Patent Publication Number: US-2006020325-A1

Title: Material for high strength, controlled recoil stent

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to alloys for use in manufacturing or fabricating implantable medical devices, and more particularly, to intravascular stents manufactured or fabricated from alloys that provide high strength and controlled recoil.  
      2. Discussion of the Related Art  
      Currently manufactured intravascular stents do not adequately provide sufficient tailoring of the microstructural properties of the material forming the stent to the desired mechanical behavior of the device under clinically relevant in-vivo loading conditions. Any intravascular device should preferably exhibit certain characteristics, including maintaining vessel patency through a chronic outward force that will help to remodel the vessel to its intended luminal diameter, preventing excessive radial recoil upon deployment, exhibiting sufficient fatigue resistance and exhibiting sufficient ductility so as to provide adequate coverage over the full range of intended expansion diameters.  
      Accordingly, there is a need to develop precursory materials and the associated processes for manufacturing intravascular stents that provide device designers with the opportunity to engineer the device to specific applications.  
     SUMMARY OF THE INVENTION  
      The present invention overcomes the limitations of applying conventionally available materials to specific intravascular therapeutic applications as briefly described above.  
      In accordance with one aspect, the present invention is directed to an intraluminal stent. The stent comprises a plurality of hoop components being formed as a continuous series of substantially circumferentially oriented radial strut members and alternating radial arc members and one or more flexible connectors being formed as a continuous series of substantially longitudinally oriented flexible strut members and alternating flexible arc members. The one or more flexible connectors are utilized to connect adjacent hoop components to form a substantially tubular structure having a luminal surface and an abluminal surface and an unexpanded and expanded inside diameter. The wall thickness is defined as the radial distance between the luminal surface and the abluminal surface of the substantially tubular structure. The intraluminal stent may be fabricated from a metallic material processed to have a microstructure, in at least the radial arc and flexible arc members, with a granularity of about 32 microns or less and comprise from about 2 to about 10 substantially equiaxed grains as measured across the wall thickness.  
      The biocompatible material for implantable medical devices of the present invention offers a number of advantages over currently utilized materials. The biocompatible material of the present invention is magnetic resonance imaging compatible, is less brittle than other metallic materials, has enhanced ductility and toughness, and has increased durability. The biocompatible material also maintains the desired or beneficial characteristics of currently available metallic materials, including strength and flexibility.  
      The biocompatible material for implantable medical devices of the present invention may be utilized for any number of medical applications, including vessel patency devices such as vascular stents, biliary stents, ureter stents, vessel occlusion devices such as atrial septal and ventricular septal occluders, patent foramen ovale occluders and orthopedic devices such as fixation devices.  
      The biocompatible material of the present invention is simple and inexpensive to manufacture. The biocompatible material may be formed into any number of structures or devices. The biocompatible alloy may be thermomechanically processed, including cold-working and heat treating, to achieve varying degrees of strength and ductility. The biocompatible material of the present invention may be age hardened to precipitate one or more secondary phases.  
      The intraluminal stent of the present invention may be specifically configured to optimize the number of discrete equiaxed grains that comprise the wall dimension so as to provide the intended user with a high strength, controlled recoil device as a function of expanded inside diameter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The foregoing and other features and advantages of the invention will be apparent from the following, more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.  
       FIG. 1  is a graphical representation of the transition of critical mechanical properties as a function of thermomechanical processing for Cobalt-Chromium alloys in accordance with the present invention.  
       FIG. 2  is a graphical representation of the endurance limit chart as a function of thermomechanical processing for a Cobalt-Chromium alloy in accordance with the present invention.  
       FIG. 3  is a planar representation of an exemplary stent fabricated from the biocompatible alloy in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Biocompatible, solid-solution strengthened alloys such as iron-based alloys, cobalt-based alloys and titanium-based alloys as well as refractory metals and refractory-based alloys may be utilized in the manufacture of any number of implantable medical devices. The biocompatible alloy for implantable medical devices in accordance with the present invention offers a number of advantages over currently utilized medical grade alloys. The advantages include the ability to engineer the underlying microstructure in order to sufficiently perform as intended by the designer without the limitations of currently utilized materials and manufacturing methodologies.  
      For reference, a traditional stainless steel alloy such as 316L (i.e. UNS S31603) which is broadly utilized as an implantable, biocompatible device material may comprise chromium (Cr) in the range from about 16 to 18 wt. %, nickel (Ni) in the range from about 10 to 14 wt. %, molybdenum (Mo) in the range from about 2 to 3 wt. %, manganese (Mn) in the range up to 2 wt. %, silicon (Si) in the range up to 1 wt. %, with iron (Fe) comprising the balance (approximately 65 wt. %) of the composition.  
      Additionally, a traditional cobalt-based alloy such as L605 (i.e. UNS R30605) which is also broadly utilized as an implantable, biocompatible device material may comprise chromium (Cr) in the range from about 19 to 21 wt. %, tungsten (W) in the range from about 14 to 16 wt. %, nickel (Ni) in the range from about 9 to 11 wt. %, iron (Fe) in the range up to 3 wt. %, manganese (Mn) in the range up to 2 wt. %, silicon (Si) in the range up to 1 wt. %, with cobalt (cobalt) comprising the balance (approximately 49 wt. %) of the composition.  
      In general, elemental additions such as chromium (Cr), nickel (Ni), tungsten (W), manganese (Mn), silicon (Si) and molybdenum (Mo) where added to iron- and/or cobalt-based alloys, where appropriate, to increase or enable desirable performance attributes, including strength, machinability and corrosion resistance within clinically relevant usage conditions.  
      In accordance with one exemplary embodiment, a cobalt-based alloy may comprise from about nil to about metallurgically insignificant trace levels of elemental iron (Fe) and elemental silicon (Si), elemental iron only, or elemental silicon only. For example, the cobalt-based alloy may comprise chromium in the range from about 10 weight percent to about 30 weight percent, tungsten in the range from about 5 weight percent to about 20 weight percent, nickel in the range from about 5 weight percent to about 20 weight percent, manganese in the range from about 0 weight percent to about 5 weight percent, carbon in the range from about 0 weight percent to about 1 weight percent, iron in an amount not to exceed 0.12 weight percent, silicon in an amount not to exceed 0.12 weight percent, phosphorus in an amount not to exceed 0.04 weight percent, sulfur in an amount not to exceed 0.03 weight percent and the remainder cobalt. Alternately, the cobalt-based alloy may comprise chromium in the range from about 10 weight percent to about 30 weight percent, tungsten in the range from about 5 weight percent to about 20 weight percent, nickel in the range from about 5 weight percent to about 20 weight percent, manganese in the range from about 0 weight percent to about 5 weight percent, carbon in the range from about 0 weight percent to about 1 weight percent, iron in an amount not to exceed 0.12 weight percent, silicon in an amount not to exceed 0.4 weight percent, phosphorus in an amount not to exceed 0.04 weight percent, sulfur in an amount not to exceed 0.03 weight percent and the remainder cobalt. In yet another alternative composition, the cobalt-based alloy may comprise chromium in the range from about 10 weight percent to about 30 weight percent, tungsten in the range from about 5 weight percent to about 20 weight percent, nickel in the range from about 5 weight percent to about 20 weight percent, manganese in the range from about 0 weight percent to about 5 weight percent, carbon in the range from about 0 weight percent to about 1 weight percent, iron in an amount not to exceed 3 weight percent, silicon in an amount not to exceed 0.12 weight percent, phosphorus in an amount not to exceed 0.04 weight percent, sulfur in an amount not to exceed 0.03 weight percent and the remainder cobalt.  
      In accordance with another exemplary embodiment, a cobalt-based alloy may comprise nickel in the range from about 1 weight percent to about 42 weight percent, chromium in the range from about 4 weight percent to about 36 weight percent, molybdenum in the range from about 4 weight percent to about 16 weight percent, iron in the range from about 0 weight percent to about 1 weight percent, titanium in the range from about 0 weight percent to about 1 weight percent, manganese in the range from about 0 weight percent to about 0.15 percent, silicon in the range from about 0 weight percent to about 0.15 weight percent, carbon in the range from about 0 weight percent to about 0.025 weight and the remainder cobalt.  
      It is important to note that any number of alloys and engineered metals, including iron-based alloys, cobalt-based alloys, refractory-based alloys, refractory metals, and titanium-based alloys may be used in accordance with the present invention. However, for ease of explanation, a detailed description of a cobalt-based alloy will be utilized in the following detailed description.  
      An exemplary embodiment may be processed from the requisite elementary raw materials, as set-forth above, by first mechanical homogenization (i.e. mixing) and then compaction into a green state (i.e. precursory) form. If necessary, appropriate manufacturing aids such as hydrocarbon based lubricants and/or solvents (e.g. mineral oil, machine oils, kerosene, isopropanol and related alcohols) be used to ensure complete mechanical homogenization. Additionally, other processing steps such as ultrasonic agitation of the mixture followed by cold compaction to remove any unnecessary manufacturing aides and to reduce void space within the green state may be utilized. It is preferable to ensure that any impurities within or upon the processing equipment from prior processing and/or system construction (e.g. mixing vessel material, transfer containers, etc.) be sufficiently reduced in order to ensure that the green state form is not unnecessarily contaminated. This may be accomplished by adequate cleaning of the mixing vessel before adding the constituent elements by use of surfactant-based cleaners to remove any loosely adherent contaminants.  
      Initial melting of the green state form into a ingot of desired composition, is achieved by vacuum induction melting (VIM) where the initial form is inductively heated to above the melting point of the primary constituent elements within a refractory crucible and then poured into a secondary mold within a vacuum environment (e.g. typically less than or equal to 10 −4  mmHg). The vacuum process ensures that atmospheric contamination is significantly minimized. Upon solidification of the molten pool, the ingot bar is substantially single phase (i.e. compositionally homogenous) with a definable threshold of secondary phase impurities that are typically ceramic (e.g. carbide, oxide or nitride) in nature. These impurities are typically inherited from the precursor elemental raw materials.  
      A secondary melting process termed vacuum arc reduction (VAR) is utilized to further reduce the concentration of the secondary phase impurities to a conventionally accepted trace impurity level (i.e. &lt;1,500 ppm). Other methods maybe enabled by those skilled in the art of ingot formulation that substantially embodies this practice of ensuring that atmospheric contamination is minimized. In addition, the initial VAR step may be following followed by repetitive VAR processing to further homogenize the solid-solution alloy in the ingot form. From the initial ingot configuration, the homogenized alloy will be further reduced in product size and form by various industrially accepted methods such as, but not limited too, ingot peeling, grinding, cutting, forging, forming, hot rolling and/or cold finishing processing steps so as to produce bar stock that may be further reduced into a desired raw material form.  
      In this exemplary embodiment, the initial raw material product form that is required to initiate the thermomechanical processing that will ultimately yield a desired small diameter, thin-walled tube, appropriate for interventional devices, is a modestly sized round bar (e.g. one inch in diameter round bar stock) of predetermined length. In order to facilitate the reduction of the initial bar stock into a much smaller tubing configuration, an initial clearance hole must be placed into the bar stock that runs the length of the product. These tube hollows (i.e. heavy walled tubes) may be created by ‘gun-drilling’ (i.e. high depth to diameter ratio drilling) the bar stock. Other industrially relevant methods of creating the tube hollows from round bar stock may be utilized by those skilled-in-the-art of tube making.  
      Consecutive mechanical cold-finishing operations such as drawing through a compressive outer-diameter (OD), precision shaped (i.e. cut), circumferentially complete, diamond die using any of the following internally supported (i.e. inner diameter, ID) methods, but not necessarily limited to these conventional forming methods, such as hard mandrel (i.e. relatively long traveling ID mandrel also referred to as rod draw), floating-plug (i.e. relatively short ID mandrel that ‘floats’ within the region of the OD compressive die and fixed-plug (i.e. the ID mandrel is ‘fixed’ to the drawing apparatus where relatively short workpieces are processed) drawing. These process steps are intended to reduce the outer-diameter (OD) and the corresponding wall thickness of the initial tube hollow to the desired dimensions of the finished product.  
      When necessary, tube sinking (i.e. OD reduction of the workpiece without inducing substantial tube wall reduction) is accomplished by drawing the workpiece through a compressive die without internal support (i.e. no ID mandrel). Conventionally, tube sinking is typically utilized as a final or near-final mechanical processing step to achieve the desired dimensional attributed of the finished product.  
      Although not practically significant, if the particular compositional formulation will support a single reduction from the initial raw material configuration to the desired dimensions of the finished product, in process heat-treatments will not be necessary. Where necessary to achieve intended mechanical properties of the finished product, a final heat-treating step is utilized.  
      Conventionally, all metallic alloys in accordance with the present invention will require incremental dimensional reductions from the initial raw material configuration to reach the desired dimensions of the finished product. This processing constraint is due to the material&#39;s ability to support a finite degree of induced mechanical damage per processing step without structural failure (e.g. strain-induced fracture, fissures, extensive void formation, etc.).  
      In order to compensate for induced mechanical damage (i.e. cold-working) during any of the aforementioned cold-finishing steps, periodic thermal heat-treatments are utilized to stress-relieve (i.e. minimization of deleterious internal residual stresses that are the result of processes such as cold-working) thereby increasing the workability (i.e. ability to support additional mechanical damage without measurable failure) the workpiece prior to subsequent reductions. These thermal treatments are typically, but not necessarily limited to, conducted within a relatively inert environment such as an inert gas furnace (e.g. nitrogen, argon, etc.), a oxygen rarified hydrogen furnace, a conventional vacuum furnace and under less common process conditions, atmospheric air. When vacuum furnaces are utilized, the level of vacuum (i.e. subatmospheric pressure), typically measured in units of mmHg or torr (where 1 mmHg is equal to 1 unit torr), shall be sufficient to ensure that excessive and deteriorative high temperature oxidative processes are not functionally operative during heat treatment. This process may usually be achieved under vacuum conditions of 10 −4  mmHg (0.0001 torr) or less (i.e. lower magnitude).  
      The stress relieving heat treatment temperature is typically held constant between 82 to 86% of the conventional melting point (i.e. industrially accepted liquidus temperature, 0.82 to 0.86 homologous temperatures) within an adequately sized isothermal region of the heat-treating apparatus. The workpiece undergoing thermal treatment is held within the isothermal processing region for a finite period of time that is adequate to ensure that the workpiece has reached a state of thermal equilibrium and for that sufficient time is elapsed to ensure that the reaction kinetics (i.e. time dependent material processes) of stress-relieving and/or process annealing, as appropriate, is adequately completed. The finite amount of time that the workpiece is held within the processing is dependent upon the method of bringing the workpiece into the process chamber and then removing the working upon completion of heat treatment. Typically, this process is accomplished by, but not limited to, use of a conventional conveyor-belt apparatus or other relevant mechanical assist devices. In the case of the former, the conveyor belt speed and appropriate finite dwell-time, as necessary, within the isothermal region is controlled to ensure that sufficient time at temperature is utilized so as to ensure that the process is completed as intended.  
      When necessary to achieve desired mechanical attributes of the finished product, heat-treatment temperatures and corresponding finite processing times may be intentionally utilized that are not within the typical range of 0.82 to 0.86 homologous temperatures. Various age hardening (i.e. a process that induces a change in properties at moderately elevated temperatures, relative to the conventional melting point, that does not induce a change in overall chemical composition change in the metallic alloy being processed) processing steps may be carried out, as necessary, in a manner consistent with those previously described at temperatures substantially below 0.82 to 0.86 homologous temperature. For cobalt-based alloys in accordance with the present invention, these processing temperatures may be varied between and inclusive of approximately 0.29 homologous temperature and the aforementioned stress relieving temperature range. The workpiece undergoing thermal treatment is held within the isothermal processing region for a finite period of time that is adequate to ensure that the workpiece has reached a state of thermal equilibrium and for that sufficient time is elapsed to ensure that the reaction kinetics (i.e. time dependent material processes) of age hardening, as appropriate, is adequately completed prior to removal from the processing equipment.  
      In some cases for cobalt-based alloys in accordance with the present invention, the formation of secondary-phase ceramic compounds such as carbide, nitride and/or oxides will be induced or promoted by age hardening heat-treating. These secondary-phase compounds are typically, but not limited to, for cobalt-based alloys in accordance with the present invention, carbides which precipitate along thermodynamically favorable regions of the structural crystallographic planes that comprise each grain (i.e. crystallographic entity) that make-up the entire polycrystalline alloy. These secondary-phase carbides can exist along the intergranular boundaries as well as within each granular structure (i.e. intragranular). Under most circumstances for cobalt-based alloys in accordance with the present invention, the principal secondary phase carbides that are stoichiometrically expected to be present are M 6 C where M typically is Cobalt (cobalt). When present, the intermetallic M 6 C phase is typically expected to reside intragranularly along thermodynamically favorable regions of the structural crystallographic planes that comprise each grain within the polycrystalline alloy in accordance with the present invention. Although not practically common, the equivalent material phenomena can exist for a single crystal (i.e. monogranular) alloy.  
      Additionally, another prominent secondary phase carbide can also be induced or promoted as a result of age hardening heat treatments. This phase, when present, is stoichiometrically expected to be M 23 C 6  where M typically is Chromium (Cr) but is also commonly observed to be Cobalt (cobalt) especially in cobalt-based alloys. When present, the intermetallic M 23 C 6  phase is typically expected to reside along the intergranular boundaries (i.e. grain boundaries) within a polycrystalline alloy in accordance with the present invention. As previously discussed for the intermetallic M 6 C phase, the equivalent presence of the intermetallic M 23 C 6  phase can exist for a single crystal (i.e. monogranular) alloy, albeit not practically common.  
      In the case of the intergranular M 23 C 6  phase, this secondary phase is conventionally considered most important, when formed in a manner that is structurally and compositionally compatible with the alloy matrix, to strengthening the grain boundaries to such a degree that intrinsic strength of the grain boundaries and the matrix are adequately balanced. By inducing this equilibrium level of material strength at the microstructural level, the overall mechanical properties of the finished tubular product can be further optimized to desirable levels.  
      In addition to stress relieving and age hardening related heat-treating steps, solutionizing (i.e. sufficiently high temperature and longer processing time to thermodynamically force one of more alloy constituents to enter into solid solution—‘singular phase’, also referred to as full annealing) of the workpiece may be utilized. For cobalt-based alloys in accordance with the present invention, the typical solutionizing temperature can be varied between and inclusive of approximately 0.88 to 0.90 homologous temperatures. The workpiece undergoing thermal treatment is held within the isothermal processing region for a finite period of time that is adequate to ensure that the workpiece has reached a state of thermal equilibrium and for that sufficient time is elapsed to ensure that the reaction kinetics (i.e. time dependent material processes) of solutionizing, as appropriate, is adequately completed prior to removal from the processing equipment.  
      The sequential and selectively ordered combination of thermomechanical processing steps that may comprise but not necessarily include mechanical cold-finishing operations, stress relieving, age hardening and solutionizing can induce and enable a broad range of measurable mechanical properties as a result of distinct and determinable microstructural attributes. This material phenomena can be observed in  FIG. 1 . which shows a chart that exhibits the affect of thermomechanical processing (TMP) such as cold working and in-process heat-treatments on measurable mechanical properties such as yield strength and ductility (presented in units of percent elongation) in accordance with the present invention. In this example, thermomechanical (TMP) groups one (1) through five (5) were subjected to varying combinations of cold-finishing, stress relieving and age hardening and not necessarily in the presented sequential order. In general, the principal isothermal age hardening heat treatment applied to each TMP group varied between about 0.74 to 0.78 homologous temperatures for group (1), about 0.76 to 0.80 homologous temperatures for group (2), about 0.78 to 0.82 homologous temperatures for group (3), about 0.80 to 0.84 homologous temperatures for group (4) and about 0.82 to 0.84 homologous temperatures for group (5). The each workpiece undergoing thermal treatment was held within the isothermal processing region for a finite period of time that was adequate to ensure that the workpiece reached a state of thermal equilibrium and to ensure that sufficient time was elapsed to ensure that the reaction kinetics of age hardening was adequately completed.  
      More so, the effect of thermomechanical processing (TMP) on cyclic fatigue properties is on cobalt-based alloys, in accordance with the present invention, is reflected in  FIG. 2 . Examination of  FIG. 2 , shows the affect on fatigue strength (i.e. endurance limit) as a function of thermomechanical processing for the previously discussed TMP groups (2) and (4). TMP group (2) from this figure as utilized in this specific example shows a marked increase in the fatigue strength (i.e. endurance limit, the maximum stress below which a material can presumably endure an infinite number of stress cycles) over and against the TMP group (4) process.  
      Once the all intended processing is complete, the tubular product may be configured into any number of implantable medical devices including intravascular stents, filters, occlusionary devices, shunts and embolic coils. In accordance with the present invention, the tubular product is configured into a stent. Preferred material characteristics of a stent include strength, fatigue robustness and sufficient ductility.  
      Strength is an intrinsic mechanical attribute of the raw material. As a result of prior thermomechanical processing, the resultant strength attribute can be assigned primarily to the underlying microstructure that comprises the raw material. The causal relationship between material structure, in this instance, grain size, and the measurable strength, in this instance yield strength, is explained by the classical Hall-Petch relationship where strength is inversely proportional the square of grain size as given by,
 
σ y ∝1/√ {square root over (G.S.)}′   (1)
 
 wherein σ y  is the yield strength as measured in MPa and G.S. is grain size as measured in millimeters as the average granular diameter. The strength attribute specifically affects the ability of the intravascular device to maintain vessel patency under in-vivo loading conditions. 
 
      The causal relationship between balloon-expandable device recoil (i.e. elastic “spring-back” upon initial unloading by deflation of the deployment catheter&#39;s balloon) and strength, in this instance yield strength, is principally affected by grain size. As previously described, a decrement in grain-size results in higher yield strength as shown above. Accordingly, the measurable device recoil is inversely proportional to the grain size of the material.  
      The causal relationship between fatigue resistance, in this instance endurance limit or the maximum stress below which a material can presumably endure an infinite number of stress cycles, and strength, in this instance yield strength, is principally affected by grain size. Although fatigue resistance is also affected by extrinsic factors such as existing material defects, for example, stable cracks and processing flaws, the principal intrinsic factor affecting fatigue resistance for a given applied load is material strength. As previously described, a decrement in grain-size results in higher yield strength as shown above. Accordingly, the endurance limit (i.e. fatigue resistance) is inversely proportional to the grain size of the material.  
      The causal relationship between ductility, in this instance the material&#39;s ability to support tensile elongation without observable material fracture (i.e. percent elongation), is significantly affected by grain size. Typically, ductility is inversely proportional to strength that would imply a direct relationship to grain size.  
      In accordance with the exemplary embodiment described wherein, microstructural attributes, in this instance, grain-size, may be configured to be equal to or less than about 32 microns in average diameter. In order to ensure that all of the measurable mechanical attributes are homogenous and isotropic within the intended stent, an equiaxed distribution of granularity is preferable. So as to ensure that the structural properties of the intended stent are configured in the preferred manner, a minimum of about two structurally finite intergranular elements (i.e. grains) to a maximum of about ten structurally finite intergranular elements shall exist within a given region of the stent. In particular, the number of grains may be measured as the distance between the abluminal and the luminal surface of the stent (i.e. wall thickness). While these microstructural aspects may be tailored throughout the entirety of the stent, it may be particularly advantageous to configure the deformable regions of the stent with these microstructural aspects as described in detail below.  
      Referring to  FIG. 3 , there is illustrated a partial planar view of an exemplary stent  100  in accordance with the present invention. The exemplary stent  100  comprises a plurality of hoop components  102  interconnected by a plurality of flexible connectors  104 . The hoop components  102  are formed as a continuous series of substantially circumferentially oriented radial strut members  106  and alternating radial arc members  108 . Although shown in planar view, the hoop components  102  are essentially ring members that are linked together by the flexible connectors  104  to form a substantially tubular stent structure. The combination of radial strut members  106  and alternating radial arc members  108  form a substantially sinusoidal pattern. Although the hoop components  102  may be designed with any number of design features and assume any number of configurations, in the exemplary embodiment, the radial strut members  106  are wider in their central regions  110 . This design feature may be utilized for a number of purposes, including, increased surface area for drug delivery.  
      The flexible connectors  104  are formed from a continuous series of substantially longitudinally oriented flexible strut members  112  and alternating flexible arc members  114 . The flexible connectors  104 , as described above, connect adjacent hoop components  102  together. In this exemplary embodiment, the flexible connectors  104  have a substantially N-shape with one end being connected to a radial arc member on one hoop component and the other end being connected to a radial arc member on an adjacent hoop component. As with the hoop components  102 , the flexible connectors  104  may comprise any number of design features and any number of configurations. In the exemplary embodiment, the ends of the flexible connectors  104  are connected to different portions of the radial arc members of adjacent hoop components for ease of nesting during crimping of the stent. It is interesting to note that with this exemplary configuration, the radial arcs on adjacent hoop components are slightly out of phase, while the radial arcs on every other hoop component are substantially in phase. In addition, it is important to note that not every radial arc on each hoop component need be connected to every radial arc on the adjacent hoop component.  
      The substantially tubular structure of the stent  100  provides the scaffolding for maintaining the patentcy of substantially tubular organs, such as arteries. The stent  100  comprises a luminal surface and an abluminal surface. The distance between the two surfaces defines the wall thickness as is described in detail above. The stent  100  has an unexpanded diameter for delivery and an expanded diameter which roughly corresponds to the normal diameter of the organ into which it is delivered. As tubular organs such as arteries may vary in diameter, different size stents having different sets of unexpanded and expanded diameters may be designed without departing from the spirit of the present invention. As described herein, the stent  100  may be formed form any number of metallic materials, including cobalt-based alloys, iron-based alloys, titanium-based alloys, refractory-based alloys and refractory metals.  
      In the exemplary stent described above, a number of examples may be utilized to illustrate the relationship of equiaxed granularity to wall thickness. In the first example, the wall thickness may be varied in the range from about 0.0005 inches to about 0.006 inches for a stent having an expanded inside diameter of less than about 2.5 millimeters. Accordingly, for a maximal number of equiaxed grains, which in the exemplary embodiment is substantially not more than ten (10) discrete grains across the thickness of the wall, the equiaxed grain size shall be equal to or greater than substantially 1.25 microns. This dimensional attribute may be arrived at by simply dividing the minimal available wall thickness by the maximal number of available equiaxed grains. In another example, the wall thickness may be varied in the range from about 0.002 inches to about 0.008 inches for a stent having an expanded inside diameter from about 2.5 millimeters to about 5.0 millimeters. Accordingly, for a maximal number of equiaxed grains, which in the exemplary embodiment is substantially not more than ten (10) discrete grains across the thickness of the wall, the equiaxed grain size shall be equal to or greater than substantially 5.0 microns. In yet another example, the wall thickness may be varied in the range from about 0.004 inches to about 0.012 inches for a stent having an expanded inside diameter from about 5.0 millimeters to about 12.0 millimeters. Accordingly, for a maximal number of equiaxed grains, which in the exemplary embodiment is substantially not more than ten (10) discrete grains across the thickness of the wall, the equiaxed grain size shall be equal to or greater than substantially 10.0 microns. In yet still another example, the wall thickness may be varied in the range from about 0.006 inches to about 0.025 inches for a stent having an expanded inside diameter from about 12.0 millimeters to about 50.0 millimeters. Accordingly, for a maximal number of equiaxed grains, which in the exemplary embodiment is substantially not more than ten (10) discrete grains across the thickness of the wall, the equiaxed grain size shall be equal to or greater than substantially 15.0 microns. In making the above calculations, it is important to maintain rigorous consistency of dimensional units.  
      Although shown and described is what is believed to be the most practical and preferred embodiments, it is apparent that departures from specific designs and methods described and shown will suggest themselves to those skilled in the art and may be used without departing from the spirit and scope of the invention. The present invention is not restricted to the particular constructions described and illustrated, but should be constructed to cohere with all modifications that may fall within the scope for the appended claims.