Patent Publication Number: US-RE42244-E

Title: Flexible stent having a pattern formed from biocompatible material

Description:
This is a reissue of application Ser. No.  08 / 748 , 669 , now U.S. Pat. No.  6 , 409 , 752  B 1 , which is a continuation of Ser. No. 08/378,073 filed on Jan. 25, 1995 now U.S. Pat. No. 5,632,771 which is a file wrapper continuation of Ser. No. 08/097,392 filed on Jul. 23, 1993, now abandoned. 
    
    
     TECHNICAL FIELD 
     This invention relates generally to balloon expandable stents and, in particular, to a flexible stent having a waveform pattern formed from a sheet of  biocompatible material and  into a cylindrical surface or tubular shape. References herein to forming the stent from a “sheet” or “sheets” describe preferred embodiments of the invention, and are not to be construed to limit the claims.  
     BACKGROUND OF THE INVENTION 
     Vascular stents are deployed at a narrowed site in a blood vessel of a patient for widening the vessel lumen and circumferentially supporting the vessel wall. Vascular stents desirably present a small cross-sectional dimension or profile for introducing the stent into the affected vessel lumen. 
     One approach to providing a vascular stent is the use of a piece of wire bent into a number of turns. Although suitable for its intended use, a problem with these bent wire stents is that stress points are formed at each wire bend or turn. As a result, the wire stent is structurally compromised at a number of points. Furthermore, bent wire stents lack longitudinal stability. For example, a wire stent is typically positioned in a blood vessel over an inflatable balloon. The balloon expands first at opposite ends, where the balloon is not in contact with the wire stent. As a result, the wire stent is longitudinally compressed between the inflated balloon ends. With continued inflation, the middle of the balloon expands, thereby unevenly expanding the wire bends of the longitudinally compressed wire stent. In an attempt to remedy the problem, the stent wire material has been formed to cross over or attach to itself. A problem with this attempted remedy is that the cross-sectional dimension of the stent, or stent profile, is increased, and the stent intrudes into the effective lumen of the blood vessel. The effective lumen of the blood vessel is further constricted by the growth of endothelial tissue layers over the stent wire. As a result, the stent and tissue growth impede fluid flow and cause turbulence in the vessel lumen. Another problem with this attempted remedy is that galvanic action, exposure to a reactive surface, or ion migration, occurs at the wire-to-wire contact points. The wire stent material rubs when movement occurs during ordinary blood flow and pulsation as well as patient muscle movement. 
     Another approach to providing a vascular stent is the use of a piece of metal cannula with a number of openings formed in the circumference thereof. A problem with the use of a metal cannula stent is that the stent is rigid and inflexible. As a result, the stent is difficult, if not impossible, to introduce through the tortuous vessels of the vascular system for deployment at a narrowed site. Furthermore, the stent is too rigid to conform with a curvature of a blood vessel when deployed at an occlusion site. Another problem with the use of a metal cannula stent is that the stent longitudinally shrinks during radial expansion. As a result, the position of the metal cannula stent shifts, and the stent supports a shorter portion of the blood vessel wall than anticipated merely by stent length. 
     Yet another approach to providing a vascular stent is the use of a wire mesh that is rolled into a generally tubular shape. A problem with the use of a wire mesh stent is that the overlapping wires forming the mesh increase the stent profile, thereby reducing the effective lumen of the blood vessel. The growth of endothelial tissue layers over the wire mesh further reduces the effective blood vessel lumen. Another problem with this approach is that ion migration also occurs at the wire-to-wire contact points. 
     Still yet another approach to providing a vascular stent is the use of a flat metal sheet with a number of openings formed in rows therein. The flat metal sheet stent also includes three rows of fingers or projections positioned on one edge of the stent along the axis thereof. When expanded, a row of the fingers or projections is positioned through a row of openings on the opposite edge is of the stent for locking the expanded configuration of the stent. A problem with the use of the flat metal sheet stent is that the overlapping edges of the stent increase the stent profile. Again, the stent profile and endothelial growth reduce the effective blood vessel lumen. Another problem with the use of the flat metal sheet stent is that the fingers or projections along one edge of the stent make wire-to-wire contact with the opposite edge of the stent. As a result, the metal edges of the stent rub during movement caused by blood flow, pulsation, and muscle movement. Yet another problem with the use of the flat metal sheet stent is that the fingers or projections extend radially outwardly and into the vessel wall. As a result, the intimal layer of the vessel wall is scraped, punctured, or otherwise injured. Injury and trauma to the intimal layer of the vessel wall result in hyperplasia and cell proliferation, which in turn effect stenosis or further narrowing of the vessel at the stent site. 
     SUMMARY OF THE INVENTION 
     The foregoing problems are solved and a technical advance is achieved in an illustrative embodiment of a flexible stent comprising a waveform pattern that is formed from a sheet of malleable, biocompatible material having a specified uniform thickness. The pattern is formed into a tubular shape and into an overlapping state around a delivery catheter balloon for introduction through tortuous vessels to, for example, an occlusion site in a coronary vessel. To provide longitudinal flexibility while preventing longitudinal contraction or expansion of the stent during radial expansion of the stent, the pattern advantageously includes a reinforcing member extending longitudinally therealong. A plurality of cells extends laterally from the reinforcing member with selected of the closed cells each having a fixedly sized aperture therein. The closed cells are interposed when the stent is in the tubular shape. To minimize the thickness of the stent and the growth of endothelial cells therearound, each segment of the cells extends laterally from the reinforcing member and does not overlap itself or any adjacent laterally extending segment of the cells. The sheet of biocompatible material with the pattern formed therein is formed into a radially alterable tubular shape around a delivery catheter balloon for introduction to the occlusion site. The balloon radially expands the stent to engage the vessel wall surface and to maintain the vessel lumen in an open condition. The expanded stent in a nonoverlapping state advantageously has a minimal thickness for endothelial tissue to form thereover. As a result, the vessel lumen is advantageously maintained with the largest diameter possible. 
     The pattern of the stent when in the tubular shape includes an overlapping state in which at least one segment of the selected cells overlaps the reinforcing member and forms a combined thickness with and along the reinforcing member of no more than substantially twice the thickness of the sheet of material. A deflated, delivery catheter balloon is positioned within the tubular-shaped stent to radially expand the stent to a nonoverlapping, expanded state when positioned at the occlusion site. The outermost longitudinal edges of the tubular stent move radially and circumferentially relative to each other when the stent is being radially altered. These outermost edges advantageously engage the surface of the lumen wall to maintain the stent in the expanded state. These outermost edges are most evident on the curved end segments of the interposed cells of the pattern when in the tubular shape. To aid expansion of the stent with the delivery balloon, the stent surface material is treated to lower its coefficient of friction. In one instance, the treatment comprises a coating of parylene on the surface of the sheet of material. Other coating materials include polytetrafluoroethylene. Furthermore, the surface of the stent may be ion beam bombarded to advantageously change the surface energy density and the coefficient of friction. 
     To maintain the moment of inertia or stiffness of the stent, each segment of the cells has a width substantially greater than the specified thickness of the sheet material. Increasing the width of the laterally extending segments also increases the surface area of the stent and support of the vessel wall. 
     To increase the expansion ratio of the stent, the laterally extending cells may be formed around the reinforcing member more than once and within the aperture of a closed cell without each segment overlapping itself or any adjacent cell segment. The width of the cell along the reinforcing member is advantageously selected so that each laterally extending segment forms a predetermined angle so as not to overlap itself or any adjacent cell segment. This is to advantageously maintain a combined thickness with and along the reinforcing member of no more than substantially twice the thickness of the sheet of material. 
     Radiopaque markers are advantageously positioned at one or more ends of the waveform pattern to aid the physician in positioning the stent at the occlusion site. 
     The method of making the balloon expandable stent includes the steps of providing a sheet of malleable material having an initial surface area and removing a majority of the material so that the sheet becomes a framework of integrated support members having a small surface area relative to the initial surface area of the sheet of material.. The method also includes positioning the framework around a cylindrical mandrel so that the framework defines at least a partially cylindrical surface or tubular shape. The removing step also includes removing isolated portions of the sheet so that the framework includes a plurality of closed cells bounded by the integrated support members. The removing step is also carried out so that the framework has a fixed length despite a reduction or expansion of the radius of the cylindrical surface or tubular shape. The cylindrical surface or tubular shape has a longitudinal axis and a substantially circular cross-section. The removing step is carried out so that the cylindrical surface or tubular shape is sufficiently flexible about the longitudinal axis to adapt the stent to curved passages within a body vessel without significantly altering the circular cross-section. 
     The stent of the present invention may also be characterized as a sheet of malleable material which has had a portion of the material removed so that the sheet becomes a framework of integrated support members arranged around a longitudinal axis to define a cylindrical surface. The cylindrical surface has a reduced diameter for delivery of the stent into a passage within a body vessel. The cylindrical surface is also plastically expandable from the reduced diameter to an expanded diameter for holding the passage open. The cylindrical surface has a range of diameters between the reduced diameter and the expanded diameter that are free from overlapping material. Each of the support members of the stent has a width and a thickness significantly less than the width. The support members are integrated in a way that the framework maintains a fixed length when the cylindrical surface is expanded from the reduced to the expanded diameter. One of the support members is a reinforcing member that extends from a first to a second end of the stent. The remaining support members extend laterally on each side of the reinforcing member. The cylindrical surface of the stent also defines a cylindrical surface when expanded to the expanded diameter. In addition, the cylindrical surface is sufficiently flexible about the longitudinal axis so that the stent can advantageously adapt to curved passages within a body vessel without significantly altering its circular cross section. The framework of the stent also includes a plurality of closed cells bounded on all sides by the integrated support members. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  depicts a pictorial view of the preferred illustrative embodiment of an unmounted flexible stent of the present invention having a waveform pattern formed from a sheet of material into a tubular shape and an overlapping state; 
         FIG. 2  depicts a pictorial view of another illustrative embodiment of an unmounted flexible stent of the present invention in an overlapping state with segments of the closed cells crossing over the reinforcing member of the stent more than once; 
         FIG. 3  depicts a pictorial view of the stent of  FIG. 1  in an expanded, nonoverlapping state and positioned in a blood vessel; 
         FIG. 4  depicts a partially sectioned, longitudinal view of the stent of  FIG. 1  in an overlapping state positioned about a delivery catheter balloon and introduced to an occlusion site; 
         FIG. 5  depicts an enlarged, partial view of the stent of  FIG. 4  in an overlapping state; 
         FIG. 6  depicts a cross-sectional view of the stent of  FIG. 5  taken along the line  6 — 6 ; 
         FIG. 7  depicts an enlarged, partial view of the stent of  FIG. 4  in a partially expanded state; 
         FIG. 8  depicts a cross-sectional view of the stent of  FIG. 7  taken along the line  8 — 8 ; 
         FIG. 9  depicts a partially sectioned, pictorial view of the stent of  FIG. 1  in a flat configuration as formed from a sheet of malleable material; 
         FIG. 10  depicts a cross-sectional view of a segment of the stent of  FIG. 9  taken along the line  10 — 10 ; 
         FIGS. 11-19  depict alternative embodiments of the stent of the present invention with different waveform patterns formed in a sheet of material; and 
         FIGS. 20 and 21  depict the method of forming the stent of  FIG. 1  into a tubular shape and around a delivery catheter balloon. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  depicts a pictorial view of a preferred illustrative embodiment of unmounted flexible stent  10  in an overlapping state and comprising a waveform pattern  13  formed from a flat sheet of biocompatible material and into a tubular shape  12 . The tubular shaped pattern is expanded with a delivery catheter balloon in a body passage such as a blood vessel to maintain an open lumen therethrough. As depicted in  FIG. 9 , pattern  13  is formed from a flat sheet  11  of malleable, biocompatible material such as stainless steel by, for example, photochemically removing a majority of the sheet material and leaving a framework of integrated support members that has a small surface area relative to the initial surface area of the sheet. After formation from the sheet, the pattern is formed into a partially cylindrical or U-shaped surface around a cylindrical mandrel and then positioned in the overlapping state around a deflated, delivery catheter balloon and into tubular shape  12 . 
     Pattern  13  includes a reinforcing member  14  extending longitudinally between opposite ends  15  and  16  for providing longitudinal stability thereof, particularly during radial expansion of the stent in the body passage. The length of prior art stents that are formed from a tube typically shorten as the stent is radially expanded. When formed into a tubular shape, pattern  13  includes a plurality of interposed closed cells  17 - 19  that extend laterally from the reinforcing member for providing vessel wall support. The tubular shaped pattern also exhibits longitudinal flexibility for introducing the stent through tortuous vessels to, for example, a coronary artery. Unlike a wire stent in which a wire is bent into a waveform pattern, waveform pattern  13  is formed from a flat sheet of material without any stresses being introduced at the curved segments thereof. As a result, thickness  23  of stent  10 , as well as sheet  11 , can be made extremely thin in comparison to that of a wire stent to minimize endothelial tissue buildup in the vessel. The same well-known moment of inertia or stillness of a wire stent is maintained by adjusting the width of each pattern member segment for a given sheet thickness. 
     When stent  10  is positioned in the overlapping state around a deflated catheter balloon, any segment of a laterally extending cell that overlaps the reinforcing member only forms a combined thickness  29  with and along the reinforcing member of no more than substantially twice the thickness of the sheet material. Accordingly, the stent is formed with an extremely small outside diameter while maximizing the inside diameter of the stent for receiving the delivery catheter balloon. When the stent is expanded in a blood vessel, the tubular shape is radially altered from a reduced, minimal inside diameter to an expanded diameter for holding the blood vessel open. In addition, oppositely facing, outermost longitudinal edges  20  and  21  of the pattern move radially and circumferentially relative to each other so that the pattern in the sheet of material is not stressed or deformed in the plane of the sheet of material. The substantially cylindrical surface of the tubular shape has a range of diameters between the expanded diameter and the reduced diameter for holding the vessel open in which the interposed cells are free of overlapping sheet material. The pattern is integrated in such a way that the framework thereof maintains a fixed length when the cylindrical surface of the tubular shape is expanded from the reduced diameter of the overlapping state to a larger diameter. 
     Interposed cell  17  includes fixedly sized aperture  22  with segments  24 - 26  and reinforcing member  14  disposed around the aperture. Substantially straight segments  24  and  25  extend laterally from reinforcing member  14 , and curved segment  26  interconnects straight segments  24  and  25 . Straight segments  24  and  25  are positioned circumferentially around the tubular shape in the space of aperture  22  without overlapping themselves or other adjacent cell segments for minimizing the stent profile or thickness in the overlapping state. Straight segments  24  and  25  extend laterally from the reinforcing member at angles  27  and  28 , for example, both slightly acute at  82 - 83  degrees. Therefore, as depicted in  FIG. 1 , the straight segments cross over reinforcing member  14  but do not overlap themselves or each other. Furthermore, interposed cell  17  is separated from adjacent cells so that the segments of other cells do not overlap segments of cell  17 . The width of the segments is greater than the thickness of the segments, or the sheet of biocompatible material, to maintain an acceptable moment of inertia and to increase the effective vessel wall support area. For example, segments  24 - 26  are approximately 0.014″ wide and 0.005″ thick. 
       FIG. 2  depicts a pictorial view of another illustrative embodiment of stent  10  in an overlapping state with laterally extending segments  24 - 26  wrapped into tubular shape  12  and into the space of aperture  22 . However, straight segments  24  and  25  cross over reinforcing member  14  at least two times in the overlapping state. Combined thickness  29  of the overlapping state stent with and along the reinforcing member is still no more than the thickness of straight segment  24  and reinforcing member  14 , or no more than substantially twice the thickness of the sheet of biocompatible material. Although, straight segments  24  and  25  cross over the reinforcing member more than once, the segments do not overlap themselves or any segments of adjacent cells. The expansion ratio of stent  10  from being in an overlapping state with a reduced or minimal diameter to an expanded, nonoverlapping state with an expanded diameter for holding a body vessel open can be varied by appropriately selecting the width of the cell along the reinforcing member and the height or extension of the cell from the reinforcing member. After the desired moment of inertia is selected for straight segments  24  and  25 , the width of each segment is calculated based on the thickness of the sheet material. With a desired expansion ratio, the reduced and expanded diameters along with the circumference of the expanded stent are calculated. From this, the number of times the straight segments must cross the reinforcing member is determined. The width and height of each cell is derived based on the width of each segment and the desired spacing to ensure that the segments do not overlap themselves or segments from adjacent cells. With this determination, angles  27  and  28  that segments  24  and  25  make with reinforcing member  14  will vary. On a more practical basis, the expanded and reduced diameters along with the sheet material thickness are more commonly selected to determine the remaining parameters of the cells. 
       FIG. 3  depicts a pictorial view of stent  10  of  FIG. 1  positioned in blood vessel  30 . The laterally extending segments of alternatingly interposed cells  17 - 19  support vessel wall  31 . Cells  17 - 19  are interposed about respective curved segments  26 ,  32 , and  33  thereof for providing an expanded tubular shape or a complete cylindrical surface  34 . Upon inflation of the ballon of a balloon catheter, the stent is expanded with oppositely facing, outermost longitudinal edges  20  and  21  moving radially outward and circumferentially toward each other. Curved segments  26  and  33 , each with edge  20 , and interposed curved segment  32  with oppositely facing edge  21 , also move radially, and circumferentially apart. However, the interposed cells are urged circumferentially together under the closing force of the vessel wall when the balloon is deflated. When the interposed cells move together, oppositely facing edges  20  and  21  of curved segments  26  and  33  and interposed curved segment  32  engage vessel wall  31  and securely lodge the stent in an expanded state. 
       FIG. 4  depicts a partially sectioned, longitudinal view of stent  10  of  FIG. 1  in an overlapping state positioned about balloon  35  of delivery catheter  36  and introduced to a partially occluded region  37  of blood vessel  30 . Well-known side arms  118  and  119  of catheter  36  are provided with lumens extending through the catheter for inserting the catheter over a wire guide and for inflating balloon  35 . 
       FIG. 5  depicts an enlarged, partial view of stent  10  of  FIG. 4  in an overlapping state with interposed cells  17  and  18  positioned about balloon  35 . Straight segments  24  and  25  of cell  17  and the straight segments of cell  18  overlap reinforcing member  14 . 
       FIG. 6  depicts a cross-sectional view of stent  10  of  FIG. 5  positioned around delivery catheter balloon  35  taken along the line  6 — 6 . The overlapping state stent has an outside diameter of approximately 0.055″, and the deflated balloon has an outside diameter of approximately 0.039″. Combined thickness  29  of segment  24  and member  14  is no more than substantially twice the thickness of the sheet material. 
       FIG. 7  depicts an enlarged, partial view of stent  10  of  FIG. 4  in a partially expanded, but still overlapping state positioned about partially inflated balloon  35 . Stent  10  has a 3 mm inside diameter when fully expanded, and balloon  35  has an outside diameter of approximately 3.5 mm when fully expanded. Pattern  13  with interposed cells  17  and  18  and respective fixedly sized apertures  22  and  47  remain stable while oppositely facing, outermost edges  20  and  21  move radially outward and circumferentially toward each other for expanding the tubular shape of the stent. Pattern  13  is not longitudinally shortened during positioning or expansion of the stent. 
       FIG. 8  depicts a cross-sectional view of stent  10  of  FIG. 7  positioned around inflated delivery catheter balloon  35  taken along the line  8 — 8 . 
       FIG. 9  depicts a partially sectioned, pictorial view of stent  10  of  FIG. 1  in a flat configuration as formed from sheet  11  of malleable, biocompatible material  39 . Biocompatible material  39  is preferably a commercially available, malleable material such as Series 316L (low carbon) stainless steel that is typically annealed for minimizing the tendency to recoil once the stent is expanded by an inflated balloon. Biocompatible material  39  commonly has a surface with a coefficient of friction capable of holding an unexpanded stent lightly around a balloon, thereby occasionally inhibiting the ability to expand the balloon and stent. Therefore, the biocompatible material of the stent preferably includes a surface treatment for lowering the coefficient of friction such as coating  40  formed of, for example, parylene material, as depicted in FIG.  10 . Parylene material is a polymer used in coating pacemaker leads and is commercially available from Specialty Coating Systems of Union Carbide in Clear Lake, Wis. Coating  40  alternatively comprises polytetrafluoroethylene or another antithrombotic material. The surface and the surface energy density of the material can also be changed by ion beam bombardment, which is commercially available from the spire Corporation of Bedford, Mass. 
     Pattern  13  is formed from sheet  11  of biocompatible material by photochemically etching, stamping, laser cutting, or any other of a number of well-known methods. Forming pattern  13  in a thin sheet of material provides a stent with an increased vessel contact surface area without increasing the metal mass of the stent, which is a limitation of prior art wire stents. A stent with an expanded inside diameter of 3 mm, for example, is formed from a sheet of material approximately 0.371″ wide, 0.7865″ long, and 0.003″ thick. Reinforcing member  14  and straight segments  24  and  25  are approximately 0.012″ wide. Curved segment  26  has a 0.010″ inside radius along oppositely facing, outermost edges  20  and  21 . Reinforcing member  14  is positioned along the centerline of pattern  13  and has a .005″ radius at intersections with the straight segments of the interposed cells. Centerlines through apertures  22  and  50  transverse to the reinforcing member are positioned 0.143″ apart. Opposite stent ends  15  and  16  have a 0.017″ radius formed thereon defining projections that extend therefrom to free ends to define the furthest extents of the unopposed proximal and distal ends of the stent, and the projections include openings (see  FIG. 3 ) through the thickness of the sheet that define closed rings therearound or eyelets for positioning radiopaque markers  41  and  42  therein or fixedly attaching them thereto. Radiopaque markers  41  and  42  are affixed by placing a 0.010″ diameter piece of radiopaque material such as gold, platinum, tungsten, or iridium in the eyelet and heating the material to melt it in place so that the material extends across the oepnings of the eyelets, as in FIG.  1 . Alternatively, radiopaque markers are positioned in the eyelets by crimping or any other well-known fastening method. 
       FIG. 10  depicts a cross-sectional view of straight segment  25  of stent  10  of  FIG. 9  taken along the line  10 — 10 . Straight  25  segment has a rectangular cross-section and includes outer surface treatment  40  for decreasing the coefficient of friction on the stent surface. The coated segment is approximately 0.005″ thick and 0.014″ wide. 
     A method of making balloon expandable stent  10  includes providing a sheet  11  of material with an initial surface area  43  and removing a majority  44  of the sheet material so that the remaining sheet  49  becomes a framework of integrated support members such as waveform pattern  13  having a small surface area relative to the initial surface area. The method also includes positioning the framework around a cylindrical mandrel so that the framework defines a cylindrical surface  34  or tubular shape  12 . The cylindrical surface has a radius that can be expanded or reduced; however, the length of the surface and stent remains fixed despite a reduction or expansion of the radius. Cylindrical surface  34  also has a longitudinal axis and a substantially circular cross section. The surface is sufficiently flexible about the longitudinal axis so that the stent can adapt to curved passages within a body vessel without significantly altering the circular cross section. By way of example, the material removed from sheet  11  to form stent  10  includes isolated portions  45  and  46  resulting in respective apertures  22  and  47  for providing respective closed cells  17  and  18 , each bounded by integrated support members such as straight segments  24  and  25 , interconnecting curved segment  26 , and reinforcing member  14 . The stent framework is formed into at least a partially cylindrical or U-shaped surface with, for examples cylindrical mandrel  38  and U-shaped form  48  as depicted in  FIGS. 20 and 21 . The stent is then formed into cylindrical surface  34  or tubular shape  12  around a delivery catheter balloon with the aid of form  48 . 
       FIGS. 20 and 21  depict U-shaped form  48  as a flat plate having a straight U-shaped channel  120  formed therein. In the cross-sectional view of  FIG. 21 , it is seen that channel  120  has a U-shaped or semi-circular surface. After stent  10  is formed from a sheet of biocompatible material and into the framework or waveform pattern  13 , the stent is placed flat upon the plate such that reinforcing member  14  is coincident with the center line of the channel. Stent  10  in its flat configuration is then pressed into U-shaped channel  120  and against the semi-circular surface with cylindrical mandrel  38 . Closed interposed cells such as  17 - 19  extend out of the channel and away from the cylindrical mandrel as 
       5  shown. The cylindrical mandrel is removed, and a delivery catheter balloon inserted in the U-shaped stent. The stent is then formed into cylindrical surface  34  or tubular shape  12  around the delivery catheter balloon with the aid of U-shaped form  48  forming the U-shaped interposed cells around the balloon and into the cylindrical surface or tubular shape. Well-known pulling tools with hooks at the ends thereof can be used to engage the interposed cells to pull the cells tightly around the delivery catheter balloon. These pulling tools are disclosed with a very similar forming method in U.S. Pat. No. 4,907,336 of Gianturco, which is incorporated by reference herein. 
       FIGS. 11-19  each depict a partial, longitudinal view of an alternative embodiment of the stent of the present invention in a flat configuration and with a different waveform pattern formed from a sheet of material. 
       FIG. 11  depicts stent  51  with herringbone pattern  52  formed therein. Substantially straight segments  53  and  54  extend laterally from reinforcing member  55  at angles  56  and  57 , both approximately 82-83 degrees, so that when the stent assumes a tubular shape, the segments overlap only the reinforcing member. 
       FIG. 12  depicts closed cell  58  of another embodiment of the stent of the present invention with mirror image curved segments  59  and  60  interconnected by curved segment  61 . In a different configuration, closed cell  62  with mirror image curved segments  63  and  64  are interconnected by bulbous curved segment  65 . In yet another configuration, closed cell  66  with mirror image curved segments  67  and  68  are interconnected by acutely curved segment  69 . A pattern of any one or more of these closed cells can be formed from a sheet of biocompatible material and interposed to provide a stent of the present invention wherein the segments of the cells overlap only the reinforcing member when the stent is in an unexpanded, overlapping state. 
       FIG. 13  depicts stent  70 , which is still another embodiment of the present invention, including waveform pattern  71  with a discontinuous longitudinal reinforcing member  72 . Pattern  71  provides increased longitudinal flexibility about segments  73  and  74 , which are not bounded or closed by reinforcing member  72 . 
       FIG. 14  depicts stent  75 , which is yet another embodiment of the present invention, including waveform pattern  76  with curved segment  77  wider than straight segments  78  and  79 . The additional width and surface area of the curved segments provide for increased support of a vessel wall. 
       FIG. 15  depicts stent  80 , which is still yet another embodiment of the present invention, with waveform pattern  81  including transverse steps  82  and  83  along the longitudinal axis of the stent. The transverse steps provide for uniform longitudinal flexibility around the circumference of the stent. The transverse steps are positioned after every pair of two interposed cells. Step  82  is positioned after cells  84  and  85 , and step  83  is positioned after cells  86  and  87 . Reinforcing member  88  extends longitudinally along the stent coincidentally with the transverse steps. 
       FIG. 16  depicts stent  89 , which is another embodiment of the present invention, with waveform pattern  91  including reinforcing member  92  that is gradually angled with respect to the longitudinal axis of the stent. When stents  80  and  89  assume their expanded, tubular shapes, the interposed curved segments are positioned in a spiral about the circumference of the tubular shapes. This configuration also provides for more uniform longitudinal flexibility of the stent about the circumference of the stent. 
       FIG. 17  depicts stent  92 , which is yet another embodiment of the present invention, including pattern  93  without any curved segments interconnecting straight segments  94 - 96 . 
       FIG. 18  depicts stent  97 , which is still another embodiment of the present invention, with waveform pattern  98  including generally egg-shaped aperture such as  99  in a closed cell and reinforcing member  100  with a repeating curvature formed therein. Pattern  98  provides increased surface area for supporting a vessel wall. Stent  97  is formed of a sheet of biocompatible material approximately 0.372″ wide, 0.955″ long, and 0.003″ thick. Curved segment  101  has a 0.021″ radius along oppositely facing, outermost edges  102  and  103 . Opposite stent ends  104  and  105  extend 0.0205″ from the centerline of the stent, and each has a 0.041″ radius formed thereon. Egg-shaped aperture  99  has approximately a 0.011″ radius and a 0.020″ radius with a length of 0.086″ extending between the centers of the two radii and a length of 0.059″ extending from the centerline of the stent and the center of the largest radius. A transverse centerline through aperture  99  is positioned 0.1658″ from the transverse centerline through aperture  106 . 
       FIG. 19  depicts stent  107 , which is still yet another embodiment of the present invention, with waveform pattern  108  including increasing width reinforcing members  109 - 111  positioned along oppositely facing, outermost stent edges  112 . Stent  107  is formed of a sheet of biocompatible material approximately 0.381″ wide, 1.026″ long, and 0.003″ thick. Curved segment  113  has a 0.010″ radius along outermost edge  112 . Reinforcing members  109 - 111  are positioned 0.033″ from outermost edges  112  and have a 0.005″ radius at intersections with the curved and straight segments. Opposite stent ends  114  and  115  have a 0.006″ radius formed thereon. Segments  116  and  117  are approximately 0.012″ wide. Reinforcing members  109 - 111  gradually increase in width from 0.006″ at the center of the stent to 0.018″ at the opposite ends. 
     It is to be understood that the above-described stent is merely an illustrative embodiment of the principles of this invention and that other stents may be devised by those skilled in the art without departing from the spirit and scope of this invention. It is contemplated that any overlapping state stent formed from a sheet of material to minimize endothelial tissue growth is within the spirit and scope of this invention. Any equivalent shape of the waveform as illustrated by the preferred and alternative embodiments of the stent is also contemplated.