Superelastic stent and method for manufacturing the same, and alloy tube and method for manufacturing the same

Abstract

To provide a superelastic stent which is excellent in mechanical characteristics and a method for manufacturing the same, and an alloy tube which is an original material for a stent and a method for manufacturing the same.SOLUTION: A superelastic stent is composed of a Ti-Ni-based alloy, wherein in an extension force hysteresis curve composed of a recovery curve indicating outer diameter extension force at the time of a recovery diameter from a minimum outer diameter to a maximum outer diameter of the superelastic stent, and a diameter reduction curve indicating outer diameter extension force at the time of diameter reduction from a maximum outer diameter to a minimum outer diameter of the superelastic stent, the outer diameter extension force with an outer diameter deformation amount of 1 / 2 of the superelastic stent in the recovery curve is 0.50 or more with respect to the outer diameter extension force with the outer diameter deformation amount of 1 / 2 of the superelastic stent in the diameter reduction curve, and a maximum change ratio of the outer diameter extension force within a range of the outer diameter deformation amount of 1 / 2 to the outer diameter deformation amount of 3 / 4 of the superelastic stent in the recovery curve is within 50.0% with respect to the outer diameter extension force with the outer diamter deformation amount of 1 / 2 of the superelastic stent in the recovery curve.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to stents used in percutaneous transluminal angioplasty for the treatment of stenotic blood vessels, and in particular to a self-expanding stent made of a superelastic Ti-Ni alloy as its base material and its manufacturing method, as well as an alloy tube as the base material for the stent and its manufacturing method. [Background technology]

[0002] It is well known that Ti-Ni alloys exhibit superelasticity, in which recovery of applied deformation occurs simultaneously with the release of external force, and a shape memory effect that requires heating. In this case, the former occurs when the alloy is used in the austenite phase (high-temperature phase) above the shape recovery temperature, and the latter occurs when the alloy is used in the martensite phase (low-temperature phase) at low temperatures.

[0003] Stenting is a medical technology whose use has expanded rapidly in recent years. A stent is a metal mesh placed inside a blood vessel to prevent restenosis after vasodilation treatment, and is incorporated into the tip of a catheter. During treatment, the catheter is introduced into the stenotic area, then detached from the catheter and attached to the inner wall of the lumen.

[0004] Their functions can be divided into balloon-expandable types that use a balloon and self-expandable types that utilize their own spring properties. The former are primarily used to treat coronary artery stenosis, and the raw material of the unprocessed tube is primarily stainless steel or cobalt alloy. The latter are used to treat diseases of the brain and lower limbs, and the raw material is a Ti-Ni alloy superelastic material.

[0005] It is well known that shape memory alloys, including Ti-Ni alloys, exhibit a remarkable shape memory effect associated with the reverse transformation of martensitic transformation, and also exhibit excellent superelasticity due to the appearance of stress-induced martensitic phases caused by severe deformation after the reverse transformation. These properties are particularly evident in Ti-Ni alloys and Ti-Ni-X alloys (X = V, Cr, Nb, Co, etc.) among the many shape memory alloys, and are widely used in medicine, construction, automobiles, etc.

[0006] For medical applications of Ti-Ni alloys for placement within the body, the compositional specifications are specified in Non-Patent Documents 1 and 2 as 53.5 to 57.5 mass% Ni, with the remainder being Ti. Therefore, all base tubes for commercial stent applications are Ti-Ni alloys conforming to these specifications, and Ti-Ni-X alloys, in which a third element X is added to a Ti-Ni alloy, are determined through mutual consultation between the parties involved. Meanwhile, in research materials, proposals for using Ti-Ni alloys in self-expanding stents are shown in Patent Documents 1 and 2, and Patent Document 3 proposes improving the properties of the base tubes, similar to the present invention. Many proposals have been made, including the addition of Nb to the Ti-Ni alloy used as the base material for stents to improve properties, and Patent Document 4 proposes imparting gradient functionality by combining shape memory and superelasticity.

[0007] Here, we will further explain the technology related to Ti-Ni alloy tubes before they are processed into stents. Many Ti-Ni alloys have been established as industrial standards (JIS), and are used as important technical information when commercializing products.

[0008] Non-Patent Document 1 defines the chemical composition of a Ti—Ni alloy for use in tubes as an alloy containing 53.5 to 57.5 mass % (48.5 to 52.5 at %) of Ni.

[0009] Furthermore, Non-Patent Document 3 has shown that Ti-Ni-X alloys (X=V, Cr, Co, Cu, Nb, Ta, Hf, etc.) containing a third element can exhibit similar properties to Ti-Ni alloys depending on the amount of addition.

[0010] In terms of tube processing technology, seamless metal tubes, the base material for stents, are generally manufactured by drilling holes in metal rods using a gun drill. Next, small-diameter tubes with a large aspect ratio are manufactured by inserting a mandrel (core metal) into the material to be drilled and then rolling or drawing the composite material (tube + mandrel). During processing, the mandrel is removed from the tube after each processing pass, and for smaller diameters, this is often combined with dry drawing and then heat treated for straightening.

[0011] On the other hand, a known method for manufacturing metal tubes with uniform cross-sectional shapes involves inserting a metal core (mandrel) into the punched tube material (raw material) described above to form a composite (tube + core) material, which is then drawn to form a composite tube, and then removing the core material from the composite material. However, in this manufacturing process, removing the core material in the final step remains a major problem. The tube material and core material of the drawn clad tube are tightly adhered to each other, and the smaller the diameter and longer the clad tube, the greater the frictional resistance when removing the core material, making it difficult to simply pull it out and remove it.

[0012] Therefore, methods proposed for removing the core material include selectively dissolving only the core material at a temperature lower than the melting point of the tube material, and heat treating the core material at a temperature above its recrystallization temperature while stretching and reducing its diameter to make it easier to extract the core material.

[0013] Patent Document 5 describes a technology for reducing the diameter of a seamless metal tube using a shape memory alloy such as a Ti-Ni alloy as the base material. Specifically, a composite material is constructed using a core material with the same ductility as the metal tube (base material), and the composite material is stretched to form a composite wire (assembly). Subsequently, only the core material of the composite material is stretched to reduce its diameter, and the reduced-diameter core material is drawn out to produce a seamless metal tube. Furthermore, the document describes that the core material can be easily stretched by heat treating the composite tube at approximately 700°C.

[0014] Patent Document 6 also discloses a method for manufacturing a shape memory alloy pipe (tube). A mandrel is inserted into a cylinder formed from a shape memory alloy, and the cylinder and mandrel are subjected to a surface reduction process together. After heat treatment, the mandrel is withdrawn. This document discloses a technique for simultaneously reducing the surface area of a tubular nickel-titanium shape memory alloy blank and a stainless steel core, utilizing the shape memory effect of the tube material (a rolled, welded, thinned sheet), thereby expanding the tube and removing the core. Furthermore, this document describes the tube manufacturing conditions, specifying a processing rate of 15% or more and a straightening heat treatment at 500°C for 20 minutes. [Prior art documents] [Patent documents]

[0015] [Patent Document 1] Japanese Patent Application Publication No. 06-054913 [Patent Document 2] Japanese Patent Application Publication No. 08-000738 [Patent Document 3] Patent Application No. 2004-062664 [Patent Document 4] Patent Application No. 2005-148995 [Patent Document 5] U.S. Patent No. 5,709,021 [Patent Document 6] Japanese Unexamined Patent Publication No. 62-199218 [Non-patent literature]

[0016] [Non-Patent Document 1] JIS H-7107-2009 "Ti-Ni shape memory alloy wire, strip and tube" [Non-patent document 2] ASTM F2063-18 “Standard Specification for Wrought Nickel-Titanium Shape Memory Alloys for Medical Devices and Surgical Implants” [Non-patent document 3] Kiyoshi Yamauchi, "Shape memory alloys exhibiting the highest operating temperature", Materia, 1996, Vol. 35, No. 11, pp. 1195-1198 Summary of the Invention [Problem to be solved by the invention]

[0017] The superelasticity of Ti-Ni alloys is manifested through a phase transformation of the alloy crystals, and this transformation requires subsequent strain-relief heat treatment to restore movement that is trapped by processing strain induced by wire drawing or rolling. For practical medical devices such as guidewires and stents, aging conditions (treatment below the recrystallization temperature that leaves processing strain) are set according to their purpose. Furthermore, Ti-Ni alloy materials include wires for coil stents and tubes for laser-processed stents. The problem addressed by this invention relates to tubes that can be widely used for coronary artery stents, peripheral stents, and other applications.

[0018] The stent is made by laser groove processing a material tube with an outer diameter of 0.3 mm to 6.0 mm based on a unique design, expanding it to a size with an outer diameter of 1.5 mm to 30.0 mm, and then measuring its radial mechanical properties (stent expansion force hysteresis) using a stent mechanical properties device (e.g., MSI-RX550 / 650) to evaluate its performance.

[0019] The problem with current stent processing is that the current processing process does not offer options for imparting optimal properties. In other words, medical manufacturers simply apply a predetermined expansion process to a straightened tube provided by a material manufacturer to create a stent of the desired shape, and are unable to consider processing methods tailored to the application, such as improving properties.

[0020] As is clear from the above-mentioned literature on tubes and stents, the manufacture of tubes and stents has been verified separately. For example, if a tube is manufactured by focusing only on the transformation temperature and wire drawing performance as shown in the above literature, the processing rate will be insufficient under the manufacturing conditions before stent processing, and the temperature of the straightening process at the tube stage will usually be 500°C or higher.

[0021] Figure 4 shows the stress hysteresis curves of a commercially available rubber band and the hyperelastic material (hyperelastic stent) tested in a uniaxial tensile test. The commercially available rubber band shown in Figure 4(a) is a typical example of an elastic body, and the loss due to interstitial friction and other factors generated by strain loading appears as a bend (deflection) in the hysteresis curve. The hyperelastic material tested in this study, shown in Figure 4(b), has a flat curve associated with the superelasticity induced by strain loading, but also shows a broader hysteresis curve with greater loss than Figure 4(a).

[0022] The important problem to be solved by the present invention is to improve the hysteresis that is inevitable in these spring materials, and to reduce the width of this hysteresis.

[0023] In order to solve the above-mentioned problems, the present invention provides a stent with improved properties and processing technology for it, by examining comprehensive process technologies from tube processing, which is a material that breaks away from the previous use of heat-treated tubes as commercial materials, to stent processing. [Means for solving the problem]

[0024] [1] A superelastic stent made of a Ti-Ni alloy, wherein in an expansion force hysteresis curve consisting of a recovery curve showing the outer diameter expansion force of the superelastic stent when it recovers from its minimum outer diameter to its maximum outer diameter, and a diameter reduction curve showing the outer diameter expansion force of the superelastic stent when it is reduced from its maximum outer diameter to its minimum outer diameter, the outer diameter expansion force of the superelastic stent at 1 / 2 of the outer diameter deformation in the recovery curve is 0.50 or more compared to the outer diameter expansion force of the superelastic stent at 1 / 2 of the outer diameter deformation in the diameter reduction curve, and the maximum rate of change in the outer diameter expansion force of the superelastic stent within the range from 1 / 2 of the outer diameter deformation to 3 / 4 of the outer diameter deformation in the recovery curve is within 50.0% based on the outer diameter expansion force of the superelastic stent at 1 / 2 of the outer diameter deformation in the recovery curve. [2] An alloy tube made of a Ti-Ni alloy, which is the base material for the superelastic stent described in [1] above. [3] A method for manufacturing a superelastic stent, in which a heat treatment is performed after a stent processing process in which an alloy tube made of a Ti-Ni alloy is processed into a superelastic stent, the heat treatment temperature is higher than 475°C and lower than the recrystallization temperature of the alloy tube, and the heat treatment time is 1 minute or more and 20 minutes or less. [4] The method for manufacturing a superelastic stent according to [3] above, wherein the heat treatment temperature is higher than 475°C and not higher than 500°C. [5] A method for manufacturing a superelastic stent according to [3] or [4] above, wherein the processing rate of the alloy tube in the stent processing is more than 30% and not more than 65%. [6] A method for manufacturing alloy tubes, in which a straightening heat treatment at 490°C or less is performed after processing to produce alloy tubes made of Ti-Ni alloys. [Effects of the Invention]

[0025] According to the present invention, it is possible to provide a superelastic stent having excellent mechanical properties and a method for manufacturing the same, as well as an alloy tube that is the base material for the stent and a method for manufacturing the same. [Brief explanation of the drawings]

[0026] [Figure 1] FIG. 2 is a schematic diagram illustrating the expansion force hysteresis curve of the superelastic stent of the present invention. [Figure 2] 1 shows the results of the expansion force hysteresis curves of the superelastic stents, where (a) shows the expansion force hysteresis curve of the superelastic stent of Example 1, (b) shows the expansion force hysteresis curve of the superelastic stent of Comparative Example 4, and (c) shows the expansion force hysteresis curves of the superelastic stents of Example 2 and Comparative Example 6. [Figure 3] 1 is a conceptual diagram of a self-expanding stent according to the present invention, in which (a) shows the alloy tube processing stage and (b) shows the diameter expansion process after placement. [Figure 4] Figure 1 shows stress hysteresis curves of tensile tests of the superelastic stent according to the present invention, where (a) shows the stress hysteresis curve of a commercially available rubber band, and (b) shows the stress hysteresis curve of a superelastic stent made of Ti-Ni alloy superelastic material treated at 500°C. DETAILED DESCRIPTION OF THE INVENTION

[0027] Hereinafter, a detailed description will be given based on an embodiment.

[0028] (Alloy composition range) The alloy tube and superelastic stent of the present invention are made of a Ti-Ni alloy. Ti-Ni alloys exhibit superelasticity, in which recovery from applied deformation occurs simultaneously with the release of external force. In this case, the Ti-Ni alloy used in the present invention has a primary structure in the austenite phase (high-temperature phase) at temperatures higher than the shape recovery temperature. Therefore, the Ti-Ni alloy required in the present invention has a Ni concentration in the austenite phase of 55.6% by mass or more and 56.3% by mass or less. Furthermore, as long as the properties required in the present invention are within the range, the Ti-Ni alloy may have a composition containing one or more of V, Cr, Co, Cu, Nb, Ta, and Hf in a total amount of 0.1 to 2.0% by mass, with the remainder consisting of Ti and unavoidable impurities.

[0029] Among these, in order to achieve the desired workability and expansion force, it is preferable that the Ti-Ni alloy contains more than 0% by mass but not more than 0.040% by mass of C, more than 0% by mass but not more than 0.040% by mass of O, with the balance being Ti and inevitable impurities. Here, inevitable impurities refer to impurities at a level that may be unavoidably contained in the manufacturing process. For example, the inevitable impurity may contain 0.005% by mass or less of N.

[0030] In the present invention, Ni (nickel) is an element necessary for exhibiting superelastic properties, but if the Ni content is outside the range of 55.6 to 56.3 mass%, it becomes difficult for the Ti-Ni alloy to exhibit superelasticity at body temperature. Therefore, when using a Ti-Ni alloy for medical purposes where the transformation temperature is particularly important and when the alloy is to be placed in the body, the Ni content is preferably in the range of 55.6 to 56.3 mass%.

[0031] C (carbon) is an element that forms nonmetallic inclusions. As the C content increases, the number of nonmetallic inclusions present in the matrix increases, and the proportion of nonmetallic inclusions in the Ti-Ni alloy (product) increases, making fatigue fracture more likely to occur. For this reason, it is preferable to keep the C content as low as possible. Specifically, based on the provisions of Non-Patent Document 2, it is preferable to keep the C content to 0.040 mass% or less.

[0032] O (oxygen) is an element that forms nonmetallic inclusions. If the O content is high, the particle size of the nonmetallic inclusions increases, making fatigue fracture more likely to occur. For this reason, it is preferable to keep the O content as low as possible. Specifically, based on the provisions of Non-Patent Document 2, it is preferable to keep the O content to 0.040 mass% or less.

[0033] The alloy composition of Ti-Ni alloys can be measured using a conventionally known analytical device, and is specified in Non-Patent Document 2. In the present invention, the measured values conforming to Non-Patent Document 2 are shown as the alloy composition range of Ti-Ni alloys. The alloy compositions of the Ti-Ni alloys used in the present invention are shown in Table 1. Table 5 also shows the results of an investigation into the relationship between the correspondence change rate CR and the curve contrast rate FR, which will be described later, and the alloy composition.

[0034] [Table 1]

[0035] (Alloy tube size and superelastic stent size) The superelastic stent of the present invention uses the alloy tube (blank tube) of the present invention as the base material (raw material). The alloy tube of the present invention is laser grooved based on a unique design, and then subjected to expansion treatment (characteristic treatment) to produce the superelastic stent of the present invention. For example, an alloy tube with an outer diameter of 1.5 mm to 2.0 mm is often expanded to produce a superelastic stent with an outer diameter of approximately 10.0 mm. In this invention, however, to confirm whether or not the properties change with size change, the properties of alloy tubes with outer diameters of 0.3 mm and 6.0 mm were also evaluated, and the outer diameters of the superelastic stents after expansion treatment were 1.5 mm and 30.0 mm, respectively.

[0036] The alloy tube of the present invention is made of the above Ti-Ni alloy and is the base material for the superelastic stent of the present invention.

[0037] 1 is a schematic diagram illustrating the expansion force hysteresis curve of the superelastic stent of the present invention. In the expansion force hysteresis curve, which is composed of a recovery curve showing the outer diameter expansion force of the superelastic stent when it recovers from its minimum outer diameter to its maximum outer diameter, and a diameter contraction curve showing the outer diameter expansion force of the superelastic stent when it contracts from its maximum outer diameter to its minimum outer diameter, the outer diameter expansion force of the superelastic stent at 1 / 2 (one-half) of the outer diameter deformation amount (point A) on the recovery curve is 0.50 or more (curve contrast FR) relative to the outer diameter expansion force of the superelastic stent at 1 / 2 (one-half) of the outer diameter deformation amount (point C) on the diameter contraction curve. Furthermore, in the expansion force hysteresis curve of the superelastic stent of the present invention, the maximum rate of change (CR) in the external diameter expansive force within the range from 1 / 2 (one-half) of the external diameter deformation (point A) to 3 / 4 (three-quarters) of the external diameter deformation (point B) on the recovery curve is within 50.0% of the external diameter expansive force at 1 / 2 (one-half) of the external diameter deformation (point A) on the recovery curve. That is, in the expansion force hysteresis curve of the superelastic stent of the present invention, the ratio (external diameter expansive force at point A) / (external diameter expansive force at point C) is 0.50 or greater, and the maximum rate of change in the external diameter expansive force within the range from point A to point B ((external diameter expansive force farthest from the external diameter expansive force at point A) × 100 / (external diameter expansive force at point A)) is within 50.0% of the external diameter expansive force at point A.

[0038] The outer diameter deformation amount 1 / 2 (point A) and the outer diameter deformation amount 3 / 4 (point B) on the recovery curve refer to the points where the diameter has expanded (recovered) by 1 / 2 (one-half) from the minimum outer diameter and the point where the diameter has expanded (recovered) by 3 / 4 (three-quarters) from the minimum outer diameter, when the value obtained by subtracting the minimum outer diameter from the maximum outer diameter of the superelastic stent on the recovery curve is set to 1. Furthermore, the outer diameter deformation amount 1 / 2 (point C) on the diameter reduction curve refers to the point where the diameter has contracted by 1 / 2 (one-half) from the maximum outer diameter, when the value obtained by subtracting the minimum outer diameter from the maximum outer diameter of the superelastic stent on the diameter reduction curve is set to 1.

[0039] (Contrast FR, Change CR) For example, taking a superelastic stent with an outer diameter of 2.0 mm as an example, characteristics can be evaluated based on the following equations 1 and 2 by measuring the outer diameter expansion force (N) in the expansion force hysteresis curve when the stent is reduced to an outer diameter of 1.6 mm, which is the size before processing, and the outer diameter expansion force (N) in the recovery curve at outer diameters of 6.3 mm and 8.6 mm.

[0040] The calculation formula for the expansion processing stroke 1 / 2 points (points A and C) is as follows: A+(BA) / 2···equation-1 Therefore, if the outer diameter A before expansion is 1.6 mm and the outer diameter B after expansion is 11 mm, the point is when the outer diameter is 6.3 mm.

[0041] Similarly, the calculation formula for the expansion processing stroke 3 / 4 point (point B) is as follows: A+(BA)×3 / 4···Eq.-2 Therefore, if the outer diameter A before expansion is 1.6 mm and the outer diameter B after expansion is 11 mm, the point is when the outer diameter is 8.6 mm.

[0042] In order to ensure the same evaluation level after expansion, tubes with an outer diameter of φ0.3 mm before stent processing were expanded to 1.5 mm, with the minimum outer diameter at the time of measurement being 0.2 mm and the maximum outer diameter being 1.6 mm, so that the expansion force can be measured and evaluated at 0.9 mm diameter for diameter reduction curve 1 and recovery curve 2 and 1.25 mm diameter for recovery curve 2, and tubes with an outer diameter of φ6.0 mm before stent processing were expanded to 30.0 mm, with the minimum outer diameter at the time of measurement being 5.0 mm and the maximum outer diameter being 31 mm, so that the expansion force can be measured and evaluated at 18.0 mm diameter for diameter reduction curve 1 and recovery curve 2 and 24.5 mm diameter for recovery curve 2.

[0043] The size of the superelastic stent of the present invention depends on the wire diameter of the Ti-Ni alloy tube used. For example, when a Ti-Ni alloy tube with a diameter of 1.6 mm is used, the superelastic stent can be manufactured in sizes ranging from 5.0 mm to 30.0 mm. In the present invention, a Ti-Ni alloy tube with a diameter of 0.3 mm to 6.0 mm before the characteristic treatment was used and expanded to produce a superelastic stent. It has been confirmed that the characteristics required by the present invention are satisfied with a superelastic stent size in the range of 1.5 mm to 30.0 mm.

[0044] Furthermore, in the method for manufacturing a superelastic stent of the present invention, in the heat treatment performed after the stent processing in which an alloy tube made of a Ti-Ni alloy is processed into a superelastic stent, the heat treatment temperature is higher than 475°C but lower than the recrystallization temperature of the alloy tube, preferably higher than 475°C but lower than 500°C, and more preferably 490°C or higher but lower than 500°C, and the heat treatment time is 1 minute or longer but 20 minutes or shorter. Specifically, the heat treatment performed after the stent processing of the alloy tube is performed at the above temperature range for the above period of time. In this way, a superelastic stent with excellent mechanical properties can be manufactured. The superelastic stent obtained by the method for manufacturing a superelastic stent of the present invention is the above-mentioned superelastic stent of the present invention. Furthermore, the alloy tube made of a Ti-Ni alloy used in the method for manufacturing a superelastic stent of the present invention is preferably the above-mentioned alloy tube of the present invention.

[0045] Furthermore, from the viewpoint of improving the mechanical properties of the superelastic stent, it is preferable that the heat treatment temperature in the heat treatment carried out after the stent processing is higher than 475°C and not higher than 500°C.

[0046] Furthermore, the processing rate in the stent processing of the alloy tube is preferably more than 30% and less than 65%. Here, the processing rate of the alloy tube is the ratio of the cross-sectional area reduced by processing to the original cross-sectional area, as defined in JIS H0500:1998. The processing rate is usually expressed as a percentage (%) obtained by dividing the difference between the cross-sectional area Ao of the material before processing and the cross-sectional area A after processing by the cross-sectional area Ao of the material before processing (processing rate = (Ao - A) / Ao × 100%). In the present invention, the cross-sectional area Ao of the pre-processing composite material (tube material in the case of plain wire drawing) measured before the heat treatment step and the cross-sectional area A after processing were evaluated, and the processing rate was calculated using the above formula. At a processing rate of 30% or less, the product can be processed, but the processing rate is insufficient, resulting in insufficient values for both the change in radial force (CR) and the radial force ratio (FR) required in the present invention. On the other hand, at a processing rate exceeding 65%, cracks and breakages occur depending on the outer diameter, making the processing conditions unstable. This is because the processing temperature in each step of the present invention is limited to a limited temperature range.

[0047] Furthermore, the method for producing an alloy tube of the present invention involves performing a straightening heat treatment at 490°C or less after processing to produce an alloy tube made of a Ti-Ni alloy. The alloy tube thus obtained is suitable for use in the method for producing a superelastic stent described above. The alloy tube is made of a Ti-Ni alloy and is preferably seamless. Furthermore, in the method for producing an alloy tube, the straightening heat treatment is preferably the final step. The alloy tube obtained by the method for producing an alloy tube of the present invention is the alloy tube of the present invention described above.

[0048] In the method for producing an alloy tube of the present invention, the treatment temperature for each diameter expansion is preferably less than 475°C, more preferably less than 380°C, based on the characteristic treatment temperature performed after stent processing. If the treatment temperature for each diameter expansion is too low, the stent cannot be expanded, so the treatment temperature is preferably 150°C or higher.

[0049] According to the present invention as described above, by subjecting the alloy tube, which is the base material for the superelastic stent, to cold- or warm-working to introduce a processing texture (processing strain), or by subjecting the cold- or warm-worked tube to straightening at a temperature of 150°C to 490°C, and then subjecting the subsequent post-stent processing treatment to a temperature above 475°C and below the recrystallization temperature of the alloy tube, it is possible to provide a superelastic stent with excellent mechanical properties and a method for manufacturing the same.

[0050] Here, "cold worked" refers to a state in which no temperature adjustment was performed in the straightening treatment before the characteristic treatment. Therefore, examples of cold worked are shown as "none" in the column for straightening treatment before the characteristic treatment in Tables 2 and 3.

[0051] In the present invention, it has been revealed that the excellent properties of a superelastic stent can be achieved by controlling the processing conditions before stent processing, the straightening processing conditions, and the temperature range and cumulative time in the property processing after stent processing.

[0052] Although the embodiments have been described above, the present invention is not limited to the above embodiments, but includes all aspects encompassed by the concept of the present invention and the scope of the claims, and can be modified in various ways within the scope of the present invention. [Example]

[0053] Next, examples and comparative examples will be described, but the present invention is not limited to these examples.

[0054] As shown in Tables 1 to 5, alloy tubes with alloy numbers and outer diameters (processing rates: 60%, 50%, 40%, and 30%) were used. Seamless Ti-Ni alloy tubes were fabricated as described in Patent Document 3, and the manufacturing conditions were as described in Patent Document 6, in which a bar material called a mandrel was inserted into the base material and processed. Here, since small diameter tubes make it difficult for the mandrel material to be removed from the composite material, the tube was sometimes fabricated to the desired size by mandrel drawing or open drawing, a commonly known manufacturing method in which a mandrel material is inserted after each pass to form the composite material. In Tables 2 to 5, RT stands for room temperature. As mentioned above, "none" in Tables 2 and 3 indicates that no temperature adjustment was performed in the straightening process before the characteristic processing, and the tube was cold-worked.

[0055] The examples are cold-worked materials with working rates of 40%, 50%, and 60%, or materials warm-worked at similar working rates at 150°C to 280°C. The present invention tubes include as-worked or warm-worked materials, but also test materials heat-treated at 150°C to 490°C as a post-work straightening treatment. Furthermore, the present invention stents were obtained by subjecting the tubes of the present invention manufactured under the above conditions to a characteristic treatment during stent fabrication. As described above, the temperature conditions for the characteristic treatment are above 475°C but below the recrystallization temperature, preferably above 475°C but not exceeding 500°C, and more preferably between 490°C and 500°C. The cumulative holding time for the heat treatment during the characteristic treatment is highly dependent on the wire diameter of the material, but is preferably between 1 and 20 minutes, with holding times exceeding 20 minutes being unsuitable.

[0056] In Comparative Examples 6, 7, and 8, a Ti-56.1% by mass Ni alloy tube with a diameter of 2.0 mm and a wall thickness of 0.15 mm was used. The processing conditions and straightening conditions were unknown, but the subsequent stent processing conditions were the same as those of the present invention. The characteristic processing conditions for the comparative materials were 500°C, and the cumulative holding times were 30 minutes, 15 minutes, and 5 minutes, respectively.

[0057] In the stent processing of the alloy tube, after a straightening process, the inventor's design was processed using a fiber laser to produce a stent (Reference Figure 3(a)). After laser processing of the alloy tube, a sequential stent expansion process was performed. Here, the diameter was expanded by approximately 1 mm increments, from an outer diameter of 2.0 mm to an outer diameter of 3.0 mm, with the temperature at 370°C and the holding time for approximately 3 minutes. As mentioned above, the processing temperature for each expansion step is preferably less than 475°C, and more preferably less than 380°C, based on the characteristic processing temperature performed after stent processing. Furthermore, if the processing temperature for each expansion step is too low, the stent cannot be expanded, so a temperature of 150°C or higher is preferred.

[0058] After the diameter expansion process was completed, the stent was subjected to a characteristic treatment at a temperature range of more than 475°C and less than 500°C for an accumulated time of 1 to 20 minutes, to obtain a superelastic stent with an outer diameter of 10.0 mm (Reference Figure 3(b)).

[0059] The radial stent expansion force of the superelastic stent was measured using a stent testing device, MSI-RX550 / 650, to obtain the expansion force hysteresis curve of the superelastic stent. Figure 2(a) shows the expansion force hysteresis curve of the superelastic stent of Example 1. Here, the outer diameter conditions for the expansion force hysteresis measurement were as follows: for example, if a 2.0 mm alloy tube was laser-machined to form slots and a superelastic stent with an expanded diameter of 10.0 mm was fabricated, the minimum outer diameter during contraction was slightly smaller than the tube due to the slot space, and the data was measured by expanding the diameter slightly larger to confirm the outer diameter upon recovery during expansion force measurement. This is because allowing a slight error relative to the stent size allows for confirmation of the characteristic behavior at the stent size. Therefore, for the superelastic stent with an deployed outer diameter of 10.0 mm fabricated by expanding from 2.0 mm, the diameter contraction curve was measured in the range of φ1.6 mm, and the recovery curve was measured again in the range of φ11 mm. Furthermore, for the rate of change CR and the rate of contrast FR, the positions of the 1 / 2 point (points A and C) and 3 / 4 point (point B) were determined based on the values at the maximum and minimum outer diameters during measurement of the expansive force hysteresis curve. This was the result of considering how to measure samples processed into stents under the same conditions. For example, since the minimum dimension (pre-processing tube size) of the product of the present invention is known before stent processing, the measurement points for points A, B, and C can be determined using the minimum contracted outer diameter and the maximum recovered outer diameter. On the other hand, when measuring a product after stent processing, the dimensions of the original tube are unknown. Therefore, the conditions for determining the minimum and maximum outer diameters are as follows. If it is difficult to confirm the tube size before processing, the minimum contracted outer diameter is set to the limit size to which the stent can be contracted after laser processing, and the outer diameter at which the expansive force (N) becomes 0 during expansion is set to the maximum recovered outer diameter for expansive force measurement, and points A, B, and C are determined accordingly.

[0060] These test data will be used for preliminary clinical evaluations. In particular, recovery curve 2 represents the actual movement of the stent after the catheter is released within the blood vessel, and allows us to see the expansion force at the target placement site and the expansion force-diameter tracking of the stent as it moves within the blood vessel.

[0061] (Sustainable stent expansion force) The recovery curve of the superelastic stent of Example 1 shown in Figure 2(a) shows the maximum deflection at 1 / 2 (1.6 + (11.0 - 1.6) / 2) of the expansion stroke from φ1.6 to φ11.0 mm, near the midpoint of the deployed stent diameter, φ6.3 mm. The resulting slope of the recovery curve 2 inflects from approximately 10° to 5° around φ6.3 mm and continues beyond φ8.6 mm, which is 3 / 4 (1.6 + (11.0 - 1.6) × 3 / 4) of the expansion stroke. To verify the sustainability of the stent's expansive force after deployment, the rate of change (CR) of expansive force (N) corresponding to the change in diameter from φ6.3 mm to φ8.6 mm, at which the change in deflection can be confirmed, was calculated using the following equation 3. CR = {(φ6.3(N)(Point A) - φ8.6(N)(Point B)) / φ8.6(N)(Point B)} x 100...Equation 3

[0062] Incidentally, the change rate CR of the superelastic stent of Example 1 was {(0.58N-0.40N / 0.40N}×100=45.0%.

[0063] On the other hand, the slope of the recovery curve 2 for the stent treated at 450°C for 15 minutes (Comparative Example 4) shown in Figure 2(b) is constant at approximately 8°, which is thought to narrow the range of blood vessel diameters in which the expansive force of the implanted stent can be maintained. As described above, the rate of change CR in expansive force (N) from φ6.3 mm to φ8.6 mm, at which change in deflection can be confirmed, was calculated for Comparative Example 4 as {(0.75 N - 0.47 N) / 0.47 N} x 100 = 59.6%. Tables 2 and 3 show the expansive force (N) CR% of the stent according to Equation 3.

[0064] (Increasing stent expansion strength) The superelastic stent was created by laser-grooving the inventor's original design into an alloy tube (see Figure 3) (a), followed by expansion into a cellular structure (b). The expansion force was the radial force generated by the stent, which reflected the multiaxial deformations (bending, twisting, compression, etc.) introduced during the stent deployment and expansion process (see Figure 3), as well as the stent design. This differed from the uniaxial stress evaluation of the equivalent material wire tensile test (see Figure 4(b)). In other words, the expansion force hysteresis curve of the test stent (see Figure 2) showed a deflection curve similar to that of a rubber band (see Figure 4(a)), but did not exhibit the flat curve characteristic of superelasticity as seen in the Ti-Ni alloy superelastic material wire (see Figure 4(b)).

[0065] In this experiment, in order to make each stent expansion force test data dimensionless, the degree of proximity between recovery curve 2 and diameter reduction curve 1 of each stent expansion force hysteresis curve was evaluated by calculating the ratio (FR) of recovery curve 2 expansion force (N) (point A) to diameter reduction curve 1 expansion force (N) (point C) at a stent diameter of 6.3 mm for recovery curve 2 using the following equation 4. Contrast ratio FR = φ6.3(N)(point A) / φ6.3(N)(point C) Equation 4

[0066] For example, the contrast FR of Example 2 was 0.73N / 1.19N=0.61.

[0067] The expansive force of the specially processed stents is significantly affected by the alloy tube processing rate and processing time. The expansive force of superelastic stents with processing rates of 60%, 50%, and 40% decreases as the alloy tube processing rate decreases. Furthermore, the expansive force of the straightened commercial stents (Comparative Examples 6, 7, and 8) is governed by the alloy tube straightening temperature (details are unknown, but it is assumed to be above the recrystallization temperature). No difference in expansive force was observed with subsequent processing (5, 15, and 30 minutes) after stent processing. In other words, the expansive force is at a minimum level for 500°C-treated stents after the special processing. Furthermore, Figure 2(c) shows a comparison of the stents of Example 2 and Comparative Example 6, revealing that the stent of Example 2 has two to three times the expansive force of Comparative Example 6. The curve contrast ratio FR of the example stents was calculated using the aforementioned formula 4 and is listed in Tables 2 and 3.

[0068] The object of the present invention is to propose a technology that contributes to increasing the strength of stents, and the range of the present invention is defined as a curve contrast FR of the above formula 4 of 0.50 or more, which exceeds the stents of Comparative Examples 6, 7, and 8, and a recovery force change rate CR of the above formula 3 of 50.0% or less. Comparative Examples were identified as stents that did not satisfy either or both of the above, with FR<0.50 and RC>50.0%. In the present invention, stents that satisfied both the properties of FR≧0.50 and RC≦50.0% were defined as Examples, and those that did not satisfy either or both of the above were defined as Comparative Examples.

[0069] [Table 2]

[0070] [Table 3]

[0071] [Table 4]

[0072] [Table 5]

[0073] (Application of the present invention) As mentioned above, the purpose of this invention is to use a stent expansion processing technology to develop superelasticity with a flat recovery curve and a desired expansive force in a stent. While the scope of the claims of this invention is limited by numerical values, such as expansive force, recovery curve slope, and processing conditions, in order to achieve high strength, it is believed that anyone skilled in the art would be able to easily examine and modify these limits to suit their purposes. Furthermore, Ti-Ni-X alloys, which also exhibit superelasticity in this invention and in which a portion of the Ni or Ti in the Ti-Ni alloy is replaced by X with one or more of the elements Cu, Fe, Cr, Al, V, Pd, Ag, Mn, Co, Nb, Hf, and Zr, are widely applicable, including the uses described herein. [Explanation of symbols]

[0074] 1 Diameter reduction curve 2 Recovery curve B. Expansion force of the recovery curve at 1 / 2 deformation B. Expansion force of recovery curve 2 at 3 / 4 of the deformation C) Expansion force of diameter reduction curve 1 at the point of deformation 1 / 2

Claims

[Claim 1] A superelastic stent made of a Ti-Ni alloy, The Ti—Ni-based alloy contains 55.6% by mass or more and 56.3% by mass or less of Ni, more than 0% by mass but not more than 0.040% by mass of C, more than 0% by mass but not more than 0.040% by mass of O, with the remainder being Ti and inevitable impurities; The superelastic stent is a recovery curve showing the outer diameter expansion force of the superelastic stent when it recovers from the minimum outer diameter to the maximum outer diameter, and a diameter contraction curve showing the outer diameter expansion force of the superelastic stent when it contracts from the maximum outer diameter to the minimum outer diameter, The external diameter expansion force of the superelastic stent at 1 / 2 of the external diameter deformation amount in the recovery curve is 0.50 or more relative to the external diameter expansion force of the superelastic stent at 1 / 2 of the external diameter deformation amount in the diameter reduction curve; and the maximum rate of change in the outer diameter expansion force of the superelastic stent within a range from 1 / 2 of the outer diameter deformation to 3 / 4 of the outer diameter deformation in the recovery curve is within 50.0% based on the outer diameter expansion force of the superelastic stent at 3 / 4 of the outer diameter deformation in the recovery curve. Has outer diameter expansion force characteristics, Superelastic stent.

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