BIOABSORBABLE STENT

Abstract

Provided are a magnesium alloy stent with improved corrosion resistance, and a method for producing same. The bioabsorbable stent including a core structure of a magnesium alloy, the stent is composed of: a first anticorrosive layer containing magnesium fluoride as a main component formed on the core structure, and a second anticorrosive layer coated with a diamond-like carbon on the first anticorrosive layer.

Claims

1. A bioabsorbable stent comprising a core structure of a magnesium alloy, the stent comprising: a first anticorrosive layer containing magnesium fluoride as a main component formed on the core structure, and a second anticorrosive layer of a carbon-coated layer containing a diamond-like carbon on the first anticorrosive layer.

2. The bioabsorbable stent according to claim 1, wherein the magnesium alloy contains, in % by mass, 0.95 to 2.00% of Zn, 0.05% to 0.30% of Zr, and 0.05 to 0.20% of Mn, and the balance consisting of Mg and unavoidable impurities, and has a grain size distribution with an average crystal grain size from 1.0 to 3.0 μm and a standard deviation of 0.7 μm or lower.

3. The bioabsorbable stent according to claim 1, wherein the first anticorrosive layer is formed by fluorination of a surface of the magnesium alloy.

4. The bioabsorbable stent according to claim 1, wherein the first anticorrosive layer has a layer thickness of 0.1 to 3 μm.

5. The bioabsorbable stent according to claim 1, wherein the diamond-like carbon of the second anticorrosive layer is a silicon-containing diamond-like carbon.

6. The bioabsorbable stent according to claim 1, wherein the second anticorrosive layer has a layer thickness of 10 nm to 5 μm.

7. The bioabsorbable stent according to claim 1, wherein a biodegradable polymer layer is formed on at least a part of the surface of the second anticorrosive layer.

8. The bioabsorbable stent according to claim 7, wherein the biodegradable polymer layer contains an intimal thickening inhibitor.

9. The bioabsorbable stent according to claim 8, wherein the intimal thickening inhibitor is a limus type drug.

10. A method for producing a bioabsorbable stent, comprising (1) fluorinating a surface of a core structure made of a magnesium alloy to form a first anticorrosive layer containing magnesium fluoride as a main component, and then, (2) subjecting the core structure with the first anticorrosive layer to be placed in a high-frequency plasma CVD apparatus such that a diamond-like carbon film is coated on the core structure via introduction of a carbon-containing source gas so as to form a second anticorrosive layer.

11. The production method according to claim 10, wherein a source gas containing carbon and silicon is introduced as the source gas, so that the surface of the core structure is coated with a silicon-containing diamond-like carbon film to form the second anticorrosive layer.

Description

[0054] The present invention will be more clearly understood from the following description of preferred embodiments thereof, when taken in conjunction with the accompanying drawings. However, the embodiments and the drawings are given only for the purpose of illustration and explanation, and are not to be taken as limiting the scope of the present invention in any way whatsoever, which scope is to be determined by the appended claims.

[0055] FIG. 1 shows a schematic view illustrating constituents of a stent according to the present invention;

[0056] FIG. 2 shows a plan view illustrating an example of a scaffold structure of a stent according to the present invention;

[0057] FIG. 3 shows a plan view illustrating another example of a scaffold structure of a stent according to the present invention;

[0058] FIG. 4 shows a schematic cross-sectional view illustrating an example of an apparatus for forming a second anticorrosive layer.

DESCRIPTION OF THE EMBODIMENTS

[0059] Basic Structure of Stent

[0060] As shown in FIG. 1, an example of a stent of the present invention comprises: a core structure (a) comprising a magnesium alloy (Mg alloy); a first anticorrosive layer (b) formed on an entire surface of the core structure (a) and comprising magnesium fluoride (MgF.sub.2) [the first anticorrosive layer contains Mg(OH).sub.2 etc. formed by oxidation of Mg on the layer surface and thus exhibits hydrophilicity]; a second anticorrosive layer (c) of a carbon-coated layer formed on the first anticorrosive layer (b) and comprising diamond-like carbon (preferably silicon-containing diamond-like carbon); a biodegradable resin layer (d) formed at least a part of a surface of the second anticorrosive layer (c); and a biodegradable resin layer (e) formed on the biodegradable resin layer (d) and containing a medicine or a drug (it should be noted that the biodegradable resin layer (d) may contain a medicine or a drug, instead of providing a biodegradable resin layer (e) containing a medicine).

[0061] The following technical elements are provided to obtain the above configuration: an element for selecting a composition of the magnesium alloy for constituting the core structure having a biodegradability and excellent deformability; an element for forming the first anticorrosive layer containing MgF.sub.2 as a main component over the entire surface of the core structure so as to control corrosion of the core structure comprising the selected magnesium alloy; an element for forming the second anticorrosive layer of a carbon-coated layer comprising diamond-like carbon (preferably silicon-containing diamond-like carbon) on the first anticorrosive layer; and optionally an element for forming a bioabsorbable material layer coated on the core structure and containing a medicine or a drug.

[0062] Magnesium Alloy

[0063] The core structure of the stent according to the present invention comprises a bioabsorbable magnesium alloy.

[0064] In the present invention, the core structure of the stent comprises a magnesium alloy that contains 90 wt % or more of magnesium (Mg) as a main component; and zinc (Zn), zirconium (Zr), and manganese (Mn) as accessary components and is free of aluminum (Al) and at least one rare earth element(s) selected from the group consisting of scandium (Sc), Yttrium (Y), dysprosium (Dy), samarium (Sm), cerium (Ce), gadolinium (Gd), lantern (La), neodymium (Nd), and 30 ppm or less of unavoidable impurity selected from the group consisting of iron (Fe), nickel (Ni), cobalt (Co) and copper (Cu). Such composition enables to ensure safety to the human body and mechanical properties.

[0065] In order to enhance safety to the human body and mechanical properties, the content of Mg may more suitably be 93 wt % or more and further suitably be 95 wt % or more.

[0066] Absence of at least one rare earth element selected from the group consisting of scandium (Sc), yttrium (Y), dysprosium (Dy), samarium (Sm), cerium (Ce), gadolinium (Gd), and lantern (La) preferably all of the rare earth element(s), as well as absence of aluminum can prevent a potential harmful effect to the human body.

[0067] A magnesium alloy of the present invention contains, in % by mass, 0.95 to 2.00% of Zn, 0.05% or more and less than 0.30% of Zr, 0.05 to 0.20% of Mn, and the balance consisting of Mg and unavoidable impurities, wherein the magnesium alloy has a particle size distribution with an average crystal particle size from 1.0 to 3.0 μm and a standard deviation of 0.7 μm or smaller.

[0068] The present invention has revealed that controlling the composition of the magnesium alloy within the above range improves plastic workability, and that finer and more uniform particle size of the alloy improves the properties such as elongation at break.

[0069] The magnesium alloy having the above features can avoid formation of coarse precipitates which may be triggers (starting points) of fractures and thereby reduce the possibility of breakage during and after deformation. It should be noted that although Zr, which is added in order to reduce the crystal particle size of the alloy, may form precipitates, the precipitates are typically dispersed at a nanometer scale (in a size smaller than 100 nm) in the matrix phase and thus has a negligible impact on deformation and corrosion of the alloy.

[0070] Zinc (Zn): In % by Mass, 0.95% or More and 2.00% or Less

[0071] Zn is added in order to enhance the strength and elongation ability of the alloy by forming a solid solution with Mg. Where the content of Zn is less than 0.95%, a desired effect cannot be obtained. An amount of Zn exceeding 2.00% is not preferred because such an amount may exceed a solid solubility limit of Zn in Mg so that Zn-rich precipitates are formed, resulting in reduced corrosion resistance. For this reason, Zn content is regulated to 0.95% or more and 2.00% or less. The content of Zn may be less than 2.00%.

[0072] Zirconium (Zr): In % by Mass, 0.05% or More and Less than 0.30%

[0073] Zr hardly forms a solid solution with Mg and forms fine precipitates, providing an effect of preventing formation of coarse crystal particles of the alloy. Addition of Zr at an amount less than 0.05% cannot provide a sufficient effect. Addition of Zr at an amount equal to or exceeding 0.30% leads to formation of a large amount of precipitates, with a reduced effect of particle size reduction. In addition, corrosion and breakage would start occurring at portions where the precipitates are biased. For this reason, content of Zr is regulated to 0.05% or more and less than 0.30%. The content of Zr may be 0.10% or more and less than 0.30%.

[0074] Manganese (Mn): In % by Mass, 0.05% or More and 0.20% or Less

[0075] Mn allows the alloy to have extremely fine particle size and have improved corrosion resistance. Where an amount of Mn is less than 0.05%, a desired effect cannot be obtained. An amount of Mn exceeding 0.20% is not preferred because plastic workability of the alloy tends to decrease. For this reason, Mn content is regulated to 0.05% or more and 0.20% or less. A preferable content of Mn may be 0.10% or more and 0.20% or less.

[0076] Unavoidable Impurities

[0077] Preferably, the content of unavoidable impurities is also controlled in the magnesium alloy for medical use. Since Fe, Ni, Co, and Cu promote corrosion of the magnesium alloy, the content of each of these unavoidable impurities is preferably lower than 10 ppm, further preferably 5 ppm or lower, and preferably substantially absent. The total content of the unavoidable impurities is preferably 30 ppm or less, and further preferably 10 ppm or less. Preferably, the magnesium alloy is substantially free from rare-earth elements and aluminum. Where an amount of an impurity element in the alloy is less than 1 ppm, it is regarded that the alloy is substantially free from the impurity element. The amount of impurity may be determined, for example, by ICP optical emission spectrometry.

[0078] Production of Magnesium Alloy

[0079] In accordance with an ordinal production method of a magnesium alloy, the magnesium alloy may be produced by throwing ground metals or alloys of Mg, Zn, Zr, Mn into a crucible, melting the ground metals and/or alloys in the crucible at a temperature from 650 to 800° C., and casting the molten alloy. Where necessary, the cast alloy may be subjected to solution heat treatment. The ground metals do not contain rare-earth elements (and aluminum). It is possible to suppress the amounts of Fe, Ni, Co, and Cu in the impurities by the use of high purity ground metals. Fe, Ni, and Co in the impurities may be removed by de-ironing treatment to the molten alloy. In addition, or alternatively, it is possible to use ground metals produced by distillation refining.

[0080] Metal Microstructure and Mechanical Properties

[0081] By the above-described controls of composition and production process, the magnesium alloy can have a fine and uniform structure as seen in a particle size distribution with an average crystal particle size from 1.0 to 3.0 μm (for example, from 1.0 to 2.0 μm) and a standard deviation of 0.7 μm or smaller (for example, from 0.5 to 0.7 μm). The standard deviation is preferably 0.65 μm or smaller. Fine precipitates containing Zr may each have a particle size smaller than 500 nm (preferably smaller than 100 nm). A matrix phase excluding the Zr precipitates may preferably be a single-phase solid solution of Mg—Zn—Mn ternary alloy.

[0082] The alloy has the following mechanical properties: a tensile strength from 230 to 380 MPa (for example, from 250 to 300 MPa), a proof strength from 145 to 220 MPa, and an elongation at breakage from 15 to 50% (for example, from 25 to 40%) in accordance with JIS Z2241. The alloy preferably has a tensile strength exceeding 280 MPa. The alloy preferably has an elongation at breakage exceeding 30%.

[0083] Shape of Stent Scaffold

[0084] The ingots prepared in the above-described manner are subjected to hot extrusion to produce a magnesium alloy tubular material, and the thus-obtained magnesium alloy tubular material is laser-processed to form a stent scaffold (core structure).

[0085] The stent of the present invention may be formed into various scaffold shapes including conventional ones. For example, the stent may have the scaffold shape shown in FIG. 2 or FIG. 3.

[0086] Electropolishing

[0087] As a pretreatment for forming a corrosion-resistant layer having a smooth surface, a preferable method of producing a core structure having an arbitrary size includes: connecting a laser-processed stent scaffold and a metal plate to an anode and a cathode, respectively, via a direct current (DC) power source in an electrolytic solution; and applying a voltage to them so as to electropolish the stent scaffold on the side of the anode.

[0088] Formation of First Anticorrosive layer

[0089] In order to form a first anticorrosive layer, the core structure is subjected to fluorination. As long as a MgF.sub.2 layer can be formed, the condition of fluorination is not particularly limited. For example, the core structure may be immersed in a treatment liquid such as an aqueous solution of hydrofluoric acid (HF) to carry out fluorination. It is preferable to shake the core structure at a speed of, for example, 50 to 200 rpm (preferably 80 to 150 rpm) during immersion. Then, the core structure on the surface of which the MgF.sub.2 layer is formed is taken out and sufficiently washed with a cleaning solution (for example, acetone and water). The core structure is, for example, washed by ultrasonic cleaning. Where the core structure is dried after cleaning, the core structure is preferably dried at a temperature from 50 to 60° C. under reduced pressure for 24 hours or longer. Further in order to form an anticorrosive layer with a smooth surface, the mirror-finished core structure obtained by electro-polishing may be subjected to fluorination.

[0090] Feature of First Anticorrosive Layer

[0091] The first anticorrosive layer of the stent according to the present invention contains magnesium fluoride as a main component. For example, the first anticorrosive layer may contain 90% or more of MgF.sub.2 as a main component. The first anticorrosive layer may also contain oxides and hydroxides such as MgO and Mg(OH).sub.2 as accessary components. It should be noted that the first anticorrosive layer may also contain oxides and hydroxides of metals other than magnesium which constitute the stent.

[0092] Layer Thickness of First Anticorrosive Layer

[0093] The first anticorrosive layer of the stent according to the present invention suitably has a layer thickness of 0.1 μm or larger (preferably 1 μm or larger) in order to exhibit corrosion resistance and a layer thickness of 3 μm or smaller, preferably 2 μm or smaller in order to exhibit deformation followability.

[0094] Formation of Second Anticorrosive Layer

[0095] The diamond-carbon coating layer is formed by using a chemical vapor deposition (CVD) method.

[0096] FIG. 4 is a schematic cross-sectional view of an apparatus used for forming a second anticorrosive layer, and shows a plasma CVD apparatus comprising a high frequency power source as discharge power source. The plasma CVD apparatus 1 is provided with a vacuum vessel 3 in which an electrode plate 2 that also serves as a substrate holder is installed at a lower portion. On the electrode plate 2 a core structure 4 coated with a first anticorrosive layer is placed. The electrode plate 2 is connected to a radio frequency (RF) power supply 5 and a blocking capacitor 6.

[0097] The vacuum vessel 3 is provided with a gas-introducing line 7 and a gas-exhausting port 8. The gas-introducing line 7 introduces a gas containing a carbon-containing gas [C-based gas such as acetylene] or a silicon- and carbon-containing gas [Si—C-based gas such as tetramethylsilane (TMS)], which is a source gas, and a bombard treatment gas (an inert gas such as Ar). The gas-exhausting port 8 is connected to an exhaust system (not shown). The gas-introducing line 7 is connected to a source gas supply device 9 and a bombard gas supply device 10 are connected to the gas-introducing line 7 via mass flow controllers 11 and 12, respectively. The vacuum vessel 3 is grounded.

[0098] The core structure 4 coated with the first anticorrosive layer is placed on the electrode plate 2, then the pressure inside of the vacuum vessel 3 is adjusted to a predetermined pressure by exhausting gas from the exhaust port 8 using an exhaust system (not shown). A C-based gas (for example, acetylene) or a Si—C-based gas (for example, tetramethylsilane), which is a source gas (raw material gas), is supplied from a source gas supply device 9, and the flow rate is adjusted using a mass flow controller 11 so as to be introduced into the vacuum vessel 3. During this time, high frequency (RF) is applied from the high frequency power source 5 to the electrode plate 2 to make the C-based gas or Si—C-based gas introduced into the vacuum vessel 3 into the plasma CVD apparatus.

[0099] By applying self-bias to the electrode plate 2 on which the core structure 4 coated with the first anticorrosive layer is placed, positive ions in the plasma apparatus are attracted to the core structure 4, so that a dense diamond-like thin film or a dense silicon-containing diamond-like thin film is locally formed on the surface of the core structure 4.

[0100] Specifically, a C-based gas containing carbon or a Si—C-based gas containing silicon and carbon used as a source gas is introduced into the chamber on which the base substrate is placed at a flow rate of 50 to 250 cm.sup.3/min (1 atm, 0° C.), preferably 100 to 200 sccm, so as to give a pressure of 1 to 5 Pa, and a high frequency power of 100 to 500 W is applied to the RE electrode. Accordingly, a diamond-like carbon coat layer (DLC layer) or a silicon-containing diamond-like carbon coat layer (Si-DLC layer) is preferably formed.

[0101] Examples of the C-based gas containing carbon may include a gas containing acetylene, methane, and the like as main components. Examples of the Si—C-based gas containing silicon and carbon may include monomethylsilane, triethylsilane, tetramethylsilane and the like as main components. Alternatively, as the Si—C-based source gas, it may be used a mixture containing one or more of silicon-based gas containing at least silicon and one or more of carbon gas (alkane or the like).

[0102] Accordingly, the C-based gas (for example, acetylene) or the Si—C-based gas (for example, tetramethylsilane), as the source gas, is ionized by the plasma CVD method so as to form a DLC film or a silicon-containing DLC film on the surface of the core structure 4, resulting in a core structure (bioabsorbable stent) in which a second anticorrosive layer is formed on the first anticorrosive layer.

[0103] Structure and Layer Thickness of Second Anticorrosive Layer

[0104] According to the present invention, by forming a second anticorrosive layer composed of a diamond-like carbon coat layer or a silicon-containing diamond-like carbon coat layer on the first anticorrosive layer, corrosion resistance of the Mg alloy can be significantly improved without deteriorating bioabsorption property of the stent structure. The second anticorrosive layer has a thickness of 10 nm to 5 μm, preferably 20 nm to 2 μm, and more preferably 20 nm to 500 nm. Too thin thickness may cause a tendency of insufficient anticorrosion effect, while too thick thickness may cause a tendency to inhibit bioabsorbability.

[0105] Biodegradable Resin Layer

[0106] In the stent of the present invention, a cover layer comprising a biodegradable polymer and an intimal thickening inhibitor may be preferably formed on the entire surface or a part of the surface of the second anticorrosive layer. Examples of the biodegradable polymers may include polyesters, such as a poly-L-lactic acid (PLLA), a poly-D,L-lactic acid (PDLLA), a poly(lactic acid-glycolic acid) (PLGA), a polyglycolic acid (PGA), a polycaprolactone (PCL), a polylactic acid-ε-caprolactone (PLCL), a poly(glycolic acid-ε-caprolactone) (PGCL), a poly-p-dioxanone, a poly(glycolic acid-trimethylene carbonate), a poly-β-hydroxybutyric acid, and others.

[0107] Intimal Thickening Inhibitor

[0108] Examples of the intimal thickening inhibitor may include sirolimus, everolimus, biolimus A9, zotarolimus, paclitaxel, etc.

[0109] Performance of Stent

[0110] The stent on which the first and second anticorrosive layer having the smooth surface is formed as described above can have a significantly suppressed temporal reduction of a radial force in a simulated plasma solution (EMEM+10% FBS) at 37° C. under 5% CO.sub.2 atmosphere as well as in pig coronary arteries, when compared with a stent that does not fall within the scope of the present invention or a stent without an anticorrosive layer (core structure alone).

[0111] Preparation of Magnesium Alloy

[0112] High purity ground metals of Mg, Zn, Mn, and Zr were prepared as initial materials. Each of the metals was weighed so as to have a component concentration as described in Table 1 and was thrown into a crucible. Then, at 730° C. the metals were molten with stirring, and a thus-obtained melt was cast to form ingots. Thus-obtained magnesium alloys of Example 1 and Example 2 contained the main components at formulation ratios which fall within the present invention. The initial materials used did not contain rare earth elements and aluminum even as unavoidable impurities. In this regard, 99.99% pure magnesium ground metal having a low concentration of impurity Cu was used. De-ironing treatment was carried out in the furnace in order to remove iron and nickel from the melt. Concentrations of impurities in the thus-obtained samples were determined using an ICP optical emission spectrometer (AGILENT 720 ICP-OES manufactured by AGILENT). Table 1 shows the compositions of Example 1 and Example 2. The concentrations of Fe, Ni, and Cu were all lower than 8 ppm (Ni and Cu were lower than 3 ppm). Al and the rare-earth elements were not detected, and Co was also below a detection limit. The total content of the unavoidable impurities was 11 or 12 ppm.

TABLE-US-00001 TABLE 1 Component concen- Impurity concen- tration (%) tration (ppm) Mg Zn Mn Zr Fe Ni Cu Total Production the 1.86 0.14 0.12 5 3 3 11 Example 1 balance Production the 0.95 0.11 0.24 8 3 1 12 Example 2 balance

[0113] Measurement of Mechanical Properties

[0114] Each alloy according to the examples was formed into a round bar material through hot extrusion. In accordance with JIS Z2241, a tensile strength, a proof strength, and an elongation at breakage of the round bar material were determined. Table 2 shows the results.

TABLE-US-00002 TABLE 2 Average Tensile Proof crystal strength strength Elongation particle Standard (MPa) (MPa) (%) size (μm) deviation Production 288 213 38 1.97 0.62 Example 1 Production 297 217 97 1.97 0.63 Example 2

Example

[0115] Hereinafter, the present invention will be specifically described with reference to Examples. The present invention, however, is not limited to the following Examples.

[0116] Evaluation of Anticorrosive Property

[0117] A stent sample obtained in Examples and Comparative Examples described below was immersed in a 37° C. simulated plasma solution (EMEM+10% FBS), then was uniformly expanded to have an inner diameter of 3 mm, and was shaken at 100 rpm with keeping immersion at 37° C. under 5% CO.sub.2 atmosphere. The sample was taken out at 28 days after immersion, and the radial force of the sample was measured (n=4). Further, the sample was cleaned ultrasonically with tetrahydrofuran (THF) and chromic acid solution to completely remove coating polymers and corrosion products such as magnesium hydroxide, etc., and the weight change of the core structure was evaluated (n=5). As to the radial force measurement, RX550/650 (produced by Machine Solutions Inc.) was used.

Example 1

[0118] A core structure comprising the above-described stent scaffold formed from the magnesium alloy obtained in the Production Example 1 was immersed in a 27 M hydrofluoric-acid aqueous solution (2 mL) and reciprocally moved at a rate of 100 rpm. Then, the stent was taken out after 24 hours, and subjected to ultrasonic cleaning sufficiently with water and acetone followed by drying the core structure for 24 hours at 60° C. under vacuum to prepare a core structure on which a first corrosion resistant layer (thickness: 1 μm) was formed. A diamond-like carbon coat layer having a thickness of 50 nm was formed on this structure so as to form a second corrosion resistant layer. Onto a surface of thus-obtained structure, a first cover layer containing 400 μg of a first polymer PCL was spray-coated, and then a second cover layer containing 150 μg of a second polymer PDLLA and 100 μg of sirolimus was spray-coated so as to obtain a stent sample shown in FIG. 1.

Comparative Example 1

[0119] Onto a surface of a core structure having the stent scaffold (without fluorination), a first cover layer containing 400 μg of a first polymer PCL was spray-coated, and then a second cover layer containing 150 μg of a second polymer PDLLA and 100 μg of sirolimus was spray-coated so as to obtain a stent sample.

Comparative Example 2

[0120] A core structure comprising the above-described stent scaffold was immersed in a 27 M hydrofluoric-acid aqueous solution (2 mL) and reciprocally moved at a rate of 100 rpm. Then, the stent was taken out after 24 hours, and subjected to ultrasonic cleaning sufficiently with water and acetone followed by drying the core structure for 24 hours at 60° C. under vacuum to prepare a core structure on which a first corrosion resistant layer (thickness: 1 μm) was formed. Onto a surface of thus-obtained structure, a first cover layer containing 400 μg of a first polymer PCL was spray-coated, and then a second cover layer containing 150 μg of a second polymer PDLLA and 100 μg of sirolimus was spray-coated so as to obtain a stent sample.

TABLE-US-00003 TABLE 3 Components of stent samples in Example 1 and Comparative Examples 1 to 2 First Second Core Anticorrosive Anticorrosive First Coating Second Coating Structure Layer Layer Polymer Layer Polymer Layer Example 1 Mg alloy Magnesium fluoride DLC PCL PDLLA/Sirolimus 100 μm 1 μm 50 nm 400 μg 150 μg/100 μg Comparative Mg alloy None None PCL PDLLA/Sirolimus Example 1 100 μm 400 μg 150 μg/100 μg Comparative Mg alloy Magnesium fluoride None PCL PDLLA/Sirolimus Example 2 100 μm 1 μm 400 μg 150 μg/100 μg

[0121] Weight Change of Core Structure Before and after Immersion

[0122] The core structure weights of each of the samples before immersion as well as after immersion for 28 days in the simulated plasma solution were measured. Table 4 shows the result of the weight residual ratio of the core structure before and after immersion calculated based on the weight of the core structure before immersion. The weight of the core structure before immersion was 6.13 mg.

TABLE-US-00004 TABLE 4 Weight Change of Core Structure Before and After Immersion (Weight Residual Ratio [%]) Before After immersion immersion for 28 days Example 1 100 98.0 ± 3.5 Com. Ex. 1 100 80.1 ± 5.7 Com. Ex. 2 100 90.1 ± 4.3

[0123] Relative Evaluation of Weight Change after Immersion for 28 Days

[0124] The sample (Comparative Example 2) with the magnesium fluoride layer as the first anticorrosive layer had higher weight residual ratio than the comparative sample (Comparative Example 1) without the anticorrosive layer. Further, it was confirmed that the sample (Example 1) comprising the diamond-like carbon coat layer as the second anticorrosive layer in addition to the first anticorrosive layer had further increase in weight residual ratio. That is, the weight residual ratio was higher in the order of Example 1>Comparative Example 2>Comparative Example 1.

[0125] Change in Radial Force of Core Structure Before and after Immersion

[0126] The core structure radial force of each of the samples before immersion as well as after immersion for 28 days in a simulated plasma solution was measured. Table 8 shows the result of the radial force residual ratio of the core structure before and after immersion calculated based on the radial force of the core structure before immersion. The radial force of the core structure before immersion was 65.4 N/mm

TABLE-US-00005 TABLE 5 Change in Physical Properties of Core Structure Before and After Immersion (Radial force residual ratio [%]) Before After immersion immersion for 28 days Example 1 100 94.0 ± 4.3 Com. Ex. 1 100 23.5 ± 6.7 Com. Ex. 2 100 89.1 ± 4.5

[0127] Relative Evaluation of Radial Force at after Immersion for 28 Days

[0128] It was confirmed that the sample (Example 1) comprising the diamond-like carbon coat layer as the second anticorrosive layer in addition to the first anticorrosive layer had the highest radial force residual ratio, followed by the sample (Comparative Example 2) with the magnesium fluoride layer as the first anticorrosive layer. As is the case of the weight residual ratio, the radial force residual ratio was higher in the order of Example 1>Comparative Example 2>Comparative Example 1. That is, it was clarified that the corrosion was suppressed by using two anticorrosive layers.

Example 2

[0129] A core structure comprising the above-described stent scaffold formed from the magnesium alloy obtained in the Production Example 1 was immersed in a 27 M hydrofluoric-acid aqueous solution (2 mL) and reciprocally moved at a rate of 100 rpm. Then, the stent was taken out after 24 hours, and subjected to ultrasonic cleaning sufficiently with water and acetone followed by drying the core structure for 24 hours at 60° C. under vacuum to prepare a core structure on which a first corrosion resistant layer (thickness: 1 μm) was formed. This structure was placed in the plasma CVD apparatus shown in FIG. 4, and then tetramethylsilane was introduced as a source gas using the apparatus shown in FIG. 4 to form a silicon-containing diamond-like carbon coat layer as a second corrosion resistant layer having a thickness of 50 nm on this structure. Onto a surface of thus-obtained structure, a first cover layer containing 400 μg of a first polymer PCL was spray-coated, and then a second cover layer containing 150 μg of a second polymer PDLLA and 100 μg of sirolimus was spray-coated so as to obtain a stent sample shown in FIG. 1.

Comparative Example 3

[0130] Onto a surface of a core structure having the stent scaffold (without fluorination), a first cover layer containing 400 μg of a first polymer PCL was spray-coated, and then a second cover layer containing 150 μg of a second polymer PDLLA and 100 μg of sirolimus was spray-coated so as to obtain a stent sample.

Comparative Example 4

[0131] A core structure comprising the above-described stent scaffold was immersed in a 27 M hydrofluoric-acid aqueous solution (2 mL) and reciprocally moved at a rate of 100 rpm. Then, the stent was taken out after 24 hours, and subjected to ultrasonic cleaning sufficiently with water and acetone followed by drying the core structure for 24 hours at 60° C. under vacuum to prepare a core structure on which a first corrosion resistant layer (thickness: 1 μm) was formed. Onto a surface of thus-obtained structure, a first cover layer containing 400 μg of a first polymer PCL was spray-coated, and then a second cover layer containing 150 μg of a second polymer PDLLA and 100 μg of sirolimus was spray-coated so as to obtain a stent sample.

TABLE-US-00006 TABLE 6 Components of stent samples in Example 2 and Comparative Examples 3 to 4 First Second Core Anticorrosive Anticorrosive First Coating Second Coating Structure Layer Layer Polymer Layer Polymer Layer Example 2 Mg alloy Magnesium fluoride Si- containing DLC PCL PDLLA/Sirolimus 100 μm 1 μm 50 nm 400 μg 150 μg/100 μg Comparative Mg alloy None None PCL PDLLA/Sirolimus Example 3 100 μm 400 μg 150 μg/100 μg Comparative Mg alloy Magnesium fluoride None PCL PDLLA/Sirolimus Example 4 100 μm 1 μm 400 μg 150 μg/100 μg

[0132] Weight Change of Core Structure Before and after Immersion

[0133] The core structure weights of each of the samples before immersion as well as after immersion for 28 days in the simulated plasma solution were measured. Table 7 shows the result of the weight residual ratio of the core structure before and after immersion calculated based on the weight of the core structure before immersion. The weight of the core structure before immersion was 6.13 mg.

TABLE-US-00007 TABLE 7 Weight Change of Core Structure Before and After Immersion (Weight Residual Ratio [%]) Before After immersion immersion for 28 days Example 2 100 98.0 ± 3.5 Com. Ex. 3 100 80.1 ± 5.7 Com. Ex. 4 100 90.1 ± 4.3

[0134] Relative Evaluation of Weight Change after Immersion for 28 Days

[0135] The sample (Comparative Example 4) with the magnesium fluoride layer as the first anticorrosive layer had higher weight residual ratio than the comparative sample (Comparative Example 3) without the anticorrosive layer. Further, it was confirmed that the sample (Example 2) comprising the silicon-containing diamond-like carbon coat layer as the second anticorrosive layer in addition to the first anticorrosive layer had further increase in weight residual ratio. That is, the weight residual ratio was higher in the order of Example 2>Comparative Example 4>Comparative Example 3.

[0136] Change in Radial Force of Core Structure Before and after Immersion

[0137] The core structure radial force of each of the samples before immersion as well as after immersion for 28 days in a simulated plasma solution was measured. Table 8 shows the result of the radial force residual ratio of the core structure before and after immersion calculated based on the radial force of the core structure before immersion. The radial force of the core structure before immersion was 65.4 N/mm

TABLE-US-00008 TABLE 8 Change in Physical Properties of Core Structure Before and After Immersion (Radial force residual ratio [%]) Before After immersion immersion for 28 days Example 2 100 98.0 ± 3.5 Com. Ex. 3 100 23.5 ± 6.7 Com. Ex. 4 100 89.1 ± 4.5

[0138] Relative Evaluation of Radial Force after Immersion for 28 Days

[0139] It was confirmed that the sample (Example 2) comprising the silicon-containing diamond-like carbon coat layer as the second anticorrosive layer in addition to the first anticorrosive layer had the highest radial force residual ratio, followed by the sample (Comparative Example 4) with the magnesium fluoride layer as the first anticorrosive layer. Likewise, the weight residual ratio, the radial force residual ratio was higher in the order of Example 2>Comparative Example 4>Comparative Example 2. That is, it was clarified that the corrosion was suppressed by using two anticorrosive layers.

INDUSTRIAL APPLICABILITY

[0140] The present invention can provide a stent comprising a first corrosion resistant layer and a second corrosion resistant layer that effectively delay decrease in mechanical strength associated with accelerated corrosion of the core structure. Therefore, the present invention contributes to development of medical technology and thus has remarkable industrial applicability.

[0141] Although the preferred examples of the present invention have been described with reference to the drawings, those skilled in the art would easily arrive at various changes and modifications in view of the specification and drawings without departing from the scope of the invention. Accordingly, such changes and modifications are included within the scope of the present invention.

REFERENCE NUMERALS

[0142] a . . . Core structure (Mg alloy) [0143] b . . . First anticorrosive layer (magnesium fluoride layer) [0144] c . . . Second anticorrosive layer (DLC layer or Si-DLC layer) [0145] d . . . Biodegradable resin layer [0146] e . . . Biodegradable resin layer (containing a medicine) [0147] 1 . . . Apparatus used for forming a second anticorrosive layer [0148] 2 . . . Electrode plate [0149] 3 . . . Vacuum vessel [0150] 4 . . . Core structure comprising the first anticorrosive layer [0151] 5 . . . RF (high frequency) power supply [0152] 6 . . . Blocking condenser [0153] 7 . . . Gas-introducing line [0154] 8 . . . Gas-exhausting port [0155] 9 . . . Source gas supply device [0156] 10 . . . Bombard gas supply device [0157] 11 . . . Mass flow controller [0158] 12 . . . Mass flow controller