Abstract:
Stented tubular grafts of expanded, sintered polytetrafluoroethylene (PTFE). The stented PTFE grafts of the present invention include an integrally stented embodiment, an externally stented embodiment, and an internally stented embodiment. In each embodiment, the stent may be either self-expanding or pressure-expandable. Also, in each embodiment, the stent may be coated or covered with a plastic material capable of being affixed (e.g., heat fused) to PTFE. Manufacturing methods are also disclosed by the individual components of the stented grafts are preassembled on a mandrel and are subsequently heated to facilitate attachment of the PTFE layer(s) to one another and/or to the stent Optionally, the stented graft may be post-flexed and post-expanded following it&#39;s removal from the mandrel to ensure that the stented graft will be freely radially expandable and/or radially contractible over it&#39;s full intended range of diameters.

Description:
This is a divisional application of Ser. No. 09/358,350 filed Jul. 21, 1999, which is a divisional application of U.S. Pat. No. 5,928,279, Ser. No. 08/675,644, filed Jul. 3, 1996. 
    
    
     FIELD OF THE INVENTION 
     The present invention pertains generally to medical devices and their methods of manufacture, and more particularly to tubular, polytetrafluoroethylene (PTFE) grafts having integral, radially expandable stents, for implantation in a cavities or passageways (e.g., ducts or blood vessels) of the body. 
     BACKGROUND OF THE INVENTION 
     A. Stents 
     The prior art has included a number of radially expandable stents which may be initially deployed in a radially collapsed state suitable for transluminal insertion via a delivery catheter, and subsequently transitioned to a radially expanded state whereby the stent will contact and engage the surrounding wall or the anatomical duct or body cavity within which the stent has been positioned. Such stents have been used to support and maintain the patency of blood vessel lumens (e.g., as an adjuvant to balloon angioplasty) and to structurally support and/or anchor other apparatus, such as a tubular endovascular grafts, at desired locations within a body cavity or passageway (e.g., to anchor a tubular endovascular graft within a blood vessel such that the graft forms an internal conduit through an aneurysm or site of traumatic injury to the blood vessel wall). 
     Many stents of the prior art have been formed of individual member(s) such as wire, plastic, metal strips, or mesh which have been bent, woven, interlaced or otherwise fabricated into a generally cylindrical configuration. These stents of the prior art have generally been classified into two major categories-a) “self-expanding” stents, and b) “pressure expandable” stents. 
     i) Self-expanding Stents 
     Self-expanding stents are typically formed of spring metal, shape memory alloy, or other material which is resiliently biased toward fully radially expanded configuration or otherwise capable of self-expanding to its fully radially expanded configuration without the need for the exertion of outwardly directed radial force upon the stent by some extraneous expansion apparatus (e.g., a balloon or mechanical expander tool). These self-expanding stents may be initially radially compressed and loaded into a small diameter delivery catheter or alternatively mounted upon the outer surface of a delivery catheter equipped with some means for restraining or maintaining the stent in its radially compressed state. Thereafter, the delivery catheter is inserted into the body and is advanced to a position where the stent is located at or near the site at which it is to be implanted. Thereafter, the stent is expelled out of (or released from) the delivery catheter and allowed to self-expand to its full radial diameter. Such expansion of the stent causes the stent to frictionally engage the surrounding wall of the body cavity or passageway within which the stent has been positioned. The delivery catheter is then extracted, leaving the self-expanded stent at intended site of implantation. Some examples of self-expanding stents of the prior art include those described in U.S. Pat. No. 4,655,771 (Wallsten et al.); U.S. Pat. No. 4,954,126 (Wallsten): U.S. Pat. No. 5,061,275 (Wallsten et al.); U.S. Pat. No. 4,580,568 (Gianturco); U.S. Pat. No. 4,830,003 (Wolf et al.); U.S. Pat. No. 5,035,706 (Gianturco et al.) and U.S. Pat. No. 5,330,400 (Song). 
     ii) Pressure-Expandable Stents 
     The pressure-expandable stents of the prior art are typically formed of metal wire, metal strips, or other malleable or plastically deformable material, fabricated into a generally cylindrical configuration. The pressure-expandable stent is initially disposed in a collapsed configuration having a diameter which is smaller than the desired final diameter of the stent, when implanted in the blood vessel. The collapsed stent is then loaded into or mounted upon a small diameter delivery catheter. The delivery catheter is then advanced to its desired location within the vasculature, and a balloon or other stent-expansion apparatus (which may be formed integrally of or incorporated into the delivery catheter) is utilized to exert outward radial is force on the stent, thereby radially expanding and plastically deforming the stent to intended operative diameter whereby the stent frictionally engages the surrounding blood vessel wall. The material of the stent undergoes plastic deformation during the pressure-expansion process. Such plastic deformation of the stent material causes the stent to remain in its radially expanded operative configuration. The balloon or other expansion apparatus is then deflated/collapsed and is withdrawn from the body separately from, or as part of, the delivery catheter, leaving the pressure-expanded stent at its intended site of implantation. 
     Some examples of pressure-expandable stents of the prior art include those described in U.S. Pat. No. 5,135,536 (Hillstead); U.S. Pat. No. 5,161,547 (Tower); U.S. Pat. No. 5,292,331 (Boneau); U.S. Pat. No. 5,304,200 (Spaulding) and U.S. Pat. No. 4,733,665 (Palmaz). 
     B. PTFE Vascular Grafts 
     Fluoropolymers, such as polytetrafluoroethylene, have been heretofore used for the manufacture of various types of prosthetic vascular grafts. These vascular grafts are typically of tubular configuration so as to be useable to replace an excised segment of blood vessel. 
     The tubular PTFE vascular grafts of the prior art have traditionally been implanted, by open surgical techniques, whereby a diseased or damaged segment of blood vessel is surgically excised and removed, and the tubular bioprosthetic graft is then anastomosed into the host blood vessel as a replacement for the previously removed segment thereof. Alternatively, such tubular prosthetic vascular grafts have also been used as bypass grafts wherein opposite ends of the graft are sutured to a host blood vessel so as to form a bypass conduit around a diseased, injured or occluded segment of the host vessel. 
     In general, many tubular prosthetic vascular grafts of the prior art have been formed of extruded, porous PTFE tubes. In some of the tubular grafts of the prior art a PTFE tape is wrapped about and laminated to the outer surface of a tubular base graft to provide reinforcement and additional all burst strength. Also, some of the prior tubular prosthetic vascular grafts have included external support member(s), such as a PTFE beading, bonded or laminated to the outer surface of the tubular graft to prevent the graft from becoming compressed or kinked during implantation. These externally supported tubular vascular grafts have proven to be particularly useful for replacing segments of blood vessel which pass through, or over, joints or other regions of the body which undergo frequent articulation or movement. 
     One commercially available, eternally-supported, tubular vascular graft is formed of a PTFE tube having a PTFE filament helically wrapped around, and bonded to, the outer surface of the PTFE tube. (IMPRA Flex™ Graft, IMPRA, Inc., Tempe, Ariz.) 
     One other commercially available, eternally-supported, tubular vascular graft comprises a regular walled, PTFE tube which has PTFE reinforcement tape helically wrapped around, and bonded to, the outer surface of the PTFE tube and individual rings of Fluorinated Ethylene Propylene (FEP) rings disposed around, and bonded to, the outer surface of the reinforcement tape. (FEP ringed ePTFE vascular graft, W. L. Gore &amp; Associates, Inc., Flagstaff, Ariz.) 
     C. Stented Grafts 
     The prior art has also included a number of “stented grafts”. These stented grafts typically comprise a self-expanding or pressure-expandable stent which is affixed to or formed within a pliable tubular graft. Because of their radial compressibility/expandability, these stented grafts are particularly useable in applications wherein it is desired to insert the graft into an anatomical passageway (e.g., blood vessel) while the graft is in a radially compact state, and to subsequently expand and anchor the graft to the surrounding wall of the anatomical passageway. More recently, methods have been developed for introducing and implanting tubular prosthetic vascular grafts within the lumen of a blood vessel, by percutaneous or minimal incision means. Such endovascular implantation initially involves translumenal delivery of the graft, in a compacted state, by way of a catheter or other transluminally advancable delivery apparatus. Thereafter, the graft is radially expanded and anchored to the surrounding blood vessel wall, thereby holding the graft at its intended site of implantation within the host blood vessel. An affixation apparatus such as a stent, is typically utilized to anchor at least the opposite ends of the tubular graft to the surrounding blood vessel wall. One particular application for endovascular grafts of this type is in the treatment of vascular aneurysms without requiring open surgical access and resection of the aneurysmic blood vessel. Also, such stented grafts may also be useable to treat occlusive vascular disease-especially in cases where the stented graft is constructed in such a manner that the tubular graft material forms a complete barrier between the stent and the blood which is flowing through the blood vessel. In this manner the tubular graft material may serve as a smooth, biologically compatible, inner “covering” for the stent, thereby preventing a) turbulent blood-flow as the blood flows over the wire members or other structural material of which the stent is formed, b) immunologic reaction to the metal or other material of which the stent is formed, and c) a barrier to separate a diseased or damaged segment of blood vessel from the blood-flow passing therethrough. Such prevention of turbulent blood-flow and/or immunologic reaction to the stent material is believed to be desirable as both of these phenomena are believed to be associated with thrombus formation and/or restenosis of the blood vessel. 
     Other uses for stented grafts may include restoring patency to, or re-canalizing, other anatomical passageways such as ducts of the biliary tract, digestive tract and/or genito-urinary tract. 
     Many of the stented grafts known in the prior art have utilized woven or knitted material, such as polyester fiber, as the graft material. 
     There exists a need for the development of a radially expandable, stented graft formed of a continuous, tubular ePTFE tube because the inherent properties of PTFE may offer various clinical advantages over the woven polyester and other graft materials which have been previously used in stented grafts of the prior art. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to stented, tubular, PTFE grafts and their methods of manufacture. In general, the present invention may exist in any of three (3) separate embodiments, depending upon whether the stent component of the graft is formed integrally (i.e., within) the tubular PTFE graft, externally (i.e., on the outer surface of) the tubular PTFE graft, or internally (i.e., on the inner lumenal surface) of the PTFE tubular graft. Each of these three separate embodiments of the invention may be self expanding (i.e., incorporating a self-expanding stent) or pressure-expandable (i.e., incorporating a pressure-expandable stent. 
     In accordance with a first embodiment of the invention, there is provided an integrally stented PTFE graft which comprises a tubular PTFE base graft preferably of a density less than 1.6 g/cc, a radially expandable stent surrounding the outer surface of the tubular base graft, and an outer PTFE layer having a density of less than 1.6 g/cc. The tubular outer layer is fused to the tubular base graft through lateral openings or perforations formed in the stent. A polymer coating, such as a PTFE coating, may be disposed on the stent to further facilitate fusing or boding of the stent to the base tube and/or outer tubular layer. 
     In accordance with a second embodiment of the invention, there is provided an externally stented, tubular PTFE graft which comprises a radially compressible/expandable stent having a ePTFE tube of less than 1.6 g/cc density coaxially disposed within the stent, with the outer surface of the tubular ePTFE graft being fused or attached to the stent. A polymer coating, such as PTFE or any other plastic which may be fused or adhered to PTFE, may be applied to or formed on the stent to facilitate the desired fusion or attachment of the tube graft to the stent, and/or to improve the biocompatibility of the stent. 
     In accordance with a third embodiment of the invention, there is provided an internally stented, tubular PTFE graft comprising a tubular outer layer formed of ePTFE having a density of less than 1.6 g/cc, and a radially expandable stent. The stent is coaxially disposed within the lumen of the tubular outer layer, and fused or attached thereto. The stent may be covered with a polymer coating, such as PTFE or other biocompatable plastic capable of adhering or fusing to PTFE, to facilitate the desired fusion or attachment of the stent to the outer tubular layer, and/or to improve the biocompatability of the stent. Additionally or alternatively, PTFE particles may be disposed between the tubular outer layer and the tubular base graft to facilitate adhering or fusing of these two layers to one another, and/or to the stent. Such PTFE particles may be disposed between the inner base graft and outer tubular layer by applying or depositing PTFE liquid dispersion therebetween, or by depositing dry PTFE resin powder therebetween. 
     Any of above-summarized three (3) separate embodiments of the invention may be manufactured by a method which comprises the steps of: a) initially positioning a generally cylindrical stent of either the self-expanding or pressure-expandable variety in contacting coaxial relation with the tubular ePTFE base graft and/or the tubular ePTFE outer layer, upon a cylindrical mandrel or other suitable support surface, and b) subsequently fusing the fuse (i.e., heating to a lamination temperature) assembled components (i.e., the stent in combination with the inner base graft and/or outer tubular layer) of the stented graft into a unitary stented graft structure. In integrally stented embodiments where both the tubular ePTFE base graft and the tubular ePTFE outer layer are present, such heating will additionally cause the tubular outer layer to fuse to the inner tubular base graft, through lateral openings or perforations which exist in the stent. The stent may be surface treated, abraded, or coated with a plastic capable of adhering or fusing to ePTFE to facilitate attachment of the stent to the adjacent outer layer and/or inner base graft upon subsequent heating, application of solvent or other suitable adhesion promoting technique. In instances where a plastic coating is formed on the stent, such coating may be in the nature of a tube or film which is applied to the stent prior to assembly and mounting of the stented graft components on the mandrel or other support surface. Also, in embodiments where both the outer tubular layer and tubular base graft are used, aqueous PTFE dispersion, powdered PTFE resin or other flowable plastic material, may be deposited between the outer tubular layer and inner tubular base graft at the time of assembly (prior to heating) to further facilitate fusion of the outer tubular layer and/or inner tubular base graft to the stent and/or to one another. 
     By the above-described materials and methods of construction, the stented PTFE grafts of the present invention are capable of radially expanding and contracting without excessive puckering, wrinkling or invagination of the PTFE graft material. Furthermore, in embodiments wherein the stent is constructed of individual members which move or reposition relative to one another during respective expansion and contraction of the stented graft, the manufacturing methods and materials of the present invention render the PTFE sufficiently strong and sufficiently firmly laminated or fused so as to permit such relative movement of the individual members of the stent without tearing or rupturing of the tubular PTFE graft. 
     Further objects and advantages of the invention will become apparent to those skilled in the art upon reading and understanding the following detailed description and the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an integrally stented PTFE tubular graft of the present invention, wherein a portion of the graft has been inserted into a tubular catheter. 
     FIG. 1 a  is an enlarged perspective view of a segment of FIG.  1 . 
     FIG. 2 is an enlarged, cut-away, elevational view of a preferred, integrally stented, tubular PTFE graft of the present invention. 
     FIG. 3 a  is an enlarged perspective view of a portion of the stent incorporated in the graft of FIG.  2 . 
     FIG. 3 b  is an enlarged cross-sectional view through line  3   b - 3   d  of FIG. 3 a.    
     FIGS. 4 a - 4   f  are a step-by-step illustration of a preferred method for manufacturing an integrally stented PTFE graft of the present invention. 
     FIG. 5 is a schematic illustration of an alternative electron beam deposition method which is usable for depositing PTFE coating on the stent portion of the integrally stented PTFE grafts of the present invention. 
     FIG. 6 is a perspective view of an alternative heating apparatus which is useable in the manufacture of the integrally stented PTFE grafts of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following detailed description is provided for the purpose of describing and illustrating presently preferred embodiments of the invention only, and is not intended to exhaustively describe all possible embodiments in which the invention may be practiced. 
     A. The Structure of an Integrally Stented PTFE Graft 
     With reference to FIGS. 1-3 b , there is shown an integrally stented tubular PTFE graft  10  of the present invention. The preferred integrally stented graft  10  comprises a tubular PTFE base graft  12 , a PTFE-coated stent  14  and an outer layer of PTFE  16 . 
     One of the many types of stents which may be used to form the stent  14  component of a stented graft  10  of the present invention, is shown in the drawings. This particular stent  14  is formed of individual elements or wires  18  which have been coated with a PTFE coating  20 . Gaps or lateral openings  19  exist between adjacent ones or bundles of the wires  18 . The configuration, construction, and function of this stent  14  is described in detail in U.S. Pat. No. 4,655,771 Wallsten); U.S. Pat. No. 4,954,126 (Wallsten); and U.S. Pat. No. 5,061,275 (Wallsten et al.), the entireties of which are hereby expressly incorporated herein by reference. As shown in the figures of this patent application, this particular stent  14  is composed of rigid but resiliently flexible thread elements or wires  18 . These thread elements or wires  18  are formed of metal, such as an alloy of cobalt, chromium, nickel or molybdenum, wherein the alloying residue is iron. One specific example of a commercially available alloy which may is usable to form the wires  18  of the stent  14  is Elgiloy (The Elgiloy Company, 1565 Fleetwood Drive, Elgin, Ill. 60120). The wires  18  of this stent  14  are arranged in helical configuration about a common longitudinal axis LA. A number of the wires  18  are positioned in substantially parallel relation to one another, but are axially displaced relative to each other. By such arrangement, some of the wires  18  are wound in a first helical direction, while others are wound in a second or opposite helical direction such that they cross on opposite sides of adjacent ones of the wires wound in the first helical direction so as to form a helically braided wire stent as shown in the Figures. This results in the formation of a braided wire stent  14  of generally tubular configuration which is self-expanding and biased to its radially expanded diameter D 2 . However, this stent  14  may be radially compressed to a smaller diameter D 1  and radial constraint, as may be applied by the surrounding wall of the tubular delivery catheter  22  shown in FIG. 1, may be applied to hold the stent  14  In such radially compressed state (diameter D 1 ). Thereafter, when the radial constraint is removed from the stent  14 , the stent  14  will resiliently spring back to its radially expanded diameter D 2 . The individual, helically wound wires  18  of this particular braided stent  14  move and articulate such that the angular dispositions of the wires  18 , relative to one another, will change radial expansion and compression of the stent  14 . Also, the longitudinal length of the stent  14  will increase as the stent  14  is radially compressed toward its radially compact configuration D 1 , and such length will shorten as the stent  14  expands toward its radially expanded configuration D 2 . Thus, is the optional PTFE coating  20  is applied to the wires  18  of the stent  14 , such coating (described in detail herebelow) is preferably flexible enough to withstand the flexing and movement of the individual wires  18  without cracking or degrading. 
     The tubular base graft  12  is initially coaxially positioned within the hollow inner bore of the tubular stent  14  while the stent  14  is in its radially expanded configuration, after the stent  14  has been coated with the PTFE coating  20 , if desired. Thereafter, the outer PTFE layer  16  is formed by any suitable means, such as by wrapping PTFE tape  17  upon the outer surface of the PTFE coated stent  14  to form the generally tubular outer PTFE layer  16 . Thereafter, heat or other means are utilized to fuse the outer PTFE layer  16  to the inner base graft  12 , through the gaps or openings  19  which exist in the stent  14 . In embodiments wherein the optional PTFE coating  20  has been applied to the stent  14 , such heating will also facilitate bonding of the PTFE coating  20  of the stent  14  to the adjacent base graft  12  and outer PTFE layer  16 . In this manner, there is formed a self-expanding, tubular, integrally stented, PTFE graft  10  of substantially unitary construction. The stent  14  forms an integral structural framework within the tubular graft  10 , and the fused PTFE body of the graft is low enough in density and sufficiently pliable to allow the stent  14  incorporated into the graft  10  to continue to undergo substantially the same range of radial expansion and contraction that such stent  14  was capable of before disposition of the PTFE graft thereon. In this regard, the internally stented graft  10  is radially compressible to the stent&#39;s first diameter D 1  and may be may be inserted into the lumen of a small diameter tubular catheter  22 . The external constraint provided by the wall of the catheter  22  will maintain the stented graft  10  in its radially compressed configuration of diameter D 1  until such time as the graft  10  is expelled or ejected out of the catheter  22 . After the graft  10  has been expelled or ejected out of the catheter  22 , the graft will self-expand to a diameter which is substantially equal to the original expanded diameter D 2  of the stent  14 . 
     B. Preparation of the Tubular Base Graft 
     i.) Preparation of Paste 
     The manufacture of the tubular base graft begins with the step of preparing a PTFE paste dispersion for subsequent extrusion. This PTFE paste dispersion may be prepared by known methodology whereby a fine, virgin PTFE powder (e.g., F-104 or F-103 Virgin PTFE Fine Powder, Dakin America, 20 Olympic Drive, Orangebury, N.Y. 10962) is blended with a liquid lubricant, such as odorless mineral spirits (e.g., Isopar®, Exxon Chemical Company, Houston, Tex. 77253-3272), to form a PTFE paste of the desired consistency. 
     ii.) Extrusion of Tube 
     The PTFE-lubricant blend dispersion is subsequently passed through a tubular extrusion dye to form a tubular extrudate. 
     iii.) Drying 
     The wet tubular extrudate is then subjected to a drying step whereby the liquid lubricant is removed. This drying step may be accomplished at room temperature or by placing the wet tubular extrudate in an oven maintained at an elevated temperature at or near the lubricant&#39;s dry point for a sufficient period of time to result in evaporation of substantially all of the liquid lubricant. 
     iv.) Expansion 
     Thereafter, the dried tubular extrudate is longitudinally expanded or longitudinally drawn at a temperature less than 327° C. and typically in the range of 250-326° C. This longitudinal expansion of the extrudate may be accomplished through the use of known methodology, and may be implemented by the use of a device known as a batch expander. Typically, the tubular extrudate is longitudinally expanded by an expansion ratio of more than two to one (2:1) (i.e., at least two (2) times its original length). 
     Preferably, the base graft  12  is formed of expanded, sintered PTFE having a density of less than 1.6 grams per cubic centimeter. 
     v.) Sintering 
     After the longitudinal expansion step has been completed, the expanded PTFE tube is subjected to a sintering step whereby it is heated to a temperature above the sintering temperature of PTFE (i.e., 350-370° C.) to effect amorphous-locking of the PTFE polymer. The methodology used to effect the sintering step, and the devices used to implement such methodology, are known in the art. 
     Completion of the sintering step marks the completion of the preparation of the expanded, sintered PTFE base graft  12 . 
     The PTFE tape  16  may be manufactured by any suitable method, including the general method for manufacturing expanded PTFE tape, as follows: 
     C. Preparation of PTFE Tape 
     i.) Preparation of Paste Dispersion 
     The usual manufacture of an expanded, sintered PTFE tape  17  useable to form the outer PTFE layer  16  of the stented graft  10  begins with the preparation of a PTFE paste dispersion. This PTFE paste dispersion may be prepared in the same manner as described hereabove for preparation of the PTFE paste dispersion used to form the tubular base graft. 
     ii.) Extrusion of Film 
     The PTFE paste dispersion is subsequently passed through the film extrusion dye to form a wet film extrudate. The wet film extrudate is taken up or wound upon a rotating core so as to form a roll of the wet film extrudate. 
     iii.) Calendaring 
     The wet film extrudate is subsequently unrolled and subjected to an initial cold (i.e., &lt;100° C.) calendaring step by passing the film through at least one set of opposing stainless steel calendaring rollers having an adjustable gap thickness therebetween. The calendaring rollers are preferably maintained at a temperature between room temperature and 60° C. The width of the wet extrudate is held constant as it passes through these calendaring rollers. The thickness of the wet film extrudate is reduced to its desired final thickness (e.g., 0.004-0.005 inches) while the width of the film is maintained constant. It will be appreciated that, since the width of the film is maintained constant, the passage of the film through the calendaring machine will result in a longitudinal lengthening of the film. The amount of longitudinal lengthening will be a function of the decrease in film thickness which occurs as the film passes between the calendaring rollers. 
     One example of a commercially available calendaring machine useable for this purpose is the small Killion 2 Roll Stack, (Killion Extruders, Inc., Cedar Grove, N.J. 07009.) 
     iv) Drying 
     Thereafter, the wet film is subjected to a drying step. This drying step may be accomplished by permitting or causing the liquid lubricant to evaporate from the matrix of the film. Such evaporation of the liquid lubricant may be facilitated by passing the film over a drum or roller which is maintained in an elevated temperature sufficient to cause the liquid lubricant to fully evaporate from the film matrix. 
     v) Expansion 
     Separately, or concurrently with the drying step the film is subjected to an expansion step. Such expansion step comprises expanding the PTFE film in at least one direction (e.g., longitudinally). Such expansion of the film serves to a) increase the porosity of the film, b) increase the strength of the film, and c) orient the PTFE fibrils in the direction of the axis of expansion. 
     This expansion step is typically carried out with some heating of the film during such expansion, but such heating does not exceed the crystalline melting point of the PTFE polymer. 
     vi) Sintering of the Film 
     After the drying step and expansion step have been completed, the film is subjected to a sintering step wherein the film is heated to a temperature above the melting point of PTFE to accomplish sintering or amorphous locking of the PTFE polymer. This sintering step may be carried out by passing the film over a drum or roller which is maintained at a high surface temperature (e.g., 350-420° C.) to cause the desired heating of the PTFE film above the melting point of the PTFE polymer for a sufficient period of time to effect the desired sintering of the film. 
     vii) Cutting the Film Into Strips 
     After the film has been dried, the film is cut into strips, each strip typically having a width of 0.25-0.50 inches, thereby creating strips of expanded, sintered PTFE tape  14 . 
     D. Coating of the Stent and/or Deposition of PTFE between Layers to Enhance Bonding 
     Prior to assembly of the components of the integrally stented graft  10 , the stent  14  may be coated with a polymer coating  20 . 
     The polymer coating formed on the stent  14  may be any suitable type of polymer which will adhere to PTFE. Examples of polymers which may be used for such polymer coating or covering include polytetrafluoroethylene(PTFE), fluorinated ethylene propylene (FEP), polytetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), polyvinyl chloride (PVC), polypropylene (PP), polyethylene terephthalate (PET) polyuinylidene fluoride (PVDF) and other biocompatable plastics. 
     One manner in which such coating of the stent  14  may be carried out is illustrated in FIG. 4 a . As shown in FIG. 4 a , the stent  14  may be immersed in a vessel  30  containing an aqueous dispersion of PTFE  32 . One aqueous PTFE dispersion which may be useable for coating of the stent  14  is DuPont T- 30  Aqueous PTFE Dispersion, available commercially from the E.I. DuPont de Numoris Co., (Wilmington, Del.). Another commercially available PTFE dispersion  32  which may be utilized for coating of the stent is Daikin-Polyflon TFE Dispersion, available from Daikin Industries, Ltd., Chemical Division (Umeda Center Bldg., 4-12 chome, Nakazaki-nishi, Kita-ku, Osaka, Japan). 
     The time in which the stent  14  must remain immersed in the liquid PTFE dispersion  32  may vary, depending on the construction of the stent  14  and the chemical composition of the PTFE dispersion  32 . However, in most cases, an immersion time of 10-15 seconds will be sufficient to obtain uniform deposition of the PTFE coating  20  on the wire members  18  of the stent  14 . 
     After the stent  14  has been removed from the liquid PTFE dispersion  32 , it will be permitted to air dry such that a dry PTFE coating  20  remains deposited upon the outer surface of each wire  18  of the stent  14 . 
     Optionally, after the air drying has been completed, the PTFE coated stent  14  may be placed in an oven at 350° C. for approximately 10 minutes to sinter the PTFE coating and/or to enhance the bonding of the PTFE coating  20  to wire members  18  of the stent  14 . Sintering of the PTFE coating renders the coating more resistant to abrasion or peeling during the subsequent handing of the stent and/or the ensuing manufacture and use of the stented graft  10 . It will be appreciated that various alternative methods, other than immersion, may be used for depositing the PTFE coating  20  on the stent  14 . One alternative method is electron beam deposition, as illustrated in FIG.  5 . In accordance with this alternative PTFE deposition method, the stent  14  is positioned within a closed vacuum chamber  36  wherein a mass of PTFE  38  is located. An electron beam apparatus  40  is then utilized to project electron beam radiation onto the PTFE  38  within the chamber  36  so as to cause sublimation of the PTFE and resultant deposition of the layer  20  of PTFE on the outer surface of the stent  14 . The apparatus and specific methodology useable to perform this electron beam deposition of the PTFE coating  20  are well known to those of skill in the relevant art. 
     As with the above-described immersion process (FIG. 4 a ), the stent  14  whereupon the PTFE coating  20  has been deposited may be subjected to optional heating at 350° C. for a period of approximately ten minutes in order to sinter the PTFE coating and/or to enhance the bonding of the PTFE coating  20  to the wire members  18  of the stent  14 . 
     As an alternative to coating of the stent, or in addition thereto, such PTFE aqueous dispersion may be painted onto the outer surface of the base graft  12 , or the inner surface of the outer tubular layer  16 , or may be otherwise disposed between the base graft  12  and outer tubular layer  16  to facilitate fusion or bonding of the inner base graft  12  to the outer tubular layer  16 . Or, such PTFE aqueous dispersion may be sprayed or otherwise applied to the outer surface of the outer tubular layer  16  provided that the PTFE present in the dispersion are small enough to migrate inwardly through pores in the outer tubular layer  16 , thereby becoming deposited between the outer tubular layer  16  and the inner base graft  12 . 
     Another alternative or additional means by which adherence or fusion of the base graft  12 , outer tubular layer  16  and/or stent  14  may be facilitated or enhanced includes the deposition of raw PTFE resin powder between the outer tubular layer  16  and inner base graft  12 , and/or upon the stent  14 . 
     It will be appreciated that in many cases, it will be desirable to apply the polymer coating  20  to the stent  14  while the stent  14  is in its fully radially expanded configuration of diameter D 2 . In this manner, after the coating  20  has been applied and formed on the fully radially expanded stent  14 , the stent  14  may subsequently be contracted to its radially compact configuration of diameter D 1  without tearing or disrupting of the previously-applied coating  20 . In embodiments which utilize a pressure-expandable stent  14 , it may thus be necessary to volitionally or purposely expand the stent  14  to its fully radially expanded diameter D 2  prior to application of the coating  20 . Alternatively, when the stent  14  is of the self-expanding variety it will, in most cases, automatically assume its fully radially expanded configuration of diameter D 2  and no such volitional or purposeful pre-expansion of the stent  14  will be required. 
     The preferred means by which liquid PTFE dispersion and/or solid PTFE powder may be deposited between the outer tubular layer  16 , and inner base graft  12  will be discussed in more detail herebelow with reference to the manufacturing methodology. 
     E. Assembly and Construction of the Integrally Stented PTFE Graft 
     FIGS. 4 b - 4   f  show, in step-wise fashion, the preferred method for assembling and constructing the integrally stented PTFE graft  10 . 
     As shown in FIG. 4 b , the tubular base graft  12  is initially disposed on a rod or mandrel  50 . Such rod or mandrel  50  may comprise a stainless steel rod having an outer diameter which is only slightly smaller than the inner diameter of the tubular base graft  12 . In this manner, the tubular base graft  12  may be slidably advanced onto the outer surface of the mandrel  50  without undue effort or damage to the base graft  12 . 
     Thereafter, the PTFE-coated stent  14  is axially advanced onto the outer surface of the tubular base graft  12 , as shown in FIG. 4 c.    
     At this point in the process, PTFE liquid dispersion or powdered PTFE resin may be additionally (optionally) applied to the stent  14  and/or outer surface of the base graft  12  to promote further bonding and fusion of the base graft  12  to the stent  14  and/or subsequently applied outer layer  16 . In this regard, the mandrel-borne tubular base graft  12  and stent  14  may be rolled in powdered PTFE resin to accomplish the desired deposition of PTFE powder thereon. Alternatively, the above-described PTFE liquid dispersion may be painted sprayed or otherwise applied to the surface of the stent  14  and/or outer surface of the tubular base graft  12  prior to subsequent application of the outer tubular layer  16 . 
     Thereafter, as shown in FIG. 4 d , the tape  17  is initially helically wrapped in overlapping fashion, on the outer surface of the stent  14 , in a first direction. In the preferred embodiment, tape of ½ inch width is used. The tape is helically wrapped about the stent at a pitch angle whereby 6 to 8 revolutions of the tape are applied per linear inch of the stent  14 . 
     Thereafter, as shown In FIG. 4 e , a second tape wrap in the opposite direction is accomplished, preferably using the same width of tape at the same pitch angle, thereby applying another 6-8 revolutions of tape  17  per linear inch of stent  14 . In this manner, both wrappings of the tape  17  (FIGS. 4 d  and  4   e ) combine to form a tubular, outer PTFE layer  16  which preferably has a thickness of less than 0.1 inches, and which may be formed of 1 to 10 consecutive (e.g., laminated) layers of the tape  17 . for example, when using ePTFE tape of less than 1.6 g/cc density and ½ inch width, the first helical wrap (FIG. 4 d ) may deposit four consecutive layers of tape  17  and the second helical wrap (FIG. 4 e ) may deposit an additional 4 layers of tape  17 , thereby resulting in an outer tubular layer  16  which is made up of a total of 8 layers of such tape  17 . 
     Optionally, to further promote bonding of the outer tubular layer  16  to the stent  14  and/or inner base graft  12 , liquid PTFE dispersion may be sprayed, painted or otherwise applied to and dried upon the tape  17  prior to wrapping, or such liquid PTFE dispersion may be deposited by any suitable means (spraying, painting, etc.) between the outer tubular layer  16  formed by the helically wrapped tape  17  and the inner base graft  12 . Or such liquid PTFE dispersion may be sprayed onto or otherwise applied to the outer surface of the helically wrapped tape  17  such the small particles of PTFE contained within the liquid dispersion will migrate inwardly through pores in the layers of tape  17 , and will thereby become deposited between the outer at tubular layer  16  and the inner base graft  12  prior to subsequent heating of the assembly, as described herebelow. Another alternative (and optional) method for depositing polymer (e.g., PTFE) particles between the base graft  12  and outer tubular layer  16  is by rolling the mandrel  50 , having the base graft  12  and stent  14  disposed thereon, in dry, powdered polymer resin (e.g., the above-described PTFE resin) to cause such dry polymer resin to become deposited on the outer surface of the base graft  12  and/or stent  14  prior to application of the tape  17  as shown in FIGS. 4 d  and  4   e.    
     Thereafter, as shown in FIG. 4 f , ligatures  52  of stainless steel wire are tied about the opposite ends of the graft  10  so as to securely hold the base graft  12 , PTFE-coated stent  14  and outer layer  16  on the mandrel  50 . The mandrel, having the graft  10  disposed thereon is then heated to a temperature of 363°±2° C. for thirty minutes. Such heating will cause the outer PTFE layer  16  to heat fuse to the inner base graft  12  through the openings  19  which exist in the stent  14 , and will further facilitate bonding or fusing of the PTFE coating  20  of the stent  14  to the adjacent base graft  12  and outer tape layer  16 . In this manner, the desired integrally-stented PTFE tubular graft  10  is formed. 
     The heating step illustrated schematically in FIG. 4 f  may be carried out by any suitable means. For example, the mandrel  50  having the graft and ligatures  52  disposed thereon may be placed in an oven preheated to the desired temperature, for the desired period of time. Alternatively, the mandrel, having the graft  10  and ligatures  52  disposed thereon may be rolled on a hot plate or heated surface to accomplish the desired heat fusing or bonding of the outer layer  16 , base graft  12  and PTFE coating  20  of the stent  14 . 
     Another alternative apparatus which may be utilized for the heating step shown schematically in FIG. 4 f , is the U-shaped aluminum block heater shown in FIG.  6 . This aluminum block heater is formed of a solid aluminum plate or block  54  formed into a generally U-shaped configuration, and having a plurality of bore holes  60  formed longitudinally therein and extending at least part way therethrough. Elongate, cylindrical, electric heaters  62 , such as those commercially available from the Watlow Electric Company, 12001 Lackland Road, St. Louis, Mo. 63146, are inserted into the bore holes  60 , and such heaters  62  are heated to a temperature which will cause the inner surface of the U-shaped aluminum heater block  54  to be maintained at approximately 300° C. or greater. It will be appreciated that other types of heating apparatus, such as electrical strip heaters mounted on the outer surface of the U-shaped block  54 , may be useable as an alternative to the bore holes  60  and cylindrical heaters  62  described herein. 
     After the U-shaped block  54  has been heated to the desired temperature, the mandrel  50 , having the graft  10  and ligatures  52  disposed thereon, is inserted into the U-shaped inner region of the block  54 , and is rotated, therein so as to accomplish the desired heat fusing of the tubular base graft  12 , outer tape layer  16  and PTFE coating  20  of the stent  14 . 
     In many applications, it will be desirable to post-flex and re-expand the stented graft  10  to ensure that the stented graft  10  is capable of undergoing full radial compression and full radial expansion, over its complete range of intended diameters. 
     To accomplish this post-flexing and re-expansion of the stented graft  10 , the stented graft  10  is removed from the mandrel  50  and is held in a heated environment, such as in the inner space of the U-shaped heater device shown in FIG.  6 . Thereafter, the opposite ends of the stent  14  are pulled longitudinally away from each other to thereby radially contract the stented graft  10  to its minimal radially compressed diameter D 1 . Thereafter, the stented graft  10  is allowed to self-expand. If this self-expansion of the stented graft  10  does not result in return of the stented graft  10  to its fully radially expanded diameter D 2 , the stented graft  10  may then be re-advanced onto the mandrel  50  to thereby force the stented graft  10  to reassume its full radially expanded configuration of diameter D 2 . 
     Thereafter, when the graft is again removed from the mandrel  50 , the stented graft  10  will be rendered capable of being radially compressed to it&#39;s fully compressed diameter D 1 , and subsequently self-expanded to its full radially expanded diameter D 2 . 
     F. Assembly and Construction of Internally Stented PTFE Tube Graft 
     In a first alternative embodiment of the invention, the inner base graft  12  may be eliminated or excluded, thereby providing a modified version of the stented graft  10  comprising only the stent  14  and outer tubular layer  16 . 
     In this first alternative embodiment, the above-described manufacturing method is performed as described without the tubular base graft  12 , thereby forming a modified version of the stented graft  10  wherein the outer tubular layer  16  of PTFE is fused only to the stent  14 . 
     In embodiments wherein the stent  14  is coated with a polymer coating such as PTFE, the presence of such coating on the stent  14  will provide lubricity and biocompatability, which may render such internally stented graft suitable for use in applications wherein the exposed stent  14  will come in direct contact with biological fluid or blood flowing through the graft, thereby avoiding the need for use of the internal base graft  12 . 
     Thus, this first alternative embodiment of the present invention includes all possible embodiments wherein only the outer tubular layer  16  is utilized in conjunction with the stent  14 , to provide an internally stented graft  10  which is devoid of any internal tubular base graft  12 . 
     G. Assembly and Construction of Externally Stented PTFE Tube Graft 
     In a second alternative embodiment of the invention, the outer tubular layer  16  may be excluded or eliminated, thereby providing an externally stented PTFE tube graft which comprises only the stent  14  and the inner-base tube  12 . 
     In this second alternative embodiment, the above-described manufacturing method is performed as described without the outer tubular layer  16 . This results in the formation of a modified version of the stented graft  10 , comprising only the inner base graft  12  and the stent  14 . In embodiments wherein the stent  14  is coated with a polymer coating, such as PTFE, the presence of such coating on the stent  14  will provide for enhanced biocompatability, which may render such externally stented graft suitable for implantation in blood vessels or other tubular anatomical passageways wherein the exposed exterior of the coated stent  14  comes in direct contact with vascular tissue or other tissue of the body, thereby avoiding the need for use of the outer tubular layer  16 . 
     Thus, this second alternative embodiment of the present invention includes all possible embodiments wherein only the inner base graft  12  is utilized in conjunction with the stent  14 , to provide an externally stented graft  10  which is devoid of any outer tubular layer  16 . 
     It will be appreciated that the invention has been described hereabove with reference to certain presently preferred embodiments of the invention. Various additions, deletions, alterations and modifications may be made to the above-described embodiments without departing from the intended spirit and scope of the invention. Accordingly, it is intended that all such reasonable additions, deletions, modifications and alterations to the above described embodiments be included within the scope of the following claims.