Superelastic stent, manufacturing method of superelastic stent, alloy tube and manufacturing method of alloy tube

Abstract

To provide a stent with improved characteristics through a study of a comprehensive process technique from tube processing to stent processing, and a processing technique thereof.SOLUTION: A superelastic stent is composed of a Ti-Ni-based alloy. In an expansion force hysteresis curve composed of a recovery curve showing an outer diameter expansion force at a diameter recovery of the superelastic stent from a minimum outer diameter to a maximum outer diameter, and a diameter reduction curve showing an outer diameter expansion force at a diameter reduction of the superelastic stent from the maximum outer diameter to the minimum outer diameter, an outer diameter expansion force of the superelastic stent at an outer diameter deformation volume 1 / 2 in the recovery curve is 0.50 or more with regard to an outer diameter expansion force of the superelastic stent at an outer diameter deformation volume 1 / 2 in the diameter reduction curve. A maximum rate of change of an outer diameter expansion force in the recovery curve of the superelastic stent within a range from the outer diameter deformation volume 1 / 2 to an outer diameter deformation volume 3 / 4 is 50.0% or less based on the outer diameter expansion force of the superelastic stent at the outer diameter deformation volume 1 / 2 in the recovery curve.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a stent used in percutaneous transluminal angioplasty for the treatment of stenotic blood vessels, and in particular to a self-expanding stent and its manufacturing method made of a Ti-Ni alloy exhibiting superelasticity as its base material, 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, where 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] Stent therapy is a medical technology whose applications have expanded rapidly in recent years. A stent is a metal mesh that is placed in a blood vessel to prevent restenosis after vasodilation therapy, and is incorporated into the tip of a catheter. During therapy, the catheter is introduced into the stenosis, and then the stent is detached from the catheter and attached to the inner wall of the lumen.

[0004] Their functions can be divided into balloon expansion types that use a balloon and self-expanding types that utilize their own spring properties. The former are mainly used to treat coronary artery stenosis, and the raw material of the tube before processing is mainly 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 such as Ti-Ni alloys exhibit a remarkable shape memory effect accompanying the reverse transformation of martensitic transformation, and also exhibit good superelasticity due to the appearance of stress-induced martensite phase caused by strong deformation after reverse transformation. These functions are particularly evident in Ti-Ni alloys and Ti-Ni-X alloys (X = V, Cr, Nb, Co, etc.) among many other shape memory alloys, and are widely used in the medical field, construction, automobiles, etc.

[0006] For medical use of Ti-Ni alloys for placement in the body, the composition is specified as Ni: 53.5-57.5 mass%, with the remainder being Ti, as specified in Non-Patent Documents 1 and 2. For this reason, all base tubes for commercial stent use are Ti-Ni alloys of the above-mentioned standard, and Ti-Ni-X alloys, in which a third element X is added to a Ti-Ni alloy, are to be discussed between the parties involved. Meanwhile, in the study materials, proposals for using Ti-Ni alloys in self-expanding stents are shown in Patent Documents 1 and 2, and a proposal for improving the properties of the base tubes, as in the present invention, is described in Patent Document 3, which proposes improving the properties by changing the base Ti-Ni alloy for stents to an alloy containing Nb, 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] In addition, it is known from Non-Patent Document 3 that Ti-Ni-X alloys (X=V, Cr, Co, Cu, Nb, Ta, Hf, etc.) containing a third element can exhibit properties similar to those of Ti-Ni alloys depending on the amount of addition.

[0010] In terms of tube processing technology, seamless metal tubes, which are 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 rolling or drawing (wire drawing) the composite material (tube + mandrel). During processing, the mandrel is removed from the tube for each processing pass, and in the case of smaller diameters, it is often processed in combination with dry drawing and drawing, and then heat treated for the purpose of straightening.

[0011] On the other hand, a method for manufacturing a metal tube with a uniform cross-sectional shape is also known in which a composite (tube + core) material (tube + core) is formed by inserting a metallic core material into the tube material (original material) that has been punched as described above, and then the composite material is drawn to form a tube, and the core material is removed from the composite material to form a tube. However, in this manufacturing process, the removal of the core material in the final step remains a major problem. The tube material and core material of the drawn clad tube are firmly attached to each other, and the smaller the diameter and the longer the clad tube is, the greater the frictional resistance when removing the core material becomes, making it difficult to simply pull out and remove the core material.

[0012] Therefore, methods proposed for removing the core material include selectively melting 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 processing a metal seamless tube into a small diameter, using a shape memory alloy such as a Ti-Ni alloy as the base material. Specifically, a composite material is formed using a core material having the same ductility as the metal tube (base material), and the composite material is stretched to form a composite wire (assembly). Thereafter, only the core material of the composite material is stretched to reduce its diameter, and the reduced core material is pulled out to produce a metal seamless tube. Furthermore, it describes that the core material can be easily stretched by heat treating the composite tube at about 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 of a shape memory alloy, the cylinder and the mandrel are subjected to area reduction processing together, and the mandrel is removed after heat treatment. This document discloses a technology that allows the tube to be expanded and the core to be removed by simultaneous area reduction processing 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). Furthermore, this document describes the manufacturing conditions for the tube, 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 Patent Application 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 by the phase transformation of the alloy crystals, and for this transformation, the movement trapped by the processing strain caused by wire drawing or rolling must be restored by subsequent strain relief heat treatment. For practical medical devices such as guide wires and stents, aging treatment conditions (treatment below the recrystallization temperature that leaves processing strain) are set according to their purpose. In addition, Ti-Ni alloy materials include wires for coil stents and tubes for laser-processed stents, but the problem to be solved by this invention is related to tubes that can be widely applied to coronary artery stents, peripheral stents, etc.

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

[0019] The problem with current stent processing is that there is no option to add suitable properties to the current processing process. In other words, medical manufacturers simply repeat the prescribed expansion process on the straightened tube provided by the material manufacturer to make a stent of the desired shape, and are unable to consider processing methods that suit 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 are 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 more.

[0021] Figure 4 shows the stress hysteresis curves of a commercially available rubber band and the superelastic material (superelastic stent) used in this experimental test 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 caused by interstitial friction and other factors resulting from strain loading appears as a bend (deflection) in the hysteresis curve. The superelastic material used in this experimental test shown in Figure 4(b) has a flat curve associated with the superelasticity induced by strain loading, but shows a broader hysteresis curve with a larger loss than that shown in Figure 4(a).

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

[0023] In order to solve the above-mentioned problems, the present invention provides a stent with improved characteristics and a processing technique for the same, by examining comprehensive process techniques from tube processing to stent processing, which is a material that breaks away from the use of heat-treated tubes as commercial materials up to now. [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 the outer diameter deformation amount in the recovery curve is 0.50 or more relative to the outer diameter expansion force of the superelastic stent at 1 / 2 the outer 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 the outer diameter deformation amount to 3 / 4 the outer diameter deformation amount in the recovery curve is within 50.0% based on the outer diameter expansion force of the superelastic stent at 1 / 2 the outer diameter deformation amount in the recovery curve. [2] An alloy tube made of a Ti-Ni alloy and serving as the base material for the superelastic stent described in [1] above. [3] A method for manufacturing a superelastic stent, comprising the steps of: (a) processing a Ti-Ni alloy tube into a superelastic stent; (b) conducting a heat treatment after the stent processing, the heat treatment temperature is higher than 475°C and lower than the recrystallization temperature of the alloy tube; and (c) conducting a heat treatment time of 1 minute or more and 20 minutes or less. [4] A method for producing a superelastic stent as described in [3] above, wherein the heat treatment temperature is higher than 475°C and lower than 500°C. [5] A method for manufacturing a superelastic stent as described in [3] or [4] above, wherein the processing rate of the alloy tube in the stent processing is greater than 30% and less than 65%. [6] A method for manufacturing an alloy tube, comprising the steps of: performing a straightening heat treatment at 490°C or lower after processing; and producing an alloy tube made of a Ti-Ni alloy. Effect 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 producing the same, as well as an alloy tube which is a base material for a stent and a method for producing the same. [Brief description 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. [Diagram 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. [Diagram 3] 1 is a conceptual diagram of a self-expanding stent according to the present invention, in which (a) shows the alloy tube after processing, and (b) shows the process of expanding the diameter of the stent after placement. [Figure 4] 1 shows stress hysteresis curves in tensile tests of the superelastic stent of 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 a Ti-Ni alloy superelastic material treated at 500°C. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[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. The Ti-Ni alloy exhibits superelasticity, in which recovery from applied deformation occurs simultaneously with the release of external force. In this case, the main structure of the Ti-Ni alloy used in the present invention is an austenite phase (high temperature phase) at a temperature higher than the shape recovery temperature. Therefore, the Ti-Ni alloy required in the present invention has a Ni concentration of 55.6 mass% or more and 56.3 mass% or less to become an austenite phase. Furthermore, if it is within the range of the properties required in the present invention, 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 mass%, with the remainder being Ti and unavoidable impurities.

[0029] Among them, in order to realize the desired workability and expansion force, it is preferable that the Ti-Ni alloy contains more than 0 mass% and 0.040 mass% or less of C, more than 0 mass% and 0.040 mass% or less of O, and the balance is composed of Ti and inevitable impurities. Here, inevitable impurities refer to impurities that may be inevitably contained in the manufacturing process. For example, as an inevitable impurity, N may be contained in an amount of 0.005 mass% or less.

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

[0031] C (carbon) is an element that forms nonmetallic inclusions. When the C content is high, the number of nonmetallic inclusions in the matrix increases, and the occupancy rate of the 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. When 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 by a conventionally known analytical device, and is specified in Non-Patent Document 2. The present invention shows the measured values ​​conforming to Non-Patent Document 2 as the alloy composition range of Ti-Ni alloys. The alloy composition of the Ti-Ni alloys used in the present invention is shown in Table 1. Table 5 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 (base tube) of the present invention as the base material (material). The alloy tube of the present invention is laser grooved based on a unique design, and then expanded (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 about 10.0 mm. On the other hand, in the present invention, in order to confirm whether or not there is a change in characteristics due to size change, the characteristics of alloy tubes with an outer diameter of 0.3 mm and an outer diameter of 6.0 mm were also evaluated, and the outer diameters of the superelastic stents after expansion 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 of the superelastic stent of the present invention, which is composed of 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 outer diameter expansion force of the superelastic stent at 1 / 2 (one half) the outer diameter deformation amount of the superelastic stent in the recovery curve (point A) is 0.50 or more (curve contrast FR) relative to the outer diameter expansion force of the superelastic stent at 1 / 2 (one half) the outer diameter deformation amount of the superelastic stent in the diameter contraction curve (point C). Furthermore, in the above-mentioned expansion force hysteresis curve of the superelastic stent of the present invention, the maximum rate of change (rate of change CR) in the external diameter expansion force within the range from 1 / 2 (one half) of the external diameter deformation of the superelastic stent (point A) to 3 / 4 (three quarters) of the external diameter deformation (point B) in the recovery curve is within 50.0% based on the external diameter expansion force at 1 / 2 (one half) of the external diameter deformation of the superelastic stent (point A) in the recovery curve. That is, in the expansion force hysteresis curve of the superelastic stent of the present invention, (external diameter expansion force at point A) / (external diameter expansion force at point C) is 0.50 or more, and the maximum rate of change in the external diameter expansion force within the range from point A to point B ((external diameter expansion force farthest from the external diameter expansion force at point A)×100 / (external diameter expansion force at point A)) is within 50.0% based on the external diameter expansion 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 at which the diameter has expanded (recovered) by 1 / 2 (one half) from the minimum outer diameter and the points at which 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. Also, the outer diameter deformation amount 1 / 2 (point C) on the diameter reduction curve refers to the point at which 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 recovery curve at the outer diameter of 6.3 mm and 8.6 mm.

[0040] The formula for calculating the expansion processing stroke 1 / 2 points (points A and C) is as follows, when the outer diameter before expansion is A and the outer diameter after expansion is B: 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 will be 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···Formula-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 will be when the outer diameter is 8.6 mm.

[0042] In order to ensure that the evaluation levels after expansion are the same, the tube outer diameter of φ0.3 mm before stent processing was 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 on diameter reduction curve 1 and recovery curve 2 and 1.25 mm diameter on recovery curve 2, and the tube outer diameter of φ6.0 mm before stent processing was 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 on diameter reduction curve 1 and recovery curve 2 and 24.5 mm diameter on 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, but for example, when a Ti-Ni alloy tube with a diameter of φ1.6 mm is used, the size of the superelastic stent can be manufactured from φ5.0 mm to φ30.0 mm. In the present invention, a Ti-Ni alloy tube with a diameter of φ0.3 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 φ1.5 mm to φ30.0 mm.

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

[0045] 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 500°C or lower.

[0046] In addition, 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 described 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 x 100%). In the present invention, the cross-sectional area Ao of the composite material before processing (tube material in the case of blank wire drawing) measured before the heat treatment process and the cross-sectional area A after processing are evaluated, and the processing rate is determined using the above calculation formula. At a processing rate of 30% or less, the product can be processed, but the processing rate is insufficient, so that both the change in radial-force (CR) and the radial force ratio (FR) required in the present invention are insufficient. On the other hand, at a processing rate exceeding 65%, cracks and breaks occur depending on the outer diameter size, and the processing conditions are not stable. This is because the processing temperature in each step of the present invention is limited to a limited temperature range.

[0047] In addition, in the method for producing an alloy tube of the present invention, a straightening heat treatment is performed at 490°C or less after processing to produce an alloy tube made of a Ti-Ni alloy. The alloy tube thus obtained is suitably used in the method for producing a superelastic stent described above. The alloy tube is made of a Ti-Ni alloy and is preferably seamless. 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 a straightening process at a temperature of 150°C to 490°C, and then setting the temperature condition for the subsequent property treatment after the stent processing 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, the term "cold worked" refers to a state in which no temperature adjustment was performed in the straightening treatment before the characteristic treatment. Therefore, examples of the cold worked state are shown as "none" in the column for the straightening treatment before the characteristic treatment in Tables 2 and 3.

[0051] In the present invention, it has become clear that it is possible to develop the excellent properties of a superelastic stent by controlling the processing conditions and straightening processing conditions before the stent is processed, as well as the temperature range and accumulated time in the property processing after the stent is processed.

[0052] Although the embodiments have been described above, the present invention is not limited to the above-described embodiments, and 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. EXAMPLES

[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 having alloy numbers and outer diameters (processing rates: 60%, 50%, 40%, 30%) were used. Seamless Ti-Ni alloy tubes were produced as shown in Patent Document 3, and the manufacturing conditions were as shown in Patent Document 6, in which a bar material called a core metal was inserted into the base material and processed. Here, in the case of a small diameter, the core metal material cannot be removed from the composite material, so in some cases, a commonly known manufacturing method in which a core metal material is inserted in each pass to form a composite material, such as mandrel drawing or empty drawing, was performed to produce a tube to a predetermined size. In Tables 2 to 5, RT is room temperature. Also, as mentioned above, "none" in Tables 2 to 3 means that there was no temperature adjustment in the straightening treatment before the characteristic treatment, 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 150°C to 280°C with similar working rates. The present invention tube material includes as-worked or warm-worked materials, and further includes test materials heat-treated in the range of 150°C to 490°C as a straightening treatment after working. In addition, the present invention stent was obtained by performing a characteristic treatment during stent processing using the present invention tube produced under the above conditions. As described above, the temperature condition of the characteristic treatment is more than 475°C and less than the recrystallization temperature, preferably more than 475°C and less than 500°C, and more preferably 490°C or more and less than 500°C. Here, the cumulative holding time of the heat treatment during the characteristic treatment is largely dependent on the wire diameter of the material, but is preferably 1 to 20 minutes, and a holding time of more than 20 minutes is not suitable.

[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, and the processing conditions and straightening processing conditions were unknown, but the subsequent processing conditions for the stent were the same as those of the present invention. The characteristic processing conditions for the comparative material 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 the straightening process, the stent processing was finished by the fiber laser with the design held by the inventor (Reference Figure 3 (a)). After the alloy tube was laser processed, a sequential stent expansion process was performed. Here, the diameter was expanded by about 1 mm at a time, such as from an outer diameter of 2.0 mm to an outer diameter of 3.0 mm, and the processing was performed at 370°C and held for about 3 minutes for each expansion. As described above, the processing temperature for each expansion is preferably less than 475°C, and more preferably less than 380°C, based on the characteristic processing temperature performed after the stent processing. In addition, the processing temperature for each expansion is preferably 150°C or more, because if it is too low, the stent cannot be expanded.

[0058] After that, after the diameter expansion process was completed, the material 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 tester MSI-RX550 / 650, and the expansion force hysteresis curve of the superelastic stent was obtained. Figure 2(a) shows the expansion force hysteresis curve of the superelastic stent of Example 1. Here, the outer diameter condition during expansion force hysteresis measurement is, for example, when a 2.0 mm alloy tube is laser processed to have a slot (slot) and a superelastic stent with an expansion diameter of 10.0 mm is produced, the minimum outer diameter during contraction is slightly smaller than the tube by the slot space, and the data is measured by expanding it slightly larger to confirm the outer diameter during expansion force measurement recovery. This is because having a slight error compared to the stent size makes it possible to confirm the characteristic behavior at the stent size. Therefore, the present invention of the superelastic stent with an indwelling outer diameter of 10.0 mm produced by expanding from 2.0 mm was measured as a contraction curve with a diameter range of φ1.6 mm, and a recovery curve with an indwelling diameter of 11 mm again. In addition, for the rate of change CR and the rate of contrast FR, the positions of the 1 / 2 point (point A, point C) and the 3 / 4 point (point B) were confirmed from the values ​​of the maximum outer diameter point and the minimum outer diameter point when measuring the expansion force hysteresis curve. This is the result of considering so that the sample after processing into a stent can be measured under the same conditions. For example, since the minimum dimension (tube size before processing) of the product of the present invention is known before processing into a stent, the measurement points of points A, B, and C can be determined using the minimum outer diameter when reduced in diameter and the maximum outer diameter when recovered. On the other hand, when measuring a product after stent processing is completed, the dimensions of the original tube are unknown. Therefore, the determination conditions for determining the numerical values ​​of the minimum outer diameter and the maximum outer diameter are shown below. If it is difficult to confirm the tube size before processing, the limit size at which the stent can be reduced by reducing the diameter after laser processing is set as the minimum outer diameter when reduced in diameter, and the outer diameter at the point when the expansion force (N) becomes 0 when expanded is set as the maximum outer diameter when recovered in expansion force measurement, and points A, B, and C are determined.

[0060] These test data will be used for pre-clinical evaluation. In particular, recovery curve 2 represents the actual movement of the stent after the catheter is released within the blood vessel, and allows the observation of the expansive force at the target placement site and the expansive force-diameter tracking of intravascular movement.

[0061] (Sustainability of 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 diameter of the deployed stent, φ6.3 mm. The slope of the resulting recovery curve 2 inflects from approximately 10 ° to 5 ° at around φ6.3 mm, and continues beyond φ8.6 mm, which is 3 / 4 (1.6 + (11.0 - 1.6) x 3 / 4) of the expansion stroke. In order to verify the sustainability of the stent expansive force after deployment in the body, the change rate (CR) of the expansive force (N) corresponding to the diameter change from φ6.3 mm to φ8.6 mm, which allows confirmation of the change in the amount of deflection, was calculated using the following formula-3. CR = {(φ6.3(N)(point A)-φ8.6(N)(point B)) / φ8.6(N)(point B)}×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 stent treated at 450°C for 15 minutes (Comparative Example 4) shown in Figure 2(b) has a constant slope of recovery curve 2 of approximately 8°, which is considered to narrow the range of blood vessel diameter in which the expansive force of the placed stent can be maintained. As described above, when the degree of change in expansive force (N) CR from φ6.3mm to φ8.6mm, at which the change in deflection amount can be confirmed, was calculated, the degree of change CR for Comparative Example 4 was {(0.75N-0.47N) / 0.47N}×100=59.6%. Tables 2-3 show the expansive force (N) CR% of the stent according to the present embodiment based on formula 3.

[0064] (Increasing stent expansion strength) The superelastic stent was expanded into a cellular structure (b) after laser groove processing (a) of the inventor's original design on an alloy tube as shown in Figure 3. The expansion force was the radial force generated by the stent, which reflected the multiaxial deformations such as bending, twisting, and compression that were incorporated during the indwelling stent expansion process shown in Figure 3, as well as the stent design, and was different from the uniaxial stress evaluation of the equivalent material wire tensile test shown in Figure 4(b). In other words, the expansion force hysteresis curve of this test stent in Figure 2 showed a deflection curve similar to that of a rubber band in Figure 4(a), and did not show the flat curve characteristic of superelasticity as shown in Figure 4(b) for the Ti-Ni alloy superelastic material wire.

[0065] In this experiment, in order to non-dimensionalize the stent expansion force test data, 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 φ6.3 mm for recovery curve 2 using the following formula 4. Contrast ratio FR = φ6.3(N)(point A) / φ6.3(N)(point C)···Equation 4

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

[0067] The expansion force of the characteristically treated stent is greatly affected by the alloy tube processing rate and processing time. The superelastic stents with processing rates of 60%, 50% and 40% have a lower expansion force as the processing rate of the alloy tube decreases. In addition, the expansion force of the straightened commercial material stent (Comparative Example 6, Comparative Example 7, Comparative Example 8) is governed by the straightening processing temperature of the alloy tube (details are unknown, but it is assumed to be above the recrystallization temperature), and no difference in expansion force is observed in the additional processing (5 minutes, 15 minutes and 30 minutes) after the stent processing, that is, it can be said to be the minimum level in the 500 ° C. treated product after the characteristic processing. Furthermore, from the stent comparison diagram of Example 2 and Comparative Example 6 in Figure 2 (c), it can be seen that the stent of Example 2 has 2 to 3 times the expansion force of Comparative Example 6. The curve contrast degree FR of the example stent is calculated from the above-mentioned formula-4 and is listed in Tables 2 to 3.

[0068] The object of the present invention is to propose a technique that contributes to increasing the strength of stents, and the scope of the present invention is that the curve contrast FR of the above formula-4 is 0.50 or more, which exceeds the stents of Comparative Examples 6, 7, and 8, and the expansion force change rate CR of the recovery curve 2 of the above formula-3 is 50.0% or less. As comparative examples, stents with FR<0.50 and RC>50.0% were confirmed as not satisfying either or both of the above. In the present invention, those that satisfy both characteristics of FR≧0.50 and RC≦50.0% are considered examples, and those that do not satisfy either or both of the above are considered 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 the present invention is to make a stent exhibit superelasticity with a flat recovery curve and an arbitrary expansion force by using a stent expansion processing technique. In the present invention, the claims are numerically limited in terms of expansion force, recovery curve slope angle, processing conditions, etc., in order to achieve high strength, but it is believed that anyone skilled in the art can easily examine and scrutinize deviations and modifications according to the purpose. In addition, Ti-Ni-X alloys, which are obtained by replacing a part of Ni or Ti in a Ti-Ni alloy with X by one or more elements selected from Cu, Fe, Cr, Al, V, Pd, Ag, Mn, Co, Nb, Hf, and Zr, and which also exhibit superelasticity in the present invention, can be widely applied, including the uses of the present invention. [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 deformation C) Expansion force of diameter reduction curve 1 at the point of deformation amount 1 / 2

Claims

1. An alloy tube made of a Ti-Ni alloy containing 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 unavoidable impurities, is processed into a superelastic stent, and the processing rate before the stent processing process is, as described in JIS H0500:1998, a ratio of the cross-sectional area reduced by processing to the original cross-sectional area is more than 30% but not more than 65%, The temperature during laser processing exceeds 100°C. In the heat treatment performed after the stent processing, The heat treatment temperature is higher than 475° C. and not higher than 500° C. The heat treatment time is from 1 minute to 20 minutes. A method for manufacturing a superelastic stent.

2. A method for manufacturing a superelastic stent as described in claim 1, in which a straightening heat treatment of 490°C or less is performed after the stent processing treatment.

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