Patent Publication Number: US-2011054589-A1

Title: Stent with variable cross section braiding filament and method for making same

Description:
RELATED APPLICATION DATA 
     This application claims the benefit under 35 U.S.C. §119 to provisional application Ser. No. 61/237,431, filed Aug. 27, 2009, which is incorporated by reference into the present application in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The field of the invention generally relates to devices, such as stents, for reinforcing the structural integrity of vessels of a human or veterinary patient. More particularly, the field of the invention relates to stents with variable porosity. 
     BACKGROUND OF THE INVENTION 
     Stents, grafts, stent-grafts, vena cava filters and similar implantable medical devices, collectively referred to hereinafter as stents, are radially expandable endoprostheses which are typically intravascular implants capable of being implanted transluminally and enlarged radially after being introduced percutaneously. Stents may be implanted in a variety of body lumens or vessels such as within the vascular system, urinary tracts, bile ducts, etc. Stents may be used to reinforce body vessels and to prevent restenosis following angioplasty in the vascular system. They may be self-expanding, mechanically expandable or hybrid expandable. 
     Stents are generally tubular devices for insertion into body lumens. However, it should be noted that stents may be provided in a wide variety of sizes and shapes. Balloon expandable stents require mounting over a balloon, positioning, and inflation of the balloon to expand the stent radially outward. Self-expanding stents expand into place when unconstrained, without requiring assistance from a balloon. A self-expanding stent may be biased so as to expand upon release from the delivery catheter and/or include a shape-memory component which allows the stent to expand upon exposure to a predetermined condition. Some stents may be characterized as hybrid stents which have some characteristics of both self-expandable and balloon expandable stents. 
     Due to the branching nature of the human vasculature it is not uncommon for stenoses to form at any of a wide variety of vessel bifurcations. A bifurcation is an area of the vasculature or other portion of the body where a first (or parent) vessel is bifurcated into two or more branch vessels. In some cases it may be necessary to implant multiple stents at the bifurcation in order to address a stenosis located thereon. Alternatively, a stent may be provided with multiple sections or branches that may be deployed within the branching vessels of the bifurcation. 
     Stents may be constructed from a variety of materials such as stainless steel, Elgiloy, nickel, titanium, nitinol, shape memory polymers, etc. Stents may also be formed in a variety of manners as well. For example a stent may be formed by etching or cutting the stent pattern from a tube or sheet of stent material; a sheet of stent material may be cut or etched according to a desired stent pattern whereupon the sheet may be rolled or otherwise formed into the desired substantially tubular, bifurcated or other shape of the stent; one or more wires or ribbons of stent material may be woven, braided or otherwise formed into a desired shape and pattern. The density of the braid in braided stents is measured in picks per inch. Stents may include components that are welded, bonded or otherwise engaged to one another. 
     Typically, a stent is implanted in a blood vessel or other body lumen at the site of a stenosis or aneurysm by so-called “minimally invasive techniques” in which the stent is compressed radially inwards and is delivered by a catheter to the site where it is required through the patient&#39;s skin or by a “cut down” technique in which the blood vessel concerned is exposed by minor surgical means. When the stent is positioned at the correct location, the stent is caused or allowed to expand to a predetermined diameter in the vessel. 
     Flow diverting stents may treat a brain aneurysm by providing resistance to blood in-flow to the aneurysm. Subsequently, the blood in the aneurysm stagnates and, in time, forms a thrombosis to close the aneurysm. To increase the therapeutic effectiveness of a flow diverting stent, the middle segment of the stent, which impedes blood flow into the aneurysm, has a low porosity. 
     Porosity of stent material is a measure of the tendency of that material to allow passage of a fluid. A stent material&#39;s porosity index (PI) is defined as one minus the ratio of stent metal surface area to artery surface area covered by the stent. Higher porosity means that the stent material has less metal surface area compared to artery surface area and lower porosity means that the stent has more metal surface area compared to artery surface area. 
       FIG. 13  shows a stent that has been cut open along its length and unrolled into a flat sheet. The proximal to distal longitudinal axis stretches from left to right. The braid angle of a stent between two braid filaments is labeled as alpha. There are three states in which a stent&#39;s braid angle is measured: (1) when the stent is fully expanded with no restriction; (2) when the stent is compressed to fit into a catheter; and (3) when the stent is expanded in a vessel. Flaring the ends of a stent can add a fourth state. 
     The number of wires in a stent determines the type of braiding apparatus, i.e. 32 wires vs. 48 wires. Wire diameter also affects porosity, radial pressure, and stiffness of a stent. 
     Perceived problems with current stents include increasing radial stiffness with decreasing porosity by increasing picks per inch. The increased radial stiffness results in resistance to radial compression, which is needed to collapse the stent for insertion through an intravascular catheter. Stents have been braided with ribbons instead of wire with a circular cross section to decrease porosity without an undue increase in radial stiffness, but such stents have unacceptably low radial pressure at the anchoring ends. Further, such stents do not form desirable looped end designs well, because it is challenging to maintain the ribbon in a single plane while forming a loop. Another perceived problem with current stents is that braiding stents from either ribbon or wire with a circular cross section results in limited porosity gradient between ends, where high porosity is desirable, and the middle, where low porosity is desirable. 
     SUMMARY 
     In accordance with a general aspect of the inventions disclosed herein, a braided stent is formed from a filament having at least one circular zone and at least two non-circular zones. Embodiments of the braided stent may have a proximal segment, a middle segment, and a distal segment. In one such embodiment, a porosity of the middle segment is lower than a respective porosity of the proximal and distal segments. In another such embodiment, a radial pressure of the middle segment may be controlled separately from, e.g., so that it is less than, a radial pressure of the distal segment. By way of another example, a stiffness of the middle segment may also be controlled separately from, e.g., so that it is less than, a stiffness of the distal segment. 
     In one embodiment, the filament comprising a single circular zone and two non-circular zones, wherein the circular zone is disposed between the two non-circular zones. Optionally, the circular zone may have at least one looped end. In one embodiment, the filament has three circular zones and two non-circular zones, wherein the three circular zones and the two non-circular zones are alternately disposed on the filament. 
     In accordance with another aspect of the disclosed inventions, a method of braiding a stent includes providing a filament having at least one circular zone and at least two non-circular zones; and braiding the filament into a stent. In one such embodiment, the method further comprises wrapping at least one circular zone of the filament around a mandrel to form a distal loop of the stent. In one such embodiment, the method further comprises braiding at least one non-circular zone of the filament into a low porosity stent segment. In one such embodiment, the method further comprises braiding at least one circular zone of the filament into a high radial pressure stent segment. 
     In one embodiment, the filament comprises a single circular zone and two non-circular zones, the method further comprising braiding the circular zone into a high porosity distal stent segment, braiding respective medial portions of the two non-circular zones into a low porosity middle stent segment, and braiding respective lateral portions of the two non-circular zones into a high porosity proximal stent segment. 
     Other and further aspects and embodiments will become apparent from the figures and following detailed description thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring now to the drawings in which like reference numbers represent corresponding parts throughout, and in which: 
         FIG. 1  is a perspective view of a stent filament in accordance with one embodiment of the invention. 
         FIGS. 2A ,  2 B, and  2 C are cross-sectional views through the lines  2 A- 2 A,  2 B- 2 B, and  2 C- 2 C in  FIG. 1 , respectively. 
         FIG. 3  is a perspective view of a stent in accordance with one embodiment of the invention. 
         FIGS. 4A ,  4 B, and  4 C are cross-sectional views through the filament zones in the proximal, middle, and distal segments of the stent in  FIG. 3 , respectively. 
         FIG. 5  is a perspective view of a stent filament in accordance with another embodiment of the invention. 
         FIGS. 6A ,  6 B,  6 C,  6 D, and  6 E are cross-sectional views through the lines  6 A- 6 A,  6 B- 6 B,  6 C- 6 ,  6 D- 6 D, and  6 E- 6 E in  FIG. 5 , respectively. 
         FIG. 7  is a perspective view of a stent in accordance with another embodiment of the invention. 
         FIGS. 8A ,  8 B, and  8 C are cross-sectional views through the filament zones in the proximal, middle, and distal segments of the stent in  FIG. 7 , respectively. 
         FIG. 9  is a perspective view of a stent filament and a mandrel used to braid a stent in accordance with one embodiment of the invention, where the portion of the stent filament behind the mandrel is shown in shadow for clarity. 
         FIGS. 10-12  are detailed perspective views of braids in accordance with various embodiments of the invention. 
         FIG. 13  shows (for purposes of illustration) a stent that has been cut open along its length and unrolled into a flat sheet. 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
       FIG. 1  illustrates a stent filament  100  according to an embodiment of the invention. The filament  100  may be formed from both metallic and non-metallic materials. 
     Metallic filament materials include, without limitation, nitinol, stainless steel, cobalt-based alloy such as Elgiloy, platinum, gold, titanium, tantalum, niobium, and combinations thereof and other biocompatible materials, as well as polymeric materials. The filament  100  or zones thereof may have an inner core of tantalum, gold, platinum, iridium or combinations thereof and an outer member or layer of nitinol to provide a composite filament for improved radiopacity or visibility. Non-metallic materials include, without limitation, polyesters, such as polyethylene terephthalate (PET) polyesters, polypropylenes, polyethylenes, polyurethanes, polyolefins, polyvinyls, polymethylacetates, polyamides, naphthalane dicarboxylene derivatives, natural silk, and polytetrafluoroethylenes. Non-metallic materials also include carbon, glass, and ceramics. Stents braided from filament  100  made from memory material, e.g. nitinol, could be biased to take on an expanded form due to the memory property of the filament material. The expanded form of the stent could be a generally tubular shape with flared ends. The flared ends increase radial pressure and stent stiffness for better anchoring at the ends of the stent, especially the distal end. 
     The filament  100  has three zones, one circular zone  102  and two non-circular zones  104 ,  106 . The cross section of the filament  100  in the circular zone  102  is circular, as shown in  FIG. 2B . The cross section of the filament  100  in the non-circular zones  104 ,  106  is non-circular, including rectangular, concave, and ovoid, as shown in  FIGS. 2A and 2C . The filament  100  in the circular zone  102  may be shaped like a wire and the filament  100  in the non-circular zone  102  may be shaped like a ribbon. The cross sectional shapes of the various filament zones  102 ,  104 , and  106  may be configured either during or after formation of the filament  100 . 
     The filament  100  in the non-circular zones  104 ,  106  has a lower moment of area in the flat direction, making it more flexible than filament  100  in the circular zone  102 . Increasing flexibility reduces the radial pressure exerted by a stent segment braided from filament  100  in the non-circular zones  104 ,  106  compared to a stent segment braided from filament  100  in the circular zone  102  with the same braid angle and braid diameter. Also, the filament  100  in the non-circular zones  104 ,  106  is wider than filament  100  in the circular zone  102 . For instance, the diameter  108  of the circular cross section measures 0.002 inches and the long axis  110  of the ovoid cross section measures 0.003 inches. Increasing width decreases the porosity of a stent segment braided from filament  100  in the non-circular zones  104 ,  106  compared to a stent segment braided from filament  100  in the circular zone  102 . 
     The stent  200  braided from the filament  100  is shown in  FIG. 3 . The stent  200  has three segments, a proximal segment  202 , a middle segment  204 , and a distal segment  206 . The distal segment  206  ends in distal loops  208 . The distal segment  206  of the stent  200  is braided from filament  100  in the circular zone  102 . The middle segment  204  of the stent  200  is braided from filament  100  in the non-circular zones  104 ,  106 . As such, the middle segment  204  of the stent  200  has lower porosity and exerts lower radial pressure compared to the distal segment  206  of the stent  200 , given the same braid angle and braid diameter. The lower porosity of the middle segment  204  increases the flow diverting effectiveness of the stent  200 . The higher radial pressure exerted by the distal segment  206  provides a better anchor for the stent  200 . 
     The non-circular shaped cross section of the filament  100  in the non-circular zones  104 ,  106  also reduces the stiffness, both radial and axial, of the middle segment  204  of the stent  200 , which is braided from filament  100  in the non-circular zones  104 ,  106 . The reduced radial pressure and stiffness allow the middle segment  204  of the stent  200  to be braided more densely, i.e., higher picks per inch, while maintaining a radial pressure and a stiffness respectively less than or equal to the radial pressure and stiffness of the distal segment  206  of the stent  200 , which has fewer picks per inch. This allows the middle segment  204  of the stent  200  to have higher braid density, and therefore lower porosity, than the other segments of the stent  200 , while maintaining the ability to radially collapse the stent for insertion through a catheter and reducing radial stiffness. 
     Like the middle segment  204 , the proximal segment  202  of the stent  200  is also braided from the non-circular zones  104 ,  106  of the filament  100 . The middle segment  204  is braided from the medial portions  112 ,  114  of the non-circular zones  104 ,  106  of the filament  100 . The proximal segment  202  is braided from the lateral portions  116 ,  118  of the non-circular zones  104 ,  106  of the filament  100 . Unlike the middle segment  204 , the braid density of the proximal segment  202  is lower due to a smaller braid angle or lower picks per inch. The resulting high porosity in the proximal segment  202  reduces the likelihood of side branch blockage. 
     In another embodiment of the invention shown in FIGS.  5  and  6 A- 6 E, the filament  100  has five zones, three circular zones  102 ,  120 ,  122 , and two non-circular zones  104 ,  106 . As shown in FIGS.  7  and  8 A- 8 C, the stent  200  braided from this filament  100  is similar to the stent  200  discussed above, except that the proximal segment  202  of the stent  200  is braided from the lateral circular zones  120 ,  122  of the filament  100 . Only the middle segment  204  of the stent  200  is braided from the non-circular zones  104 ,  106  of the filament  100 . 
     As shown in  FIG. 7 , the proximal segment  202  of the stent  200  is identical to the distal segment  206  of the stent with the exception of the distal loops  208 , which are only present in the distal segment  206 . Both the proximal segment  202  and distal segment  206  of the stent  200  are braided from circular filament zones  102 ,  120 ,  122 , as shown in  FIGS. 8A and 8C . The middle segment  204  of the stent  200  is braided from non-circular filament zone  104 ,  106 , as shown in  FIG. 8B . Further, the middle segment  204  of the stent  200  has a higher braid density (i.e., higher picks per inch or larger Alfa angle) than the proximal segment  202  and distal segment  206  of the stent  200 . 
     Accordingly, the middle  204  segment of the stent  200  has lower porosity than the proximal segment  202  and distal segment  206  of the stent  200 . Notwithstanding the higher braid density in the middle segment  204  of the stent  200 , that segment of the stent  200  has a radial pressure and a stiffness respectively less than or equal to the radial pressure and stiffness of the proximal segment  202  and distal segment  206  of the stent  200 . The middle segment  204  of the stent  200  is able to maintain lower radial pressure and lower stiffness due to the non-circular shape of the filament  100  at non-circular zones  104 ,  106  from which it is braided. 
     The filament  100  is braided into a stent  200  as shown in  FIGS. 9-12 . Braiding a filament  100  into a stent  200  begins by placing a mandrel pin  210  adjacent to the approximate middle of the middle circular zone  102  of the filament  100 , as shown in  FIG. 9 . The filament  100  is first wrapped around the mandrel pin  210  to form a distal loop  208 . The various zones of the filament  100  are then braided together to form the distal, middle, and proximal segments  206 ,  204 ,  202  of the stent  200 . 
     As depicted in  FIGS. 3 and 7 , braiding of filaments  100  includes the interlacing of at least two sections of filament  100  such that the paths of the filament sections are diagonal to the stent delivery direction, forming a tubular structure. Useful braids include, but are not limited to, a diamond braid having a 1/1 intersection repeat (i.e., braid  212  as depicted in  FIG. 10 ), a regular braid having a 2/2 intersection repeat (i.e., braid  214  as depicted in  FIG. 11 ), and a Hercules braid having a 3/3 intersection repeat (i.e., braid  216  as depicted in  FIG. 12 ). U.S. Pat. No. 5,653,746, the contents of which are incorporated herein by reference, further describes such braids. Moreover, a triaxial braid may also be used. A triaxial braid has at least one filament section that typically runs in the longitudinal direction or axial direction of the stent to limit filament movement. The axial or longitudinal filament section is not interlaced or interwound with the other braid filament sections, but is trapped between the different sections of filament in the braided structure. Moreover, an interlocking three-dimensional braided structure or a multi-layered braided structure is also useful. A multi-layered braided structure is defined as a structure formed by braiding wherein the structure has a plurality of distinct and discrete layers. 
     Generally, a braided structure is formed having a braid angle from about 30° to about 90° with respect to the longitudinal axis of the braided structure, desirably about 54.5° to about 75°. The braid angle is set by heat setting. When deploying the stent  200  into a vessel with a smaller diameter than the expanded stent  200 , the angle is reduced as the stent  200  is compressed radially to fit into the vessel. 
     While various embodiments of the present invention have been shown and described, they are presented for purposes of illustration, and not limitation. Various modifications may be made to the illustrated and described embodiments without departing from the scope of the present invention, which is to be limited and defined only by the following claims and their equivalents.