Patent Application
20130096669
1. The Field of the Invention
The present invention is generally directed to a method of manipulating the performance characteristics of a metal stent, and more particularly pertains to a heat treatment process for achieving a desired combination of strength and ductility.
2. The Relevant Technology
A focus of recent development work in the treatment of heart disease has been directed to endoprosthetic devices referred to as stents. Stents are generally tubular shaped devices that function to maintain patency of a segment of a blood vessel or other body lumen such as a coronary artery. They also are suitable for use to support and hold back a dissected arterial lining that can occlude the fluid passageway. At present, there are numerous commercial stents being marketed throughout the world. Intraluminal stents implanted via percutaneous methods have become a standard adjunct to balloon angioplasty in the treatment of atherosclerotic disease. Stents prevent acute vessel recoil and improve the long term outcome by controlling negative remodeling and supporting vessel dissections. Amongst their many properties, stents must have adequate mechanical strength, flexibility, minimal recoil, and occupy the least amount of arterial surface area possible while not having large regions of unsupported area.
One method and system developed for delivering stents to desired locations within the patient's body lumen involves crimping a stent about an expandable member, such as a balloon on the distal end of a catheter, advancing the catheter through the patient's vascular system until the stent is in the desired location within a blood vessel, and then inflating the expandable member on the catheter to expand the stent within the blood vessel. The expandable member is then deflated and the catheter withdrawn, leaving the expanded stent within the blood vessel, holding open the passageway thereof.
Stents are typically formed from biocompatible metals and alloys, such as stainless steel, nickel titanium, platinum iridium alloys, cobalt chromium alloys and tantalum. Such stents provide sufficient hoop strength to perform the scaffolding function. Furthermore, stents should have minimal wall thicknesses in order to minimize blood flow blockage. Starting stock for manufacturing stents is frequently in the form of stainless steel or cobalt-chromium alloy tubing, although the technology has began to explore other alloys and metals in search of the optimum balance of desirable characteristics and costs.
The performance characteristics of a stent are largely driven by the material properties of the stent material. Material properties such as strength and ductility are key in determining how the stent will behave under implanted conditions. As an example, a stent material with greater ductility will generally result in a stent that is capable of higher allowable deformation during expansion while a stent material with increased strength will usually result in a stent with increased radial rigidity. Other properties, such as elastic modulus and yield strength also have significant impacts on stent performance characteristics. Typically, however, strength and ductility are inversely related, and it is necessary to find a way to balance them by either changing the stent dimensions, configuration, or using a different material in its construction.
One important principle concerning the metallurgical consequences of processing the metals is that the structural properties of the material used for stents can improve with a decrease in the grain size of the substrate material. For example, it has been observed that stents cut from fully annealed 316L stainless steel tubing having less than seven grains across a strut thickness can display micro cracks in the high strain regions of the stent. Such cracks are suggestive of undesirable heavy slip band formation, with subsequent decohesion of the atoms along the slip planes. Reduction of the grain size in the substrate material will reduce the occurrence of such cracks and/or heavy slip band formation in the finished medical device.
Thus, in this case smaller grain size, leading to more grains across the strut thickness, limit the formation of slip bands. The grain size of a finished stainless steel or similar metal tube depends on numerous factors, including the length of time the material is heated above a temperature that allows significant grain growth. For a metallic tube, if the grain size is larger than desired, the tube may be swaged to introduce heavy dislocation densities, then heat treated to recrystallize the material into finer grains. Alternatively, different material forms may be taken through a drawing or other working and heat treat processes to recrystallize the tubing and smaller grains. The type and amount of working allowed depends on the material, e.g., ceramics may require a high temperature working step while metals and composites may be workable at room temperature. Grain-size strengthening occurs where there is an increase in strength of a material due to a decrease in the grain size. The outer diameter of the tube used to form the stent usually requires a machining step of some sort to smooth the surface after the swaging process, and the same may be true before the tubing can be properly drawn.
Commercially available 316L stainless steel tubing contains average grain sizes ranging from approximately 0.0025 inch (sixty four microns), ASTM grain size 5 to around 0.00088 inch (twenty two microns), ASTM grain size 8. These grain sizes result in anywhere from two to five grains across the tube thickness, and the stent subsequently manufactured from the tubing, depending on the tube and stent strut thicknesses. Part of the limitation in achieving a finer grain size in this material arises from the number of draws and anneals the tubing must go through to achieve its final size.
As indicated above, stents have been formed in the past by laser-cutting a small mesh structure from a tube of material. The tubing is typically formed to given dimensions through a drawing process that imparts a significant amount of work-hardening in the material. This involves an introduction of dislocations in the grains of the material through cold or warm working below a stress-relief temperature. In the case of large dimension reduction, the internal metallic grains become compacted and elongated. Both work hardening and grain size reduction limits dislocation mobility (the “Hall-Petch” relationship), causing an increase in material strength, but a severe loss of ductility. Therefore, internal stress caused by this process is then relieved through a heat treatment termed “full annealing” that greatly reduces the dislocation density and creates a homogeneous grain structure.
Stents have heretofore been formed of materials that have been fully annealed (and the material recrystallizes) either before or after grain growth. That is, the material is heated beyond its stress-relief temperature for a period of time sufficient to ensure recrystallization and a homogeneous grain structure. This process has been effective for the manufacture of common stent materials such as stainless steel and cobalt-chromium alloys, but may not be adequate to balance stent characteristics using newer materials such as tantalum and other refractory metal alloys. Therefore, there is a need for an improved method of manufacture of stent implants that provides a better balance of stent material strength and ductility.
The present invention provides for a stent manufacturing process which obviates the need to alter the stent configuration or to select a different material for its manufacture in order to achieve a desired balance of strength and ductility. Moreover, such a process allows materials to be used in the manufacture of stents that have previously been found to exhibit an undesirable balance of strength and ductility. The process results in a material that is only partially recrystallized and that has an inhomogenous grain structure which has unexpectedly been found to yield a more desired balance of physical characteristics.
One method of the present invention provides for the partial annealing of the stent material. In one embodiment, the tubing is partially annealed before laser cutting a pattern in the tubing. In another embodiment, a stent pattern is laser cut into tubing after which the structure is partially annealed. As a further embodiment, manufacturing sequences may include a full annealing step as long as it is followed by further cold working and a final partial annealing step.
The method of the present invention allows stents to be manufactured from a wider assortment of materials including certain refractory metals and refractory metal alloys that have heretofore been found to be unsuitable for stent applications. Such materials include, but are not limited to, tantalum alloys, niobium alloys, and molybdenum alloys, including tantalum-niobium-tungsten alloys.
In one embodiment, a tantalum alloy may includes a tantalum content of about 77 weight % (“wt %”) to about 92 wt %, a niobium content of about 7 wt % to about 13 wt %, and a tungsten content of about 1 wt % to about 10 wt %.
These and other features and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments which, taken in conjunction with the accompanying drawings, illustrate by way of example the principles of the present invention.
To further clarify the above and other advantages and features of the present invention, a more particular description of the invention will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. It is appreciated that these drawings depict only illustrated embodiments of the invention and are therefore not to be considered limiting of its scope. The invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:
Stents are well known in the art and can have many different types of patterns and configurations. The following description of intravascular stents include typical stent patterns made from a metallic tubing. Many stent patterns are well known in the art, and the description herein of stents and delivery systems is by way of example and is not meant to be limiting.
Referring to
The proximal end of the catheter assembly 10 may be fitted with an adapter 17 that includes a guide wire port and an inflation port at a sidearm 24. The distal end of the guide wire 23 exits the catheter distal end so that the catheter advances along the guide wire. As is known in the art, a guide wire lumen 22 is configured and sized for receiving various diameter guide wires to suit a particular application. The partially annealed stent 16 is typically mounted on an expandable member (balloon) 14 positioned proximate the catheter distal end 13. The stent 16 is crimped tightly thereon, so that the stent and expandable member 14 present a low profile diameter for delivery through the patient's vasculature. The stent 16 may be used to repair a diseased or damaged arterial wall 18, a dissection or a flap that are commonly found in the coronary arteries, peripheral arteries and other vessels. The presence of arterial plaque (not shown) may be treated by an angioplasty or other repair procedure prior to stent implantation.
In a typical procedure to implant a stent 16 formed from a partially annealed material, the guide wire 23 is advanced through the patient's vascular system by well known methods so that the distal end of the guide wire is in the body lumen 18 at the designated area. Prior to implanting the stent, the cardiologist may wish to perform an angioplasty procedure or other procedure (e.g., atherectomy) in order to open the vessel and remodel the diseased area. Thereafter, the stent delivery catheter assembly 10 is advanced over the guide wire 23 so that the stent is positioned in the target area. During positioning and throughout the procedure, the partially annealed stent 16 may be visualized through x ray fluoroscopy and/or magnetic resonance angiography.
As shown in
The stent 16 formed from the partially annealed material serves to hold open the artery wall 18 after the catheter assembly 10 is withdrawn, as illustrated by
As shown in
As shown in
The partially annealed stent 60 of the present invention can be made in many ways. One method of making the stent is to cut a thin walled tube of partially annealed material to remove portions of the tubing in the desired pattern for the stent, leaving relatively untouched the portions of the metallic tubing that are to form the stent. In accordance with the invention, it is preferred to cut the tubing in the desired pattern by means of a machine controlled laser, as is well known in the art. Other methods of forming the stent of the present invention can be used, such as chemical etching; electric discharge machining; laser cutting a flat sheet and rolling it into a cylinder with a longitudinal weld; and the like, all of which are well known in the art at this time. In addition, the stent and/or its struts may be formed from a wire or elongated fiber constructed from a partially annealed material. The cross section of such struts may be round, rectangular or any other suitable shape for constructing a stent.
In the present invention, during the stent manufacturing process the stent material is only partially annealed prior to forming the stent or in which the stent itself is partially annealed after manufacture from work-hardened tubing. This process will somewhat decrease the internal dislocation density caused by drawing, and allow only partial recrystallization. By creating an inhomogeneous grain structure, the partial annealing provides a controllable and optimized balance between strength and ductility of the stent material, resulting in beneficial performance characteristics. This method can be used broadly with any stent material, including stainless steel and cobalt-chromium alloys. And more particularly, testing has shown that the method is particularly useful for some novel refractory metals such as tantalum based alloys. In order to reach an optimal state, the dislocation structure, the recrystallization amount, and the end grain size will be adjusted as necessary to achieve a balance of these properties.
Referring to
Samples annealed fully using a process that lasted 80 minutes at 1300° C. resulted in material properties that were near a maximum for elongation, but near a minimum for strength. Since stent tubing in the past has been fully annealed, the present invention demonstrates that there is much to be gained by partial annealing.
A method is described as illustrative of the present invention. A stent material such as TaNbW is drawn into a tubing form with a residual cold-working of between zero and one hundred percent. The tubing is then annealed to less than full anneal using known annealing processes having time and temperature parameters. The stent tubing is formed into a stent while in the partially annealed state, such as by laser cutting, micromachining, EDM, or photolithography/etching processes. The stent can be fully annealed prior to the final drawing step(s). After the full annealing, there can be at least one or more steps to achieve additional cold work.
While testing was conducted on a TaNb10W7.5 alloy, the partial annealing process of the invention can be applied to any metallic materials used to form stents including stainless steel, cobalt-based alloys, cobalt-chromium alloys, titanium-based alloys, and tantalum alloys.
One example of a tantalum alloy includes a tantalum content of about 77 wt % to about 92 wt %, a niobium content of about 7 wt % to about 13 wt % (e.g., about 7 wt % to about 12 wt %), and a tungsten content of about 1 wt % to about 10 wt %. However, the tantalum alloy may also include other alloying elements, such as one or more grain-refining elements in an amount up to about 5 wt % of the tantalum alloy. For example, the one or more grain-refining elements may include at least one of hafnium, cerium, or rhenium. Tungsten is provided to solid-solution strengthen tantalum, and niobium is provided to improve the ability of tantalum to be drawn. The tantalum alloy is a substantially single-phase, solid-solution alloy having a body-centered cubic crystal structure. However, some secondary phases may be present in small amounts (e.g., inclusions) depending upon the processing employed to fabricate the tantalum alloy.
The composition of the tantalum alloy may be selected from a number of alloy compositions according to various embodiments. In an embodiment, the niobium content is about 8 wt % to about 12 wt % (e.g., about 9 wt % to about 11 wt %), the tungsten content is about 6 wt % to about 9 wt % (e.g., about 6.5 wt % to about 8.5 wt %), and the balance may include tantalum (e.g., the tantalum content being about 80 wt % to about 83 wt %) and, if present, other minor alloying elements and/or impurities. In a more detailed embodiment, the niobium content is about 10 wt %, the tungsten content is about 7.5 wt %, and the balance may include tantalum (e.g., the tantalum content being about 82.5 wt %) and, if present, other minor alloying elements and/or impurities. In another more detailed embodiment, the niobium content is about 10 wt %, the tungsten content is about 2.5 wt %, and the balance may include tantalum (e.g., the tantalum content being about 87.5 wt %) and, if present, other minor alloying elements and/or impurities.
In another embodiment, the niobium content is about 10.5 wt % to about 13 wt %, the tungsten content is about 5.0 wt % to about 6 wt %, and the balance may include tantalum (e.g., the tantalum content being about 80 wt % to about 82 wt %) and, if present, other minor alloying elements and/or impurities. In a more detailed embodiment, the niobium content is about 12.5 wt %, the tungsten content is about 5.8 wt %, and the balance may include tantalum (e.g., the tantalum content being about 81 wt % to about 81.5 wt %) and, if present, other minor alloying elements and/or impurities.
Further embodiments of the process of the present invention may be used for partially annealing materials using other metals and alloys, by varying the annealing temperature and time to achieve the desired degree of partial annealing. Additional example alloys for which the partial annealing manufacturing method of the present invention may be applied include, but are not limited to:
Stainless steels (e.g., 316L stainless steel) may be partially annealed by heating the metal to an annealing temperature ranging between about 800° C. and about 1100° C. and holding the metal at the annealing temperature for a period of time sufficient to achieve the desired degree of partial annealing. L 605 (ASTM F90 and AMS 5759), a Co—Cr—W—Ni alloy also available as STELLITE 25 (Deloro Stellite Company, Inc., South Bend, Ind., U.S.A.) and HAYNES 25 (Haynes International Inc., Kokomo, Ind., U.S.A.), which may be heated to an annealing temperature ranging between about 1120° C. and about 1230° C., and must have rapid cooling (e.g., air) in order to avoid precipitation of undesirable phases.
ELGILOY (ASTM F1058), a Co—Cr—Mo—Ni alloy available from Elgiloy Specialty Metals Division of Elgin, Ill., U.S.A., which may be heated to an annealing temperature ranging from about 1090° C. to about 1150° C.
Platinum iridium (Pt Ir) alloys, which may be heated to an annealing temperature ranging from about 1000° C. to about 1200° C. for alloys having up to ten percent iridium, and ranging from about 1300° C. to about 1500° C. for alloys having greater than ten percent iridium.
Nickel-titanium (Ni Ti) alloys (e.g., nitinol having stoichiometry around 50-50 for shape memory properties), which may be heated to an annealing temperature ranging from about 650° C. to about 950° C., with longer hold times for the lower temperatures
Titanium (Ti) and titanium based alloys, such that pure titanium is heated to an annealing temperature ranging from about 650° C. to about 750° C., with temperatures for titanium alloys depending on the particular alloy.
Instead of working with a semi-annealed tube, it is also within the scope of the present invention to start with a semi- or full-hard tube and control and only partially anneal the tube in the post processing. Alternatively, post-processing steps such as polishing and passivation methods may be used to improve the stent surface finish, as is well known in the art. It may also be necessary to perform a post-processing annealing step. This post-processing annealing step could also be a partially annealing step in accordance with the invention.
While a particular form of the invention has been illustrated and described, it will be apparent to those skilled in the art that various modifications can be made without departing from the spirit and scope of the invention. Accordingly, it is not intended that the invention be limited except by the appended claims.
