Patent Publication Number: US-2021177584-A1

Title: Skirt assembly for implantable prosthetic valve

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional application of U.S. patent application Ser. No. 16/059,913, filed on Aug. 9, 2018, which claims the benefit of U.S. Provisional Application No. 62/545,916, filed Aug. 15, 2017, which is incorporated herein by reference. 
    
    
     FIELD 
     The present disclosure concerns embodiments of a prosthetic valve for implantation into body ducts, such as native heart valve annuluses. 
     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. 
     Various surgical techniques may be used to replace or repair a diseased or damaged valve. Due to stenosis and other heart valve diseases, thousands of patients undergo surgery each year wherein the defective native heart valve is replaced by a prosthetic valve. Another less drastic method for treating defective valves is through repair or reconstruction, which is typically used on minimally calcified valves. The problem with surgical therapy is the significant risk it imposes on these chronically ill patients with high morbidity and mortality rates associated with surgical repair. 
     When the native valve is replaced, surgical implantation of the prosthetic valve typically requires an open-chest surgery during which the heart is stopped and patient placed on cardiopulmonary bypass (a so-called “heart-lung machine”). In one common surgical procedure, the diseased native valve leaflets are excised and a prosthetic valve is sutured to the surrounding tissue at the valve annulus. Because of the trauma associated with the procedure and the attendant duration of extracorporeal blood circulation, some patients do not survive the surgical procedure or die shortly thereafter. It is well known that the risk to the patient increases with the amount of time required on extracorporeal circulation. Due to these risks, a substantial number of patients with defective native valves are deemed inoperable because their condition is too frail to withstand the procedure. By some estimates, more than 50% of the subjects suffering from valve stenosis who are older than 80 years cannot be operated on for valve replacement. 
     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 instance, U.S. Pat. Nos. 5,411,522 and 6,730,118, which are incorporated herein by reference, describe collapsible transcatheter heart valves that can be percutaneously introduced in a compressed state on a catheter and expanded in the desired position by balloon inflation or by utilization of a self-expanding frame or stent. 
     Known prosthetic valves include a frame with a valvular structure (e.g., leaflets) mounted therein, an inner skirt secured to the inside of the frame, and optionally, an outer skirt secured to the exterior of the frame. The inner skirt can serve several functions. For example, the inner skirt can function as a seal member to prevent (or decrease) perivalvular leakage, to anchor the leaflets to the frame, and to protect the leaflets against damage caused by contact with the frame during crimping and during working cycles of the valve. The outer skirt can cooperate with the inner skirt to further reduce or avoid perivalvular leakage after implantation of the valve. The inner skirt desirably includes a tough, tear resistant material such as polyethylene terephthalate (PET), although various other synthetic or natural materials can be used. 
     The inner and outer skirts are frequently secured to the frame by suturing or stitching the fabric of the respective skirts to the frame. Suturing of the inner skirt to the frame can expose the leaflets to the sutures. During working cycles of the valve, repetitive contact between the leaflets and the exposed sutures as well as contact between the leaflets and the fabric material of the skirt can cause abrasion of the leaflets. Accordingly, improvements to skirts for prosthetic valves are desirable. 
     SUMMARY 
     The present disclosure is directed toward methods and apparatuses relating to prosthetic valves, such as prosthetic heart valves. 
     Certain embodiments of the disclosure concern methods for making an implantable prosthetic valve. One exemplary embodiment of the method includes forming a laminate comprising a fabric layer disposed between first and second covering members. The second covering member can include one or more windows where the fabric layer is exposed. The method further includes placing the laminate against an annular frame, and suturing the laminate to the annular frame by threading a suture through the fabric layer at the one or more windows of the second covering member and around a portion of the frame. 
     In certain embodiments, the laminate can comprise an annular skirt sized and shaped to cover openings in the frame to prevent blood from flowing through the frame openings. 
     In certain embodiments, the skirt can be positioned inside of the annular frame. 
     In the foregoing embodiments, the first and second covering members can be fused to each other through openings in the fabric layer. 
     In the foregoing embodiments, forming the laminate can include forming the first covering member by electrospinning, placing the fabric layer on the electrospun first covering member, and forming the second covering member on the fabric layer by electrospinning. 
     In certain embodiments, forming the laminate can further include masking one or more areas on the fabric layer prior to forming the second covering member so as to form the one or more windows in the second covering member when the second covering member is formed over the fabric layer. 
     In certain embodiments, forming the laminate can include masking one or more areas on at least one side of the fabric layer with a masking material, dipping the fabric layer in a liquefied polymeric material, allowing the liquefied polymeric material to cure, and removing the masking material to form the one or more windows in the laminate. 
     In certain embodiments, the one or more windows in the second covering member can extend continuously around the laminate in a circumferential direction. 
     In certain embodiments, the first and second covering members comprise elastomeric material. 
     In certain embodiments, the elastomeric material can include expanded polytetrafluoroethylene (ePTFE) or ultrahigh molecular weight polyethylene (UHMWPE) or polyurethane. 
     Certain embodiments of the disclosure concern also concern implantable prosthetic valves. One representative implantable prosthetic valve can include an annular frame having a plurality of frame members and a skirt assembly. The skirt assembly can include a laminate having a fabric layer sandwiched between a first covering member and a second covering member. The fabric layer can be exposed at one or more windows in the second covering member. The skirt assembly can be coupled to the annular frame by a suture extending through the fabric layer at the one or more windows and around at least one of the plurality of frame members. 
     In certain embodiments, the skirt assembly can be positioned inside of the annular frame. 
     In certain embodiments, the implantable prosthetic valve can further include a plurality of leaflets sutured to the skirt assembly. The leaflets can be configured to permit blood flow in a first direction through the prosthetic valve and block blood flow through the prosthetic valve in a second direction, opposite the first direction. 
     In the foregoing embodiments of the implantable prosthetic valve, the first and second covering members can be fused to each other through openings in the fabric layer. 
     In the foregoing embodiments of the implantable prosthetic valve, the first covering member or the second covering member can include a membrane of non-woven fibers. 
     In the foregoing embodiments of the implantable prosthetic valve, the first or second covering member can be non-absorbable and have a porous microstructure that promotes ingrowth of surrounding tissue to assist in securing the prosthetic valve in a body lumen. 
     In the foregoing embodiments of the implantable prosthetic valve, the annular frame can have an inlet end and an outlet end, and be configured to be radially collapsible and expandable. The plurality of frame members can define a plurality of gaps between the frame members, and the skirt assembly can be configured to prevent blood from flowing through those gaps in the frame that are covered by the skirt assembly. 
     In certain embodiments, the skirt assembly can be sutured to the frame only at the one or more windows in the second covering member. 
     In certain embodiments, the first covering member and the second covering member can comprise elastomeric material. 
     In certain embodiments, the elastomeric material can include ePTFE or UHMWPE or polyurethane. 
     The foregoing and other objects, features, and advantages of the invention will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a side elevation view of an exemplary embodiment of an implantable prosthetic valve. 
         FIG. 2  shows a top perspective view of the prosthetic valve of  FIG. 1 . 
         FIG. 3  shows an exemplary frame of the prosthetic valve of  FIG. 1 . 
         FIG. 4  shows is a flattened view of the frame shown in  FIG. 3 . 
         FIG. 5  shows is a cross-sectional view of the prosthetic valve of  FIG. 1  taken along line  5 - 5  of  FIG. 1 . 
         FIG. 6  shows the formation of a first covering member on a mandrel according to an exemplary process of making the inner skirt of the prosthetic valve of  FIG. 1 . 
         FIG. 7  shows the placement of a fabric layer over the first covering member shown in  FIG. 6  according to the exemplary process of making the inner skirt. 
         FIG. 8  shows the placement of a plurality of masks over the fabric layer shown in  FIG. 7  according to the exemplary process of making the inner skirt. 
         FIG. 9  shows the formation of a second covering member over the fabric layer having masks shown in  FIG. 8  according to the exemplary process of making the inner skirt. 
         FIG. 10  shows the removal of the masks after forming the second covering member shown in  FIG. 9  according to the exemplary process of making the inner skirt. 
         FIG. 11  shows a cross-sectional view of a portion of an inner skirt having a window on one side of the inner skirt exposing the underlying woven fabric. 
         FIG. 12  shows threading a suture through the woven fabric at the window shown in  FIG. 11 . 
         FIG. 13  shows suturing the inner skirt shown in  FIG. 11  to an adjacent strut of a prosthetic valve&#39;s frame. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1-2  show two different views of a prosthetic valve  10 , according to one embodiment. The illustrated 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. The valve  10  can have several main components: a stent, or frame,  12 , a valvular structure  14 , and a skirt assembly  15 . The skirt assembly  15  can include an inner skirt  16 , and optionally, an outer skirt  18 . 
     The valvular structure  14  (or leaflet structure) can include three leaflets  40  (although a greater or fewer number of leaflets can be used), collectively forming the leaflet structure, which can be arranged to collapse in a tricuspid arrangement. The valvular structure  14  is configured to permit blood to flow through the prosthetic valve  10  in a direction from an inlet end  48  of the prosthetic valve to an outlet end  50  of the prosthetic valve and to block the flow of blood through the prosthetic valve in a direction from the outlet end  50  to the inlet end  48 . 
     Each leaflet  40  desirably has a curved, generally U-shaped inlet or cusp edge  52 . In this manner, the inlet edge of the valvular structure  14  has an undulating, curved scalloped shape. By forming the leaflets with this scalloped geometry, stresses on the leaflets can be reduced, which in turn improves durability of the 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 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. 3 . In the depicted embodiment, the frame  12  has an annular shape defining an inlet end  54  and an outlet end  56  and includes a plurality of struts (or frame members). 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  58  of the valvular structure  14  to the frame, as described more fully in U.S. Patent Publication No. 2012/0123529, which is incorporated herein by reference. 
     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., Nitinol) as known in the art. When constructed of a plastically-expandable material, the frame  12  (and thus the valve  10 ) can be crimped to a radially compressed state on a delivery catheter and then expanded inside a patient by an inflatable balloon or another suitable expansion mechanism. When constructed of a self-expandable material, the frame  12  (and thus the valve  10 ) can be crimped to a radially compressed state and restrained in the compressed state by insertion into a sheath or equivalent mechanism of a delivery catheter. Once inside the body, the valve can be advanced from the delivery sheath, which allows the 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 nickel based alloy (e.g., a cobalt-chromium or a nickel-cobalt-chromium alloy), polymers, or combinations thereof. In particular embodiments, frame  12  is made of a nickel-cobalt-chromium-molybdenum alloy, such as MP35N™ (tradename of SPS Technologies), which is equivalent to UNS R30035 (covered by ASTM F562-02). MP35N™/UNS R30035 comprises 35% nickel, 35% cobalt, 20% chromium, and 10% molybdenum, by weight. It has been found that the use of MP35N to form frame  12  provides superior structural results over stainless steel. In particular, when MP35N 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 valve assembly for percutaneous delivery to the treatment location in the body. 
     Referring to  FIGS. 3 and 4 , the frame  12  in the illustrated embodiment includes 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  56  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 . 
     Each commissure window frame portion  30  mounts a respective commissure  58  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 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 valve. In particular embodiments, the thickness T of the frame  12  ( FIG. 3 ) 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  60 . The openings  60  are relatively large and are sized to allow portions of the leaflet structure  14  to protrude, or bulge, into and/or through the openings  60  when the frame  12  is crimped in order to minimize the crimping profile. 
     As shown in  FIG. 4 , 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 that is less than the thicknesses of the junctions  44 ,  46 . The junctions  44 ,  46 , along with junctions  64 , each of which links two adjacent struts  32 , prevent full closure of openings  60  when the frame  12  is in a crimped state. Thus, the geometry of the struts  31 , and junctions  44 ,  46  and  64  assists in creating enough space in openings  60  in the crimped state to allow portions of the leaflets to protrude (i.e., bulge) outwardly through openings. This allows the valve to be crimped to a relatively smaller diameter than if all of the leaflet material is constrained within the crimped frame. 
     The frame  12  is configured to prevent or at least minimize possible over-expansion of the 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. 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 at least 120 degrees or greater when the frame is expanded to its functional size. U.S. Patent Publication No. 2012/0123529 further describes the frame  12  as well as other configurations for frames that can be incorporated in a prosthetic heart valve. 
     As shown in  FIGS. 1-2 , the skirt assembly  15  can include an inner skirt  16  that is located inside the frame  12  and an outer skirt  18  that is located outside the frame  12 . The outer skirt  18  can include a plurality of circumferentially spaced apart extension portions or projections  66  and recesses  68  between adjacent projections formed along the outflow edge (the upper edge in the illustrated embodiment) of the outer skirt. In other embodiments, the outer skirt  18  can have a straight outflow edge without any projections or recesses. 
     The inflow (lower) and the outflow (upper) edges of the outer skirt  18  can be secured to the frame  12  and/or the inner skirt  16  by, for example, heat bonding, adhesive, and/or suturing. As shown in the illustrated embodiment, the projections  66  along the outflow edge of the outer skirt  18  can be secured to struts of the frame with sutures  70  while the recesses  68  between adjacent projections can be left unattached to the frame  12  and the inner skirt  16 . The outer skirt  18  functions as a sealing member for the prosthetic valve  10  by sealing against the tissue of the native valve annulus, helping to reduce paravalvular leakage past the prosthetic valve  10 . 
     In some embodiments, as shown in  FIGS. 1-2 , the outer skirt  18  can be configured to extend radially outward from the frame  12  when the prosthetic valve  10  is in a radially expanded configuration. Alternatively, the outer skirt  18  can be configured to form a snug fit with the frame  12  such that it lies against the outer surface of the frame  12  when the prosthetic valve  10  is in the radially expanded configuration. The outer skirt  18  can be formed from any of various synthetic materials or natural tissue (e.g., pericardial tissue). Suitable synthetic materials include any of various biocompatible fabrics (e.g., PET fabric) or non-fabric films, including any of the materials disclosed below for the reinforcing layer  88  of the inner skirt  16 . Further details of the outer skirt  18  are also disclosed in U.S. Patent Publication No. 2012/0123529. 
     As further shown in  FIGS. 1-2 , the inner skirt  16  in the illustrated embodiment extends from the inlet end  54  of the frame to the fourth row IV of angled struts  28 . In other embodiments, the inner skirt  16  can extend from the inlet end  54  of the frame to a location short of the fourth row IV of struts (e.g., to the second row II or the third row III of struts), or the inner skirt can extend the entire height of the frame  12  (e.g., from the inlet end  54  to the outlet end  56 ). In alternative embodiments, the inner skirt  16  can be positioned and/or sized to extend over a different portion of the frame  12  than the configuration shown in  FIGS. 1-2 . For example, in some embodiments, the inflow end of the inner skirt  16  can be axially spaced from the inlet end  54  of the frame  12 . 
     Although the inner skirt  16  is typically tubular or cylindrical in shape (forming a complete circle in a cross-sectional profile in a plane perpendicular to the longitudinal axis of the valve), the inner skirt  16  need not extend along the inner surface of the frame  12  in the circumferential direction through 360 degrees. In other words, the inner skirt  16  can have a cross-sectional profile (in a plane perpendicular to the axis of the lumen of the valve) that is not a complete circle. The inner skirt  16  can be initially formed as a flat strip, and then formed to the annular shape by coupling together opposing edge portions, for example, by sewing, thermal bonding, and/or adhesive. Alternatively, the inner skirt  16  can be formed directly in an annular shape, for example, by constructing the inner layer  16  on a cylindrically shaped mandrel as described below. 
     Referring to  FIG. 5 , the inner skirt  16  has a first side  72  defining an inner surface of the inner skirt and a second side  74  defining an outer surface of the inner skirt  16 . As described more fully below, the frame-facing side  74  of the inner skirt  16  can have one or more windows or openings, through which an otherwise encapsulated fabric layer is exposed. Sutures can be threaded through the fabric layer of the inner skirt  16  at those windows to secure the inner skirt  16  to the frame  12 . The outer skirt  18  is omitted in  FIG. 5  for purposes of illustration. 
     When the inner skirt  16  is mounted to the frame  12 , the first side  72  of the inner skirt  16  faces inwardly toward the leaflet structure  14  located interior of the prosthetic valve  10 , and the second side  74  of the inner skirt  16  faces outwardly against the inner surface of the frame  12 . In particular embodiments, the inner skirt  16  can comprise a reinforcing layer  88  sandwiched between a first covering member  84  and a second covering member  86 . In a representative embodiment, the reinforcing layer  88  can be a fabric layer. The first and second covering members  84 ,  86  can also be referred to as encapsulating layers and form the inner and outer layers, respectively, of the illustrated inner skirt  16 . In particular embodiments, the inner surface of the reinforcing layer  88  can be completely covered by the first covering member  84  on the first side  72 , and the outer surface of the reinforcing layer  88  is partially covered by the second covering member  86  on the second side  74 , with the second covering member  86  defining one or more windows or openings  90  (see  FIG. 10 ) that expose the reinforcing layer  88  on the second side  74 . 
     The reinforcing layer  88  can strengthen the inner skirt  16  to resist tearing. It can also serve as an anchor layer for suturing the inner skirt  16  to the frame  12  and for supporting the cusp edge portions of the leaflets  40 , as described more fully below. In addition, the reinforcing layer  88 , in cooperation with the encapsulating layers  84 ,  86 , can help decrease (or prevent) paravalvular leakage past the prosthetic valve  10  when in the expanded configuration. 
     In some embodiments, the reinforcing layer  88  can comprise a woven fabric that is woven from various types of natural or synthetic fibers (or filaments, or yarns, or strands), including but are not limited to: gauze, PET fibers (e.g., Dacron), polyester fibers, polyamide fibers, etc. In certain embodiments, the reinforcing layer  88  can have a knitted or braided structure rather than the woven structure. In certain embodiments, the reinforcing layer  88  can include any of various non-woven fabrics, such as felt. The thickness of the reinforcing layer  88  can vary, but can be less than 6 mil, and desirably less than 4 mil, and even more desirably about 2 mil. 
     Alternatively, the reinforcing layer  88  can include one or more layers or films formed from any of various semi-crystalline polymeric materials or thermoplastics having aligned or partially aligned (e.g., parallel) molecular chains. Such materials can exhibit anisotropic mechanical properties, such as increased mechanical strength along the longitudinal direction of the molecular chains. Suitable semi-crystalline polymeric materials can include, for example, PTFE, PET, polypropylene, polyamide, polyetheretherketone (PEEK), etc., layers or films of which can be situated between and encapsulated by the encapsulating layers  84 ,  86 , to reinforce the inner skirt  16 . Unless otherwise specified, a fabric layer is described in the following description as an exemplary reinforcing layer for illustration purposes, while it is to be understood that a non-fabric layer having sufficiently high tensile strength can also be used as the reinforcing layer. 
     The encapsulating layers  84 ,  86  can be made of any suitable biocompatible material. Desirably, the encapsulating layers  84 ,  86  comprise a material that is relatively less abrasive than the fabric layer to reduce abrasion of the leaflets  40 . The encapsulating layers  84 ,  86  can comprise, for example, a membrane or film formed from non-woven fibers or a non-fibrous material. The biocompatible material used to form the layers  84 ,  86  may be a non-absorbable polymeric material (i.e., a material that does not dissolve once implanted in the body), and the material may be elastomeric. In addition, any of the encapsulating layers  84 ,  86  can have a porous microstructure that promotes ingrowth of surrounding tissue to assist in securing the prosthetic valve  10  in a body lumen. 
     Examples of encapsulating layer materials include, without limitation, ePTFE, unexpanded porous PTFE, polyester or expanded PTFE yarns, PTFE, ultrahigh molecular weight polyethylene (UHMWPE), other polyolefins, composite materials such as ePTFE with PTFE fibers, or UHMWPE film with embedded UHMWPE fibers, polyimides, silicones, polyurethane, hydrogels, fluoroethylpolypropylene (FEP), polypropylfluorinated amines (PFA), other related fluorinated polymers, or various combinations of any of these materials. In particular embodiments, the encapsulating layers  84 ,  86  can be formed from respective tubes made of a suitable polymeric material (e.g., ePTFE tubes or UHMWPE tubes) that can be bonded to each other when subjected to heat treatment. In some embodiments, the encapsulating layers  84 ,  86  can be formed from the same type of materials, although different materials can be used to form the encapsulating layers depending on the particular application. 
     Microporous ePTFE tubes can be made by a number of well-known methods. Expanded PTFE is frequently produced by admixing particulate dry polytetrafluoroethylene resin with a liquid lubricant to form a viscous slurry. The mixture can be poured into a mold, typically a cylindrical mold, and compressed to form a cylindrical billet. The billet can then be ram extruded through an extrusion die into either tubular or sheet structures, termed extrudates in the art. The extrudates comprise an extruded PTFE-lubricant mixture called “wet PTFE.” Wet PTFE has a microstructure of coalesced, coherent PTFE resin particles in a highly crystalline state. Following extrusion, the wet PTFE can be heated to a temperature below the flash point of the lubricant to volatilize a major fraction of the lubricant from the PTFE extrudate. The resulting PTFE extrudate without a major fraction of lubricant is known in the art as dried PTFE. The dried PTFE can then be either uniaxially, biaxially, or radially expanded using appropriate mechanical apparatus known in the art. Expansion is typically carried out at an elevated temperature, e.g., above room temperature but below 327° C., the crystalline melt point of PTFE. Uniaxial, biaxial, or radial expansion of the dried PTFE causes the coalesced, coherent PTFE resin to form fibrils emanating from nodes (regions of coalesced PTFE), with the fibrils oriented parallel to the axis of expansion. Once expanded, the dried PTFE is referred to as expanded PTFE (“ePTFE”) or microporous PTFE. 
     UHMWPE is made up of very long chains of polyethylene, with molecular weight numbering in the millions, usually between 2 and 6 million. It is highly resistant to corrosive chemicals, has extremely low moisture absorption and a very low coefficient of friction. It is self-lubricating and highly resistant to abrasion. UHMWPE is processed using compression molding, ram extrusion, gel spinning, and sintering. UHMWPE is available commercially as a powder, in sheets or rods, and as fibers. 
     The encapsulating layers  84 ,  86  can be formed by a number of means. For example, in one example, the encapsulating layers  84 ,  86  can be formed using an electrospinning process, which uses electric force to draw charged threads of polymer solutions or polymer melts up to fiber diameters in the order of some hundred nanometers. In another example, the encapsulating layers  84 ,  86  can be formed using the centrifugal spinning technique. In centrifugal spinning, the spinning fluid is placed in a rotating spinning head. When the rotating speed reaches a critical value, the centrifugal force overcomes the surface tension of the spinning fluid to eject a liquid jet from the nozzle tip of the spinning head. The jet then undergoes a stretching process and is eventually deposited on the collector, forming solidified nanofibers. Yet in a further example, the encapsulating layers  84 ,  86  can be formed using an atmospheric plasma spray (APS) technique, which is a special variation of the thermal spray process. APS uses an electric arc to ionize flowing process gases, the hot gas stream can be controlled to melt a very wide range of powder feedstock materials to apply high-quality coatings to a target object. In other embodiments, the encapsulating layers  84 ,  86  can be formed using any other suitable method including, such as dip coating, spray coating, or melt-spinning. For example, any one of the encapsulating layers  84 ,  86  can be formed by dipping the fabric layer  88  in a liquefied polymeric material, and then allowing the liquefied polymeric material to cure. 
       FIGS. 6-10  show one exemplary process of forming the inner skirt  16 . Although the use of electrospinning is described below, it is exemplary in nature and is not intended as a limitation. It is to be understood that other processes for depositing a polymeric layer, such as centrifugal spinning, APS, dip coating, and others as described above, can also be used. 
     First, as depicted in  FIG. 6 , a first covering member  84  comprising a first coating material can be deposited circumferentially around the outer surface of a cylindrically shaped mandrel  100  by means of eletrospinning (or using other techniques). As known in the art, the electrospinning system can include a spinneret that is used to extrude a polymer solution or polymer melt to form fibers. To deposit the first covering member  84  over the mandrel  100 , the electrospinning system can be configured to rotate the fiber-extruding spinneret in a circular motion around the mandrel  100 . Alternatively, the fiber-extruding spinneret can be configured to be stationary while the mandrel  100  is placed in front of the spinneret and rotates about its longitudinal axis. 
     Second, as depicted in  FIG. 7 , the fabric layer  88  can be placed over the first covering member  84 . The fabric layer  88  can be in the form of a sheet of fabric material that is tightly wrapped around the first covering member  84 . For example, as described above, the fabric layer  88  can have a woven structure that includes threads of warp and weft fibers extending perpendicular to each other. In alternative embodiments, the fabric layer  88  may also be deposited over the first covering member  84 . For example, as described above, the fabric layer  88  can include a non-woven fabric, which itself can be formed by means of electrospinning and which desirably has a relatively higher tensile strength than the layers  84 ,  86 . In another example, layer  88  can be a pre-formed braided material that is wrapped around the first covering member  84 . In yet another example, layer  88  can be formed by braiding one or more yarns or filaments around the first covering member  84  to form a braided layer around the first covering member. 
     Third, as depicted in  FIG. 8 , one or more masks  92  can be placed over the fabric layer  88  at selected areas  94  of the fabric. Fourth, as depicted in  FIG. 9 , a second covering member  86  comprising of a second coating material can be deposited over the masked fabric layer  88  by means of eletrospinning (or using other techniques). Next, as depicted in  FIG. 10 , the masks  92  are removed after the deposition of the second covering member  86 . Accordingly, one or more windows  90  corresponding to the selected areas  94  are created such that the windows  90  expose the underlying fabric layer  88 . 
     In the embodiment shown  FIGS. 8-9 , the masks  92  can temporarily cover selected areas  94  of the fabric layer  88 , and prevent those selected areas  94  from being deposited by the second coating material of the second covering member  86 . Alternatively, the selected areas  94  may be functionally masked without the need of applying the physical masks  94 . For example, the relevant movement and operation (e.g., activation and/or deactivation) of the fiber-extruding spinneret with respect to the mandrel  100  may be programmed so that the second coating material of the second covering member  86  can only be deposited on portions of the fabric layer  88  that are outside the selected areas  94 . 
     In the embodiment depicted in  FIGS. 8-9 , three annular bands of masks  92  are shown, respectively corresponding to three selected areas  94  along the outer circumference of the fabric layer  88 . As a result, three annular windows  90  are created after the removal of the masks  92 , as depicted in  FIG. 10 . In other embodiments, any one of the masks  92  can have a non-annular shape, so that the corresponding selected area  94  and the resulting window  90  do not completely encircle the fabric layer  88 . For example, any one of the masks  92  may have a customized shape at a customized location so as to create a customized window  90 . In addition, although three windows  90  are shown in the illustrated embodiment, the inner skirt can be formed with a fewer or greater number of windows and the windows can be positioned at any locations along the skirt. For example, in some embodiments, the inner skirt can be formed with one or more circumferentially extending rows of windows, with each row comprising of a plurality of circumferentially spaced-apart windows. In other embodiments, one or more windows can be formed along the inlet and/or the outlet edge of the inner skirt. 
     Although not shown, it is to be understood that in each of the steps described above, an anchoring mechanism can be provided to temporarily secure the position of the each layer. As a non-limiting example, layers of PTFE tape can be wrapped around one or both ends of the second covering member  86  to help secure the position of the second covering member  86  to the underlying layers of the assembly and to the mandrel  100  during subsequent processing. 
     In a representative embodiment, the fabric layer  88  has a plurality of openings that allow the first covering member  84  and the second covering member  86  to fuse together through those openings. In one example, the openings in a fabric layer  88  may be created by weaving, braiding, or knitting fibers or yarns to form the fabric layer. In another example, the fabric layer  88  may have a non-woven, porous structure with openings. In another example, such as when a non-woven fabric (e.g., a felt) is used to form the fabric layer, openings in the fabric layer  88  can be formed by cutting (e.g., laser cutting) openings in the fabric layer. 
     In one exemplary embodiment, fusion between the first and second covering members  84 ,  86  through openings in the fabric layer  88  can occur simultaneously during the process of depositing the second covering member  86  over the masked fabric layer  88 . As the second coating material extruded from the spinneret is deposited on the fabric layer  88  to form the second covering member  86 , some of the second coating material may penetrate through those openings in the fabric layer  88 , and fuse with the fibers in the first covering member  84 . 
     In other embodiments, fusion between the first and second covering members  84 ,  86  may occur after the deposition of the second covering member  86  over the masked fabric layer  88 . For example, the assembly shown in  FIG. 10  can undergo an encapsulation process whereby the assembly is subjected to heat and/or pressure to cause the first and second covering members  84 ,  86  to bond to each other through the openings in the fabric layer  88 . In addition, the fabric layer  88  may have an axial length that is shorter than the first and second covering members  84 ,  86  to facilitate bonding of the first and second covering members  84 ,  86  at their respective ends to encapsulate the fabric layer  88  therebetween. A similar encapsulation process is described in U.S. Patent Publication Nos. 2014/0209238 and 2016/0317305, which are both incorporated herein by reference. 
     In an exemplary embodiment, ePTFE can be used as the first coating material for depositing the first covering member  84  and/or the second coating material for depositing the second covering member  86 . Alternatively, other materials such as UHMWPE, polyurethane composite materials, or any other non-absorbable polymeric materials described above can be used. The inner skirt  16  desirably can have a laminate structure in which the fabric layer  88  is sandwiched between the two fused layers: the first covering member  84  and and second covering member  86 . In some embodiments, the same material can be used for depositing the first and second covering members  84 ,  86 . Due to the inter-layer fusion or bonding, the first and second covering members  84 ,  86  can be merged together, effectively creating a unitary structure (i.e., there is no physical inter-layer boundary), in which the fabric layer  88  is encapsulated. The density of the first covering member  84  can be the same as or different from the density of the second covering member  86 . In other embodiments, the first coating material used for depositing the first covering member  84  can be different from the second coating material used for depositing the second covering member  86 . 
     After the first and second covering members  84 ,  86  are firmly fused together to encapsulate the fabric layer  88 , the inner skirt  16  can be removed from the mandrel  100 . One or both end portions of the inner skirt  16  can be trimmed to achieve the desired height of the inner skirt. The inner skirt  16  can then be mounted to the frame  12 . 
     Although  FIGS. 6-10  and the description above illustrate the process of forming an annularly shaped inner skirt  16 , it is to be understood that the same process can be used to form the outer skirt  18 . Further, as described above, the inner skirt  16  may be initially formed as a flat strip, and then formed to the annular shape by coupling together its two opposing edges. To form a flat strip, the first and second covering members  84 ,  86  as well as the fabric layer  88  may be constructed on a flat substrate, instead of the cylindrically shaped mandrel  100  as described above. 
     Although the process described above uses masking to create the windows  90  on the second covering member  86  of the inner skirt  16 , it is to be understood that other methods can be used to create those windows  90 . For example, the second covering member  86  can be initially deposited over the entire surface of the fabric layer  88 . Then selected areas  94  on the second covering member  86  can be located and removed, for example, by means of laser cutting, chemical erosion, or other means. As a result, windows  90  can be created at the selected areas  94  on the second covering member  86 , exposing the underlying fabric layer  88  therein. In another example, the second covering member  86  can be pre-fabricated so that it is devoid of the second coating material in the selected areas  94 . Then the pre-fabricated second covering member  86  can be wrapped around the fabric layer  88 . As a result, the fabric layer  88  can be exposed through windows  90  created at the selected areas  94 . Then, the assembly (the first covering member  84 , the fabric layer  88 , and the second covering member  86 ) can undergo the heat- and/or pressure-based encapsulation process as described above, causing the first and second covering members  84 ,  86  to bond to each other. 
     The inner skirt  16  can be sutured to the frame  12  at the locations of the windows  90 . For example, the inner skirt  16  can be placed on the inside of frame  12 . The positions of the windows  90  can be arranged so as to generally correspond to the first, third, and fourth rows of struts  22 ,  26  and  28 , respectively, although other configurations can be used. The inner skirt  16  can also be secured to the struts of the first, third, and fourth rows of struts with sutures extending around the struts and through the fabric layer  88  at the locations of the windows  90 , as further described below in connection with  FIGS. 11-13 . Since the windows in the illustrated embodiment extend continuously around the entire circumference of the inner skirt, a continuous, circumferentially extending whip stitch can be formed along each row of struts and the fabric layer  88  at each window  90 . 
     As described above, the windows  90  can be created at selected locations and can have any of various shapes, allowing the inner skirt to be sutured to the frame at different locations. For example, as noted above, the inner skirt can be formed with rows of circumferentially spaced-part windows, which can allow placement of individual sutures or stitching that does not extend continuously along an entire row of struts, such as at selected portions on the inner skirt that are more prone to tension or stress. 
       FIGS. 11-13  illustrate an exemplary method of suturing the inner skirt  16  to the frame  12 .  FIG. 11  shows a cross-sectional view of a portion of an inner skirt  16  having a first side  72  and a second side  74 . As described above, the inner skirt  16  has an encapsulated fabric layer  88  sandwiched between the first covering member  84  on the first side  72  and the second covering member  86  on the second side  74 .  FIG. 11  also depicts a window  90  on the second covering member  86  of the inner skirt  16 , exposing the underlying fabric layer  88 . The inter-layer boundary (as indicated by the dashed line) may be absent when the same material is used to form the encapsulating layers  84 ,  86 , which can be fused or bonded together to form a single, unitary structure encapsulating the fabric. In the depicted embodiment, the fabric layer  88  is shown to have a woven structure comprising woven filaments, fibers, or yarns  96 . The woven filaments  96  desirably have sufficient strength to serve as anchors to retain sutures  98 , as described below. 
       FIG. 12  schematically shows how to thread a suture  98  between the fabric layer  88  and the first covering member  84  through the window  90 . In the depicted embodiment, the suture  98  is attached to a needle  102 . The tip  108  of the needle  102  is desirably blunted. By sliding the needle  102  from the second side  74  and through the window  90  while applying light force at the needle tip  108 , the first covering member  84  can be slightly pushed away from the fabric layer  88 , thus creating a space for the needle  102  to be inserted between the first covering member  84  and the fabric layer  88 . As shown, the needle  102  and the attached suture  98  can slide into the fabric layer  88  from a first end  104  of the window  90 , pass behind one or more filaments  96  that are exposed by the window  90  (e.g.,  96   a  and  99   b ), and then slide out of fabric layer  88  at a second end  106  of the window  80 . In this manner, the suture  98  does not extend through the entire thickness of the inner skirt  16 . In some embodiments, the first covering member  84  may not separate from the fabric layer  88  by insertion of the needle  102  (as depicted in  FIG. 12 ), in which case the needle  102  and the attached suture  98  may be threaded partially through the thickness of the first covering member  84  but does not extend through the entire thickness of the inner skirt. 
       FIG. 13  schematically illustrates suturing the fabric layer  88  of the inner skirt  16  to an adjacent strut  22  of the frame  12 . Desirably, the first side  72  of the inner skirt  16  faces inwardly toward the leaflet structure  14  located interior of the prosthetic valve  10 , and the second side  74  of the inner skirt  16  faces outwardly against the frame  12 . By threading the needle  102  and the attached suture  98  between the fabric layer  88  and the first covering member  84  through the window  90 , the woven filaments  96  between the first and second ends  104 ,  106  of the window  80  (e.g.,  96   a  and  99   b ) can collectively serve as anchors to hold the suture  98 . The suture  98  can then be wrapped around the adjacent strut  22 , thus securing those woven filaments (e.g.,  96   a  and  99   b ) to the adjacent strut  22 . Accordingly, the inner skirt  16  can be securely attached to the frame  12 .  FIG. 13  depicts a strut  22  for purposes of illustration. It should be understood that the inner skirt  16  can be sutured to other struts of the frame (e.g.,  26 ,  28 ,  32 ) in a similar manner. 
     As described above, the fabric layer  88  may also have a non-woven structure that has no distinct woven threads  96 . In such case, the suture  98  may be attached to a needle that has a pointed tip. The needle can be used to pierce the fabric layer  88  and thread the suture  98  through the fabric layer. In this manner, the portion of the fabric layer  88  that is between the first and second ends  104 ,  106  of the window  90  can function as an anchor to hold the suture  98 , which in turns secures that portion of the fabric to the adjacent strut  22 . Accordingly, the inner skirt  16  can be securely attached to the frame  12 . 
     Because the suture  98  is routed between the first covering member  84  and the fabric layer  88 , it is not exposed on the first side  72  of the inner skirt  16 . In other words, the suture  98  is covered by the first covering member  84 . The inner surface of the fabric layer is also covered by the first covering member. Therefore, abrasion of leaflets  40  due to repetitive contact between the leaflets  40  and inner skirt  16  and between the leaflets  40  and the sutures  98  during working cycles of the prosthetic valve  10  can be avoided. Desirably, the inner skirt  16  is sutured to the frame  12  only at the one or more windows  90  on the second covering member  86 , so that contact between the moveable portions of the leaflets  40  and the sutures  98  can be avoided. Additionally, the first covering member  84  desirably covers the entire extent of the inner surface of the fabric layer, or at least the portions of the fabric layer that would otherwise contact the moveable portions of the leaflet during working cycles of the prosthetic valve. In some embodiments, sutures  98  may pass through the entire thickness of the inner skirt, such as at locations on the inner skirt that would not come in contact with moveable portions of the leaflets. 
     As noted above, the leaflets  40  can be secured to one another at their adjacent sides to form commissures  58 . Each commissure  58  can be secured to a corresponding commissure window  20  of the frame  12 , as described in U.S. Patent Publication No. 2012/0123529. The inflow or cusp edges  52  of the leaflets  40  can be sutured to the inner skirt  16  along a suture line that tracks the curvature of the scalloped inflow edge of the leaflet structure. The fabric layer  88  can provide the strength required to retain the sutures. Any suitable suture, such as an Ethibond suture, can be used to secure the leaflets  40  to the fabric layer  88  of the inner skirt. 
     In some embodiments, the inflow edges  52  of the leaflets  40  are secured to the inner skirt  16  prior to mounting the inner skirt  16  to the frame. After securing the leaflets  40  to the inner skirt  16 , the inner skirt is then secured to the frame as described above and the commissures  58  of the leaflets are mounted to the frame. In other embodiments, the inner skirt  16  can be mounted to the frame without the leaflets, after which the inflow edges  52  of the leaflets are then secured to the inner skirt. 
     In certain embodiments, the inflow edges  52  of the leaflets  40  can be secured to the inner skirt via a thin PET reinforcing strip (not shown), as disclosed in U.S. Pat. No. 7,993,394, which is incorporated herein by reference. As described in U.S. Pat. No. 7,993,394, the reinforcing strip can be sutured to the inflow edges of the leaflets. The reinforcing strip and the lower edges of the leaflets can then be sutured to the inner skirt  16 . The reinforcing strip desirably is secured to the inner surfaces of the leaflets  40  such that the inflow edges  52  of the leaflets become sandwiched between the reinforcing strip and the inner skirt when the leaflets and the reinforcing strip are secured to the inner skirt. The reinforcing strip enables a secure suturing and protects the pericardial tissue of the leaflet structure from tears. 
     As described above, the outer skirt  18  can be constructed in a similar manner as the inner skirt  16 . That is, the outer skirt  18  can also have a reinforcing layer (e.g., a fabric layer  88 ) sandwiched between encapsulating layers  84 ,  86 . Similarly, windows  90  can be created on one of the encapsulating layers  84 ,  86 . Because the outer skirt  18  is attached to the outside of the frame  12 , the outer layer  18  is desirably arranged so that the frame  12  faces the side of the outer skirt  18  that has the windows  90 . In such arrangement, the outer skirt  18  can be attached to the frame  12  by suturing the encapsulated fabric layer  88  to the frame  12  through the frame-facing windows  90 . 
     Yet in another embodiment, the outer skirt  18  can have the fabric layer  88  being coated with only one of the encapsulating layers  84 ,  86 . While attaching the outer skirt  18  to the frame  12 , the outer skirt  18  can be arranged so that the uncoated side of the fabric layer  88  faces inwardly toward the frame  12 , so that the outer skirt  18  can be attached to the frame  12  by suturing the exposed fabric layer  88  to the frame  12 . 
     Alternatively, the outer skirt  18  can comprise only the fabric layer  88  without any of the encapsulating layers  84 ,  86 . As such, the outer skirt  18  can be directly sutured to the frame  12 . Because the sutures on the outer skirt  18  are not subject to repetitive contact by the moving leaflets  40 , abrasion of the leaflets due to the sutures on the outer skirt  18  may be less a concern than the sutures on the inner skirt  16 . By eliminating one or both encapsulating layers  84 ,  86 , the outer layer  18  may be constructed thinner, thus reducing the overall profile of the valve  10  when it is crimped to a radially compressed state. 
     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 delivery approaches (e.g., retrograde, antegrade, trans septal, transventricular, transatrial, etc.). 
     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. The technologies from any example can be combined with the technologies described in any one or more of the other examples. In view of the many possible embodiments to which the principles of the disclosed technology may be applied, it should be recognized that the illustrated embodiments are only preferred examples and should not be taken as limiting the scope of the disclosed technology. 
     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 “connected” 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. 
     Directions and other relative references (e.g., inner, outer, upper, lower, etc.) may be used to facilitate discussion of the drawings and principles herein, but are not intended to be limiting. For example, certain terms may be used such as “inside,” “outside,”, “top,” “down,” “interior,” “exterior,” and the like. Such terms are used, where applicable, to provide some clarity of description when dealing with relative relationships, particularly with respect to the illustrated embodiments. Such terms are not, however, intended to imply absolute relationships, positions, and/or orientations. For example, with respect to an object, an “upper” part can become a “lower” part simply by turning the object over. Nevertheless, it is still the same part and the object remains the same. As used herein, “and/or” means “and” or “or”, as well as “and” and “or”. 
     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 of these claims.