US20110071617A1

US20110071617A1 - Stent With Improved Flexibility

Info

Publication number: US20110071617A1
Application number: US12/693,576
Authority: US
Inventors: Matthew Baldwin; Richard Bliss; Justin Goshgarian; Erik Griswold; Rui Lam
Original assignee: Medtronic Vascular Inc
Current assignee: Medtronic Vascular Inc
Priority date: 2009-09-18
Filing date: 2010-01-26
Publication date: 2011-03-24
Also published as: WO2011034720A1; US20110071619A1; US20110071620A1; US20150239032A1; US9421601B2; US8366765B2; US9060889B2; US20110067471A1; US20110071615A1; US8597343B2; US20110071618A1; US8226705B2

Abstract

A stent includes a continuous wave form wrapped around a longitudinal axis of the stent at a pitch to define a helix comprising a plurality of turns. The wave form includes a plurality of struts and a plurality of crowns. Each crown connects adjacent struts within a turn to define the continuous wave form. The stent also includes a plurality of connections configured to connect selected crowns of adjacent turns. Unconnected crowns of adjacent turns that substantially face each other are spaced from each other and define a gap therebetween. The gap between the unconnected crowns of adjacent turns is variable around a circumference of the stent.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority from U.S. Provisional Patent Application Ser. No. 61/243,592, filed on Sep. 18, 2009, the entire content of which is incorporated herein by reference. This application also claims the benefit of priority from U.S. Provisional Patent Application Ser. Nos. 61/243,578, 61/243,581, 61/243,582, 61/243,597, and 61/243,600, all filed on Sep. 18, 2009, the entire contents of all of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention is generally related to a stent having improved flexibility along the length of the stent, and a method for manufacturing a stent having improved flexibility along the length of the stent.
2. Background of the Invention
A stent is typically a hollow, generally cylindrical device that is deployed in a body lumen from a radially contracted configuration into a radially expanded configuration, which allows it to contact and support a vessel wall. A plastically deformable stent can be implanted during an angioplasty procedure by using a delivery system that includes a balloon catheter bearing a compressed or “crimped” stent, which has been loaded onto the balloon. The stent radially expands as the balloon is inflated, forcing the stent into contact with the body lumen, thereby forming a support for the vessel wall. Deployment is effected after the stent has been introduced percutaneously, transported transluminally, and tracked and positioned at a desired location by means of the balloon catheter.
Stents may be formed from wire(s), may be cut from a tube, or may be cut from a sheet of material and then rolled into a tube-like structure. While some stents may include a plurality of connected rings that are substantially parallel to each other and are oriented substantially perpendicular to a longitudinal axis of the stent, others may include a helical coil that is wrapped around the longitudinal axis at a non-perpendicular angle.

SUMMARY OF THE INVENTION

It is desirable to provide a stent that is flexible to minimize the tracking effort through tortuous vessel anatomy, and a stent that is conformable to the vessel wall, yet provides adequate radial strength to support the vessel.
In an embodiment of the present invention, a stent includes a continuous wave form wrapped around a longitudinal axis of the stent at a pitch angle to define a helix comprising a plurality of turns. The wave form includes a plurality of struts and a plurality of crowns. Each crown connects adjacent struts within a turn to define the continuous wave form. The stent also includes a plurality of connections configured to connect selected crowns of adjacent turns. Unconnected crowns of adjacent turns that substantially face each other are spaced from each other and define a gap therebetween. The gap between the unconnected crowns of adjacent turns is variable around a circumference of the stent.
In an embodiment of the present invention, there is provided a method of manufacturing a stent. The method includes forming a wave form comprising a plurality of struts and a plurality of crowns. Each crown connects adjacent struts. The method also includes wrapping the wave form around a longitudinal axis at a pitch angle relative to the longitudinal axis to define a helix that includes a plurality of turns substantially centered about the longitudinal axis, connecting selected crowns of adjacent turns, and forming a variable gap between unconnected crowns of adjacent turns that substantially face each other around a circumference of the stent along the pitch angle.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

FIG. 1 schematically depicts a conventional stent;

FIG. 2 schematically depicts a stent in accordance with an embodiment of the present invention;

FIG. 3 schematically depicts a stent in accordance with an embodiment of the present invention in an unrolled condition;

FIG. 4 is a more detailed view of a portion of the stent of FIG. 3;

FIG. 5 schematically depicts a portion of a stent in accordance with an embodiment of the present invention;

FIG. 6 schematically depicts a portion of a stent in accordance with an embodiment of the present invention;

FIG. 7 schematically depicts a portion of a stent in accordance with an embodiment of the present invention; and

FIG. 8 schematically depicts a portion of a stent in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and use of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description.
FIG. 1 illustrates a conventional stent 10 that includes a plurality of turns 20. Each turn 20 includes a plurality of struts 22 and a plurality of crowns 24. Each crown 24 connects two adjacent struts 22 within a turn 20. The turns 20 may be formed individually as rings, and then may be connected together with connections 30 that connect selected crowns 24 of adjacent turns 20 of the stent 10, as illustrated in FIG. 1. The turns 20 are placed along a common longitudinal axis LA so that the crowns 24 of adjacent turns 20 that face each other either contact each other or almost contact each other, i.e., a spacing between the adjacent crowns may be less than about 0.001″. As illustrated, the turns 20 are substantially parallel to each other and are oriented substantially perpendicular to the longitudinal axis LA of the stent 10.
When the stent 10 of FIG. 1 is crimped onto a delivery system, such as a balloon catheter, and tracked through curved lumens, the stent 10 substantially conforms with the curves in the lumens by bending. The crowns 24 have a tendency to rub against each other and even push against each other, thereby restricting movement of the unconnected crowns 24 and decreasing flexibility along the length of the stent 10.
To overcome the decrease in flexibility and yet still track the stent 10 through a curve in the lumen, the operator typically increases the amount of force applied to the delivery system so that an additional amount of force may be applied to the interfering crowns 24 to generally conform the interfering crowns with the curve in the lumen.
FIG. 2 illustrates a stent 110 in accordance with an embodiment of the present invention. As illustrated, the stent 110 includes a plurality of turns 120. Each turn 120 includes a plurality of struts 122 and a plurality of crowns 124 or turns. Each crown 124 connects two adjacent struts 122 within a turn 120. The turns 120 may be formed individually as rings, and then may be connected together with connections 130 that connect selected crowns of adjacent turns 120, as illustrated in FIG. 2.
The connections 130 may be created by fusing the selected crowns 124 together. As used herein, “fusing” is defined as heating the target portions of materials to be fused together, without adding any additional material, to a level where the material in the target portions flow together, intermix with one another, and form a fusion when the materials cool down to, for example, room temperature. A suitable laser may be used to create the fusion.
In an embodiment, the connections 130 may be created by welding or soldering the selected crowns 124 together. As used herein, “welding” and “soldering” are defined as heating an additional material that is separate from the selected crowns and applying the heated additional material to the selected crowns 124 so that when the additional material cools, the selected crowns 124 are welded or soldered together.
In an embodiment, the connections 130 may be created by fusing, welding, or soldering an additional piece of material (not shown) that extends between selected crowns 124. The additional piece of material may resemble a strut or a portion of a strut, and may be sized to provide spacing between the selected crowns of two adjacent turns, if desired. In an embodiment, the stent 110 may be cut from a tube, and the connections 130 may include material from the tube. The illustrated embodiments are not intended to be limiting in any way.
In contrast to the stent 10 of FIG. 1, the turns 120 of the stent 110 of FIG. 2 are placed along a common longitudinal axis LA so that the crowns 124 of adjacent turns 120 that face each other, but are not connected, are spaced from one another to create a gap 140. In an embodiment, the gap 140 may be in the range of between 0.001″ and 0.003″. The size of the gap 140 can vary based on the physical characteristics of the stent, such as strut length, crown design, stent diameter, strut diameter, and other characteristics. In an embodiment, the gap 140 may be in the range from just above zero, i.e. greater than zero, to as high as a distance that is equal to the length of the longest strut, i.e., less than or equal to the length of the longest strut. In an embodiment, the gap 140 may be in the range of between about 0.0005″ and about 0.010″.
It has been found that the stent 110 illustrated in FIG. 2 is generally more flexible than the stent 10 illustrated in FIG. 1, because the unconnected crowns 124 of the adjacent turns 120 that face each other do not interfere with each other when the stent 110 is flexed or bent relative to the longitudinal axis LA.
FIG. 3 illustrates a stent 210 according to an embodiment of the present invention. The stent 210 is generally cylindrical in shape and has a longitudinal axis LA extending through the center of the stent 210. FIG. 3 illustrates the stent 210 in an “unrolled” state, which may be created when the stent 210 is slit along an axis that is substantially parallel to the longitudinal axis and then unrolled. The stent 210 includes a continuous wave form 212 that includes a plurality of turns 220 that are created when the wave form 212 is wrapped around the longitudinal axis LA during manufacturing of the stent 210. The stent 210 generally includes a central portion 224 and two end portions, a first end portion 226 and a second end portion 228, that are located on opposite sides of the central portion 224.
As illustrated in FIG. 3, the wave form 212 includes a plurality of struts 230 and a plurality of crowns 232. Each crown 232 is a curved portion or turn within the wave form 212 that connects adjacent struts 230 to define the continuous wave form 212. As shown in FIG. 3, the struts 230 are substantially straight portions of the wave form 212. In other embodiments, the struts 230 may be slightly bent or have other shapes, such as a sinusoidal wave, for example.
As illustrated in FIG. 3, the wave form 212 is wrapped around the longitudinal axis LA at a pitch so that the wave form 212 generally defines a helical coil in the central portion 224 having a first helical angle, or first pitch angle α, to define a first helix FH. In the illustrated embodiment, the wave form 212 is also wrapped around the longitudinal axis LA so the ends of the stent are substantially square or perpendicular to the longitudinal axis LA. The number of turns 222 about the longitudinal axis and the first helical angle α may be determined by the particular specifications of the stent 210, such as the desired unexpanded and expanded diameters and the length of the stent, as well as the size (e.g., diameter) and particular material of the wire or strip of material. The illustrated embodiments are not intended to be limiting in any way.
The stent 210 also includes a plurality of connections 240 that are configured to connect selected crowns 232 of adjacent turns 222 so that when the stent is in an unexpanded condition, the plurality of connections 240 generally lie along a connection helix CH defined by a connection helical angle β relative to the longitudinal axis LA. As illustrated in FIG. 3, the connection helix CH is oriented substantially opposite to the first helix FH described above such that the connection helix CH angle β is between 0° and 90° when using a coordinate system that is opposite the coordinate system depicted in FIG. 3 (i.e., the positive x axis runs from left to right rather than from right to left).
Like the connections 130 discussed above, the connections 240 may be created by fusing the selected crowns 232 together, as “fusing” is defined above. In an embodiment, the connections 240 may be created by welding or soldering the selected crowns 232 together, as “welding” and “soldering” are defined above. In an embodiment, the connections 240 may be created by fusing, welding, or soldering an additional piece of material (not shown) that extends between selected crowns 232. The additional piece of material may resemble a strut or a portion of a strut, and may be sized to provide spacing between the selected crowns of two adjacent turns, if desired. The illustrated embodiments are not intended to be limiting in any way.
The size of the connections 240 may also be varied according to the desired flexibility and rate of expansion for a given area of the stent 210. In general, the larger the connection 240, i.e. the larger the fusion or weld, the greater the stiffness.
As illustrated in FIG. 3, the struts 230 and the crowns 232 are formed so that the unconnected crowns 232 of adjacent turns 220 that face each other are spaced from one another so as to form gaps 250 in between the facing unconnected crowns 232. As illustrated, the gaps 250 are generally not uniform and are variable around the circumference of the stent 210, as defined by the pitch angle α, and may also be variable along the length of the stent 210.
A more detailed view of the end portion 228 of the stent 210 of FIG. 3 is illustrated in FIG. 4. As shown, the size of the gaps 250 vary between the unconnected crowns 232 of adjacent turns 220. For example, for some of the unconnected crowns 232, there is a relatively small gap 250 a, and for some of the unconnected crowns 232, there is a relatively large gap 250 b. In the illustrated embodiment, there other gaps, represented by 250 c and 250 d, that are in between the small gap 250 a and the large gap 250 b in size. The size of the gap 250 can vary based on the physical characteristics of the stent, such as strut length, crown design, stent diameter, strut diameter, and other characteristics. In an embodiment, the gap 250 may be in the range from just above zero, i.e., greater than zero, to as high as a distance that is equal to the length of the longest strut, i.e., less than or equal to the length of the longest strut. In an embodiment, the gap 250 may be in the range of between about 0.0005″ and about 0.010″. In an embodiment, the gap 250 may be in the range of between about 0.001″ and about 0.003″.
A method that may be used to create the gaps 250 between unconnected crowns 232 of adjacent turns 220 is to vary the length of the struts 230 and/or size of the crowns 232 in the wave form 212. By varying the length of the struts 230, the amplitude of the waves of the wave form may be varied. For example, to increase the gap 250 between unconnected crowns 232 of adjacent turns 220, a longer strut 230 a than average and/or a larger crown than average may be used to form a so-called extended crown 232. Extended crowns are discussed in further detail below with respect to FIGS. 5-7.
Another method that may be used to create the gaps 250 between unconnected crowns 232 of adjacent turns 220 is to electro-polish the crowns 232 after the crowns 232 have been formed. This may be done by electro-polishing the stent 210 for a pre-determined amount of time until the desire spacing, or gap 250, is achieved between each pair of unconnected crowns 232. The pre-electro-polished dimensions of the crowns should be equal to the desired crown dimensions plus the desired spacing. The desired spacing can vary based on the physical characteristics of the stent, such as strut length, crown design, stent diameter, strut diameter, and other characteristics. In an embodiment, the spacing may be in the range from just above zero, i.e., greater than zero, to as high as a distance that is equal to the length of the longest strut, i.e., less than or equal to the length of the longest strut. In an embodiment, the spacing may be in the range of between about 0.0005″ and about 0.010″. In an embodiment, the spacing may be in the range of between about 0.001″ and about 0.003″.
Another method that may be used to create spaces between crowns is to customize the kerf of a laser. When laser cutting stents, the width of the beam of the laser can be used to create extended crowns. The kerf may be adjusted by power, speed, and focus of the laser. A wider kerf may be used just between the crowns and a smaller kerf may be used to cut the remaining parts of the stent. This method may be used in conjunction with electro-polishing.
FIG. 5 illustrates a portion of an embodiment of a stent 410 that includes a plurality of turns 420 that are oriented substantially perpendicular relative to the longitudinal axis LA of the stent 410. Each turn 420 includes a plurality of struts 430 and a plurality of crowns 432. Each crown 432 connects adjacent struts 430 within a turn 420 to each other. As illustrated, each turn 420 includes an extended crown 432 e that extends into a gap 450 that is defined by the remaining crowns 432 that face each other. In the embodiment illustrated in FIG. 5, the gap 450 is substantially the same around the circumference of the stent 410 and has a length or width of “e” in the Figure. The extended crowns may be connected or unconnected in different embodiments. For example, in the illustrated embodiment, the extended crowns 432 e are not connected to each other, but in other embodiments, the extended crowns 432 e may be connected to each other.
In accordance with an embodiment of the present invention, a stent 510 includes a plurality of turns 520 that are oriented at a pitch angle α relative to the longitudinal axis LA of the stent 510 to define a first helix FH, as illustrated in FIG. 6. Each turn 520 includes a plurality of struts 530 and a plurality of crowns 532. Each crown 532 connects adjacent struts 530 within a turn 520 to each other. As illustrated, each turn 520 includes an extended crown 532 e that extends into a gap 550 that is defined by the remaining crowns 532 that face each other. In the embodiment illustrated in FIG. 6, the gap 550 is substantially the same around the circumference of the stent 510, as defined by the pitch angle α, and has a length or width “e” in the Figure. Similar to the embodiment of FIG. 5, the extended crowns 532 e are not connected to each other, but in other embodiments, the extended crowns 532 e may be connected to each other. In other words, in some embodiments, the crowns 532 e are connected and in other embodiments, the crowns 532 e are not connected.
In the embodiments illustrated in FIGS. 5 and 6, the same spacing is used between the crowns that are not extended. It has been found that in such embodiments, especially the embodiment illustrated in FIG. 6, non-uniform crimping and/or expansion of the stent may be seen. This may be due to the longer struts, which are known to bend more easily when subjected to the same forces as shorter struts having similar cross-sectional dimensions. To improve the uniformity of the crimping and expansion behavior of the stents of FIGS. 5 and 6, it has been found that the amount of extension of the crowns and/or gaps formed between the crowns of adjacent turns may be varied and shifted throughout the stent, i.e., around the circumference and/or along the length of the stent, as illustrated in FIGS. 7 and 8.
FIG. 7 illustrates an embodiment of a stent 610 that includes a plurality of turns 620 that are generally aligned substantially perpendicularly to the longitudinal axis of the stent 610. Each turn 620 includes a plurality of struts 630 and a plurality of crowns 632, with each crown 632 connecting adjacent struts 630 within a turn 620 to each other. As illustrated, each turn 620 includes variable crown extensions so that two sets of opposing crowns 632 e extend into a gap 650 that is defined by the remaining crowns 632 that face each other. In the embodiment illustrated in FIG. 7, the gap 650 may be generally the same around the circumference of the stent 610, but defines a zig-zag-like pattern, as compared to the gap 450 illustrated in FIG. 5. In an embodiment, the gap 650 may be variable around the circumference of the stent 610.
FIG. 8 illustrates an embodiment of a stent 710 that includes a wave form 712 that includes a plurality of turns 720 that are generally oriented at a pitch angle α relative to the longitudinal axis LA of the stent 710 to define a first helix FH. Each turn 720 includes a plurality of struts 730 and a plurality of crowns 732, and each crown 732 connects adjacent struts 730 within a turn 720 to each other. As illustrated, each turn 720 includes an extended crown 732 e that extends into a gap 750 that is defined by the remaining crowns 732 that face each other. In the embodiment illustrated in FIG. 8, the gap 750 is not constant between the turns 720, as illustrated by lines A and B, and is instead variable around the circumference of the stent 710 along the pitch angle α.
The variable gap 750 may be created by varying the amplitude of waves of the wave form 712 within each of the turns 720 (a wave being defined by two adjacent crowns and two adjacent struts connected to the adjacent crowns). The amplitudes of the waves of the wave form 712 may be varied by varying the lengths of the struts 730 and/or varying the size of the crowns 732. For example, the outer radius of one crown 732 facing the gap 750 may be larger or smaller than the outer radius of the next crown 732 that faces the gap 750 within the same turn 720. The radii of the crowns 732 may be altered via electro-polishing, as described above. The radii of the crowns may also be altered during the forming process. Also, other ways of creating/changing the variable spacing include: changing the way that the crowns are connected such as using bars/bridges between crowns, including bars with sinusoidal shapes between crowns, using additional material for welds, or by having smaller crown radii on the crowns that are being fused.
Other variations of the embodiments illustrated by FIGS. 7 and 8 may be used to create the desired flexibility, crimping, and expansion properties of the stent. In addition, embodiments of the variable crown spacing described above may be applied with other design attributes, including but not limited to stent material, strut cross section geometry and dimensions, strut length, crown radius, number of struts in the cross section of the stent, and number of connection points along the stent, to achieve optimal balance between stent deployment symmetry, radial strength upon deployment, and stent flexibility.
The embodiments of the stents discussed above may be formed from a wire or a strip of suitable material. In certain embodiments, the stents may be formed, i.e., etched or cut, from a thin tube of suitable material, or from a thin plate of suitable material and rolled into a tube. Suitable materials for the stent include but are not limited to stainless steel, iridium, platinum, gold, tungsten, tantalum, palladium, silver, niobium, zirconium, aluminum, copper, indium, ruthenium, molybdenum, niobium, tin, cobalt, nickel, zinc, iron, gallium, manganese, chromium, titanium, aluminum, vanadium, and carbon, as well as combinations, alloys, and/or laminations thereof. For example, the stent may be formed from a cobalt alloy, such as L605 or MP35N®, Nitinol (nickel-titanium shape memory alloy), ABI (palladium-silver alloy), Elgiloy® (cobalt-chromium-nickel alloy), etc. It is also contemplated that the stent may be formed from two or more materials that are laminated together, such as tantalum that is laminated with MP35N®. The stents may also be formed from wires having concentric layers of different metals, alloys, or other materials. Embodiments of the stent may also be formed from hollow tubes, or tubes that have been filled with other materials. The aforementioned materials and laminations are intended to be examples and are not intended to be limiting in any way.
While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient roadmap for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of members described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.

Claims

1. A stent comprising:

a continuous wave form wrapped around a longitudinal axis of the stent at a pitch angle to define a helix comprising a plurality of turns, the wave form comprising a plurality of struts and a plurality of crowns, each crown connecting adjacent struts within a turn to define the continuous wave form; and

a plurality of connections configured to connect selected crowns of adjacent turns, wherein unconnected crowns of adjacent turns that substantially face each other are spaced from each other and define a gap therebetween, wherein the gap between the unconnected crowns of adjacent turns is variable around a circumference of the stent along the pitch angle.

2. The stent according to claim 1, wherein the gap is greater than zero and less than or equal to a length of the longest strut in the wave form.

3. The stent according to claim 2, wherein the gap is between about 0.0005″ and about 0.010″.

4. The stent according to claim 3, wherein the gap is between about 0.001″ and about 0.003″.

5. The stent according to claim 1, wherein the connections are fusions of the selected crowns.

6. The stent according to claim 1, wherein the connections are welds.

7. The stent according to claim 1, wherein the amplitude of the wave form varies around the circumference of the stent within at least one of the turns.

8. The stent according to claim 7, wherein the lengths of at least some of the struts within the at least one of the turns are different.

9. The stent according to claim 7, wherein the sizes of at least some of the crowns within the at least one of the turns are different.

10. A method of manufacturing a stent, the method comprising:

forming a wave form comprising a plurality of struts and a plurality of crowns, each crown connecting adjacent struts;

wrapping the wave form around a longitudinal axis at a pitch angle relative to the longitudinal axis to define a helix that includes a plurality of turns substantially centered about the longitudinal axis;

connecting selected crowns of adjacent turns; and

forming a variable gap between unconnected crowns of adjacent turns that substantially face each other around a circumference of the stent along the pitch angle.

11. The method according to claim 10, wherein the gap is greater than zero and less than or equal to a length of the longest strut in the wave form.

12. The method according to claim 11, wherein the gap is between about 0.0005″ and about 0.010″.

13. The method according to claim 12, wherein the gap is between about 0.001″ and about 0.003″.

14. The method according to claim 10, wherein the connecting comprises fusing the selected crowns to each other.

15. The method according to claim 10, wherein the connecting comprises welding the selected crowns to each other.

16. The method according to claim 10, wherein the forming the gap comprises electro-polishing the unconnected crowns.

17. The method according to claim 10, wherein the forming the wave form comprises forming extended crowns so that the struts connected to the extended crowns are longer than an average strut length of the wave form, and wherein the extended crowns extend into the gap.

18. The method according to claim 10, wherein the forming the gap occurs during the wrapping the wave form.