Patent Publication Number: US-2022218468-A1

Title: Sealing member for prosthetic heart valve

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/120,112, filed Aug. 31, 2018, which is continuation-in-part of U.S. patent application Ser. No. 15/991,325 filed on May 29, 2018, which claims the benefit of U.S. Patent Application No. 62/513,348, filed on May 31, 2017. The entire contents of the foregoing applications are incorporated herein by reference. 
    
    
     FIELD 
     The present disclosure relates to implantable, expandable prosthetic devices and to methods and apparatuses for such prosthetic devices. 
     BACKGROUND 
     The human heart can suffer from various valvular diseases. These valvular diseases can result in significant malfunctioning of the heart and ultimately require replacement of the native valve with an artificial valve. There are a number of known artificial valves and a number of known methods of implanting these artificial valves in humans. Because of the drawbacks associated with conventional open-heart surgery, percutaneous and minimally-invasive surgical approaches are garnering intense attention. In one technique, a prosthetic valve is configured to be implanted in a much less invasive procedure by way of catheterization. For example, collapsible transcatheter prosthetic heart valves can be crimped to a compressed state and percutaneously introduced in the compressed state on a catheter and expanded to a functional size at the desired position by balloon inflation or by utilization of a self-expanding frame or stent. 
     A prosthetic valve for use in such a procedure can include a radially collapsible and expandable frame to which leaflets of the prosthetic valve can be coupled. For example, U.S. Pat. Nos. 6,730,118, 7,393,360, 7,510,575, and 7,993,394, which are incorporated herein by reference, describe exemplary collapsible transcatheter heart valves (THVs). 
     A challenge in catheter-implanted prosthetic valves is the process of crimping such a prosthetic valve to a profile suitable for percutaneous delivery to a subject. Another challenge is the control of paravalvular leakage around the valve, which can occur for a period of time following initial implantation. 
     Paravalvular leakage has been a known problem since the first replacement valves were introduced. The earliest prosthetic heart valves, those that were implanted surgically, included a circumferential sewing ring that was adapted to extend into spaces in the tissue surrounding the implanted prosthesis to prevent paravalvular leaking. For example, U.S. Pat. No. 3,365,728 describes a prosthetic heart valve for surgical implantation that includes a rubber “cushion ring” that conforms to irregularities of the tissue to form an effective seal between the valve and the surrounding tissue. From there, vascular stents or stent grafts were developed that could be implanted by non-surgical catheterization techniques. These stents included a fabric covering that allowed the stent to be used to isolate and reinforce the wall of a blood vessel from the lumen of the vessel. These fabric coverings served essentially the same purpose on stents as did the sealing rings on surgical heart valves-they reduced the risk of blood leaking between the prosthesis and the surrounding tissue. Multiple graft designs were developed that further enhanced the external seal to prevent blood from flowing between the graft and surrounding cardiovascular tissue. For example, U.S. Pat. No. 6,015,431 to Thornton discloses a seal secured to the outer surface of a stent that is adapted to occlude leakage flow externally around the stent wall between the outer surface and the endolumenal wall when the stent is deployed, by conforming to the irregular surface of the surrounding tissue. U.S. Patent Publication 2003/0236567 to Elliot similarly discloses a tubular prosthesis having a stent and one or more fabric “skirts” to seal against endoleaks. U.S. Patent Publication 2004/0082989 to Cook et al. also recognized the potential for endoleaks, and describes a stent graft having a cuff portion that has an external sealing zone that extends around the body of the stent to prevent leakage. The cuff portion could be folded over to create a pocket that collects any blood passing around the leading edge of the graft to prevent an endoleak. 
     Building on this technology, in the late 1980&#39;s, the first permanent bioprosthetic heart valve was implanted using transcatheter techniques. U.S. Pat. No. 5,411,552 to Andersen describes a THV comprising a valve mounted within a collapsible and expandable stent structure. Certain embodiments have additional graft material used along the external and internal surface of the THV. As with stent grafts, the covers proposed to be used with THVs were designed to conform to the surface of the surrounding tissue to prevent paravalvular leaks. 
     Like with stents, “cuffs” or other outer seals were used on THVs. U.S. Pat. No. 5,855,601 to Bessler describes a self-expanding THV having a cuff portion extending along the outside of the stent. Upon collapse of the stent for delivery, the outer seal collapses to form pleats, then expands with the stent to provide a seal between the THV and the surrounding tissue. 
     Thereafter, a different THV design was described by Pavcnik in U.S. Patent Application Publication 2001/0039450. The enhanced sealing structure of Pavcnik is in the form of corner “flaps” or “pockets” secured to the stent at the edges of each “flap” or “pocket” and positioned at discrete locations around the prosthesis. The corner flap was designed to catch retrograde blood flow to provide a better seal between the THV and the vessel wall, as well as to provide an improved substrate for ingrowth of native tissue. 
     Thus, fabric and other materials used to cover and seal both internal and external surfaces of THVs and other endovascular prostheses such as stents and stent grafts are well known. These covers can be made with low-porosity woven fabric materials, as described, for example, by U.S. Pat. No. 5,957,949 to Leonhardt et al., which describes a valve stent having an outer cover that can conform to the living tissue surrounding it upon implantation to help prevent blood leakage. 
     Several more recent THV designs include a THV with an outer covering. U.S. Pat. No. 7,510,575 to Spenser discloses a THV having a cuff portion wrapped around the outer surface of the support stent at the inlet. The cuff portion is rolled up over the edge of the frame so as to provide a “sleeve-like” portion at the inlet to form a cuff over the inlet that helps prevent blood leakage. U.S. Pat. No. 8,002,825 to Letac and Cribier describes an internal cover that extends from the base of the valve to the lower end of the stent and then up the external wall of the stent so as to form an external cover. The single-piece cover could be made with any of the materials disclosed for making the valve structure, which include fabric (e.g., Dacron), biological material (e.g., pericardium), or other synthetic materials (e.g., polyethylene). 
     While covers used on the external surface of an endovascular prosthesis to prevent paravalvular leaking are well known, there remains a need for improved coverings that provide enhanced sealing while still providing a small profile suitable for percutaneous delivery to a patient. 
     SUMMARY 
     Embodiments of a radially collapsible and expandable prosthetic valve are disclosed herein that include an improved outer skirt for reducing perivalvular leakage, as well as related methods and apparatuses including such prosthetic valves. In several embodiments, the disclosed prosthetic valves are configured as replacement heart valves for implantation into a subject. 
     In one representative embodiment, a prosthetic heart valve comprises an annular frame that comprises an inflow end and an outflow end and is radially compressible and expandable between a radially compressed configuration and a radially expanded configuration. The prosthetic heart valve further includes a leaflet structure positioned within the frame and secured thereto, and an outer sealing member mounted outside of the frame and adapted to seal against surrounding tissue when the prosthetic heart valve is implanted within a native heart valve annulus of a patient. The sealing member can comprise a mesh layer and pile layer comprising a plurality of pile yarns extending outwardly from the mesh layer. 
     In some embodiments, the mesh layer comprises a knit or woven fabric. 
     In some embodiments, the pile yarns are arranged to form a looped pile. 
     In some embodiments, the pile yarns are cut to form a cut pile. 
     In some embodiments, the height of the pile yarns varies along a height and/or a circumference of the outer skirt. 
     In some embodiments, the pile yarns comprise a first group of yarns along an upstream portion of the outer skirt and a second group of yarns along a downstream portion of the outer skirt, wherein the yarns of the first group have a height that is less than a height of the yarns of the second group. 
     In some embodiments, the pile yarns comprise a first group of yarns along an upstream portion of the outer skirt and a second group of yarns along a downstream portion of the outer skirt, wherein the yarns of the first group have a height that is greater than a height of the yarns of the second group. 
     In some embodiments, the pile yarns comprise a first group of yarns along an upstream portion of the outer skirt, a second group of yarns along a downstream portion of the outer skirt, and a third group of yarns between the first and second group of yarns, wherein the yarns of the first and second groups have a height that is greater than a height of the yarns of the third group. 
     In some embodiments, the prosthetic heart valve further comprises an inner skirt mounted on an inner surface of the frame, the inner skirt having an inflow end portion that is secured to an inflow end portion of the outer sealing member. 
     In some embodiments, the inflow end portion of the inner skirt is wrapped around an inflow end of the frame and overlaps the inflow end portion of the outer sealing member on the outside of the frame. 
     In some embodiments, the mesh layer comprises a first mesh layer and the outer sealing member further comprises a second mesh layer disposed radially outside of the pile layer. 
     In some embodiments, the outer sealing member is configured to stretch axially when the frame is radially compressed to the radially compressed state. 
     In some embodiments, the mesh layer comprises warp yarns and weft yarns woven with the warp yarns, and the pile layer comprises the warp yarns or the weft yarns of the mesh layer that are woven or knitted to form the pile yarns. 
     In some embodiments, the mesh layer comprises a woven fabric layer and the pile layer comprises a separate pile layer that is stitched to the woven fabric layer. 
     In some embodiment, the mesh layer has a first height extending axially along the frame and the pile layer comprises a second height extending axially along the frame, wherein the first height is greater than the second height. 
     In some embodiment, the mesh layer extends closer to the outflow end of the frame than the pile layer. 
     In another representative embodiment, a prosthetic heart valve comprises an annular frame that comprises an inflow end and an outflow end and is radially compressible and expandable between a radially compressed configuration and a radially expanded configuration. The prosthetic heart valve further comprises a leaflet structure positioned within the frame and secured thereto, an outer sealing member mounted outside of the frame and adapted to seal against surrounding tissue when the prosthetic heart valve is implanted within a native heart valve annulus of a patient. The sealing member can comprise a fabric having a variable thickness. 
     In some embodiments, the thickness of the fabric layer varies along a height and/or a circumference of the outer sealing member. 
     In some embodiments, the fabric comprises a plush fabric. 
     In some embodiments, the fabric comprises a plurality of pile yarns and the height of the pile yarns varies along a height and/or a circumference of the outer skirt. 
     In some embodiments, the pile yarns comprise a first group of yarns along an upstream portion of the outer skirt and a second group of yarns along a downstream portion of the outer skirt, wherein the yarns of the first group have a height that is less than a height of the yarns of the second group. 
     In some embodiments, the pile yarns comprise a first group of yarns along an upstream portion of the outer skirt and a second group of yarns along a downstream portion of the outer skirt, wherein the yarns of the first group have a height that is greater than a height of the yarns of the second group. 
     In some embodiments, the pile yarns comprise a first group of yarns along an upstream portion of the outer skirt, a second group of yarns along a downstream portion of the outer skirt, and a third group of yarns between the first and second group of yarns, wherein the yarns of the first and second groups have a height that is greater than a height of the yarns of the third group. 
     In another representative embodiment, a prosthetic heart valve comprises an annular frame that comprises an inflow end and an outflow end and is radially compressible and expandable between a radially compressed configuration and a radially expanded configuration. The prosthetic heart valve further comprises a leaflet structure positioned within the frame and secured thereto, an outer sealing member mounted outside of the frame and adapted to seal against surrounding tissue when the prosthetic heart valve is implanted within a native heart valve annulus of a patient. The sealing member can comprise a pile fabric comprising a plurality of pile yarns, wherein the density of the pile yarns varies in an axial direction and/or a circumferential direction along the sealing member. 
     In some embodiments, the pile yarns are arranged in circumferentially extending rows of pile yarns and the density of the pile yarns varies from row to row. 
     In some embodiments, the pile yarns are arranged in axially extending rows pile yarns and the density of the pile yarns varies from row to row. 
     In some embodiments, the sealing member comprises a mesh layer and a pile layer comprising the pile yarns. In some embodiments, the weave density of the mesh layer varies in an axial direction and/or a circumferential direction along the sealing member. In some embodiments, the mesh layer comprises one or more rows of higher-density mesh portions and one or more rows of lower-density mesh portions. The one or more rows of higher-density mesh portions and the one or more rows of lower-density mesh portions can be circumferentially extending rows and/or axially extending rows. 
     In another representative embodiment, a prosthetic heart valve comprises an annular frame that comprises an inflow end and an outflow end and is radially compressible and expandable between a radially compressed configuration and a radially expanded configuration. The prosthetic heart valve further comprises a leaflet structure positioned within the frame and secured thereto, an outer sealing member mounted outside of the frame and adapted to seal against surrounding tissue when the prosthetic heart valve is implanted within a native heart valve annulus of a patient. The sealing member comprises a textile formed from a plurality fibers arranged in a plurality of axially extending rows of higher stitch density interspersed between a plurality of axially extending rows of lower stitch density. The sealing member is configured to stretch axially between a first, substantially relaxed, axially foreshortened configuration when the frame is the radially expanded configuration and a second, axially elongated configuration when the frame is in the radially compressed configuration. 
     In some embodiments, each of the rows of higher stitch density can extend in an undulating pattern when the sealing member is in the axially foreshortened configuration. When the sealing member is in the axially elongated configuration, the rows of higher stitch density move from the undulating pattern toward a straightened pattern. 
     In another representative embodiment, a prosthetic heart valve comprises an annular frame that comprises an inflow end and an outflow end and is radially compressible and expandable between a radially compressed configuration and a radially expanded configuration. The prosthetic heart valve further comprises a leaflet structure positioned within the frame and secured thereto, an outer sealing member mounted outside of the frame and adapted to seal against surrounding tissue when the prosthetic heart valve is implanted within a native heart valve annulus of a patient. The sealing member comprises a fabric comprising a plurality of axially extending filaments and a plurality of circumferentially extending filaments. The sealing member is configured to stretch axially when the frame is radially compressed from the radially expanded configuration to the radially compressed configuration. The axially extending filaments move from a deformed or twisted state when the frame is in the radially expanded configuration to a less deformed or less twisted state when the frame is in the radially compressed configuration. 
     In some embodiments, the axially extending filaments are heat set in the deformed or twisted state. 
     In some embodiments, the thickness of the sealing member decreases when the axially extending filaments move from the deformed or twisted state to the less deformed or twisted state. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a prosthetic heart valve, according to one embodiment. 
         FIG. 2  is an enlarged, perspective view of the inflow end portion of the prosthetic heart valve of  FIG. 1 . 
         FIG. 3  is a cross-sectional view of the prosthetic heart valve of  FIG. 1 , showing the attachment of the outer skirt to the inner skirt and the frame. 
         FIGS. 4-10  show an exemplary frame of the prosthetic heart valve of  FIG. 1 . 
         FIGS. 11-12  show an exemplary inner skirt of the prosthetic heart valve of  FIG. 1 . 
         FIGS. 13-15  show the assembly of the inner skirt of  FIG. 11  with the frame of  FIG. 4 . 
         FIGS. 16-17  show the assembly of an exemplary leaflet structure. 
         FIG. 18  shows the assembly of commissure portions of the leaflet structure with window frame portions of the frame. 
         FIGS. 19-20  show the assembly of the leaflet structure with the inner skirt along a lower edge of the leaflets. 
         FIGS. 21-23  are different views of an exemplary outer skirt of the prosthetic heart valve of  FIG. 1 . 
         FIG. 24-26  are cross-sectional views similar to  FIG. 3  but showing different embodiments of the outer skirt. 
         FIGS. 27-28  show an alternative way of securing an outer skirt to an inner skirt and/or the frame of a prosthetic heart valve. 
         FIGS. 29-32  show another way of securing an outer skirt to an inner skirt and/or the frame of a prosthetic heart valve. 
         FIGS. 33-35  show another embodiment of an outer sealing member for a prosthetic heart valve. 
         FIG. 36  shows another embodiment of an outer sealing member, shown mounted on the frame of a prosthetic heart valve. 
         FIG. 37  is a flattened view of a woven mesh layer of the sealing member of  FIG. 36 . 
         FIG. 38  is a flattened view of a pile layer of the sealing member of  FIG. 36 . 
         FIG. 39  is a flattened view of the outer surface of an outer sealing member for a prosthetic heart valve, according to another embodiment. 
         FIG. 39A  is a magnified view of a portion of the sealing member of  FIG. 39 . 
         FIG. 40  is a flattened view of the inner surface of the sealing member of  FIG. 39 . 
         FIG. 40A  is a magnified view of a portion of the sealing member of  FIG. 40 . 
         FIG. 41  is flattened view of an outer sealing member for a prosthetic heart valve shown in a relaxed state when the prosthetic heart valve is radially expanded to its functional size, according to another embodiment. 
         FIG. 42  is a flattened view of the outer sealing member of  FIG. 41  shown in an axially elongated, tensioned state when the prosthetic heart valve is in a radially compressed state for delivery. 
         FIG. 43A  is a magnified view of a portion of another embodiment of an outer sealing member for a prosthetic heart valve, wherein the sealing member is shown in a relaxed state when the prosthetic heart valve is radially expanded to its functional size. 
         FIG. 43B  is a magnified view of the sealing member of  FIG. 43A  shown in an axially elongated, tensioned state when the prosthetic heart valve is in a radially compressed state for delivery. 
         FIG. 44A  is a cross-sectional view of the fabric of the sealing member of  FIG. 43A  in a relaxed state. 
         FIG. 44B  is a cross-sectional view of the fabric of the sealing member of  FIG. 43B  in a tensioned state. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a prosthetic heart valve  10 , according to one embodiment. The illustrated prosthetic valve is adapted to be implanted in the native aortic annulus, although in other embodiments it can be adapted to be implanted in the other native annuluses of the heart (e.g., the pulmonary, mitral, and tricuspid valves). The prosthetic valve can also be adapted to be implanted in other tubular organs or passageways in the body. The prosthetic valve  10  can have four main components: a stent or frame  12 , a valvular structure  14 , an inner skirt  16 , and a perivalvular outer sealing member or outer skirt  18 . The prosthetic valve  10  can have an inflow end portion  15 , an intermediate portion  17 , and an outflow end portion  19 . 
     The valvular structure  14  can comprise three leaflets  40  ( FIG. 17 ), collectively forming a leaflet structure, which can be arranged to collapse in a tricuspid arrangement. The lower edge of leaflet structure  14  desirably has an undulating, curved scalloped shape (suture line  154  shown in  FIG. 20  tracks the scalloped shape of the leaflet structure). By forming the leaflets with this scalloped geometry, stresses on the leaflets are reduced, which in turn improves durability of the prosthetic valve. Moreover, by virtue of the scalloped shape, folds and ripples at the belly of each leaflet (the central region of each leaflet), which can cause early calcification in those areas, can be eliminated or at least minimized. The scalloped geometry also reduces the amount of tissue material used to form leaflet structure, thereby allowing a smaller, more even crimped profile at the inflow end of the prosthetic valve. The leaflets  40  can be formed of pericardial tissue (e.g., bovine pericardial tissue), biocompatible synthetic materials, or various other suitable natural or synthetic materials as known in the art and described in U.S. Pat. No. 6,730,118, which is incorporated by reference herein. 
     The bare frame  12  is shown in  FIG. 4 . The frame  12  can be formed with a plurality of circumferentially spaced slots, or commissure windows,  20  (three in the illustrated embodiment) that are adapted to mount the commissures of the valvular structure  14  to the frame, as described in greater detail below. The frame  12  can be made of any of various suitable plastically-expandable materials (e.g., stainless steel, etc.) or self-expanding materials (e.g., nickel titanium alloy (NiTi), such as nitinol) as known in the art. When constructed of a plastically-expandable material, the frame  12  (and thus the prosthetic valve  10 ) can be crimped to a radially collapsed configuration on a delivery catheter and then expanded inside a patient by an inflatable balloon or equivalent expansion mechanism. When constructed of a self-expandable material, the frame  12  (and thus the prosthetic valve  10 ) can be crimped to a radially collapsed configuration and restrained in the collapsed configuration by insertion into a sheath or equivalent mechanism of a delivery catheter. Once inside the body, the prosthetic valve can be advanced from the delivery sheath, which allows the prosthetic valve to expand to its functional size. 
     Suitable plastically-expandable materials that can be used to form the frame  12  include, without limitation, stainless steel, a biocompatible, high-strength alloys (e.g., a cobalt-chromium or a nickel-cobalt-chromium alloys), polymers, or combinations thereof. In particular embodiments, frame  12  is made of a nickel-cobalt-chromium-molybdenum alloy, such as MP35N® alloy (SPS Technologies, Jenkintown, Pa.), which is equivalent to UNS R30035 alloy (covered by ASTM F562-02). MP35N® alloy/UNS R30035 alloy comprises 35% nickel, 35% cobalt, 20% chromium, and 10% molybdenum, by weight. It has been found that the use of MP35N® alloy to form frame  12  provides superior structural results over stainless steel. In particular, when MP35N® alloy is used as the frame material, less material is needed to achieve the same or better performance in radial and crush force resistance, fatigue resistances, and corrosion resistance. Moreover, since less material is required, the crimped profile of the frame can be reduced, thereby providing a lower profile prosthetic valve assembly for percutaneous delivery to the treatment location in the body. 
     Referring to  FIGS. 4 and 5 , the frame  12  in the illustrated embodiment comprises a first, lower row I of angled struts  22  arranged end-to-end and extending circumferentially at the inflow end of the frame; a second row II of circumferentially extending, angled struts  24 ; a third row III of circumferentially extending, angled struts  26 ; a fourth row IV of circumferentially extending, angled struts  28 ; and a fifth row V of circumferentially extending, angled struts  32  at the outflow end of the frame. A plurality of substantially straight axially extending struts  34  can be used to interconnect the struts  22  of the first row I with the struts  24  of the second row II. The fifth row V of angled struts  32  are connected to the fourth row IV of angled struts  28  by a plurality of axially extending window frame portions  30  (which define the commissure windows  20 ) and a plurality of axially extending struts  31 . Each axial strut  31  and each frame portion  30  extends from a location defined by the convergence of the lower ends of two angled struts  32  to another location defined by the convergence of the upper ends of two angled struts  28 .  FIGS. 6, 7, 8, 9 , and  10  are enlarged views of the portions of the frame  12  identified by letters A, B, C, D, and E, respectively, in  FIG. 5 . 
     Each commissure window frame portion  30  mounts a respective commissure of the leaflet structure  14 . As can be seen each frame portion  30  is secured at its upper and lower ends to the adjacent rows of struts to provide a robust configuration that enhances fatigue resistance under cyclic loading of the prosthetic valve compared to known, cantilevered struts for supporting the commissures of the leaflet structure. This configuration enables a reduction in the frame wall thickness to achieve a smaller crimped diameter of the prosthetic valve. In particular embodiments, the thickness T of the frame  12  ( FIG. 4 ) measured between the inner diameter and outer diameter is about 0.48 mm or less. 
     The struts and frame portions of the frame collectively define a plurality of open cells of the frame. At the inflow end of the frame  12 , struts  22 , struts  24 , and struts  34  define a lower row of cells defining openings  36 . The second, third, and fourth rows of struts  24 ,  26 , and  28  define two intermediate rows of cells defining openings  38 . The fourth and fifth rows of struts  28  and  32 , along with frame portions  30  and struts  31 , define an upper row of cells defining openings  40 . The openings  41  are relatively large and are sized to allow portions of the leaflet structure  14  to protrude, or bulge, into and/or through the openings  40  when the frame  12  is crimped in order to minimize the crimping profile. 
     As best shown in  FIG. 7 , the lower end of the strut  31  is connected to two struts  28  at a node or junction  44 , and the upper end of the strut  31  is connected to two struts  32  at a node or junction  46 . The strut  31  can have a thickness S 1  that is less than the thicknesses S 2  of the junctions  44 ,  46 . The junctions  44 ,  46 , along with junctions  64 , prevent full closure of openings  40 . The geometry of the struts  31 , and junctions  44 ,  46 , and  64  assists in creating enough space in openings  41  in the collapsed configuration to allow portions of the prosthetic leaflets to protrude or bulge outwardly through openings. This allows the prosthetic valve to be crimped to a relatively smaller diameter than if all of the leaflet material were constrained within the crimped frame. 
     The frame  12  is configured to reduce, to prevent, or to minimize possible over-expansion of the prosthetic valve at a predetermined balloon pressure, especially at the outflow end portion of the frame, which supports the leaflet structure  14 . In one aspect, the frame is configured to have relatively larger angles  42   a ,  42   b ,  42   c ,  42   d ,  42   e  between struts, as shown in  FIG. 5 . The larger the angle, the greater the force required to open (expand) the frame. As such, the angles between the struts of the frame can be selected to limit radial expansion of the frame at a given opening pressure (e.g., inflation pressure of the balloon). In particular embodiments, these angles are at least 110 degrees or greater when the frame is expanded to its functional size, and even more particularly these angles are up to about 120 degrees when the frame is expanded to its functional size. 
     In addition, the inflow and outflow ends of a frame generally tend to over-expand more so than the middle portion of the frame due to the “dog-boning” effect of the balloon used to expand the prosthetic valve. To protect against over-expansion of the leaflet structure  14 , the leaflet structure desirably is secured to the frame  12  below the upper row of struts  32 , as best shown in  FIG. 1 . Thus, in the event that the outflow end of the frame is over-expanded, the leaflet structure is positioned at a level below where over-expansion is likely to occur, thereby protecting the leaflet structure from over-expansion. 
     In a known prosthetic valve construction, portions of the leaflets can protrude longitudinally beyond the outflow end of the frame when the prosthetic valve is crimped if the leaflets are mounted too close to the distal end of the frame. If the delivery catheter on which the crimped prosthetic valve is mounted includes a pushing mechanism or stop member that pushes against or abuts the outflow end of the prosthetic valve (for example, to maintain the position of the crimped prosthetic valve on the delivery catheter), the pushing member or stop member can damage the portions of the exposed leaflets that extend beyond the outflow end of the frame. Another benefit of mounting the leaflets at a location spaced away from the outflow end of the frame is that when the prosthetic valve is crimped on a delivery catheter, the outflow end of the frame  12  rather than the leaflets  40  is the proximal-most component of the prosthetic valve  10 . As such, if the delivery catheter includes a pushing mechanism or stop member that pushes against or abuts the outflow end of the prosthetic valve, the pushing mechanism or stop member contacts the outflow end of the frame, and not leaflets  40 , so as to avoid damage to the leaflets. 
     Also, as can be seen in  FIG. 5 , the openings  36  of the lowermost row of openings in the frame are relatively larger than the openings  38  of the two intermediate rows of openings. This allows the frame, when crimped, to assume an overall tapered shape that tapers from a maximum diameter at the outflow end of the prosthetic valve to a minimum diameter at the inflow end of the prosthetic valve. When crimped, the frame  12  has a reduced diameter region extending along a portion of the frame adjacent the inflow end of the frame that generally corresponds to the region of the frame covered by the outer skirt  18 . In some embodiments, the reduced diameter region is reduced compared to the diameter of the upper portion of the frame (which is not covered by the outer skirt) such that the outer skirt  18  does not increase the overall crimp profile of the prosthetic valve. When the prosthetic valve is deployed, the frame can expand to the generally cylindrical shape shown in  FIG. 4 . In one example, the frame of a 26-mm prosthetic valve, when crimped, had a first diameter of 14 French at the outflow end of the prosthetic valve and a second diameter of 12 French at the inflow end of the prosthetic valve. 
     The main functions of the inner skirt  16  are to assist in securing the valvular structure  14  to the frame  12  and to assist in forming a good seal between the prosthetic valve and the native annulus by blocking the flow of blood through the open cells of the frame  12  below the lower edge of the leaflets. The inner skirt  16  desirably comprises a tough, tear resistant material such as polyethylene terephthalate (PET), although various other synthetic materials or natural materials (e.g., pericardial tissue) can be used. The thickness of the skirt desirably is less than about 0.15 mm (about 6 mil), and desirably less than about 0.1 mm (about 4 mil), and even more desirably about 0.05 mm (about 2 mil). In particular embodiments, the skirt  16  can have a variable thickness, for example, the skirt can be thicker at at least one of its edges than at its center. In one implementation, the skirt  16  can comprise a PET skirt having a thickness of about 0.07 mm at its edges and about 0.06 mm at its center. The thinner skirt can provide for better crimping performances while still providing good perivalvular sealing. 
     The inner skirt  16  can be secured to the inside of frame  12  via sutures  70 , as shown in  FIG. 20 . Valvular structure  14  can be attached to the skirt via one or more reinforcing strips  72  (which collectively can form a sleeve), for example thin, PET reinforcing strips, discussed below, which enables a secure suturing and protects the pericardial tissue of the leaflet structure from tears. Valvular structure  14  can be sandwiched between skirt  16  and the thin PET strips  72  as shown in  FIG. 19 . Sutures  154 , which secure the PET strip and the leaflet structure  14  to skirt  16 , can be any suitable suture, such as Ethibond Excel® PET suture (Johnson &amp; Johnson, New Brunswick, N.J.). Sutures  154  desirably track the curvature of the bottom edge of leaflet structure  14 , as described in more detail below. 
     Known fabric skirts may comprise a weave of warp and weft fibers that extend perpendicularly to each other and with one set of the fibers extending longitudinally between the upper and lower edges of the skirt. When the metal frame to which the fabric skirt is secured is radially compressed, the overall axial length of the frame increases. Unfortunately, a fabric skirt with limited elasticity cannot elongate along with the frame and therefore tends to deform the struts of the frame and to prevent uniform crimping. 
     Referring to  FIG. 12 , in contrast to known fabric skirts, the skirt  16  desirably is woven from a first set of fibers, or yarns or strands,  78  and a second set of fibers, or yarns or strands,  80 , both of which are non-perpendicular to the upper edge  82  and the lower edge  84  of the skirt. In particular embodiments, the first set of fibers  78  and the second set of fibers  80  extend at angles of about 45 degrees relative to the upper and lower edges  82 ,  84 . Alternatively, the first set of fibers  78  and the second set of fibers  80  extend at angles other than about 45 degrees relative to the upper and lower edges  82 ,  84 , e.g., at angles of 15 and 75 degrees, respectively, or 30 and 60 degrees, respectively, relative to the upper and lower edges  82 ,  84 . For example, the skirt  16  can be formed by weaving the fibers at 45 degree angles relative to the upper and lower edges of the fabric. Alternatively, the skirt  16  can be diagonally cut (cut on a bias) from a vertically woven fabric (where the fibers extend perpendicularly to the edges of the material) such that the fibers extend at 45 degree angles relative to the cut upper and lower edges of the skirt. As further shown in  FIG. 12 , the opposing short edges  86 ,  88  of the skirt desirably are non-perpendicular to the upper and lower edges  82 ,  84 . For example, the short edges  86 ,  88  desirably extend at angles of about 45 degrees relative to the upper and lower edges and therefore are aligned with the first set of fibers  78 . Therefore the overall general shape of the skirt is that of a rhomboid or parallelogram. 
       FIGS. 13 and 14  show the inner skirt  16  after opposing short edge portions  90 ,  92  have been sewn together to form the annular shape of the skirt. As shown, the edge portion  90  can be placed in an overlapping relationship relative to the opposite edge portion  92 , and the two edge portions can be sewn together with a diagonally extending suture line  94  that is parallel to short edges  86 ,  88 . The upper edge portion of the inner skirt  16  can be formed with a plurality of projections  96  that define an undulating shape that generally follows the shape or contour of the fourth row of struts  28  immediately adjacent the lower ends of axial struts  31 . In this manner, as best shown in  FIG. 15 , the upper edge of the inner skirt  16  can be tightly secured to struts  28  with sutures  70 . The inner skirt  16  can also be formed with slits  98  to facilitate attachment of the skirt to the frame. Slits  98  are dimensioned so as to allow an upper edge portion of the inner skirt  16  to be partially wrapped around struts  28  and to reduce stresses in the skirt during the attachment procedure. For example, in the illustrated embodiment, the inner skirt  16  is placed on the inside of frame  12  and an upper edge portion of the skirt is wrapped around the upper surfaces of struts  28  and secured in place with sutures  70 . Wrapping the upper edge portion of the inner skirt  16  around struts  28  in this manner provides for a stronger and more durable attachment of the skirt to the frame. The inner skirt  16  can also be secured to the first, second, and/or third rows of struts  22 ,  24 , and  26 , respectively, with sutures  70 . 
     Due to the angled orientation of the fibers relative to the upper and lower edges, the skirt can undergo greater elongation in the axial direction (i.e., in a direction from the upper edge  82  to the lower edge  84 ). Thus, when the metal frame  12  is crimped, the inner skirt  16  can elongate in the axial direction along with the frame and therefore provide a more uniform and predictable crimping profile. Each cell of the metal frame in the illustrated embodiment includes at least four angled struts that rotate towards the axial direction on crimping (e.g., the angled struts become more aligned with the length of the frame). The angled struts of each cell function as a mechanism for rotating the fibers of the skirt in the same direction of the struts, allowing the skirt to elongate along the length of the struts. This allows for greater elongation of the skirt and avoids undesirable deformation of the struts when the prosthetic valve is crimped. 
     In addition, the spacing between the woven fibers or yarns can be increased to facilitate elongation of the skirt in the axial direction. For example, for a PET inner skirt  16  formed from 20-denier yarn, the yarn density can be about 15% to about 30% lower than in a typical PET skirt. In some examples, the yarn spacing of the inner skirt  16  can be from about 60 yarns per cm (about 155 yarns per inch) to about 70 yarns per cm (about 180 yarns per inch), such as about 63 yarns per cm (about 160 yarns per inch), whereas in a typical PET skirt the yarn spacing can be from about 85 yarns per cm (about 217 yarns per inch) to about 97 yarns per cm (about 247 yarns per inch). The oblique edges  86 ,  88  promote a uniform and even distribution of the fabric material along inner circumference of the frame during crimping so as to reduce or minimize bunching of the fabric to facilitate uniform crimping to the smallest possible diameter. Additionally, cutting diagonal sutures in a vertical manner may leave loose fringes along the cut edges. The oblique edges  86 ,  88  help minimize this from occurring. Compared to the construction of a typical skirt (fibers running perpendicularly to the upper and lower edges of the skirt), the construction of the inner skirt  16  avoids undesirable deformation of the frame struts and provides more uniform crimping of the frame. 
     In alternative embodiments, the skirt can be formed from woven elastic fibers that can stretch in the axial direction during crimping of the prosthetic valve. The warp and weft fibers can run perpendicularly and parallel to the upper and lower edges of the skirt, or alternatively, they can extend at angles between 0 and 90 degrees relative to the upper and lower edges of the skirt, as described above. 
     The inner skirt  16  can be sutured to the frame  12  at locations away from the suture line  154  so that the skirt can be more pliable in that area. This configuration can avoid stress concentrations at the suture line  154 , which attaches the lower edges of the leaflets to the inner skirt  16 . 
     As noted above, the leaflet structure  14  in the illustrated embodiment includes three flexible leaflets  40  (although a greater or a smaller number of leaflets can be used). Additional information regarding the leaflets, as well as additional information regarding skirt material, can be found, for example, in U.S. patent application Ser. No. 14/704,861, filed May 5, 2015, which is incorporated by reference in its entirety. 
     The leaflets  40  can be secured to one another at their adjacent sides to form commissures  122  of the leaflet structure ( FIG. 20 ). A plurality of flexible connectors  124  (one of which is shown in  FIG. 16 ) can be used to interconnect pairs of adjacent sides of the leaflets and to mount the leaflets to the commissure window frame portions  30  ( FIG. 5 ).  FIG. 16  shows the adjacent sides of two leaflets  40  interconnected by a flexible connector  124 . Three leaflets  40  can be secured to each other side-to-side using three flexible connectors  124 , as shown in  FIG. 17 . Additional information regarding connecting the leaflets to each other, as well as connecting the leaflets to the frame, can be found, for example, in U.S. Patent Application Publication No. 2012/0123529, which is incorporated by reference herein in its entirety. 
     As noted above, the inner skirt  16  can be used to assist in suturing the leaflet structure  14  to the frame. The inner skirt  16  can have an undulating temporary marking suture to guide the attachment of the lower edges of each leaflet  40 . The inner skirt  16  itself can be sutured to the struts of the frame  12  using sutures  70 , as noted above, before securing the leaflet structure  14  to the skirt  16 . The struts that intersect the marking suture desirably are not attached to the inner skirt  16 . This allows the inner skirt  16  to be more pliable in the areas not secured to the frame and minimizes stress concentrations along the suture line that secures the lower edges of the leaflets to the skirt. As noted above, when the skirt is secured to the frame, the fibers  78 ,  80  of the skirt (see  FIG. 12 ) generally align with the angled struts of the frame to promote uniform crimping and expansion of the frame. 
       FIG. 18  shows one specific approach for securing the commissure portions  122  of the leaflet structure  14  to the commissure window frame portions  30  of the frame. The flexible connector  124  ( FIG. 17 ) securing two adjacent sides of two leaflets is folded widthwise and the upper tab portions  112  are folded downwardly against the flexible connector. Each upper tab portion  112  is creased lengthwise (vertically) to assume an L-shape having a first portion  142  folded against a surface of the leaflet and a second portion  144  folded against the connector  124 . The second portion  144  can then be sutured to the connector  124  along a suture line  146 . Next, the commissure tab assembly is inserted through the commissure window  20  of a corresponding window frame portion  30 , and the folds outside of the window frame portion  30  can be sutured to portions  144 . 
       FIG. 18  also shows that the folded down upper tab portions  112  can form a double layer of leaflet material at the commissures. The first portions  142  of the upper tab portions  112  are positioned flat against layers of the two leaflets  40  forming the commissures, such that each commissure comprises four layers of leaflet material just inside of the window frames  30 . This four-layered portion of the commissures can be more resistant to bending, or articulating, than the portion of the leaflets  40  just radially inward from the relatively more-rigid four-layered portion. This causes the leaflets  40  to articulate primarily at inner edges  143  of the folded-down first portions  142  in response to blood flowing through the prosthetic valve during operation within the body, as opposed to articulating about or proximal to the axial struts of the window frames  30 . Because the leaflets articulate at a location spaced radially inwardly from the window frames  30 , the leaflets can avoid contact with and damage from the frame. However, under high forces, the four layered portion of the commissures can splay apart about a longitudinal axis adjacent to the window frame  30 , with each first portion  142  folding out against the respective second portion  144 . For example, this can occur when the prosthetic valve  10  is compressed and mounted onto a delivery shaft, allowing for a smaller crimped diameter. The four-layered portion of the commissures can also splay apart about the longitudinal axis when the balloon catheter is inflated during expansion of the prosthetic valve, which can relieve some of the pressure on the commissures caused by the balloon, reducing potential damage to the commissures during expansion. 
     After all three commissure tab assemblies are secured to respective window frame portions  30 , the lower edges of the leaflets  40  between the commissure tab assemblies can be sutured to the inner skirt  16 . For example, as shown in  FIG. 19 , each leaflet  40  can be sutured to the inner skirt  16  along suture line  154  using, for example, Ethibond Excel® PET thread. The sutures can be in-and-out sutures extending through each leaflet  40 , the inner skirt  16 , and each reinforcing strip  72 . Each leaflet  40  and respective reinforcing strip  72  can be sewn separately to the inner skirt  16 . In this manner, the lower edges of the leaflets are secured to the frame  12  via the inner skirt  16 . As shown in  FIG. 19 , the leaflets can be further secured to the skirt with blanket sutures  156  that extend through each reinforcing strip  72 , leaflet  40  and the inner skirt  16  while looping around the edges of the reinforcing strips  72  and leaflets  40 . The blanket sutures  156  can be formed from PTFE suture material.  FIG. 20  shows a side view of the frame  12 , leaflet structure  14  and the inner skirt  16  after securing the leaflet structure  14  and the inner skirt  16  to the frame  12  and the leaflet structure  14  to the inner skirt  16 . 
       FIG. 21  is a flattened view of the outer skirt  18  prior to its attachment to the frame  12 , showing the outer surface of the skirt.  FIG. 22  is a flattened view of the outer skirt  18  prior to its attachment to the frame  12 , showing the inner surface of the skirt.  FIG. 23  is a perspective view of the outer skirt prior to its attachment to the frame  12 . The outer skirt  18  can be laser cut or otherwise formed from a strong, durable material such as PET or various other suitable synthetic or natural materials configured to restrict and/or prevent blood-flow therethrough. The outer skirt  18  can comprise a substantially straight lower (inflow or upstream) edge portion  160  and an upper (outflow or downstream) edge portion  162  defining a plurality of alternating projections  164  and notches  166 , or castellations, that generally follow the shape of a row of struts of the frame. The lower and upper edge portions  160 ,  162  can have other shapes in alternative embodiments. For example, in one implementation, the lower edge portion  160  can be formed with a plurality of projections generally conforming to the shape of a row of struts of the frame  12 , while the upper edge portion  162  can be straight. 
     In particular embodiments, the outer skirt  18  can comprise at least one soft, plush surface  168  oriented radially outward so as to cushion and seal against native tissues surrounding the prosthetic valve. In certain examples, the outer skirt  18  can be made from any of a variety of woven, knitted, or crocheted fabrics wherein the surface  168  is the surface of a plush nap or pile of the fabric. Exemplary fabrics having a pile include velour, velvet, velveteen, corduroy, terrycloth, fleece, etc. As best shown in  FIG. 23 , the outer skirt can have a base layer  170  (a first layer) from which a pile layer  172  (a second layer) extends. The base layer  170  can comprise warp and weft yarns woven or knitted into a mesh-like structure. For example, in a representative configuration, the yarns of the base layer  170  can be flat yarns and can have a denier range of from about 7 dtex to about 100 dtex, and can be knitted with a density of from about 20 to about 100 wales per inch and from about 30 to about 110 courses per inch. The yarns can be made from, for example, biocompatible thermoplastic polymers such as PET, PTFE (polytetrafluoroethylene), Nylon, etc., or any other suitable natural or synthetic fibers. 
     The pile layer  172  can comprise pile yarns  174  woven or knitted into loops. In certain configurations, the pile yarns  174  can be the warp yarns or the weft yarns of the base layer  170  woven or knitted to form the loops. The pile yarns  174  can also be separate yarns incorporated into the base layer, depending upon the particular characteristics desired. In a representative configuration, the pile yarns  174  can be flat yarns and can have a denier range of from about 7 dtex to about 100 dtex, and can be knitted with a density of from about 20 to about 100 wales per inch and from about 30 to about 110 courses per inch. The pile yarns can be made from, for example, biocompatible thermoplastic polymers such as PET, PTFE, Nylon, etc., or any other suitable natural or synthetic fibers. 
     In certain embodiments, the loops can be cut such that the pile layer  172  is a cut pile in the manner of, for example, a velour fabric.  FIGS. 1 and 21  illustrate a representative embodiment of the outer skirt  18  configured as a velour fabric. In other embodiments, the loops can be left intact to form a looped pile in the manner of, for example, terrycloth.  FIG. 23  illustrates a representative embodiment of the outer skirt  18  in which the pile yarns  174  are knitted to form loops  176 . 
     The height of the pile yarns  174  (e.g., the loops  176 ) can be the same for all pile yarns across the entire extent of the outer skirt so as to provide an outer skirt having a constant thickness. In alternative embodiments, the height of the pile yarns  174  can vary along the height and/or circumference of the outer skirt so as to vary the thickness of the outer skirt along its height and/or circumference, as further described below. 
     The pile layer  172  has a much greater surface area than similarly sized skirts formed from flat or woven materials, and therefore can enhance tissue ingrowth compared to known skirts. Promoting tissue growth into the pile layer  172  can decrease perivaluvular leakage, increase retention of the valve at the implant site and contribute to long-term stability of the valve. In some configurations, the surface area of the pile yarns  174  can be further increased by using textured yarns having an increased surface area due to, for example, a wavy or undulating structure. In configurations such as the looped pile embodiment of  FIG. 23 , the loop structure and the increased surface area provided by the textured yarn of the loops  176  can allow the loops to act as a scaffold for tissue growth into and around the loops of the pile. 
     The outer skirt embodiments described herein can also contribute to improved compressibility and shape memory properties of the outer skirt over known valve coverings and skirts. For example, the pile layer  172  can be compliant such that it compresses under load (e.g., when in contact with tissue, other implants, or the like), and returns to its original size and shape when the load is relieved. This can help to improve sealing between the outer skirt and the tissue of the native annulus, or a surrounding support structure in which the prosthetic valve is deployed. Embodiments of an implantable support structure that is adapted to receive a prosthetic valve and retain it within the native mitral valve are disclosed in co-pending Application No. 62/449,320, filed Jan. 23, 2017, and application Ser. No. 15/876,053, filed Jan. 19, 2018, which are incorporated herein by reference. The compressibility provided by the pile layer  172  of the outer skirt  18  is also beneficial in reducing the crimp profile of the valve. Additionally, the outer skirt  18  can prevent the leaflets  40  or portions thereof from extending through spaces between the struts of the frame  12  as the prosthetic valve is crimped, thereby protecting against damage to the leaflets due to pinching of the leaflets between struts. 
     In alternative embodiments, the outer skirt  18  be made of a non-woven fabric such as felt, or fibers such as non-woven cotton fibers. The outer skirt  18  can also be made of porous or spongey materials such as, for example, any of a variety of compliant polymeric foam materials, or woven fabrics, such as woven PET. 
     Various techniques and configurations can be used to secure the outer skirt  18  to the frame  12  and/or the inner skirt  16 . As best shown in  FIG. 3 , a lower edge portion  180  of the inner skirt  16  can be wrapped around the inflow end  15  of the frame  12 , and the lower edge portion  160  of the outer skirt  18  can be attached to the lower edge portion  180  of the inner skirt  16  and/or the frame  12 , such as with one or more sutures or stitches  182  (as best shown in  FIG. 2 ) and/or an adhesive. In lieu of or in addition to sutures, the outer skirt  18  can be attached to the inner skirt  16 , for example, by ultrasonic welding. In the illustrated embodiment, the lower edge portion  160  of the outer skirt  18  can be free of loops, and the lower edge portion  180  of the inner skirt  16  can overlap and can be secured to the base layer  170  of the outer skirt  18 . In other embodiments, the lower edge portion  180  of the inner skirt  16  can extend over one or more rows of loops  176  of the pile layer  172  (see  FIG. 27 ), as further described below. In other embodiments, the lower edge portion  180  of the inner skirt  18  can be wrapped around the inflow end of the frame and extend between the outer surface of the frame and the outer skirt  18  (i.e., the outer skirt  18  is radially outward of the lower edge portion  180  of the inner skirt  18 ). 
     As shown in  FIG. 1 , each projection  164  of the outer skirt  18  can be attached to the third row III of struts  26  ( FIG. 5 ) of the frame  12 . The projections  164  can, for example, be wrapped over respective struts  26  of row III and secured with sutures  184 . The outer skirt  18  can be further secured to the frame  12  by suturing an intermediate portion of the outer skirt (a portion between the lower and upper edge portions) to struts of the frame, such as struts  24  of the second row II of struts. 
     The height of the outer skirt (as measured from the lower edge to the upper edge) can vary in alternative embodiments. For example, in some embodiments, the outer skirt can cover the entire outer surface of the frame  12 , with the lower edge portion  160  secured to the inflow end of the frame  12  and the upper edge portion secured to the outflow end of the frame. In another embodiment, the outer skirt  18  can extend from the inflow end of the frame to the second row II of struts  24 , or to the fourth row IV of struts  28 , or to a location along the frame between two rows of struts. In still other embodiments, the outer skirt  18  need not extend all the way to the inflow end of the frame, and instead the inflow end of the outer skirt can secured to another location on the frame, such as to the second row II of struts  24 . 
     The outer skirt  18  desirably is sized and shaped relative to the frame such that when the prosthetic valve  10  is in its radially expanded state, the outer skirt  18  fits snugly (in a tight-fitting manner) against the outer surface of the frame. When the prosthetic valve  10  is radially compressed to a compressed state for delivery, the portion of the frame on which the outer skirt is mounted can elongate axially. The outer skirt  18  desirably has sufficient elasticity to stretch in the axial direction upon radial compression of the frame so that it does not to prevent full radial compression of the frame or deform the struts during the crimping process. 
     Known skirts that have material slack or folds when the prosthetic valve is expanded to its functional size are difficult to assemble because the material must be adjusted as it is sutured to the frame. In contrast, because the outer skirt  18  is sized to fit snugly around the frame in its fully expanded state, the assembly process of securing the skirt to the frame is greatly simplified. During the assembly process, the outer skirt can be placed around the frame with the frame in its fully expanded state and the outer skirt in its final shape and position when the valve is fully functional. In this position, the skirt can then be sutured to the frame and/or the inner skirt. This simplifies the suturing process compared to skirts that are designed to have slack or folds when radially expanded. 
     As shown in  FIG. 3 , the height of the loops of the pile layer  172  can be constant across the entire extent of the outer skirt such that the outer skirt  18  has a constant thickness, except along the upper and lower edge portions which can be free of loops to facilitate attachment of the outer skirt to the frame and/or the inner skirt  16 . The “height” of the loops is measured in the radial direction when the skirt is mounted on the frame. In another embodiment, as shown in  FIG. 24 , the loops can comprise lower loops  176   a  along the lower or upstream portion of the skirt that are relatively shorter in height (as represented by a thinner cross-sectional area) than upper loops  176   b  (as represented by a thicker cross-sectional area) along the upper or downstream portion of the skirt. The skirt  18  can further include a group of intermediate loops  176   c  that gradually increase in height from the lower loops  176   a  to the upper loops  176   b . Thus, in the embodiment of  FIG. 24 , the thickness of outer skirt  18  increases from a minimum thickness along the lower portion to a maximum thickness along the upper portion. 
       FIG. 25  shows another embodiment in which the loops of the outer skirt comprise lower loops  176   d  along the lower portion of the skirt that are relatively higher or longer in height than upper loops  176   e  along the upper portion of the skirt. The skirt  18  can further include a group of intermediate loops  176   f  that gradually decrease in height from the lower loops  176   d  to the upper loops  176   e . Thus, in the embodiment of  FIG. 25 , the thickness of outer skirt  18  decreases from a maximum thickness along the lower portion to a minimum thickness along the upper portion. 
       FIG. 26  shows another embodiment in which the loops comprise lower loops  176   g , upper loops  176   h , and intermediate loops  176   i  that are relative shorter in height than the lower and upper loops. As shown, the lower loops  176   g  can gradually decrease in height from the lower edge of the skirt toward the intermediate loops  176   i , and the upper loops  176   h  can gradually decrease in height from the upper edge of the skirt toward the intermediate loops  176   i . Thus, in the embodiment of  FIG. 26 , the thickness of the outer skirt decreases from a maximum thickness along the lower portion to a minimum thickness along the intermediate portion, and then increases from the intermediate portion to the maximum thickness along the upper portion. In the illustrated embodiment, the upper portion of the skirt containing the upper loops  176   h  has the same thickness as the lower portion of the skirt containing the lower loops  176   g . In other embodiments, the thickness of the upper portion of the skirt containing the upper loops  176   h  can be greater or less than the same thickness of the lower portion of the skirt containing the lower loops  176   g.    
     Further, in any of the embodiments described above where the height of the loops vary along the height of the skirt, the height of the loops need not vary gradually from one section of the skirt to another section of the skirt. Thus, an outer skirt can have loops of different heights, wherein the height of the loops change abruptly at locations along the skirt. For example, in the embodiment of  FIG. 24 , the lower portion of the skirt containing the lower loops  176   a  can extend all the way to the upper portion of the skirt containing the upper loops  176   g  without the intermediate loops  176   c  forming a transition between the upper and lower portions. 
     In lieu of or in addition to having loops that vary in height along the height of the skirt, the height of the loops  176  (and therefore the thickness of the outer skirt) can vary along the circumference of the outer skirt. For example, the height of the loops can be increased along circumferential sections of the skirt where larger gaps might be expected between the outer skirt and the native annulus, such as circumferential sections of the skirt that are aligned with the commissures of the native valve. 
       FIGS. 27 and 28  show an alternative configuration for mounting the outer skirt  18  to the frame  12 . In this embodiment, as best shown in  FIG. 27 , the lower edge portion  180  of the inner skirt  16  is wrapped around the inflow end of the frame and extended over one or more rows of loops along the lower edge portion  160  of the outer skirt. The lower edge portion  180  of the inner skirt  16  can then be secured to the lower edge portion  160  of the outer skirt, such as with sutures or stitching  186  ( FIG. 28 ), an adhesive, and/or welding (e.g., ultrasonic welding). The stitching  186  can also extend around selected struts adjacent the inflow end of the frame. The lower edge portion  180  of the inner skirt is effective to partially compress the loops of the pile layer  172 , which creates a tapered edge at the inflow end of the prosthetic valve. The tapered edge reduces the insertion force required to push the prosthetic valve through an introducer sheath when being inserted into a patient&#39;s body. In one specific implementation, the stitching  186  secures the lower edge portion  180  of the inner skirt to the outer skirt  18  at a distance of at least 1 mm from the lowermost edge of the outer skirt. The upper edge portion  162  and the intermediate portion of the outer skirt can then be secured to the frame as previously described. 
       FIGS. 29-32  show another configuration for mounting the outer skirt  18  to the frame  12 . In this embodiment, the outer skirt  18  is initially placed in a tubular configuration with the base layer  170  facing outwardly and the lower edge portion  160  (which can be free of loops  176 ) can be placed between the inner surface of the frame  12  and the lower edge portion  180  of the inner skirt  16 , as depicted in  FIG. 30 . The lower edge portions of the outer skirt and the inner skirt can be secured to each other, such as with stitches, an adhesive, and/or welding (e.g., ultrasonic welding). In one implementation, the lower edge portions of the outer skirt and the inner skirt are secured to each other with in-and-out stitches and locking stitches. The outer skirt  18  is then inverted and pulled upwardly around the outer surface of the frame  12  such that the base layer  170  is placed against the outer surface of the frame and the pile layer  172  faces outwardly, as depicted in  FIG. 29 . In this assembled configuration, the lower edge portion  160  of the outer skirt wraps around the inflow end of the frame and is secured to the inner skirt inside of the frame. The upper edge portion  162  and the intermediate portion of the outer skirt can then be secured to the frame as previously described. 
     The prosthetic valve  10  can be configured for and mounted on a suitable delivery apparatus for implantation in a subject. Several catheter-based delivery apparatuses are known; a non-limiting example of a suitable catheter-based delivery apparatus includes that disclosed in U.S. Patent Application Publication No. 2013/0030519, which is incorporated by reference herein in its entirety, and U.S. Patent Application Publication No. 2012/0123529. 
     To implant a plastically-expandable prosthetic valve  10  within a patient, the prosthetic valve  10  including the outer skirt  18  can be crimped on an elongated shaft of a delivery apparatus. The prosthetic valve, together with the delivery apparatus, can form a delivery assembly for implanting the prosthetic valve  10  in a patient&#39;s body. The shaft can comprise an inflatable balloon for expanding the prosthetic valve within the body. With the balloon deflated, the prosthetic valve  10  can then be percutaneously delivered to a desired implantation location (e.g., a native aortic valve region). Once the prosthetic valve  10  is delivered to the implantation site (e.g., the native aortic valve) inside the body, the prosthetic valve  10  can be radially expanded to its functional state by inflating the balloon or equivalent expansion mechanism. 
     The outer skirt  18  can fill-in gaps between the frame  12  and the surrounding native annulus to assist in forming a good, fluid-tight seal between the prosthetic valve  10  and the native annulus. The outer skirt  18  therefore cooperates with the inner skirt  16  to avoid perivalvular leakage after implantation of the prosthetic valve  10 . Additionally, as discussed above, the pile layer of the outer skirt further enhances perivalvular sealing by promoting tissue ingrowth with the surrounding tissue. 
     Alternatively, a self-expanding prosthetic valve  10  can be crimped to a radially collapsed configuration and restrained in the collapsed configuration by inserting the prosthetic valve  10 , including the outer skirt  18 , into a sheath or equivalent mechanism of a delivery catheter. The prosthetic valve  10  can then be percutaneously delivered to a desired implantation location. Once inside the body, the prosthetic valve  10  can be advanced from the delivery sheath, which allows the prosthetic valve to expand to its functional state. 
       FIG. 33  illustrates a sealing member  200  for a prosthetic valve, according to another embodiment. The sealing member  200  in the illustrated embodiment is formed from a spacer fabric. The sealing member  200  can be positioned around the outer surface of the frame  12  of a prosthetic valve (in place of the outer skirt  18 ) and secured to the inner skirt  16  and/or the frame using stitching, an adhesive, and/or welding (e.g., ultrasonic welding). 
     As best shown in  FIG. 34 , the spacer fabric can comprise a first, inner layer  206 , a second, outer layer  208 , and an intermediate spacer layer  210  extending between the first and second layers to create a three-dimensional fabric. The first and second layers  206 ,  208  can be woven fabric or mesh layers. In certain configurations, one or more of the first and second layers  206 ,  208  can be woven such that they define a plurality of openings  212 . In some examples, openings such as the openings  212  can promote tissue growth into the sealing member  200 . In other embodiments, the layers  206 ,  208  need not define openings, but can be porous, as desired. 
     The spacer layer  210  can comprise a plurality of pile yarns  214 . The pile yarns  214  can be, for example, monofilament yarns arranged to form a scaffold-like structure between the first and second layers  206 ,  208 . For example,  FIGS. 34 and 35  illustrate an embodiment in which the pile yarns  214  extend between the first and second layers  206 ,  208  in a sinusoidal or looping pattern. 
     In certain examples, the pile yarns  214  can have a rigidity that is greater than the rigidity of the fabric of the first and second layers  206 ,  208  such that the pile yarns  214  can extend between the first and second layers  206 ,  208  without collapsing under the weight of the second layer  208 . The pile yarns  214  can also be sufficiently resilient such that the pile yarns can bend or give when subjected to a load, allowing the fabric to compress, and return to their non-deflected state when the load is removed. For example, when the prosthetic valve is radially compressed for delivery into a patient&#39;s body and placed in a delivery sheath of a delivery apparatus or advanced through an introducer sheath, the pile yarns  214  can compress to reduce the overall crimp profile of the prosthetic valve, and then return to their non-deflected state when deployed from the delivery sheath or the introducer sheath, as the case may be. 
     The spacer fabric can be warp-knitted, or weft-knitted, as desired. Some configurations of the spacer cloth can be made on a double-bar knitting machine. In a representative example, the yarns of the first and second layers  206 ,  208  can have a denier range of from about 10 dtex to about 70 dtex, and the yarns of the monofilament pile yarns  214  can have a denier range of from about 2 mil to about 10 mil. The pile yarns  214  can have a knitting density of from about 20 to about 100 wales per inch, and from about 30 to about 110 courses per inch. Additionally, in some configurations (e.g., warp-knitted spacer fabrics) materials with different flexibility properties may be incorporated into the spacer cloth to improve the overall flexibility of the spacer cloth. 
       FIG. 36  shows an outer sealing member  18 ′ mounted on the outside of the frame  12  of a prosthetic heart valve  10 , according to another embodiment.  FIG. 37  shows the base layer  170  of the sealing member  18 ′ in a flattened configuration.  FIG. 38  shows the pile layer  172  of the sealing member  18 ′ in a flattened configuration. The outer sealing member  18 ′ is similar to the sealing member  18  of  FIGS. 1 and 21-23 , except that the height (H 1 ) of the base layer  170  is greater than the height (H 2 ) of the pile layer  172 . Like the previously described embodiments, the sealing member  18 ′ desirably is sized and shaped relative to the frame  12  such that when the prosthetic valve is in its radially expanded state, both layers  170 ,  172  of the sealing member  18  fit snugly (in a tight-fitting manner) around the outer surface of the frame. 
     In the illustrated configuration, the base layer  170  extends axially from the inlet end of the frame  12  to the third row III of struts  26  of the frame  12 . The upstream and downstream edges of the base layer  170  can be sutured to the struts  22  of the first row I and to the struts  26  of the third row III with sutures  182  and  184 , respectively, as previously described. The pile layer  172  in the illustrated configuration extends from the inlet end of the frame  12  to a plane that intersects the frame at the nodes formed at the intersection of the upper ends of struts  24  of the second row II and the lower ends of struts  26  of the third row III, wherein the plane is perpendicular to the central axis of the frame. 
     The pile layer  172  can be separately formed from and subsequently attached to the base layer  170 , such as with sutures, an adhesive, and/or welding. Alternatively, the pile layer  172  can be formed from yarns or fibers woven into the base layer  170 . The pile layer  172  can have any of the configurations shown in  FIGS. 24-26 . 
     In particular embodiments, the height H 1  of the base layer  170  can be about 9 mm to about 25 mm or about 13 mm to about 20 mm, with about 19 mm being a specific example. The height  12  of the pile layer  172  can be at least 2 mm less than H 1 , at least 3 mm less than H 1 , at least 4 mm less than H 1 , at least 5 mm less than H 1 , at least 6 mm less than H 1 , at least 7 mm less than H 1 , at least 8 mm less than H 1 , at least 9 mm than H 1 , or at least 10 mm less than H 1 . The height of the frame  12  in the radially expanded state can be about 12 mm to about 27 mm or about 15 mm to about 23 mm, with about 20 inn being a specific example. 
     The relatively shorter pile layer  172  reduces the crimp profile along the mid-section of the prosthetic valve  10  but still provides for enhanced paravalvular sealing along the majority of the landing zone of the prosthetic valve. The base layer  170  also provides a sealing function downstream of the downstream edge of the pile layer  172 . 
       FIGS. 39-40  show an outer sealing member  300  for a prosthetic heart valve (e.g., a prosthetic heart valve  10 ), according to another embodiment.  FIGS. 39A and 40A  are magnified views of portions of the sealing member shown in  FIGS. 39 and 40 , respectively. The sealing member  300  can be mounted on the outside of the frame  12  of a prosthetic valve  10  in lieu of sealing member  18  using, for example, sutures, ultrasonic welding, or any other suitable attachment method. Like the previously described embodiments, the sealing member  300  desirably is sized and shaped relative to the frame  12  such that when the prosthetic valve is in its radially expanded state, the sealing member  300  fits snugly (in a tight-fitting manner) against the outer surface of the frame. 
     The sealing member  300 , like sealing members  18 ,  18 ′, can be a dual-layer fabric comprising a base layer  302  and a pile layer  304 .  FIG. 39  shows the outer surface of the sealing member  300  defined by the pile layer  304 .  FIG. 40  shows the inner surface of the sealing member  300  defined by the base layer  302 . The base layer  302  in the illustrated configuration comprises a mesh weave having circumferentially extending rows or stripes  306  of higher-density mesh portions interspersed with rows or stripes  308  of lower-density mesh portions. 
     In particular embodiments, the yarn count of yarns extending in the circumferential direction (side-to-side or horizontally in  FIGS. 40 and 40A ) is greater in the higher-density rows  306  than in the lower-density rows  308 . In other embodiments, the yarn count of yarns extending in the circumferential direction and the yarn count of yarns extending in the axial direction (vertically in  FIGS. 40 and 40A ) is greater in the higher-density rows  306  than in the lower-density rows  308 . 
     The pile layer  304  can be formed from yarns woven into the base layer  302 . For example, the pile layer  304  can comprise a velour weave formed from yarns incorporated in the base layer  302 . The pile layer  304  can comprise circumferentially extending rows or stripes  310  of pile formed at axially-spaced locations along the height of the sealing member  300  such that there are axial extending gaps between adjacent rows  310 . In this manner, the density of the pile layer varies along the height of the sealing member. In alternative embodiments, the pile layer  304  can be formed without gaps between adjacent rows of pile, but the pile layer can comprise circumferentially extending rows or stripes of higher-density pile interspersed with rows or stripes  312  of lower-density pile. 
     In alternative embodiments, the base layer  302  can comprise a uniform mesh weave (the density of the weave pattern is uniform) and the pile layer  304  has a varying density. 
     Varying the density of the pile layer  304  and/or the base layer  302  along the height of the sealing member  300  is advantageous in that it facilitates axially elongation of the sealing member  300  caused by axial elongation of the frame  12  when the prosthetic heart valve is crimped to a radially compressed state for delivery. The varying density also reduces the bulkiness of the sealing member in the radially collapsed state and therefore reduces the overall crimp profile of the prosthetic heart valve. 
     In alternative embodiments, the density of the sealing member  300  can vary along the circumference of the sealing member to reduce the bulkiness of the sealing member in the radially collapsed state. For example, the pile layer  304  can comprise a plurality of axially-extending, circumferentially-spaced, rows of pile yarns, or alternatively, alternating axially-extending rows of higher-density pile interspersed with axially-extending rows of lower-density pile. Similarly, the base layer  302  can comprise a plurality axially-extending rows of higher-density mesh interspersed with rows of lower-density mesh. 
     In other embodiments, the sealing member  300  can include a base layer  302  and/or a pile layer  304  that varies in density along the circumference of the sealing member and along the height of the sealing member. 
     In other embodiments, a sealing member can be knitted, crocheted, or woven to have rows or sections of higher stitch density and rows or sections of lower stitch density without two distinct layers.  FIG. 41 , for example, shows a sealing member  400  comprising a fabric having a plurality of axially-extending rows  402  of higher-density stitching alternating with axially-extending rows  404  of lower-density stitching. The sealing member  400  can be formed, for example, by knitting, crocheting, or weaving a single layer fabric having rows  402 ,  404  formed by increasing the stitch density along the rows  402  and decreasing the stitch density along the rows  404  while the fabric is formed. The sealing member  400  can be mounted on the outside of the frame  12  of a prosthetic valve  10  in lieu of sealing member  18  using, for example, sutures, ultrasonic welding, or any other suitable attachment method. Like the previously described embodiments, the sealing member  400  desirably is sized and shaped relative to the frame  12  such that when the prosthetic valve is in its radially expanded state, the sealing member  400  fits snugly (in a tight-fitting manner) against the outer surface of the frame. 
     The sealing member  400  can be resiliently stretchable between a first, substantially relaxed, axially foreshortened configuration ( FIG. 41 ) corresponding to a radially expanded state of the prosthetic valve, and a second, axially elongated, or tensioned configuration ( FIG. 42 ) corresponding to a radially compressed state of the prosthetic valve. As shown in  FIG. 41 , when the prosthetic valve is radially expanded and the sealing member  400  is in the first configuration, the higher-density rows  402  extend in an undulating pattern from the lower (upstream edge) to the upper (downstream edge) of the sealing member  400 . In the illustrated embodiment, for example, each of the higher-density rows  402  comprises a plurality of straight angled sections  406   a ,  406   b  arranged end-to-end in a zig-zag or herringbone pattern extending from the lower (upstream edge) to the upper (downstream edge) of the sealing member  400 . In alternative embodiments, the rows  402  can be sinusoidal-shaped rows having curved longitudinal edges. 
     When the prosthetic valve is crimped to its radially compressed state, the frame  12  elongates, causing the sealing member to stretch in the axial direction, as depicted in  FIG. 42 , to its second configuration. The lower-density rows  404  facilitate elongation of the sealing member and permit straightening of the higher-density rows  402 .  FIG. 42  depicts the higher-density rows  402  as straight sections extending from the inflow edge to the outflow edge of the sealing member. However, it should be understood that the higher-density rows  402  need not form perfectly straight rows when the prosthetic valve is in the radially compressed state. Instead, “straightening” of the higher-density rows  402  occurs when the angle  408  between adjacent angled segments  406   a ,  406   b  of each row increases upon axial elongation of the sealing member. 
     The varying stitch density of the sealing member  400  reduces overall bulkiness of the sealing member to minimize the crimp profile of the prosthetic valve. The zig-zag or undulating pattern of the higher-density rows  402  in the radially expanded state of the prosthetic valve facilitates stretching of the sealing member in the axial direction upon radial compression of the prosthetic valve and allows the sealing member to return to its pre-stretched state in which the sealing member fits snugly around the frame upon radial expansion of the prosthetic valve. Additionally, the zig-zag or undulating pattern of the higher-density rows  402  in the radially expanded state of the prosthetic valve eliminates any straight flow paths for blood between adjacent rows  402  extending along the outer surface of the sealing member from its outflow edge to its inflow edge to facilitate sealing and tissue ingrowth with surrounding tissue. 
     In alternative embodiments, a sealing member  400  can have a plurality of circumferentially extending higher-density rows (like rows  402  but extending in the circumferential direction) interspersed with a plurality of circumferentially extending lower-density rows (like rows  404  but extending in the circumferential direction). In some embodiments, a sealing member  400  can have axially-extending and circumferential-extending higher-density rows interspersed with axially-extending and circumferential-extending lower-density rows. 
       FIGS. 43A, 43B, 44A, and 44B  illustrate an outer sealing member  500  for a prosthetic heart valve (e.g., a prosthetic heart valve  10 ), according to another embodiment. The sealing member  500  can have a plush exterior surface  504 . The sealing member  500  can be secured to a frame  12  of the prosthetic valve using, for example, sutures, ultrasonic welding, or any other suitable attachment method as previously described herein. For purposes of illustration, enlarged or magnified portions of the sealing member  500  are shown in the figures. It should be understood that the overall size and shape of the sealing member  500  can be modified as needed to cover the entire outer surface of the frame  12  or portion of the outer surface of the frame, as previously described herein. 
     The sealing member  500  can comprise a woven or knitted fabric. The fabric can be resiliently stretchable between a first, natural, or relaxed configuration ( FIG. 43A ), and a second, axially elongated, or tensioned configuration ( FIG. 43B ). When disposed on the frame  12 , the relaxed configuration can correspond to the radially expanded, functional configuration of the prosthetic valve, and the elongated configuration can correspond to the radially collapsed delivery configuration of the prosthetic valve. Thus, with reference to  FIG. 43A , the sealing member  500  can have a first length L 1  in the axial direction when the prosthetic valve is in the radially expanded configuration, and a second length L 2  ( FIG. 43B ) in the axial direction that is longer than L 1  when the valve is crimped to the delivery configuration, as described in greater detail below. 
     The fabric can comprise a plurality of circumferentially extending warp yarns  512  and a plurality of axially extending weft yarns  514 . In some embodiments, the warp yarns  512  can have a denier of from about 1 D to about 300 D, about 10 D to about 200 D, or about 10 D to about 100 D. In some embodiments, the warp yarns  512  can have a thickness t 1  ( FIG. 44A ) of from about 0.01 mm to about 0.5 mm, about 0.02 mm to about 0.3 mm, or about 0.03 mm to about 0.1 mm. In some embodiments, the warp yarns  512  can have a thickness t 1  of about 0.03 mm, about 0.04 mm, about 0.05 mm, about 0.06 mm, about 0.07 mm, about 0.08 mm, about 0.09 mm, or about 0.1 mm. In a representative embodiment, the warp yarns  512  can have a thickness of about 0.06 mm. 
     The weft yarns  514  can be texturized yarns comprising a plurality of texturized filaments  516 . For example, the filaments  516  of the weft yarns  514  can be bulked, wherein, for example, the filaments  516  are twisted, heat set, and untwisted such that the filaments retain their deformed, twisted shape in the relaxed, non-stretched configuration. The filaments  516  can also be texturized by crimping, coiling, etc. When the weft yarns  514  are in a relaxed, non-tensioned state, the filaments  516  can be loosely packed and can provide compressible volume or bulk to the fabric, as well as a plush surface. In some embodiments, the weft yarns  514  can have a denier of from about 1 D to about 500 D, about 10 D to about 400 D, about 20 D to about 350 D, about 20 D to about 300 D, or about 40 D to about 200 D. In certain embodiments, the weft yarns  514  can have a denier of about 150 D. In some embodiments, a filament count of the weft yarns  514  can be from 2 filaments per yarn to 200 filaments per yarn, 10 filaments per yarn to 100 filaments per yarn, 20 filaments per yarn to 80 filaments per yarn, or about 30 filaments per yarn to 60 filaments per yarn. Additionally, although the axially-extending textured yarns  514  are referred to as weft yarns in the illustrated configuration, the fabric may also be manufactured such that the axially-extending textured yarns are warp yarns and the circumferentially-extending yarns are weft yarns. 
       FIGS. 44A and 44B  illustrate a cross-sectional view of the sealing member in which the weft yarns  512  extend into the plane of the page. With reference to  FIG. 44A , the fabric of the sealing member  500  can have a thickness t 2  of from about 0.1 mm to about 10 mm, about 1 mm to about 8 mm, about 1 mm to about 5 mm, about 1 mm to about 3 mm, about 0.5 mm, about 1 mm, about 1.5 mm, about 2 mm, about 2.5 mm, or about 3 mm when in a relaxed state and secured to a frame. In some embodiments, the sealing member  500  can have a thickness of about 0.1 mm, about 0.2 mm, about 0.3 mm, about 0.4 mm, or about 0.5 mm as measured in a relaxed state with a weighted drop gauge having a presser foot. In a representative example, the sealing member can have a thickness of about 1.5 mm when secured to a prosthetic valve frame in the relaxed state. The texturized, loosely packed filaments  516  of the weft yarns  514  in the relaxed state can also promote tissue growth into the sealing member  500 . 
     When the fabric is in the relaxed state, the textured filaments  516  of the weft yarns  514  can be widely dispersed such that individual weft yarns are not readily discerned, as depicted in  FIG. 43A . When tensioned in the axial direction, the filaments  516  of the weft yarns  514  can be drawn together as the weft yarns elongate and the kinks, twists, etc., of the filaments are pulled straight such that the fabric is stretched and the thickness decreases. In certain embodiments, when sufficient tension is applied to the fabric in the axial direction (the weft direction in the illustrated embodiment), such as when the prosthetic valve is crimped onto a shaft of a delivery apparatus, the textured fibers  516  can be pulled together such that individual weft yarns  514  become discernable, as best shown in  FIG. 43B . 
     Thus, for example, when fully stretched, the sealing member can have a second thickness t 3 , as shown in  FIG. 44B  that is less than the thickness t 2 . In certain embodiments, the thickness of the tensioned weft yarns  514  may be the same or nearly the same as the thickness t 1  of the warp yarns  512 . Thus, in certain examples, when stretched the fabric can have a thickness t 3  that is the same or nearly the same as three times the thickness t 1  of the warp yarns  512  depending upon, for example, the amount of flattening of the weft yarns  514 . Accordingly, in the example above in which the warp yarns  512  have a thickness of about 0.06 mm, the thickness of the sealing member can vary between about 0.2 mm and about 1.5 mm as the fabric stretches and relaxes. Stated differently, the thickness of the fabric can vary by 750% or more as the fabric stretches and relaxes. 
     Additionally, as shown in  FIG. 44A , the warp yarns  512  can be spaced apart from each other in the fabric by a distance y 1  when the outer covering is in a relaxed state. As shown in  FIGS. 43B and 44B , when tension is applied to the fabric in the direction perpendicular to the warp yarns  512  and parallel to the weft yarns  514 , the distance between the warp yarns  512  can increase as the weft yarns  514  lengthen. In the example illustrated in  FIG. 44B , in which the fabric has been stretched such that the weft yarns  514  have lengthened and narrowed to approximately the diameter of the warp yarns  512 , the distance between the warp yarns  512  can increase to a new distance y 2  that is greater than the distance y 1 . 
     In certain embodiments, the distance y 1  can be, for example, about 1 mm to about 10 mm, about 2 mm to about 8 mm, or about 3 mm to about 5 mm. In a representative example, the distance y 1  can be about 3 mm. In some embodiments, when the fabric is stretched as in  FIGS. 43B and 44B , the distance y 2  can be about 6 mm to about 10 mm. Thus, in certain embodiments, the length of the sealing member  500  in the axial direction can vary by 100% or more between the relaxed length L 1  and the fully stretched length (e.g., L 2 ). The fabric&#39;s ability to lengthen in this manner facilitates crimping of the prosthetic valve. Thus, the sealing member  500  can be soft and voluminous when the prosthetic valve is expanded to its functional size, and relatively thin when the prosthetic valve is crimped to minimize the overall crimp profile of the prosthetic valve. 
     General Considerations 
     It should be understood that the disclosed embodiments can be adapted to deliver and implant prosthetic devices in any of the native annuluses of the heart (e.g., the pulmonary, mitral, and tricuspid annuluses), and can be used with any of various approaches (e.g., retrograde, antegrade, transseptal, transventricular, transatrial, etc.). The disclosed embodiments can also be used to implant prostheses in other lumens of the body. Further, in addition to prosthetic valves, the delivery assembly embodiments described herein can be adapted to deliver and implant various other prosthetic devices such as stents and/or other prosthetic repair devices. 
     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 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. 
     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. 
     As used herein, the terms “integrally formed” and “unitary construction” refer to a construction that does not include any welds, fasteners, or other means for securing separately formed pieces of material to each other. 
     As used herein, operations that occur “simultaneously” or “concurrently” occur generally at the same time as one another, although delays in the occurrence of one operation relative to the other due to, for example, spacing, play or backlash between components in a mechanical linkage such as threads, gears, etc., are expressly within the scope of the above terms, absent specific contrary language. 
     In view of the many possible embodiments to which the principles of the disclosed invention may be applied, it should be recognized that the illustrated embodiments are only preferred examples of the invention and should not be taken as limiting the scope of the invention. Rather, the scope of the invention is defined by the following claims. We therefore claim as our invention all that comes within the scope and spirit of these claims.