Patent Publication Number: US-11045317-B2

Title: Expandable sheath for introducing an endovascular delivery device into a body

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is a continuation of U.S. patent application Ser. No. 15/997,587, filed on Jun. 4, 2018, which is a continuation of U.S. patent application Ser. No. 15/057,953, filed Mar. 1, 2016, now U.S. Pat. No. 9,987,134, which is a continuation of U.S. patent application Ser. No. 14/324,894, filed Jul. 7, 2014, now U.S. Pat. No. 9,301,841, which is a continuation of U.S. patent application Ser. No. 13/312,739, filed Dec. 6, 2011, now U.S. Pat. No. 8,790,387, which is a continuation-in-part of U.S. patent application Ser. No. 12/249,867, filed Oct. 10, 2008, now U.S. Pat. No. 8,690,936, the contents of which are hereby incorporated by reference herein in their entirety. 
    
    
     FIELD 
     The present application concerns embodiments of a sheath for use with catheter-based technologies for repairing and/or replacing heart valves, as well as for delivering a prosthetic device, such as a prosthetic valve to a heart via the patient&#39;s vasculature. 
     BACKGROUND 
     Endovascular delivery catheter assemblies are used to implant prosthetic devices, such as a prosthetic valve, at locations inside the body that are not readily accessible by surgery or where access without invasive surgery is desirable. For example, aortic, mitral, tricuspid, and/or pulmonary prosthetic valves can be delivered to a treatment site using minimally invasive surgical techniques. 
     An introducer sheath can be used to safely introduce a delivery apparatus into a patient&#39;s vasculature (e.g., the femoral artery). An introducer sheath generally has an elongated sleeve that is inserted into the vasculature and a housing that contains one or more sealing valves that allow a delivery apparatus to be placed in fluid communication with the vasculature with minimal blood loss. A conventional introducer sheath typically requires a tubular loader to be inserted through the seals in the housing to provide an unobstructed path through the housing for a valve mounted on a balloon catheter. A conventional loader extends from the proximal end of the introducer sheath, and therefore decreases the available working length of the delivery apparatus that can be inserted through the sheath and into the body. 
     Conventional methods of accessing a vessel, such as a femoral artery, prior to introducing the delivery system include dilating the vessel using multiple dilators or sheaths that progressively increase in diameter. This repeated insertion and vessel dilation can increase the amount of time the procedure takes, as well as the risk of damage to the vessel. 
     Radially expanding intravascular sheaths have been disclosed. Such sheaths tend to have complex mechanisms, such as ratcheting mechanisms that maintain the shaft or sheath in an expanded configuration once a device with a larger diameter than the sheath&#39;s original diameter is introduced. 
     However, delivery and/or removal of prosthetic devices and other material to or from a patient still poses a significant risk to the patient. Furthermore, accessing the vessel remains a challenge due to the relatively large profile of the delivery system that can cause longitudinal and radial tearing of the vessel during insertion. The delivery system can additionally dislodge calcified plaque within the vessels, posing an additional risk of clots caused by the dislodged plaque. 
     Accordingly, there remains a need in the art for an improved introducer sheath for endovascular systems used for implanting valves and other prosthetic devices. 
     SUMMARY 
     Embodiments of the present expandable sheath can minimize trauma to the vessel by allowing for temporary expansion of a portion of the introducer sheath to accommodate a delivery system, followed by a return to the original diameter once the delivery system passes through. Some embodiments can comprise a sheath with a smaller profile than that of prior art introducer sheaths. Furthermore, certain embodiments can reduce the length of time a procedure takes, as well as reduce the risk of a longitudinal or radial vessel tear, or plaque dislodgement because only one sheath is required, rather than several different sizes of sheaths. Embodiments of the present expandable sheath can require only a single vessel insertion, as opposed to requiring multiple insertions for the dilation of the vessel. 
     One embodiment of a sheath for introducing a prosthetic device comprises an inner layer and an outer layer. At least a portion of the sheath can be designed or configured to locally expand from a first diameter to a second diameter as the prosthetic device is pushed through a lumen of the sheath, and then at least partially return to the first diameter once the prosthetic device has passed through. Some embodiments can additionally include an elastic outer cover disposed about the outer layer. 
     The inner layer can comprise polytetrafluoroethylene (PTFE), polyimide, polyetheretherketone (PEEK), polyurethane, nylon, polyethylene, polyamide, or combinations thereof. The outer layer can comprise PTFE, polyimide, PEEK, polyurethane, nylon, polyethylene, polyamide, polyether block amides, polyether block ester copolymer, thermoset silicone, latex, poly-isoprene rubbers, high density polyethylene (HDPE), Tecoflex, or combinations thereof. In one exemplary embodiment, the inner layer can comprise PTFE and the outer layer can comprise a combination of HDPE and Tecoflex. If present, the elastic outer cover can include any suitable materials, such as any suitable heat shrink materials. Examples include Pebax, polyurethane, silicone, and/or polyisoprene. 
     Disclosed embodiments of a sheath comprise a proximal end and a distal end opposite one another. Some embodiments can include a hemostasis valve at or near the proximal end of the sheath. In some embodiments, the outer diameter of the sheath decreases along a gradient from the proximal end to the distal end of the sheath. In other embodiments, the outer diameter of the sheath is substantially constant along at least a majority of the length of the sheath. 
     One embodiment of a sheath for introducing a prosthetic device into a body can comprise a continuous inner layer defining a lumen therethrough, the inner layer having a folded portion and a discontinuous outer layer having an overlapping portion and an underlying portion. In some embodiments, the inner layer can have at least two folded portions. The outer layer can be configured so that the overlapping portion overlaps the underlying portion, wherein at least a portion of the folded portion of the inner tubular layer is positioned between the overlapping and underlying portions. At least a portion of the sheath is configured to expand to accommodate the prosthetic device. 
     In some embodiments, at least a portion of the sheath is configured such that a plurality of segments of the sheath each locally expands one at a time from a rest configuration having a first diameter to an expanded configuration having a second diameter that is larger than the first diameter to facilitate passage of the prosthetic device through the lumen of the inner layer. Each segment can have a length defined along the longitudinal axis of the sheath, and each segment of the sheath can be configured to at least partially return to the first diameter once the prosthetic device has passed through. In some embodiments, when each segment of the sheath is in the expanded configuration, a length of the folded portion corresponding to the length of the segment at least partially unfolds (e.g., by separating and/or straightening). A length of the overlapping portion corresponding to the length of the segment can be configured to move with respect to the underlying portion when each segment of the sheath expands from the rest configuration to the expanded configuration. 
     In one specific embodiment, the inner layer comprises PTFE and the outer layer comprises HDPE and/or Tecoflex. The inner and outer layers can be thermally fused together in some embodiments. In some embodiments, the inner layer comprises a woven fabric and/or braided filaments such as yarn filaments of PTFE, PET, PEEK, and/or nylon. 
     Some disclosed expandable sheaths can further include an elastic outer cover disposed on an external surface of the outer layer. The elastic outer cover can comprise, for example, heat shrink tubing. Some sheaths include one or more radiopaque marker or fillers, such as a C-shaped band positioned between the inner and outer layers near the distal end of the sheath. Some embodiments include a soft tip secured to the distal end of the sheath. 
     In some embodiments, the inner layer can include at least one folded portion and at least one weakened portion. A discontinuous outer layer can have an outer surface and an inner surface and a longitudinal gap, and a portion of the inner layer can extend through the longitudinal gap. The at least one folded portion of the inner layer can be positioned adjacent a portion of the outer surface of the outer layer. In some embodiments, the weakened portion can comprise a score line along at least a portion of the inner layer and/or a slit along at least a portion of the inner layer. The weakened portion can be positioned at the at least one folded portion of the inner layer. In some embodiments, the longitudinal gap can be positioned between a first end and a second end of the outer layer. 
     In some embodiments, an expandable sheath can include a hydrophilic inner liner defining a generally horseshoe-shaped lumen therethrough, the inner liner including at least two weakened portions and an elastic cover positioned radially outward of the inner liner. In some embodiments, when the sheath is in the expanded configuration, the inner liner splits apart at the weakened portions so as to form a discontinuous inner liner. 
     Methods of making a sheath are also disclosed. One method includes providing a mandrel having a first diameter, providing a first tube having a second diameter, the second diameter being larger than the first diameter, mounting the first tube on the mandrel, gathering excess material of the first tube and folding the excess material to one side to form a folded portion of the inner layer. A second tube can then be provided, and the second tube can be cut to form a coiled layer. An adhesive can be applied to at least a portion of the coiled layer and the coiled layer can be mounted on the first tube such that the adhesive is positioned between the first tube and the coiled layer. The folded portion can be lifted in order to position a portion of the coiled layer under the folded portion. 
     Some methods include applying heat to the first tube, coiled layer, and mandrel so as to thermally fuse the first tube and the coiled layer together. In some methods, an elastic outer cover can be secured to the outer surface of the coiled layer. In some methods, a soft tip portion can be coupled to a distal end of the expandable sheath to facilitate passing the expandable sheath through a patient&#39;s vasculature. 
     The foregoing and other features and advantages of the invention will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an elevation view of a sheath according to the present disclosure along with an endovascular delivery apparatus for implanting a prosthetic valve. 
         FIGS. 2A , B, and D are section views of embodiments of a sheath for introducing a prosthetic device into a patient, and  FIG. 2C  is a perspective view of one component of such a sheath. 
         FIG. 3  is an elevation view of the sheath shown in  FIG. 2 . 
         FIGS. 4A-4B  are elevation views of two embodiments of a sheath according to the present disclosure, having varying outer diameters. 
         FIG. 5  illustrates an elevation view of one embodiment of a sheath, expanded at a first location to accommodate a delivery system. 
         FIG. 6  shows an elevation view of the sheath of claim  5 , expanded at a second location, farther down the sheath. 
         FIG. 7  shows a section view of another embodiment of a sheath that further comprises an outer covering or shell. 
         FIG. 8  illustrates an elevation view of one embodiment of a sheath with an outer covering or shell. 
         FIG. 9  illustrates a partial elevation view of one embodiment of an intermediate tubular layer that can be used to construct a sheath according to the present disclosure. 
         FIG. 10  illustrates a partial elevation view of another embodiment of an intermediate tubular layer having a variable diamond design. 
         FIG. 11  illustrates a partial elevation view of another embodiment of an intermediate tubular layer having a diamond design with spring struts. 
         FIG. 12  illustrates a partial elevation view of another embodiment of an intermediate tubular layer having a diamond design with straight struts. 
         FIG. 13  illustrates a partial elevation view of another embodiment of an intermediate tubular layer having a saw tooth design with spring struts. 
         FIG. 14  illustrates a partial elevation view of another embodiment of an intermediate tubular layer having a saw tooth design with straight struts. 
         FIG. 15  illustrates a partial elevation view of another embodiment of an intermediate tubular layer having a diamond design with straight struts. 
         FIG. 16  illustrates a partial elevation view of another embodiment of an intermediate tubular layer having a helical or spiral design. 
         FIG. 17  illustrates a partial elevation view of another embodiment of an intermediate tubular layer having a diamond design with non-straight struts. 
         FIG. 18  illustrates a partial elevation view of another embodiment of an intermediate tubular layer having an alternative diamond design with non-straight struts. 
         FIG. 19  illustrates a partial elevation view of another embodiment of an intermediate tubular layer having yet another diamond design with non-straight struts. 
         FIG. 20  illustrates a partial elevation view of another embodiment of an intermediate tubular layer having a diamond design with struts. 
         FIG. 21  illustrates a partial elevation view of another embodiment of an intermediate tubular layer having a design similar to that shown in  FIG. 20 , but with additional struts. 
         FIG. 22  illustrates a partial elevation view of another embodiment of an intermediate tubular layer having a diamond design with spiral struts. 
         FIG. 23  illustrates a partial elevation view of another embodiment of an intermediate tubular layer having a diamond design with adjacent struts. 
         FIG. 24  illustrates a section view of one embodiment of a sheath having a longitudinal notch. 
         FIG. 25  shows a section view of one embodiment of a sheath having a longitudinal cut in the inner layer. 
         FIG. 26  shows a perspective view of one embodiment of a sheath having a plurality of notches or cuts in the outer tubular layer. 
         FIG. 27A  illustrates a section view of one embodiment of a sheath, wherein the outer tubular layer contains a longitudinal cut, and the inner layer extends into the gap created by the cut in the outer tubular layer, in an unexpanded configuration; and  FIGS. 27B-27E  show section views of various embodiments of a sheath in the unexpanded configuration. 
         FIG. 28  shows a section view of the sheath of  FIG. 27A  in an expanded configuration. 
         FIGS. 29A-29D  show section views of various embodiments of a sheath having overlapping sections. 
         FIG. 30  illustrates a block diagram of one embodiment of a method of making a sheath according to the present disclosure. 
         FIG. 31  illustrates a block diagram of another embodiment of a method of making a sheath according to the present disclosure. 
         FIGS. 32A-32H  illustrates section or elevation views of various method steps of the methods shown in  FIGS. 30-31 . 
         FIG. 33  illustrates a plan view of one embodiment of a sheath having a partial slit or score line. 
         FIG. 34  illustrates a plan view of another embodiment of a sheath having a partial slit or score line. 
         FIG. 35  is an elevation view of an expandable sheath according to the present disclosure and a representative housing. 
         FIG. 36  is an enlarged cutaway view of the distal end of the sheath of  FIG. 35 . 
         FIG. 37  is a section view of the distal end of the sheath of  FIG. 35 , taken along line  37 - 37  in  FIG. 36 . 
         FIG. 38  is a section view of a proximal section of the sheath of  FIG. 35 , taken along line  38 - 38  in  FIG. 35 . 
         FIG. 39  is a section view of the sheath of  FIG. 35  in a rest (unexpanded) configuration, taken along line  39 - 39  in  FIG. 35 . 
         FIG. 40  is the section view of the sheath of  FIG. 39 , in an expanded configuration. 
         FIG. 41  shows an elevation view of an expandable sheath having an elastic outer cover, according to another embodiment. 
         FIG. 42  illustrates a section view of the sheath of  FIG. 41 , taken along line  42 - 42  in  FIG. 41 . 
         FIG. 43  illustrates the section view of the sheath shown in  FIG. 42 , in an expanded configuration. 
         FIG. 44  illustrates a section view of another embodiment of an expandable sheath. 
         FIG. 45  shows an expanded configuration of the sheath of  FIG. 44 . 
         FIG. 46  illustrates a section view of another embodiment of an expandable sheath. 
         FIG. 47  shows an expanded configuration of the sheath of  FIG. 46 . 
         FIG. 48  illustrates a section view of another embodiment of an expandable sheath according to the present disclosure. 
         FIG. 49  illustrates a section view of another embodiment of an expandable sheath. 
     
    
    
     DETAILED DESCRIPTION 
     As used in this application and in the claims, the singular forms “a,” “an,” and “the” include the plural forms unless the context clearly dictates otherwise. Additionally, the term “includes” means “comprises.” Further, the terms “coupled” and “associated” generally means electrically, electromagnetically, and/or physically (e.g., mechanically or chemically) coupled or linked and does not exclude the presence of intermediate elements between the coupled or associated items. 
     Although the operations of exemplary embodiments of the disclosed method may be described in a particular, sequential order for convenient presentation, it should be understood that disclosed embodiments can encompass an order of operations other than the particular, sequential order disclosed. For example, operations described sequentially may in some cases be rearranged or performed concurrently. Further, descriptions and disclosures provided in association with one particular embodiment are not limited to that embodiment, and may be applied to any embodiment disclosed. 
     Moreover, for the sake of simplicity, the attached figures may not show the various ways (readily discernable, based on this disclosure, by one of ordinary skill in the art) in which the disclosed system, method, and apparatus can be used in combination with other systems, methods, and apparatuses. Additionally, the description sometimes uses terms such as “produce” and “provide” to describe the disclosed method. These terms are high-level abstractions of the actual operations that can be performed. The actual operations that correspond to these terms can vary depending on the particular implementation and are, based on this disclosure, readily discernible by one of ordinary skill in the art. 
     Disclosed embodiments of an expandable sheath can minimize trauma to the vessel by allowing for temporary expansion of a portion of the introducer sheath to accommodate the delivery system, followed by a return to the original diameter once the device passes through. Some embodiments can comprise a sheath with a smaller profile (e.g., a smaller diameter in the rest configuration) than that of prior art introducer sheaths. Furthermore, present embodiments can reduce the length of time a procedure takes, as well as reduce the risk of a longitudinal or radial vessel tear, or plaque dislodgement because only one sheath is required, rather than several different sizes of sheaths. Embodiments of the present expandable sheath can avoid the need for multiple insertions for the dilation of the vessel. Such expandable sheaths can be useful for many types of minimally invasive surgery, such as any surgery requiring introduction of an apparatus into a subject&#39;s vessel. For example, the sheath can be used to introduce other types of delivery apparatus for placing various types of intraluminal devices (e.g., stents, prosthetic heart valves, stented grafts, etc.) into many types of vascular and non-vascular body lumens (e.g., veins, arteries, esophagus, ducts of the biliary tree, intestine, urethra, fallopian tube, other endocrine or exocrine ducts, etc.). 
       FIG. 1  illustrates a sheath  8  according to the present disclosure, in use with a representative delivery apparatus  10 , for delivering a prosthetic device  12 , such as a tissue heart valve to a patient. The apparatus  10  can include a steerable guide catheter  14  (also referred to as a flex catheter), a balloon catheter  16  extending through the guide catheter  14 , and a nose catheter  18  extending through the balloon catheter  16 . The guide catheter  14 , the balloon catheter  16 , and the nose catheter  18  in the illustrated embodiment are adapted to slide longitudinally relative to each other to facilitate delivery and positioning of the valve  12  at an implantation site in a patient&#39;s body, as described in detail below. Generally, sheath  8  is inserted into a vessel, such as the transfemoral vessel, passing through the skin of patient, such that the distal end of the sheath  8  is inserted into the vessel. Sheath  8  can include a hemostasis valve at the opposite, proximal end of the sheath. The delivery apparatus  10  can be inserted into the sheath  8 , and the prosthetic device  12  can then be delivered and implanted within patient. 
       FIGS. 2A, 2B, and 2D  show section views of embodiments of a sheath  22  for use with a delivery apparatus such as that shown in  FIG. 1 .  FIG. 2C  shows a perspective view of one embodiment of an inner layer  24  for use with the sheath  22 . Sheath  22  includes an inner layer, such as inner polymeric tubular layer  24 , an outer layer, such as outer polymeric tubular layer  26 , and an intermediate tubular layer  28  disposed between the inner and outer polymeric tubular layers  24 ,  26 . The sheath  22  defines a lumen  30  through which a delivery apparatus can travel into a patient&#39;s vessel in order to deliver, remove, repair, and/or replace a prosthetic device. Such introducer sheaths  22  can also be useful for other types of minimally invasive surgery, such as any surgery requiring introduction of an apparatus into a subject&#39;s vessel. For example, the sheath  22  also can be used to introduce other types of delivery apparatus for placing various types of intraluminal devices (e.g., stents, stented grafts, etc.) into many types of vascular and non-vascular body lumens (e.g., veins, arteries, esophagus, ducts of the biliary tree, intestine, urethra, fallopian tube, other endocrine or exocrine ducts, etc.). 
     The outer polymeric tubular layer  26  and the inner polymeric tubular layer  24  can comprise, for example, PTFE (e.g. Teflon®), polyimide, PEEK, polyurethane, nylon, polyethylene, polyamide, polyether block amides (e.g. PEBAX®), polyether block ester copolymer, polyesters, fluoropolymers, polyvinyl chloride, thermoset silicone, latex, poly-isoprene rubbers, polyolefin, other medical grade polymers, or combinations thereof. The intermediate tubular layer  28  can comprise a shape memory alloy such as Nitinol, and/or stainless steel, cobalt chromium, spectra fiber, polyethylene fiber, aramid fiber, or combinations thereof. 
     The inner polymeric tubular layer  24  can advantageously be provided with a low coefficient of friction on its inner surface. For example, the inner polymeric tubular layer  24  can have a coefficient of friction of less than about 0.1. Some embodiments of a sheath  22  can include a lubricious liner on the inner surface  32  of the inner polymeric tubular layer  24 . Such a liner can facilitate passage of a delivery apparatus through the lumen  30  of the sheath  22 . Examples of suitable lubricious liners include materials that can reduce the coefficient of friction of the inner polymeric tubular layer  24 , such as PTFE, polyethylene, polyvinylidine fluoride, and combinations thereof. Suitable materials for a lubricious liner also include other materials desirably having a coefficient of friction of about 0.1 or less. 
     The inner diameter of the intermediate tubular layer  28  varies depending on the application and size of the delivery apparatus and prosthetic device. In some embodiments, the inner diameter ranges from about 0.005 inches to about 0.400 inches. The thickness of the intermediate tubular layer  28  can be varied depending on the desired amount of radial expansion, as well as the strength required. For example, the thickness of the intermediate tubular layer  28  can be from about 0.002 inches to about 0.025 inches. The thicknesses of the inner polymeric tubular layer  24  and the outer polymeric tubular layer  26  can also be varied depending on the particular application of the sheath  22 . In some embodiments, the thickness of the inner polymeric tubular layer  24  ranges from about 0.0005 inches to about 0.010 inches, and in one particular embodiment, the thickness is about 0.002 inches. Outer polymeric tubular layers  26  can have a thickness of from about 0.002 inches to about 0.015 inches, and in one particular embodiment the outer polymeric tubular layer  26  has a thickness of about 0.010 inches. 
     The hardness of each layer of the sheath  22  can also be varied depending on the particular application and desired properties of the sheath  22 . In some embodiments, the outer polymeric tubular layer  26  has a Shore hardness of from about 25 Durometer to about 75 Durometer. 
     Additionally, some embodiments of a sheath  22  can include an exterior hydrophilic coating on the outer surface  34  of the outer polymeric tubular layer  26 . Such a hydrophilic coating can facilitate insertion of the sheath  22  into a patient&#39;s vessel. Examples of suitable hydrophilic coatings include the Harmony™ Advanced Lubricity Coatings and other Advanced Hydrophilic Coatings available from SurModics, Inc., Eden Prairie, Minn. DSM medical coatings (available from Koninklijke DSM N.V, Heerlen, the Netherlands), as well as other hydrophilic coatings, are also suitable for use with the sheath  22 . 
     In some embodiments, the outer surface  34  of the outer polymeric tubular layer  26  can be modified. For example, surface modifications such as plasma etching can be performed on the outer surface  34 . Similarly, other surfaces, both outer and inner, can be surface modified according to certain embodiments and desired application. In some embodiments, surface modification can improve adhesion between the layers in the areas of the modification. 
     The sheath  22  also can have at least one radiopaque filler or marker. The radiopaque filler or marker can be associated with the outer surface  34  of the outer polymeric tubular layer  26 . Alternatively, the radiopaque filler or marker can be embedded or blended within the outer polymeric tubular layer  24 . Similarly, the radiopaque filler or marker can be associated with a surface of the inner polymeric tubular layer  24  or the intermediate tubular layer  28  or embedded within either or both of those layers. 
     Suitable materials for use as a radiopaque filler or marker include, for example, barium sulfite, bismuth trioxide, titanium dioxide, bismuth subcarbonate, or combinations thereof. The radiopaque filler can be mixed with or embedded in the material used to form the outer polymeric tubular layer  26 , and can comprise from about 5% to about 45% by weight of the outer polymeric tubular layer. More or less radiopaque material can be used in some embodiments, depending on the particular application. 
     In some embodiments, the inner polymeric tubular layer  24  can comprise a substantially uniform cylindrical tube. In alternative embodiments, the inner polymeric tubular layer  24  can have at least one section of discontinuity along its longitudinal axis to facilitate radial expansion of the inner polymeric tubular layer  24 . For example, the inner polymeric tubular layer  24  can be provided with one or more longitudinal notches and/or cuts  36  extending along at least a portion of the length of the sheath  22 . Such notches or cuts  36  can facilitate radial expansion of the inner polymeric tubular layer  24 , thus accommodating passage of a delivery apparatus or other device. Such notches and/or cuts  36  can be provided near the inner surface  32 , near the outer surface  37 , and/or substantially through the entire thickness of the inner polymeric layer  24 . In embodiments with a plurality of notches and/or cuts  36 , such notches and/or cuts  36  can be positioned such that they are substantially equally spaced from one another circumferentially around the inner polymeric layer  24 . Alternatively, notches and cuts  36  can be spaced randomly in relation to one another, or in any other desired pattern. Some or all of any provided notches and/or cuts  36  can extend longitudinally along substantially the entire length of the sheath  22 . Alternatively, some or all of any provided notches and/or cuts  36  can extend longitudinally only along a portion of the length of the sheath  22 . 
     As shown in  FIGS. 2B and 2C  (which illustrates only the inner polymeric tubular layer  24 ), in some embodiments, the inner polymeric tubular layer  24  contains at least one notch or cut  36  that extends longitudinally and parallel to an axis defined by the lumen  30 , extending substantially the entire length of the sheath  22 . Thus, upon introduction of a delivery apparatus, the inner polymeric tubular layer  24  can split open along the notch and/or cut  36  and expand, thus accommodating the delivery apparatus. 
     Additionally or alternatively, as shown in  FIG. 2D , the outer polymeric tubular layer  26  can comprise one or more notches and/or cuts  36 . Notches and/or cuts  36 , in some embodiments, do not extend through the entire thickness of the outer tubular layer  26 . The notches and/or cuts  36  can be separable upon radial expansion of the sheath  22 . The outer polymeric tubular layer  26  can be retractable longitudinally, or able to be pulled back away from the intermediate tubular layer  28  and the inner polymeric tubular layer  24 . In embodiments with a retractable outer polymeric tubular layer  26 , the outer polymeric tubular layer  26  can be retracted to accommodate or facilitate passage of a delivery apparatus through the lumen  30 , and then can be replaced to its original position on the sheath  22 . 
       FIG. 3  illustrates an elevation view of the sheath  22  shown in  FIG. 2A . In this view, only the outer polymeric tubular layer  26  is visible. The sheath  22  comprises a proximal end  38  and a distal end  40  opposite the proximal end  38 . The sheath  22  can include a hemostasis valve inside the lumen of the sheath  22 , at or near the proximal end  38  of the sheath  22 . Additionally, the sheath  22  can comprise a soft tip  42  at the distal end  40  of the sheath  22 . Such a soft tip  42  can be provided with a lower hardness than the other portions of the sheath  22 . In some embodiments, the soft tip  42  can have a Shore hardness from about 25 D to about 40 D. 
     As shown in  FIG. 3 , the unexpanded original outer diameter of the sheath  22  can be substantially constant across the length of the sheath  22 , substantially from the proximal end  38  to the distal end  40 . In alternative embodiments, such as the ones illustrated in  FIGS. 4A-4B , the original unexpanded outer diameter of the sheath  22  can decrease from the proximal end  38  to the distal end  40 . As shown in the embodiment in  FIG. 4A , the original unexpanded outer diameter can decrease along a gradient, from the proximal end  38  to the distal end  40 . In alternative embodiments, such as the one shown in  FIG. 4B , the original unexpanded outer diameter of sheath  22  can incrementally step down along the length of the sheath  22 , wherein the largest original unexpanded outer diameter is near the proximal end  38  and the smallest original unexpanded outer diameter is near the distal end  40  of the sheath  22 . 
     As shown in  FIGS. 5-6 , the sheath  22  can be designed to locally expand as the prosthetic device is passed through the lumen of the sheath  22 , and then substantially return to its original shape once the prosthetic device has passed through that portion of the sheath  22 . For example,  FIG. 5  illustrates a sheath  22  have a localized bulge  44 , representative of a device being passed through the internal lumen of the sheath  22 .  FIG. 5  shows the device close to the proximal end  38  of the sheath  22 , close to the area where the device is introduced into the sheath  22 .  FIG. 6  shows the sheath  22  of  FIG. 5 , with the device having progressed further along the sheath  22 . The localized bulge  44  is now closer to the distal end  40  of the sheath  22 , and thus is about to be introduced to a patient&#39;s vessel. As evident from  FIGS. 5 and 6 , once the localized bulge associated with the device has passed through a portion of the lumen of the sheath  22 , that portion of the sheath  22  can automatically return to its original shape and size, at least in part due to the materials and structure of the sheath  22 . 
     The sheath  22  has an unexpanded inner diameter equal to the inner diameter of the inner polymeric tubular layer (not visible in  FIGS. 5-6 ), and an unexpanded outer diameter  46  equal to the outer diameter of the outer polymeric tubular layer  26 . The sheath  22  is designed to be expanded to an expanded inner diameter and an expanded outer diameter  48  which are larger than the unexpanded inner diameter and the unexpanded outer diameter  46 , respectively. In one representative embodiment, the unexpanded inner diameter is about 16 Fr and the unexpanded outer diameter  46  is about 19 Fr, while the expanded inner diameter is about 26 Fr and the expanded outer diameter  48  is about 29 Fr. Different sheaths  22  can be provided with different expanded and unexpanded inner and outer diameters, depending on the size requirements of the delivery apparatus for various applications. Additionally, some embodiments can provide more or less expansion depending on the particular design parameters, the materials, and/or configurations used. 
     In some embodiments of a sheath according to the present disclosure, and as shown in section in  FIG. 7  and in elevation in  FIG. 8 , the sheath  22  can additionally comprise an outer covering, such as outer polymeric covering  50 , disposed on the outer surface  52  of the outer polymeric tubular layer  26 . The outer polymeric covering  50  can provide a protective covering for the underlying sheath  22 . In some embodiments, the outer polymeric covering  50  can contain a self-expandable sheath in a crimped or constrained state, and then release the self-expandable sheath upon removal of the outer polymeric covering  50 . For example, in some embodiments of a self-expandable sheath, the intermediate layer  28  can comprise Nitinol and/or other shape memory alloys, and the intermediate layer  28  can be crimped or radially compressed to a reduced diameter within the outer polymeric tubular layer  26  and the outer polymeric covering  50 . Once the self-expandable sheath is at least partially inserted into a patient&#39;s vessel, the outer polymeric covering  50  can be slid back, peeled away, or otherwise at least partially removed from the sheath. To facilitate removal of the outer polymeric covering  50 , a portion of the outer polymeric covering  50  can remain outside the patient&#39;s vessel, and that portion can be pulled back or removed from the sheath to allow the sheath to expand. In some embodiments, substantially the entire outer polymeric covering  50  can be inserted, along with the sheath, into a patient&#39;s vessel. In these embodiments, an external mechanism attached to the outer polymeric covering  50  can be provided, such that the outer polymeric covering can be at least partially removed from the sheath once the sheath is inserted into a patient&#39;s vessel. 
     Once no longer constrained by the outer polymeric covering  50 , the radially compressed intermediate layer  28  can self-expand, causing expansion of the sheath along the length of the intermediate layer  28 . In some embodiments, portions of the sheath can radially collapse, at least partially returning to the original crimped state, as the sheath is being withdrawn from the vessel after completion of the surgical procedure. In some embodiments, such collapse can be facilitated and/or encouraged by an additional device or layer that, in some embodiments, can be mounted onto a portion of the sheath prior to the sheath&#39;s insertion into the vessel. 
     The outer polymeric covering  50 , in some embodiments, is not adhered to the other layers of the sheath  22 . For example, the outer polymeric covering  50  may be slidable with respect to the underlying sheath, such that it can be easily removed or retracted from its initial position on the sheath  22 . 
     As seen in  FIG. 8 , the outer polymeric covering  50  can include one or more peel tabs  54  to facilitate manual removal of the outer polymeric covering  50 . The outer polymeric covering  50  can be automatically or manually retractable and/or splittable to facilitate radial expansion of the sheath  22 . Peel tabs  54  can be located approximately 90 degrees from any cut or notch present in the outer polymeric covering  50 , and approximately 180 degrees offset from one another. In alternative embodiments, the peel tabs  54  can extend substantially around the circumference of the outer polymeric covering  50 , thus resulting in a single circular peel tab  54 . 
     Suitable materials for the outer polymeric covering  50  are similar to those materials suitable for the inner polymeric tubular layer and the outer polymeric tubular layer, and can include PTFE and/or high density polyethylene. 
     Turning now to the intermediate tubular layer  28 , several different configurations are possible. The intermediate tubular layer  28  is generally a thin, hollow, substantially cylindrical tube comprising an arrangement, pattern, structure, or configuration of wires or struts, however other geometries can also be used. The intermediate tubular layer  28  can extend along substantially the entire length of the sheath  22 , or alternatively, can extend only along a portion of the length of sheath  22 . Suitable wires can be round, ranging from about 0.0005 inches thick to about 0.10 inches thick, or flat, ranging from about 0.0005 inches×0.003 inches to about 0.003 inches×0.007 inches. However, other geometries and sizes are also suitable for certain embodiments. If braided wire is used, the braid density can be varied. Some embodiments have a braid density of from about thirty picks per inch to about eighty picks per inch and can include up to thirty-two wires in various braid patterns. 
     One representative embodiment of an intermediate tubular layer comprises a braided Nitinol composite which is at least partially encapsulated by an inner polymeric tubular member and an outer polymeric tubular member disposed on inner and outer surfaces of the intermediate tubular layer, respectively. Such encapsulation by polymeric layers can be accomplished by, for example, fusing the polymeric layers to the intermediate tubular layer, or dip coating the intermediate tubular layer. In some embodiments, an inner polymeric tubular member, an intermediate tubular layer, and an outer polymeric tubular layer can be arranged on a mandrel, and the layers can then be thermally fused or melted into one another by placing the assembly in an oven or otherwise heating it. The mandrel can then be removed from the resulting sheath. In other embodiments, dip coating can be used to apply an inner polymeric tubular member to the surface of a mandrel. The intermediate tubular layer can then be applied, and the inner polymeric tubular member allowed to cure. The assembly can then be dip coated again, such as to apply a thin coating of, for example, polyurethane, which will become the outer polymeric tubular member of the sheath. The sheath can then be removed from the mandrel. 
     Additionally, the intermediate tubular layer  28  can be, for example, braided or laser cut to form a pattern or structure, such that the intermediate tubular layer  28  is amenable to radial expansion.  FIGS. 9-23  illustrate partial elevation views of various structures for the intermediate tubular layer. Some illustrated structures, such as those shown in  FIGS. 11-14 and 23 , include at least one discontinuity. For example, the struts  56 ,  58 ,  60 ,  62 ,  64  shown in  FIGS. 11, 12, 13, 14, and 23 , respectively, result in a discontinuous intermediate tubular layer  28  in that the struts  56 ,  58 ,  60 ,  62 ,  64  separate adjacent sections of the intermediate tubular layer  28  from each other, where the sections are spaced apart from each other along a longitudinal axis parallel to the lumen of the sheath. Thus, the structure of the intermediate tubular layer  28  can vary from section to section, changing along the length of the sheath. 
     The structures shown in  FIGS. 9-23  are not necessarily drawn to scale. Components and elements of the structures can be used alone or in combination within a single intermediate tubular layer  28 . The scope of the intermediate tubular layer  28  is not meant to be limited to these particular structures; they are merely exemplary embodiments. 
     Alternative embodiments of a sheath for introducing a prosthetic device are also described. For example,  FIGS. 24-26  illustrate a section view and a perspective view, respectively, of a sheath  66  for introducing a prosthetic device into a body. The sheath  66  comprises an inner layer, such as inner polymeric layer  68 , an outer layer, such as polymeric tubular layer  70 , and a hemostasis valve (not shown). The inner polymeric layer  68  and the outer polymeric tubular layer  70  at least partially enclose a lumen  72 , through which a delivery apparatus and prosthetic device can pass from outside the patient&#39;s body into the patient&#39;s vessel. Either or both of the inner polymeric layer  68  and the outer polymeric layer  70  can be provided with at least one longitudinal notch and/or cut to facilitate radial expansion of the sheath. 
     For example,  FIG. 24  illustrates a longitudinal notch  74  in the inner polymeric layer  68  that can facilitate radial expansion of the sheath  66 . The longitudinal notch  74  can separate or split open completely upon application of a radial force due to insertion of a delivery apparatus or prosthetic device. Similarly,  FIG. 25  illustrates a longitudinal cut  76  in the inner polymeric layer  68  that can also facilitate radial expansion of the sheath  66 . The outer polymeric layer  70  can, additionally or alternatively, comprise one or more longitudinal cuts  76  or notches  74 . Such cuts and/or notches, whether in the inner polymeric layer  68  or the outer polymeric layer  70 , can extend substantially through the entire thickness of the layer, or can extend only partially through the thickness of the layer. The cuts and/or notches can be positioned at or near the inner or outer surface, or both surfaces, of the inner and/or outer polymeric layers  68 ,  70 . 
       FIG. 26  illustrates a perspective view of one embodiment of an inner polymeric layer  68  with longitudinal notches  74  and a longitudinal cut  76 . More or fewer notches  74  and/or cuts  76  can be provided. For clarity, the outer polymeric layer  70  is not shown in  FIG. 26 . As shown in  FIG. 26 , longitudinal notches  74  and/or cuts  76  can extend only along a portion of the length of sheath  66 . In alternative embodiments, one or more notches  74  and/or cuts  76  can extend substantially along the entire length of the sheath  66 . Additionally, notches  74  and/or cuts  76  can be positioned randomly or patterned. 
     One particular embodiment of a sheath  66  comprises a sheath having a notch or cut in the outer polymeric layer  70  or the inner polymeric layer  68  that extends longitudinally along approximately 75% of the length of the sheath  66 . If such a notch or cut extends only partially through the associated layer, it can have a relatively low tear force, such as a tear force of about 0.5 lbs., so that the notch splits open relatively easily during use. 
     The inner polymeric layer  68  and the outer polymeric layer  70  can optionally be adhered together or otherwise physically associated with one another. The amount of adhesion between the inner polymeric layer  68  and the outer polymeric layer  70  can be variable over the surfaces of the layers. For example, little to no adhesion can be present at areas around or near any notches and/or cuts present in the layers, so as not to hinder radial expansion of the sheath  66 . Adhesion between the layers can be created by, for example, thermal bonding and/or coatings. Embodiments of a sheath  66  can be formed from an extruded tube, which can serve as the inner polymeric layer  68 . The inner polymeric layer  68  can be surface treated, such as by plasma etching, chemical etching or other suitable methods of surface treatment. By treating the surface of the inner polymeric layer  68 , the outer surface of the inner polymeric layer  68  can have areas with altered surface angles that can provide better adhesion between the inner polymeric layer  68  and the outer polymeric layer  70 . The treated inner polymeric layer can be dip coated in, for example, a polyurethane solution to form the outer polymeric layer  70 . In some configurations, the polyurethane may not adhere well to untreated surface areas of the inner polymeric layer  68 . Thus, by surface treating only surface areas of the inner polymeric layer  68  that are spaced away from the areas of expansion (e.g. the portion of the inner polymeric layer  68  near notches  74  and/or cuts  76 ), the outer polymeric layer  70  can be adhered to some areas of the inner polymeric layer  68 , while other areas of the inner polymeric layer  68  remain free to slide relative to the outer polymeric layer  70 , thus allowing for expansion of the diameter of the sheath  66 . Thus, areas around or near any notches  74  and/or cuts  76  can experience little to no adhesion between the layers, while other areas of the inner and outer polymeric layers  68 ,  70  can be adhesively secured or otherwise physically associated with each other. 
     As with previously disclosed embodiments, the embodiments illustrated in  FIGS. 24-26  can be applied to sheaths having a wide variety of inner and outer diameters. Applications can utilize a sheath of the present disclosure with an inner diameter of the inner polymeric layer  68  that is expandable to an expanded diameter of from about 3 Fr to about 26 Fr. The expanded diameter can vary slightly along the length of the sheath  66 . For example, the expanded outer diameter at the proximal end of the sheath  66  can range from about 3 Fr to about 28 Fr, while the expanded outer diameter at the distal end of the sheath  66  can range from about 3 Fr to about 25 Fr. Embodiments of a sheath  66  can expand to an expanded outer diameter that is from about 10% greater than the original unexpanded outer diameter to about 100% greater than the original unexpanded outer diameter. 
     In some embodiments, the outer diameter of the sheath  66  gradually decreases from the proximal end of the sheath  66  to the distal end of the sheath  66 . For example, in one embodiment, the outer diameter can gradually decrease from about 26 Fr at the proximal end to about 18 Fr at the distal end. The diameter of the sheath  66  can transition gradually across substantially the entire length of the sheath  66 . In other embodiments, the transition or reduction of the diameter of the sheath  66  can occur only along a portion of the length of the sheath  66 . For example, the transition can occur along a length from the proximal end to the distal end, where the length can range from about 0.5 inches to about the entire length of sheath  66 . 
     Suitable materials for the inner polymeric layer  68  can have a high elastic strength and include materials discussed in connection with other embodiments, especially Teflon (PTFE), polyethylene (e.g. high density polyethylene), fluoropolymers, or combinations thereof. In some embodiments, the inner polymeric layer  68  preferably has a low coefficient of friction, such as a coefficient of friction of from about 0.01 to about 0.5. Some preferred embodiments of a sheath  66  comprise an inner polymeric layer  68  having a coefficient of friction of about 0.1 or less. 
     Likewise, suitable materials for the outer polymeric layer  70  include materials discussed in connection with other embodiments, and other thermoplastic elastomers and/or highly elastic materials. 
     The Shore hardness of the outer polymeric layer  70  can be varied for different applications and embodiments. Some embodiments include an outer polymeric layer with a Shore hardness of from about 25 A to about 80 A, or from about 20 D to about 40 D. One particular embodiment comprises a readily available polyurethane with a Shore hardness of 72 A. Another particular embodiment comprises a polyethylene inner polymeric layer dipped in polyurethane or silicone to create the outer polymeric layer. 
     The sheath  66  can also include a radiopaque filler or marker as described above. In some embodiments, a distinct radiopaque marker or band can be applied to some portion of the sheath  66 . For example, a radiopaque marker can be coupled to the inner polymeric layer  68 , the outer polymeric layer  70 , and/or can be positioned in between the inner and outer polymeric layers  68 ,  70 . 
       FIGS. 27A-27E and 28  illustrate section views of various embodiments of unexpanded ( FIGS. 27A-27E ) and expanded ( FIG. 28 ) sheaths  66  according to the present disclosure. The sheath  66  includes a split outer polymeric tubular layer  70  having a longitudinal cut  76  through the thickness of the outer polymeric tubular layer  70  such that the outer polymeric tubular layer  70  comprises a first portion  78  and a second portion  80  separable from one another along the cut  76 . An expandable inner polymeric layer  68  is associated with an inner surface  82  of the outer polymeric tubular layer  70 , and, in the unexpanded configuration shown in  FIG. 27A , a portion of the inner polymeric layer  68  extends through a gap created by the cut  76  and can be compressed between the first and second portions  78 ,  80  of the outer polymeric tubular layer  70 . Upon expansion of the sheath  66 , as shown in  FIG. 28 , first and second portions  78 ,  80  of the outer polymeric tubular layer  70  have separated from one another, and the inner polymeric layer  68  is expanded to a substantially cylindrical tube. In some embodiments, two or more longitudinal cuts  76  may be provided through the thickness of the outer polymeric tubular layer  70 . In such embodiments, a portion of the inner polymeric layer  68  may extend through each of the longitudinal cuts  76  provided in the outer polymeric tubular layer  70 . 
     Preferably, the inner polymeric layer  68  comprises one or more materials that are elastic and amenable to folding and/or pleating. For example,  FIG. 27A  illustrates an inner polymeric layer  68  with folded regions  85 . As seen in  FIGS. 27A-27E , the sheath  66  can be provided with one or more folded regions  85 . Such folded regions  85  can be provided along a radial direction and substantially conform to the circumference of the outer polymeric tubular layer  70 . At least a portion of the folded regions  85  can be positioned adjacent the outer surface  83  of the outer polymeric tubular layer  70 . Additionally, as shown in  FIGS. 27B and 27E , at least a portion of the folded region or regions  85  can be overlapped by an outer covering, such as outer polymeric covering  81 . The outer polymeric covering  81  can be adjacent at least a portion of the outer surface  83  of the outer polymeric tubular layer  70 . The outer polymeric covering  81  serves to at least partially contain the folded regions  85  of the inner polymeric layer  68 , and can also prevent the folded regions  85  from separating from the outer polymeric tubular layer  70  when, for example, the sheath  66  undergoes bending. In some embodiments, the outer polymeric covering  81  can be at least partially adhered to the outer surface  83  of the outer polymeric tubular layer  70 . The outer polymeric covering  81  can also increase the stiffness and/or durability of the sheath  66 . Additionally, as shown in  FIGS. 27B and 27E , the outer polymeric covering  81  may not entirely overlap the circumference of the sheath  66 . For example, the outer polymeric covering  81  may be provided with first and second ends, where the ends do not contact one another. In these embodiments, only a portion of the folded region  85  of the inner polymeric layer  68  is overlapped by the outer polymeric covering  81 . 
     In embodiments having a plurality of folded regions  85 , the regions can be equally displaced from each other around the circumference of the outer polymeric tubular layer  70 . Alternatively, the folded regions can be off-center, different sizes, and/or randomly spaced apart from each other. While portions of the inner polymeric layer  68  and the outer tubular layer  70  can be adhered or otherwise coupled to one another, the folded regions  85  preferably are not adhered or coupled to the outer tubular layer  70 . For example, adhesion between the inner polymeric layer  68  and the outer tubular layer  70  can be highest in areas of minimal expansion. 
     One particular embodiment of the sheath illustrated in  FIGS. 27A-28  comprises a polyethylene (e.g. high density polyethylene) outer polymeric tubular layer  70  and a PTFE inner polymeric layer  68 . However, other materials are suitable for each layer, as described above. Generally, suitable materials for use with the outer polymeric tubular layer  70  include materials having a high stiffness or modulus of strength that can support expansion and contraction of the inner polymeric layer  68 . 
     In some embodiments, the outer polymeric tubular layer  70  comprises the same material or combination of materials along the entire length of the outer polymeric tubular layer  70 . In alternative embodiments, the material composition can change along the length of the outer polymeric tubular layer  70 . For example, the outer polymeric tubular layer can be provided with one or more segments, where the composition changes from segment to segment. In one particular embodiment, the Durometer rating of the composition changes along the length of the outer polymeric tubular layer  70  such that segments near the proximal end comprise a stiffer material or combination of materials, while segments near the distal end comprise a softer material or combination of materials. This can allow for a sheath  66  having a relatively stiff proximal end at the point of introducing a delivery apparatus, while still having a relatively soft distal tip at the point of entry into the patient&#39;s vessel. 
     As with other disclosed embodiments, the embodiments of sheath  66  shown in  FIGS. 27A-28  can be provided in a wide range of sizes and dimensions. For example, the sheath  66  can be provided with an unexpanded inner diameter of from about 3 Fr to about 26 Fr. In some embodiments, the sheath  66  has an unexpanded inner diameter of from about 15 Fr to about 16 Fr. In some embodiments, the unexpanded inner diameter of the sheath  66  can range from about 3 Fr to about 26 Fr at or near the distal end of sheath  66 , while the unexpanded inner diameter of the sheath  66  can range from about 3 Fr to about 28 Fr at or near the proximal end of sheath  66 . For example, in one unexpanded embodiment, the sheath  66  can transition from an unexpanded inner diameter of about 16 Fr at or near the distal end of the sheath  66  to an unexpanded inner diameter of about 26 Fr at or near the proximal end of the sheath  66 . 
     The sheath  66  can be provided with an unexpanded outer diameter of from about 3 Fr to about 30 Fr, and, in some embodiments has an unexpanded outer diameter of from about 18 Fr to about 19 Fr. In some embodiments, the unexpanded outer diameter of the sheath  66  can range from about 3 Fr to about 28 Fr at or near the distal end of sheath  66 , while the unexpanded outer diameter of the sheath  66  can range from about 3 Fr to about 30 Fr at or near the proximal end of sheath  66 . For example, in one unexpanded embodiment, the sheath  66  can transition from an unexpanded outer diameter of about 18 Fr at or near the distal end of the sheath  66  to an unexpanded outer diameter of about 28 Fr at or near the proximal end of the sheath  66 . 
     The thickness of the inner polymeric layer  68  can vary, but in some preferred embodiments is from about 0.002 inches to about 0.015 inches. In some embodiments, expansion of the sheath  66  can result in expansion of the unexpanded outer diameter of from about 10% or less to about 430% or more. 
     As with other illustrated and described embodiments, the embodiments shown in  FIGS. 27A-28  can be provided with a radiopaque filler and/or a radiopaque tip marker as described above. The sheath  66  can be provided with a radiopaque tip marker provided at or near the distal tip of the sheath  66 . Such a radiopaque tip marker can comprise materials such as those suitable for the radiopaque filler, platinum, iridium, platinum/iridium alloys, stainless steel, other biocompatible metals, or combinations thereof. 
       FIGS. 29A-29D  show section views of other possible configurations of a sheath  66  for introducing a prosthetic device into a patient&#39;s vasculature. The sheath  66  comprises a polymeric tubular layer  84  having an inner surface  86  and an outer surface  88 . The thickness of the polymeric tubular layer  84  extends from the inner surface  86  to the outer surface  88 . As shown in  FIGS. 29B-29D , the polymeric tubular layer  84  can be formed with at least a first angular portion  90  of reduced thickness adjacent the inner surface  86  and a second angular portion  92  of reduced thickness adjacent the outer surface  88 , with the second portion  92  at least partially overlapping the first portion  90 .  FIG. 29A  illustrates a similar configuration, where a second portion  92  at least partially overlaps a first portion  90  in a partial coil configuration. In the embodiment of  FIG. 29A , the second portion  92  and the first portion  90  can have the same thickness. 
     In preferred embodiments, the first and second portions  90 ,  92  are not adhered to one another. In some embodiments, and best seen in  FIG. 29A , there can be a small gap  94  between the first and second portions  90 ,  92  that can give the sheath  66  the appearance of having two interior lumens  72 ,  94 .  FIGS. 29A-29D  illustrate the sheath  66  in unexpanded configurations. Preferably, upon expansion of the sheath  66 , the ends of the first and second portions  90 ,  92  abut or are in close proximity to each other to reduce or eliminate any gap between them. 
     In some embodiments, a sheath  66  can comprise a partial slit or score line along at least a portion of its length. For example, as shown in  FIG. 33 , a sheath  66  can comprise an outer polymeric tubular layer  70  over an inner polymeric layer  68 . The inner polymeric layer can extend through a cut in the outer polymeric tubular layer  70 , to form a folded region  85  on the outer surface of the outer polymeric tubular layer  70 , such as also shown in  FIG. 27C . The folded region  85  of the inner layer, in some embodiments, terminates before the outer polymeric tubular layer  70  (i.e. the outer polymeric tubular layer  70  is longer than the inner layer). As shown in  FIG. 33 , in these embodiments, the sheath  66  can comprise a partial slit or score line  77  that can extend from the termination (distal end)  75  of the folded region  85  to the distal end  40  of the sheath  66 . In some embodiments, score line  77  can facilitate expansion of the sheath  66 . 
     Score line  77  can be substantially centrally located with respect to the folded region  85 . In alternative embodiments, score line  77  can be positioned in other locations relative to the folded region  85 . Also, sheath  66  can comprise one or more score lines  77 . For example, as shown in  FIG. 34 , one or more score lines  77  can be peripherally located with respect to the folded region  85 . The one or more score lines  77  can be positioned anywhere around the circumference of the outer polymeric tubular layer  70 . In embodiments comprising a radiopaque marker  69  as seen in  FIG. 33 , a score line  77  can extend from, for example, the distal end of the radiopaque marker  69  substantially to the distal end  40  of the sheath  66 . 
       FIGS. 35 and 36  illustrate an expandable sheath  100  according to the present disclosure, which can be used with a delivery apparatus for delivering a prosthetic device, such as a tissue heart valve into a patient. In general, the delivery apparatus can include a steerable guide catheter (also referred to as a flex catheter), a balloon catheter extending through the guide catheter, and a nose catheter extending through the balloon catheter (e.g., as depicted in  FIG. 1 ). The guide catheter, the balloon catheter, and the nose catheter can be adapted to slide longitudinally relative to each other to facilitate delivery and positioning of the valve at an implantation site in a patient&#39;s body. However, it should be noted that the sheath  100  can be used with any type of elongated delivery apparatus used for implanting balloon-expandable prosthetic valves, self-expanding prosthetic valves, and other prosthetic devices. Generally, sheath  100  can be inserted into a vessel (e.g., the femoral or iliac arteries) by passing through the skin of patient, such that a soft tip portion  102  at the distal end  104  of the sheath  100  is inserted into the vessel. The sheath  100  can also include a proximal flared end portion  114  to facilitate mating with an introducer housing  101  and catheters mentioned above (e.g., the proximal flared end portion  114  can provide a compression fit over the housing tip and/or the proximal flared end portion  114  can be secured to the housing  101  via a nut or other fastening device or by bonding the proximal end of the sheath to the housing). The introducer housing  101  can house one or more valves that form a seal around the outer surface of the delivery apparatus once inserted through the housing, as known in the art. The delivery apparatus can be inserted into and through the sheath  100 , allowing the prosthetic device to be advanced through the patient&#39;s vasculature and implanted within the patient. 
     Sheath  100  can include a plurality of layers. For example, sheath  100  can include an inner layer  108  and an outer layer  110  disposed around the inner layer  108 . The inner layer  108  can define a lumen through which a delivery apparatus can travel into a patient&#39;s vessel in order to deliver, remove, repair, and/or replace a prosthetic device, moving in a direction along the longitudinal axis X. As the prosthetic device passes through the sheath  100 , the sheath locally expands from a first, resting diameter to a second, expanded diameter to accommodate the prosthetic device. After the prosthetic device passes through a particular location of the sheath  100 , each successive expanded portion or segment of the sheath  100  at least partially returns to the smaller, resting diameter. In this manner, the sheath  100  can be considered self-expanding, in that it does not require use of a balloon, dilator, and/or obturator to expand. 
     The inner and outer layers  108 ,  110  can comprise any suitable materials. Suitable materials for the inner layer  108  include polytetrafluoroethylene (PTFE), ethylene tetrafluoroethylene (ETFE), nylon, polyethylene, polyether block amide (e.g., Pebax), and/or combinations thereof. In one specific embodiment the inner layer  108  can comprise a lubricious, low-friction, or hydrophilic material, such as PTFE. Such low coefficient of friction materials can facilitate passage of the prosthetic device through the lumen defined by the inner layer  108 . In some embodiments, the inner layer  108  can have a coefficient of friction of less than about 0.1. Some embodiments of a sheath  100  can include a lubricious liner on the inner surface of the inner layer  108 . Examples of suitable lubricious liners include materials that can further reduce the coefficient of friction of the inner layer  108 , such as PTFE, polyethylene, polyvinylidine fluoride, and combinations thereof. Suitable materials for a lubricious liner also include other materials desirably having a coefficient of friction of about 0.1 or less. 
     Suitable materials for the outer layer  110  include nylon, polyethylene, Pebax, HDPE, polyurethanes (e.g., Tecoflex), and other medical grade materials. In one embodiment, the outer layer  110  can comprise high density polyethylene (HDPE) and Tecoflex (or other polyurethane material) extruded as a composite. In some embodiments, the Tecoflex can act as an adhesive between the inner layer  108  and the outer layer  110  and may only be present along a portion of the inner surface of the outer layer  110 . Other suitable materials for the inner and outer layers are also disclosed in U.S. Patent Application Publication No. 2010/0094392, which is incorporated herein by reference. 
     Additionally, some embodiments of a sheath  100  can include an exterior hydrophilic coating on the outer surface of the outer layer  110 . Such a hydrophilic coating can facilitate insertion of the sheath  100  into a patient&#39;s vessel. Examples of suitable hydrophilic coatings include the Harmony™ Advanced Lubricity Coatings and other Advanced Hydrophilic Coatings available from SurModics, Inc., Eden Prairie, Minn. DSM medical coatings (available from Koninklijke DSM N.V, Heerlen, the Netherlands), as well as other hydrophilic coatings (e.g., PTFE, polyethylene, polyvinylidine fluoride), are also suitable for use with the sheath  100 . 
     Best seen in  FIG. 36 , the soft tip portion  102  can comprise, in some embodiments, low density polyethylene (LDPE) and can be configured to minimize trauma or damage to the patient&#39;s vessels as the sheath is navigated through the vasculature. For example, in some embodiments, the soft tip portion  102  can be slightly tapered to facilitate passage through the vessels. The soft tip portion  102  can be secured to the distal end  104  of the sheath  100 , such as by thermally bonding the soft tip portion  102  to the inner and outer layers of the sheath  100 . Such a soft tip portion  102  can be provided with a lower hardness than the other portions of the sheath  100 . In some embodiments, the soft tip  102  can have a Shore hardness from about 25 D to about 40 D. The tip portion  102  is configured to be radially expandable to allow a prosthetic device to pass through the distal opening of the sheath  100 . For example, the tip portion  102  can be formed with a weakened portion, such as an axially extending score line or perforated line that is configured to split and allow the tip portion to expand radially when the prosthetic device passes through the tip portion (such as shown in the embodiments of  FIGS. 33 and 34 ). 
       FIG. 37  shows a cross-section view of the sheath  100  taken near the distal end  104  of the sheath  100 . As shown in  FIGS. 36 and 37 , the sheath  100  can include at least one radiopaque filler or marker, such as a discontinuous, or C-shaped, band  112  positioned near the distal end  104  of the sheath  100 . The marker  112  can be associated with the inner and/or outer layers  108 ,  110  of the sheath  100 . For example, as shown in  FIG. 37 , the marker  112  can be positioned between the inner layer  108  and the outer layer  110 . In alternative embodiments, the marker  112  can be associated with the outer surface of the outer layer  110 . In some embodiments, the marker  112  can be embedded or blended within the inner or outer layers  108 ,  110 . 
     The C-shaped band  112  can serve as a radiopaque marker or filler, to enable visibility of the sheath  100  under fluoroscopy during use within a patient. The C-shaped band  112  can comprise any suitable radiopaque material, such as barium sulfite, bismuth trioxide, titanium dioxide, bismuth subcarbonate, platinum, iridium, and combinations thereof. In one specific embodiment, the C-shaped band can comprise 90% platinum and 10% iridium. In other embodiments, the marker  112  need not be a C-shaped band. Other shapes, designs, and configurations are possible. For example, in some embodiments, the marker  112  can extend around the entire circumference of the sheath  100 . In other embodiments, the marker  112  can comprise a plurality of small markers spaced around the sheath  100 . 
       FIGS. 38 and 39  show additional cross sections taken at different points along the sheath  100 .  FIG. 38  shows a cross-section of a segment of the sheath near the proximal end  106  of the sheath  100 , as indicated by line  38 - 38  in  FIG. 35 . The sheath  100  at this location can include inner layer  108  and outer layer  110 . At this location, near the proximal end of the sheath, the layers  108 ,  110  can be substantially tubular, without any slits or folded portions in the layers. By contrast, the layers  108 ,  110  at different locations along the sheath  100  (e.g., at the point indicated by line  39 - 39  in  FIG. 35 ) can have a different configuration. 
     As shown in  FIG. 39 , the inner layer  108  can be arranged to form a substantially cylindrical lumen  116  therethrough. Inner layer  108  can include one or more folded portions  118 . In the embodiment shown in  FIG. 39 , inner layer  108  is arranged to have one folded portion  118  that can be positioned on either side of the inner layer  108 . Inner layer  108  can be continuous, in that there are no breaks, slits, or perforations in inner layer  108 . Outer layer  110  can be arranged in an overlapping fashion such that an overlapping portion  120  overlaps at least a part of the folded portion  118  of the inner layer  108 . As shown in  FIG. 39 , the overlapping portion  120  also overlaps an underlying portion  122  of the outer layer  110 . The underlying portion  122  can be positioned to underlie both the overlapping portion  120  of the outer layer  110 , as well as the folded portion  118  of the inner layer  108 . Thus, the outer layer  110  can be discontinuous, in that it includes a slit or a cut in order to form the overlapping and underlying portions  120 ,  122 . In other words, a first edge  124  of the outer layer  110  is spaced apart from a second edge  126  of the outer layer  110  so as not to form a continuous layer. 
     As shown in  FIG. 39 , the sheath  100  can also include a thin layer of bonding or adhesive material  128  positioned between the inner and outer layers  108 ,  110 . In one embodiment, the adhesive material  128  can comprise a polyurethane material such as Tecoflex. The adhesive material  128  can be positioned on an inner surface  130  of at least a portion of the outer layer  110  so as to provide adhesion between selected portions of the inner and outer layers  108 ,  110 . For example, the outer layer  110  may only include a Tecoflex layer  128  around the portion of the inner surface  130  that faces the lumen-forming portion of the inner layer  108 . In other words, the Tecoflex layer  128  can be positioned so that it does not contact the folded portion  118  of the inner layer  108  in some embodiments. In other embodiments, the Tecoflex layer  128  can be positioned in different configurations as desired for the particular application. For example, as shown in  FIG. 39 , the Tecoflex layer  128  can be positioned along the entire inner surface  130  of the outer layer  110 . In an alternative embodiment, the Tecoflex layer can be applied to the outer surface of the inner liner  108  instead of the inner surface of the outer layer. The Tecoflex layer can be applied to all or selected portions on the inner layer; for example, the Tecoflex layer can be formed only on the portion of the inner layer that faces the lumen-forming portion of the outer layer and not on the folded portion. The configuration of  FIG. 39  allows for radial expansion of the sheath  100  as an outwardly directed radial force is applied from within (e.g., by passing a medical device such as a prosthetic heart valve through the lumen  116 ). As radial force is applied, the folded portion  118  can at least partially separate, straighten, and/or unfold, and/or the overlapping portion  120  and the underlying portion  122  of the outer layer  110  can slide circumferentially with respect to one another, thereby allowing the diameter of lumen  116  to enlarge. 
     In this manner, the sheath  100  is configured to expand from a resting configuration ( FIG. 39 ) to an expanded configuration shown in  FIG. 40 . In the expanded configuration, as shown in  FIG. 40 , an annular gap  132  can form between the longitudinal edges of the overlapping portion  120  and the underlying portion  122  of the outer layer  110 . As the sheath  100  expands at a particular location, the overlapping portion  120  of the outer layer  110  can move circumferentially with respect to the underlying portion  122  as the folded portion  118  of the inner layer  108  unfolds. This movement can be facilitated by the use of a low-friction material for inner layer  108 , such as PTFE. Further, the folded portion  118  can at least partially separate and/or unfold to accommodate a medical device having a diameter larger than that of lumen  116  in the resting configuration. As shown in  FIG. 40 , in some embodiments, the folded portion of the inner layer  108  can completely unfold, so that the inner layer  108  forms a cylindrical tube at the location of the expanded configuration. 
     The sheath  100  can be configured such that it locally expands at a particular location corresponding to the location of the medical device along the length of the lumen  116 , and then locally contracts once the medical device has passed that particular location. Thus, a bulge may be visible, traveling longitudinally along the length of the sheath as a medical device is introduced through the sheath, representing continuous local expansion and contraction as the device travels the length of the sheath  100 . In some embodiments, each segment of the sheath  100  can locally contract after removal of any radial outward force such that it regains the original resting diameter of lumen  116 . In some embodiments, each segment of the sheath  100  can locally contract after removal of any radial outward force such that it at least partially returns to the original resting diameter of lumen  116 . 
     The layers  108 ,  110  of sheath  100  can be configured as shown in  FIG. 39  along at least a portion of the length of the sheath  100 . In some embodiments, the layers  108 ,  110  can be configured as shown in  FIG. 39  along the length A ( FIG. 35 ) extending from a location adjacent the soft tip portion  102  to a location closer to the proximal end  106  of the sheath  100 . In this matter, the sheath is expandable and contractable only along a portion of the length of the sheath corresponding to length A (which typically corresponds to the section of the sheath inserted into the narrowest section of the patient&#39;s vasculature). 
       FIGS. 41-49  illustrate additional embodiments and variations on the general sheath  100  described above. It is to be understood that the variations (e.g., materials and alternate configurations) described above with reference to  FIGS. 35-40  can also apply to the embodiments shown in  FIGS. 41-49 . Furthermore, the variations described below with reference to  FIGS. 41-49  can also be applied to the sheath described in  FIGS. 35-40 . 
       FIGS. 41-43  illustrate a sheath  700  that additionally includes a strain relief cover, also referred to as an elastic outer cover, or an elastic cover  702  positioned around at least a part of an inner layer  704  and outer layer  706 . As shown in  FIG. 41 , the elastic cover  702  can extend for a length L along at least a portion of the main body of the sheath  700 . In some embodiments, the elastic cover  702  can extend from the proximal end  708  of the sheath  700  and towards the distal end  709  of the sheath. In some embodiments, the elastic cover  702  extends only part way down the length of the sheath  700 . In alternate embodiments, the elastic cover  702  can extend to a point adjacent the distal end  709 , or can extend all the way to the distal end  709  of sheath  700 . Furthermore, the elastic outer cover  702  need not extend all the way to the proximal end  708  of the sheath  700 . In some embodiments, the elastic outer cover  702  may extend only part way towards the proximal end  708 . In some embodiments, the longitudinal length L of the elastic cover  702  can range from about 10 cm to the entire length of the sheath  700 . 
     As shown in  FIGS. 42 and 43 , the elastic cover  702  can be a continuous tubular layer, without slits or other discontinuities. The elastic cover  702  can be positioned to surround the entire circumference of outer layer  706 , and can extend longitudinally along any portion of the length of the sheath  700 . The elastic outer cover  702  can comprise any pliable, elastic material(s) that expand and contract, preferably with a high expansion ratio. Preferably, the materials used can include low durometer polymers with high elasticity, such as Pebax, polyurethane, silicone, and/or polyisoprene. Materials for the elastic outer cover  702  can be selected such that it does not impede expansion of the sheath  700 . In fact, the elastic outer cover  702  can stretch and expand as the sheath  700  expands, such as by movement of the folded or scored inner liner with respect to itself. 
     The elastic outer cover  702  can, in some embodiments, provide hemostasis (e.g., prevent blood loss during implantation of the prosthetic device). For example, the elastic outer cover  702  can be sized or configured to form a seal with the patient&#39;s artery when inserted, such that blood is substantially prevented from flowing between the elastic outer cover  702  and the vessel wall. The elastic outer cover  702  can be inserted such that it passes the arteriotomy. For example, in embodiments where the elastic outer cover  702  does not extend all the way to the distal end  709  of the sheath  700 , the elastic cover  702  can extend distally far enough such that when the sheath  700  is fully inserted into the patient, at least part of the elastic outer cover extends through the ateriotomoy site. 
     The elastic outer cover can have a thickness ranging from, for example, about 0.001″ to about 0.010.″ In some embodiments, the outer cover can have a thickness of from about 0.003″ to about 0.006.″ The elastic outer over can be configured to expand as the sheath expands, as shown in the expanded configuration in  FIG. 43 . 
       FIG. 42  shows a cross-section of the sheath  700  in a resting configuration having an inner diameter D 1 .  FIG. 43  shows a cross-section of the sheath  700  in an expanded configuration, having an inner diameter D 2 , where D 2  is greater than D 1 . Similar to the embodiment of  FIGS. 35-40 , the sheath  700  can include an inner layer  704  having a folded portion  710 , and an outer layer  706  having an overlapping portion  712  and an underlying portion  714 . The overlapping portion  712  overlaps at least a portion of the folded portion  710  of the inner layer, and the underlying portion  714  underlies at least a portion of the folded portion  710 . As shown in  FIGS. 42-43 , in some embodiments, the overlapping portion  712  does not overlap the entire folded portion  710  of the inner layer  704 , and thus a portion of the folded portion  710  can be directly adjacent to the elastic outer cover  702  in locations where the elastic cover  702  is present. In locations where the elastic cover  702  is not present, part of the folded portion  710  may be visible from the outside of the sheath  700 , as seen in  FIG. 41 . In these embodiments, the sheath  700  can include a longitudinal seam  722  where the overlapping portion  712  terminates at the folded portion  710 . In use, the sheath can be positioned such that the seam  722  is posterior to the point of the sheath that is 180 degrees from the seam  722  (e.g., facing downward in the view of  FIG. 41 ). The seam  722  can also be seen in  FIG. 41 , which shows that the seam  722  need not extend the entire length of the sheath. In some embodiments, the proximal end portion of the sheath includes two layers without a folded portion (e.g., similar to  FIG. 38 ) while the distal end portion of the sheath includes two layers with a folded portion (e.g., similar to  FIG. 39 ). In some embodiments, the seam  722  can end at a transition point between portions of the sheath having a folded inner layer and portions of the sheath not having a folded inner layer. 
     In some embodiments, the folded portion  710  can include a weakened portion, such as a longitudinal perforation, score line, and/or slit  716  along at least a portion of the length of the inner layer  704 . The slit  716  can allow for two adjacent ends  718 ,  720  of the folded portion  710  to move relative to one another as the sheath  700  expands to the expanded configuration shown in  FIG. 43 . As a device having an outer diameter device larger than the initial resting inner diameter of the sheath  700  is inserted through the sheath  700 , the device can cause local expansion of the sheath  700  and cause the sheath  700  to expand at the partial score or split line location  716 . The weakened portion  716  can extend longitudinally along any portion of the expandable sheath  700 . 
       FIGS. 44 and 45  show another embodiment of an expandable sheath  800  having an initial diameter in a resting configuration ( FIG. 44 ) and a larger expanded diameter in an expanded configuration ( FIG. 45 ). The sheath  800  can include an elastic outer cover  802 , an inner layer  804 , and an outer layer  806 . Inner layer  804  can include first and second folded portions  808 ,  810 . The folded portions  808 ,  810  can be arranged such that they fold away from one another in opposite directions around the circumference of the sheath  800 . For example, folded portion  808  can be folded to the right in the view of  FIG. 44  and folded portion  810  can be folded to the left such that they do not overlap one another, but share a common segment  812  which is part of both folded portions  808 ,  810 . In contrast to previous embodiments, the outer layer  806  does not include an overlapping portion in this embodiment, but rather has first and second underlying portions  814 ,  816 , which underlie the first and second folded portions  808 ,  810 , respectively. The inner layer  804  can extend through a gap between the ends of the adjacent underlying portions  814 ,  816  (e.g., between a first end and a second end of discontinuous outer layer  806 ). 
     Each folded portion  808 ,  810  can include a weakened portion  818 , such as a slit, score line, and/or perforation. Weakened portion  818  can allow the expandable sheath  800  to expand easily without a high radial force. As the sheath  800  expands, segment  812  along the top of the folded portions  808 ,  810  of inner layer  804  can be configured to split apart from the rest of the folded portions  808 ,  810  and the first and second underlying portions  814 ,  816  can move away from one another so as to create an enlarged lumen within the inner layer  804 . Weakened portions  818  can allow for the segment  812  to easily split apart from the inner layer  804  as the sheath  800  expands. 
       FIGS. 46-47  show another embodiment of an expandable sheath  900 . Sheath  900  can be provided with an inner layer  902  and an elastic cover  904  surrounding the inner layer  902 . While not shown, sheath  900  can additionally include an intermediate layer positioned between the inner layer  902  and the elastic cover  904 . If present, the intermediate layer can closely follow the contour of the inner layer  902 . 
     Inner layer  902  can be shaped to include one or more folded portions  906  arranged to form a generally horseshoe-shaped lumen  908  that extends longitudinally through sheath  900  along the inner surface of the inner layer  902 . The folded portions  906  can be arranged to form an area  910  positioned with the lumen  908  and radially inward from the elastic cover  904 . In some embodiments, the area  910  can include one or more voids (e.g., smaller lumens or openings extending through portion  910 ). In some embodiments, the area  910  can be filled with material (e.g., HDPE) reflowed from an intermediate layer while the sheath is being made. In some embodiments, the area  910  can be filled with material reflowed from the elastic cover  904  during the sheath manufacturing process. 
     The inner layer  902  can include one or more weakened portions  912 , such as score lines, perforations, or slits. The weakened portions  912  can be configured to split apart, separate, or widen as the sheath expands from its initial resting configuration ( FIG. 46 ) to an expanded configuration ( FIG. 47 ) in the presence of a radial force. As the sheath  900  expands, material from the area  910  can cover any gaps  914  formed at the weakened portions  912 , thereby keeping the lumen  908  substantially sealed. 
       FIG. 48  shows another embodiment of an expandable sheath  1000  having an inner layer  1002  and a discontinuous outer layer  1004 . Sheath  1000  is similar to the sheath  800  of  FIG. 44 , except that sheath  1000  is shown without an elastic outer cover and further, the inner layer  1002  is continuous, without weakened portions at the folds  1006 . As shown in  FIG. 48 , the inner layer  1002  can be configured to have one or more folds  1006  (e.g., two folds positioned on the outer surface of the outer layer  1004 ), with portions  1008  of the outer layer  1004  extending between the folds  1006  and the outer surface  1010  of the inner layer  1002  underlying the folds  1006 . 
       FIG. 49  shows yet another embodiment of an expandable sheath  1100  having an inner layer  1102  and an outer layer  1104 . The sheath  1100  is similar to the sheath  100  shown in  FIG. 39  in that the inner layer  1102  can be continuous with a folded portion  1106 , and the outer layer  1104  can be discontinuous with an overlapping portion  1108  overlapping at least a part of the folded portion  1106  and an underlying portion  1110  underlying at least a part of the folded portion  1106 . The underlying portion  1110  can thus be positioned between an outer surface  1112  of the lumen-forming portion of the inner layer  1102  and the folded portion  1106 . 
     The inner layers  1002 ,  1102  of the sheaths  1000 ,  1100 , respectively, of  FIGS. 48-49  can be optimized to perform slightly differently than the inner layers of sheaths described above. For example, different materials can be used for the inner liner to increase durability and softness of the seam (although such materials can also be used with the other embodiments of expandable sheaths described above). For example, materials such as woven fabrics or braid filaments can be used. Such fabrics, filaments, or yarns can comprise, for example, PTFE, PET, PEEK, and/or nylon yarns or filaments. These materials can advantageously provide a soft and flexible layer that can be easily formed into the desired shapes or folded portions. Additionally, such materials can withstand high temperatures, as well as can possess high tensile strength and tear resistance. Nonetheless, these materials can also be elastic, experience minimal kinking, and provide soft distal edges for less traumatic insertion into a patient&#39;s vessels. 
     Various methods can be used to produce the sheaths discussed above and below, throughout the present disclosure. For example, a method of making the sheath shown in  FIGS. 2A-2D  can comprise providing a mandrel and applying an inner layer on the mandrel, such as by spray coating or dip coating the mandrel. An intermediate layer, such as a mesh structure, can then be mounted on the inner layer. An outer layer can be applied over the intermediate layer, such as by a second spray coating or dip coating step. Methods can comprise etching or surface treating at least a portion of the inner layer. Also, methods can comprise providing one or more notches and/or cuts in the inner layer and/or the outer layer. Cuts and/or notches can be provided by, for example, laser cutting or etching one or more layers. 
     In some embodiments of methods of making a sheath such as the sheaths illustrated in  FIGS. 2A-2D , layers can be pre-formed and mounted on a mandrel, and then fused or thermally bonded together. For example, in one method, an inner layer is applied to a mandrel. An intermediate layer can be applied to the outer surface of the inner layer. An outer layer can be applied to the outer surface of the intermediate layer. Heat shrink tubing can be applied, and the assembly heated, such that the inner layer, the intermediate layer, and/or the outer layer are thermally bonded and compressed together under the heat shrink tubing. 
       FIG. 30  illustrates a block diagram of one method of producing a sheath for use with a delivery apparatus in minimally invasive surgery. One or more mandrels can be provided (step  300 ). The mandrel can be provided with an exterior coating, such as a Teflon® coating, and the mandrel&#39;s diameter can be predetermined, based on the desired size of the resulting sheath. A liner that will become the inner polymeric layer of the sheath, such as a PTFE or high density polyethylene liner, can be mounted on the mandrel (step  302 ). The liner can be etched and/or surface treated prior to being mounted on the mandrel, according to conventional etching and surface treatment methods.  FIG. 32A  illustrates a section view of a sheath at steps  300  and  302  of  FIG. 30 . A coated mandrel  96  is inserted within the lumen  72  of the inner polymeric layer  68 . The circumference of the inner polymeric layer  68  is larger than the circumference of the mandrel  96 , such that an excess portion of the inner polymeric layer  68  can be gathered above the mandrel  96 . 
     A layer of material that will become the outer polymeric tubular layer, such as a layer comprising polyurethane or polyolefin, can be cut or notched through all, substantially all, or a part of the thickness of the layer (step  304 ). Such a cut or notch can extend longitudinally along the length of the layer and can extend along substantially the entire length of the outer polymeric tubular layer. In alternative embodiments, the cut or notch can be provided along only a portion of the outer polymeric tubular layer. For example, the outer polymeric tubular layer can be cut starting at the distal end of the outer polymeric tubular layer, with the cut ending before the proximal end of the outer polymeric tubular layer. In one embodiment, the cut can end at a transition, where the outer diameter of the outer polymeric tubular layer increases or decreases. In one specific embodiment, the cut or notch can extend longitudinally along about 75% of the length of the sheath. 
     The cut or notched outer polymeric tubular layer can be applied, positioned, adhered, mounted, thermally fused or bonded, dip coated, and/or otherwise coupled to the etched inner liner (step  306 ).  FIG. 32B  shows a section view of the sheath at step  306  of  FIG. 30 , with outer polymeric tubular layer  70  applied to the inner polymeric layer  68  such that a portion of the inner polymeric layer  68  extends between the cut formed between first and second portions  78 ,  80  of the outer polymeric tubular layer  70 . 
     In alternative embodiments, the outer polymeric tubular layer can be notched or cut after being mounted on the inner liner/mandrel assembly. The outer polymeric tubular layer can optionally be provided with a hydrophilic coating and/or provided with additional layers, such as being dip coated with polyurethane. Some portion of the inner liner can protrude through the cut in the outer polymeric tubular layer after such outer polymeric tubular layer is mounted onto the inner liner/mandrel arrangement. Using, for example, a split tool, the protruding portion of the inner liner can be folded down onto the outer surface of the outer polymeric tubular layer (step  308 ). In some embodiments, the protruding portion of the inner liner is folded down along the entire length of the resulting sheath, while in other embodiments, the protruding portion of the inner liner is only present along a portion of the length of the sheath, or is only folded down along a portion of the length of the resulting sheath.  FIG. 32C  shows a section view of the sheath at step  308  of  FIG. 30 . A split tool  98  is used to fold the excess portion of inner polymeric layer  68  over a portion of the outer surface  83  of the outer polymeric tubular layer  70 .  FIG. 32D  shows a section view of the sheath after completion of step  308  of  FIG. 30 . Split tool  98  has been removed, and folding of the excess portion of the inner polymeric layer  68  has been completed.  FIG. 32E  shows a section view of an outer covering, such as outer polymeric covering  99 , that can be applied such that it overlaps a portion of the folded portion of inner polymeric layer  68 . The outer polymeric covering  99  contacts at least a portion of the outer surface  83  of the outer polymeric tubular layer  70 . 
     A soft, atraumatic tip can be provided at the distal end of the resulting sheath (step  310 ). Additional outer layers can also be applied, if desired. Then, a layer of heat shrink tubing, such as fluorinated ethylene propylene (FEP) heat shrink tubing, can be positioned over the entire assembly (step  312 ). An appropriate amount of heat is applied, thus shrinking the heat shrink tubing and compressing the layers of the sheath together, such that components of the sheath can be thermally bonded or fused together where desired. Once the components of the sheath have been bonded together, the heat shrink tubing can be removed (step  314 ). Finally, the proximal end of the sheath can be adhered or otherwise attached to a housing of a catheter assembly, and the sheath can be removed from the mandrel (step  316 ). 
       FIG. 31  illustrates a block diagram of an alternative embodiment of a method of making a sheath. An inner liner, such as an etched PTFE tubing can be applied to a tapered mandrel, such as a 16 Fr tapered mandrel, and trimmed to an appropriate length (step  200 ). A second mandrel, such as a 0.070 inches diameter mandrel, can be inserted in the lumen of the inner liner such that the mandrels are arranged side by side in the inner liner (step  202 ).  FIG. 32F  shows a section view of a sheath at steps  200  and  202  of  FIG. 31 . An inner liner or inner polymeric layer  68  is applied on a first, tapered, mandrel  96 . A second mandrel  97  is inserted into the lumen  72  of the inner polymeric layer  68  created by the excess portion of the inner polymeric layer  68 , as described. 
     A notched or cut outer polymeric tubular layer, such as high density polyethylene tubing that has been notched or cut longitudinally, can be slid onto the tapered mandrel and a portion of the inner liner, starting at the distal end of the tapered mandrel (step  204 ). The second mandrel can then be removed (step  206 ).  FIG. 32G  illustrates a perspective view of the sheath at steps  204  and  206  of  FIG. 31 . A polymeric outer tubular layer  70  having a longitudinal cut is applied over the tapered mandrel  96  and inner polymeric layer  68 . The outer tubular layer conforms to the portion of the inner polymeric layer around the tapered mandrel  96 , and the portion of the inner polymeric layer  68  around the second mandrel  97  extends through the longitudinal cut in the outer polymeric tubular layer  70 . 
     A split tool can be inserted into the portion of the lumen of the inner liner that was previously occupied by the second mandrel (step  208 ). The split tool can then be used to form folds and/or pleats in the excess portion of the inner liner which now extends through the longitudinal cut in the outer polymeric tubular layer (step  210 ). A radiopaque marker band can optionally be applied at the distal end of the sheath (step  212 ). Heat shrink tubing, such as FEP heat shrink tubing, can be applied over the entire sheath, and heat can be applied to compress the components of the sheath and bond or fuse them together (step  214 ). The split tool, heat shrink tubing, and second mandrel can then be removed (step  216 ). The sheath can then be utilized with a delivery apparatus, such as by bonding the proximal end of the sheath to a polycarbonate housing of a delivery apparatus or catheter assembly (step  218 ). 
       FIG. 32H  illustrates an elevation view of the sheath at step  218  of  FIG. 31 . The sheath  66 , made according to described methods and processes, can be attached or bonded to a housing  101 , such as by bonding the proximal end of the sheath  66  to the polycarbonate housing  101 . 
     In another example, disclosed expandable sheaths can be made using a reflowed mandrel process. A mandrel can be provided, with the size of the mandrel defining the inner diameter of the sheath lumen in its resting configuration. A tube of material, such as a PTFE tube that will become the sheath&#39;s inner liner, can be provided with an inner diameter greater than that of the mandrel (e.g., a 9 mm PTFE tube can be mounted on a 6 mm mandrel). The PTFE tube can be mounted on the mandrel and prepared into the final folded configuration by folding the excess material of the PTFE tube over to one or both sides. An HDPE tube that will serve as the outer layer can then be placed over the PTFE liner. The two layer assembly can then be thermally fused together. For example, a reflow process can be performed where the assembly is heated to a temperature high enough such that the inner and/or outer layers are at least partially melted and are then fused together as the heat is removed and the assembly cools. 
     An elastic cover can be placed over at least part of the fused layers (e.g., over a proximal section of the sheath) and held in place using a thermal process. In some embodiments, the same thermal process can bond the layers of the sheath and the elastic cover. In other embodiments, a first thermal process can be used to fuse the layers of the sheath, and a second thermal process can be used to secure the elastic cover to the sheath. In some embodiments, the elastic cover can be heat shrink tubing that is applied over the expandable sheath, and heated to a temperature high enough to cause the tubing to shrink around the sheath. In some embodiments, a distal soft tip can then be attached to the shaft of the expandable sheath. 
     In some embodiments, the outer layer can be co-extruded with an adhesive layer, such as a layer formed from Tecoflex, such that the Tecoflex is positioned on an inner surface of the outer layer—in this manner the Tecoflex will be positioned between the inner and outer layers in the completed sheath. In these embodiments, an HDPE tube can be provided with a coating of Tecoflex on the inner surface. The HDPE tube can be slit along the length of the tube to open and flatten it, and then cut using a template in some embodiments. For example, for specific applications, portions of the outer layer can be cut and removed using a template. The cut HDPE can then be placed on the inner layer on the mandrel. In some embodiments, only a portion of the outer layer will have the adhesive Tecoflex. In these embodiments, the sections without Tecoflex will only be partially fused to the inner layer. In some embodiments, the entire inner surface of the outer layer will have the Tecoflex, and the inner surface of the outer layer can be positioned so that it contacts the inner layer on the mandrel. To position the inner and outer layers as shown in the sheath of  FIG. 39 , the folded portion of the inner layer can be lifted up, and an edge of the outer layer can be tucked beneath the fold. 
     Sheaths of the present disclosure can be used with various methods of introducing a prosthetic device into a patient&#39;s vasculature. One such method comprises positioning an expandable sheath in a patient&#39;s vessel, passing a device through the introducer sheath, which causes a portion of the sheath surrounding the device to expand and accommodate the profile of the device, and automatically retracting the expanded portion of the sheath to its original size after the device has passed through the expanded portion. In some methods, the expandable sheath can be sutured to the patient&#39;s skin at the insertion site so that once the sheath is inserted the proper distance within the patient&#39;s vasculature, it does not move once the implantable device starts to travel through the sheath. 
     Disclosed embodiments of an expandable sheath can be used with other delivery and minimally invasive surgical components, such as an introducer and loader. In one embodiment, the expandable sheath can be flushed to purge any air within the sheath, using, for example, flush port  103  ( FIG. 35 ). An introducer can be inserted into the expandable sheath and the introducer/sheath combination can be fully inserted into vasculature over a guiding device, such as a 0.35″ guidewire. Preferably, the seam formed by the intersection of the folded portion of the inner layer and the overlapping portion of the outer layer can be positioned such it is oriented downward (posterior). Once the sheath and introducer are fully inserted into a patient&#39;s vasculature, in some embodiments, the expandable sheath can be sutured in place at the insertion site. In this manner, the expandable sheath can be substantially prevented from moving once positioned within the patient. 
     The introducer can then be removed and a medical device, such as a transcatheter heart valve can be inserted into the sheath, in some instances using a loader. Such methods can additionally comprise placing the tissue heart valve in a crimped state on the distal end portion of an elongated delivery apparatus, and inserting the elongated delivery device with the crimped valve into and through the expandable sheath. Next, the delivery apparatus can be advanced through the patient&#39;s vasculature to the treatment site, where the valve can be implanted. 
     Typically, the medical device has a greater outer diameter than the diameter of the sheath in its original configuration. The medical device can be advanced through the expandable sheath towards the implantation site, and the expandable sheath can locally expand to accommodate the medical device as the device passes through. The radial force exerted by the medical device can be sufficient to locally expand the sheath to an expanded diameter (e.g., the expanded configuration) just in the area where the medical device is currently located. Once the medical device passes a particular location of the sheath, the sheath can at least partially contract to the smaller diameter of its original configuration. The expandable sheath can thus be expanded without the use of inflatable balloons or other dilators. Once the medical device is implanted, the sheath and any sutures holding in place can be removed. In some embodiments, it is preferable to remove the sheath without rotating it. 
     In view of the many possible embodiments to which the principles of the disclosed invention can be applied, it should be recognized that the illustrated embodiments are only preferred examples of the invention and should not be taken as limiting the scope of the invention. Rather, the scope of the invention is defined by the following claims. We therefore claim as our invention all that comes within the scope and spirit of these claims.