Wires of nickel-titanium alloy and methods of forming the same

Abstract

To provide good wires or the like.SOLUTION: There is disclosed a wire of a nickel-titanium alloy having a permanent set of less than 5% when 11% strain is applied to the wire. The wire may be formed by applying a first heat treatment to the wire, the first heat treatment includes applying heat of a first temperature for a first period of time, applying a strain deformation to the wire to set a shape for the wire during the first heat treatment, and applying a second heat treatment to the wire. The second heat treatment includes applying heat of a second temperature different from the first temperature for a second period of time, and the second temperature is between 210°C and 290°C. The wire may have a modulus of at least 53 GPa when 200 MPa of stress is applied to the wire, and the wire is bonded to a secondary component.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of Provisional Application No. 62 / 907,490, filed September 27, 2019, which is incorporated by reference in its entirety for all purposes.

[0002] Field The present disclosure relates generally to metal alloys and methods for making same. [Background technology]

[0003] background There has been much interest in shape memory and superelastic alloys such as nickel titanium. This family of alloys is also known as Nitinol (Nickel Titanium Naval Weapons Laboratory). The key to harnessing the performance of Nitinol alloys is the phase transformation of their crystal structure, which transitions between the austenite and martensite phases. The austenite phase is commonly referred to as the high temperature phase, and the martensite phase is commonly referred to as the low temperature phase. The specific phase characteristics and the repeatable transformation from one phase to another are the mechanism for achieving Nitinol's unique superelastic and shape memory properties.

[0004] Nickel titanium alloys are frequently used in medical devices for diagnostics and therapy. For example, minimally invasive implant procedures for medical devices require devices that can be contracted to a smaller size for delivery to a target treatment site and then released and expanded to a functional configuration, taking advantage of the superelastic and / or shape memory properties of Nitinol. There is a need for Nitinol material property improvements and manufacturing methods to produce nickel titanium alloys with properties particularly suited for medical devices where a smaller contraction profile is preferred, while maintaining or improving the properties of the medical device at a larger expanded profile. Summary of the Invention

[0005] Abstract Various disclosed concepts relate to nickel-titanium alloy wire. According to one example ("Example 1"), the nickel-titanium alloy wire has a permanent set of less than 5% when an 11% strain is applied to the wire.

[0006] According to Example 1 plus another example ("Example 2"), when a strain of 11% is applied to the wire, the permanent set is less than 1%.

[0007] According to another example ("Example 3") in addition to Examples 1 or 2, the wire has an elastic modulus of at least 53 GPa when a stress of 200 MPa is applied to the wire.

[0008] According to Example 3 as well as another example ("Example 4"), the elastic modulus is at least 55 GPa.

[0009] According to another example ("Example 5") in addition to Example 3, the elastic modulus is 53 GPa to 64 GPa.

[0010] According to any one of Examples 3-5 plus another Example ("Example 6"), the elastic modulus decreases by at least 5 GPa when a stress of 400 MPa is applied to the wire.

[0011] According to Example 6 as well as another example ("Example 7"), the modulus of elasticity decreases by at least 10 GPa when a stress of 400 MPa is applied to the wire.

[0012] According to another example ("Example 8") in addition to Example 6, the elastic modulus decreases by a value between 5 GPa and 16 GPa when a stress of 400 MPa is applied to the wire compared to when a stress of 200 MPa is applied to the wire.

[0013] According to another example ("Example 9") in addition to any of the preceding examples, the wire further has a lower plateau stress (LPS) of at least 350 MPa.

[0014] According to another example ("Example 10") in addition to any of the preceding examples, the wire is processed into an implantable medical device.

[0015] According to another example ("Example 11") in addition to Example 10, the medical device is a stent graft, an embolic filter, a septal occluder, or a heart valve.

[0016] According to another example ("Example 12"), a wire is formed by a method including subjecting the wire to a first heat treatment, subjecting the wire to a strain deformation during the first heat treatment to fix the shape of the wire, and subjecting the wire to a second heat treatment. The first heat treatment includes applying heat at a first temperature for a first time. The second heat treatment includes applying heat at a second temperature, different from the first temperature, for a second time. The second temperature is between 210°C and 290°C.

[0017] According to another example ("Example 13") in addition to Example 12, the first temperature is between 400°C and 550°C.

[0018] According to another example ("Example 14") in addition to Examples 12 or 13, the first heat treatment includes one or more additional heat treatment processes operable to fix the shape of the wire.

[0019] According to any one of Examples 12-14 as well as another example ("Example 15"), the second period of time is between 5 minutes and 40 minutes.

[0020] According to any one of Examples 12-15 plus another example ("Example 16"), the first elasticity of the wire resulting from the first heat treatment is within 2% of the second elasticity of the wire resulting from the second heat treatment.

[0021] According to any one of Examples 12-16 plus another example ("Example 17"), the second hysteresis of the wire resulting from the second heat treatment is at least 40 MPa lower than the first hysteresis of the wire resulting from the first heat treatment.

[0022] According to any one of Examples 12-17 plus another example ("Example 18"), the second lower plateau stress (LPS) of the wire resulting from the second heat treatment is at least 30 MPa higher than the first LPS of the wire resulting from the first heat treatment.

[0023] According to any one of Examples 12-18 plus another Example ("Example 19"), a second permanent strain of the wire resulting from the second heat treatment is at least 85% lower than a first permanent strain of the wire resulting from the first heat treatment, and each of the first permanent strain and the second permanent strain is defined by a length of the wire after an 11% strain is applied to the wire.

[0024] According to any one of Examples 12-18 as well as another example ("Example 20"), the wire has a permanent set of less than 5% when an 11% strain is applied to the wire.

[0025] According to Example 20 plus another example ("Example 21"), when an 11% strain is applied to the wire, the permanent set is less than 1%.

[0026] According to another example ("Example 22") in addition to any one of Examples 12-21, after the first heat treatment, when the wire is stressed at 200 MPa, the wire has a first elastic modulus of less than 42 GPa, and after the second heat treatment, when the wire is stressed at 200 MPa, the wire has a second elastic modulus of at least 55 GPa.

[0027] According to another example ("Example 23") in addition to Example 22, the second elastic modulus is 55 GPa to 64 GPa.

[0028] According to any one of Examples 12-23 plus another example ("Example 24"), the wire has a lower plateau stress (LPS) of at least 350 MPa.

[0029] According to any one of Examples 12-24 plus another Example ("Example 25"), the wire is processed into an implantable medical device.

[0030] According to another example ("Example 26") in addition to Example 25, the medical device is a stent graft, an embolic filter, a septal occluder, or a heart valve.

[0031] According to another example ("Example 27"), a method of forming a nickel-titanium alloy wire includes straining a wire to fix a shape of the wire, subjecting the wire to a first heat treatment, and subjecting the wire to a second heat treatment. The first heat treatment includes applying heat at a first temperature for a first time, and the second heat treatment includes applying heat at a second temperature, different from the first temperature, for a second time. The second temperature is between 210°C and 290°C.

[0032] According to another example ("Example 28") in addition to Example 27, the resulting wire has an LPS of at least 350 MPa after said second heat treatment.

[0033] According to another example ("Example 29") in addition to Example 27 or 28, the second time period is 5 minutes to 40 minutes.

[0034] According to any one of Examples 27-29 plus another example ("Example 30"), the first elastic modulus of the wire resulting from the first heat treatment is within 2% of the second elastic modulus of the wire resulting from the second heat treatment.

[0035] According to any one of Examples 27-30 plus another example ("Example 31"), the first elasticity of the wire resulting from the first heat treatment is within 2% of the second elasticity of the wire resulting from the second heat treatment.

[0036] According to any one of Examples 27-31 plus another example ("Example 32"), the second hysteresis of the wire resulting from the second heat treatment is at least 40 MPa lower than the first hysteresis of the wire resulting from the first heat treatment.

[0037] According to any one of Examples 27-32 plus another example ("Example 33"), the second lower plateau stress (LPS) of the wire resulting from the second heat treatment is at least 30 MPa higher than the first LPS of the wire resulting from the first heat treatment.

[0038] According to any one of Examples 27-33 plus another Example ("Example 34"), a second permanent set of the wire resulting from the second heat treatment is at least 85% lower than a first permanent set of the wire resulting from the first heat treatment, and each of the first permanent set and the second permanent set is defined by a length of the wire after an 11% strain is applied to the wire.

[0039] According to any one of Examples 27-33 plus another Example ("Example 35"), a second permanent set of the wire resulting from the second heat treatment is less than 1%. Each of the first permanent set and the second permanent set is defined by the length of the wire after an 11% strain is applied to the wire.

[0040] According to any one of Examples 27-33 as well as another example ("Example 36"), the resulting wire has a permanent set of less than 5% when an 11% strain is applied to the wire.

[0041] According to Example 36 plus another example ("Example 37"), when an 11% strain is applied to the wire, the permanent set is less than 1%.

[0042] According to another example ("Example 38") in addition to any one of Examples 27-37, after the first heat treatment, when a stress of 200 MPa is applied to the wire, the wire has a first modulus of elasticity of less than 42 GPa, and after the second heat treatment, when a stress of 200 MPa is applied to the wire, the wire has a second modulus of elasticity of at least 55 GPa.

[0043] According to another example ("Example 39") in addition to Example 38, the second elastic modulus is 55 GPa to 64 GPa.

[0044] According to any one of Examples 27-39 as well as another example ("Example 40"), the resulting wire has a lower plateau stress (LPS) of at least 350 MPa.

[0045] According to any one of Examples 27-40 plus another Example ("Example 41"), the method further includes processing the wire into an implantable medical device after the second heat treatment is applied.

[0046] According to another example ("Example 42") in addition to Example 41, the medical device is a stent graft, an embolic filter, a septal occluder, or a heart valve.

[0047] The foregoing examples are merely examples and should not be read to limit or otherwise narrow the scope of any of the concepts otherwise provided by this disclosure. While multiple examples have been disclosed, still other embodiments will become apparent to those skilled in the art from the following detailed description, which shows and describes illustrative examples. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not as restrictive in nature. [Brief description of the drawings]

[0048] BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings are included to provide a further understanding of the disclosure, and are incorporated in and constitute a part of this specification, illustrate embodiments and, together with the description, serve to explain the principles of the disclosure.

[0049] [Figure 1] FIG. 1 illustrates a flow diagram of a method for imposing a heat treatment to form a wire according to an embodiment disclosed herein.

[0050] [Diagram 2] FIG. 2 shows an example stress-strain curve for a wire formed according to an embodiment of the method disclosed herein.

[0051] [Diagram 3] FIG. 3 shows a graph of applied strain versus permanent set for wires formed using different heat treatment methods according to embodiments disclosed herein.

[0052] [Figure 4] FIG. 4 shows a graph of applied strain versus permanent set for wires formed using different heat treatment methods according to embodiments disclosed herein.

[0053] [Diagram 5] FIG. 5 shows a graph comparing different strain vs. stress curves obtained from different heat treatment methods, as known in the art and according to embodiments disclosed herein.

[0054] [Figure 6] 6A-6D show examples of implantable medical devices that can be formed using wires as disclosed herein according to various embodiments.

[0055] Those skilled in the art will readily appreciate that the various aspects of the present disclosure may be implemented by any number of methods and devices configured to perform the intended functions. It should also be noted that the accompanying drawings referred to herein are not necessarily drawn to scale and may be exaggerated to illustrate various aspects of the present disclosure, and in that regard, the drawings should not be construed as limiting. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0056] Detailed Description Definitions and Terminology This disclosure is not intended to be read in a limiting manner, for example, the terms used in this application should be read broadly with respect to the meaning that terms of the art ascribe to such terms.

[0057] With respect to imprecision terms, the terms "about" and "approximately" may be used interchangeably to refer to measurements that include the stated measurement and that are reasonably close to the stated measurement. A measurement that is reasonably close to the stated measurement deviates from the stated measurement by a reasonably small amount, as would be understood and readily ascertained by one of ordinary skill in the relevant art. Such deviations may be due, for example, to measurement error or small adjustments made to optimize performance.

[0058] Description of Various Embodiments Disclosed herein is a process for a wire or wire structure made from a nickel-titanium alloy, where the wire is obtained by a cold working process and subsequently undergoes a first heat treatment during which a strain is applied to the wire to shape-set it, the wire further undergoing a second heat treatment after the first heat treatment, where the second heat treatment is performed at a temperature between 210°C and 290°C.

[0059] The disclosed treatment processes enable the wire to have one or more advantageous physical properties. For example, wire subjected to the second heat treatment process has one or more of increased lower plateau stress (LPS), reduced hysteresis, and / or reduced permanent set compared to wires formed using other known processes, as described in more detail herein below. Furthermore, in some instances, wires obtained through the disclosed treatment processes are capable of more superelastic recovery than wires formed by known processes.

[0060] FIG. 1 illustrates a process 200 for treating a wire or wire structure made from a nickel-titanium alloy according to an embodiment disclosed herein. In a first step 100 of the process 200, a cold worked wire is obtained. It is noted that step 100 may include any cold working process as known in the art. Such processes are referred to as "cold working" because they involve forming a metal at a temperature below the recrystallization temperature. In some instances, the cold working process may be carried out at room temperature.

[0061] In a second step 202 of process 200 following step 100, while the wire is being subjected to strain, a first heat treatment is applied to the wire to permanently shape-set the wire into a desired geometry. The first heat treatment includes applying heat at a first temperature for a first time. In some examples, the first temperature can be between 450°C and 550°C. In other examples, the first temperature can be between 450°C and 470°C, between 470°C and 500°C, between 500°C and 520°C, between 520°C and 550°C, or any other range therebetween. In some examples, the first heat treatment can include one or more additional heating processes to shape-set the wire.

[0062] In step 204, a second heat treatment is applied to the wire, the second heat treatment including applying heat at a second temperature different from the first temperature for a second time period. In some examples, the second temperature is between 210° C. and 290° C. In some examples, the second heat treatment is in a temperature range of 210° C. to 220° C., 220° C. to 230° C., 230° C. to 240° C., 240° C. to 250° C., 250° C. to 260° C., 260° C. to 270° C., 270° C. to 280° C., 280° C. to 290° C., or any range or combination thereof. In some examples, the second heat treatment is effective when it is in a time length range of 5 to 15 minutes, 15 to 30 minutes, 30 to 40 minutes, or any range or combination thereof.

[0063] During each of the aforementioned steps, the physical properties of the wire undergo changes, including modulus of elasticity, permanent set, recoverable strain, plateau stress, as well as other properties discussed further herein. These properties are measured using a test method known as ASTM F2516 Standard Test Method for Tensile Testing of Nickel-Titanium Superelastic Materials. In the ASTM F2516 method, a sample of Nitinol wire is loaded by clamping both ends of the wire in a fixture of a measuring device. After the wire is firmly secured to the measuring device, the ends of the wire are pulled away from each other at a specified rate until the wire reaches a specified strain percentage. In some examples, this specified strain percentage is 6% of the length of the wire. The direction of movement of the ends of the wire is then reversed and the ends are moved toward each other until the load falls below a predetermined limit. Finally, the ends of the wire are again pulled away until the wire breaks. The measurements made at each step are used to create a stress-strain curve that represents the physical properties of the Nitinol being tested. An example of such a stress-strain curve is shown in FIG. 2.

[0064] 2 shows an example of a stress-strain curve 300 that includes some of the properties of Nitinol wire that can be determined using the ASTM F2516 method. First, when an initial strain is applied to the wire by pulling both ends in opposite directions, the wire undergoes a strain curve called E, which defines the ratio of tensile stress (in Pascals) to tensile strain (percent change in length). AThe wire undergoes a linear elastic (non-permanent) deformation 302 with a gradient in the strain (i.e., Young's or elastic modulus). The tensile stress then experiences a flat region or plateau 304 where the wire undergoes tensile strain without any additional stress being applied to the wire. Per ASTM F2516 method specifications, the upper plateau stress (UPS) is defined as the stress measured at this plateau 304, specifically at a strain of 3% when the tensile stress is initially applied. Next, per ASTM F2516 method specifications, the tensile stress is applied until the strain reaches the applied strain 305, which per ASTM F2516 specifications is 6%, the initial load strain or recoverable (i.e., reversible) strain for testing Nitinol. The direction is then reversed during the unloading process 306, and the ends of the wire move closer to each other. The wire goes through another plateau 308 in this process and the LPS is measured according to the ASTM F2516 method specifications at 2.5% strain after the wire reaches the initial 6% strain. The difference between the UPS and LPS values ​​is defined as the mechanical hysteresis. It is noted that for different values ​​of applied strain 305, different amounts of permanent strain 311 are observed. The permanent strain 311 is defined by the length of the wire after strain 305 is applied to the wire and subsequently released. When the resulting length of the wire after release of strain is longer than the initial length of the wire, the difference in length determines the amount of permanent strain 311 that the wire has undergone. The smaller the increase in length of the wire, the less its amount of permanent strain 311, which indicates that the wire is more capable of superelastic recovery.

[0065] The wire is then stressed to 5 MPa, as specified by the ASTM F2516 method. In the absence of other stresses, the wire is allowed to return to its initial length. In many Nitinol wires, the length of the wire does not change or changes very little, e.g., less than 0.5% change in length before and after reaching an initial strain of 6%, but in some cases the resulting length of the wire is longer than the initial length. In these cases, the wire is said to have undergone a permanent deformation 310, with a permanent set 311 determined by comparing the resulting length of the wire to the initial length. Finally, the ends of the wire are again pulled in opposite directions, and the wire undergoes an elastic deformation 302 and an upper plateau 304. However, this time the wire is pulled beyond a strain of 6%, and undergoes a plastic (permanent) deformation 312. The wire then breaks or fractures when it is at its ultimate permanent set, and the ultimate tensile strength (UTS) of the wire is measured.

[0066] In a conventional process for imparting physical properties to nickel-titanium alloy wire, the wire is subjected to a strain deformation to form the desired shape, followed by a shape-setting annealing process, typically by heat treatment at a temperature used for shape setting. Examples of methods used to process Nitinol wire and the resulting properties of each wire prepared using the methods are disclosed below. The first example ("Exemplary Process 1") relates to methods of wire processing known in the art, while the second example ("Exemplary Process 2") relates to the present disclosure. The Exemplary Process 2 method presented below is illustrative and not limiting, and further uses will be recognized by those skilled in the art.

[0067] Exemplary Process 1: A Nitinol wire (e.g., a superelastic ASTM F2063 compliant wire with a wire diameter of 0.0206 inches) is obtained, which is prepared via a cold working process and reduced to a wire diameter of 0.0206 inches. Specifically, the Nitinol wire undergoes a drawing process at room temperature (about 25°C), which is well below the recrystallization temperature of the wire (e.g., 550°C). Next, after the strain deformation is imposed, the wire undergoes a shape-setting annealing process in which the wire is heated to a temperature typically used for shape setting of the alloy (e.g., 470°C) and below the melting temperature (1310°C), the wire is maintained at that temperature for a period of time, and then the wire is cooled to room temperature. In this example, the wire undergoes a shape-setting annealing process at 470°C for 17 minutes, resulting in a permanent set 311 of 5.6% after an applied strain 305 of 11%.

[0068] Exemplary Process 2: Nitinol wire is subjected to a first heat treatment under step 202 of process 200, where the first heat treatment is performed at 470° C., which is the shape-setting temperature of the wire, for 17 minutes. This is the same temperature and time as the shape-setting annealing process described above in Exemplary Process 1. The wire is shape-set in a straight section by applying a strain during the annealing process (step 202) and holding the shape during the first heat treatment. Thereafter, a second heat treatment is subjected to the wire in step 204, where the second heat treatment involves a lower temperature than the first heat treatment. The second heat treatment in one example involves holding the wire at 250° C. for 40 minutes. In this example, the wire had a permanent set 311 of 0.6% after an applied strain 305 of 11%.

[0069] FIG. 3 compares the different permanent strains 311 induced by the same type of Nitinol wire after different amounts of applied strain 305 in the exemplary processes 1 and 2. In the figure, specifically at an applied strain 305 of 11%, the first sample 400 based on the procedure performed in the exemplary process 1 (i.e., only the first heat treatment at 470° C.) has a permanent strain 311 of 5.6%, which is higher than the second sample 402 based on the procedure performed in the exemplary process 2 (i.e., the first heat treatment at 470° C. and the second heat treatment at 250° C.), which has a permanent strain of 0.6%. In this example, the permanent strain 311 is reduced by 89% from the first sample 400 to the second sample 402. Therefore, since the permanent strain 311 is reduced in the second sample 402 compared to the first sample 400, it can be said that the second sample 402 has a greater superelastic recovery than the first sample 400. In some examples, the applied strain 305 is between 10.5% and 11.5%. In some instances, the second heat treatment using a temperature between 210°C and 290°C causes the wire to experience a reduction in permanent set of at least 50%, 60%, 70%, 80%, 85%, or any range therebetween, compared to wire subjected to only the first heat treatment.

[0070] Exemplary Process 3: Nitinol wire is subjected to a first heat treatment under step 202 of process 200, where the first heat treatment is at 470° C., the shape-setting temperature of the wire, for 17 minutes. This is the same temperature and time as the shape-setting annealing process described above in Exemplary Process 1. A strain deformation is imposed on the wire during the first heat treatment in step 202, after which a second heat treatment is imposed on the wire in step 204, where the second heat treatment involves a lower temperature than the first heat treatment. The second heat treatment involves holding the wire at 320° C. for 40 minutes. In this example, the wire had a permanent set 311 of 6.0% after an applied strain 305 of 11%.

[0071] FIG. 4 compares the different permanent strains 311 that the same type of Nitinol wire undergoes after different amounts of applied strain 305 in exemplary processes 2 and 3. In particular, at an applied strain of 11%, the second sample 402 of FIG. 3 based on the procedure performed in exemplary process 2 has a permanent strain 311 of 0.4%, which is lower than the third sample 500 based on the procedure performed in exemplary process 3, which has a permanent strain 311 of 6.0%. Since the permanent strain 311 is reduced in the second sample 402 relative to the third sample 500, it can be said that the second sample 402 has a greater superelastic recovery than the third sample 500. In some examples, the applied strain 305 is between 10.5% and 11.5%. Thus, when the wire undergoes a second heat treatment at a temperature above the range of 210° C. to 290° C., the permanent strain 311 increases, resulting in a lower superelastic recovery of the wire compared to the wire that underwent a second heat treatment at a temperature within the range of 210° C. to 290° C.

[0072] Considering the results of Figures 3 and 4, there are certain regions of temperature ranges for the second heat treatment where the second heat treatment appears to be particularly effective in increasing the elasticity of the wire by, for example, 11%. Exemplary Process 3 shows a wire experiencing a 6.0% permanent set 311 after an applied strain 305 of 11%, which is similar to Exemplary Process 1, where the wire underwent a second heat treatment similar to Exemplary Process 2, but experienced a 5.6% permanent set 311 after the same applied strain 305. Some examples of such regions are described elsewhere herein. Also, in some examples, when the wire is subjected to a second heat treatment at a temperature between 210°C and 290°C, the second sample 402 achieves a permanent set of less than 5%, less than 4%, less than 3%, less than 2%, less than 1%, less than 0.9%, less than 0.8%, less than 0.7%, less than 0.6%, less than 0.5%, or any range therebetween, when an 11% strain 305 is applied to the wire.

[0073] Furthermore, in some examples, the wire after undergoing a second heat treatment within the temperature and time length ranges described above also has a UPS304 of at least 630 MPa and an LPS308 of at least 345 MPa. Specifically, in one example with a first heat treatment at 450° C. and a second heat treatment at 250° C., an average UPS304 value of 641.31 MPa and an average LPS308 value of 352.54 MPa were observed, resulting in hysteresis or a difference between UPS304 and LPS308 of 288.77 MPa.

[0074] Other examples having different (or no) second heat treatment temperatures are shown in Table 1 below, which shows that the second heat treatment within the specified temperature range increases the hysteresis of LPS308 and wires. Table 1 also shows the increase observed for LPS308 and the decrease in hysteresis after the second heat treatment compared to examples where only the first heat treatment was performed. [Table 1]

[0075] According to Table 1, wires subjected to a second heat treatment always result in higher LPS and lower hysteresis compared to wires subjected to only the first heat treatment. At a minimum, the second heat treatment increases LPS by 6 MPa and reduces hysteresis by 17 MPa. The temperature at which the second heat treatment is performed affects the values, as shown in the fact that, among the three temperatures used, the second heat treatment temperature of 250°C causes the largest change and has the highest LPS and lowest hysteresis among all conditions, including the condition where only the first heat treatment is performed. Thus, in some examples, the increase in LPS resulting from the second heat treatment is at least 15 MPa or 30 MPa, and the decrease in hysteresis resulting therefrom is at least 30 MPa or 40 MPa. In some examples, the temperature range of the second heat treatment from 210°C to 290°C causes an increase in LPS of at least 30 MPa and a decrease in hysteresis of at least 40 MPa compared to when only the first heat treatment is applied. This temperature range also increases LPS to at least 350 MPa.

[0076] In some examples, the secondary component can be attached to the wire, such as, but not limited to, via thermal bonding and sutures. The secondary component can be made from polymeric materials, including, but not limited to, bioabsorbable polymers (such as, for example, polylactic acid, poly(trimethylene carbonate) or PGA / TMC), fluoropolymers (such as, for example, fluorinated ethylene propylene or FEP, polytetrafluoroethylene or PTFE and expanded (expanded, expanded, stretched or foamed) fluoropolymers, such as expanded polytetrafluoroethylene or ePTFE), fluoroelastomers and elastomeric materials (such as, for example, TFE / PMVE copolymers), polyesters (such as, for example, polyethylene terephthalate or PET), polyethylene, polypropylene, polyurethane or silicone, and the like. Additionally or alternatively, a second heat treatment can be used to cure or modify the secondary component (e.g., drug coating, therapeutic or other material) that is bonded to the wire. The secondary component may be a graft member or other component of a stent graft, a heart valve leaflet, skirt, cuff or other component of a heart valve, a filter membrane or other component of a filter, or other secondary component as desired.

[0077] Furthermore, in some examples, the first elastic modulus of the wire resulting from the first heat treatment is approximately the same as the second elastic modulus of the wire resulting from the second heat treatment. That is, the wire after the second heat treatment maintains the same tensile stress / tensile strain ratio as after the first heat treatment. In other examples, the second elastic modulus measured after the second heat treatment remains within 2% of the first elastic modulus measured after the first heat treatment (i.e., increases or decreases by less than 2%). In some examples, the first elasticity of the wire resulting from the first heat treatment is approximately the same as the second elasticity of the wire resulting from the second heat treatment. In other examples, the second elasticity is within 2% of the first elasticity.

[0078] 5 is a graph 600 comparing experimental data of different stress vs. strain curves observed on wire undergoing one of four heat treatment processes. In a first process 602, shown as a solid line on the graph 600, the wire is heated to a first heat treatment temperature of 470° C. for 17 minutes and then to a second heat treatment temperature of 320° C. for 40 minutes. In a second process 604, shown as a dashed line with a longer segment on the graph 600, the wire is heated to a first heat treatment temperature of 470° C. for 17 minutes and then to a second heat treatment temperature of 270° C. for 40 minutes. In a third process 606, shown as a dashed line with a shorter segment on the graph 600, the wire is heated to a first heat treatment temperature of 470° C. for 17 minutes and then to a second heat treatment temperature of 210° C. for 40 minutes. In a fourth process 608, shown as a dotted line in graph 600, the wire is heated only to the first heat treatment temperature of 470° C. for 17 minutes without any subsequent heat treatments.

[0079] The resulting data is collected for each of the four processes above such that the wire is subjected to different values ​​of stress loads (in MPa) to measure the amount of strain (% of original wire length) caused by each stress load. The data is plotted as shown in Figure 5 and then analyzed to determine the elastic modulus (GPa), which is calculated by dividing the stress by the corresponding stress. In the table shown below, the elastic modulus is calculated at applied stresses of 200 MPa and 400 MPa.

[0080] [Table 2]

[0081] [Table 3]

[0082] As can be observed from Tables 2 and 3 above, the second and third processes 604 and 606 cause a larger change in modulus between 200 MPa and 400 MPa. Indeed, the second and third processes 604 and 606, which include a second heat treatment within the specified temperature range as disclosed herein, have a larger modulus at an applied stress of 200 MPa. More specifically, the measured modulus is in a range such as at least 53 GPa, at least 54 GPa, or at least 55 GPa. In some examples, the measured modulus is 53 GPa to 64 GPa, 55 GPa to 64 GPa, 57 GPa to 64 GPa, 60 GPa to 64 GPa, 62 GPa to 64 GPa, or any range therebetween. In comparison, the first process 602 has the lowest modulus at 41.67 GPa and the fourth process at 52.63 GPa, both of which are lower than the modulus of the second and third processes 604 and 606.

[0083] Also, the change in modulus of elasticity of the second and third processes 604 and 606 is greater than 5 GPa, greater than 7 GPa, or greater than 10 GPa, etc., between an applied stress of 200 MPa and 400 MPa. In some examples, the change in modulus of elasticity of the second and third processes 604 and 606 is 5 GPa to 16 GPa, 7 GPa to 16 GPa, 10 GPa to 16 GPa, 12 GPa to 16 GPa, 14 GPa to 16 GPa, or any range therebetween. In comparison, the first process 602 has a change in modulus of elasticity of 0.43 GPa, while the fourth process 608 has a change in modulus of elasticity of 4.44 GPa, which is much lower than that of the second and third processes 604 and 606.

[0084] 6A-6D show examples of various implantable medical devices that may be made using wires as disclosed herein. The wires may be processed into any one or more of these medical devices using any suitable method known in the art. The examples are merely illustrative and are not intended to be exhaustive, and the shapes and configurations of the medical devices are not limited to those shown in the figures.

[0085] 6A shows an example of a stent graft 700 including one or more wires or stents 701 and one or more membrane-like materials or grafts 702. The wires or stents 701 may be spirally formed around the grafts 702, although any suitable configuration may be envisioned.

[0086] 6B shows an example of an embolic filter 710 that includes wires or struts 711 and one or more membrane-like materials or filter elements 712 attached to a central support member 713. The struts 711 can be straight, curved, or have any other suitable shape and configuration.

[0087] 6C shows an example of a septal occluder or cardiac sealing device 720 that includes one or more wires forming a wire frame 721, one or more membrane-like materials or sealing members 722, and one or more eyelets 723 that may be occluded or unoccluded to control fluid flow therethrough. The wire frame 721 may be attached to or mounted with the eyelets 723, and the seal members 722 may be attached to the wire frame 721. The wire frame 721 may be of any suitable configuration as known in the art.

[0088] 6D shows an example of a heart valve or prosthesis retention element 730 including one or more wires forming a valve frame 731 and one or more membrane-like materials or covers 732. The covers 732 can be attached to or implemented with the valve frame 731. The valve frame 731 can be of any suitable configuration as known in the art.

[0089] The embodiments and methods disclosed herein can also be used to enable a variety of shape memory articles. Shape memory articles can include medical devices, such as implantable medical devices. The implantable medical devices can be Nitinol shape memory alloy devices, such as those that are superelastic at normal body temperature (about 37°C). An implantable medical device is defined as a device that is intended to remain in the body for a period of time of 24 hours or more.

[0090] The medical devices or articles may be fabricated from materials in various shapes, such as wires, in various cross-sectional shapes including round, oval, square, rectangular, etc. Alternatively, the devices or articles may be fabricated by machining precursor forms, such as sheets, tubes or rods, or by electrical discharge machining (EDM), laser cutting, chemical milling, etc.

[0091] The disclosure of the present application has been described above both generally and with reference to specific embodiments. It will be apparent to those skilled in the art that various changes and modifications can be made in the embodiments without departing from the scope of the present disclosure. Therefore, the embodiments are intended to cover the modifications and modifications of the present disclosure provided they fall within the scope of the appended claims and their equivalents.

Claims

1. A method for forming a nickel-titanium alloy wire, comprising: applying a strain to the wire to fix the shape of the wire; subjecting the wire to a first heat treatment; and subjecting the wire to a second heat treatment; Including, the first heat treatment includes applying heat at a first temperature for a first time such that the wire has a first hysteresis resulting from the first heat treatment; imposing the second heat treatment includes imposing heat at a second temperature different from the first temperature for a second time such that the wire has a second hysteresis resulting from the second heat treatment that is at least 40 MPa lower than the first hysteresis, the second temperature being between 210°C and 290°C; The resulting wire has a permanent set of less than 5% when an 11% strain is applied to said wire; The method wherein the alloy is medical grade, ASTM compliant Nitinol.

2. The method described in claim 1, wherein the wire obtained after the second heat treatment has a lower plateau stress (LPS) of at least 350 MPa.

3. The method described in claim 1, wherein the second time period is 5 minutes to 40 minutes.

4. The method of claim 1, wherein the first modulus of elasticity of the wire resulting from the first heat treatment is within 2% of the second modulus of elasticity of the wire resulting from the second heat treatment.

5. The method of claim 1, wherein the second lower plateau stress (LPS) of the wire resulting from the second heat treatment is at least 30 MPa higher than the first LPS of the wire resulting from the first heat treatment.

6. A second permanent strain of the wire resulting from the second heat treatment is at least 85% lower than a first permanent strain of the wire resulting from the first heat treatment; The method of claim 1 , wherein each of the first permanent set and the second permanent set is defined by the length of the wire after an 11% strain is applied to the wire.

7. A second permanent strain of the wire resulting from the second heat treatment is less than 1%; 7. The method of claim 6, wherein each of the first permanent set and the second permanent set is defined by the length of the wire after an 11% strain is applied to the wire.

8. The method described in claim 1, wherein the permanent strain is less than 1% when an 11% strain is applied to the wire.

9. The method of claim 1, wherein after the first heat treatment, when a stress of 200 MPa is applied to the wire, the wire has a first modulus of elasticity of less than 42 GPa, and after the second heat treatment, when a stress of 200 MPa is applied to the wire, the wire has a second modulus of elasticity of at least 55 GPa.

10. The method of claim 9, wherein the second elastic modulus is between 55 GPa and 64 GPa.

11. The method of claim 1, wherein the resulting wire has a lower plateau stress (LPS) of at least 350 MPa.

12. The method of claim 1, further comprising processing the wire into an implantable medical device after the second heat treatment has been applied.

13. The method of claim 12, wherein the medical device is a stent graft, an embolic filter, a septal occluder, or a heart valve.

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