Process for loading a stent onto a stent delivery system

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's vessel including providing a stent delivery system is also provided.

Claims

1. A stent delivery system having a contracted delivery configuration and an expanded deployed configuration, the stent delivery system comprising: 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 comprising creases extending non-uniformly within the interstices of the stent in the contracted delivery configuration, each crease defining a maximum radial height within a corresponding interstice, wherein the maximum radial heights of the creases vary.

2. The stent delivery system of claim 1, wherein the stent comprises a metal stent.

3. The stent delivery system of claim 1, wherein the stent comprises a polymer stent.

4. The stent delivery system of claim 1, wherein the stent comprises an external coating.

5. The stent delivery system of claim 1, wherein the creases include a folded first region having at least one fold of the balloon folded into at least an interstice and a second folded region having a least two folds of the balloon folded into at least a second interstice, wherein the creases are captured within the interstices to releasably affix the stent to the inflatable balloon.

6. The stent delivery system of claim 1, wherein the balloon has a working length and is free from heat-induced and solvent-induced inhomogeneities along the working length of the balloon.

7. The stent delivery system of claim 1, wherein the creases have varying slope about a circumference of the stent delivery system.

8. The stent delivery system of claim 1, wherein a first crease corresponding to a first interstice has a first maximum radial height greater than a radial midpoint of the first interstice and a second crease corresponding to a second interstice has a second maximum radial height less than a radial midpoint of the second interstice.

9. A method for stenting at a target site within a patient's vessel, the method comprising: providing a stent delivery system comprising a stent having a plurality of expandable elements and a plurality of interstices disposed between adjacent expandable elements and a delivery catheter with an inflatable balloon having creases extending non-uniformly within the interstices of the stent, each crease defining a maximum radial height within a corresponding interstice, wherein the maximum radial heights of the creases vary; percutaneously delivering the stent delivery system to the target site within the patient's vessel in the contracted delivery configuration; and expanding the stent delivery system to an expanded deployed configuration, wherein the balloon is inflated, the interstices of the stent open, and the stent engages the target site.

10. The method of claim 9, wherein the stent comprises a metal stent.

11. The method of claim 9, wherein the stent comprises a polymer stent.

12. The method of claim 9, wherein the stent comprises an external coating.

13. The method of claim 9, wherein the creases include a first folded region having at least one fold of the balloon folded into at least a first interstice and a second folded region having at least two folds of the balloon folded into at least a second interstice, and wherein the creases are captured within the interstices to releasably affix the stent to the inflatable balloon.

14. The method of claim 9, further comprising: deflating the balloon; and removing the delivery catheter from the patient's vessel.

15. The method of claim 9, wherein the balloon has a working length and is free from heat-induced and solvent-induced inhomogeneities along the working length of the balloon.

16. The method of claim 9, wherein the creases have varying slope about a circumference of the stent delivery system.

17. The method of claim 9, wherein a first crease corresponding to a first interstice has a first maximum radial height greater than a radial midpoint of the first interstice and a second crease corresponding to a second interstice has a second maximum radial height less than a radial midpoint of the second interstice.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) 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:

(2) 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;

(3) FIG. 2 is a flow chart showing the steps of the cold-molding process of the present invention;

(4) 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 BB 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;

(5) 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;

(6) 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;

(7) FIG. 6 is a cross-sectional view along section line C-c in FIG. 4A during crimping after pressure has been removed;

(8) 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;

(9) 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

(10) FIGS. 9A-9D are side views, partially in section, of the stent delivery system of FIG. 1 disposed within a patient's vasculature, depicting a method of using the apparatus in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(11) 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.

(12) 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.

(13) 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.

(14) 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'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.

(15) 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.

(16) 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.

(17) 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 Cmaydeliver therapeutic agents into-the-patient's blood stream or vessel wall.

(18) 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.

(19) 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 inflatedpreferably only partiallywith 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.

(20) 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.

(21) 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.

(22) 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 12preferably 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.

(23) 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.

(24) 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.

(25) 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.

(26) 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.

(27) 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 suitableelastic 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.

(28) 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, .sub.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.

(29) 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, .sub.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.

(30) 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'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.

(31) 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.

(32) 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.

(33) 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, .sub.crimp. Stent 20 and elastic crimping tool 30, meanwhile, theoretically resist the crimping stress with an outwardly-directed radial stress, .sub.resistance. Thus, the composite inwardly-directed radial stress, .sub.in, applied to balloon 14 may be idealized as:
.sub.in=.sub.crimp.sub.resistance(1)

(34) Pressurization/inflation of balloon 14 similarly may be modeled as an evenly distributed, outwardly-directed radial stress, .sub.o and it may be assumed that the rated burst pressure of balloon 14 is the yield stress of the balloon, .sub.y. A stress balance provides:
.sub.in.sub.out<.sub.y(2)

(35) Thus, a theoretical upper bound for the radial stress, .sub.y that may be applied to balloon 14 is:
.sub.in<.sub.y+.sub.out(3)

(36) A theoretical lower bound for .sub.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, .sub.crimp, that is greater than the net stress provided by resistance of stent 20 and crimping tube 30, .sub.resistance, and by the inflation of balloon 14, .sub.out:
.sub.crimp>.sub.out+.sub.resistance(4)

(37) Combining Equation (1) and (4) provides a lower bound for .sub.in:
.sub.in>.sub.out(5)

(38) Finally, combining Equations (3) and (5) provides a range for .sub.in:
.sub.out<.sub.in<.sub.y+.sub.out(6)

(39) As an example, assuming a burst pressure, .sub.y, of 12 atm and a balloon pressurization, .sub.out, of 8 atm, the balloon will theoretically withstand an inwardly-directed stress, .sub.in, of up to 20 atm. Furthermore, in order to ensure that stent 20 is crimped onto balloon 14, .sub.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, .sub.resistance, of 2 atm, crimping tool 40 must apply a crimping stress, .sub.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.

(40) 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'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.

(41) 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.

(42) 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.

(43) 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'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.

(44) 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.

(45) 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's blood stream or a portion of the vessel wall adjacent to the stent.

(46) 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.

(47) 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.

(48) 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.

(49) 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.

(50) 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.