Patent Application
20130096666
The human body includes various lumens, such as blood vessels or other passageways. A lumen may sometimes become at least partially blocked or weakened. For example, a lumen may be at least partially blocked by a tumor, by plaque, or both. An at least partially blocked lumen may be reopened or reinforced with an implantable stent.
A stent is typically a tubular body that is placed in a lumen in the body. A stent may be delivered inside the body by a catheter that supports the stent in a reduced-size configuration as the stent is delivered to a desired deployment site within the body. At the deployment site, the stent may be expanded so that, for example, the stent contacts the walls of the lumen to expand the lumen.
Advancement of the stent through the body may be monitored during deployment. After the stent is delivered to the target site, the stent can be monitored to determine whether the placement thereof is correct and/or the stent is functioning properly. Methods of tracking and monitoring stent after delivery include X-ray fluoroscopy and magnetic resonance imaging (“MRI”).
Stents made from tantalum alloys have been identified as being easily detectable using X-ray fluoroscopy and MRI because of the high density of tantalum. Furthermore, tantalum alloys are typically compatible with MRI techniques because they do not produce substantial amounts of magnetic artifacts and/or image distortions or voids during MRI imaging. Additionally, tantalum alloys have proven to be biocompatible and corrosion resistant.
The present disclosure is directed to a drug-eluting implantable medical devices (e.g., stents, guide wires, closure elements, etc.) that includes a tantalum-alloy body having a drug-eluting coating thereon for delivering a drug to treat, for example, restenosis, and methods of processing such tantalum-alloy products to modify at least one of a microstructural, a mechanical, or a chemical property thereof prior to application of the drug-eluting coating. In an embodiment, a tantalum-alloy product is disclosed. The tantalum-alloy product includes a body comprising a tantalum alloy. The tantalum alloy 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 %. The tantalum alloy exhibits at least one mechanical property modified by heat treatment thereof.
In an embodiment, an implantable medical device is disclosed. The implantable medical device includes a body sized and configured to be implanted in a living subject. At least a portion of the body may comprise a tantalum alloy. The tantalum alloy 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 %. The tantalum alloy exhibits at least one mechanical property modified by heat treatment thereof. The implantable device further includes drug-eluting coating that at least partially coats the body. In an embodiment, the body may be configured as a stent body, a guide wire, a closure device, embolic coils, pacemaker leads, sutures, prosthetic heart valves, mitral valve repair coils, or other implantable structure.
In an embodiment, an implantable medical device includes a body configured to be implanted into a living subject. At least a portion of the body may comprise a tantalum alloy. The tantalum alloy may exhibit a tensile elongation of about 5% to about 50%, a tensile yield strength of about 440 MPa to about 840 MPa, an ultimate tensile strength of about 490 MPa to about 880 MPa, and a radiopacity less than or equal to substantially pure tantalum having a thickness of about 55.88 μm (0.0022 inch).
In an embodiment, a method of processing an implantable device is disclosed. The method includes providing the implantable medical device including a drawn tantalum-alloy body comprising a tantalum alloy. The 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 %, and a tungsten content of about 1 wt % to about 10 wt %. In an embodiment, the implantable device may also be characterized by a tensile elongation of about 5% to about 50%, a tensile yield strength of about 440 MPa to about 840 MPa, an ultimate tensile strength of about 490 MPa to about 880 MPa, and a radiopacity less than or equal to substantially pure tantalum having a thickness of about 55.88 μm (0.0022 inch). The method further includes heat treating the tantalum-alloy body to modify at least one mechanical property thereof. The method also includes coating at least part of the tantalum-alloy body with a drug-eluting coating.
In an embodiment, a method for implanting an implantable medical device (e.g., a stent) into a living subject is disclosed. The method includes delivering the implantable medical device in a delivery device to a selected deployment site within the living subject. In an embodiment, the implantable medical device includes a body comprising a tantalum alloy having a tantalum content of about 77 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 %. The tantalum alloy exhibits at least one mechanical property modified by heat treatment thereof. In an embodiment, the implantable medical device may also be characterized by a tensile elongation of about 5% to about 50%, a tensile yield strength of about 440 MPa to about 840 MPa, an ultimate tensile strength of about 490 MPa to about 880 MPa, and a radiopacity less than or equal to substantially pure tantalum having a thickness of about 55.88 μm (0.0022 inch). The method further includes expanding the implantable device The implantable device further includes a drug-eluting coating that at least partially coats the body. The method further includes expanding the body at the selected deployment site. The method additionally includes removing the implantable device from the delivery device.
Features from any of the disclosed embodiments may be used in combination with one another, without limitation. In addition, other features and advantages of the present disclosure will become apparent to those of ordinary skill in the art through consideration of the following detailed description and the accompanying drawings.
To further clarify at least some of the advantages and features of the present disclosure, a more particular description of the disclosure will be rendered by reference to various embodiments thereof that are illustrated in the appended drawings. It is appreciated that these drawings depict only various embodiments of the disclosure and are therefore not to be considered limiting of its scope. The various embodiments will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:
The present disclosure is directed to drug-eluting implantable medical devices (e.g., stents, closure devices, etc.) including a tantalum-alloy body having a drug-eluting coating thereon for delivering a drug, and methods of processing such tantalum-alloy bodies to enhance at least one of a microstructural, a mechanical, or a chemical property thereof prior to applying the drug-eluting coating. The description below is directed mainly to a stent including a stent body made from a coated tantalum alloy that is composed and processed to impart at least one of certain microstructural, mechanical, or chemical properties to the tantalum alloy. However, other implantable medical devices besides stents may employ a tantalum alloy exhibiting one or more of the disclosed properties, such as guide wires, closure elements, or other implantable medical devices.
Referring to
The stent body 102 is made from a tantalum alloy that is composed and heat-treated to obtain one or more of certain desirable microstructural, mechanical, or chemical properties. For example, the tantalum alloy may be heat treated to modify at least one mechanical property thereof, such as ductility, yield strength, or ultimate tensile strength. The 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.
In an embodiment, the tantalum alloy may exhibit a grain microstructure characteristic of being formed by heat treating a precursor product of the stent body 102 or the stent body 102 itself, both of which may be severely plastically deformed in a drawing process. Tantalum alloys readily oxidize and form an oxide layer on the inner and outer diameter surfaces during the tubing fabrication process. Such oxide layers can be removed by subjecting the tubes to a chemical etching process (e.g., in a solution of HF and HNO3). The inventors in the present case have found that if the oxide layer is not removed prior to the heat treatment, the surface oxide can diffuse into the body of the alloy and decrease ductility, which can yield artificially high numbers for ultimate tensile strength and yield strength.
Depending upon the extent of recrystallization process, the grain microstructure may be only partially recrystallized. In some embodiments, the recrystallization process may substantially completely recrystallize the grain microstructure with the new recrystallized grains having consumed substantially all of the old deformed grains. Even when the grain microstructure is partially recrystallized, it will be apparent from microstructural analysis using optical and/or electron microscopy that the grain microstructure includes some recrystallized grains. An average grain size of the tantalum alloy may be about 10 μm to about 20 μm and, more particularly, about 13 μm to about 16 μm in the transverse orientation depending on the extent of recyrstallization and the amount of the optional one or more grain-refining alloy elements in the tantalum alloy.
In other embodiments, the tantalum alloy may be stress relieved at a temperature below a recrystallization temperature of the tantalum alloy so that the grain microstructure is relatively unchanged from the as-drawn condition. Thus, in the stress-relieved condition, the grain microstructure may include essentially only non-equiaxed, deformed, cold-worked grains. However, the stress-relief heat treatment may at least partially remove at least one of hydrogen, oxygen, or nitrogen from the tantalum alloy, which can detrimentally embrittle the tantalum alloy. Thus, the tantalum alloy in the stress-relieved condition may exhibit an improved ductility relative to the as-drawn condition, while the tensile yield strength and tensile ultimate tensile strength may be relatively lowered by the stress-relief heat treatment.
The heat-treated tantalum alloy from which the stent body 102 is made may exhibit combination of strength (e.g., tensile yield strength and ultimate tensile strength) and ductility (e.g., percent elongation) suitable to withstand loading conditions encountered when implanted and utilized in a lumen of a living subject. The tensile yield strength may be the 0.2% offset yield strength determined in a uniaxial tensile test when no yield point is present, and the yield point if the tantalum alloy exhibits a yield point. For example, the heat treated tantalum alloy may exhibit a tensile elongation of about 5% to about 50%, a tensile yield strength of about 440 MPa to about 840 MPa, and an ultimate tensile strength of about 490 MPa to about 880 MPa as determined by, for example, tensile testing a tubular body from which the stent body 102 may be cut from or a drawn wire in a uniaxial tensile test. In an embodiment, the tantalum alloy (e.g., about 82.5 wt % tantalum, about 10 wt % niobium, and about 7.5 wt % tungsten) may exhibit a tensile elongation of about 9% to about 40%, a tensile yield strength of about 455 MPa to about 810 MPa, and an ultimate tensile strength of about 515 MPa to about 850 MPa. In another embodiment, the tantalum alloy may exhibit a tensile elongation of about 20% to about 40%, a tensile yield strength of about 460 MPa to about 480 MPa, and an ultimate tensile strength of about 500 MPa to about 520 MPa. In one embodiment, the tantalum alloy may exhibit a tensile elongation of about 23% to about 27%, a tensile yield strength of about 450 MPa to about 470 MPa, and an ultimate tensile strength of about 505 MPa to about 515 MPa.
In an embodiment, a heat-treated tantalum alloy from which the stent body 102 is made having a tantalum content of about 87.5 wt %, a niobium content of about 10 wt %, and a tungsten content of about 2.5 wt % and an at least partially recrystallized grain microstructure may exhibit a tensile elongation of about 5% to about 50%, a tensile yield strength of about 440 MPa to about 840 MPa, and an ultimate tensile strength of about 490 MPa to about 880 MPa. In one embodiment, the heat-treated tantalum alloy may exhibit a tensile elongation of about 20% to about 40%, a tensile yield strength of about 440 MPa to about 500 MPa, and an ultimate tensile strength of about 490 MPa to about 540 MPa.
In an embodiment, a stress-relieved tantalum alloy from which the stent body 102 is made having a tantalum content of about 82.5 wt %, a niobium content of about 10 wt %, and a tungsten content of about 7.5 wt % may exhibit a percent elongation of about 5% to about 15% (e.g., about 9% to about 11%), a tensile yield strength of about 580 MPa to about 840 MPa (e.g., about 680 MPa to about 810 MPa), and an ultimate tensile strength of about 600 MPa to about 880 MPa (e.g., about 715 MPa to about 850 MPa). In the stress-relieved condition, the percent elongation of the tantalum alloy may increase by at least about 100%, at least about 200%, at least about 300%, at least about 400%, or about 300% to about 400% compared to the same tantalum alloy in the as-drawn (i.e., un-stress-relieved condition), while the tensile yield strength and ultimate tensile strength are reduced. As yield strength and ultimate tensile strength go down, the ductility of the tantalum alloy tends to increase. The reduction in tensile yield strength and ultimate tensile strength and the increase in ductility needs to be balanced, but, in general, increasing ductility tends to yield a more durable medical device fabricated from the tantalum alloy. For example, an alloy having increased ductility is less likely to crack when radially stressed. The grain microstructure may also be relatively un-changed from the as-drawn condition and may include deformed, non-equiaxed grains.
Other mechanical properties of the stent body 102 suitable for characterizing the combination of strength and ductility exhibited by the tantalum alloy include, but are not limited to, percent recoil and radial strength of the stent body 102. Such mechanical properties may be determined by crimping the stent body 102 on a mandrel, expanding the crimped stent body 102 to a specific outer diameter using a balloon catheter or a similar device, and inflating the expanded and crimped stent body 102 to a specific pressure. ASTM F2079 provides one suitable standard for determining percent recoil of the stent body 102. Radial strength may be determined using a commercially available machine for radially expanding a stent, such as an MSI radial strength tester. For example, the percent recoil may be about 1% to about 5% (e.g., about 2% to about 3%) and the radial strength may be about 845 mm Hg to about 1050 mm Hg (e.g., about 880 mm Hg to about 1000 mm Hg) when the stent 100 is expanded to an outer diameter of at least about 3 mm (e.g., about 3 mm to about 7 mm). More particularly, the percent recoil may be about 2.5% to about 3.2% and the radial strength may be about 950 mm Hg to about 1000 mm Hg when the stent 100 is expanded to an outer diameter of at least about 3 mm (e.g., about 3 mm to about 7 mm).
The disclosed heat-treated tantalum alloys are sufficiently radiopaque and stronger (e.g., greater yield strength) than substantially pure tantalum (e.g., commercially pure tantalum). Consequently, the struts 104 of the stent body 102 may be thinner in a radial direction than a stent made from substantially pure tantalum and having a similar configuration, while still providing adequate imaging characteristics under X-ray fluoroscopy and MRI. Commercially pure tantalum exhibits a relatively greater radiopacity. However, since commercially pure tantalum is much weaker than the tantalum alloys disclosed herein, a stent made from commercially pure tantalum typically could be excessively thick for structural reasons thereby resulting in the stent being excessively radiopaque and making it difficult to distinguish surrounding body tissue during imaging.
Referring still to
In an embodiment, for a thickness of about 60.96 μm (0.0024 inch), any of the tantalum alloy embodiments disclosed herein may have a radiopacity about equal to a radiopacity of substantially pure tantalum having a thickness of about 55.88 μm (0.0022 inch). In other embodiments, for a thickness about equal to or less than about 60.96 μm (0.0024 inch), any of the tantalum alloy embodiments disclosed herein may have a radiopacity of about 101% or less, about 100% or less, about 98% or less, about 95% or less, 93% or less, about 90% or less, or about 85% or less than the radiopacity of substantially pure tantalum having a thickness of 55.88 μm (0.0022 inch) and measured using cine equipment with an x-ray energy value of about 80 kVp to about 120 kVp. Radiopacity may be calculated by the equation Radiopacity=eμ
In one or more embodiments, the stent body 102 may be etched in an acid (e.g., hydrofluoric acid) to remove features (e.g., slag, remelt, heat-affected zones, etc) associated with forming the struts 104 via laser cutting and/or electropolished to improve a surface finish of the stent body 102. In such embodiments, the stent body 102 may be heat treated (e.g., a stress-relief heat treatment and/or recrystallization heat treatment) so that at least one of hydrogen, oxygen, or nitrogen introduced to the tantalum alloy from the acid and/or the electropolishing solution is at least partially removed. Following heat treatment, the stent body 102 may include one or more etched and/or one or more electropolished surfaces, and the tantalum alloy that forms the stent body 102 may substantially free of at least one of hydrogen, oxygen, or nitrogen or include at least one of hydrogen, oxygen, or nitrogen in an amount below a threshold concentration sufficient to cause environmental cracking in the tantalum alloy, such as hydrogen that causes hydrogen embrittlement. For example, oxygen may be present in the tantalum alloy in a concentration of about 400 ppm or less (e.g., about 100 ppm to about 300 ppm) without causing embrittlement.
As in the previous example, the stent 150 is made from a tantalum alloy that is composed and heat-treated to obtain one or more of certain desirable microstructural, mechanical, or chemical properties. For example, the tantalum alloy may be heat treated to modify at least one mechanical property thereof, such as ductility, yield strength, or ultimate tensile strength. The 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 one embodiment, the tantalum alloy used to form the stent 150 may be heat treated as described above with reference to stent 100. As a result, the tantalum alloy used to form stent 150 should have substantially the same tensile and elongation properties as the alloy described with reference to stent 100.
Other mechanical properties of the stent 150 suitable for characterizing the combination of strength and ductility exhibited by the tantalum alloy include, but are not limited to, percent recoil and radial strength of the stent 150. Such mechanical properties may be determined by crimping the stent 150 on a mandrel, expanding the crimped stent 150 to a specific outer diameter with a balloon or a similar device, and inflating the expanded and crimped stent body 102 to a specific pressure. ASTM F2079 provides one suitable standard for determining percent recoil of the stent 150. Radial strength may be determined using a commercially available machine for radially expanding a stent, such as an MSI radial strength tester. For example, the percent recoil for this stent design may be about 1% to about 5% (e.g., about 2% to about 4%) and the radial strength may be about 1000 mm Hg to about 880 mm Hg (e.g., about 950 mm Hg to about 880 mm Hg) when the stent 150 is expanded to an outer diameter of about 3 mm (e.g., about 3 mm to about 7 mm). More particularly, the percent recoil may be about 2% to about 3% and the radial strength may be about 950 mm Hg to about 880 mm Hg when the stent 150 is expanded to an outer diameter of about 3 mm.
In one or more embodiments, the stent 150 may be etched in an acid (e.g., hydrofluoric acid) to remove features associated with forming the struts 152 and connectors 154 via laser cutting and/or electropolished to improve a surface finish of the stent 150. In such embodiments, the stent 150 may be heat treated (e.g., a stress-relief heat treatment and/or recrystallization heat treatment) so that at least one of hydrogen, oxygen, or nitrogen introduced to the tantalum alloy from the acid and/or the electropolishing solution is at least partially removed. Following heat treatment, the stent 150 may include one or more etched and/or one or more electropolished surfaces, and the tantalum alloy that forms the stent 150 may substantially free of at least one of hydrogen, oxygen, or nitrogen or include at least one of hydrogen, oxygen, or nitrogen in an amount below a threshold concentration sufficient to cause environmental cracking in the tantalum alloy, such as hydrogen that causes hydrogen embrittlement. For example, oxygen may be present in the tantalum alloy in a concentration of about 400 ppm or less (e.g., about 100 ppm to about 300 ppm) without causing embrittlement.
Other implantable medical devices besides stents may employ a tantalum alloy exhibiting one or more of the disclosed tailored properties, such as guide wires, closure elements, pacemaker leads, orthopedic devices, embolic coils, sutures, prosthetic heart valves, mitral valve repair coils, or other medical devices or portions thereof for deploying the foregoing medical devices. Such implantable medical devices may also be coated with any of the drug-eluting coatings disclosed herein and discussed in more detail below. For example,
Referring again to
In an embodiment, the drug-eluting coating 106 has a thickness of about 2 μm to about 75 μm, about 2 μm to about 50 μm, or about 10 μm to about 50 μm. Referring to
In an embodiment, the drug may be present in the drug-eluting coating 106 in an amount from about 10 μg/cm2 (micrograms drug/area of stent) to about 2000 μg/cm2, in an amount from about 100 μg/cm2 to about 1000 μg/cm2, in an amount from about 200 μg/cm2 to about 500 μg/cm2, in an amount greater or equal to about 150 μg/cm2, in an amount greater or equal to about 175 μg/cm2, in an amount greater or equal to about 200 μg/cm2, or in an amount greater or equal to about 225 μg/cm2.
In an embodiment, the amount of drug of the drug-eluting coating 106 may be described as the total amount of drug per stent 100. For example, the amount of drug per stent 100 may be about 0.5 mg to about 12 mg, about 0.75 mg to about 10 mg, or about 1 mg to about 5 mg.
In an embodiment, the drug that is eluted from the drug-eluting coating 106 produces a systemic blood concentration of the drug that produces at least one of the following: a maximum kidney concentration of less than or about 50 ng/g, less than or about 40 ng/g, or less than or about 30 ng/g; a maximum lung concentration of less than or about 45 ng/g, less than or about 35 ng/g, or less than or about 25 ng/g; a maximum muscle concentration of less than or about 35 ng/g, less than or about 30 ng/g, or less than or about 25 ng/g; a maximum liver concentration of less than or about 30 ng/g, less than or about 25 ng/g, or less than or about 17 ng/g; or a maximum spleen concentration of less than or about 35 ng/g, less than or about 30 ng/g, or less than or about 25 ng/g.
As discussed above, the pharmaceutically acceptable carrier may include a polymeric material. Examples of such polymeric materials include a suitable hydrogel, hydrophilic polymer, hydrophobic polymer biodegradable polymers, bioabsorbable polymers, monomers thereof, and combinations thereof. More specific examples of such polymers may include nylons, poly(alpha-hydroxy esters), polylactic acids, polylactides, poly-L-lactide, poly-DL-lactide, poly-L-lactide-co-DL-lactide, polyglycolic acids, polyglycolide, polylactic-co-glycolic acids, polyglycolide-co-lactide, polyglycolide-co-DL-lactide, polyglycolide-co-L-lactide, polyanhydrides, polyanhydride-co-imides, polyesters, polyorthoesters, polycaprolactones, polyesters, polyanydrides, polyphosphazenes, polyester amides, polyester urethanes, polycarbonates, polytrimethylene carbonates, polyglycolide-co-trimethylene carbonates, poly(PBA-carbonates), polyfumarates, polypropylene fumarate, poly(p-dioxanone), polyhydroxyalkanoates, polyamino acids, poly-L-tyrosines, poly(beta-hydroxybutyrate), polyhydroxybutyrate-hydroxyvaleric acids, polyethylenes, polypropylenes, polyaliphatics, polyvinylalcohols, polyvinylacetates, hydrophobic/hydrophilic copolymers, alkylvinylalcohol copolymers, ethylenevinylalcohol copolymers (“EVAL”), propylenevinylalcohol copolymers, polyvinylpyrrolidone (“PVP”), polycarboxylic acids, cellulosic polymers, gelatin, polyvinylpyrrolidone, maleic anhydride polymers, polyamides, polyvinyl alcohols, polyethylene oxides, glycosaminoglycans, polysaccharides, polyesters, polyurethanes, silicones, polyorthoesters, polyanhydrides, polycarbonates, polypropylenes, polylactic acids, polyglycolic acids, polycaprolactones, polyhydroxybutyrates, polyacrylamides, polyethers, mixtures thereof, derivatives thereof, copolymers thereof, polymers having monomers thereof, or combinations of any of the foregoing.
Some specific biodegradable polymers for use as the pharmaceutically acceptable carrier in the drug-eluting coating 106 may include poly(L-lactic acids), poly(DL-lactic acids), polycaprolactones, polyhydroxybutyrates, polyglycolides, poly(diaxanones), poly(hydroxy valerates), polyorthoesters, poly(lactide-co-glycolides), polyhydroxy(butyrate-co-valerates), polyglycolide-co-trimethylene carbonates, polyanhydrides, polyphosphoesters, polyphosphoester-urethanes, polyamino acids, polycyanoacrylates, biomolecules, fibrin, fibrinogen, cellulose, starch, collagen, hyaluronic acid, mixtures thereof, derivatives thereof, copolymers thereof, and combinations of any of the foregoing.
Some specific biostable polymers for use as the pharmaceutically acceptable carrier in the drug-eluting coating 106 may include polyurethanes, silicones, polyesters, polyolefins, polyamides, polycaprolactams, polyimides, polyvinyl chlorides, polyvinyl methyl ethers, polyvinyl alcohols, acrylic polymers, polyacrylonitriles, polystyrenes, vinyl polymers, polymers including olefins (e.g., styrene acrylonitrile copolymers, ethylene methyl methacrylate copolymers, ethylene vinyl acetate, and other like polymers), polyethers, rayons, cellulosics (e.g., cellulose acetate, cellulose nitrate, cellulose propionate, and other like polymers), parylene, mixtures thereof, derivatives thereof, copolymers thereof, and combinations of any of the foregoing.
The drug of the drug-eluting coating 106 may be selected to improve the use of the stent 100, such as inhibiting restenosis. Such drugs may include antithrombotics, anticoagulants, antiplatelet agents, thrombolytics, antiproliferatives, anti-inflammatories, agents that inhibit hyperplasia, inhibitors of smooth muscle proliferation, antibiotics, growth factor inhibitors, cell adhesion inhibitors, or combinations of the foregoing, as well as antineoplastics, antimitotics, antifibrins, antioxidants, agents that promote endothelial cell recovery, antiallergic substances, radiopaque agents, viral vectors having beneficial genes, genes, siRNA, antisense compounds, oligionucleotides, cell permeation enhancers, or combinations thereof.
As an alternative to or in addition to the aforementioned drugs, the drug-eluting coating 106 may include one or more of the following drugs: anti-proliferative/antimitotic agents including natural products such as vinca alkaloids (e.g., vinblastine, vincristine, vinorelbine, or combinations thereof), paclitaxel, epidipodophyllotoxins (e.g., etoposide, teniposide, or combinations thereof), antibiotics (e.g., dactinomycin (actinomycin D), daunorubicin, doxorubicin, idarubicin, or combinations thereof), anthracyclines, mitoxantrone, bleomycins, plicamycin (mithramycin) and mitomycin, enzymes (e.g., L-asparaginase that systemically metabolizes L-asparagine and deprives cells that do not have the capacity to synthesize their own asparagine); antiplatelet agents such as G(GP) IIb/IIIa inhibitors and vitronectin receptor antagonists; anti-proliferative/antimitotic alkylating agents such as nitrogen mustards (e.g., mechlorethamine, cyclophosphamide, melphalan, chlorambucil, analogs thereof, or combinations thereof), ethylenimines and methylmelamines (e.g., hexamethylmelamine, thiotepa, or combinations thereof), alkyl sulfonates-busulfan, nirtosoureas (e.g., carmustine (“BCNU”), streptozocin, analogs thereof, or combinations thereof), trazenes-dacarbazinine (“DTIC”); anti-proliferative/antimitotic antimetabolites such as folic acid analogs (methotrexate), pyrimidine analogs (e.g., fluorouracil, floxuridine, cytarabine, or combinations thereof), purine analogs and related inhibitors (e.g., mercaptopurine, thioguanine, pentostatin, 2-chlorodeoxyadenosine (cladribine), or combinations thereof); platinum coordination complexes (e.g., cisplatin, carboplatin, or combinations thereof), procarbazine, hydroxyurea, mitotane, aminoglutethimide; hormones (e.g., estrogen); anti-coagulants (e.g., heparin, synthetic heparin salts, other inhibitors of thrombin, or combinations thereof); fibrinolytic agents (e.g., tissue plasminogen activator, streptokinase, urokinase, or combinations thereof), aspirin, dipyridamole, ticlopidine, clopidogrel, abciximab; antimigratory; antisecretory (breveldin); anti-inflammatory agents such as adrenocortical steroids (e.g., cortisol, cortisone, fludrocortisone, prednisone, prednisolone, 6α-methylprednisolone, triamcinolone, betamethasone, dexamethasone, or combinations thereof), non-steroidal agents (e.g., salicylic acid derivatives i.e. aspirin; para-aminophenol derivatives i.e. acetaminophen; indole and indene acetic acids (indomethacin, sulindac, and etodalac), heteroaryl acetic acids (e.g., tolmetin, diclofenac, ketorolac, or combinations thereof), arylpropionic acids (e.g., ibuprofen and/or derivatives thereof), anthranilic acids (e.g., mefenamic acid, meclofenamic acid, or combinations thereof), enolic acids (e.g., piroxicam, tenoxicam, phenylbutazone, oxyphenthatrazone, or combinations thereof), nabumetone, gold compounds (e.g., auranofin, aurothioglucose, gold sodium thiomalate, or combinations thereof); immunosuppressives: (e.g., cyclosporine, tacrolimus (FK-506), sirolimus (rapamycin), analogs of rapamycin, everolimus, analogs of everolimus, azathioprine, mycophenolate mofetil, or combinations thereof); angiogenic agents; vascular endothelial growth factor (“VEGF”); fibroblast growth factor (“FGF”); angiotensin receptor blockers; nitric oxide donors; antisense oligionucleotides; cell cycle inhibitors, mTOR inhibitors, and growth factor receptor signal transduction kinase inhibitors; retenoids; cyclin/CDK inhibitors; HMG co-enzyme reductase inhibitors (statins); protease inhibitors, and any combination of the foregoing. In addition, it should be recognized that many active agents have multiple pharmaceutical uses other than those specifically recited.
In some embodiments, the drug of the drug-eluting coating 106 may include a lipophilic drug, a hydrophilic drug, an amphipathic drug, hydrophobic drug, or combinations of the foregoing drugs. For example, the drug-eluting coating 106 may be formulated for preferential drug delivery into the tissue defining the lumen in which the stent 100 is disposed. For example, the preferential drug delivery may be facilitated by a hydrophobic component being included in the drug-eluting coating 106 that is in contact with the tissue. Hydrophobic components of the tissue may cooperate with the hydrophobic component of the drug-eluting coating 106 to facilitate preferential diffusion of a hydrophilic drug into the tissue over into systemic blood. Similarly, the drug may be hydrophilic or amphipathic by having both lipophilic and hydrophilic portions. The drug-eluting coating 106 may include a hydrophilic component for the hydrophilic drug, and hydrophilic and/or hydrophobic components for the lipophilic, hydrophilic, or amphipathic drugs.
In an embodiment, the drug may be a lipophilic drug mixed with the polymeric pharmaceutically acceptable carrier and the lipophilic drug/polymeric material combination cooperates to form a lipophilic diffusion pathway with tissue defining a body lumen when the stent 100 is disposed in the body lumen so that the lipophilic drug preferentially diffuses into the tissue over a body fluid passing through the body lumen. In such an embodiment, a maximum systemic blood concentration of the drug may be less than or about 30 ng/ml, less than or about 20 ng/ml, or less than or about 10 ng/ml after the stent 100 is deployed in the body lumen.
In an embodiment, the drug-eluting coating 106 includes at least one polymeric coating (e.g., a polymer coating including EVAL copolymer), which helps controls the release of a drug contained therein. The drug contained within the polymer coating may be an anti-restinoic drug, such as rapamycin, everolimus, analogs thereof, prodrugs thereof, combinations thereof, or other suitable anti-restinoic drug. For example, the thickness of an EVAL polymeric coating may be about 2 μm to about 50 μm, about 4 μm to about 25 μm, about 5 μm to about 20 μm, or about 13 μm to about 15 μm. The polymer coating that contains the drug may also be coated by another layer of the same or different polymer that further controls the drug release profile from the stent. The EVAL polymeric coating may be capable of releasing everolimus relatively slowly to thereby eluting approximately 80% of its drug load over the first days 90 days.
In a more specific embodiment, the drug-eluting coating 106 may be loaded with a relatively high overall drug content (e.g., 225 μg everolimus/cm2 stent area). Everolimus (40-O-(2-hydroxyethyl)-rapamycin available from Novartis Pharmaceuticals Corporation of Basel, Switzerland) is a macrolide immunosuppressant analog of rapamycin (i.e., sirolimus) that, in conjunction with cyclosporine, has been shown to be effective in inhibiting chronic rejection episodes of solid organ transplants. Its oral formulation is marketed outside the United States under the trade name Certican®. Everolimus effectively inhibits neointimal hyperplasia in animal models and, when provided on coronary stents at a dose of, for example, about 150 μg everolimus/cm2 stent area, it reduces restenosis as compared to bare metal or paclitaxel-eluting stents.
Referring again to
The drug-loaded coating 120 and the topcoat layer 122 may control elution of the drug from the stent 100. This may include facilitating elution into the tissue adjacent to the stent 100 and inhibiting elution into the bloodstream. The controlled elution may be accomplished by the coatings and artery tissues establishing a diffusion pathway having a steep concentration gradient with respect to the drug to induce the drug to diffuse through the diffusion pathway. The steep concentration gradient may be accomplished by the drug-eluting coating 106 having a high concentration of drug and the tissue having a low concentration of drug, which thereby promotes diffusion through the diffusion pathway.
Additionally, the diffusion pathway into the vascular tissue can be enhanced by the stent 100 being placed in a blood vessel that passes blood. Blood, while containing some lipid-based components, is significantly more aqueous that than lipidic because the blood includes a significant amount of water. As such, in an embodiment, the drug may include a lipophilic drug that preferentially diffuses through a lipophilic diffusion pathway over an aqueous pathway. The lipophilic drug preferentially diffusing through the lipophilic diffusion pathway into the tissue adjacent to the stent 100 over diffusion into the blood attributes to the vascular tissue adjacent to the stent obtaining a therapeutic concentration of drug and the system concentration being significantly below a therapeutic concentration and toxic concentration. Accordingly, systemic effects of the drug may be inhibited by maintaining an extremely low systemic drug concentration to thereby inhibit the adverse effects of prolonged systemic drug. This may also be accomplished with hydrophilic and/or amphophilic drugs and polymer components because tissues inherently have water as a major component.
In an embodiment, the coating/drug combination is configured to provide an extended elution profile that may elute substantially constant levels of drug over 3 months, over 6 months, or over 9 months. The slow elution kinetics may be attributed to the significantly inhibited systemic elution of the drug and helps to maintain the systemic of the drug below any therapeutic and/or toxic index. Additionally, the slow elution kinetics attributes to the drug preferentially diffusing through the lipophilic diffusion pathway because slow elution kinetics further drive the lipophilic drug through a lipophilic diffusion pathway over diffusing into the blood. In addition, the slow elution kinetics may enable the tissue to retain sink-like properties with respect to the drug to provide a continuously steep concentration gradient through the lipophilic diffusion pathway.
It is currently believed that the coating/drug combination that provides preferential diffusion of the drug through the lipophilic diffusion pathway over diffusion into the systemic blood supply cooperates with natural physiological processes in order to further differentiate the amount of drug in the vascular tissue adjacent to the sent 100 compared to systemic drug. The difference in drug diffusion pathways that result in extremely low systemic concentrations is supplemented by the physiological functions of drug metabolism. Drug metabolism occurs mainly in organs that are removed from the vascular tissue, and preferentially not in the vascular tissue. This physiological process naturally further reduces the systemic concentration of drug without reducing the concentration of drug in the vascular tissue.
In an embodiment, the drug-eluting coating 106 may comprise a porous metallic coating including a pharmaceutically acceptable carrier and a drug incorporated therein. The combination of the porosity of the metallic coating and the pharmaceutically acceptable carrier and the drug controls the elution rate of the drug from the drug-eluting coating 106. The pharmaceutically acceptable carrier and the drug are disposed in at least a portion of the pores defined by the porous metallic coating. For example, the porous metallic coating may formed on the stent body 102 by chemical vapor deposition, physical vapor deposition, another suitable deposition technique, sintering metallic particles on the stent body 102, or other suitable technique. The porous metallic coating may be made from a suitably biocompatible metal or alloy, such as tantalum, tungsten, niobium, or alloys thereof. The drug and pharmaceutically acceptable carrier may be selected from any of the polymeric materials and drugs disclosed herein.
Implantable medical devices disclosed herein, such as the stent 100 shown in
The delivery catheter 200 may be a conventional balloon dilatation catheter commonly used for angioplasty procedures. The balloon 202 may be formed of, for example, polyethylene, polyethylene terephthalate, polyvinylchloride, nylon, or another suitable polymeric material. To facilitate the stent 100 remaining in place on the balloon 202 during delivery to the site of the damage within the body lumen 204, the stent 100 may be compressed onto the balloon 202. Other techniques for securing the stent 100 onto the balloon 202 may also be used, such as providing collars or ridges on edges of a working portion (i.e., a cylindrical portion) of the balloon 202.
In use, the stent 100 may be mounted onto the inflatable balloon 202 on the distal extremity of the delivery catheter 200. The balloon 202 may be slightly inflated to secure the stent 100 onto an exterior of the balloon 202. The catheter/stent assembly may be introduced within a living subject using a conventional Seldinger technique through a guiding catheter 206. A guide wire 208 may be disposed across the damaged arterial section with the detached or dissected lining 207 and then the catheter/stent assembly may be advanced over the guide wire 208 within the body lumen 204 until the stent 100 is directly under the detached lining 207. For example, the guide wire 208 may be made from a superelastic nickel-titanium alloy, any of the heat-treated tantalum alloys disclosed herein, or another suitable material. The balloon 202 of the catheter 200 may be expanded, expanding the stent 100 against the interior surface defining the body lumen 204 by, for example, permanent plastic deformation of the stent 100. When deployed, the stent 100 holds open the body lumen 204 after the catheter 200 and the balloon 202 are withdrawn. A drug is controllably eluted from the drug-eluting coating of the stent 100 into the tissue defining the body lumen 204 while it is deployed.
Referring again to
The drawn tantalum-alloy tube or the stent body 102 may be heat treated at a temperature and for a time sufficient to at least partially recrystallize the grain microstructure of the tantalum alloy to impart the above-described mechanical properties to the tantalum alloy. In some embodiments, the drawn tantalum-alloy tube or the stent body 102 of the stent 150 may be etched in a chemical etching solution (e.g., a solution containing HF and HNO3) prior to heat treating in order to remove an oxide layer present on the drawn tantalum-alloy tube or the stent body 102. The heat treatment may be performed in a vacuum furnace at a vacuum level of about 1×10−4 torr to 1×10−6 torr to help prevent impurities from dissolving in and/or reacting with the tantalum alloy. In an embodiment, the temperature and the time may be selected so that the grain microstructure is only partially recrystallized. In another embodiment, the temperature and the time may be selected so that the grain microstructure is substantially completely recrystallized. In such an embodiment, the recrystallization process may be terminated before proceeding past the stage of complete recrystallization. In yet another embodiment, the temperature and the time may be selected so that the grain microstructure includes grains having experienced grain growth characteristic of the heat-treatment process proceeding past the stage of complete recrystallization.
In an embodiment, the heat-treatment temperature is selected to be above the recrystallization temperature of the tantalum alloy. In various embodiments, the heat-treatment temperature may be about 1000° C. to about 1350° C., 1200° C. to about 1350° C., about 1250° C. to about 1300° C., or about 1275° C. While it is difficult to precisely determine the recrystallization temperature of the tantalum alloys disclosed herein, it is currently believed that the recrystallization temperature is about 1275° C., but it may be lower in tantalum alloys having relatively lower amounts of tungsten. The heat-treatment time may vary depending upon the extent of recrystallization and grain size that is desired in the tantalum alloy of the drawn tantalum-alloy tube. The heat-treatment time may be about 2 min to about 100 min, about 5 min to about 50 min, about 5 min to about 40 min, about 5 min to about 15, about 5 min to about 10 min, or about 6 min to about 8 min for any of the disclosed heat-treatment temperatures. In a specific embodiment, the heat treatment time may be about 40 min at 1275° C. The heat-treatment time may be the time at which the drawn tantalum-alloy tube is at a selected heat-treatment temperature and does not include the heat-up time necessary for the drawn tantalum-alloy tube to reach the heat-treatment temperature. The recrystallization time may decrease as the tungsten content in the disclosed tantalum alloys decreases.
Heat treating a tantalum alloy having about 80 wt % to about 83 wt % tantalum (e.g., about 82.5%), about 8 wt % to about 12 wt % niobium (e.g., about 10 wt %), and about 6 wt % to about 9 wt % tungsten (e.g., about 7.5 wt %) at about 1250° C. to about 1300° C. (e.g., about 1275° C.) for about 5 min to about 180 min (e.g., about 20 min to about 180 min or about 20 min to about 80 min) may provide for a combination of tensile strength properties and ductility that is suitable for the stent 100. For example, such tantalum alloys subjected to one or more of the aforementioned heat-treatment processes may exhibit a tensile elongation of about 20% to about 50% (e.g., about 23% to about 27%), a tensile yield strength of about 440 MPa to about 500 MPa (e.g., about 460 MPa to about 480 MPa), and an ultimate tensile strength of about 490 MPa to about 540 MPa (e.g., about 500 MPa to about 515 MPa).
Before or after heat treatment, the drawn tantalum-alloy tube may be cut using, for example, a laser-cutting process, electro-discharge machining, or another suitable cutting process to form the stent body 102 shown in
In an embodiment, the drawn tantalum-alloy tube may be electropolished to polish exterior and interior surfaces thereof prior to the stent body 102 being formed therefrom. In another embodiment, after being formed, the stent body 102 may be electropolished and/or chemically etched in an acid (e.g., hydrofluoric acid) to remove features (e.g., heat-affected zones, slag, remelt, and the like) associated with defining the struts 104 of the stent body 102 via laser cutting. In such embodiments, the electropolished tantalum-alloy tube or the electropolished and/or etched stent body 102 may be heat treated at a temperature below a recrystallization temperature of the tantalum alloy to remove at least a portion of at least one of hydrogen, nitrogen, or oxygen dissolved in the tantalum alloy from the electropolishing solution used in the electropolishing process and/or the acid. Such a heat treatment is referred to as a stress-relief heat treatment.
For example, the stress relief heat treatment may be performed at a temperature of about 700° C. to about 1100° C., more particularly about 700° C. to about 1000° C., and even more particularly about 1000° C. After heat treatment, the tantalum alloy may be substantially free of at least one of hydrogen, nitrogen, or oxygen or may include at least one of hydrogen, oxygen, or nitrogen present below a threshold concentration sufficient to cause environmental cracking in the tantalum alloy, such as hydrogen that causes hydrogen embrittlement. Removal of at least one of hydrogen, nitrogen, or oxygen by a stress-relief heat treatment may substantially improve the ductility of the tantalum alloy, while reducing the yield strength and ultimate tensile strength compared to the as-drawn condition.
For example, in the stress-relieved condition, the tantalum alloy may exhibit a percent elongation of about 5% to about 15% (e.g., about 9% to about 11%), a yield strength of about 580 MPa to about 840 MPa (e.g., about 680 MPa to about 810 MPa), and an ultimate tensile strength of about 600 MPa to about 880 MPa (e.g., about 715 MPa to about 850 MPa). In the stress-relieved condition, the percent elongation may increase by about 200% to about 1200% or about 300% to about 1200%. In another embodiment, the percent elongation may increase by at least about 100%, at least about 200%, at least about 300%, at least about 400%, or about 200% to about 400% compared to the same tantalum alloy in the as-drawn (i.e., un-stress-relieved) condition, while the yield strength and ultimate tensile strength are reduced. It is noted that heat treating at a temperature sufficient to at least partially relieve the stress of the tantalum alloy may also at least partially remove at least one of hydrogen, nitrogen, or oxygen.
It is also noted that stress relief heat treatment may be performed after recrystallization heat treatment to relieve cold work and other stresses imparted on the material during stent fabrication and to remove embrittling gasses such as hydrogen, oxygen, and nitrogen that may become dissolved in the material during one or more manufacturing processes. Such a material that has been recrystallization heat treated and then at a later stage stress relief heat treated will have typically have elongation and tensile properties similar to metal that has been subjected to recrystallization heat treatment alone.
Electropolishing of the stent body 102 may be performed by immersing the stent body 102 in a temperature-controlled bath of electrolyte, and connecting a positive terminal (anode) of a direct current (“DC”) power supply to the stent body 102 and a negative terminal of the DC power supply to an auxiliary electrode (cathode). A current passes from the anode to the cathode through the electrolyte solution. At the anode, metal on the surface of the stent body 102 is oxidized and dissolved in the electrolyte. At the cathode a reduction reaction takes place, which normally evolves hydrogen. Electrolytes used for electropolishing are most often concentrated acid solutions. To achieve electropolishing of a rough metal surface, the protruding portions of a surface profile dissolve faster than the recesses. This behavior, which is referred to as anodic leveling, may be achieved by applying a specific electrochemical condition (e.g., voltage, current, and/or acid concentration/acid makeup). In addition to smoothing the surface of the stent body 102, electropolishing may be used to adjust the dimension “t” s of the struts 104 to a desired size (e.g., about 58 μm to about 70 μm). In an alternative embodiment, the surface of the stent body 102 may be smoothed and the dimensions of the struts adjusted to the desired size using abrasive techniques such as bead blasting and the like.
In an embodiment, the electrolyte solution employed in the electropolishing may be an inaqueous acidic solution. For example, the electrolyte solution may contain methanol (or another alcohol), sulfuric acid (“H2SO4”), methanolic hydrochloric acid (methanol HCl) and, optionally, a desiccating agent such as polyethylene glycol (“PEG”) and/or ethylene glycol. In another example, the electrolyte solution may contain methanol, H2SO4, and ethylene glycol. In a specific embodiment, the H2SO4 concentration in the electrolyte solution is about 1.5 molar (“M”) to about 3 M (e.g., about 1.9 M), and the ethylene glycol concentration is about 0.8 M to about 1.1 M (e.g., about 0.9 M).
The stent body 102 may be electropolished in the electrolyte solution (i.e., methanol, H2SO4, and ethylene glycol) using a threshold current of up to about 4 amps. Preferably, the current is about 1 amp to about 3 amps, about 1.2 amps to about 2 amps, about 1.3 to about 1.6 amps, or about 1.5 amps.
The electrical current directed through the electrolyte solution is above the threshold current in order to achieve a smoothing or polishing effect on the surface of the stent body 102 as opposed to an roughening or etching effect. At lower current (e.g., about 1.5 amps) better surface finish is obtained and less damage to the stents is observed. As the electropolishing process proceeds, H2SO4 is consumed producing H2 gas and metal sulfates. Eventually, as the H2SO4 is consumed, the current will drop below the threshold value. When the current drops below the threshold value, the solution needs should discarded. 800 ml of electrolyte solution is, for example, sufficient for electropolishing about 80 tantalum-alloy stents.
While the electrolyte solution is essentially water-free in the as-prepared condition, the solution is hygroscopic and can scavenge water out of the environment. In the case of the tantalum alloys discussed herein, the electrolyte solution is formulated to be essentially water-free because water reacts the tantalum and forms an oxide passivation layer on the surface of the tantalum alloy that can interfere with the electropolishing process. In one embodiment, a desiccating agent may be added to the electropolishing electrolyte solution to mitigate the effect of water that may be introduced into the electrolyte from the atmosphere or through the chemical action of the electropolishing process. PEG, ethylene glycol, and similar desiccating agents are capable of forming multiple hydrogen bonding interactions, which may surround and effectively sequester water that may otherwise interfere with the electropolishing process.
At least one of hydrogen, nitrogen, or oxygen may also be introduced to the tantalum alloy during the drawing process used to form the precursor drawn tantalum-alloy tube. As an alternative or in addition to heat treating after electropolishing and/or chemical etching, in another embodiment, the precursor drawn tantalum-alloy tube and/or the stent body 102 may be heat treated to at least partially remove at least a portion of hydrogen, nitrogen, and/or oxygen dissolved in the tantalum alloy that was introduced during the drawing process used to form the precursor drawn tantalum-alloy tube.
After forming and heat treating the stent body 102 to modify at least one mechanical property thereof, and etching and/or electropolishing processing, the drug-eluting coating 106 may be applied to the exterior 108 of the stent body 102. For example, a selected formulation of one or more of the pharmaceutically acceptable carriers disclosed herein may be mixed with one or more of the drugs disclosed herein. The mixture may be applied to the stent body 102 to form the drug-eluting coating 106 by spraying the stent body 102 with the mixture, dipping the stent body 102 in the mixture, or another suitable coating technique.
The heating element 308 may be positionable about the furnace tube 304. For example, the heating element 308 may be supported by rollers 309 to enable movement thereof back and forth on the support 302 and over the furnace tube 304 along a longitudinal axis of the furnace tube 304. For example,
In the illustrated embodiment, the heating element 308 is substantially cylindrical and may partially enclose the furnace tube 304. However, other configurations may be employed that depart from the illustrated cylindrical configuration. The heating element 308 may extend circumferentially about the furnace tube 304 and apply uniform heating thereto. Because the heating element 308 is positionable in the heating position and the retracted position, the workpiece (shown supported on a tray 324) may be heated and rapidly cooled by retraction of the heating element 308. Such rapid cooling is difficult in a conventional vacuum-chamber furnace without purging the chamber with a cooling gas. However, even high-purity inert gases (e.g., argon) still include one or more of hydrogen, nitrogen, or oxygen impurities that may be present in an amount sufficient to embrittle the tantalum alloys disclosed herein, particularly at substantially elevated temperatures.
An interlock assembly 310 may be disposed at and proximate to the open end 306b (
The interlock body 312 may include four or more ports (not labeled) that are in communication with the inside of the furnace tube 304. One of the ports may have a vacuum line 320 coupled thereto that is operably coupled to a vacuum pump 322 configured to draw a partial vacuum inside the furnace tube 304. Another port may be coupled to a sensor 323 (e.g., a pressure sensor) configured to measure a vacuum level in the furnace tube 304. Drawing a vacuum inside of the furnace tube 304 allows the heat-treatment process to be conducted in an environment that is substantially free of at least one of oxygen, hydrogen, nitrogen, or other gases that can react and/or embrittle the tantalum alloys disclosed herein, particularly at substantially elevated temperatures.
In practice, the cap 318 may be removed, the workpiece may be placed on the tray 324, the tray 324 may be inserted inside the furnace tube 304 through the interlock assembly 310, and the cap 318 is re-attached to the interlock body 312 by screwing or otherwise securing cap 318 thereto. Once the furnace tube 304 is sealed, a vacuum may be drawn to a sufficient level (e.g., about 10−3 torr to about 10−7 ton, or less) using the vacuum pump 322 and the heating element 308 may be subsequently moved to the heating position over the furnace tube 304 and the workpiece supported by the tray 324. Once in the heating position, the heating element 308 may heat the workpiece to a selected heat-treatment temperature and for a selected heat-treatment time, as previously described. After heat treating for the selected temperature and time, the heating element 308 may be moved to the refracted position to allow the heat-treated workpiece to cool rapidly to a temperature at which safe removal of the workpiece may occur without introducing undesirable impurities to it, such as at or below 100° C. A small fan (not shown) may blow cool air or other gas on the furnace tube 304 to improve cooling efficiency, while preventing significant amounts of grain growth in the tantalum alloy upon cooling from the heat-treatment temperature.
In an embodiment, the heating element 308 may be pre-heated to a temperature of about 1100° C. to about 1300° C. The pre-heated heating element 308 may be moved over the furnace tube 304 once the vacuum level detected by the sensor 323 is sufficient. Pre-heating the heating element 308 enables the furnace tube 304 and the workpiece disposed therein to be rapidly heated to the heat-treatment temperature.
In the illustrated embodiment shown in
It is noted that the furnace system 300 is merely one of many suitable furnaces for heat treating the tantalum-alloy products disclosed herein. Other vacuum-tube furnaces may be employed.
The following working examples of the present disclosure provide further detail in connection with the various embodiments described above for tantalum-alloy products and methods of processing such tantalum-alloy products. The following working examples are for illustrative purposes only and are not meant to be limiting with regard to the scope of the specification or the appended claims.
The percent elongation to failure in the as-drawn condition was only about 3% or, in some extreme cases, about 1%. The low ductility of the as-drawn sample from the first set was attributed primarily to the high-degree of cold work in the tantalum alloy. The heat-treatment times in
As shown in
In practice, the tubing used to fabricate an implantable medical device (e.g., a stent or a closure device) may be drawn, etched in a chemical etching solution (e.g., a solution that includes HF and HNO3), and subjected to recrystallization heat treatment (e.g., at about 1250° C. to about 1275° C.) to improve ductility. The etched and heat treated tubes may then be laser cut to form the implantable medical device, etched to, for example, remove features resulting from the laser cutting process, and electropolished to produce a mirror like finish. Finally, the implantable medical devices may be subjected to stress relief heat treatment to remove any cold work and/or any gaseous impurities (e.g., H and/or N) introduced during electropolishing and other manufacturing processes.
Referring to
Referring to
Microstructural analysis in the transverse orientation showed that after heat treatment at 1275° C. for 10 min, the tantalum alloy was only partially recrystallized. After heat treatment at 1275° C. for 20 min, the tantalum alloy was still only partially recrystallized. Complete recrystallization appeared to occur after heat treating at 1275° C. for 40 min, and the average grain size was about 13 μm to about 16 μm in the transverse orientation. Increasing the heat-treatment time past 40 min lead to grain growth, with an average grain size of about 16.1 μm at 100 min and 19.1 μm at 120 min in the transverse orientation.
The first tantalum alloy composition was about 87.5 wt % tantalum, about 10 wt % niobium, and about 2.5 wt % tungsten and is referred to as TaNb10W2.5 in
The embodiments of 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. Therefore, the scope of the disclosure is indicated by the appended claims rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.
