Patent Publication Number: US-7722662-B2

Title: Expandable stent with sliding and locking radial elements

Description:
RELATED APPLICATIONS 
   This is a divisional of U.S. patent application Ser. No. 10/452,954, filed Jun. 3, 2003 now abandoned, which is a continuation of U.S. patent application Ser. No. 09/739,552, filed Dec. 14, 2000, now U.S. Pat. No. 6,623,521, which is a continuation-in-part of U.S. patent application Ser. No. 09/283,800 filed on Apr. 1, 1999, now U.S. Pat. No. 6,224,626, which is a continuation-in-part of U.S. patent application Ser. No. 09/024,571 filed on Feb. 17, 1998, now U.S. Pat. No. 6,033,436. 

   BACKGROUND OF THE INVENTION 
   This invention relates to expandable medical implants for maintaining support of a body lumen. 
   An important use of stents is found in situations where part of the vessel wall or stenotic plaque blocks or occludes fluid flow in the vessel. Often, a balloon catheter is utilized in a percutaneous transluminal coronary angioplasty procedure to enlarge the occluded portion of the vessel. However, the dilation of the occlusion can cause fissuring of atherosclerotic plaque and damage to the endothelium and underlying smooth muscle cell layer, potentially leading to immediate problems from flap formation or perforations in the vessel wall, as well as long-term problems with restenosis of the dilated vessel. Implantation of stents can provide support for such problems and prevent re-closure of the vessel or provide patch repair for a perforated vessel. Further, the stent may overcome the tendency of diseased vessel walls to collapse, thereby maintaining a more normal flow of blood through that vessel. 
   Significant difficulties have been encountered with all prior art stents. Each has its percentage of thrombosis, restenosis and tissue in-growth, as well as various design-specific disadvantages. 
   Examples of prior developed stents have been described by Balcon et al., “Recommendations on Stent Manufacture, Implantation and Utilization,” European Heart Journal (1997), vol. 18, pages 1536-1547, and Phillips, et al., “The Stenter&#39;s Notebook,” Physician&#39;s Press (1998), Birmingham, Mich. The first stent used clinically was the self-expanding “Wallstent” which comprised a metallic mesh in the form of a Chinese fingercuff. This design concept serves as the basis for many stents used today. These stents were cut from elongated tubes of wire braid and, accordingly, had the disadvantage that metal prongs from the cutting process remained at the longitudinal ends thereof. A second disadvantage is the inherent rigidity of the cobalt based alloy with a platinum core used to form the stent, which together with the terminal prongs, makes navigation of the blood vessels to the locus of the lesion difficult as well as risky from the standpoint of injury to healthy tissue along the passage to the target vessel. Another disadvantage is that the continuous stresses from blood flow and cardiac muscle activity create significant risks of thrombosis and damage to the vessel walls adjacent to the lesion, leading to restenosis. A major disadvantage of these types of stents is that their radial expansion is associated with significant shortening in their length, resulting in unpredictable longitudinal coverage when fully deployed. 
   Among subsequent designs, some of the most popular have been the Palmaz-Schatz slotted tube stents. Originally, the Palmaz-Schatz stents consisted of slotted stainless steel tubes comprising separate segments connected with articulations. Later designs incorporated spiral articulation for improved flexibility. These stents are delivered to the affected area by means of a balloon catheter, and are then expanded to the proper size. The disadvantage of the Palmaz-Schatz designs and similar variations is that they exhibit moderate longitudinal shortening upon expansion, with some decrease in diameter, or recoil, after deployment. Furthermore, the expanded metal mesh is associated with relatively jagged terminal prongs, which increase the risk of thrombosis and/or restenosis. This design is considered current state of the art, even though their thickness is 0.004 to 0.006 inches. 
   Another type of stent involves a tube formed of a single strand of tantalum wire, wound in a sinusoidal helix; these are known as coil stents. They exhibit increased flexibility compared to the Palmaz-Schatz stents. However, they have the disadvantage of not providing sufficient scaffolding support for many applications, including calcified or bulky vascular lesions. Further, the coil stents also exhibit recoil after radial expansion. 
   One stent design described by Fordenbacher, employs a plurality of elongated parallel stent components, each having a longitudinal backbone with a plurality of opposing circumferential elements or fingers. The circumferential elements from one stent component weave into paired slots in the longitudinal backbone of an adjacent stent component. By incorporating locking means within the slotted articulation, the Fordenbacher stent may minimize recoil after radial expansion. In addition, sufficient numbers of circumferential elements in the Fordenbacher stent may provide adequate scaffolding. Unfortunately, the free ends of the circumferential elements, protruding through the paired slots, may pose significant risks of thrombosis and/or restenosis. Moreover, this stent design would tend to be rather inflexible as a result of the plurality of longitudinal backbones. 
   Some stents employ “jelly roll” designs, wherein a sheet is rolled upon itself with a high degree of overlap in the collapsed state and a decreasing overlap as the stent unrolls to an expanded state. Examples of such designs are described in U.S. Pat. Nos. 5,421,955 to Lau, U.S. Pat. Nos. 5,441,515 and 5,618,299 to Khosravi, and U.S. Pat. No. 5,443,500 to Sigwart. The disadvantage of these designs is that they tend to exhibit very poor longitudinal flexibility. In a modified design that exhibits improved longitudinal flexibility, multiple short rolls are coupled longitudinally. See e.g., U.S. Pat. No. 5,649,977 to Campbell and U.S. Pat. Nos. 5,643,314 and 5,735,872 to Carpenter. However, these coupled rolls lack vessel support between adjacent rolls. 
   Another form of metal stent is a heat expandable device using Nitinol or a tin-coated, heat expandable coil. This type of stent is delivered to the affected area on a catheter capable of receiving heated fluids. Once properly situated, heated saline is passed through the portion of the catheter on which the stent is located, causing the stent to expand. The disadvantages associated with this stent design are numerous. Difficulties that have been encountered with this device include difficulty in obtaining reliable expansion, and difficulties in maintaining the stent in its expanded state. 
   Self-expanding stents are also available. These are delivered while restrained within a sleeve (or other restraining mechanism), that when removed allows the stent to expand. Self-expanding stents are problematic in that exact sizing, within 0.1 to 0.2 mm expanded diameter, is necessary to adequately reduce restenosis. However, self-expanding stents are currently available only in 0.5 mm increments. Thus, greater selection and adaptability in expanded size is needed. 
   In summary, there remains a need for an improved stent: one that has smoother marginal edges, to minimize restenosis; one that is small enough and flexible enough when collapsed to permit uncomplicated delivery to the affected area; one that is sufficiently flexible upon deployment to conform to the shape of the affected body lumen; one that expands uniformly to a desired diameter, without change in length; one that maintains the expanded size, without significant recoil; one that has sufficient scaffolding to provide a clear through-lumen; one that employs a thinner-walled design, which can be made smaller and more flexible to reach smaller diameter vessels; and one that has a thinner-walled design to permit faster endothelialization or covering of the stent with vessel lining, which in turn minimizes the risk of thrombosis from exposed stent materials. 
   SUMMARY OF THE INVENTION 
   The present invention relates to an expandable intraluminal stent, comprising a tubular member with a clear through-lumen. The tubular member has proximal and distal ends and a longitudinal length defined therebetween, and a circumference, and a diameter which is adjustable between at least a first collapsed diameter and at least a second expanded diameter. In a preferred mode, the longitudinal length remains substantially unchanged when the tubular member is adjusted between the first collapsed diameter and the second expanded diameter. The tubular member includes at least one module comprising a series of sliding and locking radial elements, wherein each radial element defines a portion of the circumference of the tubular member and wherein no radial element overlaps with itself in either the first collapsed diameter or the second expanded diameter. 
   In one aspect, each radial element may comprise at least one elongated rib disposed between first and second end portions. Preferably, the radial elements that comprise a module alternate between radial elements having an odd number of elongated ribs and radial elements having an even number of elongated ribs. In one preferred mode, the radial elements alternate between radial elements having one elongated rib and radial elements having two elongated ribs. 
   The stent also includes at least one articulating mechanism comprising a tab and at least one stop. The articulating mechanism permits one-way sliding of the radial elements from the first collapsed diameter to the second expanded diameter, but inhibits radial recoil from the second expanded diameter. 
   In variations to the stent, the tubular member may comprise at least two modules which are coupled to one another by at least one linkage element. In one variation, the tubular member may further comprise a frame element that surrounds at least one radial element in each module. In stents in which the tubular member comprises at least two modules, such frame elements from adjacent modules may be coupled. The coupling may include a linkage element extending between the frame elements. In addition or in the alternative, the frame elements from adjacent modules may be coupled by interlinking of the frame elements. In another aspect, the intermodular coupling may be degradable allowing for the independent modules to adapt to the vessel curvature. 
   In another variation to the stent of the present invention, any amount of overlap among the radial elements within in a module remains constant as the tubular member is adjusted from the first collapsed diameter to the second expanded diameter. This amount of overlap is preferably less than about 15%. 
   The radial recoil of the tubular member in accordance with one preferred embodiment is less than about 5%. The stiffness of the stent is preferably less than about 0.1 Newtons force/millimeter deflection. The tubular member also preferably provides a surface area coverage of greater than about 20%. 
   In accordance with another variation of the present stent, the tubular member is at least partially radiopaque. The radial elements may be made substantially from a material which is work hardened to between about 80% and 95%. In one preferred variation, the radial elements in the expandable intraluminal stent are made from a material selected from the group consisting of a polymer, a metal, a ceramic, and combinations thereof. In one mode, the material may be degradable. 
   In another mode of the invention, the material may also include a bioactive agent. The material is preferable adapted to deliver an amount of the bioactive agent which is sufficient to inhibit restenosis at the site of stent deployment. In one variation, the radial elements are adapted to release the bioactive agent during stent deployment when the tubular member is adjusted from the first collapsed diameter to the second expanded diameter. The bioactive agent(s) is preferably selected from the group consisting of antiplatelet agents, antithrombin agents, antiproliferative agents, and antiinflammatory agents. 
   In another variation, the tubular member further comprises a sheath, such as for example in a vessel graft. 
   In one aspect, the expandable intraluminal stent comprises at least two modules, wherein the expanded diameters of the first and second modules are different. 
   The articulating mechanism(s) of the present invention which allow the stent to expand but inhibit stent recoil, may comprise a slot and a tab on one radial element and at least one stop on an adjacent radial element which is slideably engaged in the slot, wherein the tab is adapted to engage the at least one stop. The articulating mechanism(s) may also include an expansion resistor on the slideably engaged radial element, wherein the expansion resistor resists passing through the slot during expansion until further force is applied, such that the radial elements in the module expand in a substantially uniform manner. In another variation, the articulating mechanism may include a release, such that actuation of the release permits sliding of the radial elements from the second expanded diameter back to the first collapsed diameter for possible removal of the stent. In another variation, the stent may comprise a floating coupling element having an articulating mechanism. 
   In another variation, the expandable intraluminal stent comprises a tubular member with a clear through-lumen and a diameter which is adjustable between at least a first collapsed diameter and at least a second expanded diameter. The tubular member comprises a series of sliding and locking radial elements made from a degradable material, wherein each radial element in the series defines a portion of the circumference of the tubular member and wherein no radial element overlaps itself. This stent also has at least one articulating mechanism that permits one-way sliding of the radial elements from the first collapsed diameter to the second expanded diameter, but inhibits radial recoil from the second expanded diameter. The degradable material may be selected from the group consisting of polyarylates (L-tyrosine-derived), free acid polyarylates, polycarbonates (L-tyrosine-derived), poly(ester-amides), poly(propylene fumarate-co-ethylene glycol) copolymer, polyanhydride esters, polyanhydrides, polyorthoesters, and silk-elastin polymers, calcium phosphate, magnesium alloys or blends thereof. 
   In a variation to the degradable stent, the degradable polymer may further comprise at least one bioactive agent, which is released as the material degrades. The at least one bioactive agent may be selected from the group consisting of antiplatelet agents, antithrombin agents, antiproliferative agents and antiinflammatory agents. 
   In another variation, the stent material may be fiber-reinforced. The reinforcing material may be a degradable material such as calcium phosphate (e.g., BIOGLASS). Alternatively, the fibers may be fiberglass, graphite, or other non-degradable material. 
   In another mode, the stent of the present invention comprises a tubular member having a wall and a clear through-lumen. The tubular member comprises a series of sliding and locking radial elements which do not overlap with themselves. The radial elements further comprise a ratcheting mechanism that permits one-way sliding of the radial elements from a first collapsed diameter to a second expanded diameter. The tubular member in this embodiment has a stiffness of less than about 0.1 Newtons force/millimeter deflection, and the wall of the tubular member has a thickness of less than about 0.005 inches. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A-C  are plan views of one module of an expandable stent in accordance with the present invention, illustrating a series of radial elements. The assembled module is shown in various states, from a collapsed state ( FIG. 1A ), to a partially expanded state ( FIG. 1B ), to an expanded state ( FIG. 1C ). 
       FIGS. 2A and 2B  are schematic views of the individual radial elements from  FIGS. 1A-C . A one-rib radial element is shown in  FIG. 2A  and a two-rib radial element is shown in  FIG. 2B . 
       FIG. 3  is a perspective view of a tubular member formed from one module comprising a series of one-rib and two-rib sliding and locking radial elements. 
       FIGS. 4A and 4B  are plan views of another embodiment of a module having a floating coupling element, wherein the one-rib radial elements further comprise a frame element. The module is shown in a collapsed state ( FIG. 4A ) and an expanded state ( FIG. 4B ). 
       FIG. 4C  is a perspective view of a tubular member comprising a plurality of modules shown in  FIGS. 4A and 4B . 
       FIG. 4D  is a perspective view of a tubular member comprising a plurality of the modules shown in  FIGS. 4A and 4B , wherein the expanded diameter of adjacent modules is not the same. 
       FIG. 5  is a plan view of another embodiment of a module comprising sliding and locking radial elements having two ribs each and a frame element. 
       FIG. 6  is a plan view of a variation of the stent showing the linkage of adjacent modules, each comprising alternating one-rib and a two-rib radial elements, wherein the one-rib elements have a frame element adapted to facilitate linkage of adjacent modules in the circumferential axis. 
       FIG. 7  is a plan view of a variation of the stent showing intermodule coupling through inter-linking of adjacent frame elements. 
       FIG. 8  is a plan view of a variation of the stent showing intermodule coupling through direct attachment of adjacent frame elements to one another. 
       FIG. 9  is a perspective view of a tubular member comprising one module in accordance with one aspect of the present invention. 
       FIG. 10  is a perspective view of a tubular member comprising a plurality of modules. 
       FIG. 11  is a plan view of a snap-together variation of the module design, having a floating coupling element and frame elements on the one-rib radial elements. 
       FIGS. 12A-C  are perspective views showing the steps in forming a biased or chamfered stop. 
       FIGS. 13A and 13B  show a releasable articulating mechanism in accordance with a collapsible variation of the present stent. An exploded view of the components of the releasable articulating mechanism is shown in  FIG. 13A . A perspective view of several releasable articulating mechanisms positioned on a module are shown in  FIG. 13B . 
       FIGS. 14A and 14B  show comparative longitudinal flexibility data for undeployed mounted (collapsed diameter) stents ( FIG. 14A ) and for deployed (expanded diameter) stents ( FIG. 14B ). 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Stent Design 
   The present invention relates to a radially expandable stent used to open, or to expand a targeted area in a body lumen. In one preferred embodiment of the present invention, the assembled stent comprises a tubular member having a length in the longitudinal axis and a diameter in the radial axis, of appropriate size to be inserted into the body lumen. The length and diameter of the tubular member may vary considerably for deployment in different selected target lumens depending on the number and configuration of the structural components, described below. The tubular member is adjustable from at least a first collapsed diameter to at least a second expanded diameter. One or more stops and engaging elements or tabs are incorporated into the structural components of the tubular member whereby recoil (i.e., collapse from an expanded diameter to a more collapsed diameter) is minimized to less than about 5%. 
   The tubular member in accordance with the present invention has a “clear through-lumen,” which is defined as having no structural elements protruding into the lumen in either the collapsed or expanded diameters. Further, the tubular member has smooth marginal edges to minimize the trauma of edge effects. The tubular member is preferably thin-walled (wall thickness depending on the selected materials ranging from less than about 0.006 inches for plastic and degradable materials to less than about 0.002 inches for metal materials) and flexible to facilitate delivery to small vessels and through tortuous vasculature. The thin walled design will also minimize blood turbulence and thus risk of thrombosis. The thin profile of the deployed tubular member in accordance with the present invention also facilitates more rapid endothelialization of the stent. 
   The wall of the tubular member comprises at least one module, which consists of a series of sliding and locking radial elements. Preferably, a plurality of modules are connected in the longitudinal axis via linkage elements which couple at least some of the radial elements between adjacent modules. The radial elements are configured within each module so as to define the circumference of the tubular member. Each radial element within a module is preferably a discrete, unitary structure, comprising one or more circumferential ribs bowed in the radial axis to form a fraction of the total circumference of the tubular member. The radial elements within a module are preferably assembled so that all of the circumferential ribs are substantially parallel to one another. At least one of the ribs in each radial element has one or more stops disposed along the length of the rib. At least some of the radial elements also have at least one articulating mechanism for slideably engaging the rib(s) from adjacent, circumferentially offset radial elements. In one aspect of the present invention, the articulating mechanism includes a tab for engaging the stops disposed along the slideably engaged adjacent rib. The articulation between the tab from one radial element and the stops from an adjacent radial element is such that a locking or ratcheting mechanism is formed, whereby the adjacent radial elements may slide circumferentially apart from one another, but are substantially prevented from sliding circumferentially toward one another. Accordingly, the tubular member may be radially expanded from a smaller diameter to a larger diameter, but recoil to a smaller diameter is minimized by the locking mechanism. The amount of recoil can be customized for the application by adjusting the size and the spacing between the stops along the ribs. Preferably, the recoil is less than about 5%. 
   Some aspects of the present stents are disclosed in U.S. Pat. No. 6,033,436 issued to Steinke, and co-pending U.S. application Ser. No. 09/283,800. The disclosures of which are hereby incorporated in their entirety by reference thereto. 
   Referring to  FIG. 1A-C , a plan view of one module  10  is illustrated comprising a series of sliding and locking radial elements  20  in accordance with one embodiment of the present invention. The pictured module is shown in a two-dimensional, flat plane. Each radial element has one or more elongated ribs  22  (in the vertical axis) with a generally perpendicular end portion  24  (in the horizontal axis), permanently affixed to each end of each rib. Each rib has at least one stop  30 . The radial elements in the module alternate from a one-rib configuration  20 ′ to a two-rib configuration  20 ″. The illustrated one-rib configuration  20 ′ has a single rib  22  with a plurality of stops  30 , whereas the illustrated two-rib configuration  20 ″ has two ribs, each with a plurality of stops  30 . The radial elements in accordance with the invention could have different numbers of circumferential ribs  22 , however, vertically adjacent radial elements preferably alternate between an odd-numbered rib configuration and an even-numbered rib configuration, as illustrated in  FIGS. 1A-C . 
   The odd-even alternation in adjacent radial elements facilitates nesting of the circumferential ribs  22  within a module, while maintaining a constant width (w). However, if the radial elements are configured differently, e.g., in a parallelogram shape as opposed to a rectangular shape, wherein the ribs exhibit a non-circumferential orientation, then changes in the longitudinal length of the module would be expected upon expansion of the tubular member. Such variations are encompassed within the present invention. 
   With reference to  FIGS. 1A-C , some of the end portions  24  of the radial elements  20  in the illustrated design are depicted with articulating mechanisms  34  each comprising a slot  36  for slidably engaging a rib from a vertically adjacent radial element and a tab  32  for engaging the stops  30  in the slidably engaged rib. The end portions  24  of the one-rib radial elements  20 ′ are generally adapted to articulate with each rib  22  from the slideably engaged, vertically adjacent two-rib radial element  20 ″. The end portions  24  of the two-rib radial elements  20 ″ are generally adapted to articulate with the single rib  22  of the slideably engaged, vertically adjacent one-rib radial element  20 ′. The articulating mechanism is shown in greater detail in  FIGS. 2A and 2B . The stops  30  may be evenly distributed along the entire length (as shown on the second radial element from the bottom), or the stops may be distributed unevenly along the ribs (as shown in the upper most radial element). 
   The articulation between the tab  32  from one radial element and the stops  30  from an adjacent radial element creates a locking or ratcheting mechanism, such that only one-way sliding (expansion) can take place. Accordingly, the series of radial elements in plan view, as shown in  FIGS. 1A-C , is adjustable from a collapsed state, as shown in  FIG. 1A , to a partially expanded state, as shown in  FIG. 1B , to a fully expanded state, as shown in  FIG. 1C . Expansion of the module  10  in plan view may be accomplished by application of opposing forces (arrows). The nested, sliding and locking radial elements  20  slide apart from one another, thereby increasing the height (h) of the series in the vertical axis, with no change in the width (w) of the series in the horizontal axis. The locking mechanism formed by the articulation between the tab  32  and the individual stops  30  prevents the expanded series from recoiling back to a more collapsed height. 
   When the module  10  is rolled to form a tubular member, a slideable articulation may be made between the end portion on the radial element on top of the module and the rib from the radial element on the bottom of the module. Likewise, a slideable articulation may also be made between the end portion on the radial element on the bottom of the module and the two ribs from the radial element on top of the module. In a variation, after rolling to form a tubular member, the top and bottom end portions can be connected to one another by a variety of fastening means known in the art, including welding, adhesive bonding, mechanical or snap fit mechanism, etc. In other modes, specialized structural elements may be included to facilitate coupling of the top and bottom portions of the rolled module. Examples, of specialized circumferential coupling elements are detailed below with reference to  FIGS. 4A and 4B . 
   With reference to  FIGS. 2A and 2B , individual one-rib  20 ′ and two-rib  20 ″ radial elements, respectively, are shown unassembled in greater detail. Both the one-rib radial element  20 ′ in  FIG. 2A  and the two-rib  20 ″ radial element in  FIG. 2B  have at least one circumferential rib  22  and an end portion  24  on each end of the rib. The rib has one or more stops  30  disposed along the length of the rib  22 . One end of each of the illustrated radial elements includes an articulating mechanism  34  comprising a tab  32  and a slot  36 . Also illustrated in  FIGS. 2A and 2B  are linkage elements  40 , which extend laterally from an end portion  24  of a radial element. These linkage elements  40  are used to couple radial elements between adjacent modules. The linkage elements may extend from either or both end portions  24  of either the one-rib  20 ′ or two-rib  20 ″ radial elements. In one preferred mode (as illustrated), the linkage elements  40  extend off of both end portions  24  of a one-rib radial element  20 ′. The configuration and angle of the linkage elements may vary substantially depending on the desired linkage distance between modules and the desired flexibility and surface area coverage of the stent. 
   A tubular member formed from a single module  10  comprising four one-rib radial elements  20 ′ and four two-rib radial elements  20 ″, similar to the plan view described with reference to  FIGS. 1A-D  and  FIGS. 2A-B , is shown in  FIG. 3 . The radial elements that form the wall of the tubular member alternate between radial elements having odd and even-numbers of circumferential ribs  22 . Each rib in the illustrated module has one or more stops  30 . An articulating mechanism (shown in greater detail in  FIGS. 2A and 2B ), has a tab  32  that engages the stops and prevents the tubular member from collapsing to a smaller diameter. Each radial element forms a portion of the total circumference of the tubular member (in this case ⅛ of the circumference). Preferably, the total number of radial elements that comprise a module varies between about 2 and 12. More preferably, the number of radial elements is between 4 and 8 radial elements. Linkage elements  40  are shown extending laterally away from the module on both sides. The linkage elements  40  are for coupling the module to similar modules to create a tubular member with a greater longitudinal length. 
   A variation of the basic module design described above with reference to  FIGS. 1A-D  and  FIGS. 2A-B  is shown in  FIGS. 4A and 4B . The module is illustrated in plan view in both a collapsed state ( FIG. 4A ) and an expanded state ( FIG. 4B ). In this variation of the stent, similar to the earlier design, a module  110  comprises a series of sliding and locking radial elements  120 . Each radial element has one or more elongated ribs  122  (in the vertical axis) with a substantially perpendicular end portion  124  (in the horizontal axis), permanently affixed to each end of each rib. Each rib has one or more stops  130 . The radial elements in the module alternate from a one-rib configuration  120 ′ to a two-rib configuration  120 ″. The one-rib configuration  120 ′ has a single rib  122  with one or more stops  130 , whereas the two-rib configuration  120 ″ has two ribs, each with one or more stops  130 . 
   Like the previously described module, the odd-even alternation in adjacent radial elements facilitates nesting of the circumferential ribs  122  within a module, while maintaining a constant width (w). Some of the end portions  124  of the radial elements  120  in the illustrated design are depicted with articulating mechanisms  134  each comprising a slot  136  for slidably engaging a rib from a vertically adjacent radial element and a tab  132  for engaging the stops  130  in the slidably engaged rib. The feathered edges  138  of the articulating mechanisms  134  shown in  FIGS. 4A and 4B  indicate where the articulating mechanism has been welded onto the end portions  124  of the respective radial elements, thereby creating the slot  136  through which the engaged rib can slide. The end portions  124  of the one-rib radial elements  120 ′ are generally adapted to articulate with each rib  122  from the slideably engaged, vertically adjacent two-rib radial element  120 ″. The end portions  124  of the two-rib radial elements  120 ″ are generally adapted to articulate with the single rib  122  of the slideably engaged, vertically adjacent one-rib radial element  120 ′. The stops  130  may be evenly distributed along the entire length (as shown), or the stops may be distributed unevenly along the ribs, or there may be only a single stop. 
   In  FIG. 4A and 4B , a bump  161  is also shown on the one-rib radial elements  120 ″. These bumps can be incorporated along the length of the rib(s) in order to provide a temporary stop. During expansion, the rib with the bump  161  temporarily stops sliding when the bump  161  enters the slot  136  of the articulating mechanism  138 . This temporary stop allows other elements to fully expand before the temporary stop is overcome by additional radial expansion force. The incorporation of one or more of these bumps in a module facilitates uniform expansion of the radial elements within the module. In addition or in the alternative to the temporary stop created by the bump  161 , some elements may have only one stop so that this element is expanded first to the stop, with the other elements having multiple stops providing preferred expansion steps. 
   The articulation between the tab  132  from one radial element and the stops  130  from an adjacent radial element creates a locking or ratcheting mechanism, such that only one-way sliding (expansion) can take place. The nested, sliding and locking radial elements  120  slide apart from one another, thereby increasing the height of the series in the vertical axis, with no change in the width of the series in the horizontal axis. The locking mechanism formed by the articulation between the tab  132  and the individual stop(s)  130  prevents the expanded series from recoiling back to a more collapsed height. 
   The module  110  shown in  FIGS. 4A and 4B  includes a floating coupling element  150  which is shaped like the end portion  124  of a two-rib radial element  120 ″, having one articulating mechanism  134  adapted to slideably engage the circumferential rib  122  of a one-rib radial element  120 ′. In variations to the depicted embodiment, the floating coupling element may be adapted to float over more than one rib in radial elements having two or more circumferential ribs. The coupling element  150  is also adapted to couple with the end portion  124  of the top radial element  121  in the series. Both the coupling element  150  and the end portion  124  on the top radial element  121  are configured so as to have coupling arms  152  and  154 , and  152 ′ and  154 ′, which may exhibit a complimentary configuration as illustrated. 
   Another specialization illustrated in  FIGS. 4A and 4B , are frame elements  160  from which linkage elements  140  extend laterally away from the frame elements  160 . In the module depicted in  FIGS. 4A and 4B , the frame elements  160  are only employed on the one-rib radial elements  120 ′. The frame elements are shown attached to and extending between the end portions  124  of the one-rib radial elements  120 ′, so that the circumferential rib  122  is surrounded, or framed, by the end portions  124  and frame elements  160 . The use of frame elements to facilitate coupling between adjacent modules has several advantages. The frame elements contribute additional physical support to the vessel wall. Larger surface area of the individual elements may be desirable in some instances, first to provide greater support for the surrounding lumen, and second the larger surface area provides a larger carrier for site-directed introduction of biologically active agents (discussed below). Alternatively, a smaller surface can be configured to minimize impact of the stent material on the vessel wall, for example, by using narrower ribs and frame elements. By suspending the linkage elements  140  laterally outward from the radial elements, the frame elements minimize the length of the linkage elements  140  that will be necessary to couple adjacent modules, while separating the sliding ribs from one module from those of the adjacent module. Coupling of the linkage elements  140  in adjacent modules provides for a very flexible stent. The flexure is also carried to the frame element  160 , allowing much larger movement, and thus, increased flexibility. In variations to this mode, the frame elements can be employed in radial elements have more than one rib. See e.g.,  FIG. 5 , showing a module design comprising a series of two-rib radial elements, each having frame elements. 
   With reference to  FIG. 5 , a variation of odd-even radial elements is shown, wherein each of the two illustrated radial elements  220  have two circumferential ribs  222  and two articulating mechanisms  234  disposed on at least one of the end portions  224  of the radial elements and comprising a tab  232  and a slot  236 . As in previous modes of the present invention, the circumferential ribs may have a plurality of stops  230  disposed along the length of the rib. Each of the radial elements has a frame element  260 , which is substantially rectangular in shape (linkage elements are not shown). The frame element may be any shape consistent with the function of surrounding the ribs and providing a connection point for coupling the radial elements from one module to those from an adjacent module. Preferably the frame elements permit nesting of the ribs in both collapsed and expanded states, without overlapping stent components, which would increase the thickness of the stent. 
   The shape of the frame elements can be varied to cause circumferential off-setting of the different radial elements having odd and even-numbers of ribs. For example, with reference to  FIG. 6 , the lateral coupling of one pair of radial elements (a one-rib  320 ′ and a two-rib  320 ″ radial element) from one module are connected by the linkage element  340  to another pair of radial elements from an adjacent module. The frame elements  360  are shown in this embodiment surrounding only the one-rib radial elements  320 ′. The frame elements  360  are configured so as to promote nesting (and not overlap) of ribs  322  and frame elements  360 , minimize the lateral space between the modules, and facilitate linkage by a circumferentially, rather than longitudinally, oriented linkage element  340 , thereby maximizing the circumferential scaffolding and radial support. 
   With reference to  FIG. 7 , there is illustrated a variation in the coupling mechanism between adjacent modules. No separate linking elements are employed. Instead, the frame elements  360  from adjacent modules may be assembled by weaving so as to inter-link with one another as shown. This coupling between adjacent modules allows much greater stent flexibility. 
   With reference to  FIG. 8 , there is illustrated another variation in the coupling mechanism between adjacent modules. No separate linking elements are employed. Instead, the frame elements  360  from adjacent modules are directly joined to one another as shown. The frame elements from adjacent modules may attached by any means suitable for the material, e.g., welding, etc. In one embodiment, frame elements from adjacent modules may be constructed (e.g., cut out) from a single piece of material. This direct coupling of frame elements from adjacent modules tends to produce a stent with greater axial strength. 
   A variety of different articulating mechanisms and stops are encompassed within the present invention; including but not limited to the slot and tab designs disclosed herein and illustrated in  FIGS. 1-8 , as well as those disclosed in the parent case, now U.S. Pat. No. 6,033,436 to Steinke, which is incorporated herein in its entirety by reference thereto. 
   It will be appreciated by those skilled in the art that the basic module design of a series of sliding and locking radial elements provides the manufacturer with a great deal of flexibility with regard to the collapsed and expanded diameters of the stent as well as the longitudinal length. Increased expanded diameter and expansion ratio can be achieved by increasing the number of radial elements within each module. Increased longitudinal length can be achieved by increasing the number of modules that are linked to form the tubular member (from one module as shown in  FIG. 9  to six modules as shown in  FIG. 10 ). 
   With reference to  FIG. 9 , a tubular member having only one module  410  comprising a series of four radial elements (two one-rib radial elements  420 ′ and two two-rib radial elements  420 ″). In the pictured module  410 , no specialized coupling element, like the floating coupling element described with respect to  FIGS. 4A and 4B  is employed, although such a coupling element could be used in this module without departing from the basic design. The illustrated frame elements  460  have a rectangular shape and surround only the one-rib radial elements  420 ′. The module shown in  FIG. 9  is in an expanded state and is subject to only minimum recoil or collapse (&lt;about 5%) because of the ratcheting effect created by the articulation between a tab  432  on the articulating mechanism  434  of one radial element and a stop  430  on the slideably engaged rib  422  from the adjacent radial element. The articulating mechanism is shown as a separate structural element that has been affixed, e.g., by welding, to the end portion  424  of the respective radial element, thereby entrapping and slideably engaging the rib(s) from the adjacent radial element. 
   In  FIG. 10 , a stent in accordance with the present invention is shown, comprising a tubular member  500  having six modules  510  which are linked in the longitudinal axis (for clarity, linkage elements extending between the frame elements in adjacent modules are not shown). 
   In another variation of the present invention, a series of radial elements are illustrated in  FIG. 11 , wherein the articulating mechanism is formed by a tab  632  in a one-way locking slot  633 . This design eliminates the need to attach an overlapping articulating mechanism, e.g., by. welding, to entrap and slideably engage a circumferential rib  622  from an adjacent radial element. As shown in  FIG. 11 , an entry slot  631  is provided at one end of the central locking slot  633 , which is disposed along at least a portion of the length of each rib  622  in each radial element. The entry slot  631  is adapted to permit a tab  632  on the end portion  624  of one radial element  620  to fit into and engage the locking slot  633  in the rib  622 . Once the tab(s)  632  is placed through the entry slot(s)  631  the radial elements  620  can be slid apart enough to prevent the tab  632  from coming back out of the entry slot  631 . The locking slot  633  is adapted to allow the tab to slide through the slot in only one direction (to a more expanded configuration). For example, as illustrated, the locking slot  633  has a series of serrated notches or stops  630 , which are offset on both sides of the slot and which permit the tab  632  to move through the slot  633  in one direction, but which are shaped so as to engage the tab and prevent it from moving through the slot in the opposite direction, i.e., prevent collapse of the expanded stent. Any of a variety of locking slot and stop configurations are encompassed within this snap-together design. Some alternative locking slot and stop configurations are disclosed in the parent application, now U.S. Pat. No. 6,033,436 to Steinke. 
   The weldless design module illustrated in  FIG. 11  is shown with frame elements  660  with linkage elements  640  around the one-rib radial elements and a floating coupling element  650  with coupling arms  652  and  654  for mating with complementary coupling arms  652 ′ and  654 ′ on the end portion  624  of the top radial element in the series. Because the intermodule coupling can be made to the frame elements  660 , this increased length allows the stent to be very flexible both in the collapsed and expanded states. 
   Another variation of the present invention includes varying the articulating mechanism and rib configurations so as to produce increasing friction with progressive expansion. This variation may facilitate uniform expansion of all radial elements within a module. 
   In another variation of the present stent, different modules within the stent may exhibit different expanded diameters, such that the stent may be adjustable to different luminal states along the length of the stent. Accordingly, the stent may exhibit a tapered configuration in its deployed state, having a larger diameter at one end with progressive or step-wise decreases in modular expanded diameter moving toward the other end of the stent. 
   It will be appreciated by those of skill in the art that the interlocking and sliding radial element design of the present invention provides the manufacturer with substantial flexibility in customizing the stent for different applications. Because overlap of stent components is minimized by the nesting of ribs and frame elements, the collapsed profile can be very thin without compromising radial strength. Moreover, the degree of overlap does not change substantially during expansion, unlike jelly-roll designs which expand by unraveling of a rolled sheet. Furthermore, the deployment flexibility of the present stent can be customized by changing the length, configuration and number of lateral linkage elements employed. Thus, a very flexible and ultra-thin embodiment of the present stent is deemed to be uniquely suited for deployment in small and difficult to reach vessels, such as the intercranial vessels distal to the carotids and the remote coronary vessels. 
   In another variation, the stent may be used in combination with a covering or sheath to provide a vessel graft, for example, in the treatment of an aneurysm. Materials and methods of making vessel grafts (stent and sheath) incorporating the present stent design are described in detail below. 
   In another variation of the present invention, the stops that are disposed along an elongate rib may be shaped so as to facilitate locking of the tab from the articulating member within the stop, wherein the shape of the hole is adapted to provide a channel which will have a bias for capturing parts (i.e., a tab) sliding past it. With reference to  FIGS. 12A-C , there are illustrated the steps in forming one embodiment of such a stop. In  FIG. 12A , the stent component  700  can be etched from the top  700 ′ and bottom  700 ″ surfaces. The top and bottom surfaces are coated or masked in some areas  702 ′ and  702 ″, respectively, with a layer that resists etching (e.g., by chemical, laser, etc.), leaving uncoated areas  704 ′ and  704 ″ on the top and bottom, respectively, susceptible to etching. The uncoated areas are offset by a distance  706 , which allows some overlap  708  between the top and bottom uncoated areas  704 ′ and  704 ″. As illustrated in  FIG. 12B , during the etching process wherein stent material is removed, the uncoated areas  704 ′ and  704 ″ become cavities  710  extending through the stent material. At some point during the etching process, as shown in  FIG. 12C , the cavities meet in the overlap area  708  and create a through hole or channel  712 . The stop thus formed has a chamfered edge that is biased for capturing a tab as it slides over the stop. 
   In another embodiment of the present stent, the locking mechanism may be designed to be releasable, wherein the stent may be collapsed for removal from the body lumen. Whereas the other configurations in this disclosure are designed for permanent locking of the members in the expanded state, there may be a need for a reversible, or unlocking mechanism. The components of one possible release mechanism are illustrated in exploded view in  FIG. 13A . Most aspects of the stent in accordance with the present invention remain as described in preceding sections. However, the articulating mechanism  1034  is altered to be releasable. The tab  1032  is preformed or biased (as a result of its springy material and/or angle of deployment) not to lockably engage the individual stops  1030 . Instead, a moveable slider  1080  and retainer plate  1090  are positioned over the tab  1032  to deflect the tab downward into the individual stops. The shape of tab  1032  which is deflected against the rib  1022  by the slider  1080  and retainer plate  1090  provides locking of rib  1022  against one direction of travel (collapse) while allowing travel in the opposite direction (expansion). The slider  1080  has a wide area  1082  that provides the structural interference to flex tab  1032  into the locking position. When the wide region  1082  is positioned between retainer  1090  and tab  1032  the tab is forced against the slideably engaged rib  1022  and into the passing stops  1030  as the rib slides through the articulating mechanism. The slider  1080  also has a narrow region  1084  that will permit tab  1032  to relax and pull out of the stop  1030 . By pulling the slider  1080  outward from the perpendicular plane of the ribs  1020  the narrow region  1084  is repositioned over the tab  1032 , thereby allowing the tab to disengage from the stop  1030  and spring back upward against the retainer plate  1090 . 
   With reference to  FIG. 13B  there is illustrated a partial view of a module having one-rib and two-rib radial elements and releasable articulating mechanisms  1034 . The releasable articulating mechanisms on the one-rib radial element are shown engaging the two ribs from the adjacent two-rib radial element. The slider may be modified on this releasable articulating mechanism to have two narrow regions for releasing both tabs by pulling the one side of the slider. 
   Stent Manufacture 
   Preferred materials for the making the stents of the present invention include 316 stainless steel, tantalum, titanium, tungsten, gold, platium, iridium, rhodium and alloys thereof. Also shape memory alloys such as Nitinol may be used in accordance with the present invention. Preferably, sheets are work-hardened prior to forming of the individual stent elements to increase strength. Methods of work hardening are well known in the art. Sheets are rolled under tension, annealed under heat and then re-worked. This may be continued until the desired modulus of hardness is obtained. Most stents in commercial use today employ 0% to 10% work hardened material in order to allow for “softer” material to deform to a larger diameter. In contrast, because expansion of the sliding and locking radial elements in accordance with the present invention depends on sliding rather than material deformation, it is preferred to use harder materials, preferably in the range of about 25-95% work hardened material to allow for thinner stent thickness. More preferably, the stent materials are 50-90% work hardened and most preferably, the materials are 80-85% work hardened. 
   Preferred methods of forming the individual elements from the metal sheets may be laser cutting, laser ablation, die-cutting, chemical etching, plasma etching or other methods known in the art which are capable of producing high-resolution components. The method of manufacture, in some embodiments, depends on the material used to form the stent. Chemical etching provides high-resolution components at relatively low price, particularly in comparison to high cost of competitive product laser cutting. Tack-welding, adhesives, mechanical attachment (snap-together), and other art-recognized methods of attachment, may be used to fasten the individual elements. Some methods allow for different front and back etch artwork, which could result in chamfered edges, which may be desirable to help improve engagements of lockouts. 
   In one preferred mode of the present invention, the stent is made, at least in part, from a polymeric material, which may be degradable. The motivation for using a degradable stent is that the mechanical support of a stent may only be necessary for several weeks after angioplasty, particularly if it also controls restenosis and thrombosis by delivering pharmacologic agents. Degradable polymeric stent materials are well suited for drug delivery. 
   It is believed that there is a need for short-term intervention since the majority of cardiac events occur in the first 6 months, including in-stent restenosis. The permanency of metal stents presents long-term risks and complications. With long lesions and full coverage, metal stents can also preclude surgical re-intervention. The ideal implant: (1) mimics the tissue it is designed to replace in size, shape, and material consistency; (2) neither is disposed to infection nor evokes a foreign body response; (3) is a temporary prosthesis that takes on characteristics of the natural tissue as it disappears; and (4) is a biocompatible implant that has a smooth surface to minimize the risk for thrombus formation and macrophage enzyme activity. 
   Degradable stents have the potential to perform more like an ideal implant. Degradable stents that integrate seamlessly with the living host tissue may improve tissue biocompatibility due to their temporary residence. With the initial strength to secure the diseased tissue, such stents may eliminate the concern for product migration over time and long-term product failure. They may also minimize time, costs, and complications associated with re-intervention of specific and neighboring sites. Degradable stents have a clear advantage over metal stents in that they can dose the diseased tissue with a drug; compared to drug coated metal stents, degradable stents can dose the tissue over a longer period of time. 
   Unlike restenosis after angioplasty, in-stent restenosis is a consequence almost entirely of tissue hyperplasia, occurring principally at the points where the stent&#39;s struts impinge upon the artery wall. Placement of an excessively stiff stent against the compliant vessel creates a mismatch in mechanical behavior that results in continuous lateral expansile stress on the arterial wall. This stress can promote thrombosis, arterial wall thinning, or excessive cellular proliferation. Hence, polymeric biomaterials, which are more flexible, may minimize the pathology and are more likely to approximate the mechanical profile of the native tissue. 
   The intact internal elastic lamina (IEL) of a healthy artery serves as an effective barrier to (1) protect the underlying smooth muscle cells (SMC) from exposure to mitogens that induce hyperplasia, and (2) prevent exposure to monocytes or lipid-filled macrophages and circulating elastin peptides that promote hard plaque formation and narrowing of the artery. A biomaterial stent may minimize progression of disease states by mimicking the barrier functions of the IEL: (1) by delivering a cell-cycle inhibitor to counteract the affects of mitogens, and (2) by serving as a temporary physical barrier to the trafficking immune cells. 
   In the natural disease states, arteriostenosis and atherosclerosis, arteries can have a compromised or structurally discontinuous IEL. The cause of the discontinuity is largely unknown. Elastases, circulating elastin peptides, and elastin receptors may play a pivotal role along with denudation of the endothelium. A biomaterial stent that does not grossly over expand the vessel wall may minimize the risk for further perforation of the IEL. In addition the stent surface can serve as an anchorage site for formation of an endothelial lining, the gatekeeper to blood elements and circulating molecules. 
   In one mode of the degradable stent of the present invention, the stent matrix may be formulated so as to release a pharmacologic agent. Mechanical treatment of diseased vessels by angioplasty and stenting can further damage the arterial wall. Ironically, each of these practices can promote thrombus formation and restenosis associated with reocclusion within 6- to 24-months post-operatively. These inadequate clinical outcomes are the impetus for development of many counteractive therapies. Some new treatments for restenosis are use of radioisotopes, Paclitaxel and Rapamycin all of which inhibit vascular cell proliferation. 
   It is estimated that pharmacological interventions for restenosis need to occur continuously for 2-4 weeks following angioplasty or stent implantation. It is also estimated that a polymer stent can deliver a drug dose that is ten times higher than systemic delivery. If a cell cycle inhibitor was released from a degradable stent, we may achieve optimal long-term patency in the diseased vessel. 
   Degradable biomaterial stents may improve the long-term product safety and efficacy for the patients. We believe that a completely degradable, drug-eluting stent that resides in the vessel for several weeks after deployment will be effective in controlling restenosis. Accordingly, the present invention encompasses stents having the sliding and locking geometry described above, wherein the stent components are made from a functional biomaterial. 
   The mechanical properties of the degradable biomaterial are selected in accordance with the present invention to exhibit at least one, and preferably more, of the following characteristics: (1) resist failure due to the multiaxial stress-strain behavior of native arteries and exceeds that of annealed metals, which are known to fail for stent applications; (2) retain mechanical strength during several weeks or months post-deployment; (3) degrade via hydrolytic or enzymatic degradation preferably with surface erosion whereby the implant degrades uniformly and maintains its original shape as it degrades; (4) maintains favorable hemodynamics; (5) exhibits a hydrophilic, negatively charged, smooth and uniform surface with a low critical surface tension; (6) supports endothelialization; (7) is nontoxic and eliminated from the body safely, i.e., no systemic effects; and (8) includes an anti-restenosis pharmacological agent. The pharmacologic agent may be a cell-cycle inhibitor that inhibits SMC proliferation, allows for favorable early and late remodeling, and that is stable in the biomaterial. The degradable biomaterial and pharmacologic agent preferably provide dosing of the lesion for about three to four weeks or through the degradation cycle of stent. 
   Degradable plastic or natural (animal, plant or microbial) or recombinant materials in accordance with one aspect of the present invention may include polydepsipeptides, nylon copolymides, conventional poly(amino acid) synthetic polymers, pseudo-poly(amino acids), aliphatic polyesters, such as polyglycolic acid (PGA), polylactic acid (PLA), polyalkylene succinates, polyhydroxybutyrate (PHB), polybutylene diglycolate, and poly epsilon-caprolactone (PCL), polydihydropyrans, polyphosphazenes, polyorthoesters, polycyanoacrylates, polyanhydrides, polyketals, polyacetals, poly(α-hydroxy-esters), poly(carbonates), poly(imino-carbonates), poly(β-hydroxy-esters), polypeptides, and their chemical modifications and combinations (blends and copolymers) and many other degradable materials known in the art. (See e.g., Atala, A., Mooney, D. Synthetic Biodegradable Polymer Scaffolds. 1997 Birkhauser, Boston; incorporated herein by reference). 
   In one preferred mode, the degradable materials are selected from the group consisting of poly(alkylene oxalates), polyalkanotes, polyamides, polyaspartimic acid, polyglutarunic acid polymer, poly-p-diaxanone (e.g., PDS from Ethicon), polyphosphazene, and polyurethane. 
   In a more preferred mode, the degradable materials are selected from the group consisting of poly(glycolide-trimethylene carbonate); terpolymer (copolymers of glycolide, lactide or dimethyltrimethylene carbonate); polyhydroxyalkanoates (PHA); polyhydroxybutyrate (PHB) and poly(hydroxybutyrate-co-valerate) (PHB-co-HV) and copolymer of same; poly(epsilon-caprolactone) and copolymers (e.g., lactide or glycolide); poly(epsilon-caprolactone-dimethyltrimethylene carbonate); polyglycolic acid (PGA); and poly-L and poly-D(lactic acid) and copolymers and additives (e.g., calcium phosphate glass) and lactic acid/ethylene glycol copolymers. 
   In a most preferred mode, the degradable materials are selected from the group consisting of polyarylates (L-tyrosine-derived) or free acid polyarylates, polycarbonates (L-tyrosine-derived), poly(ester-amides), poly(propylene fumarate-co-ethylene glycol) copolymer (i.e., fumarate anhydrides), polyanhydride esters (mechanically stronger) and polyanhydrides (mechanically weaker), polyorthoesters, ProLastin or silk-elastin polymers (SELP), calcium phosphate (BIOGLASS), magnesium alloys, and a composition of PLA, PCL, PGA ester commercial polymers used sigularly or in any mixture. 
   Natural polymers (biopolymers) include any protein or peptide. These can be used in a blend or copolymer with any of the other aforementioned degradable materials, as well as with pharmacologic substances, or with hydrogels, or alone. Typically, these biopolymers degrade upon the action of enzymes. Preferred biopolymers may be selected from the group consisting of aliginate, cellulose and ester, chitosan (NOCC and NOOC-G), collagen, cotton, dextran, elastin, fibrin, gelatin, hyaluronic acid, hydroxyapatite, spider silk, other polypeptides and proteins, and any combinations thereof. 
   Coatings for degradable and metal stent materials may be selected from the group consisting of hydrogels, such as: NO-carboxymethyl chitosan (NOCC), PEG diacrylate with drug (intimal layer) with second layer without drug (blood flow contact), polyethylene oxide, polyvinylalcohol (PVA), PE-oxide, polyvinylpyrolidone (PVP), polyglutarunic acid polymers, DMSO or alcohols and any combinations thereof. 
   Where plastic and/or degradable materials are used, the elements may be made using hot-stamp embossing to generate the parts and heat-staking to attach the linkage elements and coupling arms. Other preferred methods comprise laser ablation using a screen, stencil or mask; solvent casting; forming by stamping, embossing, compression molding, centripital spin casting and molding; extrusion and cutting, three-dimensional rapid prototyping using solid free-form fabrication technology, stereolithography, selective laser sintering, or the like; etching techniques comprising plasma etching; textile manufacturing methods comprising felting, knitting, or weaving; molding techniques comprising fused deposition modeling, injection molding, room temperature vulcanized (RTV) molding, or silicone rubber molding; casting techniques comprising casting with solvents, direct shell production casting, investment casting, pressure die casting, resin injection, resin processing electroforming, or reaction injection molding (RIM). These parts may be connected or attached by solvent or thermal bonding, or by mechanical attachment. Preferred methods of bonding comprise the use of ultrasonic radiofrequency or other thermal methods, and by solvents or adhesives or ultraviolet curing processes or photoreactive processes. The elements may be rolled by thermal forming, cold forming, solvent weakening forming and evaporation, or by preforming parts before linking. Soluble materials such as hydrogels which are hydrolized by water in blood could also be used, for example, cross-linked poly 2-hydroxyethyl methacrylate (PHEMA) and its copolymers, e.g., polyacrylamide, and polyvinyl alcohol. 
   The addition of radiopacifiers (i.e., radiopaque materials) to facilitate tracking and positioning of the stent could be added in any fabrication method or absorbed into or sprayed onto the surface of part or all of the implant. The degree of radiopacity contrast can be altered by implant content. Radiopacity may be imparted by covalently binding iodine to the polymer monomeric building blocks of the elements of the implant. Common radiopaque materials include barium sulfate, bismuth subcarbonate, and zirconium dioxide. Other radiopaque elements include: cadmium, tungsten, gold, tantalum, bismuth, platium, iridium, and rhodium. In one preferred embodiment, iodine may be employed for its radiopacity and antimicrobial properties. Radiopacity is typically determined by fluoroscope or x-ray film. 
   The stents in accordance with the present invention, may also be useful in vessel grafts, wherein the stent is covered with a sheath formed from either a polymeric material, such as expanded PTFE, degradable polymers, or a natural material, such as fibrin, pericardial tissue, or their derivatives, as will be known to those of skill in the art. The covering may be attached to the inner or outer surface of the stent. Alternatively, the stent may be embedded within layers of the covering material. 
   Once the stent components have been cut out and assembled into flat modules (see plan views described with respect to  FIGS. 1 ,  2 ,  4 - 8 , and  11 ), and linkage elements between adjacent modules have been connected (e.g., by welding, inter-weaving frame elements, etc.), the flat sheets of material are rolled to form a tubular member. Coupling arms from floating coupling elements and end portions are joined (e.g., by welding) to maintain the tubular shape. In embodiments that do not include coupling elements, the end portions of the top and bottom radial elements in a module may be joined. Alternatively, where sliding is desired throughout the entire circumference, a sliding and locking articulation can be made between the end portion of the top radial element and the rib(s) of the bottom radial element (e.g., by tack-welding, heat-staking or snap-together). Similarly, a corresponding articulation can be made between the end portion of the bottom radial element and the rib(s) of the top radial element. 
   Rolling of the module(s) to form a tubular member can be accomplished by any means known in the art, including rolling between two plates, which are each padded on the side in contact with the stent elements. One plate is held immobile and the other can move laterally with respect to the other. Thus, the stent elements sandwiched between the plates may be rolled about a mandrel by the movement of the plates relative to one another. Alternatively, 3-way spindle methods known in the art may also be used to roll the tubular member. Other rolling methods that may be used in accordance with the present invention include those used for “jelly-roll” designs, as disclosed for example, in U.S. Pat. Nos. 5,421,955, 5,441,515, 5,618,299, 5,443,500, 5,649,977, 5,643,314 and 5,735,872; the disclosures of which are incorporated herein in their entireties by reference thereto. 
   The construction of the stent in this fashion provides a great deal of benefit over the prior art. The construction of the locking mechanism is largely material-independent. This allows the structure of the stent to comprise high strength materials, not possible with designs that require deformation of the material to complete the locking mechanism. The incorporation of these materials will allow the thickness required of the material to decrease, while retaining the strength characteristics of thicker stents. In preferred embodiments, the frequency of locking holes or stops present on selected circumferential ribs prevents unnecessary recoil of the stent subsequent to expansion. 
   Drugs Incorporated into Stents 
   Drugs and other bioactive compounds can be incorporated into the degradable matrices themselves or coated on the non-degradable stent materials, thereby providing sustained release of such compounds at the site of the stent. In addition, degradable biomaterial can be fabricated in a various forms and processed into the stent components. Preferred biomaterials would incorporate a pharmaceutical agent blended with the degradable polymer prior to fabricating the stent. The preferred pharmaceutical agent(s) control restenosis (including neointimal thickening, intimal hyperplasia and in-stent restenosis or limits vascular smooth muscle cell overgrowth in the lumen of a stented vessel. Other body applications may require different drugs. 
   In a another aspect of the present invention, the stent biomaterial may also incorporate a hydrogel that acts to prevent adhesions of blood cells, extracellular matrix or other cell types. For instance, NOCC and NOCC-G chitosan. In another aspect, the pharmaceutical agents or hydrogels can be coated onto the surface of the biomaterial singularly or in mixtures or in combination with other binders required to adhere or absorb the pharmaceutical or hydrogel to the biomaterial surface. In addition or in the alternative, the pharmaceutical or hydrogel or genetic material may be incorporated with the biomaterial polymer, microspheres, or hydrogel. 
   Use of synthetic, natural (plant, microbial, viral or animal-derived) and recombinant forms having selected functions or chemical properties can be mixed with complementary substances (e.g., anti-thrombotic and anti-restenosis substances; nucleic acids and lipid complexes). Pharmacologic agents may also incorporate use of vitamins or minerals. For instance, those that function directly or indirectly through interactions or mechanisms involving amino acids, nucleic acids (DNA, RNA), proteins or peptides (e.g., RGD peptides), carbohydrate moieties, polysaccharides, liposomes, or other cellular components or organelles for instance receptors and ligands. 
   Pharmaceutical agents may be polar or possess a net negative or positive or neutral charge; they may be hydrophobic, hydrophilic or zwitterionic or have a great affinity for water. Release may occur by controlled release mechanisms, diffusion, interaction with another agent(s) delivered by intravenous injection, aerosolization, or orally. Release may also occur by application of a magnetic field, an electrical field, or use of ultrasound. 
   The variety of compounds which may be used for coating metallic stents or for incorporating into degradable stent materials have been disclosed by Tanguay et al.  Cardio Clin  (1994) and Nikol et al.  Atherosclerosis  (1996); these references are herein incorporated in their entirety by reference thereto. These compounds include antiplatelet agents (Table 1), antithrombin agents (Table 2), and antiproliferative agents (Table 3). Some preferred agents that fall within these classes of compounds are presented in Tables 1-3 (below). 
   
     
       
         
             
           
             
               TABLE 1 
             
           
          
             
                 
             
             
               Antiplatelet Agents 
             
          
         
         
             
             
          
             
               Compound 
               Action 
             
             
                 
             
             
               Aspirin 
               Cyclo-oxygenase inhibition 
             
             
               Dipyridamole 
               Phosphodiesterase inhibition 
             
             
               Ticlopidine 
               Blocks interaction between platelet receptors, fibrinogen, 
             
             
                 
               and von Willebrand factors 
             
             
               C7E3 
               Monoclonal antibody to the glycoprotein IIb/IIIa receptor 
             
             
               Integrelin 
               Competitive glycoprotein Iib/IIIa receptor inhibition 
             
             
               MK-852, 
               Glycoprotein IIb/IIIa receptor inhibition 
             
             
               MK-383 
             
             
               RO-44-9883 
               Glycoprotein IIb/IIIa receptor inhibition 
             
             
                 
             
          
         
       
     
   
   
     
       
         
             
           
             
               TABLE 2 
             
           
          
             
                 
             
             
               Antithrombin Agents 
             
          
         
         
             
             
          
             
               Compound 
               Action 
             
             
                 
             
             
               Heparin 
               Antithrombin III cofactor 
             
             
               Low molecular weight 
               Inhibition of factor Xa by antithrombin III 
             
             
               heparin (LMWH) 
             
             
               R-Hirudin 
               Selective thrombin inhibition 
             
             
               Hirulog 
               Synthetic direct thrombin inhibition 
             
             
               Argatroban, efegatran 
               Synthetic competitive thrombin inhibition 
             
             
               Tick anticoagulant 
               Specific thrombin inhibition 
             
             
               peptide 
             
             
               Ppack 
               Irreversible thrombin inhibition 
             
             
                 
             
          
         
       
     
   
   Additional anti-thrombogenic substances and formulations include endothelium-derived relaxing factor, prostaglandin I 2 , plasminogen activator inhibitor, tissue-type plasminogen activator (tPA), ReoPro: anti-platelet glycoprotein IIb/IIIa integrin receptor, heparin, polyamine to which dextran sulfate and heparin are covalently bonded, heparin-containing polymer coating for indwelling implants (MEDI-COAT by STS Biopolymers), polyurethaneurea/heparin, hirudin/prostacyclin and analogues, fibrin and fibrin peptide A, lipid-lowering drugs, e.g., Omega-3 fatty acids, and chrysalin (aka TRAP-508) by Chrysalis Vascular Technologies (which is a synthetically manufactured peptide portion of the human enzyme thrombin, responsible for blood clotting and initiating cellular/tissue repair). Chrysalin mimics specific attributes of thrombin by interacting with receptors on cells involved in tissue repair. 
   Other anti-restenosis substances in accordance with the present invention include INTEGRILIN® (eptifibatide) by COR Therapeutics (blocks platelet clumping), Resten-NG (NeuGene) by AVI BioPharma (synthetic version of C-MYC oncogene), and Implant Sciences Corp., BiodivYsio (phosphorylcholine (PC)) by Abbott Laboratories Inc. and Biocompatibles International PLC, Liposomal Prostaglandin El by Endovasc Ltd. and Collaborative BioAlliance, Adenovirus vectors to carry genes to vascular smooth muscle cells (Boston Scientific Corp and CardioGene Therapeutics Inc.), TAXOL (paclitaxel) by Bristol-Myers Squibb (prevents cell division by promoting the assembly of and inhibiting the disassembly of microtubules), and Raparnycin or nitric oxide. Other drugs include ceramide, tranilast, probucol, statins, cilostazol, and low molecular weight variations of heparin. 
   A variety of compounds are considered to be useful in controlling vascular restenosis and in-stent restenosis. Some of these preferred antiproliferative agents are presented in Table 3 (below). 
   
     
       
         
             
           
             
               TABLE 3 
             
           
          
             
                 
             
             
               Antiproliferative Agents 
             
          
         
         
             
             
          
             
               Compound 
               Action 
             
             
                 
             
             
               Angiopeptin 
               Somatostatin analog which inhibits IGF-I 
             
             
               Ciprostene 
               Prostacyclin analog 
             
             
               Calcium 
               Inhibition of slow calcium channels 
             
             
               blockers 
             
             
               Colchicine 
               Antiproliferative and migration inhibition 
             
             
               Cyclosporine 
               Immunosuppressive, intracellular growth signal inhibition 
             
             
               Cytorabine 
               Antineoplastic, DNA synthesis inhibition 
             
             
               Fusion proteins 
               Toxin-bounded growth factor 
             
             
               Lioprost 
               Prostacyclin analog 
             
             
               Ketaserine 
               Serotonin antagonist 
             
             
               Prednisone 
               Steroid hormone 
             
             
               Trapidil 
               Platelet-derived growth factor inhibitor (inhibitor of 
             
             
                 
               thromboxane-A2 and/or PDGF receptor antagonist) 
             
             
                 
             
          
         
       
     
   
   Specific therapeutic agents have also been identified which may modulate smooth muscle cell (SMC) proliferation. Since SMC cell proliferation has been implicated in atherosclerotic stenosis as well as post-operative restenosis, incorporation of such agents may be particularly useful. These include without limitation, regulators of SMC mitosis (e.g., TAXOL, Rapamycin, or ceramide) and stimulators and triggers for extracellular matrix production, such as anti-FGF and TGF-β 1  strategies, tissue inhibitor metalloproteinases (TIMPs), and matrix metaloproteinases (MMPs). 
   Various compounds address specific pathologic events and/or vascular diseases. Some of these therapeutic target compounds are summarized in Table 4 (below). 
   
     
       
         
             
           
             
               TABLE 4 
             
           
          
             
                 
             
             
               Specific Therapeutic Target Compounds 
             
          
         
         
             
             
          
             
               Pathologic Event 
               Therapeutic Target 
             
             
                 
             
             
               Endothelial dysfunction 
               Nitric oxide inducer or antioxidants 
             
             
               Endothelial injury 
               Administer VEGF; FGF&#39;s 
             
             
               Cell activation &amp; 
               MEF-2 &amp; Gax modulators; NFKB antagonists; 
             
             
               phenotypic modulation 
               cell cycle inhibitors 
             
             
               Dysregulated cell growth 
               E2F decoys; RB mutants; cell cycle inhibitors 
             
             
               Dysregulated apoptosis 
               Bax or CPP32 inducers; Bcl-2 inhibitors; 
             
             
                 
               integrin antagonists 
             
             
               Thrombosis 
               IIb/IIIa blockers; tissue factor inhibitors; 
             
             
                 
               antithrombin agents 
             
             
               Plaque rupture 
               Metalloproteinase inhibitors; leukocyte 
             
             
                 
               adhesion blockers 
             
             
               Abnormal cell migration 
               Integrin antagonists: PDGF blockers; 
             
             
                 
               plasminogen activator inhibitors 
             
             
               Matrix modification 
               Metalloproteinase inhibitors, plasminogen 
             
             
                 
               antagonists; matrix protein cross-linking 
             
             
                 
               modifiers 
             
             
                 
             
          
         
       
     
   
   The therapeutic agents to be bonded to or incorporated within the stent materials of the present invention may be classified in terms of their sites of action in the host. The following agents are believed to exert their actions extracellularly or at specific membrane receptor sites. These include corticoids and other ion channel blockers, growth factors, antibodies, receptor blockers, fusion toxins, extracellular matrix proteins, peptides, or other biomolecules (e.g., hormones, lipids, matrix metalloproteinases, and the like), radiation, anti-inflammatory agents including cytokines such as interleukin-1 (IL-1), and tumor necrosis factor alpha (TNF-α), gamma interferon (interferon-γ), and Tranilast, which modulate the inflammatory response. 
   Other groups of agents exert their effects at the plasma membrane. These include those involved in the signal transduction cascade, such as coupling proteins, membrane associated and cytoplasmic protein kinases and effectors, tyrosine kinases, growth factor receptors, and adhesion molecules (selectins and integrins). 
   Some compounds are active within the cytoplasm, including for example, heparin, ribozymes, cytoxins, antisense oligonucleotides, and expression vectors. Other therapeutic approaches are directed at the nucleus. These include gene integration, proto-oncogenes, particularly those important for cell division, nuclear proteins, cell cycle genes, and transcription factors. 
   Genetic approaches to control restenosis include without limitation: use of antisense oligonucleotides to PDGFR-ββ mRNA to control PDGF expression; use of antisense oligonucleotides for nuclear antigens c-myb or c-myc oncogenes (Bauters et al., 1997,  Trends CV Med ); use of antisense phosphorothioate oligodeoxynucleotides (ODN) against cdk 2 kinase (cyclin dependent kinase) to control the cell cycle of vascular SMC (Morishita et al, 1993,  Hypertension ); use of VEGF gene (or VEGF itself) to stimulate reconstructive wound healing such as endothelialization and decrease neointima growth (Asahara et al 1995); delivery of the nitric oxide synthetase gene (eNOS) to reduce vascular SMC proliferation (Von Der Leyen et al., 1995,  Proc Natl Acad Sci ); use of adenovirus expressing plasminogen activator inhibitor-1 (PAI-1) to reduce vascular SMC migration and thereby diminish restenosis (Carmeliet et al., 1997,  Circulation ); stimulation of apolipoprotein A-1 (ApoA1) over-expression to rebalance serum levels of LDL and HDL; use of apoptosis gene products to promote cell death (of SMC) and cytotactic gene products to that regulate cell division (tumor suppressor protein p53 and Gax homeobox gene product to suppress ras; p21 over expression); and inhibition of NFKB activation (e.g., p65) to control SMC proliferation (Autieri et al., 1994,  Biochem Biophys Res Commun ). 
   Other therapeutic substances that may be useful as stent coatings and/or depot formulations incorporated within degradable stents include: antibodies to ICAM-1 for inhibition of monocyte chemotactic recruitment and adhesion, macrophage adhesion and associated events (Yasukawa et al, 1996,  Circulation ); toxin based therapies such as chimeric toxins or single toxins to control vascular SMC proliferation (Epstein et al., 1991,  Circulation ); bFGF-saporin to selectively stop SMC proliferation among those cells with a large number of FGF-2 receptors (Chen et al, 1995,  Circulation ), suramin inhibits migration and proliferation by blocking PDGF-induced and/or mitogen activated protein kinase (MAPK-AP-1)-induced signaling (Hu et al.,  Circulation,  1999); Beraprost Sodium, a chemically stable prostacyclin analogue (PG I 2 ), suppresses intimal thickening and lumenal narrowing of coronary arteries. (Kurisu et al,  Hiroshima J. Med Sci,  1997); Verapamil inhibits neointimal smooth muscle cell proliferation (Brauner et al.,  J Thorac Cardiovasc Surg  1997), agents that block the CD154 or CD40 receptor may limit the progression of atherosclerosis (E Lutgens et al.,  Nature Medicine  1999), agents that control responses of shear stress response elements or mechanical stress or strain elements or heat shock genes; and anti-chemoattractants for SMC and inflammatory cells. 
   In addition or in the alternative, cells could be encapsulated in a degradable microsphere, or mixed directly with polymer, or hydrogel and serve as vehicle for pharmaceutical delivery. Living cells could be used to continuously deliver pharmaceutical type molecules, for instance, cytokines and growth factors. Nonliving cells could also serve as a limited or timed release system. Cells or any origin may be used in accordance with this aspect of the present invention. Further, preserved or dehydrated cells which retain their viability when rehydrated may be used. Native, chemically modified (processed), and/or genetically engineered cells may be used. 
   Stent Deployment 
   Stents can be deployed in a body lumen by means appropriate to their design. One such method would be to fit the collapsed stent over an inflatable element of a balloon catheter and expand the balloon to force the stent into contact with the body lumen. As the balloon is inflated, the problem material in the vessel is compressed in a direction generally perpendicular to the wall of the vessel which, consequently, dilates the vessel to facilitate blood flow therethrough. Radial expansion of the coronary artery occurs in several different dimensions and is related to the nature of the plaque. Soft, fatty plaque deposits are flattened by the balloon and hardened deposits are cracked and split to enlarge the lumen. It is desirable to have the stent radially expand in a uniform manner. 
   Alternatively, the stent may be mounted onto a catheter that holds the stent as it is delivered through the body lumen and then releases the stent and allows it to self-expand into contact with the body lumen. This deployment is effected after the stent has been introduced percutaneously, transported transluminally and positioned at a desired location by means of the catheter. The retaining means may comprise a removable sheath. 
   The popular stents in use today are stiffer than desired. Their relative flexibility is shown in  FIGS. 14A and 14B . The flexibility of undeployed/mounted stents is shown in  FIG. 14A . All deflection tests were conducted in saline at body temperature as defined in the ASTM standards for stent measurements. The S540 (2.5×18 mm) and S670 (3.0×18 mm) stents are produced by Medtronic, the TRISTAR® (2.5×18 mm) is made by Guidant, VELOCITY (2.5×13 mm) is produced by J&amp;J, and the Nir (2.5×32 mm) is marketed by Boston Scientific. The results shown in  FIG. 14A  (undeployed on a delivery catheter) indicate that the other stents tested are more than 2-fold stiffer than the stent (MD3) made in accordance with the present invention. The difference in flexibility of the deployed (expanded) stents is even more pronounced, as illustrated in  FIG. 14B . 
   Because of the very low profile, small collapsed diameter and great flexibility, stents made in accordance with the present invention may be able to navigate small or torturous paths. Thus, the low-profile stent of the present invention may be useful in coronary arteries, carotid arteries, vascular aneurysms (when covered with a sheath), and peripheral arteries and veins (e.g., renal, iliac, femoral, popliteal, sublavian, aorta, intercranial, etc.). Other nonvascular applications include gastrointestinal, duodenum, biliary ducts, esophagus, urethra, reproductive tracts, trachea, and repiratory (e.g., bronchial) ducts. These applications may or may not require a sheath covering the stent. 
   The stents of the present invention are adapted for deployment using conventional methods known in the art and employing percutaneous transluminal catheter devices. The stents are designed for deployment by any of a variety of in situ expansion means, such as an inflatable balloon or a polymeric plug that expands upon application of pressure. For example, the tubular body of the stent is first positioned to surround a portion of an inflatable balloon catheter. The stent, with the balloon catheter inside is configured at a first, collapsed diameter. The stent and the inflatable balloon are percutaneously introduced into a body lumen, following a previously positioned guidewire in an over-the-wire angioplasty catheter system, and tracked by a fluoroscope, until the balloon portion and associated stent are positioned within the body passageway at the point where the stent is to be placed. Thereafter, the balloon is inflated and the stent is expanded by the balloon portion from the collapsed diameter to a second expanded diameter. After the stent has been expanded to the desired final expanded diameter, the balloon is deflated and the catheter is withdrawn, leaving the stent in place. The stent may be covered by a removable sheath during delivery to protect both the stent and the vessels. 
   The expanded diameter is variable and determined by the desired expanded internal diameter of the body passageway. Accordingly, the controlled expansion of the stent is not likely to cause a rupture of the body passageway. Furthermore, the stent will resist recoil because the locking means resist sliding of the elongated ribs within the articulating mechanism on the end portions of the radial elements. Thus, the expanded intraluminal stent will continue to exert radial pressure outward against the wall of the body passageway and will therefore, not migrate away from the desired location. 
   While a number of preferred embodiments of the invention and variations thereof have been described in detail, other modifications and methods of using and medical applications for the same will be apparent to those of skill in the art. Accordingly, it should be understood that various applications, modifications, and substitutions may be made of equivalents without departing from the spirit of the invention or the scope of the claims.