Stent having adjacent elements connected by flexible webs

Abstract

A stent incorporating flexible, preferably polymeric, connecting elements into the stent wherein these elements connect adjacent, spaced-apart stent elements. Preferably the spaced-apart adjacent stent elements are the result of forming the stent from a helically wound serpentine wire having space provided between adjacent windings. Other stent forms such as multiple, individual spaced-apart ring-shaped or interconnected stent elements may also be used. The connecting elements are typically web-shaped and result from creating slits or apertures in a covering of graft material applied to the stent and then, for example, applying heat to cause the slits or apertures to enlarge. The remaining graft material forms the interconnecting webs between the adjacent stent elements.

Description

FIELD OF THE INVENTION

The present invention relates to the field of implantable stents having flexibly connected adjacent stent elements.

BACKGROUND OF THE INVENTION

The use of implantable stents in the vasculature and other body conduits has become commonplace since first proposed by Dotter in the 1960's. These devices are required to have a small, compacted diameter for insertion into an intended body conduit and transport, typically via a catheter, to a desired site for deployment, at which site they are expanded to a larger diameter as necessary to fit interferably with the luminal surface of the body conduit. Balloon expandable stents are expanded by plastically deforming the device with an inflatable balloon on which the expandable stent was previously mounted in the compacted state, the balloon being attached to the distal end of the catheter and inflated via the catheter. Self-expanding stents are forcibly compacted to a small diameter and restrained at that diameter by a constraining sleeve or other means. Following delivery to a desired site for deployment, they are released from the restraint and spring open to contact the luminal surface of the body conduit. These devices are typically made from nitinol metal alloys and typically rely on the superelastic and biocompatible character of this metal. Nitinol stents that rely on the shape memory attributes of that material are also known.

The evolution of implantable stents has also included the use of a tubular covering fitted to the stent, either to the outer surface, the luminal surface or to both surfaces of the stent. These covered stents have generally come to be referred to as stent-grafts. The coverings are generally of a polymeric biocompatible material such as polyethylene terephthalate (PET) or polytetrafluoroethylene (PTFE). See, for example, U.S. Pat. No. 4,776,337 to Palmaz.

The Palmaz '337 patent also describes that the covering may be optionally provided with perforations if desired for particular applications. Because of the open area provided by the perforations, such devices having perforated coverings may be considered to be a sort of hybrid stent and stent-graft, as are devices that include stent frames having metallic stent elements and polymeric elements connecting, covering or other otherwise being attached to the stent elements. The presence of the polymeric elements reduces the otherwise open space between the adjacent metallic stent elements, either very slightly or very substantially depending on the intended application and mechanical design. Perforated stent-grafts are also described elsewhere; see, for example WO00/42949.

Stents having stent elements provided with polymeric coatings or coverings are also known; see, for example, U.S. Pat. No. 5,735,892 to Myers et al. and U.S. Pat. No. 5,968,091 to Pinchuk et al.

Generally, a fully covered stent-graft can be considered to have a surface area (hereinafter A_max) equal to the outer circumference of the expanded stent multiplied by the length of the stent. For a conventional, open frame stent (as opposed to a stent-graft), the surface area represented by all of the stent elements is only a small portion of the maximum surface area A_max. The actual surface area covered by the stent, meaning the area covered by all components of the stent (including connecting elements) in their deployed state, is A_stent. The porosity index, or P.I., describes the open area (the portion of the maximum surface area not covered by all components of the stent assembly) as a percentage of maximum surface area, wherein:P.I.=(1−(A_stent/A_max))×100%.

A preferred method of measuring the actual surface area covered by the stent (A_stent), involves the use of a machine provided Visicon Inspection Technologies, LLC (Napa, Calif.). The Visicon Finescan™ Stent Inspection System (Visicon Finescan machine model 85) uses a 6000 pixel line scan camera to generate a flat, unrolled view of a stent. In operation, the stent is mounted on a sapphire mandrel with a fine diffuse surface. This mandrel is held under the linear array camera and rotated by the system electronics and is used to trigger the linear array camera to collect a line of image data in a precise line-by-line manner. After a complete revolution an entire image of the stent is acquired. When the entire stent has been imaged, the software differentiates between the stent with cover and the background. The total number of picture elements (pixels) is compared to the total number of pixels associated with the stent and cover to determine A_stent. Basic settings on the machine used for this type of determination are (for example): light, 100%; exposure, 0.3 ms/line; gain, 5; threshold, 50; noise filter, 20; smoothing, 4.

The open area may be a continuous single space, such as the space between windings of a single helically wound stent element. Likewise the open area may be represented by the space between multiple individual annular or ring-shaped stent elements. The open area may also be represented by the total area of multiple apertures provided by either a single stent element (e.g., as shown by FIGS. 1B and 2B of U.S. Pat. No. 4,776,337 to Palmaz) or by multiple stent elements providing multiple apertures. If multiple apertures are provided they may be of equal or unequal sizes. The use of a perforated graft covering or of polymeric elements in addition to metallic stent elements may also reduce the open area.

Stents having a porosity index of greater than 50% are considered to be substantially open stents.

In addition to the porosity index, the size of any aperture providing the open area must be considered if it is intended to cover only a portion of a stent area for a specific stent application. For multiple apertures, often the consideration must be for the largest size of any individual aperture, particularly if the apertures are to provide for a “filtering” effect whereby they control or limit the passage of biologic materials from the luminal wall into the flow space of the body conduit.

Various stent devices combining metallic stent elements with polymeric connecting elements are known; see, for example U.S. Pat. No. 5,507,767 to Maeda et al. Another is a stent provided with a flexible knitted sleeve having small open apertures in the fashion of chain link fencing, from InspireMD Ltd. (4 Derech Hashalom St., Tel Aviv 67892 Israel).

SUMMARY OF THE INVENTION

An open stent (a stent having open space through its thickness at locations between the ends of the stent) and method of making are described. The stent incorporates flexible, preferably polymeric connecting elements (i.e., polymeric webs) into the stent wherein these connecting elements connect adjacent, spaced-apart stent elements. The flexible, preferably polymeric connecting elements provide a means for keeping the stent elements equally spaced and allow the construction of a stent having good flexibility and a useful resistance to forces that may be applied to the device in vivo such as torsional forces, bending forces, axial tension or compression, or radial compression.

Preferably the spaced-apart adjacent stent elements are in the form of a helically wound serpentine wire having space provided between adjacent windings. Other stent forms such as multiple, individual spaced-apart ring-shaped stent elements may also be used. Ring shaped stent elements may be in the form of zig-zag elements creating a circumferential ring, or interconnected elements that provide diamond shaped openings in a circumferential sequence when the device is diametrically expanded. Alternatively, embodiments presented that utilize the helically wound serpentine forms are preferred for many applications. The stent is preferably self-expanding (made from materials such as nitinol) but may also be made from materials suitable for balloon expandable stents (e.g., stainless steel, magnesium based alloys, magnesium, cobalt chromium alloy, titanium or titanium based alloys).

Helically wound stent frames are inherently unstable in absence of a secondary linkage connecting adjacent rows. Utilization of the described polymer web linkage to interconnect adjacent rows stabilizes the helical structure and limits axial elongation, torsion and bending while allowing a high degree of flexibility.

The adjacent, spaced-apart stent elements are preferably substantially circumferentially oriented, meaning that they have a general direction of orientation perpendicular to the longitudinal axis of the stent, when the stent is in a straight, unbent state.

A method of making involves the application of a biocompatible polymeric covering to the chosen stent form to create, temporarily, a stent-graft. The covering is preferably of a strong and thin material and may be in a tubular form, although sheet forms (e.g., relatively wide films cut into narrow tapes) are preferred for manufacturing as will be described. The covering is preferably applied to the outer surface of the stent, but may be applied only to the luminal surface, or alternatively may be applied to both the luminal and abluminal (outer) surfaces of the stent. Covering both the luminal and abluminal surfaces allows for the possibility of covering substantially all of the metallic surfaces of the stent with the desired polymer. The polymeric film covering is preferably a thermoplastic film, and preferably a film with strength properties that result in relatively uniform directional shrinking properties when the film is subjected to heat above its melt point. The film-covered stent graft is provided with punctures (slits or other apertures) through the thickness of the film, preferably at locations between adjacent stent elements as will be further described. The punctured stent-graft is then exposed to heat above the melt temperature of the film which causes the film to shrink back from the edges of the previously created puncture, resulting in openings through the wall of the stent. These openings are of size, shape, quantity and orientation that are a result of the size, shape, quantity and orientation of the previously created punctures, the amount of heat subsequently applied and the thickness and type of polymeric film used. It is apparent that these are manufacturing variables that may be controlled as desired. The resulting open area of the stent (i.e., porosity index) may cover a wide range (i.e., 10%, 20%, 30%, 40%, 50%, 60%, 70%, 75%, 80%, 85%, 90%, 95%, or higher, or between any of these percentages). The remaining polymeric film following the heating step is in the form of polymeric webs extending between the adjacent stent elements.

An alternate method of making also involves the application of a biocompatible polymeric covering to the chosen stent form to create, temporarily, a stent-graft. A preferable stent form in this instance would be ring shaped stent elements made from a suitable balloon expandable material. The covering is similar to that described previously and may be applied to the chosen stent form similarly to the methods described in the previous section. The polymeric film covering is preferably a thermoplastic film, and preferably a film with unidirectional strength properties. The film-covered stent graft is provided with punctures (slits or other apertures) through the thickness of the film, preferably at locations between adjacent stent elements as will be further described. The punctured stent graft is then exposed to heat sufficient to bond the film to the stent form. When the resulting stent is diametrically expanded, these openings are of size, shape, quantity and orientation that are a result of the size, shape, quantity, and orientation of the previously created punctures. It is apparent that these are manufacturing variables that may be controlled as desired. The resulting open area of the stent (i.e., porosity index) may cover a wide range such as previously described. The remaining polymeric film following the puncturing/slitting step is in the form of polymeric webs extending between and interconnecting the adjacent stent elements.

Further, the finished open frame stent may optionally be provided with another covering of polymeric graft material to create a stent-graft if desired. This graft covering is easily adhered or bonded to the covering or coating that is provided over the stent elements (e.g., the wire) and forms the interconnecting webs.

The polymeric covering of these finished devices (that include a multiplicity of openings and a multiplicity of polymeric interconnecting webs) is generally continuous or substantially continuous between the stent ends, being the result of having been made from a continuous sheet of film or the result using helically wrapped polymeric tape with overlapping adjacent edges that are melt-bonded together. The film covering that forms these continuous webs is well adhered to the stent elements.

Still further, these devices may be provided with coatings (preferably elutable coatings) of various therapeutic agents (e.g., heparin) by various means known in the art that are suitable to the particular agent.

Stents made as described herein have good conformability enabled by the flexible interconnecting webs between adjacent stent elements that provide flexibility and anatomic apposition. They also have good flexural durability enabled by interconnecting webs between adjacent stent elements that mitigates fracture due to cyclic longitudinal bending in curved anatomies. The expandable device is scalable to accommodate a range of vessel sizes (e.g. 3 mm-55 mm).

The potential clinical applications of the expandable device described herein include but are not limited to: congenital defects (i.e., pulmonary artery stenosis, aortic coarctation), adjunctive aortic therapy (i.e., Type I endoleaks; aortic side branch stenting), peripheral artery disease (i.e., renal and iliac artery stenosis, aneurysm, and dissection) and venous applications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B describe respectively a perspective view and a plan view of a helically wound serpentine wire form (previously known) of a preferred stent as described herein.

FIG. 2A is a side perspective view of a portion of a helically wound serpentine wire stent provided with flexible interconnecting webs between adjacent stent elements.

FIG. 2B is a flattened, plan view of the stent of FIG. 2A.

FIGS. 2C and 2D are plan views wherein each single opening shown by FIG. 2B is replaced by multiple apertures, specifically four openings in FIG. 2C and six openings in FIG. 2D.

FIG. 3 is a scanning photomicrographs of a multiaxial ePTFE film useful for making the described open frame stent.

FIG. 4 shows a side view of a partially completed stent provided with slits or punctures that are part of the process of manufacturing the device.

FIGS. 5A-5C show transverse cross sectional views of a stent element as it may appear for a finished stent made as described herein.

FIG. 6A is a side perspective view of a balloon expandable stent (or a length portion of such a stent) provided with flexible interconnecting webs between adjacent stent elements.

FIG. 6B is a side perspective view of three stent rings shown without the interconnecting polymeric covering.

FIG. 6C is a side perspective view of the stent assembly comprising the stent rings shown in 6B provided with the interconnecting polymeric covering.

FIG. 6D is the upper left section of the stent assembly described by FIG. 6C, shown as a perspective detail.

FIG. 7 is a side perspective view of a balloon expandable stent (or a length portion of such a stent) provided with flexible interconnecting webs between adjacent stent elements.

FIG. 8 is a schematic side view of stent as it would appear when mounted on a balloon for subsequent deployment and expansion.

DETAILED DESCRIPTION OF THE DRAWINGS

It has been noted that a variety of stent forms may be usefully provided with the flexible connecting elements taught herein. FIG. 1A shows a perspective view of a stent 10 that is preferred for use as described herein. The stent 10 shown comprises a helical winding of a length of serpentine wire 18. Sequential windings of the helical wound serpentine wire 18 result in spaced-apart adjacent stent elements 12. The ends 17 of wire 18 may be secured by any suitable method (e.g., welding) to the adjacent helical winding. For clarity, stent 10 is shown with a mandrel 16 extending through and beyond both ends of the stent lumen, making the side closest to the viewer visually apparent while blocking the view of the side of stent 10 furthest from the viewer. Mandrel 16 is present only for clarity of visualization and is not a part of stent 10.

The helically wound serpentine wire 18 extends continuously between opposing ends of stent 10, wherein opposing apices 22a and 22b formed of wire bends of relatively small radii are interconnected by straight or relatively straight wire segments 24. The apices typically “point” in directions that are substantially parallel to the longitudinal axis 19 of the mandrel 16 and the tubular form of the stent 10, with alternating apices 22a and 22b pointing in opposite directions, that is, pointing to opposite ends of the stent. As shown by FIG. 1A, it is preferred that apices pointing in one direction (e.g., apices 22a) are aligned along a first common line while the apices pointing in the opposite direction (e.g., apices 22b) are aligned along a second common line that is parallel to the first common line.

FIG. 1B shows a plan (or flattened) view of details of the serpentine wire form described by FIG. 1A; dimensions relate to the method of making described below. Dimension 27 is considered as the height (amplitude) of adjacent opposing apices while dimension 28 is the width of adjacent opposing apices. Dimension 29 describes one full period of the serpentine form. Wire diameter 25 and bend radius 26 of the apices 22 may be chosen as appropriate.

FIG. 2A is a side perspective view of a portion of the length of an open-frame stent 10 wherein spaced-apart, adjacent stent elements 12 (e.g., two adjacent apices 22a connected to opposing apex 22b) are interconnected by a pair of flexible polymeric webs 32. FIG. 2B shows a flattened plan view of this same construction. Openings 34 exist between adjacent aligned apices 22a; the particular single openings 18 are generally in the shape of a guitar pick. If one drew a line through the center of the length of an individual, randomly selected web (i.e., extending between the adjacent wire apices joined by that web), that line would preferably form an angle of between 15 and 75 degrees with respect to a line parallel with the centerline of the stent (or parallel with the centerline 19 of mandrel 16 shown in FIG. 1). Said otherwise, for this type of stent with elements interconnected by flexible webs 32, the webs 32 preferably are oriented at an angle to the length of the stent.

The enlarged portion of FIG. 2B shows how these flexible polymeric webs 32 are narrower at the middle of their length than at the ends where they are attached to the stent element (e.g. the nitinol wire). It also shows how the webs 32 preferably merge tangentially into the stent element where they are joined to and attached to the stent element.

FIGS. 2C and 2D are plan views wherein each single opening shown by FIG. 2B is replaced by multiple apertures, specifically four openings in FIG. 2C and six openings in FIG. 2D.

While various polymeric films may be suitable for use as the stent covering (or coating) material for this device, combinations of FEP (fluorinated ethylene propylene) films used in combination with ePTFE films are preferred. The preferred ePTFE films for use with these helically wound serpentine wire stents are films having multiaxial fibrillar orientations as shown by the scanning electron photomicrograph of FIG. 3. It is seen how the fibrils are oriented in all directions within the plane of the ePTFE film. ePTFE films of this type may be made as taught by U.S. Pat. No. 7,306,729 and US Published Patent Application 2007/0012624 to Bacino et al. Films of this same type may optionally be provided with a partial covering of a thin layer of FEP (having openings through the FEP film covering; i.e., a discontinuous covering). FEP coated ePTFE films, with either a discontinuous (porous) FEP covering (coating) or a continuous (non-porous) FEP covering (coating) may be made generally as taught by U.S. Pat. No. 5,735,892 to Myers et al.

FIG. 4 shows a partially finished stent 13 of helically wound serpentine wire provided with a first outer (abluminal) covering of FEP film and an additional covering of multiaxial ePTFE film, wherein longitudinally oriented slits 41 have been made through the film between adjacent apices of the wire that are pointed in the same direction. Heat will be applied to the device having the multiple slits 41, causing the films to shrink back toward the adjacent wire stent elements, subsequently resulting the openings in the finished stent 15 (FIG. 2A). This process is described in further detail below.

While, as noted, various types of films may be used for the stent covering, the described ePTFE films is preferred because of its multiaxial (within the plane of the film) strength orientation. It is strong, thin, and has excellent biocompatibility. When suitable heat is applied following slitting, the film will retract (shrink back) with good uniformity to create the openings through the polymeric stent covering and to create the flexible polymeric interconnecting webs between adjacent stent elements.

The flexible interconnecting webs 32 that result from this process typically are of wider width at their end points where they connect with the wire apices and are of comparatively narrower width in the middle of their lengths between the apices that they interconnect. Additionally, there may be a very thin, vestigial edge (36, FIG. 2B) of film that extends outwardly away from the wire 18 in the straight portions 24 that connect the apices in the same helical winding (i.e., apices 22a and 22b). FIG. 5A shows a transverse cross section of the wire with this edge (taken at section 5 indicated in the plan view of FIG. 2B) that shows the general appearance of the edge for a single layer of graft material applied to either the outer or inner surface of the stent. FIGS. 5B and 5C show the transverse cross section as it would appear for a covering applied to both the inner and outer surfaces of the stent element.

A preferred method of making a flexible stent is as follows. A stainless steel mandrel of diameter equal to about the inside diameter of the intended stent is obtained. The surface of the mandrel is provided with a helical wrapping of a 1″ wide tape of Kapton Polyimide Film (DuPont, 0.002 inch thickness). A stent of the desired length and diameter made of helically wound serpentine nitinol wire is provided (wire diameter as desired). This is then wound around the Kapton covered surface of the mandrel. The end of the stent wires are secured to an adjacent winding of the stent wire using an FEP thread tied with a securing knot. The apices of the serpentine wire are aligned so that apices pointing in a common direction are aligned with and parallel to the longitudinal axis of the mandrel. The stent is then helically wrapped with a covering of a single layer of FEP tape that has been cut from FEP film (0.00015 inch thickness and about 0.75 inch width), stretched tight over the outer surface of the stent with minimal overlap of adjacent edges of the FEP tape. This FEP tape is then cigarette wrapped (wrapped in a direction perpendicular to the longitudinal axis of the mandrel) with an ePTFE film of the type described previously. This wrapping may be started by aligning a transverse edge of the film with the longitudinal axis of the mandrel and attaching it to the underlying FEP film by carefully melt-bonding the ePTFE film edge to the FEP using a heat source such as a clean soldering iron or appropriate equivalent. Six layers of the ePTFE film are wrapped around the outer surface of the stent and the film edge is trimmed along the length of the stent (i.e., parallel to the longitudinal axis of the mandrel). The film edge is secured with the previously-used heat source.

Longitudinal slits 41 are created between adjacent wire apices that are pointed in the same direction as shown by FIG. 4. These slits may be created by any suitable means, including the use of a scalpel blade, water jet, laser, etc. One such suitable laser is a Coherent Inc., Model: GEM-100A, CO₂, CW (continuous wave only), Santa Clara, Calif. The last row of apices at each end of the stent may be omitted from slitting if it is desired to leave these end rows covered in their entirety (i.e., in stent-graft fashion). The entire length of the wrapped stent is then provided with an additional, temporary helical wrap of the Kapton tape; the ends of this tape may be secured to the surface of the mandrel beyond each end of the stent with a mechanical clip or other temporary fastener. This layer of Kapton is then tightly wrapped with a temporary helical wrap of ePTFE tape (made from an ePTFE film having a fibrillar microstructure with fibrils oriented predominately parallel to the length of the tape and wrapped with a small pitch angle so that the orientation is primarily circumferential with respect to the mandrel). This ePTFE tape will provide circumferential compression to the underlying materials when suitably heated.

The above construction is them placed into a suitable convection oven set at 380° C. for 11 minutes, after which it is removed from the oven and allowed to cool to approximately ambient temperature. The outer layers of ePTFE film and Kapton tape are then removed. The resulting coated stent and underlying layer of Kapton tape are then carefully removed from the mandrel. The remaining layer of Kapton tape may then be removed from the stent using a suitable tool such as small forceps or tweezers. Remaining film edges protruding beyond the ends of the stent may then be carefully trimmed in a transverse direction close to the end apices of the stent wire with a scalpel blade.

FIG. 6A shows a perspective view of a balloon expandable stent 60, as it appears following diametrical expansion with a balloon that is preferred for use as described herein. The stent 60 shown comprises rings 62 wherein the balloon-expanded stent elements form multiple diamond-shaped openings 63d; stent 60 is typically comprised of one or more of these rings 62. The individual rings 62 may be constructed by any suitable means known in art but are preferably fabricated from a laser cut tube. For clarity, only the side of the tubular stent 60 closest to the viewer is shown. Stent 60 is provided with a polymeric covering 66, preferably of a flexible film. It is apparent how covering 66 interconnects the multiple rings 62 to create stent 60, via webs 32 that span the distance between apices 22a and 22b of adjacent rings 62.

While various polymeric films may be suitable for use as the stent covering (or coating) material for this device, combinations of FEP (fluorinated ethylene propylene) films used in combination with ePTFE films are preferred. The preferred ePTFE film for this device is a uni-axial film having higher strength in one direction, with the direction primarily aligned with the longitudinal axis 61 of the stent prior to balloon expansion. This type of film is similar to that described in U.S. Pat. No. 5,476,589. A further preference would be to modify the film with an application of a discontinuous coating of FEP similar to that taught in U.S. Pat. No. 6,159,565.

The arrangement of stent rings 62 are shown in FIG. 6B without polymeric covering 66 as the rings 62 would appear prior to balloon expansion. Unexpanded stent rings 62 are cut to have openings 63 which become diamond shaped openings 63d when expanded (as shown in FIG. 6A). Stent rings 62 are placed in proximity to one another with apices 22a and 22b in a typical apex to apex alignment. It is apparent that the distance between adjacent rings 62 may be as desired.

FIG. 6C illustrates the stent rings 62 as shown previously in FIG. 6B with the addition of interconnecting polymeric covering 66. Webs 32, each a portion of polymeric covering 66, are shown to interconnect adjacent rings 62. FIG. 6D is an enlarged detail perspective view of the upper left end of stent 60 described in FIG. 6C.

Also shown in FIGS. 6C and 6D are punctures or slits 68 arranged in polymeric covering 66 along the longitudinal axis of stent 60. FIGS. 6B-6D show the multiplicity of openings 63 and 64 formed between adjacent stent elements of stent rings 62. Slits 68 through polymeric covering 66 are formed of size and shape to generally correspond with the multiplicity of openings 63 and 64 in each stent ring 62. These slits 68 may be formed by various means as previously described. Slits 68 are formed through the polymeric covering 66 that covers openings 63 that extend between opposing apices 22a and 22b (openings that are enclosed between the ends of each stent ring 62). Alternate openings 64 that extend from the middle of the length of each stent ring 62 and fully to the end of each stent ring 62 (i.e. between radially adjacent apices 22a and 22a, and likewise between radially adjacent apices 22b and 22b) are also provided with slits through the covering polymeric material 66. These slits 68 extend longitudinally between adjacent rings 62 and into the corresponding opening in the adjacent ring 62. These slits 68 collectively create individual interconnecting webs 32. Slits 68 may be of width as desired; the width of a scalpel blade may be deemed sufficient even though the figures show that width of slit 68 corresponding to the width of the underlying stent openings 63 and 64.

The apices 22a and 22b of each ring 62 may be made to point toward one another as shown in FIG. 6A or may be arranged to be offset as shown in FIG. 7 (i.e. aligned peak-to-valley as shown in FIG. 7 as opposed to being aligned in peak-to-peak fashion as shown in FIGS. 1A through 2D, FIG. 4 and FIG. 6A). The apices typically “point” in directions that are substantially parallel to the longitudinal axis 61 of the tubular form of the stent 60.

FIG. 8 is a schematic side view of stent 60 as it would appear mounted on a balloon (not shown) for subsequent deployment and expansion. Stent 60 is preferably axially compressed during mounting so that Interconnecting webs 32 are bowed or wrinkled so that stent 60 is foreshortened. The advantage of mounting stent 60 in this fashion is that, during balloon expansion, stent rings 62 will foreshorten as they are deformed (with openings 63 becoming diamond shaped openings 63d). For example, this allows for less than 10% shortening with a greater than 6 times diametrical expansion. Bowed webs 32 may be tucked under adjacent stent ring 62 if it is preferred that they do not protrude outwardly. A preferred balloon is a balloon that expands diametrically from the middle of its length toward its opposing ends. Alternatively, stent rings 62 at the ends of stent 60 may be made of a thicker material than ring 62 positioned closer to the middle of the length of stent 60. These alternatives result in the application of tension during expansion to bowed webs 32 thereby pulling the slack out of them, increasing their length and compensating for foreshortening of rings 62 to maintain the length of stent 60.

A preferred method of making a stent such as a stent shown in FIGS. 6A through 7 is as follows. Standard diamond pattern geometry stents were laser machined and electro-polished at Laserage Technology Inc, Waukegan, Ill. from a 316 LVM stainless steel tube measuring 4.19 mm diameter×0.38 mm wall thickness, available from Norman Noble, Cleveland Ohio. The stents were exposed to a surface roughening step to improve adherence without degrading fatigue durability performance. Plasma treatment of the stents was performed prior to FEP powder coating for purposes of cleaning and reducing contact angle of the metal surface. Plasma treatment was performed as commonly known in the arts.

FEP powder (Daikin America, Orangeburg N.Y.) was applied to the stent component by first stirring the powder into an airborne “cloud” in a standard kitchen-type blender and suspending the frame in the cloud until a uniform layer of powder was attached to the stent frame. The stent component was then subjected to a thermal treatment of 320° C. for approximately three minutes. This caused the powder to melt and adhere as a coating over the stent component. Each ring was coated a second time while suspending it from the opposite end and placed in 320° C. oven for 3 minutes then removed and allowed to cool to room temperature.

Seventeen layers of a thin ePTFE film provided with a discontinuous coating of FEP as previously described was then wrapped around a stainless steel mandrel measuring approx 3.43 mm. The film is applied with its high strength orientation parallel to the longitudinal axis of the stent and with the FEP side facing out. Individual stent rings were placed over the film tube and aligned. In this case, the stent rings were aligned apex to apex and separated evenly with a gap of about 2.5 mm between each ring to achieve an overall device length of about 40 mm. An additional 17 layers of the same film was applied as previously described except with the FEP side oriented down, toward the outer diameter of the stent.

The entire assembly was wound with several layers of an ePTFE thread (Part #SO24T4, WL Gore, Elkton, Md.) to impart compressive forces to the underlying construct. The assembly was placed in 320° C. oven (Grieves, Model MT1000, The Grieve Corporation, Round Lake, Ill.) for approximately 40 minutes. The stent assembly was removed and allowed to cool to room temperature. The over-wrap was then removed and the slits were created and excess material was removed.

While particular embodiments of the present invention have been illustrated and described herein, the present invention should not be limited to such illustrations and descriptions. It should be apparent that changes and modifications may be incorporated and embodied as part of the present invention within the scope of the following claims.

Claims

1. A method of making a flexible stent comprising: a) providing a stent having a length between opposing ends and having metallic stent elements;b) providing at least a portion of the length of the stent with a polymeric covering;c) forming slits or apertures through the covering between adjacent stent elements;d) heating the stent and polymeric covering to cause the slits or apertures to enlarge.

2. The method of claim 1, wherein said heating step results in the formation of polymeric webs interconnecting said stent elements.

3. The method of claim 2, wherein the webs comprise ePTFE and fluorinated ethylene propylene.

4. The method of claim 1, wherein said polymeric covering comprises ePTFE.

5. The method of claim 1, wherein said polymeric covering comprises fluorinated ethylene propylene.

6. The method of claim 1, wherein forming slits or apertures through the covering between adjacent stent elements includes puncturing slits or apertures through the covering.

7. The method of claim 1, wherein forming slits or apertures through the covering between adjacent stent elements includes lasing slits or apertures through the covering.

8. A method of making a flexible stent comprising: a) providing a stent having a smaller compacted diameter prior to diametrical expansion and a larger diameter following diametrical expansion, said stent having a length between opposing ends and having stent elements;b) providing at least a portion of the length of the stent with a polymeric covering when the stent is at the smaller compacted diameter;c) heating the stent and the polymeric covering to bond the covering to the stent elements;d) forming slits through the covering between adjacent stent elements;wherein following heating of the flexible stent and the polymeric covering, the polymeric covering shrinks so that said slits form apertures, and wherein after diametrical expansion said apertures become diamond-shaped.

9. The method of claim 8, wherein forming slits through the covering between adjacent stent elements includes puncturing slits through the covering.

10. The method of claim 8, wherein forming slits through the covering between adjacent stent elements includes lasing slits through the covering.

11. A medical device comprising: a frame;a polymeric film having a multiplicity of slits there-through, said slits having a width;wherein said film has been heated sufficiently to cause said slits to become apertures by shrinking said film and to cause the film to be bonded to the frame, wherein said apertures have a width greater than the width of said slits; andwherein the polymeric film defines polymeric webs interconnecting a plurality of stent elements of the device, said polymeric webs defining a length between the stent elements, wherein the polymeric webs are narrower at a middle portion of the length relative to a width of the polymeric webs at a connection point between the polymeric webs and the stent elements.

12. A method of making a flexible stent comprising: a) providing a stent having a length between opposing ends and having metallic stent elements;b) providing at least a portion of the length of the stent with a polymeric covering;c) forming slits or apertures through the covering between adjacent stent elements;d) heating the stent and polymeric covering to cause the slits or apertures to enlarge, wherein said heating step results in the formation of polymeric webs interconnecting said stent elements.

13. The method of making a flexible stent comprising: a) providing a stent having a smaller compacted diameter prior to diametrical expansion and a larger diameter following diametrical expansion, said stent having a length between opposing ends and having stent elements;b) providing at least a portion of the length of the stent with a polymeric covering when the stent is at the smaller compacted diameter;c) heating the stent and the polymeric covering to bond the covering to the stent elements;d) forming slits through the covering between adjacent stent elements;wherein following heating of the flexible stent and the polymeric covering, the polymeric covering shrinks so that said slits form apertures, and wherein after diametrical expansion said slits or apertures become diam and-shaped; andwherein the polymeric covering defines polymeric webs interconnecting said stent elements.