Patent Publication Number: US-2002010505-A1

Title: Multilayered metal stent

Description:
FIELD OF THE INVENTION  
       [0001] The present invention relates to stents for deploying within body lumens, and more particularly, to optimizing the radiopacity of such stents.  
       BACKGROUND  
       [0002] Stents are tubular structures that are implanted inside bodily conduits, blood vessels or other body lumens to widen and/or to help keep such lumens open. Typically, stents are delivered into the body while in a compressed configuration, and are thereafter expanded to a final diameter once positioned at a target location within the lumen. Stents are often used following or substituting for balloon angioplasty to repair stenosis and to prevent future restenosis and, more generally, may be used in repairing any of a number of tubular body conduits such as those in the vascular, biliary, genitourinary, gastrointestinal, respiratory and other systems. Exemplary patents in the field of stents formed of wire, for example, include U.S. Pat. Nos. 5,019,090 to Pichuk; 5,161,547 to Tower; 4,950,227 to Savin et al.; 5,314,472 to Fontaine; 4,886,062 and 4,969,458 to Wiktor; and 4,856,516 to Hillstead; each of which is incorporated herein by reference. Stents formed of cut stock metal, for example, are described in U.S. Pat. Nos. 4,733,665 to Palmaz; 4,762,128 to Rosenbluth; 5,102,417 to Palmaz and Schatz; 5,195,984 to Schatz; WO 91 FR013820 to Meadox; and WO 96 03092 to Medinol, each of which is incorporated herein by reference. Bifurcating stents are described in U.S. Pat. No. 4,994,071 to MacGregor, and commonly-assigned U.S. Pat. application Ser. No. 08/642,297, filed May 3, 1996, each of which is incorporated herein by reference.  
       [0003] For stents to be effective, it is essential that they be accurately positioned at a target location within a desired body lumen. This is especially true where, for example, multiple stenting is required with overlapping stents to cover excessively long regions or bifurcating vessels. In these and other cases, it is often necessary to visually observe the stent both during placement in the body and after expansion of the stent. Various approaches have been attempted to achieve such visualization. For example, stents have been made from radiopaque (i.e., not allowing the passage of x-rays, gamma rays, or other forms of radiant energy) metals, such as tantalum and platinum, to facilitate fluoroscopic techniques. One of the potential problems with such stents, however, is that a useful balance of radiopacity and stent strength is difficult, if not impossible, to achieve. For example, in order to form such a stent of adequate strength, it is often necessary to increase stent dimensions such that the stent becomes overly radiopaque. Consequently, fluoroscopy of such a stent after deployment can hide the angiographic details of the vessel in which it is implanted, thus making it difficult to assess problems such as tissue prolapse and hyperplasia.  
       [0004] Another technique that has been used to achieve the visualization of stents is the joining of radiopaque markers to stents at predetermined locations. The joining of the stent and marker materials (e.g., stainless steel and gold, respectively), however, can create a junction potential or turbulence in blood and thus promote thrombotic events, such as clotting. Consequently, the size of the markers is minimized to avoid this problem, with the adverse effect of greatly decreasing fluoroscopic visibility and rendering such visibility orientation-sensitive.  
       [0005] Yet another technique that has been used to achieve the visualization of stents is to simply increase the thickness of such stents to thereby increase radiopacity. Overly thick stent struts, however, effectively create an obstruction to blood flow. In addition, design limitations for stents having thick struts often result in large gaps between these struts, thus decreasing the support of a surrounding lumen. Furthermore, overly thick stent struts could adversely affect stent flexibility.  
       [0006] There is thus a need for the increased radiopacity of stents without sacrificing stent mechanical properties or performance. The coating of stents with radiopaque materials is described in U.S. Pat. No. 5,607,442 to Fishell et al. According to this patent, the disclosed radiopaque coating is much thicker on longitudinal stent members when compared with radial stent members such that only the longitudinal stent members are visible during fluoroscopy.  
       SUMMARY OF THE INVENTION  
       [0007] The present invention provides stents of optimized radiopacity and mechanical properties.  
       [0008] In one embodiment, the present invention includes a stent comprising a tubular member which comprises struts of a first material, and a first coating on the tubular member. The first coating substantially covers the tubular member and is substantially uniform in thickness. The first coating comprises a second material that is more radiopaque than the first material comprising the struts.  
       [0009] In another embodiment of the present invention, the stent further comprises a second coating disposed between the tubular member and the first coating, wherein the second coating covers only a portion of the tubular member. When the stent is observed with fluoroscopy, the portion where the second coating exists appears darker than where only the first coating exists.  
       [0010] In yet another embodiment of the present invention, the stent is a coated bifurcated stent for positioning in a body lumen that is bifurcated into a trunk lumen and a branch lumen. The stent has trunk and branch legs for positioning in trunk and branch lumens, respectively. In this embodiment, the stent is coated with multiple layers of radiopaque materials such when the stent is observed with fluoroscopy, the branch leg appears darker than the trunk leg. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0011]FIG. 1A illustrates a coated patterned stent, in accordance with an embodiment of the present invention.  
     [0012]FIG. 1B is a cross-sectional view of a typical strut from the stent of FIG. 1A.  
     [0013]FIG. 2A illustrates a preferred stent configuration in an embodiment of the present invention.  
     [0014]FIG. 2B illustrates a most preferred configuration for a single stent cell, in accordance with an embodiment of the present invention.  
     [0015]FIG. 3A illustrates a patterned stent having multiple coatings thereon, in accordance with an embodiment of the present invention.  
     [0016]FIG. 3B is a cross-sectional view of a typical strut from the stent of FIG. 3A, at a location where two coatings have been applied to the stent.  
     [0017]FIG. 3C is a cross-sectional view of a typical strut from the stent of FIG. 3A, at a location where only one coating has been applied to the stent.  
     [0018]FIG. 4A illustrates a first coated bifurcated stent, in accordance with an embodiment of the present invention.  
     [0019] FIGS.  4 B- 4 C illustrate a second coated bifurcated stent, in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION  
     [0020] The present invention provides optimal radiopacity of stents without sacrificing mechanical properties or performance. A stent according to the present invention is made from a base material having desired mechanical properties (e.g., strength) and coated with a material to provide optimal, radiopacity to the stent. The radiopacity of the stents of the present invention is optimized in the sense that, during fluoroscopic procedures, the stents are entirely visible but are not so radiopaque that angiographic details are masked. The present invention thus provides for stents that have both the desired mechanical properties of the base material and the desired radiopacity of the coating material. The stents of the present invention have the additional benefit of being manufactured according to simple and reproducible techniques.  
     [0021] In one embodiment of the present invention, stent  100  is a tubular member  101  comprising struts  110  as shown in FIG. 1A- 1 B. The term “strut”, as used herein, is intended to mean any structural member of a stent, such as any radial, longitudinal, or other members made from wire, cut stock, or other materials. Struts  110  comprise a first material that is selected for its mechanical properties such as, for example, the ability to be delivered into the body while in a compressed configuration, the ability to expand or be expanded once positioned to a target location, the ability to resist recoil, and the ability to hold open a body lumen during the stent lifetime. Typical exemplary materials for struts  110  include stainless steel and nitinol. Stent  100  further comprises a first coating  102  of a second material that is selected for its radiopacity. Coating  102  covers the entire tubular member  101  with the result that intersections of the first and second materials are not exposed to the exterior of the stent. By not exposing intersections of the first and second materials to the exterior of the stent, the risks of creating a junction potential in the blood and causing the electrolytic corrosion of the stent are precluded. FIG. 1B shows a cross-sectional view of coating  102  on a typical strut  110  of stent  100 . Although FIG. 1B shows both the strut  110  and coating  102  to be substantially square in cross-sectional shape, the actual cross-sectional shape of either or both of these elements is any desired or suitable shape, such as circular, oval-shaped, rectangular, or any of a number of irregular shapes.  
     [0022] Coating  102  is applied to tubular member  101  according to any suitable technique such as, for example, electroplating, electroless plating, ion beam aided deposition, physical vapor deposition, chemical vapor deposition, electron beam evaporation, hot-dipping or any other suitable sputtering or evaporation process. Coating  102  comprises any suitable radiopaque material such as, for example, gold, platinum, silver and tantalum.  
     [0023] The thickness of coating  102  is an important aspect of the present invention. A coating that is too thick will result in a stent that is overly radiopaque, and angiographic details will consequently be masked during subsequent fluoroscopy. In addition, stent rigidity often increases with coating thickness, thus making it difficult to expand the stent for placement in a body lumen if the coating is too thick. On the other hand, a radiopaque coating that is too thin will not be adequately visible during fluoroscopy. Depending on the material and configuration of the tubular member  101 ; and the material of the coating  102 , the thickness of coating  102  is optimized to provide the optimum balance between radiopacity and strength. In general, however, it is preferred that coating  102  be approximately 1-20%, and more preferably approximately 5-15%, of the underlying strut thickness. In all embodiments of the present invention, coating  102  is applied to the entire stent such that it is wholly visible during fluoroscopy. Accordingly, any suboptimal expansion at any position along the stent is visible and any deviations from perfect circular expansion can be noticed.  
     [0024] The stents of the present invention are of any suitable configuration, although the patterned configurations as described in WO 96 03092 and commonly-assigned, allowed U.S. Pat. application Ser. No. 08/457,354, filed May 31, 1995 and incorporated herein by reference, are preferred for all embodiments of the present invention. As an example of such a configuration (a closeup of which is shown in FIGS. 2A and 2B), stent  100  is a tube having sides that are formed into a plurality of two orthogonal meander patterns intertwined with each other. The term “meander pattern” is used herein to describe a periodic pattern about a center line and “orthogonal meander patterns” are patterns having center lines that are orthogonal to each other.  
     [0025] As shown in FIG. 2A, stent  100  optionally includes two meander patterns  11  and  12 . Meander pattern  11  is a vertical sinusoid having a vertical center line  9 . Meander pattern  11  has two loops  14  and  16  per period wherein loops  14  open to the right while loops  16  open to the left. Loops  14  and  16  share common members  15  and  17 , where member  15  connects from one loop  14  to its following loop  16  and member  17  connects from one loop  16  to its following loop  14 . Meander pattern  12  is a horizontal pattern having a horizontal center line  13 . Meander pattern  12  also has loops, labeled  18  and  20 , which may be oriented in the same or opposite directions. The stent configuration shown in FIG. 2A, with orthogonal meander patterns  11  and  12 , provides for a high degree of stent flexibility to facilitate expansion, yet results in a high degree of rigidity once the stent is expanded. FIG. 2B illustrates a detailed view of a single cell of the most preferred stent configuration of the present invention.  
     [0026] In another embodiment of the invention as shown in FIGS.  3 A- 3 C, stent  200  includes a second coating  202  applied between the struts  110  of stent  200  and first coating  102 . In distinction to first coating  102 , however, second coating  202  covers only a portion or multiple portions of stent  200  so that isolated regions of stent  200  are most visible during fluoroscopy. For example, second coating  202  is applied to one or both of the proximate  111  and distal  112  ends of stent  100 , as shown in FIG. 3A. As in the embodiment shown in FIGS.  1 A- 1 B, however, first coating  102  covers the entire stent  200  shown in FIGS.  3 A- 3 C. FIGS. 3B and 3C show cross-sectional views of struts  110  of stent  100  where second coating  202  has and has not been applied, respectively. Such isolated marking is useful for the accurate positioning of the ends of stents, such as, for example, in the case of multiple stenting wherein the overlapping length is important, or, for example, in the case of ostial stenting wherein the position of the stent end relative to the ostium is important.  
     [0027] Second coating  202  comprises a suitable radiopaque material such as gold, platinum, silver and tantalum, and may be the same or different material as first coating  102 . Second coating  202  is applied to stent  200  by any suitable technique, such as those described for the application of first coating  102 . Second coating  202  is applied only to a portion or multiple portions of tubular member  101 , for example, by masking during the application of second coating  202  or by isolated etching after second coating  202  is applied. It is to be appreciated that although coating  202  is herein described to be a “second” coating, it is applied to stent  200  before the application of first coating  102 .  
     [0028] When used, second coating  202  has a thickness that will result in increased radiopacity at the portion(s) where second coating  202  exists when compared with the portion(s) where second coating  202  does not exist. Because second coating  202  is applied to only a portion or multiple portions of stent  200 , it can be thickly applied without significantly affecting the resistance of stent  200  to expand or affecting the visibility of arterial details during fluoroscopy. Like first coating  102 , the thickness of second coating  202  is optimized to provide a desired balance between stent radiopacity and other properties. In general, however, second coating  202  is typically as thick or thicker than first coating  102 . When both first and second coatings  102 ,  202  are applied, it is generally preferred that the thickness of first and second coatings  102 ,  202  are about 1-5% and 5-15%, respectively, of the underlying stent strut thickness. Furthermore, the combined thickness of first and second coatings  102 ,  202  typically does not exceed 25% of the underlying stent strut thickness. As an illustrative example, second coating  202  is applied to a thickness of about 10 microns onto a stent having 100 micron diameter struts. First coating  102  is then applied to a thickness of about 1 micron.  
     [0029] In another embodiment of the present invention, stent  300  is a bifurcated stent as shown in FIG. 4A. Stent  300  comprises a tubular member  301  that is bifurcated into tubular trunk and branch legs  310 ,  311  for positioning in trunk and branch lumens of a bifurcated lumen, respectively. In this embodiment, the entire stent is coated with first coating  102  as described for the embodiments shown in FIGS. 1 and 3. Branch leg  311 , however, includes second coating  202  disposed between tubular member  301  and first coating  102  such that when stent  300  is observed with fluoroscopy, branch leg  311  appears darker than the trunk leg  310 . The cross-sectional views of the struts of stent  300  thus appear as shown in FIGS. 3B and 3C for branch and trunk legs  311 ,  310 , respectively. Such a configuration is useful for aligning and inserting branch leg  311  into a branch lumen.  
     [0030] Alternatively, branch leg  311  may be selectively inserted into branch aperture  312  of tubular member  301  so that tubular member  301  and trunk leg  310  are separately delivered into a bifurcated lumen. In this case, tubular member  301  is provided with a branch aperture  312  as shown in FIG. 4B. When tubular member  301  is delivered to a bifurcated lumen, branch aperture  312  is aligned with the corresponding branch lumen. Tubular member portion  301  of stent  300  is thereafter expanded to secure its position in the lumen to be treated, and branch leg  311  is delivered through branch aperture  312  so that part of branch leg  311  is positioned into the branch lumen. Branch leg  311  is thereafter expanded as shown in FIG. 4C in an amount sufficient for its external surface to engage the portion of the tubular member  301  defining the branch aperture  312  and secure the branch leg  311  in the branch lumen and tubular member portion  301 . In this embodiment of the invention, a region  313  surrounding branch aperture  312  includes both first and second coatings  102 ,  202  such that region  313  is most visible during fluoroscopy. In other words, the cross-sectional view of the struts  110  of stent  300  appear as shown in FIG. 3B for region  313 , and as shown in FIG. 3C elsewhere. Such a configuration is useful for aligning branch aperture  312  with a branch lumen so that branch leg  310  is thereafter easily inserted into the branch lumen.  
     [0031] The present invention provides stents having optimal radiopacity without sacrificing stent properties or performance. Those with skill in the art may recognize various modifications to the embodiments of the invention described and illustrated herein. Such modifications are meant to be covered by the spirit and scope of the appended claims.