Patent Publication Number: US-2022233310-A1

Title: Expandable sheath

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/US2020/054594, filed Oct. 7, 2020, which claims benefit of U.S. Provisional Application No. 62/912,569, filed on Oct. 8, 2019, the contents of each of which are herein incorporated by reference in their entirety. 
    
    
     FIELD 
     The present application relates to expandable introducer sheaths for prosthetic devices such as transcatheter heart valves and methods of making the same. 
     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. Such introducer sheaths may be radially expandable. However, such sheaths tend to have complex mechanisms, such as ratcheting mechanisms that maintain the sheath in an expanded configuration once a device with a larger diameter than the sheath&#39;s original diameter is introduced. Existing expandable sheaths can also be prone to axial elongation as a consequence of the application of longitudinal force attendant to passing a prosthetic device through the sheath. Such elongation can cause a corresponding reduction in the diameter of the sheath, increasing the force required to insert the prosthetic device through the narrowed sheath. 
     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 
     The expandable sheaths disclosed herein include a first polymeric layer, a braided layer radially outward of the first polymeric layer (the braided layer comprising a plurality of filaments braided together), and a second polymeric layer radially outward of the braided layer. The second polymeric layer can be bonded to the first polymeric layer, such that the braided layer is encapsulated between the first and second polymeric layers. When a medical device is passed through the sheath, the diameter of the sheath expands from a first diameter to a second diameter around the medical device. 
     In some embodiments, when a medical device is passed through the sheath, the diameter of the sheath expands from a first diameter to a second diameter around the medical device while resisting axial elongation of the sheath, such that a length of the sheath remains substantially constant. 
     In some embodiments, the first and second polymeric layers include a plurality of longitudinally-extending folds when the sheath is at the first diameter. The longitudinally-extending folds create a plurality of circumferentially spaced ridges and a plurality of circumferentially spaced valleys. As a medical device is passed through the sheath, the ridges and valleys level out to allow the sheath to radially expand. 
     In some embodiments, a portion of the first polymeric layer and/or a portion of the second polymeric layer comprises an elastic coating. 
     In some embodiments, the filaments of the braided layer are movable between the first and second polymeric layers, such that the braided layer can radially expand as a medical device is passed through the sheath. The length of the sheath can stay substantially constant as the braided layer radially expands. In some embodiments, the filaments of the braided layer are resiliently buckled when the sheath is at the first diameter, and the first and second polymeric layers are attached to each other at a plurality of open spaces between the filaments of the braided layer. In some embodiments, the braided layer includes a self-contracting material. In some embodiments, at least a portion of the plurality of filaments includes an elastic coating. 
     Some embodiments of the expandable sheath can include an outer cover formed of a heat shrink material and extending over at least a longitudinal portion of the first polymeric layer, the braided layer, and the second polymeric layer. The outer cover can include one or more longitudinally extending slits, weakened portions, or scorelines. 
     Some expandable sheath embodiments include a cushioning layer positioned between the braided layer and an adjacent polymeric layer. The cushioning layer dissipates radial forces acting between filaments of the braided layer and the adjacent polymeric layer. A first cushioning layer can be positioned between the braided layer and the first polymeric layer, and a second cushioning layer can be positioned between the braided layer and the second polymeric layer. The cushioning layer(s) can have, for example, a thickness of from about 80 microns to about 1000 microns. Some embodiments of the cushioning layer can have a porous interior region. The cushioning layer can further include a sealed surface positioned between the porous interior region and the adjacent polymeric layer, with the sealed surface having a higher melting point than the adjacent polymeric layer. The sealed surface can also be thinner than the porous interior region of the cushioning layer. In some embodiments, the sealed surface is a sealing layer attached to the cushioning layer. In some embodiments, the sealed surface is a surface of the cushioning layer, and the sealed surface of the cushioning layer is continuous with and formed of the same material as the porous interior region of the cushioning layer. 
     Another expandable sheath embodiment can include a braided layer (including a plurality of filaments braided together), and a first expandable sealing layer adhered to a portion of the filaments of the braided layer. The sealing layer is impermeable to blood flow. When a medical device is passed through the sheath, the diameter of the sheath expands from a first diameter to a second diameter around the medical device. In some embodiments, a second expandable sealing layer can be adhered to a portion of the filaments of the braided layer. The second expandable sealing layer can be positioned on the opposite side of the braided layer as the first expandable sealing layer. In some embodiments, the braided layer includes a self-contracting material, and the expandable sealing layer varies in thickness according to the longitudinal position of the sheath. 
     In some embodiments, at least a portion of the plurality of filaments includes a sealing coating instead of, or in addition to, one or both of the sealing layers. 
     Methods of making expandable sheaths are also disclosed herein. One embodiment of a method of making an expandable sheath includes: placing a braided layer radially outward of a first polymeric layer situated on a mandrel (the mandrel having a first diameter), and applying a second polymeric layer radially outward of the braided layer, applying heat and pressure to the first polymeric layer, the braided layer, and the second polymeric layer such that the first and second polymeric layers bond to each other and encapsulate the braided layer to form an expandable sheath. The method further includes removing the expandable sheath from the mandrel to allow the expandable sheath to at least partially radially collapse to a second diameter that is less than the first diameter. 
     In some embodiments, an elastic coating can be applied to a portion of the plurality of filaments. In some embodiments, an elastic coating can be applied to a portion of the first polymeric layer and/or a portion of the second polymeric layer. 
     Some embodiments of the methods of making expandable sheaths can include shape-setting the braided layer to a contracted diameter prior to placing the braided layer radially outward of the first polymeric layer. 
     In some embodiments of the methods of making expandable sheaths, applying heat and pressure further includes placing the mandrel in a vessel containing a thermally-expandable material, heating the thermally-expandable material in the vessel, and applying a radial pressure of 100 MPa or more to the mandrel via the thermally-expandable material. 
     In some embodiments of the methods of making expandable sheaths, applying heat and pressure further includes applying a heat shrink tubing layer over the second polymeric layer and applying heat to the heat shrink tubing layer. 
     Some embodiments of the methods of making expandable sheaths can include resiliently buckling the filaments of the braided layer as the sheath is radially collapsed to the second diameters. 
     Some embodiments of the methods of making expandable sheaths can include sealing a surface of a cushioning layer and applying the cushioning layer such that the sealed surface contacts the first polymeric layer or the second polymeric layer. 
     Some embodiments of the methods of making expandable sheaths can include crimping the expandable sheath to a third diameter, the third diameter being smaller than the first diameter and the second diameter. 
     Some other embodiments also describe the sheath further comprising a distal end portion having a predetermined length and comprising two or more layers. 
     Yet, in other embodiments, as disclosed herein, the distal end portion can extend distally beyond a longitudinal portion of the sheath comprising the braided layer. 
     Also disclosed herein are embodiments where the distal end portion comprises an inner polymeric layer and an outer polymeric layer. 
     In still further embodiments, the distal end portion can further comprise an external covering. 
     In yet further embodiments, a portion of the distal end portion can comprise a portion of a distal end of the braided layer. 
     Also disclosed are embodiments, where the portion of the distal end of the braided layer comprises loops. 
     In some embodiments disclosed herein, the external covering can have a melting temperature lower than a melting temperature of the inner polymeric layer. 
     While in other embodiments, the external covering can have a melting temperature lower than a melting temperature of the outer polymeric layer. 
     In still further embodiments, the external covering can comprise a low density polyethylene. 
     Also described herein are embodiments, where a portion of the sheath proximal to the distal end portion of the sheath does not comprise the external covering. 
     In yet other embodiments described herein, a portion of the sheath extending from a proximal end of the sheath to a portion of the sheath proximal to the distal end portion of the sheath does not comprise the external covering. 
     Some embodiments comprise the sheath comprising at least one attachment region between the distal end portion and a portion of the sheath proximal to the distal end. 
     Yet, in other embodiments, the attachment region is a circumferential attachment region. 
     While in other embodiments, the attachment region comprises a plurality of circumferentially spaced attachment regions. 
     Also disclosed are the embodiments where the distal end portion of the sheath comprises a first plurality of folds present in the inner layer. 
     In other embodiments, the distal end portion of the sheath comprises a second plurality of folds present in the outer layer. 
     In still further embodiments, the distal end portion of the sheath can comprise a third plurality of folds present in the external covering. 
     Also disclosed are the embodiments, where folds in the third plurality of folds present in the external covering are at least partially attached to each other. 
     In certain embodiments, disclosed also are methods of forming a tip of a sheath. In such exemplary embodiments the method comprises pre-crimping a distal end portion of any of the disclosed herein sheaths to a first diameter, wherein the distal end portion extends distally beyond a longitudinal portion of the sheath comprising the braided layer and comprises an inner polymeric layer and an outer polymeric layer; wherein the inner polymeric layer and the outer layer exhibit a first melting temperature; covering the pre-crimped distal end portion with an external covering; wherein the external covering exhibits a second melting temperature, wherein the second melting temperature is lower than the first melting temperature; heating at least a portion of the pre-crimped distal end portion covered with the external covering to a first temperature, wherein the first temperature is equal or greater than the first melting temperature, thereby forming at least one attachment region between the external cover and the inner and outer polymeric layers; inserting a mandrel into a lumen of at least a portion of the distal end portion and further crimping the at least a portion of the distal end portion to a second diameter; and heating the at least a portion of the distal end portion to a second temperature; wherein the second temperature is equal or greater than the second melting temperature. 
     Also disclosed are embodiments wherein the second temperature is lower than the first melting temperature. 
     In some embodiments, wherein the second diameter is smaller than the first diameter. 
     Some embodiments of the methods disclosed herein include that the step of crimping can form a plurality of folds along the external covering. 
     In yet other embodiments, the inner polymeric layer and outer polymeric layer comprise a plurality of folds. 
     In yet further exemplary embodiments, the plurality of folds in the inner polymeric layer and the outer polymeric layer are formed at the pre-crimping step. While in other exemplary embodiments, the plurality of folds in the inner polymeric layer and the outer polymeric layer are formed at the crimping step. 
     Also disclosed herein are the embodiments, where the step of heating to the second temperature forms an attachment between at least a portion of the plurality of folds in the external covering to each other. 
     In yet other embodiments of the methods disclosed herein comprise applying a heat-shrink material to at least a portion of the crimped distal end portion. 
     In still further embodiments, the step of applying the heat-shrink material is performed prior to the step of heating to the second temperature. While in yet other embodiments, the step of applying the heat-shrink material is performed during the step of heating to the second temperature. While in still further embodiments, the step of applying the heat-shrink material is performed after to the step of heating to the second temperature. 
     In yet other embodiments of the methods disclosed herein comprise removing the heat-shrink material after the attachment between at least a portion of the plurality of folds in the external covering to each other is formed. 
     In yet further embodiments, the heat-shrink material can be a tube or a tape. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a delivery system for a cardiovascular prosthetic device, according to one embodiment. 
         FIG. 2  illustrates an expandable sheath that can be used in combination with the delivery system of  FIG. 1 , according to one embodiment. 
         FIG. 3  is a magnified view of a portion of the expandable sheath of  FIG. 2 . 
         FIG. 4  is a side elevation cross-sectional view of a portion of the expandable sheath of  FIG. 2 . 
         FIG. 5A  is a magnified view of a portion of the expandable sheath of  FIG. 2  with the outer layer removed for purposes of illustration. 
         FIG. 5B  is a magnified view of a portion of the braided layer of the sheath of  FIG. 2 . 
         FIG. 6  is a magnified view of a portion of the expandable sheath of  FIG. 2  illustrating expansion of the sheath as a prosthetic device is advanced through the sheath. 
         FIG. 7  is a magnified, partial cross-sectional view illustrating the constituent layers of the sheath of  FIG. 2  disposed on a mandrel. 
         FIG. 8  is a magnified view illustrating another embodiment of an expandable sheath. 
         FIG. 9  is a cross-sectional view of an apparatus that can be used to form an expandable sheath, according to one embodiment. 
         FIGS. 10A-10D  illustrate another embodiment of a braided layer in which the filaments of the braided layer are configured to buckle when the sheath is in a radially collapsed state. 
         FIG. 11  shows a side cross-sectional view of an assembly of an expandable sheath with a vessel dilator. 
         FIG. 12  shows the vessel dilator of the assembly embodiment of  FIG. 11 . 
         FIG. 13  shows a side view of another assembly embodiment including an expandable sheath and a vessel dilator. 
         FIG. 14  shows a side view of the assembly embodiment of  FIG. 13 , with the vessel dilator pushed partially away from the expandable sheath. 
         FIG. 15  shows a side view of the assembly embodiment of  FIG. 13 , with the vessel dilator pushed fully away from the expandable sheath. 
         FIG. 16  shows a side view of the assembly embodiment of  FIG. 13 , with the vessel dilator being retracted into the expandable sheath. 
         FIG. 17  shows a side view of the assembly embodiment of  FIG. 13 , with the vessel dilator being retracted further into the expandable sheath. 
         FIG. 18  shows a side view of the assembly embodiment of  FIG. 13 , with the vessel dilator being fully retracted into the expandable sheath. 
         FIG. 19  shows a side cross-sectional view of another assembly embodiment including an expandable sheath and a vessel dilator. 
         FIG. 20  illustrates an embodiment of a vessel dilator that may be used in combination with the expandable sheaths described herein. 
         FIG. 21  illustrates an embodiment of a vessel dilator that may be used in combination with the expandable sheaths described herein. 
         FIG. 22  shows a side view with a cutaway to cross section of an embodiment of an expandable sheath having an outer cover and an overhang. 
         FIG. 23  shows an example embodiment of an outer cover having longitudinal scorelines. 
         FIG. 24  illustrates an end portion of an embodiment of a braided layer of an expandable sheath. 
         FIG. 25A  illustrates a perspective view of a roller-based crimping mechanism embodiment for crimping an expandable sheath. 
         FIG. 25B  illustrates a side view of a disc-shaped roller and connector of the crimping mechanism shown in  FIG. 25A . 
         FIG. 25C  illustrates a top view of a disc-shaped roller and connector of the crimping mechanism shown in  FIG. 25A . 
         FIG. 26  shows an embodiment of a device for crimping an elongated expandable sheath. The encircled portion of the device is magnified in the inset at the left side of the picture. 
         FIG. 27  shows an embodiment of an expandable sheath having an inner layer with scorelines. 
         FIG. 28  shows an additional embodiment of a braided layer of an expandable sheath. 
         FIG. 29  shows a perspective view of an additional expandable sheath embodiment. 
         FIG. 30  shows a perspective view of the embodiment of  FIG. 29  with the outer heat shrink tubing layer partially torn away from the inner sheath layers. 
         FIG. 31  shows a side view of a sheath embodiment prior to movement of a delivery system therethrough. 
         FIG. 32  shows a side view of a sheath embodiment as a delivery system moves through, splitting the heat shrink tubing layer. 
         FIG. 33  shows a side view of a sheath embodiment with the delivery system fully moved through, the heat shrink tubing layer fully split along the length of the sheath. 
         FIG. 34  shows a perspective view of a sheath embodiment having a distal end portion folded around an introducer. 
         FIG. 35  shows an enlarged, cross-sectional view of the distal end portion folded around the introducer. 
         FIG. 36  shows a cross section of an additional expandable sheath embodiment. 
         FIG. 37  shows an embodiment of a cushioning layer. 
         FIG. 38  shows another embodiment of a cushioning layer. 
         FIG. 39  shows a side view of an additional expandable sheath embodiment. 
         FIG. 40  shows a longitudinal cross section of the embodiment of  FIG. 39 . 
         FIG. 41  shows a transverse cross section of an additional expandable sheath embodiment. 
         FIG. 42  shows a partial longitudinal cross section of an additional expandable sheath embodiment. 
         FIG. 43  shows a transverse cross section of an additional expandable sheath embodiment in an expanded state. 
         FIG. 44  shows a transverse cross section of the expandable sheath embodiment of  FIG. 43  during the crimping process. 
         FIG. 45  shows a perspective view of a sheath embodiment similar to the sheath of  FIG. 43 , in the expanded state. 
         FIG. 46  shows a perspective view of a sheath embodiment similar to the sheath of  FIG. 43 , in the folded and compressed state. 
         FIG. 47  shows an additional embodiment of a braided layer. 
     
    
    
     DETAILED DESCRIPTION 
     The expandable introducer sheaths described herein can be used to deliver a prosthetic device through a patient&#39;s vasculature to a procedure site within the body. The sheath can be constructed to be highly expandable and collapsible in the radial direction while limiting axial elongation of the sheath and, thereby, undesirable narrowing of the lumen. In one embodiment, the expandable sheath includes a braided layer, one or more relatively thin, non-elastic polymeric layers, and an elastic layer. The sheath can resiliently expand from its natural diameter to an expanded diameter as a prosthetic device is advanced through the sheath, and can return to its natural diameter upon passage of the prosthetic device under the influence of the elastic layer. In certain embodiments, the one or more polymeric layers can engage the braided layer and can be configured to allow radial expansion of the braided layer while preventing axial elongation of the braided layer, which would otherwise result in elongation and narrowing of the sheath. 
       FIG. 1  illustrates a representative delivery apparatus  10  for delivering a medical device, such as a prosthetic heart valve or other prosthetic implant, to a patient. The delivery apparatus  10  is exemplary only and can be used in combination with any of the expandable sheath embodiments described herein. Likewise, the sheaths disclosed herein can be used in combination with any of various known delivery apparatuses. The delivery apparatus  10  illustrated can generally include a steerable guide catheter  14  and a balloon catheter  16  extending through the guide catheter  14 . A prosthetic device, such as a prosthetic heart valve  12 , can be positioned on the distal end of the balloon catheter  16 . The guide catheter  14  and the balloon catheter  16  can be adapted to slide longitudinally relative to each other to facilitate delivery and positioning of a prosthetic heart valve  12  at an implantation site in a patient&#39;s body. The guide catheter  14  includes a handle portion  18  and an elongated guide tube or shaft  20  extending from the handle portion  18 . 
     The prosthetic heart valve  12  can be delivered into a patient&#39;s body in a radially compressed configuration and radially expanded to a radially expanded configuration at the desired deployment site. In the illustrated embodiment, the prosthetic heart valve  12  is a plastically expandable prosthetic valve that is delivered into the patient&#39;s body in a radially compressed configuration on a balloon of the balloon catheter  16  (as shown in  FIG. 1 ) and then radially expanded to a radially expanded configuration at the deployment site by inflating the balloon (or by actuating another type of expansion device of the delivery apparatus). Further details regarding a plastically expandable heart valve that can be implanted using the devices disclosed herein are disclosed in U.S. Publication No. 2012/0123529, which is incorporated herein by reference. In other embodiments, the prosthetic heart valve  12  can be a self-expandable heart valve that is restrained in a radially compressed configuration by a sheath or other component of the delivery apparatus and self-expands to a radially expanded configuration when released by the sheath or other component of the delivery apparatus. Further details regarding a self-expandable heart valve that can be implanted using the devices disclosed herein are disclosed in U.S. Publication No. 2012/0239142, which is incorporated herein by reference. In still other embodiments, the prosthetic heart valve  12  can be a mechanically expandable heart valve that comprises a plurality of struts connected by hinges or pivot joints and is expandable from a radially compressed configuration to a radially expanded configuration by actuating an expansion mechanism that applies an expansion force to the prosthetic valve. 
     Further details regarding a mechanically expandable heart valve that can be implanted using the devices disclosed herein are disclosed in U.S. Publication No. 2018/0153689, which is incorporated herein by reference. In still other embodiments, a prosthetic valve can incorporate two or more of the above-described technologies. For example, a self-expandable heart valve can be used in combination with an expansion device to assist expansion of the prosthetic heart valve. 
       FIG. 2  illustrates an assembly  90  (which can be referred to as an introducer device or assembly) that can be used to introduce the delivery apparatus  10  and the prosthetic device  12  into a patient&#39;s body, according to one embodiment. The introducer device  90  can comprise a housing  92  at a proximal end of the device and an expandable sheath  100  extending distally from the housing  92 . The housing  92  can function as a handle for the device. The expandable sheath  100  has a central lumen  112  ( FIG. 4 ) to guide passage of the delivery apparatus for the prosthetic heart valve. Generally, during use, a distal end of the sheath  100  is passed through the skin of the patient and is inserted into a vessel, such as the femoral artery. The delivery apparatus  10  with its implant  12  can then be inserted through the housing  92  and the sheath  100 , and advanced through the patient&#39;s vasculature to the treatment site, where the implant is to be delivered and implanted within the patient. In certain embodiments, the introducer housing  92  can include a hemostasis valve that forms a seal around the outer surface of the guide catheter  14  once inserted through the housing to prevent leakage of pressurized blood. 
     In alternative embodiments, the introducer device  90  need not include a housing  92 . For example, the sheath  100  can be an integral part of a component of the delivery apparatus  10 , such as the guide catheter. For example, the sheath can extend from the handle  18  of the guide catheter. Additional examples of introducer devices and expandable sheaths can be found in U.S. patent application Ser. No. 16/378,417, which is incorporated by reference in its entirety. 
       FIG. 3  illustrates the expandable sheath  100  in greater detail. With reference to  FIG. 3 , the sheath  100  can have a natural, unexpanded outer diameter D 1 . In certain embodiments, the expandable sheath  100  can comprise a plurality of co-axial layers extending along at least a portion of the length L of the sheath ( FIG. 2 ). For example, with reference to  FIG. 4 , the expandable sheath  100  can include a first layer  102  (also referred to as an inner layer), a second layer  104  disposed around and radially outward of the first layer  102 , a third layer  106  disposed around and radially outward of the second layer  104 , and a fourth layer  108  (also referred to as an outer layer) disposed around and radially outward of the third layer  106 . In the illustrated configuration, the inner layer  102  can define the lumen  112  of the sheath extending along a central axis  114 . 
     Referring to  FIG. 3 , when the sheath  100  is in an unexpanded state, the inner layer  102  and/or the outer layer  108  can form longitudinally-extending folds or creases such that the surface of the sheath comprises a plurality of ridges  126  (also referred to herein as “folds”). The ridges  126  can be circumferentially spaced apart from each other by longitudinally-extending valleys  128 . When the sheath expands beyond its natural diameter D 1 , the ridges  126  and the valleys  128  can level out or be taken up as the surface radially expands and the circumference increases, as further described below. When the sheath collapses back to its natural diameter, the ridges  126  and valleys  128  can reform. 
     In certain embodiments, the inner layer  102  and/or the outer layer  108  can comprise a relatively thin layer of polymeric material. For example, in some embodiments, the thickness of the inner layer  102  can be from 0.01 mm to 0.5 mm, 0.02 mm to 0.4 mm, or 0.03 mm to 0.25 mm. In certain embodiments, the thickness of the outer layer  108  can be from 0.01 mm to 0.5 mm, 0.02 mm to 0.4 mm, or 0.03 mm to 0.25 mm. 
     In certain examples, the inner layer  102  and/or the outer layer  108  can comprise a lubricious, low-friction, and/or relatively non-elastic material. In particular embodiments, the inner layer  102  and/or the outer layer  108  can comprise a polymeric material having a modulus of elasticity of 400 MPa or greater. Exemplary materials can include ultra-high-molecular-weight polyethylene (UHMWPE) (e.g., Dyneema®), high-molecular-weight polyethylene (HMWPE), or polyether ether ketone (PEEK). With regard to the inner layer  102  in particular, such a low coefficient of friction materials can facilitate passage of the prosthetic device through the lumen  112 . Other suitable materials for the inner and outer layers can include polytetrafluoroethylene (PTFE), expanded polytetrafluoroethylene (ePTFE), ethylene tetrafluoroethylene (ETFE), nylon, polyethylene, polyether block amide (e.g., Pebax), and/or combinations of any of the above. Some embodiments of a sheath  100  can include a lubricious liner on the inner surface of the inner layer  102 . Examples of suitable lubricious liners include materials that can further reduce the coefficient of friction of the inner layer  102 , 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 0.1 or less. 
     Additionally, some embodiments of the sheath  100  can include an exterior hydrophilic coating on the outer surface of the outer layer  108 . Such a hydrophilic coating can facilitate insertion of the sheath  100  into a patient&#39;s vessel, reducing potential damage. 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 . Such hydrophilic coatings may also be included on the inner surface of the inner layer  102  to reduce friction between the sheath and the delivery system, thereby facilitating the use and improving safety. In some embodiments, a hydrophobic coating, such as Perylene, may be used on the outer surface of the outer layer  108  or the inner surface of the inner layer  102  in order to reduce friction. 
     In certain embodiments, the second layer  104  can be a braided layer.  FIGS. 5A and 5B  illustrate the sheath  100  with the outer layer  108  removed to expose the elastic layer  106 . With reference to  FIGS. 5A and 5B , the braided layer  104  can comprise a plurality of members or filaments  110  (e.g., metallic or synthetic wires or fibers) braided together. The braided layer  104  can have any desired number of filaments  110 , which can be oriented and braided together along any suitable number of axes. For example, with reference to  FIG. 5B , the filaments  110  can include a first set of filaments  110 A oriented parallel to a first axis A, and a second set of filaments  110 B oriented parallel to a second axis B. The filaments  110 A and  110 B can be braided together in a biaxial braid such that filaments  110 A oriented along axis A form an angle θ with the filaments  110 B oriented along axis B. In certain embodiments, the angle θ can be from 5° to 70°, 10° to 60°, 10° to 50°, or 10° to 45°. In the illustrated embodiment, the angle θ is 45°. In other embodiments, the filaments  110  can also be oriented along three axes and braided in a triaxial braid, or oriented along any number of axes and braided in any suitable braid pattern. 
     The braided layer  104  can extend along substantially the entire length L of the sheath  100 , or alternatively, can extend only along a portion of the length of the sheath. In particular embodiments, the filaments  110  can be wires made from metal (e.g., Nitinol, stainless steel, etc.), or any of various polymers or polymer composite materials, such as carbon fiber. In certain embodiments, the filaments  110  can be round, and can have a diameter of from 0.01 mm to 0.5 mm, 0.03 mm to 0.4 mm, or 0.05 mm to 0.25 mm. In other embodiments, the filaments  110  can have a flat cross-section with dimensions of 0.01 mm×0.01 mm to 0.5 mm×0.5 mm, or 0.05 mm×0.05 mm to 0.25 mm×0.25 mm. In one embodiment, filaments  110  having a flat cross-section can have dimensions of 0.1 mm×0.2 mm. However, other geometries and sizes are also suitable for certain embodiments. If a braided wire is used, the braid density can be varied. Some embodiments have a braid density of from ten picks per inch to eighty picks per inch, and can include eight wires, sixteen wires, or up to fifty-two wires in various braid patterns. In other embodiments, the second layer  104  can be laser cut from a tube, or laser-cut, stamped, punched, etc., from sheet stock and rolled into a tubular configuration. The layer  104  can also be woven or knitted, as desired. 
     The third layer  106  can be a resilient, elastic layer (also referred to as an elastic material layer). In certain embodiments, the elastic layer  106  can be configured to apply force to the underlying layers  102  and  104  in a radial direction (e.g., toward the central axis  114  of the sheath) when the sheath expands beyond its natural diameter by passage of the delivery apparatus through the sheath. Stated differently, the elastic layer  106  can be configured to apply encircling pressure to the layers of the sheath beneath the elastic layer  106  to counteract expansion of the sheath. The radially inwardly directed force is sufficient to cause the sheath to collapse radially back to its unexpanded state after the delivery apparatus is passed through the sheath. 
     In the illustrated embodiment, the elastic layer  106  can comprise one or more members configured as strands, ribbons, or bands  116  helically wrapped around the braided layer  104 . For example, in the illustrated embodiment, the elastic layer  106  comprises two elastic bands  116 A and  116 B wrapped around the braided layer with opposite helicity, although the elastic layer may comprise any number of bands depending upon the desired characteristics. The elastic bands  116 A and  116 B can be made from, for example, any of a variety of natural or synthetic elastomers, including silicone rubber, natural rubber, any of various thermoplastic elastomers, polyurethanes such as polyurethane siloxane copolymers, urethane, plasticized polyvinyl chloride (PVC), styrenic block copolymers, polyolefin elastomers, etc. In some embodiments, the elastic layer can comprise an elastomeric material having a modulus of elasticity of 200 MPa or less. In some embodiments, the elastic layer  106  can comprise a material exhibiting an elongation to break of 200% or greater, or an elongation to break of 400% or greater. The elastic layer  106  can also take other forms, such as a tubular layer comprising an elastomeric material, a mesh, a shrinkable polymer layer such as a heat-shrink tubing layer, etc. In lieu of, or in addition to, the elastic layer  106 , the sheath  100  may also include an elastomeric or heat-shrink tubing layer around the outer layer  108 . Examples of such elastomeric layers are disclosed in U.S. Publication No. 2014/0379067, U.S. Publication No. 2016/0296730, and U.S. Publication No. 2018/0008407, which are incorporated herein by reference. In other embodiments, the elastic layer  106  can also be radially outward of the polymeric layer  108 . 
     In certain embodiments, one or both of the inner layer  102  and/or the outer layer  108  can be configured to resist axial elongation of the sheath  100  when the sheath expands. More particularly, one or both of the inner layer  102  and/or the outer layer  108  can resist stretching against longitudinal forces caused by friction between a prosthetic device and the inner surface of the sheath such that the length L remains substantially constant as the sheath expands and contracts. As used herein with reference to the length L of the sheath, the term “substantially constant” means that the length L of the sheath increases by not more than 1%, by not more than 5%, by not more than 10%, by not more than 15%, or by not more than 20%. Meanwhile, with reference to  FIG. 5B , the filaments  110 A and  110 B of the braided layer can be allowed to move angularly relative to each other such that the angle θ changes as the sheath expands and contracts. This, in combination with the longitudinal folds  126  in the layers  102  and  108 , can allow the lumen  112  of the sheath to expand as a prosthetic device is advanced through it. 
     For example, in some embodiments, the inner layer  102  and the outer layer  108  can be heat-bonded during the manufacturing process such that the braided layer  104  and the elastic layer  106  are encapsulated between the layers  102  and  108 . More specifically, in certain embodiments, the inner layer  102  and the outer layer  108  can be adhered to each other through the spaces between the filaments  110  of the braided layer  104  and/or the spaces between the elastic bands  116 . The layers  102  and  108  can also be bonded or adhered together at the proximal and/or distal ends of the sheath. In certain embodiments, the layers  102  and  108  are not adhered to the filaments  110 . This can allow the filaments  110  to move angularly relative to each other, and relative to the layers  102  and  108 , allowing the diameter of the braided layer  104 , and thereby the diameter of the sheath, to increase or decrease. As the angle θ between the filaments  110 A and  110 B changes, the length of the braided layer  104  can also change. For example, as the angle θ increases, the braided layer  104  can foreshorten, and as the angle θ decreases, the braided layer  104  can lengthen to the extent permitted by the areas where the layers  102  and  108  are bonded. However, because the braided layer  104  is not adhered to the layers  102  and  108 , the change in length of the braided layer that accompanies a change in the angle θ between the filaments  110 A and  110 B does not result in a significant change in the length L of the sheath. 
       FIG. 6  illustrates radial expansion of the sheath  100  as a prosthetic device  12  is passed through the sheath in the direction of arrow  132  (e.g., distally). As the prosthetic device  12  is advanced through the sheath  100 , the sheath can resiliently expand to a second diameter D 2  that corresponds to a size or diameter of the prosthetic device. As the prosthetic device  12  is advanced through the sheath  100 , the prosthetic device can apply longitudinal force to the sheath in the direction of motion by virtue of the frictional contact between the prosthetic device and the inner surface of the sheath. However, as noted above, the inner layer  102  and/or the outer layer  108  can resist axial elongation such that the length L of the sheath remains constant, or substantially constant. This can reduce or prevent the braided layer  104  from lengthening, and thereby constricting the lumen  112 . 
     Meanwhile, the angle θ between the filaments  110 A and  110 B can increase as the sheath expands to the second diameter D 2  to accommodate the prosthetic valve. This can cause the braided layer  104  to foreshorten. However, because the filaments  110  are not engaged or adhered to the layers  102  or  108 , the shortening of the braided layer  104  attendant to an increase in the angle θ does not affect the overall length L of the sheath. Moreover, because of the longitudinally-extending folds  126  formed in the layers  102  and  108 , the layers  102  and  108  can expand to the second diameter D 2  without rupturing, in spite of being relatively thin and relatively non-elastic. In this manner, the sheath  100  can resiliently expand from its natural diameter D 1  to a second diameter D 2  that is larger than the diameter D 1  as a prosthetic device is advanced through the sheath, without lengthening, and without constricting. Thus, the force required to push the prosthetic implant through the sheath is significantly reduced. 
     Additionally, because of the radial force applied by the elastic layer  106 , the radial expansion of the sheath  100  can be localized to the specific portion of the sheath occupied by the prosthetic device. For example, with reference to  FIG. 6 , as the prosthetic device  12  moves distally through the sheath  100 , the portion of the sheath immediately proximal to the prosthetic device  12  can radially collapse back to the initial diameter D 1  under the influence of the elastic layer  106 . The layers  102  and  108  can also buckle as the circumference of the sheath is reduced, causing the ridges  126  and the valleys  128  to reform. This can reduce the size of the sheath required to introduce a prosthetic device of a given size. Additionally, the temporary, localized nature of the expansion can reduce trauma to the blood vessel into which the sheath is inserted, along with the surrounding tissue, because only the portion of the sheath occupied by the prosthetic device expands beyond the sheath&#39;s natural diameter and the sheath collapses back to the initial diameter once the device has passed. This limits the amount of tissue that must be stretched in order to introduce the prosthetic device, and the amount of time for which a given portion of the vessel must be dilated. 
     In addition to the advantages above, the expandable sheath embodiments described herein can provide surprisingly superior performance relative to known introducer sheaths. For example, it is possible to use a sheath configured as described herein to deliver a prosthetic device having a diameter that is two times larger, 2.5 times larger, or even three times larger than the natural outer diameter of the sheath. For example, in one embodiment, a crimped prosthetic heart valve having a diameter of 7.2 mm was successfully advanced through a sheath configured as described above and having a natural outer diameter of 3.7 mm. As the prosthetic valve was advanced through the sheath, the outer diameter of the portion of the sheath occupied by the prosthetic valve increased to 8 mm. In other words, it was possible to advance a prosthetic device having a diameter more than two times the outer diameter of the sheath through the sheath, during which the outer diameter of the sheath resiliently increased by 216%. In another example, a sheath with an initial or natural outer diameter of 4.5 mm to 5 mm can be configured to expand to an outer diameter of 8 mm to 9 mm. 
     In alternative embodiments, the sheath  100  may optionally include the layer  102  without the layer  108 , or the layer  108  without the layer  102 , depending upon the particular characteristics desired. 
       FIGS. 10A-10D  illustrate another embodiment of the braided layer  104  in which the filaments  110  are configured to buckle. For example,  FIG. 10A  illustrates a unit cell  134  of the braided layer  104  in a configuration corresponding to the braided layer in a fully expanded state. For example, the expanded state illustrated in  FIG. 10A  can correspond to the diameter D 2  described above, and/or a diameter of the braided layer during initial construction of the sheath  100  before the sheath is radially collapsed to its functional design diameter D 1 , as described further below with reference to  FIG. 7 . The angle θ between the filaments  110 A and  110 B can be, for example, 40°, and the unit cell  134  can have a length L x  along the x-direction (note Cartesian coordinate axes shown).  FIG. 10B  illustrates a portion of the braided layer  104 , including an array of unit cells  134  in the expanded state. 
     In the illustrated embodiments, the braided layer  104  is disposed between the polymeric layers  102  and  108 , as described above. For example, the polymeric layers  102  and  108  can be adhered or laminated to each other at the ends of the sheath  100  and/or between the filaments  110  in the open spaces  136  defined by the unit cells  134 . Thus, with reference to  FIGS. 10C and 10D , when the sheath  100  is radially collapsed to its functional diameter D 1 , the diameter of the braided layer  104  can decrease as the angle θ decreases. However, the bonded polymeric layers  102  and  108  can constrain or prevent the braided layer  104  from lengthening as it radially collapses. This can cause the filaments  110  to resiliently buckle in the axial direction, as shown in  FIGS. 10C and 10D . The degree of buckling can be such that the length L x  of the unit cells  134  is the same, or substantially the same, between the collapsed and fully expanded diameters of the sheath. This means that the overall length of the braided layer  104  can remain constant, or substantially constant, between the natural diameter D 1  of the sheath and the expanded diameter D 2 . As the sheath expands from in its initial diameter D 1  during passage of a medical device, the filaments  110  can straighten as the buckling is relieved, and the sheath can radially expand. As the medical device passes through the sheath, the braided layer  104  can be urged back to the initial diameter D 1  by the elastic layer  106 , and the filaments  110  can resiliently buckle again. Using the configuration of  FIGS. 10A-10C , it is also possible to accommodate a prosthetic device having a diameter that is two times larger, 2.5 times larger, or even three times larger than the natural outer diameter D 1  of the sheath. 
     Turning now to methods of making expandable sheaths,  FIG. 7  illustrates the layers  102 - 108  of the expandable sheath  100  disposed on a cylindrical mandrel  118 , according to one embodiment. In certain embodiments, the mandrel  118  can have a diameter D 3  that is greater than the desired natural outer diameter D 1  of the finished sheath. For example, in some embodiments, a ratio of the diameter D 3  of the mandrel to the outer diameter D 1  of the sheath can be 1.5:1, 2:1, 2.5:1, 3:1, or greater. In certain embodiments, the diameter D 3  of the mandrel can be equal to the expanded diameter D 2  of the sheath. In other words, the diameter D 3  of the mandrel can be the same, or nearly the same, as the desired expanded diameter D 2  of the sheath when a prosthetic device is being advanced through the sheath. Thus, in certain embodiments, a ratio of the expanded outer diameter D 2  of the expanded sheath to the collapsed outer diameter D 1  of the unexpanded sheath can be 1.5:1, 2:1, 2.5:1, 3:1, or greater. 
     With reference to  FIG. 7 , the expandable sheath  100  can be made by wrapping or situating an ePTFE layer  120  around the mandrel  118 , followed by the first polymeric layer  102 . In some embodiments, the ePTFE layer can aid in removing the sheath  100  from the mandrel  118  upon completion of the fabrication process. The first polymeric layer  102  may be in the form of a pre-fabricated sheet that is applied by being wrapped around the mandrel  118 , or may be applied to the mandrel by dip-coating, electro-spinning, etc. The braided layer  104  can be situated around the first layer  102 , followed by the elastic layer  106 . In embodiments in which the elastic layer  106  comprises one or more elastic bands  116 , the bands  116  can be helically wrapped around the braided layer  104 . In other embodiments, the elastic layer  106  may be dip-coated, electro-spun, etc. The outer polymeric layer  108  can then be wrapped, situated, or applied around the elastic layer  106 , followed by another layer  122  of ePTFE and one or more layers  124  of heat-shrink tubing or heat-shrink tape. 
     In particular embodiments, the elastic bands  116  can be applied to the braided layer  104  in a stretched, taut, or extended condition. For example, in certain embodiments, the bands  116  can be applied to the braided layer  104  stretched to a length that is twice their natural, relaxed length. This will cause the completed sheath to radially collapse under the influence of the elastic layer when removed from the mandrel, which can cause corresponding relaxation of the elastic layer, as described below. In other embodiments, the layer  102  and the braided layer  104  can be removed from the mandrel, the elastic layer  106  can be applied in a relaxed state or moderately stretched state, and then the assembly can be placed back on the mandrel such that the elastic layer is radially expanded and stretched to a taut condition prior to application of the outer layer  108 . 
     The assembly can then be heated to a sufficiently high temperature that the heat-shrink layer  124  shrinks and compresses the layers  102 - 108  together. In certain embodiments, the assembly can be heated to a sufficiently high temperature such that the polymeric inner and outer layers  102  and  108  become soft and tacky, and bond to each other in the open spaces between the braided layer  104  and the elastic layer  106  and encapsulate the braided layer and the elastic layer. In other embodiments, the inner and outer layers  102 ,  108  can be reflowed or melted such that they flow around and through the braided layer  104  and the elastic layer  106 . In an exemplary embodiment, the assembly can be heated at 150° C. for 20-30 minutes. 
     After heating, the sheath  100  can be removed from the mandrel  118 , and the heat-shrink tubing  124  and the ePTFE layers  120  and  122  can be removed. Upon being removed from the mandrel  118 , the sheath  100  can at least partially radially collapse to the natural design diameter D 1  under the influence of the elastic layer  106 . In certain embodiments, the sheath can be radially collapsed to the design diameter with the optional aid of a crimping mechanism. The attendant reduction in circumference can buckle the filaments  110 , as shown in  FIGS. 10C and 10D , along with the inner and outer layers  102  and  108  to form the longitudinally-extending folds  126 . 
     In certain embodiments, a layer of PTFE can be interposed between the ePTFE layer  120  and the inner layer  102 , and/or between the outer layer  108  and the ePTFE layer  122 , in order to facilitate separation of the inner and outer polymeric layers  102 ,  108  from the respective ePTFE layers  120  and  122 . In further embodiments, one of the inner layer  102  or the outer layer  108  may be omitted, as described above. 
       FIG. 8  illustrates another embodiment of the expandable sheath  100 , including one or more members configured as yarns or cords  130  extending longitudinally along the sheath and attached to the braided layer  104 . Although only one cord  130  is illustrated in  FIG. 8 , in practice, the sheath may include two cords, four cords, six cords, etc., arrayed around the circumference of the sheath at equal angular spacings. The cords  130  can be sutured to the exterior of the braided layer  104 , although other configurations and attachment methods are possible. By virtue of being attached to the braided layer  104 , the cords  130  can be configured to prevent axial elongation of the braided layer  104  when a prosthetic device is passed through the sheath. The cords  130  may be employed in combination with the elastic layer  106 , or separately. The cords  130  may also be used in combination with one or both of the inner and/or outer layers  102  and  108 , depending upon the particular characteristics desired. The cords  130  may also be disposed on the inside of the braided layer  104  (e.g., between the inner layer  102  and the braided layer  104 ). 
     The expandable sheath  100  can also be made in other ways. For example,  FIG. 9  illustrates an apparatus  200 , including a containment vessel  202  and a heating system schematically illustrated at  214 . The apparatus  200  is particularly suited for forming devices (medical devices or devices for non-medical uses) comprised of two or more layers of material. Devices formed by the apparatus  200  can be formed from two or more co-axial layers of material, such as the sheath  100 , or shafts for catheters. Devices formed by the apparatus  200  alternatively can be formed by two or more non-coaxial layers, such as two or more layers stacked on top of each other. 
     The containment vessel  202  can define an interior volume or chamber  204 . In the illustrated embodiment, the vessel  202  can be a metal tube, including a closed end  206  and an open end  208 . The vessel  202  can be at least partially filled with a thermally-expandable material  210  having a relatively high coefficient of thermal expansion. In particular embodiments, the thermally-expandable material  210  may have a coefficient of thermal expansion of 2.4×10 −4 /° C. or greater. Exemplary thermally-expandable materials include elastomers such as silicones materials. Silicone materials can have a coefficient of thermal expansion of from 5.9×10 −4 /° C. to 7.9×10 −4 /° C. 
     A mandrel similar to the mandrel  118  of  FIG. 7  and including the desired combination of sheath material layers disposed around it can be inserted into the thermally-expandable material  210 . Alternatively, the mandrel  118  can be inserted into the chamber  204 , and the remaining volume of the chamber can be filled with the thermally-expandable material  210  so that the mandrel is surrounded by the material  210 . The mandrel  118  is shown schematically for purposes of illustration. As such, the mandrel  118  can be cylindrical, as depicted in  FIG. 7 . Likewise, the inner surface of the material  210  and the inner surface of the vessel  202  can have a cylindrical shape that corresponds to the shape of the mandrel  118  and the final shape of the sheath  100 . To facilitate placement of a cylindrical or rounded mandrel  118 , the vessel  202  can comprise two portions that are connected to each other by a hinge to allow the two portions to move between an open configuration for placing the mandrel inside of the vessel and a closed configuration extending around the mandrel. For example, the upper and lower halves of the vessel shown in  FIG. 9  can be connected to each other by a hinge at the closed side of the vessel (the left side of the vessel in  FIG. 9 ). 
     The open end  208  of the vessel  202  can be closed with a cap  212 . The vessel  202  can then be heated by the heating system  214 . Heating by the heating system  214  can cause the material  210  to expand within the chamber  204  and apply radial pressure against the layers of material on the mandrel  118 . The combination of the heat and pressure can cause the layers on the mandrel  118  to bond or adhere to each other to form a sheath. In certain embodiments, it is possible to apply radial pressure of 100 MPa or more to the mandrel  118  using the apparatus  200 . The amount of radial force applied to the mandrel can be controlled by, for example, the type and quantity of the material  210  selected and its coefficient of thermal expansion, the thickness of the material  210  surrounding the mandrel  118 , the temperature to which the material  210  is heated, etc. 
     In some embodiments, the heating system  214  can be an oven into which the vessel  202  is placed. In some embodiments, the heating system can include one or more heating elements positioned around the vessel  202 . In some embodiments, the vessel  202  can be an electrical resistance heating element or an induction heating element controlled by the heating system  214 . In some embodiments, heating elements can be embedded in the thermally-expandable material  210 . In some embodiments, the material  210  can be configured as a heating element by, for example, adding electrically conductive filler materials, such as carbon fibers or metal particles. 
     The apparatus  200  can provide several advantages over known methods of sheath fabrication, including uniform, highly controllable application of radial force to the mandrel  118  along its length, and high repeatability. The apparatus  200  can also facilitate fast and accurate heating of the thermally-expandable material  210 , and can reduce or eliminate the need for heat-shrink tubing and/or tape, reducing material costs and labor. The amount of radial force applied can also be varied along the length of the mandrel by, for example, varying the type or thickness of the surrounding material  210 . In certain embodiments, multiple vessels  202  can be processed in a single fixture, and/or multiple sheaths can be processed within a single vessel  202 . The apparatus  200  can also be used to produce other devices, such as shafts or catheters. 
     In one specific method, the sheath  100  can be formed by placing layers  102 ,  104 ,  106 ,  108  on the mandrel  118  and placing the mandrel with the layers inside of the vessel  202  with the thermally-expandable material  210  surrounding the outermost layer  108 . If desired, one or more inner layers  120  of ePTFE (or similar material) and one or more outer layers  122  of ePTFE (or similar material) can be used (as shown in  FIG. 7 ) to facilitate removal of the finished sheath from the mandrel  118  and the material  210 . The assembly is then heated with the heating system  214  to reflow the layers  102 ,  108 . Upon subsequent cooling, the layers  102 ,  108  become at least partially bonded to each other and at least partially encapsulate layers  104 ,  106 . 
       FIG. 11  illustrates another embodiment in which the expandable sheath  100  is configured to receive an apparatus configured as a pre-introducer or vessel dilator  300 . In particular embodiments, the introducer device  90  can include the vessel dilator  300 . Referring to  FIG. 12 , the vessel dilator  300  can comprise a shaft member  302 , including a tapered dilator member configured as a nose cone  304  located at the distal end portion of the shaft member  302 . The vessel dilator  300  can further comprise a capsule or retaining member  306  extending proximally from a proximal end portion  308  of the nose cone  304  such that a circumferential space  310  is defined between the exterior surface of the shaft member  302  and the interior surface of the retaining member  306 . In certain embodiments, the retaining member  306  can be configured as a thin polymeric layer or sheet, as further described below. 
     Referring to  FIGS. 11 and 13 , a first or distal end portion  140  of the sheath  100  can be received in the space  310  such that the sheath engages the nose cone  304 , and/or such that the retaining member  306  extends over the distal end portion  140  of the sheath. In use, the coupled or assembled vessel dilator  300  and sheath  100  can then be inserted through an incision into a blood vessel. The tapered cone shape of the nose cone  304  can aid in gradually dilating the blood vessel and access site while minimizing trauma to the blood vessel and surrounding tissue. Once the assembly has been inserted to the desired depth, the vessel dilator  300  can be advanced further into the blood vessel (e.g., distally) while the sheath  100  is held steady, as illustrated in  FIG. 14 . 
     Referring to  FIG. 15 , the vessel dilator  300  can be advanced distally through the sheath  100  until the retaining member  306  is removed from over the distal end portion  140  of the sheath  100 . In certain embodiments, the helically-wrapped elastic layer  106  of the sheath can terminate proximally of the distal end  142  of the sheath. Thus, when the distal end portion  140  of the sheath is uncovered, the distal end portion (which can be heat-set) can flare or expand, increasing the diameter of the opening at the distal end  142  from the first diameter D 1  ( FIG. 13 ) to a second, larger diameter D 2  ( FIG. 15 ). The vessel dilator  300  can then be withdrawn through the sheath  100 , as illustrated in  FIGS. 16-18 , leaving the sheath  100  in place in the vessel. 
     The vessel dilator  300  can include a variety of active and/or passive mechanisms for engaging and retaining the sheath  100 . For example, in certain embodiments, the retaining member  306  can comprise a polymeric heat-shrink layer that can be collapsed around the distal end portion of the sheath  100 . In the embodiment illustrated in  FIG. 1 , the retaining member can comprise an elastic member configured to compress the distal end portion  140  of the sheath  100 . In yet other embodiments, the retaining member  306  and the sheath  100  can be glued or fused (e.g., heat-bonded) together in a manner such that application of a selected amount of force can break the adhesive bonds between retaining member  306  free from the sheath  100  to allow the vessel dilator to be withdrawn. In some embodiments, the end portion of the braided layer  104  can be heat set to flare or expand radially inwardly or outwardly, in order to apply pressure to a corresponding portion of the vessel dilator  300 . 
     Referring to  FIG. 19 , the assembly can include a mechanically-actuated retaining mechanism, such as a shaft  312  disposed between the dilator shaft member  302  and the sheath  100 . In certain embodiments, the shaft  312  can releasably couple the vessel dilator  300  to the sheath  100 , and can be actuated from outside the body (i.e., manually deactivated). 
     Referring to  FIGS. 20 and 21 , in some embodiments, the shaft  304  can comprise one or more balloons  314  arrayed circumferentially around its exterior surface and configured to engage the sheath  100  when inflated. The balloons  314  can be selectively deflated in order to release the sheath  100  and withdraw the vessel dilator. For example, when inflated, the balloons press the captured distal end portion of the sheath  100  against the inner surface of the capsule  306  to assist in retaining the sheath in place relative to the vessel dilator. When the balloons are deflated, the vessel dilator can be more easily moved relative to the sheath  100 . 
     In another embodiment, an expandable sheath configured as described above can further comprise a shrinkable polymeric outer cover, such as a heat-shrink tubing layer  400  shown in  FIG. 22 . The heat-shrink tubing layer  400  can be configured to allow a smooth transition between the vessel dilator  300  and the distal end portion  140  of the sheath. The heat-shrink tubing layer  400  can also constrain the sheath to a selected initial, reduced outer diameter. In certain embodiments, the heat-shrink tubing layer  400  extends fully over the length of the sheath  100  and can be attached to the sheath handle by a mechanical fixation means, such as a clamp, nut, adhesive, heat welding, laser welding, or an elastic clamp. In some embodiments, the sheath is press-fit into the heat-shrink tubing layer during manufacturing. 
     In some embodiments, the heat-shrink tubing layer  400  can extend distally beyond the distal end portion  140  of the sheath as the distal overhang  408  shown in  FIG. 22 . A vessel dilator can be inserted through the sheath lumen  112  and beyond the distal edge of the overhang  408 . The overhang  408  conforms tightly to the inserted vessel dilator to give a smooth transition between the dilator diameter and the sheath diameter to ease insertion of the combined dilator and sheath. When the vessel dilator is removed, overhang  408  remains in the vessel as part of sheath  100 . The heat shrink tubing layer  400  offers the additional benefit of shrinking the overall outer diameter of the sheath along the longitudinal axis. However, it will be understood that some embodiments, such as sheath  301  shown at  FIG. 42  may have a heat-shrink tubing layer  401  that stops at the distal end of the sheath  301  or, in some embodiments, does not extend fully to the distal end of the sheath. In embodiments without distal overhangs, the heat-shrink tubing layer functions mainly as an outer shrinking layer, configured to maintain the sheath in a compressed configuration. Such embodiments will not result in a flapping overhang at the distal end of the sheath once the dilator is retrieved. 
     In some embodiments, the heat-shrink tubing layer can be configured to split open as a delivery apparatus such as the delivery apparatus  10  is advanced through the sheath. For example, in certain embodiments, the heat-shrink tubing layer can comprise one or more longitudinally extending openings, slits, or weakened, elongated scorelines  406  such as those shown in  FIG. 22  configured to initiate splitting of the layer at a selected location. As the delivery apparatus  10  is advanced through the sheath, the heat-shrink tubing layer  400  can continue to split open, allowing the sheath to expand as described above with reduced force. In certain embodiments, the sheath need not comprise the elastic layer  106  such that the sheath automatically expands from the initial, reduce diameter when the heat-shrink tubing layer splits open. The heat shrink tubing layer  400  can comprise polyethylene or other suitable materials. 
       FIG. 23  illustrates a heat-shrink tubing layer  400  that can be placed around the expandable sheaths described herein, according to one embodiment. In some embodiments, the heat-shrink tubing layer  400  can comprise a plurality of cuts or scorelines  402  extending axially along the tubing layer  400  and terminating at distal stress relief features configured as circular openings  404 . It is contemplated that the distal stress relief feature can be configured as any other regular or irregular curvilinear shape including, for example, oval and/or ovoid shaped openings. It is also contemplated various shaped distal stress relief features along and around the heat-shrink tubing layer  400 . As the delivery apparatus  10  is advanced through the sheath, the heat-shrink tubing layer  400  can split open along the scorelines  402 , and the distally positioned openings  404  can arrest further tearing or splitting of the tubing layer along the respective scorelines. As such, the heat-shrink tubing layer  400  remains attached to the sheath along the sheath length. In the illustrated embodiment, the scorelines and associated openings  404  are longitudinally and circumferentially offset from one another or staggered. Thus, as the sheath expands, the scorelines  402  can form rhomboid structures. The scorelines can also extend in other directions, such as helically around the longitudinal axis of the sheath, or in a zig-zag pattern 
     In other embodiments, splitting or tearing of the heat-shrink tubing layer may be induced in a variety of other ways, such as by forming weakened areas on the tubing surface by, for example, applying chemical solvents, cutting, scoring, or ablating the surface with an instrument or laser, and/or by decreasing the wall thickness or making cavities in the tubing wall (e.g., by femto-second laser ablation). 
     In some embodiments, the heat-shrink tubing layer may be attached to the body of the sheath by adhesive, welding, or any other suitable fixation means.  FIG. 29  shows a perspective view of a sheath embodiment including an inner layer  802 , a braided layer  804 , an elastic layer  806 , an outer layer  808 , and a heat shrink tubing layer  809 . As described below with respect to  FIG. 36 , some embodiments may not include elastic layer  806 . Heat shrink tubing layer  809  includes a split  811  and a perforation  813  extending along the heat shrink tubing layer  809 . Heat shrink tubing layer  809  is bonded to the outer layer  808  at an adhesive seam  815 . For example, in certain embodiments, the heat-shrink tubing layer  809  can be welded, heat-bonded, chemically bonded, ultrasonically bonded, and/or bonded using adhesive agents (including, but not limited to, hot glue, for example, LDPE fiber hot glue) at seam  815 . The outer layer  808  can be bonded to the heat shrink tubing layer  809  axially along the sheath at a seam  815 , or in a spiral or helical fashion.  FIG. 30  shows the same sheath embodiment with heat shrink tubing layer  809  split open at the distal end of the sheath. 
       FIG. 31  shows a sheath having a heat shrink tubing layer  809 , but prior to movement of a delivery system therethrough.  FIG. 32  shows a perspective view of a sheath wherein the heat shrink tubing layer  809  has been partially torn open and detached as a passing delivery system widens the diameter of the sheath. Heat shrink tubing layer  809  is being retained by the adhesive seam  815 . Attaching the heat-shrink tubing layer  809  to the sheath in this manner can help to keep the heat-shrink tubing layer  809  attached to the sheath after the layer splits, and the sheath has expanded, as shown in  FIG. 33 , where delivery system  817  has moved completely through the sheath and torn the heat shrink tubing layer  809  along the entire length of the sheath. 
     In another embodiment, the expandable sheath can have a distal end or tip portion comprising an elastic thermoplastic material (e.g., Pebax), which can be configured to provide an interference fit or interference geometry with the corresponding portion of the vessel dilator  300 . In certain configurations, the outer layer of the sheath may comprise polyamide (e.g., nylon) in order to provide for welding the distal end portion to the body of the sheath. In certain embodiments, the distal end portion can comprise a deliberately weakened portion, scoreline, slit, etc., to allow the distal end portion to split apart as the delivery apparatus is advanced through the distal end portion. 
     In other embodiments, the entire sheath could have an elastomeric outer cover that extends longitudinally from the handle to the distal end portion  140  of the sheath, optionally extending onward to create an overhang similar to overhang  408  shown in  FIG. 22 . The elastomeric overhang portion conforms tightly to the vessel dilator but remains a part of the sheath once the vessel dilator is removed. As a delivery system is passed through, the elastomeric overhang portion expands and then collapses to allow it to pass. The elastomeric overhang portion, or the entire elastomeric outer cover, can include deliberately weakened portions, scorelines, slits, etc. to allow the distal end portion to split apart as the delivery apparatus is advanced through the distal end portion. 
       FIG. 24  illustrates an end portion (e.g., a distal end portion) of another embodiment of the braided layer  104  in which portions  150  of the braided filaments  110  are bent to form loops  152 , such that the filaments loop or extend back in the opposite direction along the sheath. The filaments  110  can be arranged such that the loops  152  of various filaments  110  are axially offset from each other in the braid. Moving toward the distal end of the braided layer  104  (to the right in the figure), the number of braided filaments  110  can decrease. For example, the filaments indicated at 5 can form loops  152  first, followed by the filaments indicated at 4, 3, and 2, with the filaments at 1 forming the distal-most loops  152 . Thus, the number of filaments  110  in the braid decreases in the distal direction, which can increase the radial flexibility of the braided layer  104 . 
     In another embodiment, the distal end portion of the expandable sheath can comprise a polymer such as Dyneema®, which can be tapered to the diameter of the vessel dilator  300 . Weakened portions such as dashed cuts, scoring, etc., can be applied to the distal end portion such that it will split open and/or expand in a repeatable way. 
     Crimping of the expandable sheath embodiments described herein can be performed in a variety of ways, as described above. In additional embodiments, the sheath can be crimped using a conventional short crimper several times longitudinally along the longer sheath. In other embodiments, the sheath may be collapsed to a specified crimped diameter in one or a series of stages in which the sheath is wrapped in heat-shrink tubing and collapsed under heating. For example, a first heat shrink tube can be applied to the outer surface of the sheath, the sheath can be compressed to an intermediate diameter by shrinking the first heat shrink tube (via heat), the first heat shrink tube can be removed, a second heat shrink tube can be applied to the outer surface of the sheath, the second heat shrink tube can be compressed via heat to a diameter smaller than the intermediate diameter, and the second heat shrink tube can be removed. This can go on for as many rounds as necessary to achieve the desired crimped sheath diameter. 
     Crimping of the expandable sheath embodiments described herein can be performed in a variety of ways, as described above. A roller-based crimping mechanism  602 , such as the one shown in  FIGS. 25A-25C  can be advantageous for crimping elongated structures such as the sheaths disclosed herein. The crimping mechanism  602  has a first end surface  604 , a second end surface  605 , and a longitudinal axis a-a extending between the first and second end surfaces  604 ,  605 . A plurality of disc-shaped rollers  606   a - f  are radially arranged about the longitudinal axis a-a, each positioned at least partially between the first and second end surfaces of the crimping mechanism  602 . Six rollers are depicted in the embodiment shown, but the number of rollers may vary. Each disc-shaped roller  606  is attached to the larger crimping mechanism by a connector  608 . A side cross-sectional view of an individual disc-shaped roller  606  and connector  608  is shown in  FIG. 25B , and a top view of an individual disc-shaped roller  606  and connector  608  is shown in  FIG. 25C . An individual disc-shaped roller  606  has a circular edge  610 , a first side surface  612 , a second side surface  614 , and a central axis c-c extending between center points of first and second side surfaces  612 ,  614 , as shown in  FIG. 25C . The plurality of disc-shaped rollers  606   a - f  are radially arranged about the longitudinal axis a-a of the crimping mechanism  602  such that each central axis c-c of a disc-shaped roller  606  is oriented perpendicularly to the longitudinal axis a-a of the crimping mechanism  602 . The circular edges  610  of the disc-shaped rollers partially define a passage that extends axially through the crimping mechanism  602  along longitudinal axis a-a. 
     Each disc-shaped roller  606  is held in place in the radially arranged configuration by a connector  608  that is attached to crimping mechanism  602  via one or more fasteners  619 , such that the location of each of the plurality of connectors is fixed with respect to the first end surface of the crimping mechanism  602 . In the depicted embodiment, fasteners  619  are positioned adjacent an outer portion of the crimping mechanism  602 , radially outwardly of the disc-shaped rollers  606 . Two fasteners  619  are used to position each connector  608  in the embodiment shown, but the number of fasteners  619  can vary. As shown in  FIGS. 25B and 25C , a connector  608  has a first arm  616  and a second arm  618 . First and second arms  616 ,  618  extend over a disc-shaped roller  608  from a radially-outward portion of circular edge  610  to a central portion of the disc-shaped roller  608 . A bolt  620  extends through the first and second arms  616 ,  618  and through a central lumen of the disc-shaped roller  608 , the central lumen passing from a center point of front surface  612  to a center point of the back surface  614  of the disc-shaped roller  606  along central axis c-c. The bolt  620  is positioned loosely within the lumen, with substantial clearance/space to allow the disc-shaped roller  608  to rotate about central axis c-c. 
     During use, an elongated sheath is advanced from the first side  604  of the crimping mechanism  602 , through the axial passage between the rollers, and out the second side  605  of the crimping mechanism  602 . The pressure from the circular edge  610  of the disc shaped rollers  606  reduces the diameter of the sheath to a crimped diameter as it rolls along the outer surface of the elongated sheath. 
       FIG. 26  shows an embodiment of a crimping device  700  designed to facilitate crimping of elongated structures, such as sheaths. The crimping device includes an elongated base  704 , and elongated mandrel  706  positioned above the elongated base  704 , and a holding mechanism  708  attached to the elongated base  704 . The holding mechanism  708  supports the mandrel  706  in an elevated position above base  704 . The holding mechanism includes a first end piece  710  that includes a crimping mechanism  702 . The mandrel  706  includes a conical end portion  712  that nests within a first tapered portion  713  of a narrowing lumen  714  of the first end piece  710 . The conical end portion  712  of mandrel  706  is positioned loosely within the narrowing lumen  714 , with enough space or clearance between the conical end portion  712  and the lumen  714  to allow for passage of an elongated sheath over the conical end portion  712  of mandrel  706  and through the narrowing lumen  714 . During use, the conical end portion  712  helps to avoid circumferential buckling of the sheath during crimping. In some embodiments, the mandrel  706  can also include a cylindrical end portion  724  that extends outwardly from the conical end portion  712  and defines an end  726  of the mandrel  706 . 
     The first tapered portion  713  of the narrowing lumen  714  opens toward a second end piece  711  of the holding mechanism  708 , such that the widest side of the taper is located on an inner surface  722  of the first end piece  710 . In the embodiment shown, the first tapered portion  713  narrows to a narrow end  715  that connects with a narrow cylindrical portion  716  of the narrowing lumen  714 . In this embodiment, the narrow cylindrical portion  716  defines the narrowest diameter of the narrowing lumen  714 . The cylindrical end portion  724  of the mandrel  706  may nest loosely within the narrow cylindrical portion  716  of the narrowing lumen  714 , with enough space or clearance between the cylindrical end portion  724  and the narrow cylindrical portion  716  of the lumen to allow for passage of the elongated sheath. The elongated nature of the narrow cylindrical portion  716  may facilitate smoothing of the crimped sheath after it has passed over the conical end portion  712  of the mandrel. However, the length of the cylindrical portion  716  of the narrowing lumen  714  is not meant to limit the invention, and in some embodiments, the crimping mechanism  702  may only include first tapered portion  713  of the narrowing lumen  714 , and still be effective to crimp an elongated sheath. 
     At the opposite end of the first end piece  710  shown in  FIG. 26 , a second tapered portion  718  of the narrowing lumen  714  opens up from narrow cylindrical portion  716  such that the widest side of the taper located on the outer surface  720  of the first end piece  710 . The narrow end  719  of the second tapered portion  718  connects with the narrow cylindrical portion  716  of the narrowing lumen  714  in the interior of the crimping mechanism  702 . The second tapered portion  718  of the narrowing lumen  714  may not be present in some embodiments. 
     The holding mechanism  708  further includes a second end piece  711  positioned opposite the elongated base  704  from the first end piece  710 . The second end piece  711  is movable with respect to elongated base  704 , such that the distance between the first end piece  710  and the second end piece  711  is adjustable and, therefore, able to support mandrels of varying sizes. In some embodiments, elongated base  704  may include one or more elongated sliding tracks  728 . The second end piece  711  can be slidably engaged to the sliding track  728  via at least one reversible fastener  730 , such as, but not limited to, a bolt that extends into or through the second end piece  711  and the elongated sliding track  728 . To move the second end piece  711 , the user would loosen or remove the reversible fastener  730 , slide the second end piece  711  to the desired location, and replace or tighten the reversible fastener  730 . 
     In use, a sheath in an uncrimped diameter can be placed over the elongated mandrel  706  of the crimping device  700  shown in  FIG. 26 , such that the inner surface of the entire length of the uncrimped sheath is supported by the mandrel. The uncrimped sheath is then advanced over the conical end portion  712  and through the narrowing lumen  714  of the crimping mechanism  702 . The uncrimped sheath is crimped to a smaller, crimped diameter via pressure from the interior surface of the narrowing lumen  714 . In some embodiments, the sheath is advanced through both a first tapered portion  713  and a cylindrical portion  716  of the narrowing lumen  714  before exiting the crimping mechanism  702 . In some embodiments, the sheath is advanced through a first tapered portion  713 , a cylindrical portion  716 , and a second tapering portion  718  of the narrowing lumen  714  before exiting the crimping mechanism  702 . 
     In some embodiments, the crimping mechanism  602  shown in  FIG. 25A  may be positioned within a larger crimping device such as crimping device  700  shown in  FIG. 26 . For example, the crimping mechanism  602  can be positioned within the first end piece  710  of crimping device  700  instead of, or in combination with, crimping mechanism  702 . For example, the rolling crimping mechanism  602  could entirely replace the narrowing lumen  714  of crimping mechanism  702 , or the rolling crimping mechanism  602  could be nested within the narrow cylindrical portion  716  of the narrowing lumen  714  of the crimping mechanism  702 , such that the first tapered portion  713  feeds the expandable sheath through the plurality of radially arranged disc-shaped rollers  606 . 
       FIGS. 34-35  show a sheath embodiment including a distal end portion  902 , which can be an extension of an outer cover extending longitudinally along the sheath in the proximal direction.  FIG. 34  shows a distal end portion  902  folded around an introducer (in the crimped and collapsed configuration).  FIG. 35  shows a cross section of the distal end portion  902  folded around the introducer  908  (in the crimped and collapsed configuration). The distal end portion  902  can be formed of, for example, one or more layers of a similar or the same material used to form the outer layer of the sheath. In some embodiments, the distal end portion  902  includes an extension of the outer layer of the sheath, with or without one more additional layers added by separate processing techniques. The distal end portion can include anywhere from 1 to 8 layers of material (including 1, 2, 3, 4, 5, 6, 7, and 8 layers of material). In some embodiments, the distal end portion comprises multiple layers of a Dyneema® material. The distal end portion  902  can extend distally beyond a longitudinal portion of the sheath that includes braided layer  904  and elastic layer  906 . In fact, in some embodiments, the braided layer  904  may extend distally beyond the elastic layer  906 , and the distal end portion  902  may extend distally beyond both the braided layer  904  and elastic layer  906 , as shown in  FIGS. 34-35 . 
     The distal end portion  902  may have a smaller collapsed diameter than the more proximal portions of the sheath, giving it a tapered appearance. This smooths the transition between the introducer/dilator and the sheath, ensuring that the sheath does not get lodged against the tissue during insertion into the patient. The smaller collapsed diameter can be a result of multiple folds (for example, 1, 2, 3, 4, 5, 6, 7, or 8 folds) positioned circumferentially (evenly or unevenly spaced) around the distal end portion. For example, a circumferential segment of the distal end portion can be brought together and then laid against the adjacent outer surface of the distal end portion to create an overlapping fold. In the collapsed configuration, the overlapping portions of the fold extend longitudinally along the distal end portion  902 . Exemplary folding methods and configurations are described in U.S. application Ser. No. 14/880,109 and U.S. application Ser. No. 14/880,111, each of which are hereby incorporated by reference in their entireties. Scoring can be used as an alternative, or in addition to folding of the distal end portion. Both scoring and folding of the distal end portion  902  allow for the expansion of the distal end portion upon the passage of the delivery system, and ease the retraction of the delivery system back into the sheath once the procedure is complete. In some embodiments, the distal end portion of the sheath (and/or of the vessel dilator) can decrease from the initial diameter of the sheath (e.g., 8 mm) to 3.3 mm (10F), and may decrease to the diameter of a guidewire, allowing the sheath and/or the vessel dilator  300  to run on a guidewire. 
     In some embodiments, a distal end portion can be added, the sheath and tip can be crimped, and the crimping of the distal end portion and sheath can be maintained, by the following method. As mentioned above, the distal end portion  902  can be an extension of the outer layer of the sheath. It can also be a separate, multilayer tubing that is heat bonded to the remainder of the sheath prior to the tip crimping processing steps. In some embodiments, the separate, multilayer tubing is heat bonded to a distal extension of the outer layer of the sheath to form the distal end portion  902 . For crimping of the sheath after tip attachment, the sheath is heated on a small mandrel. The distal end portion  902  can be folded around the mandrel to create the folded configuration shown in  FIG. 34 . The folds be added to the distal end portion  902  prior to the tip crimping process, or at an intermediate point during the tip crimping process. In some embodiments, the small mandrel can be from about 2 millimeters to about 4 millimeters in diameter (including about 2.2 millimeters, about 2.4 millimeters, about 2.6 millimeters, about 2.8 millimeters, about 3.0 millimeters, about 3.2 millimeters, about 3.4 millimeters, about 3.6 millimeters, about 3.8 millimeters and about 4.0 millimeters). The heating temperature will be lower than the melting point of the material used. This can cause the material to shrink on its own to a certain extent. For example, in some embodiments, such as those where Dyneema® materials are utilized as a part of the sheath outer layer and/or distal end portion materials, a sheath crimping process begins by heating the sheath on a 3 millimeter mandrel to about 125 degrees Celsius (lower than Dyneema® melting point of about 140 degrees Celsius). This causes the sheath to crimp itself to about a 6 millimeter outer diameter. At this point, the sheath and distal end region  902  are allowed to cool. A heat shrink tube can then be applied. In some embodiments, the heat shrink tube can have a melting point that is about the same as the melting point of the distal end portion material. The sheath with the heat shrink tube extending over the sheath and the distal end portion  902  is heated again (for example, to about 125 degrees Celsius for sheaths including Dyneema® outer layers and distal end portions), causing the sheath to crimp to an even smaller diameter. At the distal end portion  902 , a higher temperature can be applied (for example, from about 145 degrees Celsius to about 155 degrees Celsius for Dyneema® material), causing the layers of material to melt together in the folded configuration shown in  FIG. 34  (the folds can be added at any point during this process). The bonds at the distal end portion  902  induced by the high temperature melting step will still be weak enough to be broken by a passing delivery system. As a final step, the heat shrink tube is removed, and the shape of the sheath remains at the crimped diameter. 
       FIG. 43  shows a transverse cross section taken near the distal end of another sheath embodiment, at a point longitudinally distal to the braided layer. The sheath  501  includes an inner polymeric layer  513 , an outer polymeric layer  517 , and an outer covering  561 . A method of compressing the distal portion of an expandable sheath can include: covering at pre-crimped state the distal portion of the expandable sheath  501  with an external covering layer  561  having a melting temperature TM1 which is lower than the melting temperature TM2 of the inner and outer polymeric layers; heating at least one region, which does not span the entire area of overlap between the cover layer  561  and the expandable sheath  501 , to a first temperature which is equal or higher than TM2, thereby melting both the covering layer  561  and the outer polymeric layer  517  of the expandable sheath  501 , so as to create at attachment region  569  there between; inserting a mandrel into the lumen of the expandable sheath  501  and crimping at least a portion thereof, such as the distal portion, of the expandable sheath  501 ; heating the external covering layer  561  over the distal portion of the expandable sheath  501  to a second temperature which is at least equal to or higher than the melting temperature TM1 of the external covering layer  561 , and lower than the melting temperature TM2 of the inner and outer polymeric layers, for a predefined first time window. 
     This method advantageously avoids risks that a tear initiated at a score or split line (such as perforation  813  shown in  FIG. 29 ) should divert from the intended axial direction of tear propagation due to defects (weakened points or unintended apertures) in the heat-shrink tubing. This method further enables choosing an external covering layer made of materials that may be heated to form moderately attached folds at temperatures lower than those required for the internal or external layers of the expandable sheath. 
     The crimping of the inner and outer polymeric layers  513 ,  517  and the external covering layer  561  can be, for example, from a pre-compressed diameter of about 8.3 mm to a compressed diameter of about 3 mm.  FIG. 44  shows a transverse cross section of the embodiment of  FIG. 43  during crimping. Folds  563  are created along the external layer  561  during crimping. The heating to the second temperature is sufficient to melt the external covering layer  561  so as to attach the fold  563  to each other, while avoiding similar melting and attachment of the inner and outer polymeric layers. 
     The method of compressing the distal portion of the expandable sheath can further include a step of covering the expandable sheath  501  and the external covering layer  561  with a heat-shrink tube (HST) prior to, during or following the heating to the second temperature, wherein the second temperature further acts to shrink the HST in order to retain the external covering layer  561  and the expandable sheath  501  in a compressed state. The HST can be removed from the expandable sheath  501  and the external covering layer  561  after the folds  563  of the covering layer  563  are sufficiently attached to each other in the desired compressed state, and cooled down for a sufficient period of time. 
     According to some embodiments, the HST is further utilized as a heat shrink tape, to apply the external radial pressure by wrapping and heating it over the external covering layer  561  and the expandable sheath  501 . 
     According to some embodiments, a non-heat-shrink tape can be used instead of a heat shrink tube. 
       FIG. 45  shows a distal portion of an expandable sheath  501  having an expandable braid  521 , wherein its distal portion is covered by an external covering layer  561 , which is shown to extend along a length L 1  up to the distal edge  567  of the expandable sheath  501 . D 1  denotes the distal diameter of the expandable sheath  501  in the pre-compressed state.  FIG. 46  shows the distal portion of the expandable sheath  501  in a compressed state, wherein its distal diameter D 2  is smaller than D 1 . It should be noted that compressing the external covering layer  561 , from an uncompressed state to a compressed state of the expandable sheath  501 , results in formation of folds  563  ( FIGS. 44 and 46 ) along the external covering layer  561  as well as layers  517  and  513 , when reaching the compressed state, due to the diameter reduction thereof. It is desirable to promote moderate attachment between the folds  563 . The term “moderate attachment,” as used herein, refers to an attachment force sufficient in magnitude to form a structural cover maintaining the expandable sheath  501  in a compressed state prior to advancement of a DS component through its lumen, yet low enough so that advancement of the DS component therethrough is sufficient to break or disconnect the attachments  565  between the folds  563  ( FIG. 44 ), thereby enabling expansion of the expandable sheath  501 . 
     The external covering layer  561  is chosen such that its melting temperature TM1 is lower than the melting temperature TM2 of the polymeric layers of the expandable sheath  100 , in order to promote folds  563  formation with moderate attachment in the external covering layer  561 , while avoiding melting and attaching similar folds in the polymeric layers  513  and  517  of the expandable sheath  501 . 
     According to some embodiments, the external covering layer  561  is low density polyethylene. Other suitable materials, as known in the arts, such as polypropylene, thermoplastic polyurethane, and the like, may be utilized to form the external covering layer  561 . 
       FIGS. 45 and 46  show perspective views of a sheath embodiment that is similar to or the same as  FIGS. 43 and 44 . The external covering layer  561  and expandable sheath  501  were heated to a first temperature TM2 along a circumferential interface therebetween at the proximal end of the external covering layer  561 , to form a circumferential proximal attachment region  569 . 
     According to some embodiments, the external covering layer  561  is attached different attachment regions, such as along a longitudinally oriented attachment line, to the external surface of the expandable sheath  501  (e.g., the outer polymeric layer). According to some embodiments, the external covering layer  561  is attached to the external surface of the expandable sheath  501  by a plurality of circumferentially spaced attachment regions wherein the circumferential distance between adjacent attachment regions is chosen to allow formation of folds  563  therebetween. Attachment regions, such as  569 , ensure that the external covering layer  561  always remains attached to the expandable sheath  501 , either during the compressed or expanded states thereof. 
     According to some embodiments, the covering with an external covering layer  561  is performed after crimping the expandable sheath  501 , such that the external layer  561  covers pre-formed folds of inner  513  and/or outer  517  layers of the sheath  501 . 
     According to some embodiments, the bond between the folds  563  is based on an adhesive with moderate adhesion strength. 
     Embodiments of the sheaths described herein may comprise a variety of lubricious outer coatings, including hydrophilic or hydrophobic coatings, and/or surface blooming additives or coatings. 
       FIG. 27  illustrates another embodiment of a sheath  500  comprising a tubular inner layer  502 . The inner layer  502  may be formed from an elastic thermoplastic material such as nylon, and can comprise a plurality of cuts or scorelines  504  along its length such that the tubular layer  502  is divided into a plurality of long, thin ribs or portions  506 . When the delivery apparatus  10  is advanced through the tubular layer  502 , the scorelines  504  can resiliently expand or open, causing the ribs  506  to splay apart, and allowing the diameter of the layer  502  to increase to accommodate the delivery apparatus. 
     In other embodiments, the scorelines  504  can be configured as openings or cutouts, having various geometrical shapes, such as rhombuses, hexagons, etc., or combinations thereof. In the case of hexagonal openings, the openings can be irregular hexagons with relatively long axial dimensions to reduce foreshortening of the sheath when expanded. 
     The sheath  500  can further comprise an outer layer (not shown), which can comprise a relatively low durometer, elastic thermoplastic material (e.g., Pebax, polyurethane, etc.), and which can be bonded (e.g., by adhesive or welding, such as by heat or ultrasonic welding, etc.) to the inner nylon layer. Attaching the outer layer to the inner layer  502  can reduce axial movement of the outer layer relative to the inner layer during radial expansion and collapse of the sheath. The outer layer may also form the distal tip of the sheath. 
       FIG. 28  illustrates another embodiment of a braided layer  600  that can be used in combination with any of the sheath embodiments described herein. The braided layer  600  can comprise a plurality of braided portions  602 , in which filaments of the braided layer are braided together, and unbraided portions  604 , in which the filaments are not braided, and extend axially without being intertwined. In certain embodiments, the braided portions  602  and unbraided portions  604  can alternate along the length of the braided layer  600 , or maybe incorporated in any other suitable pattern. The proportion of the length of the braided layer  600  given to braided portions  602  and unbraided portions  604  can allow the selection and control of the expansion and foreshortening properties of the braided layer. 
       FIG. 47  depicts an embodiment of a braided layer  601  having at least one radiopaque strut or filament. The expandable sheath  601  and its expandable braided layer  621  is shown without the polymeric layers, as would be visualized in the x-ray fluoroscopy, for purposes of illustration. As shown in  FIG. 47 , the expandable braided layer  621  comprises a plurality of crossing struts  623 , which can further form distal crowns  633 , for example, in the form of distal loops or eyelets at the distal portion of the expandable sheath  601 . 
     The expandable sheath  601  is configured for advancement in a pre-compressed state up to a target area, for example, along the abdominal aorta or the aortic bifurcation, at which point the clinician should cease further advancement thereof and introduce the DS through its lumen, to facilitate expansion thereof. For that end, the clinician should receive a real-time indication of the expandable sheath&#39;s position during advancement thereof. According to an aspect of the invention, there is provided at least one radio-opaque marker at or along at least one region of the expandable braided layer  621 , configured to enable visualization of the expandable sheath&#39;s position under radio fluoroscopy. 
     According to one embodiment, at least one of the distal crowns  633  comprises a radio-opaque marker. According to some embodiments, the distal crowns  633  comprise at least one gold-plated crown  635  ( FIG. 47 ), configured to serve as a radio-opaque marker. It will be clear that gold-plating is merely an example and that the crowns  635  can comprise other radio-opaque material known in the art, such as tantalum, platinum, iridium and the like. 
     Since the expandable sheath  601  comprises an expandable braided layer  621  having a plurality of crossing struts  623  disposed along its length, this structure can be advantageously utilized for more convenient incorporation of radio-opaque elements. 
     According to some embodiments, the struts  623  further comprise at least one radio-opaque strut  625 , having a radio-opaque core. For example, a drawn filled tubing (DFT) wire comprising a gold core (as may be provided by, for example, Fort Wayne Metals Research Products Corp.) may serve as a radio-opaque strut  625 .  FIG. 47  shows an exemplary expandable braided layer  621  comprising a plurality of less-opaque struts or filaments  623  and radio-opaque struts or filaments  625   a ,  625   b  and  625   c . In some instances, the struts  625   a  and  625   c  can be made of a single wire, wherein the wire extends along the path of strut  625   a , loops at the distal crown  635  and extends along the path of strut  625   c  therefrom. Thus, a single wire, such as a DFT wire, can be utilized to form radio-opaque struts  625   a  and  625   c  and radio-opaque distal crown  635 . 
     Since radio-opaque wires, such as a DFT wire, can be costly, the expandable braided layer  621  can comprise a plurality of non-radio-opaque or less radio-opaque struts  623 , for example, made of a shape-memory alloy such as Nitinol and polymer wire such as PET, respectively, intertwined with at least one radio-opaque strut  625  ( FIG. 47 ). 
     According to some embodiments, radio-opaque wires are embedded within the polymer braid, such as the outer polymeric layer  617  or the inner polymeric layer  615 , which are made of less-opaque materials. 
     Advantageously, the expandable braid embedded within the expandable sheath is utilized according to the invention, for incorporating radio-opaque markers along specific portions thereof to improve visualization of the sheath&#39;s position in real-time under radio fluoroscopy. 
     According to yet another aspect of the invention, radiopaque tubes can be threaded on the distal crowns or loops  633 , or radiopaque rivets can be swaged on the distal crowns or loops  633  to improve their visibility under fluoroscopy. 
       FIG. 36  shows a longitudinal cross section of another embodiment of expandable sheath  11  (positioned on mandrel  91  during the fabrication process, under compression by heat shrink tube  51 ). The sheath  11  comprises a braided layer  21 , but lacks the elastic layer described in the previous embodiments. The heat applied during the shrinking procedure may promote at least partial melting of the inner  31  and outer  41  polymeric layers. Since the filaments of the braid define open cells therebetween, uneven outer surfaces may be formed when the inner  31  and outer  41  polymeric layers melt into the cell openings and over the filaments of the braided layer  21 . 
     In order to mitigate uneven surface formations, cushioning polymeric layers  61   a ,  61   b  are added between the inner  31  and outer  41  layers of the sheath  11 , configured to evenly spread the forces acting in the radial direction during sheath compression. A first cushioning layer  61   a  is placed between the inner polymeric layer  31  and the braided layer  21 , and a second cushioning layer  61   b  is placed between the outer polymeric layer  41  and the braided layer  21 . 
     The cushioning layers  61   a ,  61   b  can comprise a porous material having a plurality of micropores of nanopores  63  ( FIGS. 37-38 ) in a porous interior region. One such material includes, but is not limited to, expanded polytetrafluoroethylene (ePTFE). A porous cushioning layer can advantageously be formed with a minimal thickness h 1  required to sufficiently spread the compression forces to prevent uneven surface formation along the inner  31  and outer  41  polymeric layers. Thickness h 1  is measured in the radial direction (from an inner surface to an outer surface) of the cushioning layer and can be from about 80 microns to about 1000 microns (including, for example, about 80 microns, about 90 microns, about 100 microns, about 110 microns, about 120 microns, about 130 microns, about 140 microns, about 150 microns, about 160 microns, about 170 microns, about 180 microns, about 200 microns, about 250 microns, about 300 microns, about 350 microns, about 400 microns, about 450 microns, about 500 microns, about 550 microns, about 600 microns, about 650 microns, about 700 microns, about 750 microns, about 800 microns, about 850 microns, about 900 microns, about 950 microns, and about 1000 microns). In some embodiments, the range of thickness h 1  is from about 110 to 150 microns. 
     However, when cushioning layers comprise a plurality of micropores of nanopores  63  ( FIGS. 37-38 ), the inner  31  and outer  41  polymeric layers may melt into the pores of the cushioning layers  61   a ,  61   b  upon heating during the fabrication process. In order to prevent the inner  31  and outer  41  polymeric layers from melting into the pores  63  of the cushioning layer  61 , a first sealing layer  71   a  can be placed between the inner polymeric layer  31  and the first cushioning layer  61   a , and a second sealing layer  71   b  can be placed between the outer polymeric layer  41  and the second cushioning layer  61   b . (as shown in  FIG. 36 ). The sealing layers  71   a ,  71   b  can have a higher melting point than the polymeric layers  31  and  41 , and can be formed of a non-porous material (such as, but not limited to, polytetrafluoroethylene) in order to prevent fluid flow therethrough. The thickness h 2  of each sealing layer  71  ( FIG. 37 ), measured in a radial direction from the inner to the outer surface of the sealing layer, can be much thinner than that of the cushioning layer  61 , for example, from about 15 to about 35 microns (including about 15 microns, about 20 microns, about 25 microns, about 30 microns, and about 35 microns). 
     While advantageous for the reasons described above, the addition of the cushioning and sealing can increase the complexity and time required to assemble the sheath  11 . Advantageously, providing a single sealed cushioning member, configured to provide both cushioning and sealing functionalities (instead of providing two separate cushioning and sealing layers, each configured to provide one functionality) reduces sheath assembly time and significantly simplifies the process. According to an aspect of the invention, there is provided a single sealed cushioning member, configured for placement between the inner and outer polymeric layers of the sheath and the central braided layer. The single sealed cushioning member includes a cushioning layer and a sealed surface configured to prevent leakage/melting into the pores in the radial direction. 
       FIG. 37  shows an embodiment of a single sealed cushioning member  81 ′, comprising a cushioning layer  61  having a width thickness h 1  as elaborated hereinabove, fixedly attached to a corresponding sealing layer  71  having a thinner thickness h 2  to form the sealed surface. The sealing layer  71  and the cushioning layer  61  are pre-assembled or pre-attached to each other to form together a single member  81 ′, for example, by gluing, welding and the like. 
       FIG. 38  shows one embodiment of a single sealed cushioning member  81 , comprising a cushioning layer  61  having a width thickness h 1 , wherein the cushioning layer  61  is provided with at least one sealed surface  65 , configured to face an inner  31  or an outer  41  polymeric layer when assembled in the sheath  11 . According to some embodiments, the sealed surface  65  can be formed by a surface treatment configured to fluidly seal a surface of the cushioning layer  61 . As such, the sealed surface  65  can be the same material as the cushioning layer  61 . 
     According to another aspect of the invention, and as mentioned above, with respect to  FIG. 36 , a minimum of three layers may be sufficient to retain the sheath&#39;s expandability provided with the preferable resistance to axial elongation. This is accomplished by eliminating the need to incorporate an additional elastic layer in the sheath, thereby advantageously reducing production costs and simplifying manufacturing procedures. 
     The sheath does not necessarily return to an initial diameter, but may rather remain in an expanded diameter upon passage of the valve, in the absence of the elastic layer. 
       FIGS. 39-40  show an expandable sheath  101  similar to the expandable sheath  100  shown in  FIG. 3 , but without an elastic layer  106 . The inner and outer layers  103  and  109  may be structured and configured to resist axial elongation of the sheath  101  during expansion. However, in the proposed configuration, the absence of an elastic layer results in the sheath  101  remaining in an expanded diameter along the sheath&#39;s portion proximal to the valve, without necessarily collapsing back to the initial diameter D 1  after the valve passes in in the longitudinal direction.  FIG. 39  is a schematic representation of the sheath  101  remaining in an expanded diameter D 2  along the portion proximal to the valve&#39;s passage. 
     Thus, there is provided an expandable sheath for deploying a medical device, comprising a first polymeric layer, a braided layer radially outward of the first polymeric layer, and a second polymeric layer radially outward of the braided layer. The braided layer includes a plurality of filaments braided together. The second polymeric layer is bonded to the first polymeric layer such that the braided layer is encapsulated between the first and second polymeric layers. When a medical device is passed through the sheath, the diameter of the sheath expands from a first diameter to a second diameter around the medical device, while the first and second polymeric layers resist axial elongation of the sheath such that the length of the sheath remains substantially constant. However, according to some embodiments, the first and second polymeric layers are not necessarily configured to resist axial elongation. 
     According to another aspect of the invention, the expandable sheath does include an elastic layer. But, unlike elastic layer  106  shown in  FIG. 3 , the elastic layer is not configured to apply a substantial radial force. It can still serve to provide column strength to the sheath. By limiting tangential (diametrical) expansion of the braid, the elastic layer enhances the strength of the braid and the sheath in the axial direction (column strength). As such, the use of elastic materials with higher tensile strengths (resistance to stretch) will result in a sheath with greater column strength. Likewise, elastic materials that are under greater tension in the free state will also result in a sheath with greater column strength during pushing, as they will be more resistant to stretch. The pitch of any helically wound elastic layers is another variable that contributes to the column strength of the sheath. The additional column strength ensures that the sheath does not spontaneously expand due to frictional forces applied thereto during forward movement in a distal direction, and does not buckle when the delivery system is pulled out of the sheath. 
     In another optional embodiment, the elastic layer can be applied by dip coating in an elastic material (such as, but not limited to) silicone or TPU. The dip coating can be applied to the polymeric outer layer, or to the braided layer. 
     Thus, there is provided an expandable sheath for deploying a medical device, comprising a first polymeric layer, a braided layer radially outward of the first polymeric layer, an elastic layer radially outward of the braided layer, and a second polymeric layer radially outward of the braided layer. The braided layers comprise a plurality of filaments braided together. The elastic layer is configured to provide the expandable sheath with sufficient column strength to resist buckling of spontaneous expansion due to friction forces applied thereto by a surrounding anatomical structure during the sheath&#39;s movement in an axial direction. The second polymeric layer is bonded to the first polymeric layer such that the braided layer is encapsulated between the first and second polymeric layers. When a medical device is passed through the sheath, the diameter of the sheath expands from a first diameter to a second diameter around the medical device, optionally while the first and second polymeric layers resist axial elongation of the sheath such that the length of the sheath remains substantially constant. 
     According to an aspect of the invention, there is provided a three-layered expandable sheath, comprising an inner polymeric layer, an outer polymeric layer bonded to the inner polymeric layer and a braided layer encapsulated between the inner and outer polymeric layers, wherein the braided layer comprises an elastic coating. 
       FIG. 41  shows a transverse cross section of expandable sheath  201 . The expandable sheath  201  includes inner and outer polymeric layers  203  and  209  and a braided layer  205 . Instead of the elastic layer described with reference to  FIG. 3 , above, the braided layer  205  is provided with an elastic coating  207 . The elastic coating  207  can be applied directly to the filaments of the braided layer  205 , as shown in  FIG. 41 . The elastic coating can be made of synthetic elastomers, exhibiting properties similar to those described in conjunction with the elastic layer  106 . 
     In some embodiments, the second, outer polymeric layer  209  is bonded to the first, inner polymeric layer  203  such that the braided layer  205  and the elastic coating  207  are encapsulated between the first and second polymeric layers. Moreover, the elastic coating applied directly to the braided filaments is configured to serve the same function as that of the elastic layer  106  (that is, to apply radial force on the braided layer and the first polymeric layer). 
     While the embodiment of  FIG. 41  shows the elastic coating  207  covering the entire circumference of every filament of the braided layer  205 , it will be understood that only a portion of the filaments, for example, a portion constituting essentially an outer surface of the braided layer, may be coated by the elastic coating  207 . 
     Alternatively, or additionally, an elastic coating can be applied to other layers of the sheath. 
     In some embodiments, a braided layer such as the one shown in  FIG. 40  can have a self-contractible frame made of a shape-memory material, such as, but not limited to, Nitinol. The self-contracting frame can be pre-set to have a free-state diameter equal to the sheath&#39;s initial compressed diameter D 1 , for example, prior to being placed on a mandrel around the first polymeric layer. The self-contracting frame may expand to a larger diameter D 2  while an inner device, such as a prosthetic valve, passes through the sheath&#39;s lumen and self-contract back to the initial diameter D 1  upon passage of the valve. In some embodiments, the filaments of the braid are the self-contracting frame and are made of a shape-memory material. 
     According to another aspect, an expandable sheath can include a braided expandable layer attached to at least one expandable sealing layer. In some embodiments, the braided layer and the sealing layer are the only two layers of the expandable sheath. The braided layer is passively or actively expandable relative to a first diameter, and the at least one expandable sealing layer is passively or actively expandable relative to a first diameter. An expandable sealing layer can be useful with any of the embodiments described above and may be particularly advantageous for braids having self-contracting frames or filaments. 
     The braided layer can be attached or bonded to the expandable sealing layer along its entire length, advantageously decreasing the risk of the polymeric layer being peeled off the braided layer due to frictional forces that may be applied thereon either during entry or exit through the surgical incision. The at least one sealing layer can comprise a lubricious, low-friction material, so as to facilitate passage of the sheath within the blood vessels, and or to facilitate passage of the delivery apparatus carrying a valve through the sheath. 
     A sealing layer is defined as a layer which is not permeable to the blood flow. The sealing layer can comprise a polymeric layer, a membrane, a coating and/or a fabric, such as a polymeric fabric. According to some embodiments, the sealing layer comprises a lubricious, low-friction material. According to some embodiments, the sealing layer is radially outward to the braided layer, so as to facilitate passage of the sheath within the blood vessels. According to some embodiments, the sealing layer is radially inward to the braided layer, so as to facilitate passage of the medical device through the sheath. 
     According to some embodiments, the at least one sealing layer is passively expandable and/or contractible. In some embodiments, the sealing layer is thicker at certain longitudinal positions of the sheath than at others, which can hold a self-contracting braided layer open at a wider diameter than at other longitudinal positions where the sealing layer is thinner. 
     Attaching the braided layer to at least one expandable sealing layer, instead of encapsulating it between two polymeric layers bonded to each other, may simplify the manufacturing process and reduce costs. 
     According to some embodiments, the braided layer can be attached to both an outer expandable sealing layer and an inner expandable sealing layer, so as to seal the braided layer from both sides, while facilitating passage of the sheath along the blood vessels, and facilitating passage of a medical device within the sheath. In such embodiments, the braided layer can be attached to a first sealing layer, while the other sealing layer may also be attached to the first sealing layer. For example, the braided layer and the inner sealing layer can be each attached to the outer sealing layer, or the braided layer and the outer sealing layer can be each attached to the inner sealing layer. 
     According to some embodiments, the braided layer is further coated by a sealing coating. This may be advantageous in configurations of a braided layer being attached only to a single expandable layer, wherein the coating ensures that the braided layer remains sealed from the blood flow or other surrounding tissues, even along regions which are not covered by the expandable layer. For example, if a braided layer is attached to a sealing layer on one side, the other side of the braided layer may receive a sealing coating. In some embodiments, the sealing coating can be used instead of, or in addition to, one or both of the sealing layers. 
     General Considerations 
     For purposes of this description, certain aspects, advantages, and novel features of the embodiments of this disclosure are described herein. The disclosed methods, apparatus, and systems should not be construed as being limiting in any way. Instead, the present disclosure is directed toward all novel and nonobvious features and aspects of the various disclosed embodiments, alone and in various combinations and sub-combinations with one another. The methods, apparatus, and systems are not limited to any specific aspect or feature or a combination thereof, nor do the disclosed embodiments require that any one or more specific advantages be present or problems be solved. 
     Although the operations of some of the disclosed embodiments are described in a particular, sequential order for convenient presentation, it should be understood that this manner of description encompasses rearrangement, unless a particular ordering is required by specific language set forth below. For example, operations described sequentially may, in some cases, be rearranged or performed concurrently. Moreover, for the sake of simplicity, the attached figures may not show the various ways in which the disclosed methods can be used in conjunction with other methods. Additionally, the description sometimes uses terms like “provide” or “achieve” to describe the disclosed methods. These terms are high-level abstractions of the actual operations that are performed. The actual operations that correspond to these terms may vary depending on the particular implementation and are readily discernible by one of ordinary skill in the art. 
     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 mean 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 absent specific contrary language. 
     In the context of the present application, the terms “lower” and “upper” are used interchangeably with the terms “inflow” and “outflow,” respectively. Thus, for example, the lower end of a valve is its inflow end, and the upper end of the valve is its outflow end. 
     As used herein, the term “proximal” refers to a position, direction, or portion of a device that is closer to the user and further away from the implantation site. As used herein, the term “distal” refers to a position, direction, or portion of a device that is further away from the user and closer to the implantation site. Thus, for example, proximal motion of a device is motion of the device toward the user, while distal motion of the device is motion of the device away from the user. The terms “longitudinal” and “axial” refer to an axis extending in the proximal and distal directions, unless otherwise expressly defined. 
     Unless otherwise indicated, all numbers expressing dimensions, quantities of components, molecular weights, percentages, temperatures, forces, times, and so forth, as used in the specification or claims, are to be understood as being modified by the term “about.” Accordingly, unless otherwise indicated, implicitly or explicitly, the numerical parameters set forth are approximations that can depend on the desired properties sought and/or limits of detection under test conditions/methods familiar to those of ordinary skill in the art. When directly and explicitly distinguishing embodiments from discussed prior art, the embodiment numbers are not approximates unless the word “about” is recited. Furthermore, not all alternatives recited herein are equivalents. 
     In view of the many possible embodiments to which the principles of the disclosed technology may be applied, it should be recognized that the illustrated embodiments are only preferred examples and should not be taken as limiting the scope of the disclosure. Rather, the scope of the disclosure is at least as broad as the following claims. We, therefore, claim all that comes within the scope and spirit of these claims.