FIELD OF THE INVENTION
The present invention relates generally to endoluminal grafts for stenotic or diseased lumens and methods of making such grafts. More particularly, the invention includes a stent-graft assembly comprising a thin-walled graft component and methods of making the assembly.
BACKGROUND OF THE INVENTION
A wide range of medical treatments have been previously developed using “endoluminal prostheses,” which terms are herein intended to mean medical devices which are adapted for temporary or permanent implantation within a body lumen, including both naturally occurring or artificially made lumens. Examples of lumens in which endoluminal prostheses may be implanted include, without limitation: arteries, such as those located within the coronary, mesentery, peripheral, or cerebral vasculature; veins; gastrointestinal tract; biliary tract; urethra; trachea; hepatic shunts; and fallopian tubes. Various types of endoluminal prostheses have also been developed, each providing a uniquely beneficial structure to modify the mechanics of the targeted luminal wall.
For example, various grafts, stents, and combination stent-graft prostheses have been previously disclosed for implantation within body lumens. More specifically regarding stents, various designs of these prostheses have been previously disclosed for providing artificial radial support to the wall tissue which forms the various lumens within the body, and often more specifically within the blood vessels of the body. An example of such a stent displaying optimal radial strength includes, but is not limited to, the stent disclosed in U.S. Pat. No. 5,292,331 to Boneau, the disclosure of which is herein incorporated by reference. Stents of other designs are known in the art, and may also be suitable for use in the stent-graft assembly. Other example of stents include but are not limited to those disclosed in U.S. Pat. No. 4,733,665 issued to Palmaz, U.S. Pat. No. 5,195,984 issued to Schatz, or U.S. Pat. No. 5,514,154 issued to Lau. Stents are used alone or in conjunction with grafts.
The use of an angioplasty balloon catheter is common in the art as a minimally invasive treatment to enlarge a stenotic or diseased blood vessel. This treatment is known as percutaneous transluminal angioplasty, or PTA. To provide radial support to the treated vessel in order to prolong the positive effects of PTA, a stent may be implanted in conjunction with the procedure. Under this procedure, the stent may be collapsed to an insertion diameter and inserted into a body lumen at a site remote from the diseased vessel. The stent may then be delivered to the desired site of treatment within the affected lumen and deployed to its desired diameter for treatment. Although the stents listed above are balloon expandable, stents which rely on other modes of deployment such as self-expansion, may be used to make a device according to the present invention. Because the procedure requires insertion of the stent at a site remote from the site of treatment, the device must be guided through the potentially tortuous conduit of the body lumen to the treatment site. Therefore, the stent must be capable of being reduced to a small insertion diameter and must be flexible.
During an angioplasty procedure, atheromatous plaques undergo fissuring, thereby creating a thrombogenic environment in the lumen. Excessive scarring may also occur following the procedure, potentially resulting in reocclusion of the treated lumen. Attempts to address these problems include providing a suitable surface within the lumen for more controlled healing to occur in addition to the support provided by a stent. These attempts include providing a lining or covering in conjunction with an implanted stent. A stent with such a lining or covering is known in the art as a stent-graft.
The graft component, or membrane, of a stent-graft may prevent excessive tissue prolapse or protrusion of tissue growth through the interstices of the stent while allowing limited tissue in-growth to occur to enhance the implantation. The surface of the graft material at the same time may minimize thrombosis, prevent scarring from occluding the lumen, prevent embolic events and minimize the contact between the fissured plaque and the hematological elements in the bloodstream.
A combination stent-graft may serve other objectives, such as delivering therapeutic agents via the assembly, excluding aneurysms or other malformations, occluding a side branch of a lumen without sacrificing perforator branches, conferring radiopacity on the device, and others. Various designs to achieve these objectives include stents partially or completely coated or covered with materials, some of which are impregnated with therapeutic agents, radiopaque elements, or other features designed to achieve the particular objectives of the device.
A graft component may be combined with a stent in order to achieve some or all of the foregoing objectives. However, adding a graft layer to the stent increases the challenges of delivering a stent via a catheter by increasing the crossing profile, or diameter, of the device, and by decreasing the flexibility of the device. Because the angioplasty process requires the insertion of the device into a body lumen at a site remote from the site of treatment and the guiding of the device the body lumen to the treatment site, it is required that the device be both capable of being collapsed to a relatively small diameter and be quite flexible. Moreover, flexibility and a desirable insertion diameter must be achieved without sacrificing the treatment objectives of the assembly, which include, at a minimum, radial strength. Therefore, an objective of a combination stent and graft is achieving the advantages of both a stent and a graft without significantly increasing the crossing profile of the device or significantly decreasing the flexibility of the device.
Various methods of manufacturing graft devices alone have been disclosed in the art. One such method for manufacturing a graft is disclosed in U.S. Pat. No. 5,641,373, issued to Shannon et al. The disclosed method comprises reinforcing an extruded flouropolymer tube with a second flouropolymer tube. The second tube is prepared by winding fluoropolymer tape around the exterior of a mandrel and heating it to form a tube. The graft may then be mounted on an anchoring mechanism such as a stent or other fixation device.
Another example of a graft is disclosed in U.S. Pat. No. 4,731,073, issued to Robinson. The graft disclosed therein comprises multiple layers of segmented polyether-polyurethane which form multiple zones having varying porosities.
U.S. Pat. No. 5,628,786, issued to Banas, discloses a polytetrafluoroethylene (PTFE) graft which has a reinforcing structure integrally bound to the graft. The reinforcing structure may be in the form of a rib which is sintered or otherwise integrally bound to the graft.
U.S. Pat. No. 5,207,960, issued to Moret de Rocheprise, discloses a process for the manufacture of a thin-walled tube of fluorinated resin tape. The method includes winding the tape around a mandrel and sintering the tape. While still on the mandrel, the tube is rolled to elongate the tube, to reduce the thickness of the tube, and to facilitate removal of the tube from the mandrel. The patent discloses that the tubes obtained can be used particularly as sheaths for the lining of metal tubes.
There are also numerous examples of combination stent-grafts disclosed in the art. U.S. Pat. No. 5,653,747 issued to Dereume discloses a stent to which a graft is attached. The graft component is produced by extruding polymer in solution into fibers from a spinnerette onto a rotating mandrel. A stent may be placed over the fibers while on the mandrel and then an additional layer of fibers spun onto the stent. The layer or layers of fibers may be bonded to the stent and/or one another by heat or by adhesives.
PCT Application WO 95/05132 discloses a stent around which a thin film of PTFE has been wrapped circumferentially one time and overlapped upon itself to form a seam. The stent may be alternatively or additionally placed to cover the interior of the stent. Fluorinated ethylene propylene is used as an adhesive to affix the graft to the stent.
A specific example of a coated stent is disclosed by Pinchuk in European Patent Application EP 0 797 963 A2. The objectives of Pinchuk's invention include both increasing the hoop strength and decreasing the thrombogenic potential of a criss-crossed wire stent or a zig-zag stent. Pinchuk's application also discloses covering the coated stent in the manner disclosed in U.S. patent application No. 5,653,747 issued to Dereume, discussed above.
An example of a stent and tubular graft is disclosed in U.S. Pat. No. 5,522,882 issued to Gaterud, et al. Gaterud discloses an expandable stent mounted on a balloon and a graft mounted over the stent.
U.S. Pat. No. 5,123,917 issued to Lee discusses a flexible and expandable inner tube upon which separate ring scaffold members are mounted, and a flexible and expandable outer tube enclosing the inner tube and scaffold members. The rings may be secured to the inner liner with an adhesive layer. Alternatively, the liners may be adhered to each other with the rings trapped between the layers. Lee discloses that the luminal surface of the device may be coated with various pharmacological agents.
Similarly, U.S. Pat. Nos. 5,282,823 and 5,443,496, both issued to Schwartz, et al. disclose a stent with a polymeric film extending between the stent elements, and strain relief means in the form of cuts in the film to allow the stent to fully expand and conform to the interior of the lumen. The thin polymeric film is applied to the stent while in solution and dried. Once dried, cuts are made in the film to provide strain relief means.
Another assembly includes a stent embedded in a plastic sleeve or stitched or glued to a nylon sleeve, as in U.S. Pat. No. 5,507,771, issued to Gianturco. Other prior art devices requiring stitching of the graft to the stent are disclosed in European Patent Application EP 0 686 379 A2, which teaches a perforate tubular frame having a fabric liner stitched to the frame, and World Intellectual Property Organization Application Number WO 96/21404 which indicates that the graft be stitched to the stent, and possibly to loops or eyelets which are part of the stent structure.
U.S. Pat. No. 5,637,113 issued to Tartaglia teaches a stent with a sheet of polymeric film wrapped around the exterior. Tartaglia teaches that the film is attached to the stent at one end by an adhesive, by a hook and notch arrangement, or by dry heat sealing. The polymer can also be attached to the stent by wrapping the film circumferentially around the stent and attaching the polymer film to itself to form a sleeve around the stent by heating and melting the film to itself, adhesive bonding, solvent bonding, or by mechanical fastening, such as by a clip. The film may be loaded or coated with a therapeutic agent.
U.S. Pat. No. 5,628,788, issued to Pinchuk, discloses a process of melt-attaching a graft to a stent by disposing a layer of material between the stent and graft which has a lower melting point than the graft, and heating the assembly to the melting point of the low-melting point material. Pinchuk also teaches adhering a textile graft to a stent by coating a stent with vulcanizing silicone rubber adhesive and curing the adhesive. Pinchuk discloses a similar stent-graft assembly in European Patent Application EP 0 689 805 A2 and teaches that the graft member can be bonded to the stent member thermally or by the use of adhesive agents.
Similarly, World Intellectual Property Organization Application No. WO 95/05132 discloses a stent with and inner and/or an outer liner wrapped around the stent to form a seam, with the liner(s) affixed using an adhesive or melt-attached using a layer of a material with a lower melting point.
U.S. Pat. No. 5,645,559, issued to Hachtman, et al., discloses a multiple layer stent-graft assembly comprising a first layer defining a hollow tubular construction, a second layer having a self-expanding braided mesh construction and a layer of polymeric material disposed between the first and second layers. The polymeric material may be adhered by two-sided adhesive tape. The self-expanding braided mesh, the tubular material, or both may be larger in diameter in the distal regions than in the medial region.
U.S. Pat. No. 5,534,287, issued to Lukic, discloses methods which result in a covered stent, the covering adhered via a lifting medium.
U.S. Pat. No. 5,674,241, issued to Bley, et al. teaches that a hydrophilic polymer layer may be laminated, embedded, coated, extruded, incorporated, or molded around an expandable mesh stent while the stent is in its collapsed condition, and the stent and graft permitted to expand upon hydration.
European Patent Application No. EP 0 775 472 A2 discloses a PTFE-covered stent. The stent can be covered by diagonally winding an expanded PTFE tape under tension around an at least partially expanded stent.
Challenges arising in the art which none of the prior art adequately addresses include achieving a stent-graft assembly of sufficiently small crossing profile and which is sufficiently flexible. Other challenges include minimizing, if not eliminating, migration of the stent, graft or stent-graft; minimizing, if not eliminating, delamination of the stent and graft material; and minimizing thrombogenic potential, vessel reocclusion and tissue prolapse following deployment. Shortcomings associated with the prior art include: assemblies with undesirably large crossing profiles; assemblies with insufficient flexibility; inadequate adhering of coatings and coverings to stents; inadequate adhering of coatings to coverings; failure to shield the injured vascular surface; failure to prevent tissue ingrowth from occluding the lumen; failure to minimize the embolization of particles loosely adherent to the vessel wall (especially during device placement and deployment); and increased thrombogenic potential arising from delamination of the stent and graft material.
SUMMARY OF THE INVENTION
The present invention and its varied embodiments address several problems associated with the prior art. It is a first objective of this invention to provide an improved stent-graft assembly for the repair and support of a body lumen. It is a second objective of this invention to provide an improved stent-graft assembly with ample radial strength and minimal thrombogenic potential without appreciably increasing the profile of the device over that of the stent alone. Further, the decreased profile following deployment may reduce the thrombogenic potential of the device. It is a further objective of this invention to provide a thin-walled stent-graft with both ample radial strength and ample flexibility.
It is a further objective of this invention to solve the problem in the prior art of inadequate adhesion between the stent and graft material.
An additional objective of this invention is to balance the need for some tissue in-growth against the need to minimize thrombogenic potential and excessive cell growth through the interstices of the stent. This objective is achieved by providing a non-thrombogenic, thin-walled stent-graft assembly which is a smoother device, especially in regions where the prior art stent-graft has a tendency to fray, delaminate, or exhibit a scissoring effect. This objective is also achieved by providing a thin-walled stent-graft assembly which shields the injured vascular surface, controls excessive tissue in-growth through the stent, and minimizes the embolization of particles loosely adherent to the vessel wall especially during placement and deployment of the device. The device can also be used to control the luminal protrusion of dissection planes created during PTA or spontaneous fissuring.
A stent-graft assembly according to the present invention first comprises a generally cylindrical stent which comprises at least one support member. Some or all of the support member or members comprise a coating which substantially encapsulates the coated support member or members. Further, the stent-graft includes an ultra-thin membrane or covering which is attached to the coating.
In one embodiment, the proximal and distal regions of the stent-graft have an additional coating over the first coating and the membrane. In an alternative embodiment, the proximal and distal regions of the stent may be left completely uncoated and uncovered if needed for the particular medical application of the device.
The thin membrane may be either on the inner or the outer surface of the stent or both. The material used for the membrane comprises an ultra-thin polymer. In use, the membrane may have varying degrees of distensibility depending upon the desired application of the device. The membrane is bound to the coating either as a result of defining a homogeneous material with the coating or as a result of extensions of the coating into the pores of the membrane and the resulting interlocking engagement between the coating extensions and the pores of the membrane to form a composite. An alternative embodiment of the invention comprises a stent with either a continuous membrane or more than one membrane on the interior and on the exterior of the stent, the membranes bound to one another through the interstices of the stent. The membrane(s) may be sintered or otherwise bound to itself or to one another. The invention also contemplates the use of a coating at the proximal and distal regions of the stent, which substantially encapsulates the assembly at the proximal and distal regions.
The method according to a first embodiment of the present invention comprises helically wrapping ultra-thin polymeric tape around a mandrel, adjusting the angle of orientation of the tape to the mandrel depending upon the desired distensibility of the membrane and allowing adjacent edges of the helical wrapping to overlap somewhat; sintering the tape to itself over the mandrel to produce a thin tube; removing the thin tube from the mandrel; coating a stent with a polymer; covering and/or lining the coated stent with the thin tube; introducing a solvent to attach the coating to the membrane; curing the assembly to drive off remaining solvent.
The method according to an alternative embodiment comprises winding the polymeric tape around the mandrel to form two layers, in each layer reversing the angle of orientation of the tape to the longitudinal axis of the mandrel to form a bias ply, and then following the remaining steps of the method described above. In any given embodiment, the angle of orientation of the tape to the longitudinal axis of the mandrel, dependent on the width of tape and diameter of the mandrel, can be varied depending upon the desired distensibility of the graft component of the device. The sintering parameters can also be varied to affect the distensibility of the device. Further, pressure may be utilized in conjunction with sintering to improve the adherence of the tape. And finally, the amount of overlap between adjacent edges of tape can be varied depending upon the particular indication for the device.
In yet a further embodiment of a method according to the invention, a thin-walled stent-graft assembly, having inner and outer membranes, can be fabricated utilizing pressure and heat.
In any of the methods according to the particular embodiment, the extent that the coating and membrane cover the stent can be varied. And the manner in which the membrane is wrapped about the mandrel, specifically, helically or otherwise. Also according to the particular embodiment, the proximal and distal regions of the stent-graft assembly may be coated a second or multiple times to seal the resulting layers of stent, coating and membrane, and to substantially increase bond strength due to the increased surface area of the encapsulation of the stent strut.