Abstract:
Stent delivery system having a contracted delivery configuration and an expanded deployed configuration is provided. The stent delivery system includes a stent having a plurality of expandable elements and a plurality of interstices disposed between adjacent expandable elements, and a delivery catheter having an inflatable balloon including creases extending non-uniformly within the interstices of the stent in the contracted delivery configuration. Each crease defines a maximum radial height within a corresponding interstice, and the maximum radial heights of the creases vary. A method for stenting at a target site within a patient&#39;s vessel including providing a stent delivery system is also provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 11/064,692, filed Feb. 23, 2005, which is a divisional of U.S. patent application Ser. No. 09/957,216, filed Sep. 19, 2001, now U.S. Pat. No. 6,863,683 issued Mar. 8, 2005, the contents of each of which are incorporated by reference in their entireties, and to each of which priority is claimed. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a cold-molding process for loading a stent onto a stent delivery system. More specifically, the present invention relates to a method of loading a stent onto a balloon having creases that extend non-uniformly into the interstices of the stent without the use of a heating step. 
       BACKGROUND OF THE INVENTION 
       [0003]    A stent is commonly used alone or in conjunction with angioplasty to ensure patency through a patient&#39;s stenosed vessel. Stents overcome the natural tendency of the vessel walls of some patients to restenose after angioplasty. A stent is typically inserted into a vessel, positioned across a lesion, and then expanded to create or maintain a passageway through the vessel, thereby restoring near-normal blood flow through the vessel. 
         [0004]    A variety of stents are known in the art, including self-expandable and expandable stents, as well as wire braid stents. One such stent is described, for example, in U.S. Pat. No. 4,733,665 to Palmaz. Expandable stents are typically delivered to treatment sites on delivery devices, such as balloon catheters or other expandable devices. Balloon catheters may comprise a balloon having a collapsed delivery configuration with wings that are wrapped and folded about the catheter. An expandable stent is then disposed in a collapsed delivery configuration about the balloon by compressing the stent onto the balloon. The stent and balloon assembly may then be delivered, using well-known percutaneous techniques, to a treatment site within the patient&#39;s vasculature, for example, within the patient&#39;s coronary arteries. Once the stent is positioned across a lesion at the treatment site, it is expanded to a deployed configuration by inflating the balloon. The stent contacts the vessel wall and maintains a path for blood flow through the vessel. 
         [0005]    Significant difficulties have been encountered during stent delivery and deployment, including difficulty in maintaining the stent on the balloon and in achieving symmetrical expansion of the stent when deployed. Several techniques have been developed to more securely anchor the stent to the balloon and to ensure more symmetrical expansion. These include plastically deforming the stent so that it is crimped onto the balloon, and sizing the stent such that its internal diameter provides an interference fit with the outside diameter of the balloon catheter. Such techniques have several drawbacks, including less than optimal securement of the stent to the balloon. Consequently, the stent may become prematurely dislodged from the balloon during advancement of the stent delivery system to the treatment site. 
         [0006]    Stent delivery systems utilizing a removable sheath disposed over the exterior surface of the stent, which is removed once the stent is positioned at the treatment site, have also been proposed, for example, in U.S. Pat. No. 5,690,644 to Yurek et al. Such systems may be used with or without retainer rings and are intended to protect the stent during delivery and to provide a smooth surface for easier passage through the patient&#39;s vasculature. However, the exterior sheath increases the crossing profile of the delivery system while decreasing flexibility, thereby decreasing the ability of the device to track through narrowed and tortuous anatomy. 
         [0007]    U.S. Pat. No. 6,106,530 to Harada describes a stent delivery device comprising a balloon catheter having stoppers disposed proximal and distal of a balloon on to which a stent is affixed for delivery. The stoppers are separate from the balloon and maintain the stent&#39;s position in relation to the balloon during delivery. As with the removable sheaths discussed previously, the stoppers are expected to increase delivery profile and decrease flexibility of the stent/balloon system. 
         [0008]    U.S. Pat. No. 6,110,180 to Foreman et al. provides a catheter with a balloon having pre-formed, outwardly-extending protrusions on the exterior of the balloon. A stent may be crimped onto the balloon such that the protrusions extend into the gaps of the stent, thereby securing the stent about the balloon for delivery. A drawback to this device is the added complexity involved in manufacturing a balloon with pre-formed protrusions. Additionally, if the protrusions are not formed integrally with the balloon, there is a risk that one or more of the protrusions may detach during deployment of the stent. The protrusions may also reduce flexibility in the delivery configuration, thereby reducing ability to track through tortuous anatomy. 
         [0009]    U.S. Pat. No. 5,836,965 to Jendersee et al. describes a hot-molding process for encapsulating a stent on a delivery system. Encapsulation entails placement of the stent over a balloon, placement of a sheath over the stent on the balloon, and heating the pressurized balloon to cause it to expand around the stent within the sheath. The assembly is then cooled while under pressure to cause the balloon to adhere to the stent and to set the shape of the expanded balloon, thereby providing substantially uniform contact between the balloon and the stent. This method also provides a substantially uniform delivery profile along the surface of the encapsulated balloon/stent assembly. 
         [0010]    A significant drawback of Jendersee&#39;s encapsulation method is the need to heat the balloon in order to achieve encapsulation. Such heating while under pressure may lead to localized plastic flows resulting in inhomogeneities along the length of the balloon including, for example, varying wall thickness. Varying wall thickness may, in turn, yield areas of decreased strength that are susceptible to rupture upon inflation of the balloon during deployment of the stent. Additionally, heating and cooling increases the complexity, time, and cost associated with affixing the stent to the balloon. 
         [0011]    U.S. Pat. No. 5,976,181 to Whelan et al. provides an alternative technique for stent fixation involving the use of solvents to soften the balloon material. In this method, the stent is disposed over an evacuated and wrapped balloon while in its compact delivery configuration. A rigid tube is then placed over the stent and balloon assembly, and the balloon is pressurized while the balloon is softened by application of a solvent and/or heating. The rigid tube prevents the stent from expanding but allows the balloon to deform so that its surface projects through either or both of the interstices and ends of the stent. Softening under pressure molds the balloon material such that it takes a permanent set into the stent. Once pressure is removed, the stent is interlocked with the surface of the balloon, providing substantially uniform contact between the balloon and the stent and a substantially uniform delivery profile. 
         [0012]    As with the technique in the Jendersee patent, the technique in the Whelan patent has several drawbacks. Chemically softening the balloon material under pressure is expected to introduce inhomogeneities along the length of the balloon, such as varying wall thickness, which again may lead to failure of the balloon. Additionally, chemical alteration of the balloon, via application of a solvent to the surface of the balloon, may unpredictably degrade the mechanical characteristics of the balloon, thereby making accurate and controlled deployment of a stent difficult. Softening also adds cost, complexity, and time to the manufacturing process. 
         [0013]    In view of the drawbacks associated with previously known methods and apparatus for loading a stent onto a stent delivery system, it would be desirable to provide methods and apparatus that overcome those drawbacks. 
         [0014]    It would be desirable to provide methods and apparatus for loading a stent onto a stent delivery system that enhance positional stability of the stent during delivery. 
         [0015]    It would further be desirable to provide methods and apparatus for loading a stent onto a stent delivery system wherein the delivery system comprises a crossing profile and flexibility suitable for use in tortuous and narrowed anatomy. 
         [0016]    It would still further be desirable to provide methods and apparatus for loading a stent onto a stent delivery system that provide a substantially symmetrical expansion of the stent at deployment. 
         [0017]    It would also be desirable to provide methods and apparatus for loading a stent onto a stent delivery system that do not unpredictably modify the mechanical characteristics of the balloon during fixation of the stent to the balloon. 
       SUMMARY OF THE INVENTION 
       [0018]    In view of the foregoing, it is an object of the present invention to provide methods and apparatus for loading a stent onto a stent delivery system and deployment that overcome drawbacks associated with previously known methods and apparatus. 
         [0019]    It is an object to provide methods and apparatus for loading a stent onto a stent delivery system that enhance positional stability of the stent during delivery. 
         [0020]    It is an object to provide methods and apparatus for loading a stent onto a stent delivery system wherein the delivery system comprises a crossing profile and flexibility suitable for use in tortuous and narrowed anatomy. 
         [0021]    It is also an object to provide methods and apparatus for loading a stent onto a stent delivery system that provide a substantially symmetrical expansion of the stent at deployment. 
         [0022]    It is an object to provide methods and apparatus for loading a stent onto a stent delivery system that do not unpredictably modify the mechanical characteristics of the balloon during fixation of the stent to the balloon. 
         [0023]    These and other objects of the present invention are achieved by providing methods and apparatus for cold-molding a stent to the balloon of a stent delivery system so that the balloon extends non-uniformly into the interstices of the stent. In a preferred embodiment, the stent is a balloon expandable stent and is manufactured in a fully-expanded state or in an intermediate-expanded state (i.e., having a diameter smaller than its fully-expanded, deployed diameter, but larger than its compressed delivery diameter). 
         [0024]    The stent is disposed on the balloon of a delivery catheter, and the balloon and stent are placed within an elastic crimping tube. The balloon/stent/crimping tube assembly is then placed in a crimping tool, and the balloon is inflated, preferably only partially. The crimping tool is actuated to compress the stent on the outside of the partially inflated balloon and to cause creases of the balloon to extend non-uniformly into the interstices of the stent. Crimping occurs at a substantially constant temperature, without the use of chemicals. The balloon is then deflated, and the elastic crimping tube is removed. 
         [0025]    Optionally, pillows or bumpers may be formed in the proximal and/or distal regions of the balloon during crimping that, in conjunction with the non-uniform creases of the balloon, prevent longitudinal movement of the stent with respect to the balloon during intravascular delivery. 
         [0026]    Furthermore, one or more additional, secondary crimping steps may be performed to achieve a smoother delivery profile, in which a semi-rigid crimping tube is disposed over the stent delivery system, and the assembly is again disposed within the crimping tool. During secondary crimping, the crimping tool is actuated to further compress the stent onto the unpressurized balloon. Secondary-crimping may alternatively be performed with the balloon partially or completely pressurized/inflated. 
         [0027]    Apparatus of the present invention may be used with a variety of prior art stents, such as balloon expandable stents, and may include tubular slotted stents, connected stents, articulated stents, multiple connected or non-connected stents, and bi-stable stents. In addition to methods of production, methods of using the apparatus of the present invention are provided. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    Further features of the invention, its nature and various advantages will be more apparent from the following detailed description of the preferred embodiments, taken in conjunction with the accompanying drawings, in which like reference numerals refer to like parts throughout, and in which: 
           [0029]      FIGS. 1A-1C  are, respectively, a side view of a stent delivery system in accordance with the present invention, a cross-sectional view of the system along section line A-A in  FIG. 1A , and a detail view of the balloon of the system non-uniformly extending within the interstices of the stent; 
           [0030]      FIG. 2  is a flow chart showing the steps of the cold-molding process of the present invention; 
           [0031]      FIGS. 3A-3C  are, respectively, a side view of the distal end of the delivery catheter of the system of  FIG. 1  in an expanded configuration, and cross-sectional views of the catheter along section line B-B in  FIG. 3A , showing the balloon evacuated to form radially extended wings and in a contracted configuration with the radially extended wings wrapped about the catheter; 
           [0032]      FIGS. 4A-4C  are, respectively, a side view, partially in section, of the wrapped delivery catheter of  FIG. 3C  having the stent of  FIG. 1  and an elastic crimping tube disposed thereover, the entire assembly disposed within a crimping tool; a cross-sectional view of the same along section line C-C in  FIG. 4A ; and a detail view of the expandable structure of the stent; 
           [0033]      FIGS. 5A and 5B  are, respectively, a cross-sectional view along section line C-C in  FIG. 4A  of the apparatus upon pressurization of the balloon, and a detail view of the expandable structure of the stent; 
           [0034]      FIG. 6  is a cross-sectional view along section line C-c in  FIG. 4A  during crimping after pressure has been removed; 
           [0035]      FIG. 7  is a cross-sectional view along section line C-C in  FIG. 4A  of a possible configuration of the stent delivery system after crimping and removal of the elastic crimping tube; 
           [0036]      FIG. 8  is a side view, partially in section, of the stent delivery system disposed within a semi-rigid crimping tube and within the crimping tool for optional secondary crimping; and 
           [0037]      FIGS. 9A-9D  are side views, partially in section, of the stent delivery system of  FIG. 1  disposed within a patient&#39;s vasculature, depicting a method of using the apparatus in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0038]    The present invention comprises methods and apparatus for cold-molding a stent onto a stent delivery system. More specifically, the present invention provides methods and apparatus for obtaining a balloon having creases that extend non-uniformly into the interstices of a stent loaded onto the exterior of the balloon, without the use of a heating or chemical process. 
         [0039]    With reference to  FIG. 1 , apparatus in accordance with the present invention is described. As seen in  FIG. 1A , stent delivery system  10 , illustratively shown in a collapsed delivery configuration, comprises balloon expandable stent  20  loaded on balloon  14  of delivery catheter  12 . Stent  20  comprises an illustrative balloon expandable stent and may be replaced with other stents known in the art. As seen in  FIGS. 1B and 1C , balloon  14  has creases  16  that extend non-uniformly into interstices  22  of stent  20 . 
         [0040]    In  FIG. 1B , creases  16  are shown with varying slope and height about the circumference of stent delivery system  10 .  FIG. 1C  depicts creases  16  as shaded areas and illustrates that creases  16  extend along the length of stent  20  within interstices  22 . Line L indicates the longitudinal axis of stent  20  in  FIG. 1C . It should be understood that creases  16  typically do not extend within every interstice  22  of stent  20 . 
         [0041]    Delivery catheter  12  preferably includes markers  17  disposed distal of and proximal to stent  20  that facilitate placement of stent  20  on balloon  14 , and that facilitate positioning of stent delivery system  10  at a treatment site within a patient&#39;s vasculature. Markers  17  are preferably radiopaque and fabricated from a radiopaque material, such as platinum or gold. Catheter  12  preferably also comprises guide wire lumen  13  and inflation lumen  15 , which is coupled to balloon  14 . As described hereinbelow, during the cold-molding process of the present invention, proximal and/or distal pillows  19  optionally may be formed in balloon  14  during pressurized crimping. As with creases  16 , pillows  19  act to reduce or prevent longitudinal movement of the stent on the balloon during intravascular delivery. 
         [0042]    Balloon  14  is expandable by injection of a suitable medium, such as air or saline, via inflation lumen  15 . Balloon  14  preferably expands stent  20  to a deployed configuration under application of pressure in the range of about 6-9 atm. Additionally, balloon  14  preferably has a rated burst pressure above 10 atm, and even more preferably between about 12-14 atm. Balloon  14  may be fabricated from a variety of materials, including Nylon, polyethylene terephalate, polyethylene, and polyether/polyamide block copolymers, such as PEBAX. 
         [0043]    Additionally, balloon  14  may be fabricated from an elastomeric polyester block copolymer having an aromatic polyester hard segment and an aliphatic polyester soft segment, such as “Pelprene,” which is marketed by the Toyobo Corporation of Osaka, Japan. Balloon  14  also may be fabricated from a copolymer having a polybutylene terephalate hard segment and a long chain of polyether glycol soft segment, such as “Hytrel” from the DuPont Corporation of Wilmington, Del. 
         [0044]    Illustrative stent  20  may be fabricated from a variety of materials, including polymers and metals, and may comprise any of a variety of prior art stents, such as balloon expandable stents, including tubular slotted stents, connected stents, articulated stents, multiple connected or non-connected stents, and bi-stable stents. Stent  20  also may include external coating C configured to retard restenosis or thrombus formation in the vessel region surrounding the stent. Alternatively, coating C—may—deliver therapeutic agents into-the-patient&#39;s blood stream or vessel wall. 
         [0045]    Referring now to  FIGS. 2-8 , a method of producing stent delivery system  10  is described.  FIG. 2  provides an overview of the cold-molding process of the present invention, while  FIGS. 3-8  provide detailed views of these process steps. 
         [0046]    As depicted in  FIG. 2 , the cold-molding process of the present invention involves steps of: obtaining a stent, step  102 ; obtaining a balloon catheter, step  103 ; disposing the stent on the balloon of the balloon catheter, step  104 ; and disposing an elastic crimping sleeve over the stent and balloon, step  105 . In accordance with the method of the present invention, the balloon is then inflated—preferably only partially—with an inflatable medium, such as air, at step  106 . The sleeve/stent/balloon assembly is then crimped within a crimping tool that compresses the stent onto the balloon, step  107 , while the balloon is pressurized. 
         [0047]    As described hereinbelow, this step causes the balloon to bulge into the interstices of the stent, and in addition, to form pillows  19 , proximal of, and distal to, the ends of the stent to retain the stent in place during transluminal delivery. At step  108 , the balloon is depressurized, and the elastic sleeve is removed to complete the stent loading process. 
         [0048]    If desired, a semi-rigid sleeve optionally may be disposed over the stent/balloon assembly, and one or more additional crimping steps may be performed, steps  109  and  110  of  FIG. 2 . 
         [0049]    Referring now to  FIGS. 3-8 , additional details of a preferred embodiment of the process of the present invention are illustrated and described. In  FIG. 3 , balloon  14  of delivery catheter  12 —preferably is folded prior to placement of stent  20  about balloon  14 . Balloon  14  is first expanded, as in  FIG. 3A , and then evacuated to form radially extended wings  18 , as seen in  FIG. 3B . Balloon  14  is illustratively depicted with four wings  18 , but it should be understood that any number of wings may be provided, for example, two, three or five wings. In  FIG. 3C , wings  18  are wrapped about the shaft of delivery catheter  12  to dispose catheter  12  in a contracted configuration. It should be understood that balloon  14  may alternatively be folded and/or disposed in a collapsed delivery configuration by other techniques, for example, with techniques that do not utilize wings. 
         [0050]    With reference to  FIG. 4 , stent  20  and elastic crimping tube  30  are disposed about balloon  14 , preferably with stent  20  positioned between markers  17  of delivery catheter  12  (steps  102 - 105 ,  FIG. 2 ). The balloon/stent/crimping tube assembly is inserted within crimping tool  40 , as seen in  FIG. 4A . Crimping tool  40  is preferably positioned between markers  17  to facilitate formation of optional pillows  19  during pressurization of balloon  14 . Crimping tool  40  may be any of a variety of crimping tools known in the art. An illustrative crimping tool is described, for example, in U.S. Pat. No. 6,082,990 to Jackson et al., which is incorporated herein by reference. 
         [0051]    Referring to  FIG. 4B , stent  20  may be directly placed about balloon  14 , and elastic crimping tube  30  then may be loaded over the stent/balloon assembly. Alternatively, stent  20  may be placed within elastic crimping tube  30 , and then the stent/tube assembly disposed surrounding balloon  14 . As yet another alternative, crimping tube  30 , or crimping tube  30  and stent  20 , may be positioned within crimping tool  40 ; then, balloon  14 , with or without stent  20  loaded thereon, may be positioned within crimping tool  40 . 
         [0052]    As depicted in  FIG. 4C , stent  20  preferably is manufactured in an intermediate-expanded state having a diameter smaller than its expanded deployed diameter, but larger than its compressed delivery diameter, thereby facilitating positioning of stent  20  about balloon  14 . When stent  20  is initially disposed surrounding balloon  14 , the balloon does not substantially extend into interstices  22  of stent  20 . It should be understood that stent  20  alternatively may be manufactured in a fully-expanded state. 
         [0053]    In  FIG. 5 , once stent  20  and crimping tube  30  are disposed about balloon  14  of delivery catheter  12 , and once the entire assembly is disposed within crimping tool  40 , balloon  14  is pressurized, for example, via an inflation medium delivered through inflation lumen  15  of catheter  12  (step  106 ,  FIG. 2 ). Pressure application causes balloon  14  to enter a portion of interstices  22  of stent  20  in a non-uniform manner, as seen in the cross section of  FIG. 5A  and in the detail view of  FIG. 5B . Crimping tube  30  and crimping tool  40  prevent expansion of stent  20  during partial or complete pressurization of balloon  14 , as depicted in  FIG. 5A . 
         [0054]    The inflation medium is preferably delivered at a pressure in the range of about 6-8 atm. This pressure range is below the preferred rated burst pressure of balloon  14 , which is above 10 atm, and even more preferably between about 12-14 atm, and thus ensures that the balloon does not puncture. The elasticity of crimping tube  30  allows the tube to expand slightly upon application of pressure, and to contract slightly during crimping. Tube  30  may be fabricated from any suitable—elastic material, for example, a polymer, such as PEBAX. Elastic crimping tube  30  preferably has a hardness of between about 30 and 40 Shore Hardness, and more preferably a hardness of about 35 Shore Hardness. 
         [0055]    With reference to  FIG. 6 , in conjunction with  FIG. 4A , crimping tool  40  is actuated to crimp stent  20  onto balloon  14  (step  107 ,  FIG. 2 ). Crimping tool  40  applies an inwardly-directed stress, σ crimp , to the assembly. Initially, balloon  14  is still pressurized. Stent  20  is compressed onto the outside of balloon  14 , causing the balloon to further bulge non-uniformly into interstices  22  of the stent. Crimping preferably proceeds along the length of the balloon/stent/tube assembly all at once but may alternatively proceed in sections, so that the assembly is gradually crimped along its length. 
         [0056]    Balloon  14  is then depressurized, allowing crimping tool  40  to further compress stent  20  onto balloon  14 , as seen in  FIG. 6  (step  108 ,  FIG. 2 ), which forms creases  16  of balloon  14  that extend non-uniformly within interstices  22  of the stent. Creases  16  are most clearly seen in  FIGS. 1B and 1C . Optional pillows  19  of stent delivery system  10  are also formed. Since many prior art crimping tools  40  apply an inwardly-directed stress, σ crimp , that is not uniform about the radius of balloon  14 , elastic crimping tube  30  acts to more uniformly distribute the stress about the circumference of the balloon/stent assembly. 
         [0057]    Stent delivery system  10  is removed from elastic crimping tube  30  and crimping tool  40  (step  108 ,  FIG. 2 ). Stent delivery system  10  has a low-profile delivery configuration adapted for percutaneous delivery within a patient&#39;s vasculature, as described hereinbelow with respect to  FIG. 9 . Creases  16 , as well as pillows  19 , secure stent  20  to balloon  14  between markers  17  of delivery catheter  12 . 
         [0058]    In contrast to prior art techniques described hereinabove, crimping in accordance with the present invention occurs at a substantially constant temperature, without the use of chemicals. In the context of the present invention, substantially constant temperature during crimping should be understood to include minor fluctuations in the actual temperature due to frictional losses, etc. 
         [0059]    Importantly, the system of the present invention is not actively heated to thermally remodel the balloon, as described in U.S. Pat. No. 5,836,965 to Jendersee et al. Likewise, no solvents are added to soften and mold the balloon, as described in U.S. Pat. No. 5,976,181 to Whelan et al. As described previously, both heating and solvents have significant potential drawbacks, including inhomogeneities along the length of the balloon, such as varying wall thickness. Varying wall thickness may yield areas of decreased strength that are susceptible to rupture upon inflation of the balloon during deployment of the stent. Additionally, heating and cooling, as well as addition of solvents, increases the complexity, time, and cost associated with affixing the stent to the balloon. 
         [0060]    Theoretical bounds for the radial stress that may be applied to balloon  14  during crimping, while the balloon is pressurized, may be estimated by modeling balloon  14  as an idealized tube and assuming crimping tool  40  applies an evenly distributed, inwardly-directed radial stress, σ crimp . Stent  20  and elastic crimping tool  30 , meanwhile, theoretically resist the crimping stress with an outwardly-directed radial stress, σ resistance . Thus, the composite inwardly-directed radial stress, σ in , applied to balloon  14  may be idealized as: 
         [0000]      σ in =σ crimp −σ resistance   (1)
 
         [0061]    Pressurization/inflation of balloon  14  similarly may be modeled as an evenly distributed, outwardly-directed radial stress, σ o  and it may be assumed that the rated burst pressure of balloon  14  is the yield stress of the balloon,  σy . A stress balance provides: 
         [0000]      σ in −σ out &lt;σ y   (2)
 
         [0062]    Thus, a theoretical upper bound for the radial stress, σ y  that may be applied to balloon  14  is: 
         [0000]      σ in &lt;σ y +σ out   (3)
 
         [0063]    A theoretical lower bound for σ y  in also may be found by observing that, in order to compress stent  20  onto the exterior of balloon  14 , crimping tool  40  must apply a radial stress, σ crimp , that is greater than the net stress provided by resistance of stent  20  and crimping tube  30 , σ resistance , and by the inflation of balloon  14 , σ out : 
         [0000]      σ crimp &gt;σ out +σ resistance   (4)
 
         [0064]    Combining Equation (1) and (4) provides a lower bound for σ in : 
         [0000]      σ in &gt;σ out   (5)
 
         [0065]    Finally, combining Equations (3) and (5) provides a range for σ in : 
         [0000]      σ out &lt;σ in &lt;σ y +σ out   (6)
 
         [0066]    As an example, assuming a burst pressure, σ y , of 12 atm and a balloon pressurization, σ out , of 8 atm, the balloon will theoretically withstand an inwardly-directed stress, σ in , of up to 20 atm. Furthermore, in order to ensure that stent  20  is crimped onto balloon  14 , σ in  must be greater than 8 atm. Thus, the inwardly-directed radial stress must be between 8 and 20 atm. Assuming, for example, a resistance stress, σ resistance , of 2 atm, crimping tool  40  must apply a crimping stress, σ crimp , between 10 and 22 atm. As one of ordinary skill will readily understand, the actual radial stress applied should be further optimized within this range to provide a safety factor, optimal crimping, etc. Since balloon  14  is not in reality an idealized tube, stresses applied to the balloon will have a longitudinal component in addition to the radial component, which may be, for example, accounted for in the safety factor. 
         [0067]    With reference now to  FIG. 7 , a possible configuration of the stent delivery system after crimping and removal of elastic crimping tube  30  is described. One or more struts  21  of stent  20  may be incompletely compressed against balloon  14 . Such a strut may potentially snag against the patient&#39;s vasculature during delivery, and thereby prevent positioning of stent delivery system  10  at a treatment site. Additionally, pressurized crimping may result in a delivery profile for delivery system  10  that is more polygonal than cylindrical, thereby applying undesirable stresses on the vessel wall during transluminal insertion. Accordingly, it may be desirable to perform an optional secondary crimping step after balloon  14  has been depressurized. 
         [0068]    Referring to  FIG. 8 , in order to reduce the potential for incompletely compressed individual struts  21  of stent  20 , and to provide a more uniform cylindrical delivery profile, one or more additional, secondary crimping steps may be performed on stent delivery system  10 . In  FIG. 8 , stent delivery system  10  is disposed within semi-rigid crimping tube  50 , which is disposed within crimping tool  40  (step  109 ,  FIG. 2 ). Tube  50  may be fabricated from any suitable semi-rigid material. As with elastic crimping tube  30 , semi-rigid crimping tube  50  preferably comprises a polymer, such as PEBAX. Semi-rigid crimping tube  50  preferably has a hardness of between about 50 and 60 Shore Hardness, and more preferably a hardness of about 55 Shore Hardness. 
         [0069]    With stent delivery system  10  disposed within semi-rigid tube  50  and crimping tool  40 , tool  40  is actuated to compress individual struts  21  against balloon  14  and to give delivery system  10  the substantially cylindrical delivery profile of  FIG. 1B  (step  110 ,  FIG. 2 ). As with elastic crimping tube  30 , semi-rigid tube  50  acts to evenly distribute crimping stresses applied by crimping tool  40  around the circumference of the stent/balloon assembly. Since balloon  14  is not pressurized, secondary crimping preferably proceeds in sections along the length of stent delivery system  10 . However, as will be apparent to those of skill in the art, secondary crimping may proceed in one step. Optionally, balloon  14  may be pressurized during secondary crimping. 
         [0070]    Referring now to  FIG. 9 , a method of using stent delivery system  10  of the present invention is described. Stent delivery system  10  is disposed in a contracted delivery configuration with stent  20  disposed over balloon  14  of delivery catheter  12 . Creases  16  of balloon  14  non-uniformly extend within interstices  22  of stent  20 . Creases  16 , in conjunction with optional pillows  19 , act to secure stent  20  to balloon  14 . As seen in  FIG. 9A , the distal end of catheter  12  is delivered to a target site T within a patient&#39;s vessel V using, for example, well-known percutaneous techniques. Target site T may, for example, comprise a stenosed region of vessel V. The radiopacity of markers  17  may facilitate positioning of system  10  at the target site. Alternatively, stent  20  or other portions of catheter  12  may be radiopaque to facilitate positioning. 
         [0071]    In  FIG. 9B , balloon  14  is inflated, for example, via an inflation medium delivered through inflation lumen  15  of catheter  12 . Stent  20  expands to the deployed configuration in which it contacts the wall of vessel V at target site T. Expansion of stent  20  opens interstices  22  of the stent and removes the non-uniform creases of balloon  14  from within the interstices. Additionally, stent  20  has a diameter in the deployed configuration that is larger than the diameter of optional pillows  19 , thereby facilitating removal of stent  20  from delivery catheter  12 . Balloon  14  is then deflated, as seen in  FIG. 9C , and delivery catheter  12  is removed from vessel V, as seen in  FIG. 9D . 
         [0072]    Stent  20  remains in place within vessel V in the deployed configuration in order to reduce restenosis and recoil of the vessel. Stent  20  also may comprise external coating C configured to retard restenosis or thrombus formation around the stent. Alternatively, coating C may deliver therapeutic agents into the patient&#39;s blood stream or a portion of the vessel wall adjacent to the stent. 
         [0073]    Although preferred illustrative embodiments of the present invention are described hereinabove, it will be evident to those skilled in the art that various changes and modifications may be made therein without departing from the invention. 
         [0074]    For example, stent delivery system  10  may be produced without using elastic crimping tube  30 . In this case, the stent/balloon assembly would be loaded directly into crimping tool  40 , which would limit expansion of balloon  14  during pressurization. Likewise, semi-rigid crimping tube  50  may be eliminated from the secondary crimping procedure. If crimping tubes are not used, crimping tool  40  preferably applies an inwardly-directed stress that is substantially evenly distributed about the circumference of the stent/balloon assembly. 
         [0075]    Additionally, balloon  14  may be depressurized prior to crimping stent  20  onto the balloon. This may be particularly beneficial when crimping long stents, for example, stents longer than about 50 mm. Pressurization of balloon  14  may cause the balloon to increase in longitudinal length. When crimping a long stent  20  onto a correspondingly long balloon  14 , this increase in balloon length is expected to be more significant, for example, greater than about 1 mm. 
         [0076]    If stent  20  is crimped onto balloon  14  while the balloon is pressured, significant stresses may be encountered along creases  16  after balloon  14  is depressurized, due to contraction of the balloon back to its shorter, un-inflated longitudinal length. These stresses may, in turn, lead to pinhole perforations of balloon  14 . Thus, since pressurization of balloon  14  causes the balloon to extend at least partially within interstices  22  of stent  20  in a non-uniform manner, as seen in  FIG. 5A , it is expected that crimping after depressurization will still establish creases  16  of stent delivery system  10 , in accordance with the present invention. Obviously, crimping after depressurization may be done with stents  20  of any length, not just long stents. 
         [0077]    It is intended in the appended claims to cover all such changes and modifications that fall within the true spirit and scope of the invention.