Abstract:
A radially expandable stent and methods of making the same, the stent made entirely of a bioabsorbable metal, the stent having a portion of increased radiopacity, wherein the portion of increased radiopacity has one or more of the following characteristics: 
     i. the wall thickness of the stent in the portion of increased radiopacity exceeds the wall thickness of the wall immediately adjacent thereto by at least 0.0010″; 
     ii. the width of the stent in the portion of increased radiopacity exceeds the width of the stent immediately adjacent thereto by at least 0.0005″.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is claims priority to US Patent Provisional Application No. 61/406,231 filed Oct. 25, 2010, the entire contents of which are hereby incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The disclosed invention relates generally to a medical device and more particularly to a bioabsorbable stent 
         [0003]    Intraluminal stents are typically inserted or implanted into a body lumen, for example, a coronary artery, after a procedure such as percutaneous transluminal coronary angioplasty. Such stents are used to maintain the patency of a body lumen by supporting the walls of the lumen and preventing abrupt reclosure or collapse thereof. These stents can also be provided with one or more therapeutic agents adapted to be locally released from the stent at the site of implantation. In the case of a coronary stent, the stent can be adapted to provide release of, for example, an antithrombotic agent to inhibit clotting or an antiproliferative agent to inhibit smooth muscle cell proliferation, i.e., neointimal hyperplasia, which is believed to be a significant factor leading to re-narrowing or restenosis of the blood vessel after implantation of the stent. 
         [0004]    Metallic radially expandable stents such as those formed from stainless steel or Nitinol (NiTi) are desirable because of the superior strength and flexibility. One example of a radially expandable stent formed from stainless steel or Nitinol, for example, is shown in 
         [0005]      FIG. 1 . This stent is disclosed in commonly assigned U.S. Pat. No. 6,818,014 incorporated by reference herein in its entirety. 
         [0006]    However, metallic stents can cause complications such as thrombosis and neointimal hyperplasia. Thus, physicians are becoming increasingly interested in bioabsorbable stents rather than metallic stents that are left in the body permanently. 
         [0007]    Recently, there has been significant interest in the use of bioabsorbable stents formed from iron. Iron has similar mechanical properties to stainless steel which has superior strength and flexibility which makes the stents easy to deliver through the patient&#39;s vasculature. 
         [0008]    However, the radiopacity of iron is not sufficient to allow thin wall stents to be readily visible via fluoroscopic techniques which are often used for placement and for follow up visualization of the implanted stent. Attaching conventional noble metal markers such as gold or titanium renders the stent not completely bioabsobable as desired. 
         [0009]    There remains a need in the art for a bioabsorbable stent with sufficient fluoroscopic visibility. 
       SUMMARY OF THE INVENTION 
       [0010]    In some embodiments, the present invention relates to a radially expandable stent and methods of making the same, the stent made entirely of a bioabsorbable metal, the stent having an outer wall surface and an inner wall surface and a wall extending therebetween, the wall characterized by a wall thickness as measure in a radial direction between the outer and inner wall surfaces, the wall comprising a plurality of struts and connectors which are interconnected and which define openings in the wall, each strut and each connector having two opposing sides extending between the outer and inner wall surfaces and having a length, and a width everywhere along its length, the stent having a portion of increased radiopacity, wherein the portion of increased radiopacity has one or more of the following characteristics: 
         [0011]    i. the wall thickness of the stent in the portion of increased radiopacity exceeds the wall thickness of the wall immediately adjacent thereto by at least 0.0010″; 
         [0012]    ii. the width of the stent in the portion of increased radiopacity exceeds the width of the stent immediately adjacent thereto by at least 0.0005″. 
         [0013]    In some embodiments, the present invention the present invention is directed to a radially expandable stent, the stent formed entirely from a bioabsorbable metal, the stent comprising a plurality of serpentine bands interconnected by connectors, each serpentine band comprising a plurality of struts characterized by a length and interconnected by curved end portions which define the length of the struts, wherein the struts are folded along a portion of the length, the folded portion comprising increased radiopacity. 
         [0014]    In some embodiments, the stent is formed entirely of a bioabsorbable iron. 
         [0015]    In some embodiment, the present invention is directed to a method of increasing the radiopacity of a bioabsorbable metal stent, the method including providing a tubular stent preform formed from a bioabsorbable metal, cutting a strut pattern in the tubular stent preform, partially electropolishing said tubular stent preform, focally depositing said bioabsorbable metal on portions of the tubular stent preform to form portions of increased radiopacity and electropolishing said tubular stent preform, 
         [0016]    wherein the portions of increased radiopacity and have one or more of the following characteristics: 
         [0017]    i. the wall thickness of the stent in the masked portion of increased radiopacity exceeds the wall thickness of the wall immediately adjacent thereto by at least 0.0010″; 
         [0018]    ii. the width of the stent in the masked portion of increased radiopacity exceeds the width of the stent immediately adjacent thereto by at least 0.0005″. 
         [0019]    In some embodiments, the present invention relates to a method of increasing the radiopacity of a bioabsorbable metal stent, the method including providing a tubular stent preform formed from a bioabsorbable metal, cutting a strut pattern in the tubular stent preform, partially electropolishing the tubular stent preform, focally depositing said bioabsorbable metal on portions of the tubular stent preform to form portions of increased radiopacity and electropolishing said tubular stent preform, wherein the portions of increased radiopacity and have one or more of the following characteristics: 
         [0020]    i. the wall thickness of the stent in the masked portion of increased radiopacity exceeds the wall thickness of the wall immediately adjacent thereto by at least 0.0010″; 
         [0021]    ii. the width of the stent in the masked portion of increased radiopacity exceeds the width of the stent immediately adjacent thereto by at least 0.0005″. 
         [0022]    These and other aspects, embodiments and advantages of the present disclosure will become immediately apparent to those of ordinary skill in the art upon review of the Detailed Description and Claims to follow. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  is a fragmentary flat view of a radially expandable stent in an unexpanded state. 
           [0024]      FIG. 2  is a flat view of the outer surface of a single strut. 
           [0025]      FIG. 3  is a side view of the single strut taken at section  3  in  FIG. 2  wherein the strut is shown having increased thickness. 
           [0026]      FIG. 4  is a flat view of a portion of a stent illustrating a plurality of interconnected struts similar to those shown in  FIGS. 2 and 3 . 
           [0027]      FIG. 5  is a radial cross-section of a stent illustrating a plurality of struts similar to those shown in  FIGS. 2-4 . 
           [0028]      FIG. 6  is a side view of an alternate embodiment of a single strut wherein the strut has no increased thickness but has increased width as shown in  FIG. 5 . 
           [0029]      FIG. 7  is a flat view of the outer surface of the strut shown in  FIG. 4  wherein a portion of the strut has an increased width. 
           [0030]      FIG. 8  is a flat view of a portion of a stent illustrating a plurality of interconnected struts similar to those shown in  FIGS. 6 and 7 . 
           [0031]      FIG. 9  is a radial cross-section of a stent illustrating a plurality of struts similar to those shown in  FIGS. 6-8 . 
           [0032]      FIG. 10  is a side view of the outer surface of a single strut in an alternative embodiment. 
           [0033]      FIG. 11  is a flat view of the outer surface of the strut shown in  FIG. 6  wherein the curved end portions that interconnect adjacent parallel struts have increased width. 
           [0034]      FIG. 12  is a flat view of a portion of a stent illustrating a plurality of interconnected struts similar to those shown in  FIGS. 10 and 11 . 
           [0035]      FIG. 13  is a radial cross-section of a stent illustrating a plurality of struts similar to those shown in  FIGS. 10-12 . 
           [0036]      FIG. 14  is a flat view of the outer surface of a single strut in an alternative embodiment. 
           [0037]      FIG. 15  is a side view of the strut taken at section  9  in  FIG. 8  wherein a portion of the strut has increased thickness. 
           [0038]      FIG. 16  is a flat view of a portion of a stent illustrating a plurality of struts similar to those shown in  FIGS. 14 and 15 . 
           [0039]      FIG. 17  is a radial cross-section of a stent illustrating a plurality of struts similar to those shown in  FIGS. 14-16 . 
           [0040]      FIG. 18  is a side view of an elongated strut. 
           [0041]      FIG. 19  is a side view of the elongated strut shown in  FIG. 10  after folding a portion of the strut. 
           [0042]      FIGS. 20   a - 20   d  are side views of a single strut illustrating a process for making a stent according to the invention. 
           [0043]      FIGS. 21   a - 21   d  are side views of a single strut illustrating an alternative process for making a stent according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0044]    While embodiments of the present disclosure may take many forms, there are described in detail herein specific embodiments of the present disclosure. This description is an exemplification of the principles of the present disclosure and is not intended to limit the disclosure to the particular embodiments illustrated. 
         [0045]    The present invention relates to bioabsorbable radially expandable stents. As used herein, bioabsorbable stents shall refer to those that can be advantageously eliminated from body lumens after a predetermined, clinically appropriate period of time, for example, after the traumatized tissues of the lumen have healed and a stent is no longer needed to maintain the integrity of the lumen. The conventional bioabsorbable materials from which such stents are made are selected to resorb or degrade over time, thereby eliminating the need for subsequent surgical procedures to remove the stent from the body lumen if problems arise. 
         [0046]    Suitably, the bioabsorbable stents disclosed herein lose between 0 to about 30% of their original radial force in the first 6 months after implantation, and then thereafter disintegrate into pieces (100% strength loss) in about 12 to about 36 months after implantation. Suitably, all of the original stent material is converted to the biocompatible degradation product or to chemical species already present in the body in 12-48 months after implantation. 
         [0047]    One example of a radially expandable stent construction is shown in  FIG. 1  disclosed in commonly assigned U.S. Pat. No. 6,818,014, the entire content of which is incorporated by reference herein. Stent  10  is formed from a plurality of adjacent serpentine segments  16  connected by connector elements  20 . Each serpentine band is made up of a plurality of parallel struts  18  interconnected by curved end portions  19   a,    19   b.    
         [0048]    While the connector elements  20  in this embodiment are straight, curved connector elements can also be employed. Furthermore, while connector elements are shown extending from outer curved end portions  19   a,    19   b,  they could also extend from the inner surface of troughs of the serpentine bands  16  (embodiment not shown) rather than the outer curved end portions  19   a,    19   b.  This is only one example of a radially expandable stent and is not intended as a limitation on the scope of the present invention. Those of ordinary skill in the art are well aware of various stent constructions. 
         [0049]    Suitably, the bioabsorbable stents disclosed herein have an average wall thickness that is less than current commercially available stainless steel stents. For example, strut thickness may range from 0.0020″ to 0.0055″ (about 50 microns to about 140 microns) and strut width from 0.0025″ to 0.0060″ (63.5 microns to about 152 microns). The more highly radiopaque areas of the strut may range in thickness from 0.0035″ to 0.0065″ (about 89 microns to about 165 microns) or in width from 0.0030″ to 0.0070″ (about 76 microns to about 178 microns). 
         [0050]    It is desirable to provide these bioabsorbable stents with portions having an increased wall thickness of at least about 0.0010″ (about 25 microns) or 0.0015″ (about 38 microns) or increased width of at least about 0.0005″ or 0.0010 relative to the stent wall immediately adjacent thereto so that the stents have sufficient radiopacity for visibility using fluoroscopic techniques. 
         [0051]    Preserving portions of the stent that have a thinner wall thickness increases the flexibility of the stent and increases the rate at which is absorbed. 
         [0052]    Adding radiopaque markers formed from platinum, gold, palladium, iridium and so forth would result in stents that are not completely bioabsorbable as desired herein. 
         [0053]    Each of the following figures illustrates a variety of alternate embodiments wherein at least a portion of the stent has an increased width of at least about 0.0005″ (about 12 microns) or an increased thickness of at least about 0.0010″ (about 25 microns) relative to the stent wall immediately adjacent thereto. These various embodiments are intended for illustrative purposes only, and not as a limitation on the scope of the present invention. 
         [0054]    The areas of increased thickness or width may be included on every strut  18  of every serpentine band  16 , on every strut  18  of every other serpentine band  16 , or on every strut of the proximal band, distal band and middle band or a combination thereof. Of course, this pattern can be varies so that every other strut  18 , every third strut  18  and so forth of the band  16  might include the areas of increased thickness or width. In a preferred embodiment, the all the struts  18  of the proximal, distal and middle bands  16  have areas of increased thickness or width. Of course, connectors  20  could also include the portions of increased thickness. 
         [0055]      FIG. 2  is a flat view of the outer surface of a single strut  18  according to the invention. Section  3  in  FIG. 2  illustrates a portion of the strut  18  having a portion  22  having increased wall thickness of at least about 0.0010″ as related to the wall immediately adjacent thereto for increased radiopacity in this portion. 
         [0056]      FIG. 3  is a side view taken at section  3  in  FIG. 2  to illustrate the increased thickness of a portion of the stent strut  18 . 
         [0057]      FIG. 4  is a flat view of a portion of a stent  10  wherein every strut  18  has portions  22  of increased thickness. In this case, the outer diameter only is shown with portions  22  of increased thickness. 
         [0058]      FIG. 5  is a radial cross-section illustrating a stent  10  having struts  18  as in  FIGS. 2-4 . 
         [0059]      FIG. 6  is a flat view of the outer surface of a single strut  18  wherein a portion of the strut  22  has an increased width of at least about 0.0005″ (about 12.5 microns).  FIG. 7  is a flat view of the outer surface of strut  18  shown in  FIG. 6 . 
         [0060]      FIG. 8  is a flat view of a portion of a stent  10  wherein every strut  18  is shown having portions  22  of increased width.  FIG. 9  is a radial cross-section illustrating stent  10  having struts  18  similar to those shown in  FIGS. 6-8 . 
         [0061]      FIG. 10  is a side view of a single strut in another alternative embodiment of the stent according to the invention. In this embodiment, strut  18  includes curved end portions  19   a ,  19   b  having an increased width of at least about 0.0005″ (about 25 microns) as shown in a flat view of the outer surface in  FIG. 11 . 
         [0062]      FIG. 12  is a flat view of a portion of a stent  10  having interconnected struts  12  wherein the curved end portions  19  have increased width. 
         [0063]      FIG. 13  is a radial cross-section of a stent  10  similar to that shown in  FIG. 12   
         [0064]      FIG. 14  is a flat view of the outer surface of a single strut  18  in yet another embodiment of the stent according to the invention. In this embodiment, strut  18  has a portion  22  having an increased thickness taken at section  9  in  FIG. 14 .  FIG. 15  is a side view of strut  18  illustrating the increased thickness of the strut  18 . In this embodiment, the portion  22  of increased thickness includes both the inner diameter and outer diameter of the strut. 
         [0065]      FIG. 16  is a flat view of a portion of a stent  10  having struts  18  similar to those shown in  FIGS. 14 and 15  wherein the portions  22  of increased thickness include both the inner diameter and the outer diameter of the strut. 
         [0066]      FIG. 17  is a radial cross-section of a stent  10  having struts  18  similar to those shown in  FIGS. 14-16 . 
         [0067]      FIG. 18  is a side view of an elongated strut  18 . In this embodiment, the elongated strut is folded along a portion  24  of the strut to form an area having increased radiopacity as shown in side view in  FIG. 19 . 
         [0068]    A variety of methods can be employed in order to provide the stent with thicker/wider portions. 
         [0069]    In some embodiment, portions of the stent are masked during electropolishing in order to limit the amount of metal removed from those portions of the stent. Using this technique, a strut pattern is laser machined or otherwise cut or etched into the stent preform. Post-laser finishing performed to remove laser affected metal and dross and to achieve finished stent mass and dimensions are not applied uniformly over the entire stent surface. The desired thicker stent portions can be masked. This may include the ends and/or middle serpentine bands, as well as any other pattern desired. 
         [0070]      FIGS. 20   a  through  20   d  illustrate formation of the thicker portions using this technique.  FIGS. 20   a  through  20   d  show a partial side view of a strut  18 . In a first step, a strut pattern is formed in an iron stent preform via laser cutting as illustrated by  FIG. 20   a . A maskant  22  is then provided on a portion of each strut  18  wherein it is desirable to have increased wall thickness as shown in  FIG. 20   b . The masked strut  18  is then electropolished and material is removed from any unmasked surfaces as shown in  FIG. 20   c . The maskant is removed leaving a portion  22  of strut  18  with an increased thickness of at least about 0.0010″ relative to the stent wall immediately adjacent thereto. In this case, only the outer surface of the strut was masked so that increased wall thickness is seen only on the outer or abluminal strut surface and not on the inner or luminal strut surface. 
         [0071]    In other embodiments, iron is deposited onto the stent preform after pattern formation but prior to final stent finishing steps. For example, in some embodiments, the iron is deposited after pattern formation via laser machining or other cutting or etching the pattern in the stent preform and after partial electropolishing. Deposition may be conducted via any suitable method such as laser deposition, electroplating or plasma deposition. 
         [0072]      FIGS. 21   a  through  21   d  illustrate formation of thicker stent portions using metal deposition techniques.  FIG. 21   a  is a side view of a stent strut  18  after laser cutting. The stent strut  18  is then partially electropolished represented by  FIG. 21   b . Focal deposition of iron is achieved via use of laser deposition, electroplating or plasma deposition as represented in  FIG. 21   c  to form a portion  22  on the strut  18  of increased thickness. Final electropolishing is then conducted as shown in  FIG. 21   d  leaving a portion  22  on the strut  18  of increased thickness of about 0.0010″ relative to the stent wall immediately adjacent thereto. 
         [0073]    The description provided herein is not to be limited in scope by the specific embodiments described which are intended as single illustrations of individual aspects of certain embodiments. The methods, compositions and devices described herein can comprise any feature described herein either alone or in combination with any other feature(s) described herein. Indeed, various modifications, in addition to those shown and described herein, will become apparent to those skilled in the art from the foregoing description and accompanying drawings using no more than routine experimentation. Such modifications and equivalents are intended to fall within the scope of the appended claims. 
         [0074]    All publications, patents and patent applications mentioned in this specification are herein incorporated by reference in their entirety into the specification to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated herein by reference. Citation or discussion of a reference herein shall not be construed as an admission that such is prior art.