Embolization coil and method for producing embolization coil

Abstract

The present invention is an embolization coil having an optimum morphological stability. The embolization coil includes a wire material made of an Au—Pt alloy. The wire material constituting the embolization coil has such a composition that a Pt concentration is 24 mass % or more and less than 34 mass %, with the balance being Au. The wire material has such a material structure that a Pt-rich phase of an Au—Pt alloy having a Pt concentration of 1.2 to 3.8 times a Pt concentration of an α phase is distributed in an α phase matrix. The wire material has a bulk susceptibility of −13 ppm or more and −5 ppm or less. In a material structure of a transverse cross-section of the wire material, an average value of two or more average crystal particle diameters measured by a linear intercept method is 0.20 μm or more and 0.35 μm or less.

Claims

1. An embolization coil comprising a wire material made of an Au—Pt alloy, wherein the wire material constituting the embolization coil has such a composition that a Pt concentration is 24 mass % or more and less than 34 mass %, with the balance being Au, and the wire material has such a material structure that a Pt-rich phase of an Au—Pt alloy having a Pt concentration of 1.2 to 3.8 times a Pt concentration of an α phase is distributed in an α phase matrix, wherein the wire material has a bulk susceptibility of −13 ppm or more and −5 ppm or less, wherein the wire material has a tensile strength of 800 MPa or more, wherein in a material structure of a transverse cross-section of the wire material there are a plurality of crystal particles with diameters where the average diameter of the crystal particles measured by a linear intercept method is 0.20 μm or more and 0.35 μm or less, and the standard deviation of the crystal particles' diameters is 0.025 μm or more and 0.085 μm or less, wherein the embolization coil has a spiral primary coil shape and a secondary coil shape, and wherein the embolization coil's secondary coil shape has an inner diameter D.sub.1 when the embolization coil is loaded to a 1.2 mm diameter core bar and an inner diameter D.sub.2 when unloaded from the core bar, the embolization coil has an inner-diameter return rate (K.sub.D) of 50% or less, which is represented by the following formula:
K.sub.D (%)=((D.sub.2−D.sub.1)/D.sub.1)×100.

2. The embolization coil according to claim 1, wherein the wire material made of the Au—Pt alloy has a wire diameter of 10 μm or more and 100 μm or less.

3. A method for producing an embolization coil according to claim 1, wherein the embolization coil has a secondary coil shape, the method comprising performing a winding processing step at least once, the winding processing step comprising subjecting a wire material made of an Au—Pt alloy to winding processing, the wire material having such a composition that a Pt concentration is 24 mass % or more and less than 34 mass %, with the balance being Au, the wire material comprising a Pt-rich phase of an Au—Pt alloy distributed in an α phase matrix, the Au—Pt alloy of the Pt-rich phase having a Pt concentration of 1.2 to 3.8 times a Pt concentration of an α phase, the wire material having a bulk susceptibility −13 ppm or more and −5 ppm or less, wherein the morphological stability treatment is performed when the winding processing step comprises forming the secondary coil shape, and wherein the winding processing step comprises performing morphological stability treatment at least once, the morphological stability treatment comprising, with the wire material secured in a wound state, heating and holding the wire material at a temperature of 350° C. or higher and 550° C. or lower.

4. The method for producing the embolization coil according to claim 3, comprising processing a material of the Au—Pt alloy at a processing rate of 50% or more so as to produce the wire material to be processed into the embolization coil.

5. A method for producing an embolization coil according to claim 2, the method comprising performing a winding processing step at least once, the winding processing step comprising subjecting a wire material made of an Au—Pt alloy to winding processing, the wire material having such a composition that a Pt concentration is 24 mass % or more and less than 34 mass %, with the balance being Au, the wire material comprising a Pt-rich phase of an Au—Pt alloy distributed in an α phase matrix, the Au—Pt alloy of the Pt-rich phase having a Pt concentration of 1.2 to 3.8 times a Pt concentration of an α phase, the wire material having a bulk susceptibility −13 ppm or more and −5 ppm or less, wherein the winding processing step comprises performing morphological stability treatment at least once, the morphological stability treatment comprising, with the wire material secured in a wound state, heating and holding the wire material at a temperature of 350° C. or higher and 550° C. or lower.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a phase diagram of an Au—Pt system alloy.

(2) FIG. 2 illustrates transverse cross-sectional structures of alloy wire materials that have undergone morphological stability treatment.

(3) FIG. 3 shows photographs of external appearances of secondary coils produced with morphological stability treatment performed at different treatment temperatures.

(4) FIG. 4 illustrates results of measurement of inner-diameter return ratios and bulk susceptibilities of the secondary coils produced at different treatment temperatures.

(5) FIG. 5 illustrates results of measurement of tensile strengths and bulk susceptibilities of the secondary coils produced at different treatment temperatures.

MODES FOR CARRYING OUT THE INVENTION

(6) An embodiment of the present invention will be described. In this embodiment, an Au—Pt alloy ingot having a proper magnetic susceptibility was produced, and this Au—Pt alloy ingot was used as a material processed into a wire material. Then, the wire material was processed into a secondary coil shape. In this manner, an embolization coil was produced. In this coil processing step, which involved two times of winding processing, morphological stability treatment was performed during the secondary coil processing. In this embodiment, the morphological stability treatment was performed under a plurality of different conditions to produce different embolization coils. Then, the embolization coils were evaluated as to properties such as coil morphological stability and bulk susceptibility. Also, the embolization coils were examined as to how the constituent material structures of the embolization coils differed from each other.

(7) Production of Au—Pt Alloy Material

(8) In this embodiment, an Au—Pt alloy having a Pt concentration of 30 mass % was produced. Pure Au and pure Pt (99.99% pure products of TANAKA KIKINZOKU KOGYO K.K.) were weighed to an intended composition, and the resulting alloy was subjected to high-frequency melting and casted into an alloy ingot. The alloy ingot was produced at a standard weight of 60 g. The alloy ingot obtained by melting and casting was subjected to hot forging at a forging temperature of 1000° C.

(9) Next, the alloy ingot was subjected to single-phase treatment, and thus a supersaturated solid solution alloy of single α phase was produced. First in the single-phase treatment, the alloy ingot was subjected to cold working, namely, cold groove rolling (at a processing rate of 40%). Then, the alloy ingot was heated at 1200° C. for one hour or longer. Then, the alloy ingot was put into water, where the alloy ingot was rapidly cooled. This single-phase treatment, which was a combination of cold working and heat treatment, was performed three times.

(10) Then, the alloy that has undergone the single-phase treatment was subjected to drawing processing. After the drawing processing, the alloy was subjected to heat treatment so that a Pt-rich phase was deposited. The heat treatment temperature was set at 800° C. In the heat treatment, the alloy was heated and, after a period of time, put into iced water, where the alloy was rapidly cooled. As a result of this heat treatment, an Au—Pt alloy material (wire diameter: 2 mm) was obtained.

(11) Production of Au—Pt Alloy Wire Material

(12) The Au—Pt alloy material produced in the above-described manner was processed into an Au—Pt alloy wire material that was to be processed into an embolization coil. In the processing to obtain the wire material, the Au—Pt alloy material was subjected to cold drawing processing using a plurality of dies. In this manner, a wire material (wire diameter: 38 μm) made of the Au—Pt alloy was produced. The processing rate was 99.97%.

(13) Production of Embolization Coil

(14) Next, the Au—Pt alloy wire material produced in the above-described manner was subjected to winding processing twice, and thus an embolization coil having a secondary coil shape was produced. First, primary processing was performed. In the primary processing, the Au—Pt alloy wire material (wire diameter: 38 μm) was wound around a fine core wire (outer diameter of 1 mm, made of SUS304). As a result, the Au—Pt alloy wire material was processed into a spiral wire material (primary coil) having an outer diameter of 0.25 mm and an inner diameter of 0.18 mm. This spiral wire material was wound around and fixed to a core bar (outer diameter of 1.2 mm, made of SUS304). In this secondary coil processing, the spiral wire material was processed into a secondary coil shape with reference values (target values) of the outer diameter and the inner diameter set at 2.0 mm and 1.5 mm, respectively.

(15) During the secondary coil processing, morphological stability treatment was performed. In the morphological stability treatment, the wire material (primary coil) wound around and fixed to the core bar was placed on a heat treatment boat and inserted into a horizontal tube furnace, where the wire material was subjected to heat treatment. The morphological stability treatment was attempted at a plurality of heat treatment temperatures in the range of 100° C. to 800° C. for a common heat treatment period of time of 30 minutes.

(16) In the coil processing of the Au—Pt alloy wire material, a sample obtained at each of the temperatures of the morphological stability treatment was subjected to various kinds of measurement and evaluation, namely, structure observation, measurement of average crystal particle diameter, and evaluation of magnetic susceptibility and morphological stability.

(17) Structure Observation and Measurement of Average Crystal Particle Diameter

(18) A transverse cross-section of the Au—Pt alloy wire material that has undergone the secondary coil processing was subjected to SIM (scanning ion microscope) image observation. In the SIM image observation, the Au—Pt alloy wire material was precisely cut by FIB processing (focused ion beam processing, equipment name: FB-2000A) to form an observation surface. The structure of the obtained cross-section was observed through an SIM image, and the average crystal particle diameter of the cross-section was measured based on the SIM image. In this embodiment, the average crystal particle diameter was measured based on an SIM image represented by a field of vision of 8-by-8 μm at a magnification of approximately 16000 times.

(19) How the average crystal particle diameter is measured by the linear intercept method will be described by referring to cross-sectional structures of Au—Pt alloy wire materials obtained by coil processing with morphological stability treatment performed during the coil processing at 300° C., 500° C., and 700° C. FIG. 2 illustrates transverse cross-sectional structures of alloy wire materials treated at the above-described temperatures. The average crystal particle diameter is measured by the linear intercept method according to the following work procedure. (i) On a structure photograph, draw lines along grain boundaries using a pen or another writing instrument so as to make the grain boundaries more recognizable. (ii) Draw a plurality of lines on arbitrary portions of the photograph. (iii) Count the number (n.sub.c) of points (intersections) at which the lines in (ii) and the grain boundaries meet. (iv) Compare the length of each of the lines in (ii) against a scale in an observation region and correct the length to an actual length (L). (v) Calculate an average crystal particle diameter (R.sub.A) by solving the following formula.
R.sub.A=L/n.sub.g  [Mathematical formula 1]

(20) In the above-described measurement of the average crystal particle diameter by the linear intercept method, a plurality of lines are drawn in (ii), and the average crystal particle diameter is calculated on each of the lines. In this embodiment, 30 lines were drawn, and average crystal particle diameters were calculated on the 30 lines. Then, the resulting 30 pieces of data were used to calculate an average value (X.sub.RA) and a standard deviation (σ.sub.RA). The standard deviation (σ.sub.RA) is calculated by solving the following formula.
X.sub.RA=(R.sub.A1+R.sub.A2+ . . . R.sub.An)/n
σ.sub.RA.sup.2=[(R.sub.A1−X.sub.RA).sup.2+(R.sub.A2−X.sub.RA).sup.2+ . . . (R.sub.An−X.sub.RA).sup.2]/n  [Mathematical formula 2] [R.sub.An=n-th R.sub.A (in this embodiment, n=1 to 30), n=number of pieces of data (in this embodiment, n=30)]

(21) In this embodiment, average crystal particle diameters of wire materials subjected to morphological stability treatment at 300° C., 500° C., and 700° C. were measured. Tables 1 to 3 list exemplary results of average crystal particle diameters measured. Also in this embodiment, average crystal particle diameters of wire materials subjected to morphological stability treatment at 350° C. and 550° C. were measured, and average crystal particle diameters of wire materials in pre-coil processing state (that is, wire materials without morphological stability treatment) were measured.

(22) TABLE-US-00001 TABLE 1 R.sub.A Heat Average X.sub.RA treatment crystal R.sub.A R.sub.A Average temper- particle Maximum Minimum value σ.sub.RA ature diameter value value of R.sub.A Standard (° C.) n (μm) (μm) (μm) (μm) deviation 300 1 0.19 0.25 0.15 0.19 0.023 2 0.20 3 0.22 4 0.18 5 0.19 6 0.16 7 0.21 8 0.18 9 0.20 10 0.21 11 0.16 12 0.15 13 0.20 14 0.22 15 0.23 16 0.25 17 0.22 18 0.20 19 0.21 20 0.22 21 0.18 22 0.19 23 0.16 24 0.16 25 0.17 26 0.19 27 0.20 28 0.21 29 0.18 30 0.21

(23) TABLE-US-00002 TABLE 2 R.sub.A Heat Average X.sub.RA treatment crystal R.sub.A R.sub.A Average temper- particle Maximum Minimum value σ.sub.RA ature diameter value value of R.sub.A Standard (° C.) n (μm) (μm) (μm) (μm) deviation 500 1 0.30 0.62 0.21 0.30 0.081 2 0.25 3 0.27 4 0.25 5 0.22 6 0.25 7 0.23 8 0.23 9 0.23 10 0.34 11 0.26 12 0.30 13 0.27 14 0.42 15 0.33 16 0.42 17 0.62 18 0.27 19 0.36 20 0.32 21 0.23 22 0.33 23 0.25 24 0.33 25 0.21 26 0.26 27 0.34 28 0.25 29 0.30 30 0.25

(24) TABLE-US-00003 TABLE 3 R.sub.A Heat Average X.sub.RA treatment crystal R.sub.A R.sub.A Average temper- particle Maximum Minimum value σ.sub.RA ature diameter value value of R.sub.A Standard (° C.) n (μm) (μm) (μm) (μm) deviation 700 1 0.35 0.62 0.22 0.38 0.090 2 0.36 3 0.29 4 0.33 5 0.29 6 0.22 7 0.29 8 0.38 9 0.27 10 0.43 11 0.32 12 0.46 13 0.35 14 0.38 15 0.50 16 0.44 17 0.53 18 0.42 19 0.50 20 0.62 21 0.37 22 0.52 23 0.33 24 0.36 25 0.32 26 0.29 27 0.39 28 0.34 29 0.43 30 0.29
Measurement of Bulk Susceptibility

(25) Each of the Au—Pt alloy wire materials was subjected to measurement of bulk susceptibility. The magnetic susceptibility was measured using a sensitive and portable magnetic balance, MSB-AUTO (product of Sherwood Scientific Ltd.) (measurement temperature: 27° C.). The wire materials subjected to measurement of bulk susceptibility were those subjected to morphological stability treatment at the above-described temperatures and those in pre-coil processing state (that is, wire materials without morphological stability treatment).

(26) Evaluation of Morphological Stability

(27) The morphological stability was evaluated by measuring the inner-diameter return ratio of each of the embolization coils that have undergone the secondary coil processing together with the morphological stability treatment. Each embolization coil that has undergone the secondary coil processing was removed off the core bar, and the inner diameter of the secondary coil produced was measured using a digital scope (VHX-900, product of KEYENCE CORPORATION). Based on this measured value and the following formula, an inner-diameter return ratio (K.sub.D) was calculated at the time when the secondary coil was unloaded, that is, removed off the core bar.
K.sub.D(%)=((D.sub.2−D.sub.1)/D.sub.1)×100  [Mathematical formula 3]
(D.sub.1: core bar diameter (1.2 mm), D.sub.2: inner diameter of unloaded secondary coil)

(28) In addition to measurement of the inner-diameter return ratio, a tensile test was performed to measure the tensile strength of the wire material. Measurement of the tensile strength is for the purpose of evaluating the morphological stability that the wire material would have when used as an embolization coil. If the tensile strength of the wire material changes (degrades) due to heat treatment, it is possible for the wire material to be easily deformed when used as an embolization coil. In light of this, the tensile strength is measured to examine this possibility. The tensile test was performed using Strong life Ell-L05 (product of Toyo Seiki Seisaku-sho, Ltd.) with a load cell of 50N and at a test speed of 10 mm/minute.

(29) Description will be made with regard to results of measurement and evaluation of the various items described above. First, results associated with the average crystal particle diameter are listed on Table 4.

(30) TABLE-US-00004 TABLE 4 Heat X.sub.RA treatment Average value σ.sub.RA temperature of R.sub.A Standard (° C.) (μm) deviation No heat treatment 0.19 0.023 300° C. 0.19 0.023 350° C. 0.22 0.029 500° C. 0.30 0.081 550° C. 0.34 0.084 700° C. 0.38 0.090

(31) Referring to the average value (X.sub.RA) of average crystal particle diameters on Table 4, the average value (X.sub.RA) starts changing at the coil wire material subjected to the morphological stability treatment performed at 350° C. The same applies in the standard deviation (σ.sub.RA). X.sub.RA and σ.sub.RA tend to increase as the temperature of the morphological stability treatment increases.

(32) Now, how the temperature of the morphological stability treatment is related to magnetic susceptibility and morphological stability will be examined. FIG. 3 illustrates photographs of external appearances of secondary coils (embolization coils) produced with morphological stability treatment performed at different treatment temperatures (100° C. to 800° C.). As seen from the photographs, the secondary coil processing is not complete at temperatures 100° C. and 200° C. in that the unloaded coils are shaped like primary coils, which are not subjected to the secondary coil processing yet. Also, the coils treated at 300° C. increased in inner diameter after unloading.

(33) FIG. 4 illustrates results of measurement of inner-diameter return ratios and bulk susceptibilities of the secondary coils produced at different treatment temperatures. In the heat treatments at 300° C. or lower, the inner-diameter return ratio is 100% or more, indicating that morphological stability is not obtained as early as immediately after the secondary coil processing. It is in the treatments at 350° C. or higher that the inner-diameter return ratio falls below 50%.

(34) FIG. 5 illustrates results of measurement of tensile strengths and bulk susceptibilities of the secondary coils produced at different treatment temperatures. This graph shows a sharp decrease in tensile strength at high heat treatment temperatures (600° C. and higher).

(35) Now, both FIGS. 4 and 5 will be examined. The heat treatment temperature of the morphological stability treatment influences all of the inner-diameter return ratio, tensile strength, bulk susceptibility. First, this tendency will be examined from an artifact-free standpoint. In the heat treatment performed at 600° C., which is in excess of 500° C., the bulk susceptibility is more positive than −5 ppm, which is a great shift toward the positive side from the pre-processing bulk susceptibility (−12.5 ppm). This leads to the assumption that for an embolization coil to have an artifact-free configuration (bulk susceptibility: −13 ppm or more and −5 ppm or less), the heat treatment needs to be performed at 500° C. or lower.

(36) Then, the above tendency will be examined from the standpoint of morphological stability required of an embolization coil. First, the least requirement that a secondary coil shape be maintained immediately after the secondary processing will be evaluated using return ratio. In the heat treatment performed at 300° C. or lower, the return ratio is too large. Some samples do not have a secondary coil shape in the first place, such as the samples subjected to heat treatment at 100° C. and 200° C. This leads to the requirement that in terms of shape immediately after processing (immediately after production), the heat treatment needs to be performed at a temperature of 350° C. or higher.

(37) It is noted, however, that no matter how stable the obtained secondary coil shape is, the secondary coil cannot exhibit morphological stability in actual applications without a suitable level of strength. In light of the circumstances, the tensile strength results lead to the assumption that at 600° C. or higher, the tensile strength degrades too greatly to implement an optimum embolization coil.

(38) Thus, with both return ratio and tensile strength taken into consideration, the temperature of the morphological stability treatment should be set within the temperature range of 350° C. or higher and 550° C. or lower. Coils produced within this temperature range exhibit an optimum range of bulk susceptibility (bulk susceptibility: −13 ppm or more and −5 ppm or less).

(39) Thus, with a balance between magnetic susceptibility and morphological stability taken into consideration, the temperature of the morphological stability treatment should be set within the temperature range of 350° C. or higher and 550° C. or lower. In this respect, the material structure of an Au—Pt alloy wire material treated within this temperature range will be examined. The results listed on Table 4 show an increase in average crystal particle diameter in the treatment performed at 350° C. or higher. It is also at this temperature or higher that a separate phase is presumed to be generated, judging from how the standard deviation of the average crystal particle diameter changes. Although the increase in crystal particle diameter and generation of a separate phase should basically be avoided, these phenomena are tolerable to some degree. This is because if the treatment temperature is 350° C. or higher but not excessively higher, the magnetic susceptibility is maintained within a proper range. Then, the material structure of the Au—Pt alloy wire material according to the present invention can be specified as follows, with changes in magnetic susceptibility taken into consideration: the average value of average crystal particle diameters is set at 0.20 μm or more and 0.35 μm or less.

INDUSTRIAL APPLICABILITY

(40) The embolization coil made of the Au—Pt alloy according to the present invention has a magnetic susceptibility optimum for preventing artifacts. The embolization coil also has a morphological stability optimum enough to take optimum forms while the embolization coil is being inserted and/or handled in an affected part of body when the embolization coil is in use. Also, the embolization coil serves as a medical instrument optimum in biocompatibility and anti-corrosion property.