Patent Application
20130092299
Stents, grafts and a variety of other implantable devices are well known and used in interventional procedures, such as for treating aneurysms, for lining or repairing vessel walls, for filtering or controlling fluid flow, and for expanding or scaffolding occluded or collapsed vessels. Such implantable devices may be delivered and used in virtually any accessible body lumen of a human or an animal, and may be deployed by any of a variety of recognized means.
Implantable devices, such as stents, are used for the treatment of atherosclerotic stenosis in blood vessels. For example, after a patient undergoes a percutaneous transluminal coronary angioplasty or similar interventional procedure, an implantable device, such as a stent, is often deployed at the treatment site to scaffold or support the treated blood vessel. If desired, the implantable device may also be loaded with beneficial agent to act as a delivery platform to reduce restenosis or the like.
Factors affecting the choice of the medical implant or device and the material thereof include mechanical properties and biocompatibility. For example, the implantable device should have sufficient rigidity, flexibility, and biocompatibility. In addition, it is preferred that the implantable device be visible under x-rays and the implantable device should not interfere with MRI analysis. Refractory metals and refractory metal alloys are well-suited for fabricating implantable devices because they have favorable mechanical properties, x-ray and MRI properties, and excellent biocompatibility. Moreover, refractory metals and refractory metal alloys may be made sufficiently radio-opaque to allow for good imaging of the device under x-ray without the addition of an extra layer or a portion of radio-opaque material. Nevertheless, the refractory metals and refractory metal alloys may not be overly “bright” and therefore do not obscure the image of the surrounding tissue, as would be the case with a device made from an extremely dense material. In addition, the refractory metals and refractory metal alloys may be made to be MRI safe and compatible, and visible under MRI.
However, in order to effectively employ refractory metals and refractory metal alloys in implantable medical devices, improved heat treating apparatuses and methods are needed for processing such implantable devices.
Embodiments of apparatuses for vacuum heat treating refractory metal articles (e.g., implantable medical devices) and methods for vacuum heat treating refractory metal articles are disclosed. Heat treating refractory metal articles may be used during manufacturing processes to improve overall device characteristics. For instance, a refractory metal article (e.g., a stent or another implantable medical devices) that is heat treated (e.g., the article is heat treated above the recrystallization temperature for the material) may exhibit a desirable combination of strength and ductility because internal stresses are relieved and the grain microstructure of the material is partially or fully recrystallized. A refractory metal article that is only stress relieved (e.g., the article is heat treated below the recrystallization temperature for the material) may also exhibit improved material characteristics because internal stresses are relieved while maintaining a work-hardened structure. Heat treating under high vacuum (e.g., 10−6 Torr) may also remove material contaminants (e.g., oxygen or hydrogen) that are absorbed during manufacturing processes and that may cause chemical embrittlement (e.g., hydrogen embrittlement). Heat treating material under high vacuum may also prevent additional contamination during heat exposure.
In one embodiment, a heat treating apparatus for heat treating a refractory metal article is disclosed. The heat treating apparatus may include a guide tube defining a guide tube passageway, a furnace tube fluidly coupled to the guide tube and defining a furnace-tube passageway in communication with the guide tube passageway, and a heating element disposed around at least a portion of the furnace tube. Being so disposed, the heating element defines a heated zone in a first portion of the furnace tube having the heating element disposed therearound and an unheated zone in at least a second portion of the furnace tube outside the heated zone.
The heat treating apparatus further includes a guide rod structure movable within the guide tube and/or the furnace tube including a proximal portion and a distal portion configured to support the article, and a magnetic drive system configured to move the guide rod structure axially (i.e., horizontally) within the guide tube passageway and the furnace-tube passageway. The guide rod structure and the magnetic drive system may be used, for example, to selectively position the article within the heated zone and/or the unheated zone while a partial vacuum environment is maintained inside the guide tube and the furnace tube.
In one embodiment, a method for heat treating a refractory metal article is disclosed. The method may include (1) disposing the refractory metal article in a heat treating apparatus, and (2) sealing the heat treating apparatus. With the article disposed in the heat treating apparatus and sealed against the intrusion of outside air, the method may further include (3) drawing a partial vacuum in the heat treating apparatus to a sufficient vacuum level so that chemical embrittlement does not occur in the refractory metal article before and/or during and/or after heat treating, (4) advancing the refractory metal article to a heated zone in the heat treating apparatus while maintaining the partial vacuum therein, (5) exposing the refractory metal article to a temperature in the heated zone for a period of time sufficient to improve ductility of the refractory metal article and/or at least partial removal of hydrogen therefrom, and (6) moving the refractory metal article to an unheated zone in the heat treating apparatus while maintaining the partial vacuum therein. For example, in one embodiment, the refractory metal article may be moved into and out of the heated zone using a magnetic drive system as described herein.
In one embodiment, a method for moving a refractory metal article in a vacuum environment is disclosed. The method includes (1) providing a heat treating apparatus including a guide tube defining a guide tube passageway and a furnace tube defining a furnace tube passageway, the guide tube and the furnace tube being fluidly coupled and configured to contain a partial vacuum environment therein, a heating element disposed around at least a portion of the furnace tube, and a magnetic drive system disposed in the guide tube and the furnace tube, (2) disposing a refractory metal article within the furnace tube passageway on the magnetic drive system, (3) advancing the magnetic drive system to move the refractory metal article into a heated zone of the furnace tube while maintaining the partial vacuum therein, and (4) retracting the magnetic drive system to move the refractory metal article into an unheated zone while the partial vacuum is maintained therein.
Suitable examples of refractory metal articles that may be heat treated in the heat treating apparatus disclosed herein or that may be heat treated using the methods disclosed herein may include articles formed from refractory metals or refractory metal alloys having a very high melting point (e.g., about 2600° C. to about 3400° C.) such as tungsten, molybdenum, tantalum, niobium, rhenium, and alloys thereof. Such refractory metals may be susceptible to chemical embrittlement due to adsorption of gases such as oxygen, nitrogen, and/or hydrogen.
These and other objects and features of the present disclosure will become more fully apparent from the following description and appended claims, or may be learned by the practice of the disclosure as set forth hereinafter.
To further clarify the above and other advantages and features of the present disclosure, a more particular description of the disclosure 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 disclosure and are therefore not to be considered limiting of its scope. The disclosure will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:
Embodiments of apparatuses for vacuum heat treating refractory metal articles (e.g., implantable medical devices) and methods for vacuum heat treating refractory metal articles are disclosed. Heat treating of refractory metal articles may be used during manufacturing processes to improve overall device characteristics. For instance, a refractory metal article (e.g., a stent or another implantable medical device) that is heat treated may exhibit a desirable combination of strength and ductility because internal stresses are relieved and the grain microstructure of the material is partially or fully recrystallized. A refractory metal article that is stress relieved (e.g., the article is heat treated below the recrystallization temperature) may also exhibit improved material characteristics because internal stresses are relieved while maintaining a work-hardened structure. Heat treating under high vacuum (e.g., at least about 10−6 Torr) may also remove material contaminants (e.g., oxygen or hydrogen) that are absorbed during the manufacturing process and that may cause chemical embrittlement. Heat treating material under high vacuum may also prevent additional contamination during heat exposure.
Relief of cold-work-induced internal stresses in the refractory metal article, partial or complete recrystallization, and/or removal of embrittling contaminants may improve the ductility of the refractory metal article, while maintaining a sufficient strength.
Referring now to
In one embodiment, the act 102 of providing a refractory metal article may include providing a billet of metal, a metal tube, and the like or medical implants and devices including minimal-invasive devices, such as, guide wires, intra-cavernous implants, in particular intra-esophagus, intra-urethra, intra-tracheal implants and intra-vascular implants, in particular stents, stent grafts, stent graft connectors, heart valve repair devices, or filters. In one embodiment, the article provided in act 102 may be fabricated from tantalum or a tantalum alloy. It has been found that a tantalum alloy that includes tantalum, niobium, and at least one additional element selected from the group consisting of tungsten, zirconium, molybdenum, and/or at least one of hafnium, rhenium, or cerium may fulfill the mechanical and structural requirements needed for functioning as a medical device.
The heat treating apparatus provided in act 104 will be discussed in greater detail below in reference to
The act 106 of disposing the refractory metal article in the heat treating apparatus may include disposing the refractory metal article on a portion of a support structure that is operably coupled to a magnetic drive system. For example, the support structure may include a tray configured for holding one or more refractory metal articles.
In one embodiment, the act 106 of sealing the heat treating apparatus may include sealing the apparatus such that a partial vacuum may be drawn inside the heat treating apparatus. In addition, the act of 106 of sealing the heat treating apparatus may include sealing the apparatus such that outside air cannot intrude into the heat treating apparatus when the partial vacuum is drawn therein.
Refractory metals (e.g., tantalum, tungsten, molybdenum, tantalum, niobium, rhenium, and alloys thereof) may be reactive with and/or have a high solubility for oxygen, hydrogen, nitrogen, and other gases. These gases are also found in purging gases (e.g., small percentages of oxygen and hydrogen contaminants are commonly found in commercially produced, high purity argon). It is desirable to limit the presence of gases of any type during the heat treating process to prevent exposure of the refractory metal article to such gases while at high temperature to prevent chemical embrittlement. As such, in one embodiment, the partial vacuum drawn in act 110 may be characterized as a vacuum sufficient so that the first refractory metal article does not react with and/or dissolve a sufficient amount of hydrogen to cause hydrogen embrittlement during and/or after the act of heat treating.
The partial vacuum drawn in act 110 may range from about 25 torr to about 10−12 torr in pressure (i.e., approximately 3000 Pa to approximately 10−10 Pa). For example, the pressure may range from about 1 torr to about 10−7 torr (i.e., approximately 100 Pa to approximately 10−5 Pa) or, more specifically, the pressure may range from about 10−3 ton to about 10−7 ton (i.e., from approximately 10−1 Pa to approximately 10−5 Pa).
In one embodiment, the act 112 of advancing the refractory metal article to a heated zone in the heat treating apparatus while maintaining a partial vacuum therein may include advancing the refractory metal article disposed in the furnace tube using the magnetic drive system described briefly above. For example, the interacting components of the magnetic drive system that are inside and outside the furnace tube may be used to advance the refractory metal articles disposed in the furnace from an unheated zone to a heated zone where heat treatment of the refractory metal article occurs. The details of the components and the details of the operation of the magnetic drive system will be discussed in greater detail below in reference to
In one embodiment, the act 114 of exposing the refractory metal article to a temperature in the heated zone for a period of time sufficient to improve ductility of the refractory metal article and/or at least partially remove one or more chemically embrittling elements therefrom may include exposing the refractory metal article to a temperature in a range from about 800° C. to about 1500° C. Typically, the heating element is pre-heated prior to moving the refractory metal article (or articles) into the heated zone (i.e., the region of the furnace tube defined by the heating element) such that the furnace tube is pre-heated to the heat treating temperature prior to placing the article (or articles) in the heated zone. As such, the refractory metal articles rapidly heat to the heat treating temperature once they are moved into the heated zone (e.g., about 10 seconds up to about 2 hours, depending on the articles being heat treated). This reduces the amount of time that the articles spend in an undefined temperature zone as the articles heat up to the heat treating temperature, which may have the advantage of reducing undefined crystal re-growth and/or stress relief in the metallic structure of the articles.
Once the refractory metal article (or articles) in the heated zone reach the heat treating temperature, the refractory metal articles may be heat treated for a period of time (i.e., the “dwell time”) from less than about 1 minute (e.g., about 30 seconds) to about 2 hours or more or about 5 minutes to about 25 minutes. Most specifically, the refractory metal articles may be heat treated for a period of time from about 10 minutes to about 20 minutes.
In some embodiments of the present disclosure, it may be desirable to heat treat the refractory metal articles in the heat treating apparatus described herein at a temperature in a range from about 800° C. to about 1000° C., which is below the recrystallization temperature of the refractory metal article. Typically, heat treating a refractory metal article at a temperature in a range from about 800° C. to about 1000° C. is sufficient to cause out-gassing of hydrogen, oxygen, nitrogen, and other embrittling elements from the refractory metal article without causing excessive grain growth of the grain microstructure of the refractory metal article. Heat treating a refractory metal article at a temperature in a range from about 800° C. to about 1000° C. may thus be sufficient to remove at least one of hydrogen, nitrogen, or oxygen contamination and to relieve internal stresses without significantly recrystallizing the grain microstructure and/or altering the strength (yield strength and ultimate tensile strength) of the refractory metal article.
In some embodiments of the present disclosure, it may be desirable to heat treat the refractory metal articles in the heat treating apparatus described herein at a temperature in a range from about 1000° C. to about 1500° C. In addition to being sufficient to cause the desired out-gassing, heat treating a refractory metal article at a temperature in a range from about 1000° C. to about 1500° C. is typically sufficient to partially or completely recrystallize the grain microstructure to thereby increase the ductility of the refractory metal article. In a more specific embodiment, the heat treating apparatus described herein is employed for heat treating the refractory metal article at a temperature in a range from about 1100° C. to about 1400° C., about 1200° C. to about 1400° C., or about 1200° C. to about 1300° C. Most specifically, the temperature employed in the heat treating apparatus may be in a range from about 1265° C. to about 1285° C.
In an embodiment, the act 114 includes heat treating the refractory metal article (or articles) in the heated zone of the heat treating apparatus for a period of time in a range between about 10 minutes and about 20 minutes at a temperature in a range between about 1265° C. and about 1285° C.
Once the dwell time at the heat treating temperature is complete, the method 100 includes an act 116 of retracting the refractory metal article from the heated zone to an unheated zone in the heat treating apparatus while maintaining a partial vacuum therein. Typical cool down times include a temperature drop from about 1300° C. to about 500° C. in about 5 minutes and a drop from 500° C. to room-temperature in about 30 minutes.
The act 116 may include using the magnetic drive system to withdraw the article (or articles) from the heated zone to an unheated zone in the furnace tube. In some embodiments, the unheated zone may be passively cooled to cool the heat-treated articles or the heated zone may be actively cooled with the use of, for example, a water-cooled jacket or the flow of cooled gases around the outside of the unheated zone of the furnace tube.
Referring now to
In one embodiment, the heat treating apparatus may include a guide tube defining a guide tube passageway and a furnace tube defining a furnace tube passageway, the guide tube and the furnace tube being fluidly coupled and configured to contain a partial vacuum environment therein, a heating element disposed around at least a portion of the furnace tube, and a magnetic drive system configured to move one or more refractory metal articles in the guide tube and/or the furnace tube while a partial vacuum is maintained in the guide tube and/or the furnace tube.
The method 200 may further include an act 204 of disposing a refractory metal article within the heat treating apparatus, an act 206 of advancing the refractory metal article from an unheated zone of the heat treating apparatus to a heated zone of the heat treating apparatus using the magnetic drive system while a partial vacuum is maintained therein, and an act 208 of retracting the refractory metal article from the heated zone of the heat treating apparatus to the unheated zone of the heat treating apparatus using the magnetic drive system while the partial vacuum is maintained therein.
The act 206 and/or the act 208 may further include advancing and/or retracting a guide rod structure on which the refractory metal articles are supported by magnetically actuating a magnetically responsive component that is coupled to the guide rod structure via a magnet that is outside the guide tube. The magnetically responsive component (e.g., a billet of magnetically responsive metal such as iron or an iron alloy) is configured to magnetically engage with the magnet through the guide tube in order to permit axial movement of the guide rod structure in the guide tube passageway while the partial vacuum environment is maintained in the guide tube and/or the furnace tube.
The advancing and/or retracting of acts 206 and 208 may include traversing an axial distance along the long axis of the heat treating apparatus of at least about 20 cm to about 200 cm using the magnetic drive system while maintaining the partial vacuum environment in the guide tube and/or the furnace tube. More specifically, the advancing and/or retracting of acts 206 and 208 may include traversing an axial distance along the long axis of the heat treating apparatus of at least about 50 cm to about 100 cm using the magnetic drive system.
Any of the methods discussed herein may be employed for heat treating a refractory metal article. Suitable examples of refractory metals or refractory metal alloys include, but are not limited to, tantalum and tantalum alloys. It has been found that a tantalum alloy that includes tantalum, niobium, and at least one additional element selected from the group consisting of tungsten, zirconium, molybdenum, and/or at least one of hafnium, rhenium, and cerium may fulfill the mechanical and structural requirements needed for functioning as in a medical device.
In one example, a tantalum-containing refractory metal article may include (a) about 0.1 weight-percent and 70 weight-percent niobium, (b) about 0.1 weight-percent and 30 weight-percent of at least one element selected from the group consisting of tungsten, zirconium, and molybdenum, (c) up to 5 weight-percent of at least one element selected from the group consisting of hafnium, rhenium, and cerium, (d) and tantalum.
In another example, the tantalum-containing refractory metal article disclosed herein may be made from a tantalum alloy that includes about 75 to about 80 weight percent tantalum, about 8 to about 12 weight percent niobium, and about 7 to about 10 weight percent tungsten. The tantalum-containing refractory metal article may further include at least one of a tungsten content of about 0.1 to about 15 weight percent, a zirconium content of about 0.1 to about 10 weight percent, or a niobium content of about 5 to about 25 weight percent.
In yet another example, the tantalum-containing refractory metal article disclosed herein may be made from a tantalum alloy that includes about 75 to about 90 weight percent tantalum, about 8 to about 12 weight percent niobium, and about 2 to about 10 weight percent tungsten.
In still yet another example, the tantalum-containing refractory metal article disclosed herein may be made from a tantalum alloy that includes about 75 to about 80 weight percent tantalum, about 8 to about 12 weight percent niobium, and about 7 to about 10 weight percent tungsten.
In a specific example, the tantalum-containing refractory metal article disclosed herein may be made from a tantalum alloy that includes about 82.5 weight percent tantalum, about 10 weight percent niobium, and about 7.5 weight percent tungsten.
In another specific example, the tantalum-containing refractory metal article disclosed herein may be made from a tantalum alloy that includes about 87.5 weight percent tantalum, about 10 weight percent niobium, and about 2.5 weight percent tungsten.
The tantalum alloys disclosed herein may have a passive oxide film primarily composed of tantalum-oxide (Ta2O5), which is generally more durable and more corrosion resistant than, for example, the chromium-oxide film formed during the passivation of stainless steel. The tantalum alloys disclosed herein may include a substantially single phase body centered cubic crystal structure, and have the ability for conversion oxidation or nitrodization surface hardening.
The tantalum alloys disclosed herein may easily be cold-worked to increase strength. It is possible to form a hard, abrasion resistant surface on the inventive alloy through various oxidation and nitridizing methods. The presence of a hard, inert, abrasion resistant surface layer enables fabrication of medical implants and devices having lower friction and wear, electrical insulation, and improved corrosion resistance.
To further improve the biocompatibility of a medical implant or device fabricated at least in part from the tantalum alloys disclosed herein, at least a portion of the surface of the alloy may be conversion surface hardened and/or coated. Such coatings may include, but are not limited to a polymer, a blend of polymers, a metal, a blend of metals, a ceramic and/or biomolecules, in particular peptides, proteins, lipids, carbohydrates and/or nucleic acids (e.g. collagen, heparin, fibrin, phosphorylcholine, cellulose, morphogenic proteins or peptides, growth factors). Furthermore the alloy surface or the coatings may comprise stem cells and/or bioactive substances, in particular drugs, antibiotics, growth factors, anti-inflammatory agents and/or anti-thrombogenic agents.
The refractory metals and refractory metal alloys (e.g., tantalum and tantalum-containing alloys) discussed herein are prone to dissolving oxygen, hydrogen, and other gases when heated above about 250° C. Dissolved oxygen, nitrogen, carbon and hydrogen impurities may, for example, lead to embrittlement of the refractory metals through processes such as hydrogen embrittlement.
Referring now to
The elongate tubular structure defined by the furnace tube 302 and the guide tube 310 includes a first closed end 302a and a second closed end 310a. In the illustrated embodiment, an interlock assembly 306 is mounted to the furnace tube 302 and the guide tube 310. The interlock assembly 306 is configured to couple the furnace tube 302 to the guide tube 310. Moreover, the interlock assembly 306 is configured to allow a user to separate the guide tube 310 from the furnace tube to allow access to the inside of the heat treating apparatus to allow at least one refractory metal article to be disposed in the furnace tube 302. The interlock assembly 306 includes an interlock body 308, and a furnace tube flange 309 that couples the interlock body 308 to the furnace tube 302 and a guide tube flange 307 that couples the interlock body 308 to the guide tube 310. In one embodiment, the interlock body 308 may further include a water-cooled jacket (not shown) configured to facilitate rapid cooling of the interlock body 308 heat treated articles dispersed therein.
As shown in
Referring now to the heating element 304 of apparatus 300, the heating element 304 shown in
In a typical heat treating process using the heat treating apparatus 300, the furnace tube may be opened by separating guide tube 310 from the furnace tube 302 at the interlock 306. With the furnace tube 302 opened, at least one refractory metal article (e.g., a refractory metal stent) is placed in tray 324, the furnace tube 302 and the guide tube 310 are rejoined via the interlock 306. Once the furnace tube 302 is sealed, a vacuum may be drawn to a sufficient level (e.g., about 10−3 torr to about 10−7 torr) and the tray may be positioned in the heated zone using the magnetic drive system 328, which is coupled to the guide rod structure 326 and the tray 324. The heating element 304 is typically pre-heated to a temperature of about 800° C. to about 1500° C. prior to moving the tray 324 containing the article into the heated zone. Pre-heating the heating element 304 allows the furnace tube 302 and the refractory metal article disposed therein to rapidly heat up to a selected heat treating temperature (e.g., about 800° C. to about 1500° C.). The refractory metal articles disposed in the heated zone are held for a pre-determined amount of time (e.g., about 10 min.) at the heat treating temperature in the heated zone. Following the heat treating period, the magnetic drive system is used to remove the articles from the heated zone for cooling. Once the articles are cooled, the vacuum may be released and the articles may be removed from the furnace.
Referring now to
The magnetically responsive component 402 may be coupled to the guide rod 326 by press fitting, adhesive, brazing, soldering, one or more fasteners, or other suitable techniques. As such, moving the magnetically responsive component 402 within the guide tube 310 may effect movement of the guide rod 326 and the tray 324. In the illustrated embodiment, the magnetically responsive component 402 includes a plurality of bearing elements (e.g., ball bearings, Teflon balls, or the like) 404 disposed on the magnetically responsive component 402 so as to interpose between the magnetically responsive component 402 and the inside of the guide tube 310. The bearing elements 404 are configured to facilitate movement of the guide rod 326 via movement of the magnetically responsive component. The bearing elements 404 are shown as balls in
Referring now to
Referring now to
The interlock body 308 may include four openings. A first opening may receive a portion of the guide tube 310. A second opening may receive a portion of the furnace tube 302 including the open end 302b. A third opening 314 may be used to couple the interlock body 308 to a vacuum source, while a fourth opening (not shown) may be used to couple the interlock body 308 to a vacuum sensor.
In the embodiment shown in
In the embodiment shown in
It is noted that the furnace system 300 is merely one of many suitable furnaces for heat treating the refractory metal articles disclosed herein. Other vacuum tube furnaces may be employed to practice the methods of vacuum heat treating refractory metal articles disclosed herein.
The present disclosure may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the disclosure is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.
