower than the first melting temperature. Each segment of the alternating first and second segments extend in an axial direction for substantially an entire length of the catheter shaft. 
     Embodiments hereof also relate to a system including a self-expanding prosthesis and a delivery device configured to percutaneously deliver the self-expanding prosthesis. The delivery device includes a handle having an actuator thereon, an outer sheath including a proximal end coupled to the handle, a middle shaft slidingly disposed within the outer sheath, the middle shaft having a proximal end coupled to the handle and a distal end configured to releasably couple to the self-expanding prosthesis such that the self-expanding prosthesis axially moves therewith when coupled to thereto, an inner shaft disposed within the middle shaft, wherein the self-expanding prosthesis is disposed on a distal portion of the inner shaft during delivery thereof. At least one of the outer sheath and the middle shaft include an inner layer defining an innermost circumferential surface and an outer layer defining an outermost circumferential surface. The inner layer is formed by a first polymer having a first durometer and a first melting temperature. The outer layer is formed by alternating first and second segments of the first polymer and a second polymer, respectively, that alternate in a circumferential direction. The second polymer has a second durometer softer than the first durometer and a second melting temperature lower than the first melting temperature. Each segment of the alternating first and second segments extend in an axial direction for substantially an entire length of the at least one of the outer sheath and the middle shaft. 
     Embodiments hereof also relate to a method of forming a catheter shaft. A first component is extruded, the first component being formed of a first polymer having a first durometer and a first melting temperature. The first component includes an inner layer defining an innermost circumferential surface and a plurality of segments radially extending from the inner layer. A notch extends between each pair of adjacent segments of the plurality of segments. Each segment of the plurality of segments extends in an axial direction for substantially an entire length of the inner layer. An elongated tube of a second polymer having a second durometer and a second melting temperature is positioned into each notch. The second durometer is softer than the first durometer and the second melting temperature is lower than the first melting temperature. The elongated tubes of the second polymer are heated to fuse the elongated tubes of the second polymer to the first component and thereby form the catheter shaft. The catheter shaft has a smooth and continuous outermost circumferential surface after the step of heating the elongated tubes of the second polymer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying figures, which are incorporated herein, form part of the specification and illustrate embodiments of a delivery system. Together with the description, the figures further explain the principles of and enable a person skilled in the relevant art(s) to make, use, and implant the prosthesis described herein. In the drawings, like reference numbers indicate identical or functionally similar elements. 
         FIG. 1  is a side view of a delivery system according to an embodiment hereof. 
         FIG. 1A  is a cross-sectional view of the delivery system of  FIG. 1  taken along line A-A of  FIG. 1 . 
         FIG. 1B  is an enlarged cross-sectional view of the outer sheath of the delivery system of  FIG. 1 , wherein the outer sheath is removed from the delivery system for illustrative purposes only. 
         FIG. 1C  is a perspective view of a distal portion of the delivery system of  FIG. 1 , wherein the delivery system is in the delivery configuration and an outer sheath of the delivery system is not shown for illustrative purposes only. 
         FIG. 2  is a side view of the delivery system of  FIG. 1 , wherein the delivery system is in a deployed configuration. 
         FIG. 3  is a side perspective view of a heart valve prostheses for use in embodiments hereof. 
         FIG. 4  is an end view of the heart valve prosthesis of  FIG. 3 . 
         FIG. 5  is a flow chart illustrating a method of forming a catheter shaft to have uniform bending stiffness in a circumferential direction according to an embodiment hereof. 
         FIG. 6  is a cross-sectional view illustrating a first component of a first polymer described in the method of forming a catheter shaft of  FIG. 5 . 
         FIG. 7  is a cross-sectional view illustrating the first component and a plurality of segments of a second polymer described in the method of forming a catheter shaft of  FIG. 5 . 
         FIG. 8  is a cross-sectional view illustrating heat shrink tubing positioned over the first component and a plurality of segments of a second polymer described in the method of forming a catheter shaft of  FIG. 5 . 
         FIG. 9  is a cross-sectional view illustrating the heating step of the first component and a plurality of segments of a second polymer described in the method of forming a catheter shaft of  FIG. 5 . 
         FIG. 10  is a cross-sectional view illustrating the catheter shaft formed via the method of forming a catheter shaft of  FIG. 5  after the heating step and after removal from a mandrel. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Specific embodiments of the present invention are now described with reference to the figures, wherein like reference numbers indicate identical or functionally similar elements. Unless otherwise indicated, the terms “distal” and “proximal”, when used in the following description to refer to a sheath, a delivery device, or a catheter-based delivery system are with respect to a position or direction relative to the treating clinician. Thus, “distal” and “distally” refer to positions distant from, or in a direction away from the treating clinician, and the terms “proximal” and “proximally” refer to positions near, or in a direction toward the treating clinician. The terms “distal” and “proximal”, when used in the following description to refer to a device to be implanted into a vessel, such as a heart valve prosthesis, are used with reference to the direction of blood flow. Thus, “distal” and “distally” refer to positions in a downstream direction with respect to the direction of blood flow, and the terms “proximal” and “proximally” refer to positions in an upstream direction with respect to the direction of blood flow. 
     In addition, the term “self-expanding” is used in the following description with reference to one or more stent structures of the prostheses hereof and is intended to convey that the structures are shaped or formed from a material that can be provided with a mechanical memory to return the structure from a radially compressed or constricted delivery configuration to a radially expanded deployed configuration. Non-exhaustive illustrative self-expanding materials include stainless steel, a pseudo-elastic metal such as a nickel titanium alloy or nitinol, various polymers, or a so-called super alloy, which may have a base metal of nickel, cobalt, chromium, or other metal. Mechanical memory may be imparted to a wire or stent structure by thermal treatment to achieve a spring temper in stainless steel, for example, or to set a shape memory in a susceptible metal alloy, such as nitinol. Various polymers that can be made to have shape memory characteristics may also be suitable for use in embodiments hereof to include polymers such as polynorborene, trans-polyisoprene, styrene-butadiene, and polyurethane. As well poly L-D lactic copolymer, oligo caprylactone copolymer and poly cyclo-octine can be used separately or in conjunction with other shape memory polymers. 
     Embodiments hereof relate to catheter devices or delivery systems including at least one catheter shaft that includes an inner layer defining an innermost circumferential surface and an outer layer defining an outermost circumferential surface. The inner layer is formed entirely of a first polymer having a first durometer and a first melting temperature. The outer layer is formed by alternating segments of the first polymer and a second polymer that alternate in a circumferential direction. The second polymer has a second durometer that is softer than the first durometer and a second melting temperature that is lower than the first melting temperature. As described in more detail herein, such a catheter shaft has uniform bending stiffness in a circumferential direction. As used herein, bending stiffness refers to the resistance of the catheter shaft against bending deformation and a catheter shaft constructed according to embodiments hereof has uniform or unvarying bending stiffness in all circumferential directions. The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary, or the following detailed description. Although the description of the invention is primarily in the context of a prosthetic valve delivery system, the catheter shaft construction described herein may be utilized in any type of catheter device or delivery system. 
     A delivery system including at least one catheter shaft having uniform bending stiffness in a circumferential direction will be described in more detail with reference to the figures. A delivery system  100  includes a self-expanding prosthesis  101  and a delivery device  110  configured to percutaneously deliver the self-expanding prosthesis  101 . More particularly, the delivery system  100  is shown in  FIGS. 1, 1A, 1B, 1C, and 2 .  FIG. 1  is a side view of the delivery system  100 , with an outer sheath  112  thereof shown in a delivery configuration in which the outer sheath  112  surrounds and constrains the self-expanding prosthesis  101  (not shown in  FIG. 1A ) in a compressed or delivery configuration.  FIG. 1A  is a cross-sectional view taken along line A-A of  FIG. 1A .  FIG. 1B  is an enlarged cross-sectional view of the outer sheath  112  removed from the delivery system  100  for illustrative purposes only.  FIG. 1C  is a perspective view of a distal portion of the delivery system  100  in the delivery configuration but with the outer sheath  112  not shown for illustrative purposes only.  FIG. 2  is a side view of the delivery system  100  after the outer sheath  112  has been retracted to allow the prosthesis  101  to self-expand to a deployed or expanded configuration. The delivery device  110  includes a handle  140  having an actuator  142  thereon. The handle  140  can have any shape or size appropriate for convenient handling by a user. 
     In addition to the outer sheath  112  operatively coupled to the handle  140 , the delivery device  110  further includes a middle shaft  122  slidingly disposed within the outer sheath  112  and operatively coupled to the handle  140 , and an inner shaft  132  disposed within the middle shaft  122 . As used herein, “slidably” denotes back and forth movement in a longitudinal direction along or generally parallel to a central longitudinal axis LA of the delivery system  100 . The outer sheath  112 , the middle shaft  122 , and the inner shaft  132  each distally extend from within the handle  140 . 
     The outer sheath  112  has a proximal end disposed within the handle  140  and a distal end  116 . As best shown in  FIG. 1A , the outer sheath  112  defines a lumen  118  and is slidingly and concentrically disposed over the middle shaft  122 . A distal portion of the outer sheath  112  defines a capsule  120 . The capsule  120  is configured to retain the self-expanding prosthesis  101  in a radially collapsed configuration for delivery to the desired treatment location as will be described in more detail herein. While the capsule  120  is described herein as a distal portion of the outer sheath  112 , the capsule  120  may be a separate component coupled to the distal end of the outer sheath  112 . If formed as a separate component, the capsule  120  may include a relatively short, tapered proximal end that is attached to a distal end of the outer shaft  112  by any suitable attachment means. When the capsule  120  is a separate component than the outer shaft  112 , the capsule  120  may be formed with the same constructions as outer shaft  112  such that the capsule  120  also has uniform bending stiffness in a circumferential direction. If formed as a separate component, the capsule  120  may be larger in both inner diameter and outer diameter than the outer shaft  112 . For example, the capsule  120  may be formed with an inner diameter of ranging between 0.195 and 0.215 inches and an outer diameter of ranging between 0.215 and 0.235 inches. Conversely, the outer shaft  112  may be formed with an inner diameter of approximately 0.120 inches and an outer diameter of approximately 0.160 inches. The middle shaft  122  may be formed with an inner diameter of approximately 0.050 inches and an outer diameter of approximately 0.110 inches. 
     The actuator  142  of the handle  140  is configured for retracting the capsule  120 . The actuator  142  is coupled to the outer sheath  112 , and is generally constructed to provide selective proximal retraction and distal advancement of the outer sheath  112 , and particularly of the capsule  120  attached thereto, relative to the self-expanding prosthesis  101  held in a radially compressed, delivery configuration therein for covering and uncovering the self-expanding prosthesis  101 . The actuator  142  may assume any construction that is capable of providing the desired sheath actuation functionality, such as those described in U.S. Pat. No. 8,579,963 to Tabor, which is assigned to the same assignee as the present disclosure and which is herein incorporated by reference in its entirety. 
     The middle shaft  122  has a proximal end disposed within the handle  140  and a distal end  126  disposed inside of the outer sheath  112  when the outer sheath  112  is disposed over the self-expanding prosthesis  101 . The distal end  126  of the middle shaft  122  includes a spindle  108  which is releasably coupled to an end of the self-expanding prosthesis  101 . As best shown on the perspective view of  FIG. 1C , having the outer sheath  112  removed for illustrative purposes only, the spindle  108  is a tubular component having at least one recess  107 A formed on an outer surface thereof that is configured to receive a paddle  107 B extending proximally from the self-expanding prosthesis  101 . The paddle  107 B fits within or mates with the recess  107 A of the spindle  108  such that the self-expanding prosthesis  101  is releasably coupled to middle shaft  122 . Although only one recess  107 A is visible on  FIG. 1B , it will be understood by one of ordinary skill in the art that the spindle  108  may include two or more recesses for receiving a mating paddle of the self-expanding prosthesis  101 , such as for example first and second recesses at opposing circumferential locations on the spindle  108 . As best shown in  FIG. 1A , the middle shaft  122  defines a lumen  128  and is concentrically disposed over the inner shaft  132 . The inner shaft  132  has a proximal end (not shown) which terminates within the handle  140  and a distal end  136 . A tapered flexible nosecone or distal tip  133  may be coupled to the distal end  136  of the inner shaft  132  as shown in  FIG. 1  and  FIG. 2 . As best shown in  FIG. 1A , the inner shaft  132  defines a lumen  138  such that the delivery system  100  may be slidingly disposed and tracked over a guidewire  109 . The inner shaft  132  is coupled to the middle shaft  122  at the spindle  108  such that the inner shaft  132  and the middle shaft  122  are slidingly disposed within the outer sheath  112  as an assembly. 
     The inner shaft  132  is configured to receive the self-expanding prosthesis  101  on a distal portion thereof and the outer sheath  112  is configured to compressively retain the self-expanding prosthesis  101  on the distal portion of the inner shaft  132  during delivery, as shown in  FIG. 1 . Stated another way, the outer sheath  112  surrounds and constrains the self-expanding prosthesis  101  in a radially compressed or delivery configuration. As previously described, the distal end  126  of the middle shaft  122  includes the spindle  108  to which the self-expanding prosthesis  101  is releasably coupled. The self-expanding prosthesis  101  is shown in the view of  FIG. 2  but is obscured from view by the outer sheath  112  in  FIG. 1 . During deployment of the self-expanding prosthesis  101  in situ, the outer sheath  112  is proximally retracted with respect to the self-expanding prostheses  101  via the actuator  142  on the handle  140 , thereby incrementally exposing the self-expanding prosthesis  101  until the self-expanding prothesis  101  is fully exposed and thereby released from the delivery device  110 . The middle shaft  122 , the inner shaft  132  and the self-expanding prosthesis  101  are held stationary while the outer sheath  112  is proximally retracted. When the outer sheath  112  is proximally retracted beyond the spindle  108 , the paddles  107 B of the self-expanding prosthesis  101  are no longer held within the recesses  107 A of the spindle  108  and the self-expanding prosthesis  101  is permitted to self-expand to its deployed configuration. 
       FIG. 3  and  FIG. 4  illustrate side perspective and end views, respectively, of a heart valve prosthesis  301  that may be utilized as the self-expanding prosthesis  101  according to an embodiment hereof. The heart valve prosthesis  301  is merely exemplary and is described in more detail in U.S. Patent Application Pub. No. 2011/0172765 to Nguyen et al., which is herein incorporated by reference in its entirety. It is understood that any number of alternate heart valve prostheses can be used with the delivery devices and methods described herein. In addition, the delivery device  110  may also be used with other self-expanding prostheses such as stent-graft prostheses, uncovered stents, bare metal stents, drug eluting stents, and any self-expanding structure. 
     Heart valve prosthesis  301  includes an expandable stent or frame  306  that supports a prosthetic valve component  308  within the interior of the frame  306 . In embodiments hereof, the frame  306  is self-expanding to return to a radially expanded configuration from a radially compressed or constricted delivery configuration. In the embodiment depicted in  FIGS. 3 and 4 , the frame  306  has an expanded, longitudinally asymmetric hourglass configuration including a first end or portion  302  and a relatively enlarged second end or portion  304 . Each portion of frame  306  may be designed with a number of different configurations and sizes to meet the different requirements of the location in which it may be implanted. When configured as a replacement for an aortic valve, the first end  302  functions as an inflow end of the heart valve prosthesis  301  and extends into and anchors within the aortic annulus of a patient&#39;s left ventricle, while the enlarged second end  304  functions as an outflow end of the heart valve prosthesis  301  and is positioned in the patient&#39;s ascending aorta. When configured as a replacement for a mitral valve, the enlarged second end  304  functions as an inflow end of the heart valve prosthesis  301  and is positioned in the patient&#39;s left atrium, while the first end  302  functions as an outflow end of the heart valve prosthesis  301  and extends into and anchors within the mitral annulus of a patient&#39;s left ventricle. For example, U.S. Patent Application Publication Nos. 2012/0101572 to Kovalsky et al. and 2012/0035722 to Tuval, each of which are herein incorporated by reference in their entirety, illustrate heart valve prostheses configured for placement in a mitral valve. Each portion of the frame  306  may have the same or different cross-portion which may be for example circular, ellipsoidal, rectangular, hexagonal, rectangular, square, or other polygonal shape, although at present it is believed that circular or ellipsoidal may be preferable when the valve prosthesis is being provided for replacement of the aortic or mitral valve. As alternatives to the deployed asymmetric hourglass configuration of  FIGS. 3 and 4 , the frame  306  may have a symmetric hourglass configuration, a generally tubular configuration, or other stent configuration or shape known in the art for valve replacement. 
     As previously mentioned, the heart valve prosthesis  301  includes the prosthetic valve component  308  within the interior of frame  306 . The prosthetic valve component  308  is capable of blocking flow in one direction to regulate flow there through via valve leaflets that may form a bicuspid or tricuspid replacement valve.  FIG. 4  is an end view of the heart valve prostheses  201  of  FIG. 3  and illustrates an exemplary tricuspid valve having three leaflets, although a bicuspid leaflet configuration may alternatively be used in embodiments hereof. Valve leaflets are sutured or otherwise securely and sealingly attached to the interior surface of the frame  306  and/or graft material  307  which encloses or lines the frame  306  as would be known to one of ordinary skill in the art of prosthetic tissue valve construction. Leaflets may be attached along their bases to the graft material  307 , for example, using sutures or a suitable biocompatible adhesive. Adjoining pairs of leaflets are attached to one another at their lateral ends to form commissures. The orientation of the leaflets within the frame  306  would change depending on which end of the heart valve prosthesis  301  is the inflow end and which end of the heart valve prosthesis  301  is the outflow end, thereby ensuring one-way flow of blood through the heart valve prosthesis  301 . 
     Leaflets may be made of pericardial material; however, the leaflets may instead be made of another material. Natural tissue for replacement valve leaflets may be obtained from, for example, heart valves, aortic roots, aortic walls, aortic leaflets, pericardial tissue, such as pericardial patches, bypass grafts, blood vessels, intestinal submucosal tissue, umbilical tissue and the like from humans or animals. Synthetic materials suitable for use as leaflets include DACRON® polyester commercially available from Invista North America S.A.R.L. of Wilmington, Del., other cloth materials, nylon blends, polymeric materials, and vacuum deposition nitinol fabricated materials. One polymeric material from which the leaflets can be made is an ultra-high molecular weight polyethylene material commercially available under the trade designation DYNEEMA from Royal DSM of the Netherlands. With certain leaflet materials, it may be desirable to coat one or both sides of the leaflet with a material that will prevent or minimize overgrowth. It is further desirable that the leaflet material is durable and not subject to stretching, deforming, or fatigue. 
     The graft material  307  may also be a natural or biological material such as pericardium or another membranous tissue such as intestinal submucosa. Alternatively, the graft material  307  may be a low-porosity woven fabric, such as polyester, Dacron fabric, or PTFE, which creates a one-way fluid passage when attached to the stent. In one embodiment, the graft material  307  may be a knit or woven polyester, such as a polyester or PTFE knit, which can be utilized when it is desired to provide a medium for tissue ingrowth and the ability for the fabric to stretch to conform to a curved surface. Polyester velour fabrics may alternatively be used, such as when it is desired to provide a medium for tissue ingrowth on one side and a smooth surface on the other side. These and other appropriate cardiovascular fabrics are commercially available from Bard Peripheral Vascular, Inc. of Tempe, Ariz., for example. 
     At least one of the middle shaft  122  and the outer sheath  112  is formed without or devoid of axial wires or other reinforcement structures and is constructed to have uniform bending stiffness in a circumferential direction. Since the inner shaft  132  is coupled to the middle shaft  122  at the spindle  108  such that the inner shaft  132  and the middle shaft  122  are slidingly disposed within the outer sheath  112  as an assembly as described above, it is not necessary for the inner shaft  132  to have uniform bending stiffness in a circumferential direction. However, if the inner shaft  132  is not attached to the middle shaft  122 , it also may be constructed to have uniform bending stiffness in a circumferential direction as described herein. More particularly, catheter shafts that undergo very high tensile and compressive forces during operation (i.e., during deployment of a self-expanding prosthesis or during re-sheathing of a self-expanding prosthesis) are often longitudinally or axially reinforced with one or more axial wires that are disposed at circumferentially opposite locations. As used herein, very high tensile forces include forces between 10 lbf and 50 lbf and very high compressive forces include forces between 10 lbf and 50 lbf. However, catheter shafts constructed with such axial wires cannot bend in certain circumferential directions, which can contribute to tracking difficulty thereof in tortuous vasculature or anatomies such as an aortic arch. Catheter shafts constructed as described herein have uniform bending stiffness in a circumferential direction combined with ability to withstand very high tensile and compressive forces during operation due to the properties of two different polymer materials and the configuration of the inner and outer layers thereof. In the embodiment of  FIGS. 1, 1A, 1B, 1C, and 2 , both the middle shaft  122  and the outer sheath  112  are formed to have uniform bending stiffness in a circumferential direction as described herein. The construction of the outer sheath  112  is described in detail with respect to  FIG. 1B . In an embodiment, the middle shaft  122  has the same layered construction as the outer sheath  112  as described with reference to  FIG. 1B  such that both the outer sheath  112  and the middle shaft  122  have the construction described. In another embodiment, only the middle shaft  122  is formed to have uniform bending stiffness in a circumferential direction and is formed with the layered construction described with reference to  FIG. 1B . In another embodiment, only the outer sheath  112  is formed to have uniform bending stiffness in a circumferential direction and is formed with the layered construction described with reference to  FIG. 1B . 
     As shown in  FIG. 1B , which is an enlarged cross-sectional view of the outer sheath  112  removed from the delivery system  100  for illustrative purposes only, the outer sheath  112  includes a first or inner layer  150  that defines, forms, or otherwise includes an innermost circumferential surface  152  of the outer sheath  112  and a second or outer layer  160  that defines, forms, or otherwise includes an outermost circumferential surface  164  of the outer sheath  112 . The inner layer  150  and the outer layer  160  directly contact each other with the outer layer  160  circumferentially surrounding the inner layer  150 . More particularly, the inner layer  150  includes the innermost circumferential surface  152  and an outermost circumferential surface  154 . In an embodiment, the innermost circumferential surface  152  may be textured to reduce friction. The inner layer  150  has a consistent thickness T 1  around the circumference thereof. The outer layer  160  includes an innermost circumferential surface  162  and the outermost circumferential surface  164 . The outer layer  160  has a consistent thickness T 2  around the circumference thereof. The innermost circumferential surface  162  of the outer layer  160  contacts or abuts against the outermost circumferential surface  154  of the inner layer  150 . In an embodiment, the inner layer  150  is thinner than the outer layer  160 . For example, in an embodiment, the thickness T 2  of the outer layer  160  is between 2 and 10 times the thickness T 1  of the inner layer  150 . In another embodiment, the thickness T 2  of the outer layer  160  is between 6 and 8 times the thickness T 1  of the inner layer  150 . Exemplary relative pairings of thickness values, in inches, for thickness T 1  and thickness T 2  are shown in the table below. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Thickness T 1   
                 0.002″ 
                 0.005″ 
                 0.005″ 
                 0.010″ 
               
               
                   
                 Thickness T 2   
                 0.018″ 
                 0.015″ 
                 0.025″ 
                 0.020″ 
               
               
                   
                   
               
            
           
         
       
     
     The inner layer  150  is formed entirely or solely of a first polymer having a first durometer and a first melting temperature. The first polymer is a relatively hard polymeric material that has the ability to withstand very high tensile and compressive forces during operation such as but not limited to polyetheretherketone (PEEK). Other suitable polymers for the first polymer include PPS (Polyphenylene Sulfide), PPSU (Polyphenyl Sulfone), PEI (Polyetherimide), PET (Polyethylene Terephthalate), PBT (Polybuthylene Terephthalate), and PCT (Polycyclohexylenedimethylene Terephthalate). In an embodiment hereof, at body temperature, the first polymer has a tensile strength between 9,000 and 11,000 psi, a flexural strength between 19,0000 and 21,000 psi, and a compressive strength between 14,000 and 16,000 psi. In another embodiment hereof, at body temperature, the first polymer has a tensile strength of 10,000 psi, a flexural strength of 20,000 psi, and a compressive strength of 15,000 psi. 
     The outer layer  160  is formed by alternating first and second segments  156 ,  166  of the first polymer and a second polymer, respectively, that alternate in a circumferential direction around the circumference of the outer layer  160 . The segments of the first polymer are referred to herein as first segments  156  and the segments of the second polymer are referred to herein as second segments  166 . As will be described herein in more detail with respect to  FIGS. 5-8 , in an embodiment, the inner layer  150  and the first segments  156  of the outer layer  160  are formed in a single piece construction by extrusion. The first segments  156  alternate with the second segments  166 , or stated another way, each first segment  156  is disposed between a pair of the second segments  166  and each second segment  166  is disposed between a pair of the first segments  156 . The second polymer has a second durometer softer than the first durometer of the first polymer. Further, the second polymer has a second melting temperature lower than the first melting temperature of the first polymer. The second polymer is a relatively soft polymeric material such as but not limited to thermoplastic polyurethane 80A that imparts flexibility to the outer sheath  112  and improves trackability of the delivery system  100 . Other suitable polymers for the second polymer include thermoplastic polyether urethane (ELASTHANE, TECOTHANE, TECOFLEX, TEXIN), thermoplastic polycarbonate urethane (BIONATE), thermoplastic silicone urethane (PURSIL), C-FLEX, CHRONOPRENE, AND POLYBLEND. In an embodiment hereof, at body temperature, the second polymer has a flexural strength between 4,000 and 6,000 psi. In another embodiment hereof, at body temperature, the second polymer has a flexural strength of 5,000 psi. 
     Although only a cross-sectional view is shown in  FIG. 1B , each segment of the alternating first and second segments  156 ,  166  extends in an axial direction for an entire length or substantially the entire length of the outer sheath  112 . Stated another way, the cross-section of the outer sheath  112  is the same as shown in  FIG. 1B  along an entire length of the outer sheath  112 . As used herein, “substantially the entire length” includes at least 95% of the total or entire length of the catheter shaft. The first segments  156  of the first polymer are circumferentially spaced apart at equal intervals around the circumference of the outer layer  160 , and the second segments  166  of the second polymer are circumferentially spaced apart at equal intervals around the circumference of the outer layer  160 . Each of the first segments  156  have the same size or width, and each of the second segments  166  have the same size or width, with the first segments  156  being equally circumferentially spaced apart from each other. In an embodiment, each segment of the alternating first and second segments  156 ,  166  are the same size or width although this is not required. Although  FIG. 1B  illustrates the outer layer  160  with eight first segments  156  and eight second segments  166 , the total number of alternating first and second segments  156 ,  166  may vary. In an embodiment, the alternating first and second segments  156 ,  166  include at least five first segments  156  and at least five second segments  166 . In an embodiment, the alternating first and second segments  156 ,  166  include between five and ten first segments  156  and between five and ten second segments  166 . 
     The manufacturing of the catheter shafts having the layered construction described with reference to  FIG. 1B  is simplified as compared to catheter shafts including axial wires or other reinforcement structures. More particularly, manufacture of catheter shafts that include axial wires or other reinforcement structures require multiple processes across manufacturing sites such as extrusion, braiding, and fusing processes. Further, catheter shafts that include axial wires or other reinforcement structures often include a high number (i.e., 10 or more) components per shaft and must be manufactured one at a time due to the design thereof, thereby resulting in a relatively expensive cost of manufacture per shaft. In contrast, catheter shafts having the layered construction described with reference to  FIG. 1B  may be made using only three components (i.e., a core mandrel, the first polymer, and the second polymer). Further, catheter shafts having the layered construction described with reference to  FIG. 1B  may be made using a continuous extrusion process and multiple catheter shafts may be formed during the extrusion process in a batch-style method of manufacture, thereby resulting in a relatively less expensive cost of manufacture per shaft. 
       FIG. 5  is a flow chart illustrating a method of forming a catheter shaft having the layered construction described with reference to  FIG. 1B  to have uniform bending stiffness in a circumferential direction according to an embodiment hereof. In a step  570 , a first component is extruded over a core mandrel.  FIG. 6  is a cross-sectional view illustrating a first component  680  and a core mandrel  682 . The first component  680  is formed entirely or solely of the first polymer described above having a first durometer and a first melting temperature. As described above, the first polymer is a relatively hard polymeric material that has the ability to withstand very high tensile and compressive forces during operation such as but not limited to polyetheretherketone (PEEK). Other suitable polymers for the first polymer are described in more detail herein. In an embodiment hereof, at body temperature, the first polymer has a tensile strength between 9,000 and 11,000 psi, a flexural strength between 19,0000 and 21,000 psi, and a compressive strength between 14,000 and 16,000 psi. In another embodiment hereof, at body temperature, the first polymer has a tensile strength of 10,000 psi, a flexural strength of 20,000 psi, and a compressive strength of 15,000 psi. 
     The first component  680  includes an inner layer  650  and a plurality of first segments  656  of an outer layer  660 . The plurality of first segments  656  radially extend from the inner layer  650 . A gap or notch  684  extends between each pair of adjacent first segments  656 . Each first segment  656  and each notch  684  extends in an axial direction along an entire length or substantially the entire length of the inner layer  650 . The mandrel  682  is not required to be utilized in the remaining method steps after the first component  680  is extruded, and thus the mandrel  682  is not shown in  FIGS. 7-9 . However, in another embodiment, the mandrel  682  may be left in place during the method steps of  FIGS. 7-9 . 
     In a step  572 , an elongated tube  786  of the second polymer described above having a second durometer and a second melting temperature is positioned into each notch  684  as shown in the cross-sectional view of  FIG. 7 . As described above, the second durometer of the second polymer is softer than the first durometer of the first polymer and the second melting temperature of the second polymer is lower than the first melting temperature of the first polymer. As described above, the second polymer has a second durometer softer than the first durometer of the first polymer. Further, the second polymer has a second melting temperature lower than the first melting temperature of the first polymer. The second polymer is a relatively soft polymeric material such as but not limited to thermoplastic polyurethane 80A that imparts flexibility to the catheter shaft and improves trackability of the delivery system  100 . Other suitable polymers for the second polymer are described in more detail herein. In an embodiment hereof, at body temperature, the second polymer has a flexural strength between 4,000 and 6,000 psi. In another embodiment hereof, at body temperature, the second polymer has a flexural strength of 5,000 psi. Each elongated tube  786  extends in an axial direction along an entire length or substantially the entire length of the inner layer  650 . Although shown with an oval cross-section which is sized to occupy the full thickness of the respective notch  684 , each elongated tube  786  may have other cross-sections such as but not limited to circular or rectangular as the shape of each elongate tube  786  will change upon heating thereof. 
     In a step  574 , heat shrink tubing  888  is be positioned around the outer perimeter of the subassembly of the first component  680  and the plurality of elongated tubes  786 . More particularly, as shown in  FIG. 8 , heat shrink tubing  888  is positioned over or around the first component  680  and the plurality of elongated tubes  786 . Although  FIG. 8  is a cross-sectional view, the heat shrink tubing  888  extends the full or entire length of the subassembly of the first component  680  and the plurality of elongated tubes  786 . 
     In a step  576 , with the heat shrink tubing  888  positioned thereover, the elongated tubes  786  of the second polymer are heated to fuse the elongated tubes  786  to the first component  680  and thereby form a catheter shaft  990  as shown in the cross-sectional view of  FIG. 9 . More particularly, heat is applied to the heat shrink tubing  888  in order to melt or reflow the plurality of elongated tubes  786  and thereby fuse the elongated tubes  786  to the first component  680 . 
     The catheter shaft  990  is shown in  FIG. 10  after the steps of heating the elongated tubes  786  and after removal of the heat shrink tubing  888 . The catheter shaft  990  includes an inner layer  950  that defines, forms, or otherwise includes an innermost circumferential surface  952  of the catheter shaft  990  and an outer layer  960  that defines, forms, or otherwise includes an outermost circumferential surface  964  of the catheter shaft  990 . The inner layer  950  is the same as the inner layer  150  described above with respect to the outer sheath  112 , and the outer layer  960  is the same as the outer layer  160  described above with respect to the outer sheath  112 . The inner layer  950  and the outer layer  960  directly contact each other with the outer layer  960  circumferentially surrounding the inner layer  950 . The outer layer  960  is formed by alternating first and second segments  956 ,  966  of the first polymer and the second polymer, respectively, that alternate in a circumferential direction around the circumference of the outer layer  960 . The first segments  956  alternate with the second segments  966 , or stated another way, each first segment  956  is disposed between a pair of the second segments  966  and each second segment  966  is disposed between a pair of the first segments  956 . Although only a cross-sectional view is shown in  FIG. 10 , each segment of the alternating first and second segments  956 ,  966  extends in an axial direction for an entire length or substantially the entire length of the catheter shaft  990 . Stated another way, the cross-section of the catheter shaft  990  is the same as shown in  FIG. 10  along an entire length or substantially the entire length of the catheter shaft  990 . The first segments  956  are circumferentially spaced apart at equal intervals around the circumference of the outer layer  960 , and the second segments  966  are circumferentially spaced apart at equal intervals around the circumference of the outer layer  960 . In an embodiment, each segment of the alternating first and second segments  956 ,  966  are the same size or width. The outermost circumferential surface  964  of the catheter shaft  990  is smooth and continuous after the step of heating the elongated tubes  786  of the second polymer. 
     The catheter shaft  990  may be used, for example, as the outer sheath  112  and/or the middle shaft  122  of the delivery system  100  as described herein with respect to  FIGS. 1-2 , or alternatively may be a shaft used in any type of catheter device, including but not limited to balloon catheters, diagnostic catheters, drug delivery catheters, guide catheters, or any type of catheter device which it is desirable to have uniform bending stiffness in a circumferential direction combined with ability to withstand very high tensile and compressive forces during operation. 
     The foregoing description has been presented for purposes of illustration and enablement and is not intended to be exhaustive or to limit the invention to the precise form disclosed. Other modifications and variations are possible in light of the above teachings. The embodiments and examples were chosen and described in order to best explain the principles of the invention and its practical application and to thereby enable others skilled in the art to best utilize the invention in various embodiments and various modifications as are suited to the particular use contemplated. It is intended that the appended claims be construed to include other alternative embodiments of the invention.