US20110071617A1 - Stent With Improved Flexibility - Google Patents

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 ofFIG. 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 aconventional stent10 that includes a plurality ofturns20. Eachturn20 includes a plurality ofstruts22 and a plurality of crowns24. Each crown24 connects twoadjacent struts22 within aturn20. Theturns20 may be formed individually as rings, and then may be connected together withconnections30 that connect selected crowns24 ofadjacent turns20 of thestent10, as illustrated inFIG. 1. Theturns20 are placed along a common longitudinal axis LA so that the crowns24 of adjacent turns20 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, theturns20 are substantially parallel to each other and are oriented substantially perpendicular to the longitudinal axis LA of thestent10.
• When thestent10 ofFIG. 1 is crimped onto a delivery system, such as a balloon catheter, and tracked through curved lumens, thestent10 substantially conforms with the curves in the lumens by bending. The crowns24 have a tendency to rub against each other and even push against each other, thereby restricting movement of the unconnected crowns24 and decreasing flexibility along the length of thestent10.
• To overcome the decrease in flexibility and yet still track thestent10 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 crowns24 to generally conform the interfering crowns with the curve in the lumen.
• FIG. 2 illustrates astent110 in accordance with an embodiment of the present invention. As illustrated, thestent110 includes a plurality ofturns120. Eachturn120 includes a plurality ofstruts122 and a plurality ofcrowns124 or turns. Eachcrown124 connects twoadjacent struts122 within aturn120. Theturns120 may be formed individually as rings, and then may be connected together withconnections130 that connect selected crowns ofadjacent turns120, as illustrated inFIG. 2.
• Theconnections130 may be created by fusing theselected crowns124 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, theconnections130 may be created by welding or soldering theselected crowns124 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 selectedcrowns124 so that when the additional material cools, theselected crowns124 are welded or soldered together.
• In an embodiment, theconnections130 may be created by fusing, welding, or soldering an additional piece of material (not shown) that extends betweenselected crowns124. 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, thestent110 may be cut from a tube, and theconnections130 may include material from the tube. The illustrated embodiments are not intended to be limiting in any way.
• In contrast to thestent10 ofFIG. 1, theturns120 of thestent110 ofFIG. 2 are placed along a common longitudinal axis LA so that thecrowns124 ofadjacent turns120 that face each other, but are not connected, are spaced from one another to create agap140. In an embodiment, thegap140 may be in the range of between 0.001″ and 0.003″. The size of thegap140 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, thegap140 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, thegap140 may be in the range of between about 0.0005″ and about 0.010″.
• It has been found that thestent110 illustrated inFIG. 2 is generally more flexible than thestent10 illustrated inFIG. 1, because theunconnected crowns124 of the adjacent turns120 that face each other do not interfere with each other when thestent110 is flexed or bent relative to the longitudinal axis LA.
• FIG. 3 illustrates astent210 according to an embodiment of the present invention. Thestent210 is generally cylindrical in shape and has a longitudinal axis LA extending through the center of thestent210.FIG. 3 illustrates thestent210 in an “unrolled” state, which may be created when thestent210 is slit along an axis that is substantially parallel to the longitudinal axis and then unrolled. Thestent210 includes acontinuous wave form212 that includes a plurality ofturns220 that are created when thewave form212 is wrapped around the longitudinal axis LA during manufacturing of thestent210. Thestent210 generally includes acentral portion224 and two end portions, afirst end portion226 and asecond end portion228, that are located on opposite sides of thecentral portion224.
• As illustrated inFIG. 3, thewave form212 includes a plurality ofstruts230 and a plurality ofcrowns232. Eachcrown232 is a curved portion or turn within thewave form212 that connectsadjacent struts230 to define thecontinuous wave form212. As shown inFIG. 3, thestruts230 are substantially straight portions of thewave form212. In other embodiments, thestruts230 may be slightly bent or have other shapes, such as a sinusoidal wave, for example.
• As illustrated inFIG. 3, thewave form212 is wrapped around the longitudinal axis LA at a pitch so that thewave form212 generally defines a helical coil in thecentral portion224 having a first helical angle, or first pitch angle α, to define a first helix FH. In the illustrated embodiment, thewave form212 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 turns222 about the longitudinal axis and the first helical angle α may be determined by the particular specifications of thestent210, 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.
• Thestent210 also includes a plurality ofconnections240 that are configured to connect selectedcrowns232 of adjacent turns222 so that when the stent is in an unexpanded condition, the plurality ofconnections240 generally lie along a connection helix CH defined by a connection helical angle β relative to the longitudinal axis LA. As illustrated inFIG. 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 inFIG. 3 (i.e., the positive x axis runs from left to right rather than from right to left).
• Like theconnections130 discussed above, theconnections240 may be created by fusing the selectedcrowns232 together, as “fusing” is defined above. In an embodiment, theconnections240 may be created by welding or soldering the selectedcrowns232 together, as “welding” and “soldering” are defined above. In an embodiment, theconnections240 may be created by fusing, welding, or soldering an additional piece of material (not shown) that extends between selected crowns232. 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 theconnections240 may also be varied according to the desired flexibility and rate of expansion for a given area of thestent210. In general, the larger theconnection240, i.e. the larger the fusion or weld, the greater the stiffness.
• As illustrated inFIG. 3, thestruts230 and thecrowns232 are formed so that theunconnected crowns232 ofadjacent turns220 that face each other are spaced from one another so as to formgaps250 in between the facingunconnected crowns232. As illustrated, thegaps250 are generally not uniform and are variable around the circumference of thestent210, as defined by the pitch angle α, and may also be variable along the length of thestent210.
• A more detailed view of theend portion228 of thestent210 ofFIG. 3 is illustrated inFIG. 4. As shown, the size of thegaps250 vary between theunconnected crowns232 ofadjacent turns220. For example, for some of theunconnected crowns232, there is a relativelysmall gap250a, and for some of theunconnected crowns232, there is a relativelylarge gap250b. In the illustrated embodiment, there other gaps, represented by250cand250d, that are in between thesmall gap250aand thelarge gap250bin size. The size of thegap250 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, thegap250 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, thegap250 may be in the range of between about 0.0005″ and about 0.010″. In an embodiment, thegap250 may be in the range of between about 0.001″ and about 0.003″.
• A method that may be used to create thegaps250 betweenunconnected crowns232 ofadjacent turns220 is to vary the length of thestruts230 and/or size of thecrowns232 in thewave form212. By varying the length of thestruts230, the amplitude of the waves of the wave form may be varied. For example, to increase thegap250 betweenunconnected crowns232 ofadjacent turns220, alonger strut230athan average and/or a larger crown than average may be used to form a so-calledextended crown232. Extended crowns are discussed in further detail below with respect toFIGS. 5-7.
• Another method that may be used to create thegaps250 betweenunconnected crowns232 ofadjacent turns220 is to electro-polish thecrowns232 after thecrowns232 have been formed. This may be done by electro-polishing thestent210 for a pre-determined amount of time until the desire spacing, orgap250, is achieved between each pair ofunconnected crowns232. 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 astent410 that includes a plurality ofturns420 that are oriented substantially perpendicular relative to the longitudinal axis LA of thestent410. Eachturn420 includes a plurality ofstruts430 and a plurality ofcrowns432. Eachcrown432 connectsadjacent struts430 within aturn420 to each other. As illustrated, eachturn420 includes anextended crown432ethat extends into agap450 that is defined by the remainingcrowns432 that face each other. In the embodiment illustrated inFIG. 5, thegap450 is substantially the same around the circumference of thestent410 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, theextended crowns432eare not connected to each other, but in other embodiments, theextended crowns432emay be connected to each other.
• In accordance with an embodiment of the present invention, a stent510 includes a plurality ofturns520 that are oriented at a pitch angle α relative to the longitudinal axis LA of the stent510 to define a first helix FH, as illustrated inFIG. 6. Eachturn520 includes a plurality ofstruts530 and a plurality ofcrowns532. Eachcrown532 connectsadjacent struts530 within aturn520 to each other. As illustrated, eachturn520 includes an extended crown532ethat extends into agap550 that is defined by the remainingcrowns532 that face each other. In the embodiment illustrated inFIG. 6, thegap550 is substantially the same around the circumference of the stent510, as defined by the pitch angle α, and has a length or width “e” in the Figure. Similar to the embodiment ofFIG. 5, the extended crowns532eare not connected to each other, but in other embodiments, the extended crowns532emay be connected to each other. In other words, in some embodiments, the crowns532eare connected and in other embodiments, the crowns532eare not connected.
• In the embodiments illustrated inFIGS. 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 inFIG. 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 ofFIGS. 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 inFIGS. 7 and 8.
• FIG. 7 illustrates an embodiment of astent610 that includes a plurality ofturns620 that are generally aligned substantially perpendicularly to the longitudinal axis of thestent610. Eachturn620 includes a plurality ofstruts630 and a plurality ofcrowns632, with eachcrown632 connectingadjacent struts630 within aturn620 to each other. As illustrated, eachturn620 includes variable crown extensions so that two sets of opposingcrowns632eextend into agap650 that is defined by the remainingcrowns632 that face each other. In the embodiment illustrated inFIG. 7, thegap650 may be generally the same around the circumference of thestent610, but defines a zig-zag-like pattern, as compared to thegap450 illustrated inFIG. 5. In an embodiment, thegap650 may be variable around the circumference of thestent610.
• FIG. 8 illustrates an embodiment of astent710 that includes awave form712 that includes a plurality ofturns720 that are generally oriented at a pitch angle α relative to the longitudinal axis LA of thestent710 to define a first helix FH. Eachturn720 includes a plurality ofstruts730 and a plurality ofcrowns732, and eachcrown732 connectsadjacent struts730 within aturn720 to each other. As illustrated, eachturn720 includes anextended crown732ethat extends into agap750 that is defined by the remainingcrowns732 that face each other. In the embodiment illustrated inFIG. 8, thegap750 is not constant between theturns720, as illustrated by lines A and B, and is instead variable around the circumference of thestent710 along the pitch angle α.
• Thevariable gap750 may be created by varying the amplitude of waves of thewave form712 within each of the turns720 (a wave being defined by two adjacent crowns and two adjacent struts connected to the adjacent crowns). The amplitudes of the waves of thewave form712 may be varied by varying the lengths of thestruts730 and/or varying the size of thecrowns732. For example, the outer radius of onecrown732 facing thegap750 may be larger or smaller than the outer radius of thenext crown732 that faces thegap750 within thesame turn720. The radii of thecrowns732 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 byFIGS. 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 (18)

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 toclaim 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 toclaim 2, wherein the gap is between about 0.0005″ and about 0.010″.

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

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

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

7. The stent according toclaim 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 toclaim 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 toclaim 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 toclaim 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 toclaim 11, wherein the gap is between about 0.0005″ and about 0.010″.

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

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

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

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

17. The method according toclaim 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 toclaim 10, wherein the forming the gap occurs during the wrapping the wave form.

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