Abstract:
A prosthesis, and method for forming same, are provided which includes expanded polytetrafluoroethylene (ePTFE) tubes having angularly offset node and fibril configurations. Also, the node and fibril configurations are angularly offset from the longitudinal axes of the respective tubes, providing resistance against failure in the longitudinal direction.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 10/775,442, filed Feb. 10, 2004, now U.S. Pat. No. 7,056,412, which is a division of U.S. application Ser. No. 09/990,422, filed Nov. 21, 2001, now U.S. Pat. No. 6,719,784, which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to methods of preparing tubular prostheses, and, more particularly, to techniques for forming multi-layered prostheses. 
     BACKGROUND OF THE INVENTION 
     Formation of prostheses from polytetrafluoroethylene (PTFE), particularly expanded polytetrafluoroethylene (ePTFE) is well known in the prior art. ePTFE includes a node and fibril structure, having longitudinally extending fibrils interconnected by transverse nodes. The nodes are not particularly strong in shear, and, thus, ePTFE structures are susceptible to failure in a direction parallel to the fibril orientation. ePTFE structures (tubes, sheets) are typically paste extruded, and, the fibrils are oriented in the extrusion direction. 
     Vascular grafts formed of ePTFE are well known in the art. Where sutures have been used to fix such grafts, suture hole elongation and propagation of tear lines from suture holes have been noted. 
     To overcome the deficiencies of the prior art, techniques have been developed which re-orient the node and fibril structure of an ePTFE element to be transverse to the extrusion direction. By orienting the fibrils at an angle relative to the extrusion direction, the tear strength of a respective product may be greatly improved. In one technique set forth in U.S. Pat. Nos. 5,505,887 and 5,874,032, both to Zdrahala et al., an extrusion machine is described having a counter-rotating die and mandrel arrangement. Accordingly, upon being extruded, a single-layer unitary PTFE tube is formed having an outer surface twisted in one helical direction, and an inner surface twisted in an opposite helical direction. Although tubes formed in accordance with the method of U.S. Pat. Nos. 5,505,887 and 5,874,032 are expandable to form an ePTFE structure, the fibrils of the structure are oriented generally parallel to the expansion direction after expanding as shown in the micrograph of FIG. 5 in U.S. Pat. No. 5,874,032. Also, the tube tends to thin out unevenly under expansion, and, suffers from “necking”. 
     SUMMARY OF THE INVENTION 
     To overcome the deficiencies of the prior art, a method is provided wherein ePTFE tubes are counter-rotated, coaxially disposed, and fixed one to another to form a composite multi-layer prosthesis. By rotating the tubes, the tubes each becomes helically twisted with its node and fibril configuration being angularly offset throughout from the longitudinal axis of the tube (and, thus, angularly offset from the extrusion direction of the tube). With counter-rotation, the nodes and fibrils of the two tubes are also angularly offset from each other, resulting in a relatively strong composite structure. The composite multi-layer structure is akin to plywood, where alternating layers have differently oriented grain directions. 
     These and other features will be better understood through a study of the following detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an elevational view of an ePTFE tube; 
         FIG. 2A  is an elevational view of a helically wound tube twisted in a first rotational direction; 
         FIG. 2B  is a schematic of the node and fibril orientation of the first tube in a helically wound state; 
         FIG. 3A  is an elevational view of a helically wound tube twisted in a second rotational direction; 
         FIG. 3B  is a schematic of the node and fibril orientation of the second tube in a helically wound state; 
         FIG. 4A  is an elevational view of a prosthesis formed in accordance with the subject invention; 
         FIG. 4B  is a schematic of the node and fibril orientations of the composite prosthesis; and, 
         FIG. 5  is an exploded view of a prosthesis having a radially-expandable support member. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention herein provides a multi-layer prosthesis which may be used as a graft to replace a portion of a bodily passageway (e.g., vascular graft), or within a bodily passageway to maintain patency thereof, such as an endovascular stent-graft. In addition, the prosthesis can be used in other bodily applications, such as the esophagus, trachea, colon, biliary tract, urinary tract, prostate, and the brain. 
     The prosthesis is composed of multiple layers, including coaxially disposed ePTFE tubes. To illustrate the invention, reference will be made to the use of two ePTFE tubes, although any number may be utilized consistent with the principles disclosed herein. With reference to  FIG. 1 , an ePTFE tube  10  is shown which extends along a longitudinal axis  12 . The ePTFE tube  10  is preferably formed by extrusion, thus having its fibrils generally parallel to the extrusion direction of the tube, which coincides with the longitudinal axis  12 . The ePTFE tube  10  includes a wall  14  (which is seamless if extruded), that extends about a lumen  16 . The wall  14  includes an inner luminal surface  18  facing the lumen  16 , and an outer, abluminal surface  20 . The ePTFE tube may be formed of any length and of various dimensions, although it is preferred that the dimensions be generally constant throughout the length thereof. In describing first and second tubes of the invention, like reference numerals will be used to describe like elements, but with the extensions “A” and “B” for differentiation. Elements associated with a first tube will have the extension “A”, while elements associated with a second tube will have the extension “B”. 
     Referring to  FIG. 2A , a first ePTFE tube  10 A is shown disposed along a longitudinal axis  12 A. The first tube  10 A is twisted about its longitudinal axis  12 A in a first rotational direction, such as clockwise, as shown in  FIG. 2A . The tube  10 A may be twisted over any given range of degrees, although it is preferred that the tube be twisted at least 10 degrees. Accordingly, as represented by the hypothetical reference axis  22 A, the first tube  10 A is helically wound in the first rotational direction. As a result and as shown in  FIG. 2B , fibrils  24 A are generally parallel to the reference axis  22 A, with the fibrils  24 A being angularly offset an angle α from the longitudinal axis  12 A, and, thus, being also angularly offset the angle α from the original extrusion direction of the first tube  10 A. Nodes  26 A are generally perpendicular to the fibrils  24 A. With the fibrils  24 A, and the nodes  26 A, being obliquely disposed relative to the longitudinal axis  12 A, failure along the longitudinal axis  12 A may be reduced. 
     Referring to  FIGS. 3A and 3B , a second ePTFE tube  10 B is shown being twisted in a second rotational direction different than the first rotational direction of the first tube  10 A. As shown in  FIG. 3A , the second ePTFE tube is twisted in a counterclockwise direction. The particular rotational direction of twisting may be switched for the first and second tubes  10 A and  10 B. As with the first tube  10 A, the amount of twisting of the second tube  10 B may be varied, although it is preferred that at least a 10 degree displacement be provided. The helically wound distortion of the second tube  10 B is represented by the hypothetical reference axis  22 B. As shown in  FIG. 3B , fibrils  24 B are generally parallel to the reference axis  22 B and are angularly offset an angle β from the longitudinal axis  12 B (and, thus, the extrusion direction). Nodes  26 B are generally perpendicular to the fibrils  26 A. The oblique disposition of the fibrils  24 B and the nodes  26 B resists failure along the longitudinal axis  12 B. 
       FIG. 4A  shows a prosthesis  100  including the first tube  10 A, in its twisted helical state being coaxially disposed within, and fixed to, the second tube  10 B, in its twisted helical state. It is preferred that the tubes  10 A and  10 B be generally coextensive, although the ends of the tubes need not be coterminous. Because of the different rotational orientations of the node and fibril structures of the tubes  10 A and  10 B, the node and fibril structures are angularly offset from each other. In particular, as shown schematically in  FIG. 4B , because of the coaxial arrangement of the tubes  10 A,  10 B, the longitudinal axes  12 A and  12 B are generally colinear. Also, the fibrils  24 A of the first tube  10 A are angularly offset from the fibrils  24 B of the second tube  10 B by an angle γ. The angular offset of the fibrils  24 A and  24 B provides the prosthesis  100  with resistance against failure not provided by either tube  10 A,  10 B alone. In a preferred embodiment, with the angles α and β being each at least 10 degrees, the angle γ will be at least 20 degrees. Preferably, the node and fibrils of each of the tubes  10 A,  10 B are generally-equally angularly offset throughout the respective tube  10 A,  10 B. 
     Because the first tube  10 A is disposed within the second tube  10 B, the second tube  10 B is formed dimensionally slightly larger to accommodate the first tube  10 A within its lumen  16 B. 
     As an alternative, only one of the tubes  10 A,  10 B may be twisted. The node and fibrils of the two tubes  10 A,  10 B would, nevertheless, be angularly offset. 
     In a preferred manner of preparing the prosthesis  100 , the first tube  10 A is provided and mounted onto a mandrel where it is twisted into its desired helical configuration. The twisted configuration of the first tube  10 A is maintained. The second tube  10 B is provided and twisted as desired, and in its twisted state telescoped over the first tube  10 A. The first and second tubes  10 A and  10 B are fixed together using any technique known to those skilled in the art, preferably sintering. Adhesive may also be used to bond the tubes, such as a thermoplastic fluoropolymer adhesive (e.g., FEP). Once fixed, the prosthesis  100  is prepared. 
     Although reference has been made herein to extruded ePTFE tubes, tubes formed by other techniques may also be used, such as with rolling a sheet, or wrapping a tape. Generally, with these non-extrusion techniques, the fibrils of the ePTFE will not initially be oriented parallel to the longitudinal axis of the tube, but rather transverse thereto. These non-extruded tubes may replace one or more of the tubes  10 A,  10 B in a non-twisted state or in a twisted state. 
     As shown in  FIG. 5 , the prosthesis  100  may also include a radially expandable support member  28 , which may be disposed interiorly of the first tube  10 A, exteriorly of the second tube  10 B, or interposed between the two tubes  10 A,  10 B. Additionally, multiple support members located at the aforementioned locations may be provided. The radially expandable support member  28  may be fixed to the tubes  10 A,  10 B using any technique known to those skilled in the art, such as bonding. Additionally, with the radially expandable support member  28  being interposed between the tubes  10 A,  10 B, the tubes  10 A,  10 B may be fixed together through any interstices formed in the radially expandable support member  28 . 
     The radially expandable support member  28  may be of any construction known in the prior art which can maintain patency of the prosthesis  100 . For example, as shown in  FIG. 5 , the radially-expandable support member  28  may be a stent. The particular stent  28  shown in  FIG. 5  is fully described in commonly assigned U.S. Pat. No. 5,693,085 to Buirge et al., and the disclosure of U.S. Pat. No. 5,693,085 is incorporated by reference herein. The stent may be an intraluminally implantable stent formed of a metal such as stainless steel or tantalum, a temperature-sensitive material such as Nitinol, or alternatively formed of a superelastic alloy or suitable polymer. Although a particular stent construction is shown with reference to the present invention, various stent types and stent constructions may be employed for the use anticipated herein. Among the various useful radially-expandable support members  28  include, without limitation, self-expanding stents and balloon expandable stents. The stents may be capable of radially contracting as well. Self-expanding stents include those that have a spring-like action which causes the stent to radially expand or stents which expand due to the memory properties of the stent material for a particular configuration at a certain temperature. Other materials are of course contemplated, such as stainless steel, platinum, gold, titanium, tantalum, niobium, and other biocompatible materials, as well as polymeric stents. The configuration of the radially-expandable support member  28  may also be chosen from a host of geometries. For example, wire stents can be fastened in a continuous helical pattern, with or without wave-like forms or zig-zags in the wire, to form a radially deformable stent. Individual rings or circular members can be linked together such as by struts, sutures, or interlacing or locking of the rings to form a tubular stent. 
     Furthermore, the prosthesis  100  may be used with additional layers which may be formed of polymeric material and/or fabric. Furthermore, any layer or portion of the prosthesis  100 , including the tubes  10 A,  10 B, may be impregnated with one or more therapeutic and pharmacological substances prior to implantation of the prosthesis  100  for controlled release over an extended duration. It is anticipated that the prosthesis  100  can be partially or wholly coated with hydrophilic or drug delivery-type coatings which facilitate long-term healing of diseased vessels. Such a coating is preferably bioabsorbable, and is preferably a therapeutic agent or drug, including, but not limited to, anti-thrombogenic agents (such as heparin, heparin derivatives, urokinase, and PPack (dextrophenylalanine proline arginine chloromethylketone); anti-proliferative agents (such as enoxaprin, angiopeptin, or monoclonal antibodies capable of blocking smooth muscle cell proliferation, hirudin, and acetylsalicylic acid); anti-inflammatory agents (such as dexamethasone, prednisolone, corticosterone, budesonide, estrogen, sulfasalazine, and mesalamine); antineoplastic/antiproliferative/anti-miotic agents (such as paclitaxel, 5-fluorouracil, cisplatin, vinblastine, vincristine, epothilones, endostatin, angiostatin and thymidine kinase inhibitors); anesthetic agents (such as lidocaine, bupivacaine, and ropivacaine); anti-coagulants (such as D-Phe-Pro-Arg chloromethyl keton, an RGD peptide-containing compound, heparin, antithrombin compounds, platelet receptor antagonists, anti-thrombin antibodies, anti-platelet receptor antibodies, aspirin, prostaglandin inhibitors, platelet inhibitors and tick antiplatelet peptides); vascular cell growth promotors (such as growth factor inhibitors, growth factor receptor antagonists, transcriptional activators, and translational promotors); vascular cell growth inhibitors (such as growth factor inhibitors, growth factor receptor antagonists, transcriptional repressors, translational repressors, replication inhibitors, inhibitory antibodies, antibodies directed against growth factors, bifunctional molecules consisting of a growth factor and a cytotoxin, bifunctional molecules consisting of an antibody and a cytotoxin); cholesterol-lowering agents; vasodilating agents; and agents which interfere with endogenous vascoactive mechanisms. 
     Various changes and modifications can be made in the present invention. It is intended that all such changes and modifications come within the scope of the invention as set forth in the following claims.