Abstract:
Various intravascular stents, such as intracoronary stents, include improved expansion and connecting strut designs. Such stents can be both very flexible and fully cover vessel surface inside the vascular lumen, and be well designed for both the delivery phase and the deployed phase of the stent life cycle.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims the benefit of U.S. Provisional Application 60/235,115, filed Sep. 25, 2000, which is hereby incorporated by reference in its entirety. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of Invention  
           [0003]    This invention relates to intravascular stents in general, and more particularly to intracoronary stents.  
           [0004]    2. Description of the Related Art  
           [0005]    Intracoronary stents provide intraluminal scaffolding support of the vascular wall after percutaneous angioplasty in which the balloon catheter is used to expand the stenotic vascular lesion. In both the delivery phase and the deployed phase, there are numerous performance factors that can characterize the overall clinical performance of a stent and can be improved.  
           [0006]    By the year 2000, the percutaneous balloon angioplasty and stent implant procedures have become the dominant non-surgical revascularization method of the atherosclerotic stenosis, or obstruction, of the vascular lumen, and particularly in the coronary vascular system of the heart. With balloon angioplasty alone and without stents, the restenosis rate after angioplasty has been as high as 25-45% in the first time coronary cases. With stents after balloon angioplasty, the restenosis rate has been reduced significantly. Even so, the restenosis rate after stent implantation is reported to be 15-25% range in coronary arteries, depending on the condition of the stented vessel or the specific stent. An ideal coronary stent is still elusive in the current state of the art commercial products.  
           [0007]    Some of the best selling current, second generation, stents can be divided into two categories. One category is a stent with high flexibility and the other category has full vessel coverage. The flexible stents generally have poor vessel coverage, tissue prolapse, rough surface modulation and increased restenosis rate. On the other hand, a stent with good vessel coverage in the current state of art may not be flexible enough for easy delivery and for highly efficient procedures. This means that a stent with good flexibility and good vessel coverage remains as the unreached gold standard.  
           [0008]    To further reduce the restenosis rate after stent implant, numerous means have been tried including laser, atherectomy, high frequency ultrasound, radiation device, local drug delivery, etc. Although the brachytherapy (radiation treatment) has proved to be reasonably effective in further reducing restenosis after stent implant, using brachytherpy is very cumbersome, inconvenient, and costly. Brachytherapy is a radioactive device and a radiation therapy specialist from another department has to be involved with the interventional cardiologist in the cardiac catheterization laboratory. The laser and atherectomy devices proved to be marginally useful with added costs.  
           [0009]    Local drug therapy appears to be a very promising method for the future, as better pharmaceutical, chemical, or biogenetic agents are developed and became available. Some research data, both from animal tests and human clinical studies, indicate evidence of some suppression of restenosis after stent implantation when certain growth blocking pharmaceutical agents coat the stent. In other instances, it has been speculated that certain surface modifying materials coated on the surface of the stent may be beneficial, alone or in combination with growth suppressing agents, in reducing the restenosis rate. In either instance, a drug or substance should be locally attached or coated on the stent in sufficient amounts. However, attaching or coating a sufficient amount of a substance or drug on the coronary stent may not be an easy proposition, because coating enough volume of the drug on the small surface area of a stent is a challenging task. If and when stent coating becomes practical, a good stent can still have better outcomes than a poorly designed stent when used with substance coating.  
           [0010]    A stent is a scaffolding device. When delivered to a remote vessel location via percutaneous approach it can be deployed by expanding the device inside a vessel. The vessel can have a very small caliber and sometimes has a very tortuous anatomy. When a stent is deployed, the stent should have a good radial strength, a good vessel coverage, a good internal surface modulation without tulips (i.e., sharp metal loop projections that resemble fish scale phenomena), an optimal vessel conformability, a low metal fraction, and so forth. If the stent is stiff and non-flexible, it can be very difficult to deliver to an intended lesion site inside a vessel. Easy delivery of a stent is aided by good flexibility of the stent in combination with the delivery balloon, a smooth surface modulation without or minimizing tulips and a degree of radiopacity. A good stent should have a combination of features for delivery and deployment.  
           [0011]    Although there are countless variations of vascular stent designs today, few have these desired stent features both in the delivery phase and in the postdelivery delivery phase. Today&#39;s top selling stents in the market can have undesirable characteristics, either in the delivery phase or in the deployed phase of the stent life cycle. For example, some stents may have flexibility, but lack vessel coverage or surface modulations both in delivery and deployed phases. Some stents may have good vessel coverage and surface modulations, but lack flexibility.  
           [0012]    Vascular stents, which are designed to be delivered to vessel sites via percutaneous approach, can have two elements. The first element is the expansion strut that expands circumferentially to provide the scaffolding radial force against a possible collapsing force of the vessel wall. The second element is the connecting strut that can link the expansion struts along the longitudinal axis of the stent, giving articulation or flexibility to the stent. The particular combination of expansion struts and connecting struts generally form various cells, depending on the specific configuration and shape of the expansion and connecting struts. If a cell is too large, the vessel wall support or coverage can be poor and the vessel wall tissue can prolapse through the large cells of the stent net. If the cells are too small, the vessel wall may be well covered but the metal fraction of the stent can be too high. The metal fraction is a fraction of the total metal surface area of an expanded stent (inside a blood vessel) divided by the total internal vessel wall surface area where the stent is deployed.  
           [0013]    Some very flexible stents have very large cell size with poor vessel coverage and tissue prolapse, in addition to poor (inner and/or outer) surface modulation due to large numbers of tulips directed to both ends of the stent. Most of the current flexible stents are designed to effect flexibility by using fewer or a minimal number of connecting struts, handicapping the vessel coverage, surface modulation and tissue prolapse defects.  
           [0014]    On the other hand, a stent that is designed for good vessel coverage and ideal cell size tends to be inflexible when such a stent is being delivered to a vessel lesion. A lack of flexibility during stent delivery is a very critical issue; a stiff stent often cannot be delivered to a needed location inside a blood vessel because such a stent cannot navigate through a tortuous and small vessel lumen.  
           [0015]    There is a need for a vascular stent that is very flexible for delivery and with good vessel coverage when deployed.  
         SUMMARY OF THE INVENTION  
         [0016]    Various embodiments of a stent include a combination of maximum possible flexibility and conformability in the stent, full vessel coverage with optimal metal fraction, evenly expanding stent struts, excellent radial strength and radiopacity, and smooth surface modulations in both delivery and deployed phases of the stent life cycle. To arrive at these goals, many detailed new innovations are added to the expansion and connecting strut designs of the stent. Expansion strut design is largely responsible for radial strength and radiopacity, while connecting strut design is largely responsible for flexibility and smooth surface modulations. Full vessel coverage and uniform stent expansion are largely from interaction between expansion and connecting struts. Various embodiments of the stent demonstrate a balance among these multiple qualities, using smart expansion struts and flexible connecting struts in a seamlessly integrated stent network.  
           [0017]    Various embodiments of the stent are specifically designed to be both very flexible and fully cover vessel surface inside the vascular lumen. The stent can have both characteristics of vessel coverage and flexibility, particularly for coronary use.  
           [0018]    Various embodiments of a stent are well designed for both the delivery phase and the deployed phase of the stent life cycle. Both flexibility and good vessel coverage are in a right balance in various embodiments of the stent have. Various embodiments of the stent include certain configurations in expansion and connecting struts of the stent.  
           [0019]    One embodiment includes a first expansion column, a second expansion column, and a first connecting strut column. The first expansion column and the second expansion column can include individual expansion struts. The individual expansion struts can form a plurality of expansion strut pairs. Two adjacent expansion strut pairs share a common strut. The first connecting strut column can include a plurality of individual connecting struts. The plurality of individual connecting struts can couple the first and second expansion columns. Each individual connecting strut can include at least six pivot points.  
           [0020]    One embodiment includes a first expansion column, a second expansion column, and a first connecting strut column. The first expansion column and the second expansion column can include individual stair-step expansion struts. The individual stair-step expansion struts can form a plurality of expansion strut pair loops. Two adjacent expansion strut pair loops share a common stair-step expansion strut. The first connecting strut column can include a plurality of individual symmetrical geometry connecting struts. Each of an end of an individual connecting strut can extend ipsilaterally from sides of expansion strut pair loops of the first and second expansion columns.  
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0021]    [0021]FIG. 1 shows a side elevation view of an embodiment of a stent, such as a tubular stent.  
         [0022]    [0022]FIG. 2 shows an isometric view of an embodiment of a stent, such as a tubular stent.  
         [0023]    [0023]FIG. 3 shows a cut-open view of an embodiment of a stent. Various expansion columns and connecting strut columns are shown.  
         [0024]    [0024]FIG. 4A shows a magnified view of a middle section of an embodiment of a stent, such as a stent of FIGS. 1, 2, and/or  3 . Some details are shown of expansion columns.  
         [0025]    [0025]FIG. 4B shows a magnified view of a middle section of an embodiment of a stent, such as a stent of FIGS. 1, 2, and/or  3 . Some details are shown of connecting strut columns conjoined with expansion columns.  
         [0026]    [0026]FIG. 5A shows a magnified view of an end section of one embodiment of a stent, such as a stent of FIGS. 1, 2, and/or  3 . Some details are shown of an end expansion column.  
         [0027]    [0027]FIG. 5B shows a magnified view of an end section of one embodiment of a stent, such as a stent of FIGS. 1, 2, and/or  3 . of present invention in a two-dimensional view. Some details are shown of different connecting strut columns conjoined with expansion columns.  
     
    
     DETAILED DESCRIPTION  
       [0028]    Some embodiments of stents can be in a state, such as one or more of a non-expanded state, an expanded state, a crimped state, and a non-crimped state.  
         [0029]    Some embodiments of stents can include one or more of a first expansion column, a second expansion column, a third expansion column, a first connecting strut column, and a second connecting strut column.  
         [0030]    The first expansion column, the second expansion column, and/or the third expansion column can include individual expansion struts forming a plurality of expansion strut pairs. FIG. 4B shows examples of individual expansion struts  50  and expansion strut pairs  51 . In some embodiments of the stent, one expansion strut of an expansion strut pair can have a stair-step segment at a proximal end and the other expansion strut of the expansion strut pair can have a stair-step segment at a distal end. FIG. 4B shows examples of one expansion strut  53  of an expansion strut pair  51  having a stair-step segment at a proximal end and the other expansion strut  55  of the expansion strut pair  51  having a stair-step segment at a distal end. In some embodiments of the stent, two adjacent expansion strut pairs share a common strut.  
         [0031]    The first connecting column and/or the second connecting column can include a plurality of individual connecting struts. The plurality of individual connecting struts for at least the first connecting column can couple the first and second expansion columns. In various embodiments of the stent, each connecting strut includes one or more of: at least some number of pivot points, at least some number of sections, bilateral short stems, a geometrical configuration, at least some number of radii of curvature, a center section, and a longitudinal axis. For example, each connecting strut can include at least six pivot points; at least four, five, six, or seven sections; and/or at least three, four, five, or six radii of curvature. FIG. 4B shows examples  114 ,  116 ,  118 ,  120 ,  122 , and  124  of pivot points each having some radius of curvature. Bilateral short stems can be ipsilaterally conjoined to an expansion strut of the first expansion column and to an expansion strut of the second expansion column. FIG. 4B shows examples of bilateral stems  100  and  102  on proximal and distal ends respectively of a connecting strut. Exemplary geometrical configurations include a symmetrical one and/or a quasi M-frame one. FIG. 4A shows examples of connecting struts  90  and  92  which have both a symmetrical geometrical configuration and a quasi M-frame geometrical configuration. The longitudinal axis can be non-perpendicular to a longitudinal axis of the stent, substantially perpendicular to a longitudinal axis of the stent, and/or substantially parallel to a longitudinal axis of the stent. FIGS. 1, 2, and  3  show examples of a longitudinal axis  26  of a stent  10 . Examples of longitudinal axes of connecting struts are shown in FIG. 4B as axes  94  and  96 . Other examples of axes are in FIG. 4A shown as axes  82 ,  84 , and  86  and in FIG. 5A as axes  80 ,  82 ,  88 , and  89 .  
         [0032]    In some embodiments of the stent, each connecting strut can be invaginated and/or inverted into a connector space between expansion strut pairs between the first and second expansion columns. FIG. 4A shows examples of connecting struts  90  and  92  which are invaginated and inverted into the connector space between expansion strut pairs.  
         [0033]    In some embodiments of the stent, each connecting strut can be ipsilaterally coupled to an expansion strut of the first expansion column and to an expansion strut of the second expansion column. FIG. 4A shows examples of connecting struts  90  and  92  which are ipsilaterally coupled between expansion struts of different expansion columns.  
         [0034]    In various embodiments of the stent, at least one connecting strut has a geometric configuration, such as an asymmetrical geometric configuration and/or a quasi M-frame geometric configuration.  
         [0035]    In some embodiments of the stent the center section can have a substantially truncated conical configuration. In some embodiments of the stent all center sections of the connecting struts extend in a first direction. In some embodiments of the stent the center section of at least a portion of the connecting struts extend in a first direction and/or extend in a second direction. FIG. 5A and 5B show examples of a connecting strut column  130  having center sections that extend in a first direction, and a connecting strut column  134  having center sections that extend in a first direction and a second direction.  
         [0036]    Some embodiments of the stent include a plurality of expansion columns. The plurality of expansion columns can be coupled by a plurality of connecting strut columns. Each connecting strut can have a longitudinal axis. In some embodiments of the stent, substantially every longitudinal axis of a connecting strut in a connecting column is parallel to the longitudinal axis of the connecting struts in that column. Each connecting strut can have, for example, at least six pivot points.  
         [0037]    Some embodiments of the stent include a first end expansion column and a second end expansion column. The first end expansion column and the second end expansion column can define a proximal and a distal end of the stent. The first end expansion column and the second end expansion column can be mirror images of each other.  
         [0038]    Some embodiments of the stent include a plurality of cells. Cells can have asymmetrical geometries ad/or symmetrical geometries. Some geometric shapes have a semi-hexagonal geometry in a nominally expanded state, such as within operating parameters. Cells can be defined by the first expansion column, the second expansion column, and the first connecting strut column. Cells can be defined by the second expansion column, the third expansion column and the second connecting strut column. Cells can have evenly spaced geometric shapes.  
         [0039]    Some embodiments of stents can include one or more of a first expansion column, a second expansion column, and a first connecting strut column.  
         [0040]    The first expansion column and/or the second expansion column can include individual stair-step expansion struts forming a plurality of expansion strut pair loops. In some embodiments of the stent, expansion strut pair loops couple adjacent individual expansion struts. In some embodiments of the stent, two adjacent expansion strut pair loops share a common stair-step expansion strut.  
         [0041]    The first connecting strut column can include a plurality of individual symmetrical geometry connecting struts. The plurality of individual symmetrical geometry connecting struts for at least the first connecting column can couple the first and second expansion columns. In various embodiments of the stent, each symmetrical geometry connecting strut includes one or more of: at least some number of pivot points, at least some number of sections, bilateral short stems, a geometrical configuration, at least some number of radii of curvature, a center section, and a longitudinal axis. For example, each symmetrical geometry connecting strut can include at least six pivot points; at least four, five, six, or seven sections; and/or at least three, four, five, or six radii of curvature. Bilateral short stems can be ipsilaterally conjoined to an expansion strut of the first expansion column and to an expansion strut of the second expansion column. Exemplary geometrical configurations include a symmetrical one and/or a quasi M-frame one. The longitudinal axis can be non-perpendicular to a longitudinal axis of the stent, substantially perpendicular to a longitudinal axis of the stent, and/or substantially parallel to a longitudinal axis of the stent.  
         [0042]    In some embodiments of the stent, each symmetrical geometry connecting strut can be invaginated and/or inserted into a connector space between expansion strut pair loops between the first and second expansion columns.  
         [0043]    In some embodiments of the stent, each symmetrical geometry connecting strut can be ipsilaterally coupled to an expansion strut of the first expansion column and to an expansion strut of the second expansion column.  
         [0044]    In various embodiments of the stent, at least one symmetrical geometry connecting strut has a geometric configuration, such as an asymmetrical geometric configuration and/or a quasi M-frame geometric configuration.  
         [0045]    In some embodiments of the stent the center section can have a substantially truncated conical configuration. In some embodiments of the stent all center sections of the symmetrical geometry connecting struts extend in a first direction. In some embodiments of the stent the center section of at least a portion of the symmetrical geometry connecting struts extend in a first direction and/or extend in a second direction.  
         [0046]    Some embodiments of the stent include a plurality of expansion columns. The plurality of expansion columns can be coupled by a plurality of connecting strut columns. Each symmetrical geometry connecting strut can have a longitudinal axis. In some embodiments of the stent, substantially every longitudinal axis of a symmetrical geometry connecting strut in a connecting column is parallel to the longitudinal axis of the symmetrical geometry connecting struts in that column. Each symmetrical geometry connecting strut can have, for example, at least six pivot points.  
         [0047]    Some embodiments of the stent include a first end expansion column and a second end expansion column. The first end expansion column and the second end expansion column can define a proximal and a distal end of the stent. The first end expansion column and the second end expansion column can be mirror images of each other.  
         [0048]    In some embodiments of the stent, individual expansion struts of the first and second expansion column can form a plurality of expansion strut pair loops that couple adjacent individual expansion strut pair loops. In some embodiments of the stent, adjacent individual expansion strut pair loops can be coupled in a symmetrical geometry.  
         [0049]    In some embodiments of the stent, expansion strut pair loops of the first and second expansion columns can be aligned. For example, the alignment can be in a peak-to-valley geometry, in a valley-to-peak geometry, and/or in a peak-to-peak geometry.  
         [0050]    Expansion struts can include zigzag cycles in a ring shape to form an expansion ring otherwise known as an expansion column. These expansion columns are largely responsible for optimal crimping, even and smooth expanding, and radial strength. Expansion columns by themselves may not be flexible. Each zigzag cycle in an expansion column can have a pair of expansion struts. Two expansion struts can be conjoined, for example, by a joining loop section at a proximal or a distal end. Such pairing, including conjoining at an alternating proximal to distal and distal to proximal sequence, continues in one embodiment twelve (12) times seamlessly around the circumference of an expansion column, in the one embodiment that has six zigzag cycles around the circumference of the expansion column.  
         [0051]    Various embodiments of the stent can include one or more of several different types of expansion columns. A first end expansion column in a proximal end can be a mirror image of a second expansion column in a distal end. A second expansion column nearest to the proximal end expansion column can have its mirror image in an expansion column next to a distal end expansion column. Such expansion columns can transition to a middle expansion column. The middle of the stent can include another type of expansion column that can repeat to make up the rest of the middle section of the stent of a predetermined length.  
         [0052]    In some embodiments of the stent, the proximal end expansion column can include one or more types of expansion strut patterns forming different kinds of expansion strut pairs around the circumference of the proximal end expansion column. Such types include an expansion strut with a distal stepped-down section, an expansion strut with a straight-line configuration, and an expansion strut with a distal stepped-up section. A distal end expansion column can be a mirror image of the proximal end expansion column. Different types of expansion struts can be arranged in certain sequence. Joining loop sections can form expansion strut pair loops at a proximal end and at a distal end, for example in an alternating sequence.  
         [0053]    An expansion column nearest to a proximal end expansion column and an expansion column nearest to a distal end expansion column can be mirror images. One or more types of expansion strut patterns are possible. Various embodiments of the stent can include one or more of: an expansion strut with a stepped-down section at a proximal end, an expansion strut with a stepped-down section at a distal end, an expansion strut with a straight line configuration, and an expansion strut with a stepped-up section at a proximal end and a stepped-down section at a distal end. Such types of expansion struts can be arranged in certain sequence. Joining loop sections can form expansion strut pair loops at a proximal end and at a distal end, for example in an alternating sequence. An expansion column nearest to a proximal end expansion column and an expansion column nearest to a distal end expansion column can have the same conjoining of expansion pair loops.  
         [0054]    Expansion columns in the middle may have one or more types of expansion struts, such as an expansion strut with a stepped-down section at a proximal end, and an expansion strut with a stepped-down section at a distal end. A pair of these types can be conjoined by, for example, a joining loop section at a proximal end or at a distal end, making expansion strut pair loops in a proximal end or in a distal end in an alternating sequence.  
         [0055]    Various embodiments of the stent can include multiple types of expansion columns. Particular configurations of an expansion strut pair and expansion columns can be created for specific performance purposes. The short stepped-up or stepped-down part and the longer straight part in an expansion strut with a sloped transitional zone between a long and short part can provide distinct expansion characteristic, smooth surface modulation effects, and well-formed crimping space to stent performance. A short stepped-down or stepped-up section of an expansion strut can be where a connecting strut can conjoin on a side of an expansion strut pair loop. A connecting strut can conjoin with an expansion strut as a direct extension from a side of an expansion strut pair loop and can be an integral stent structure, rather than a separate structure added, welded or attached. Separate terminology for stent elements, for example, expansion and connecting struts, conveniently describes the anatomy and function of various stent portions, and may not imply that previously separate elements are subsequently connected together.  
         [0056]    Connecting struts can have a geometric configuration, for example a symmetrical quasi M-frame configuration. In the symmetrical quasi M-frame configuration, the center element can have an invaginated truncated conical shape (or a trapezoid shape), and/or with outer arms truncated short. A longitudinal axis of a connecting strut can align with a longitudinal axis of a stent. Various embodiments of the stent can have connecting struts with one or more of horizontal segments, slant-vertical sections, and short outer arm-end sections or bilateral short stems, with some number of pivot points. In one embodiment, connecting struts include three horizontal segments, two long slant-vertical sections, and two bilateral short stems, with six pivot points. The pivot points of a connecting strut have some radii of curvature of a varying degree to make the corners smooth with a good surface modulation. In one embodiment, the pivot points makes the stent flexible while inducing a very smooth surface geometry.  
         [0057]    Connecting struts can conjoin on ipsilateral sides of expansion strut pair loops on each end. The center element, such as of an upside down trapezoid (or truncated conical shape) of the connecting strut can be invaginated into the connector space between the two apposing expansion strut pair loops that are, for example, aligned in a mirror image pattern. Some embodiments of the stent do not protrude into the main cell space. One configuration, a quasi M-frame connecting strut, divides the connector space into multiple portions. The bilateral short stems can be conjoined on ipsilateral sides of the apposed expansion strut pair loops, while the center element, the truncated conical shape or the trapezoid shape, can be invaginated into the connector space between the two apposed expansion strut pair loops.  
         [0058]    In some embodiments of the stent, when two bilateral short stems of a connecting strut conjoins expansion strut pair loops on ipsilateral sides, the connecting strut can conjoin to the two apposing expansion strut pair loops on each side of the connecting strut. A stepped-down or a stepped-up section of an expansion strut can give a connecting strut a well-planned space for crimping. Conjoining of a connecting strut on ipsilateral sides, along with an invaginated center section or sections into a connector space with multiple pivot points can create flexibility, smooth surface modulation, conformability, cell geometry (for example, hexagonally expanded) and a well formed full vessel coverage stent net mesh without an excessive metal fraction.  
         [0059]    In some embodiments of the stent, an end of a connecting strut can be conjoined to an expansion strut pair loop, making a ratio of expansion struts to connecting struts two to one.  
         [0060]    In some embodiments of the stent, when the expansion columns and connecting columns are conjoined as a single unit, the stent can have a continuous, unbroken cylindrical form without breaks or de-linking around the circumference and along the length of the stent. The unbroken link between the expansion and connecting struts can make regular and evenly spaced asymmetrical cells. The cell size can be maximized or minimized by programming of the stent (design) platform, as the clinical or application requirements may dictate.  
         [0061]    [0061]FIG. 1 shows one embodiment of a stent  10  in side elevation view, with a first expansion column  29 , a second expansion column  30 , a third expansion column  31 , a first connecting strut column  32 , and a second connecting strut column  33 . The stent  10  has a proximal end  20  and a distal end  22 . The stent  10  can have a tubular or cylindrical structure. The stent  10  can have a longitudinal length  24  and a longitudinal axis  26 .  
         [0062]    In some embodiments of the stent, an expansion column can be a zigzag and/or corrugated ring configuration of expansion struts. An expansion column, for example expansion column  30 , in a stent  10  can be an unbroken circular ring. Multiple expansion strut columns can be interconnected with connecting struts continuously along the longitudinal axis  26  of the stent  10  in an unbroken manner to form a stent  10  having a tubular shape. The interconnections among expansion columns and connecting strut columns enclose spaces, or cells, formed by expansion struts and connecting struts. In the embodiment shown in FIG. 1, many cells have symmetrical geometry, for example the middle of the stent  10 , but some cells, for example near proximal end  20  and distal end  22 , can have asymmetrical geometry.  
         [0063]    [0063]FIG. 2 shows one embodiment of a stent  10  in isometric view. A back half of the stent  10  can be seen through the front half of the stent  10 . The shown embodiment of the stent  10  has a tubular structure with a central lumen, a is proximal opening  40 , and a distal opening  42 . Stent cells  34  include open spaces in the network of expansion struts and connecting struts. The lumen includes the central, open tunnel formed by the stent. The stent  10  has two different diameters, including an outer diameter  36  and an inner diameter  38 , having a difference of a thickness of the stent  10 . Both the outer diameter  36  and inner diameter  38  can change as the stent  10  goes through a crimping stage, when the diameters  36  and  38  are narrowed, and through a deployed stage, when the diameters  36  and  38  are expanded.  
         [0064]    [0064]FIG. 3 shows one embodiment of a stent  10  in cut-open view. The stent  10  has a proximal end  20  and a distal end  22 . This view of the stent  10  is a scale drawing for a 15 mm coronary stent. There are eight expansion columns and seven connecting strut columns. At the proximal end  20  are two different expansion columns  44  and  46 , which are mirror images of two expansion columns  45  and  47  at the distal end  22 . In the middle of the stent  10 , there are four identical expansion columns  48 . Interconnecting with eight expansion columns along the longitudinal axis  26  of the stent  10  are seven connecting strut columns. The first connecting strut column  130  in a proximal end and the last connecting strut column  130  are mirror images. In the middle of the stent  10  are two upright connecting strut columns  132  and three upside down connecting strut columns  134 . There are a total of 49 cells of six different geometric configurations. Some cells have symmetrical geometry and some have asymmetrical geometry.  
         [0065]    Expansion columns  44 ,  46 ,  48 ,  47  and  45  are vertically arranged with expansion strut pair loops aligned peak-to-peak. A short distal step-down segment of one expansion column is matched with a short proximal step-down segment of another expansion column. In the middle of the stent  10 , a peak-topeak matching alignment pattern of strut pair loops repeats. Geometric configurations of expansion columns  44  and  46  in the proximal end  20  and expansion columns  47  and  45  in the distal end  22  are mirror images from expansion columns  48  in the middle of the stent. Peak-to-peak alignment of expansion strut pair loops of distal and proximal step-down segments are consistent throughout the stent  10 .  
         [0066]    Connecting strut columns  130 ,  132  and  134  interconnect expansion columns  44 ,  46 ,  48 ,  47  and  45  in a continuous and unbroken manner along the length  24  and around the circumference  28  of the stent  10 . The first and last connecting strut columns  130  use both upside down and upright quasi M-frame connecting struts. In the middle of the stent  10 , connecting strut columns  132  use upright quasi M-frame connecting struts, whereas connecting strut columns  134  have upside down quasi M-frame connecting struts. The quasi M-frame connecting struts are mounted on the ipsilateral sides of two apposed expansion strut pair loops with a distal and proximal step-down segments. This apposed arrangement of distal versus proximal step-down segments of the corrugated loops of expansion columns  30  is for a smooth and efficient crimping space for proximal and distal bilateral short stems of quasi M-frame connecting struts in the stent  10 .  
         [0067]    The stent  10  in FIG. 3 has the proximal end  20  on the left and the distal end  22  on the right. The stent  10  has a length  24  horizontally and a circumference  28  vertically, with a longitudinal axis  26  horizontally along the length  24  from the proximal end  20  to the distal end  22 .  
         [0068]    A width (horizontal dimension) of expansion columns is wider than a width of connecting strut columns. However, a width of a connecting strut column could be made the same or larger than a width of an expansion column. The variation of width ratio between a connecting strut column and an expansion column are within the scope of present invention of stent  10 . The number of expansion strut cycles in an expansion column and the number of connecting struts in a connecting strut column can be made variably different. Variable numbers of making expansion strut cycles and connecting struts are within the scope of the present invention of the stent  10 .  
         [0069]    [0069]FIG. 4A shows a magnified view of a middle section of one embodiment of a stent  10 . FIG. 4A shows identical expansion columns  48 . Each expansion column  48  can have six cycles of continuous, unbroken expansion strut pair loops with six loops on a proximal end and six loops on a distal end. Each expansion strut pair loop in an expansion column  48  can include a stair step expansion strut  54  with a stepped-down short segment  56  in a proximal end and a stair step expansion strut  54  with a short stepped-down segment  58  in a distal end, in a regularly alternating sequence. The embodiment of stent  10  of FIG. 3 includes twelve stair step expansion struts  54  in an expansion column  48 . A pair of stair step expansion struts  54  is conjoined by a joining loop  70  in a proximal end and a pair of stair step expansion struts  54  is conjoined by a joining loop  7   2  in a distal end. When a pair of stair step expansion struts  54  is conjoined by a joining loop  70  or  72 , a loop is formed.  
         [0070]    An expansion strut  54  can have a longer straight segment and a shorter stepped down segment  56  in a proximal end. A transitional slope  74  can be between a stepped down proximal segment  56  and a straight segment in a stair step expansion strut  54 . Likewise, a transitional slope  76  can be between a stepped down distal segment  58  and a straight segment in a stair step expansion strut  54 . Expansion strut pair loops of an expansion column  48  can be identical in expansion columns marked  48 .  
         [0071]    In an expansion column  48 , a straight segment of expansion strut  54  can have a longitudinal axis  82  in a horizontal direction. Similarly, a proximal short stepped down segment  56  can have a longitudinal axis  84 , which also lies horizontally and roughly parallel with an axis  80  although the axis  84  does not have to be parallel with the axis  82 . A distal short stepped down segment  58  has a longitudinal axis  86 , which also lies horizontally and may be parallel with the axis  80 , although the axis  86  does not have to be parallel with an axis  82 .  
         [0072]    Expansion columns  48  can be vertically aligned, with proximal peaks  70  of expansion strut pair loops of one expansion column  48  apposed with distal peaks  72  of expansion strut pair loops of adjacent expansion column  48 . Short stepped down segments  56  and  58  of adjacent expansion columns  48  are aligned on the ipsilateral, or same sides. Similarly, long straight segments of expansion struts  54  in an adjacent expansion column  48  can also be aligned on the ipsilateral sides. The ipsilateral apposition of stepped down segments  56  and  58  between two adjacent expansion columns  48  allows for symmetrical conjoining of a quasi M-frame connecting strut to adjacent expansion columns  48 .  
         [0073]    As expansion columns are arranged in FIG. 4A, a longitudinal axis  82  of a stair step expansion strut  54  in an expansion column  48  is roughly parallel with a longitudinal axis  82  of a stair step expansion strut  54  in adjacent expansion column  48 , although non-paralleling of these two axis  82  in expansion struts  54  in adjacent expansion columns does not have to be so. This variation is within the scope of present invention of stent  10 .  
         [0074]    An upright quasi M-frame connecting strut  90  can be conjoined on the ipsilateral sides of expansion strut pair loops in peak-to-peak apposition on the stepped down segments of the expansion strut pair loops of adjacent expansion columns  48 . The center element of quasi M-frame connecting strut  90  can be located within the confines of the connector space between two apposed expansion strut pair loops of adjacent expansion columns  48 . An upside down quasi M-frame connecting strut  92  can be conjoined on the ipsilateral sides of expansion strut pair loops in peak-to-peak apposition on the straight longer segment side of expansion strut pair loops of adjacent expansion columns  48 . The center element of upside down quasi M-frame connecting strut  92  can also be located within the confines of the connector space between two apposed expansion strut  54  pair loops of adjacent expansion columns  48 .  
         [0075]    [0075]FIG. 4B shows a magnified view of a middle section of one embodiment of a stent  10 . An upright quasi M-frame connecting strut  90  conjoins two adjacent expansion columns  48 . A connecting strut  90  has a longitudinal axis  94 , which lies horizontally along the same direction as longitudinal axis  26  of the stent  10 .  
         [0076]    An upright quasi M-frame connecting strut  90  has a proximal bilateral short stem  100  in the proximal end and a distal bilateral short stem  102  in the distal end. These two stems are anchoring roots a connecting strut  90  to conjoin, on ipsilateral sides, stepped down short segments  56  and  58  of apposed expansion strut pair loops of two adjacent expansion columns  48 . A quasi M-frame connecting strut  90  has a symmetrical geometric shape. There are three horizontal segments  104 ,  106  and  108 . A horizontal segment  104  is an extension from a proximal stem  100  through a radius of curvature  114 . A distal horizontal segment  108  along with a distal stem  102  and a radius of curvature  124  is a mirror image of a proximal horizontal segment. A middle horizontal segment  106  is at the base of a truncated cone of the quasi M-frame connecting strut  90 . On both sides of a middle horizontal segment  106  are two mirror image vertical slant segments  110  and  112 . A proximal vertical slant segment  110  is an extension of proximal horizontal segment  104  through a radius of curvature  116  and is an extension of middle horizontal segment  106  through a radius of curvature  118 . Likewise, a distal vertical slant segment  112  is an extension of a middle horizontal segment  106  through a radius of curvature  120  and is an extension of distal horizontal segment  108  through a radius of curvature  112 . There are six radii of curvature  114 ,  116 ,  118 ,  120 ,  122 , and  124  in a quasi M-frame connecting strut  90 . These six radii of curvatures serve as flexibility pivot points in the connecting strut  90 , so that the stent  10  can have more flexibility. The structure of a quasi M-frame connecting strut  90  can be substantially or entirely confined inside an imaginary “connector space” between two apposed expansion strut pair loop peaks  70  and  72  of adjacent expansion columns  48 . The central element, such as a truncated cone, of a quasi M-frame connecting strut  90 , can be inverted or invaginated into a connector space between apposed expansion strut pair loops of adjacent expansion columns  48 , instead of projecting substantially into the free space of the stent cell  34 . This can enhance stent crimping as well as smooth surface modulation during a delivery phase of stent implant procedure.  
         [0077]    The upside down quasi M-frame connecting strut  92  is a reverse image of an upright quasi M-frame connecting strut  90 . A quasi M-frame connecting strut  92  in a connector space  134  is similar to an upright quasi M-frame connecting strut  90 , but having an upside down orientation, rather than an upright orientation of the upright quasi M-frame connecting strut  90 . Designations of an upside down quasi M-frame connecting strut are similar to that of an upright quasi M-frame connecting strut  92 . In the middle of the stent  10 , connecting strut columns  32  can alternate between upright quasi M-frame connecting strut column  132  and upside down quasi M-frame connecting strut column  134 , while expansion columns  48  can repeat a same configuration. Upside down connecting struts  92  in connecting strut columns  134  can be conjoined on the ipsilateral sides of long straight segments of expansion struts  54  of expansion strut pair loop peaks  70  and  72  of adjacent expansion columns  48 .  
         [0078]    The total length of an M-frame  90  can be substantially longer than the width of a connecting strut column space  132 . This can compensate a foreshortening of the stent  10  when expanded, and enhance the flexibility of connecting strut column  132 .  
         [0079]    In connecting strut column  132 , a quasi M-frame connecting strut  90  is conjoined to ipsilateral sides on the proximal or distal stepped down segments  56  and  58  of two apposed expansion strut pair loops of adjacent expansion columns  48 .  
         [0080]    [0080]FIG. 5A shows a magnified view of an end section of one embodiment of a stent  10 , such as a proximal end  20  of stent  10 . This Figure focuses on the details of the expansion columns  44 ,  46 , and  48 .  
         [0081]    [0081]FIG. 5B shows a magnified view of an end section of one embodiment of a stent  10 , with details of connecting strut columns  130  and  134 .