Stent and process for producing the same

TECHNICAL FIELD

The present invention relates to a stent (intraluminal graft) which is recently used for intravascular therapy and surgical operation, particularly to enlarge a coronary stenosis, a carotid stenosis, a biliary duct, or an esophagus or to block an aneurysm and to a process for producing the same.

BACKGROUND ART

Conventionally, ischemic heart diseases are generally treated by percutaneous transluminal cornary angioplasty (PTCA), that is, a procedure of introducing a balloon catheter to, for example, a narrowed part through a lumen of a blood vessel and, after that, inflating a balloon with liquid such as normal saline solution. However, this procedure has a problem of high possibilities that an acute phase block of a coronary artery is caused and that the portion treated by PTCA is narrowed again (so-called post-PTCA restenosis). To solve the problem, intraluminal graft called stent has been developed. The stent recently rapidly came into practical use and are in widespread use. According to recent data, nearly 75% of procedures using balloon catheters have been already replaced by procedures using stents.

Stent matrix is an intraluminal graft which is implanted into a portion of a lumen to be treated through the lumen of a blood vessel or the like and is increased its diameter at the portion of the lumen to be treated so that the lumen is supported by action on the inside. Though the stent is mainly used in procedure for coronary artery so that the following description will be made mainly as to the procedure for coronary artery, the stent can be used for other lumens of human body such as biliary duct, esophagus, trachea, prostate, urinary duct, fallopian tube, aortic aneurysm, peripheral artery, renal artery, carotid artery, and cerebral blood vessel. As the application field of stent expands more and more, it is expected that stents will be used in many procedures including dilation of the narrowed portion, aneurysm embolization, cancer therapy, and the like, particularly that importance of microscopic stents will be increased according to the use in a field of cerebral surgery.

Through the spread of procedure using stent, restenosises have been dramatically prevented. On the other hand, however, since metallic stent matrixes are foreign substances in human body, a metallic stent matrix may thrombose a patient after several weeks from insertion of the metallic stent matrix. This is because the metallic stent itself is exposed to blood, resulting in adsorption of blood proteins such as fibrinogen and adherence or agglutination of blood platelets, thus forming thrombus. Further, thrombus may be formed because blood platelets are agglutinated on the convexes and concaves of a skeleton of the metallic stent matrix. Intimal thickening of a blood vessel due to cytokine discharged from blood platelets agglutinated on the periphery of the metallic stent matrix has been also pointed out as a problem. Accordingly, <patcit id="pcit0001" dnum="JPH11299901A"><text>JP H11-299901A</text></patcit> discloses to coat an outer periphery of a metallic stent matrix with a flexible polymer film having a number of fine pores.

<figref idref="f0001">Fig. 2</figref> is a perspective view showing such a metallic stent matrix 10 having a mesh form to be used for a stent, <figref idref="f0001">Fig. 3</figref> is a perspective view showing the stent matrix of <figref idref="f0001">Fig. 2</figref> in the expanded state 10', and <figref idref="f0002">Fig. 4</figref> is a perspective view showing a stent 20 comprising the stent matrix 10 of which outer periphery is coated with a flexible polymer film 19 having fine pores, and <figref idref="f0002">Fig. 5</figref> is a perspective view showing the stent 20 in the expanded state,

In biological tissues, inner walls of blood vessels and the like, that is, portions to be directly in contact with blood are coated with cell layer so-called endothelial cells. Since the surfaces of the endothelial cells are covered by sugar and the endothelial cells secrete substances that inhibit activation of blood platelets such as prostaglandin, thrombus is hardly formed in biological tissues. According to the stent disclosed in the aforementioned <patcit id="pcit0002" dnum="JPH11299901A"><text>JP H11-299901A</text></patcit>, the outer periphery of the metallic stent matrix is coated with a polymer film, thereby promoting proper endothelium formation with tissues and reducing thrombogenic property.

In <patcit id="pcit0003" dnum="JPH11299901A"><text>JP H11-299901A</text></patcit>, the polymer film for coating the outer periphery of the stent matrix is formed as follows. That is, a mandrel for a cover strip is first impregnated in a polymer solution, then is dried, and is perforated. After that, the mandrel is pulled out, thereby forming a membrane cover strip (envelope-shaped cover film). A stent matrix is inserted into the envelope-shaped cover film in a state that the cover film is sufficiently expanded by sending air into the cover film. After that, the sending of air is stopped so as to shrink the cover film, thereby forming a covering membrane on the outer periphery of the stent matrix.

Objects of the invention

(1) According to the stent of <patcit id="pcit0004" dnum="JPH11299901A"><text>JP H11-299901A</text></patcit>, the outer periphery of the metallic stent matrix is covered by a flexible polymer film having fine pores so as to engraft endothelium on the surface of the film on the outer periphery of the stent matrix, thereby reducing the causing of thrombus formation. However, in the stent of <patcit id="pcit0005" dnum="JPH11299901A"><text>JP H11-299901A</text></patcit>, the inner periphery of the stent matrix is not covered with the polymer film so that the metallic stent matrix is exposed. There is still a problem of causing thrombus, allergic to metal, stimulus of tissues due to metal, and rust development. Since the inner periphery of the stent has convexes formed by stent struts composing the stent matrix, the convexes disarrange bloodstream, facilitating the formation of thrombuses. The formed thrombuses exfoliate and move downstream (travels peripherad through the bloodstream) so as to cause infarction in small blood vessel on the downstream side or platelet-derived growth factor discharged from blood platelets in the thrombuses stimulate to cause thickening. Therefore, the problem of causing intimal thrombus is serious at this portion.

The method of forming a polymer film as an outer covering membrane on the outer periphery of a stent matrix by inserting the stent matrix to an envelope-shaped cover film and shrinking the envelope-shaped cover film has the following problem. That is, as shown in <figref idref="f0001">Figs. 2 and 3</figref>, the stent matrix 10 used in <patcit id="pcit0006" dnum="JPH11299901A"><text>JP H11-299901A</text></patcit> is formed of a cross-hatched lattice. When a polymer film as an outer covering membrane is formed on such a mesh stent matrix 10 by the aforementioned method, the outer covering membrane is bonded at contact points between the polymer film and the respective stent struts 11 composing the mesh stent matrix 10 as shown in <figref idref="f0002">Fig. 6</figref>. Accordingly, the integrity between the polymer film 19 and the stent matrix is poor.

Accordingly, when the stent matrix is expanded radially, the contact points between the stent struts 11 and the polymer film 19 slide and move. That is, the position of the polymer film 19 covering the outer periphery of the stent matrix is shifted when the stent is expanded.

In <patcit id="pcit0007" dnum="JPH11299901A"><text>JP H11-299901A</text></patcit>, the polymer film 19 has fine pores which are arranged to be spaced substantially equally. The purpose of the formation of fine pores is inhibiting formation of thrombuses and intimal thickening by grafting endothelial cells on the inner wall of the stent. Therefore, it is believed that the pores are formed at positions other than the position directly above the stent skeleton. When the polymer film is shifted relative to the stent matrix during expansion of the stent, however, the fine pores may be occluded by the stent struts. If the fine pores are occluded, the arrangement design of the fine pores becomes worthless.

<patcit id="pcit0008" dnum="JPH11299901A"><text>JP H11-299901A</text></patcit> also describes that the polymer film is coated with biodegradable polymer or chemicals. When the inner periphery of the stent matrix is coated with such a functioning agent, the portions of the inner periphery of the polymer film where the struts of the mesh stent matrix are positioned are not coated with such a coating of the functioning agent. However, as the position of the polymer film is shifted relative to the stent matrix during the expansion of the stent, surfaces without coating of the functioning agent are exposed. The coating also becomes worthless.

In a paragraph [0040] of <patcit id="pcit0009" dnum="JPH11299901A"><text>JP H11-299901A</text></patcit>, it is described that the adhesion of the cover strip to the outer periphery of the stent matrix 10 may be secured by heat-sealing of sending heated air during the coating of the stent matrix with the cover strip. Though this operation increases the adhesion at contact points between the polymer film 19 and the stent struts 11 composing the mesh stent matrix, it is impossible to coat entire surfaces of the stent struts 11 by the polymer film 19. Since the stent matrix is generally formed by laser beam machining of a metallic tube, shape edges of stent struts formed by cutting are rounded by chemical polishing or sonic treatment so that the surface of the stent matrix is generally mirror finish. As well known, it is difficult to bond a resin material to a smooth surface of the metal. Similarly, it is not easy to bond the polymer film to the stent matrix. To cover the entire surfaces of the stent struts with the polymer film to increase the adhesion of the polymer film 19 relative to the struts, it is required to melt the cover strip for even a moment and press the cover strip against the stent matrix. For this, it is necessary to send significantly hot air. Since the cover strip is a thin film having fine pores, however, the polymer film may not maintain its shape because of the hot air capable of melting the polymer film, resulting in bursting, breakage, defects of pin holes, cracks, or the like.

<patcit id="pcit0010" dnum="US6309413B"><text>US 6 309 413</text></patcit> discloses an endoluminal graft that is both expandable and supportive.

It is an object of the invention to overcome the problems of the stent of <patcit id="pcit0011" dnum="JPH11299901A"><text>JP H11-299901A</text></patcit> and to provide a stent in which a stent matrix is covered by a polymer layer with improved adhesion, thereby more securely preventing formation of thrombuses and overcoming a problem of drift between the stent matrix and the covering layer.

It is each object of the invention to provide a process of producing a stent having reduced thrombus formation.

Recently, as the application field of stent has expanded more and more, a stent matrix made of a flexible material having shape memory property allowing deformation of the stent into an arch has been developed in order to allow the stent to be inserted into a bent vascular channel and a stent matrix which is deformable into an arch during expansion has been developed because there is a need that the stent can be deformed while increasing its diameter according to the shape of a part (for example, a part bent into an arch) of a vascular channel where the stent will be implanted.

SUMMARY OF THE INVENTION

A stent of the invention comprises a tubular stent matrix of which diameter is extendable and a flexible polymer layer coating said stent matrix, and is characterized in that the polymer layer is closely attached to and covers the entire surface of the stent matrix.

Since the flexible polymer layer is attached to and covers the entire surface of the stent matrix not only the outer periphery of the stent matrix, the stent has no problem of causing allergic to metal, stimulus of tissues due to metal, and rust development. Since the inner periphery of the stent is a flat and smooth surface covered by the polymer layer without convexes and concaves so as not to disarrange bloodstream, the formation of thrombus can be inhibited well. In addition, there is no problem of drift between the stent matrix and the polymer layer during the expansion of the stent, thereby maintaining the positional relationship between the stent matrix and the polymer layer before and after the expansion of the stent.

A process of producing a stent of the invention is a process of producing a stent having a tubular stent matrix of which diameter is extendable and flexible polymer films which are attached to both the inner periphery and the outer periphery of said stent matrix and have a plurality of fine pores formed therein, said process comprising: a step of forming the polymer film by impregnating a mandrel into a liquid resin material for forming the polymer film and pulling up the mandrel; and a step of equalizing the thickness of the polymer film by pulling up the mandrel in the vertical direction and controlling the pulling-up speed, characterized wherein said fine pores are formed after the polymer film is formed by laser machining and the pulling-up speed is gradually lowered.

Since the flexible polymer films cover not only the outer periphery but also the inner periphery of the stent matrix in the stent produced by the process of the invention, the formation of thrombus can be inhibited well.

According to the invention, the thickness of the polymer film can be equalized all over the length in the longitudinal direction of the stent (that is, the direction of pulling up the mandrel) by controlling the pulling-up seed of the mandrel.

If the pulling-up speed is constant, the thickness of the film becomes larger the lower the position of the mandrel pulled up is, because of liquid resin material falling from the above. In the third invention, the thickness of the polymer film can be equalized all over the length in the longitudinal direction of the mandrel by gradually lowering the speed of pulling up the mandrel.

BRIEF DESCRIPTION OF THE DRAWINGS

<ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">Fig. 1</figref> is a schematic sectional view of a stent of the invention, showing a state coated with a polymer layer;</li><li><figref idref="f0001">Fig. 2</figref> is a perspective view of a stent matrix;</li><li><figref idref="f0001">Fig. 3</figref> is a perspective view of the stent matrix in a state that its diameter is increased;</li><li><figref idref="f0002">Fig. 4</figref> is a perspective view of a stent;</li><li><figref idref="f0002">Fig. 5</figref> is a perspective view of the stent in a state that its diameter is increased;</li><li><figref idref="f0002">Fig. 6</figref> is a schematic sectional view a stent of <patcit id="pcit0012" dnum="JPH11299901A"><text>JP H11-299901A</text></patcit>, showing a state coated with a polymer film;</li><li><figref idref="f0003">Fig. 7</figref> is an explanatory illustration showing relations of pattern of fine pores of a polymer layer relative to diameter, interval, and density of the fine pores;</li><li><figref idref="f0003">Fig. 8</figref> is a graph showing a relation between density of pores of the polymer layer and the thickness of the intimal thickening of a blood vessel caused by implantation of a stent into the blood vessel;</li><li><figref idref="f0004">Fig. 9</figref> is an X-ray transmission image of a stent of Example 1;</li><li><figref idref="f0004">Fig. 10</figref> is an X-ray transmission image of a stent of Comparative Example 1;</li><li><figref idref="f0005">Fig. 11a</figref> is a photomicrograph of the stent after one month from implantation in Comparative Example land <figref idref="f0005">Fig. 11b</figref> is a photomicrograph of the stent after one month from implantation in Example 1;</li><li><figref idref="f0006">Fig. 12</figref> is a photomicrograph of a section of biological tissue where the stent is implanted in Comparative Example 1;</li><li><figref idref="f0007">Fig. 13</figref> is an X-ray transmission image of the stent which is expanded after coating the inner wall of the stent in Comparative Example 2;</li><li><figref idref="f0008">Figs 14a-14f</figref> are explanatory illustrations of a process of the invention;</li><li><figref idref="f0009">Fig. 15</figref> is a schematic perspective view of a stent according to an embodiment of the fifth invention;</li></ul>

DETAILED DESCRIPTION

Preferably, a stent matrix composing a stent of the first invention is a tubular member having a length of from about 2 to 40 mm and a diameter of from about 1/10 to 1/2 of the length. The thickness of the stent matrix (wall thickness of the tubular member) is preferably from 11 to 2000 µm, more preferably from 51 to 500 µm, especially preferably from 101 to 300 µm. The stent matrix is preferably formed of a mesh so that the stent matrix can be flexibly expanded radially, particularly preferably, a cross-hatched lattice as shown in <figref idref="f0001">Fig. 2</figref> in which the lattice extends in the helical direction.

The stent matrix is preferably made of a biocompatible metal. Examples of such a biocompatible metal include stainless steel, titanium, tantalum, aluminum, tungsten, nickel-titanium alloy, cobalt-chromium-nickel-iron alloy. When the stent matrix is made of nickel-titanium alloy, cobalt-chromiuln-nickel-iron alloy, or the like, the stent matrix is preferably subjected to heat treatment for shape memory. When the stent matrix is made of nitinol as one of nickel-titanium alloys, the heat treatment is conducted by converting the crystal structure of nitinol from martensite phase to austenite phase while the stent matrix is expanded, thereby memorizing the shape and giving self-expandability to the stent matrix. Other than metals, resins having excellent mechanical strength such as polyether ether ketone, aromatic polyamide, polyimide, and the like may be used for the base material of the stent.

Suitable material of the flexible polymer layer is a high-molecular elastomer having high flexibility. Examples of such a high-molecular elastomer include elastmers of polystyrene series, of polyolefin series, of polyester series, of polyamide series, of silicone series, of urethane series, of fluorocarbon resin series, and of natural rubber series, copolymers of these, and polymer alloys of these. Among these, a segmented polyurethane, a polymer of polyolefin series, and a polymer of silicone series are preferable, particularly, a segmented polyurethane having high flexibility and excellent mechanical strength is best.

The segmented polyurethane polymer has a flexible polyether section as its soft segment and a section containing highly aromatic rings and urethane bonds as its hard segment, wherein the soft segment and the hard segment are subjected to phase separation to form a fine structure. The segmented polyurethane polymer is excellent in antithrombogenicity and is also excellent in properties such as strength and ductility so that it can be sufficiently expanded without being broken when the stent is expanded radially.

The thickness ("d" in <figref idref="f0001">Fig. 1</figref> as will be described later) of a polymer layer made of the segmented polyurethane polymer is preferably from 1 µm to 100 µm, particularly from 5 µm to 50 µm, especially from 20 µm to 50 µm.

The polymer layer is preferably provided with a plurality of fine pores. The fine pores may be arranged in random order. Preferably, the fine pores are formed to have substantially equal intervals therebetween. The phrase "substantially equal intervals" does not mean that the distances between fine pores are equal, but means that the fine pores are arranged at nearly regular intervals by a method of controlling the spaces between the fine pores. Accordingly, the arrangement with substantially equal intervals includes arrangements of oblique order, of circular order, of elliptic order, and the like which look random order at the first glance. The fine pores may have any size and any shape if the fine pore can allow passage of endothelial cells. The fine pores are preferably circular pores of which diameter is from 5 µm to 500 µm, preferably from 10 µm to 100 µm, more preferably from 20 µm to 100 µm. It should be understood that the fine pores may be pores of ellipse, square, rectangular, or other shape. This is true for the state before expansion. At a time when the stent is expanded and implanted in a lumen, the shape is varied from circle to ellipse and the diameter is also varied accordingly. The fine pores are aligned in a plurality of straight lines with intervals of from 51 µm to 10000 µm, preferably from 101 µm to 8000 µm, more preferably from 201 µm to 5000 µm. The plurality of straight lines are arranged in the axial direction of the stent with a predetermined constant angular interval from each other and are from 10 to 50 in number.

It should be noted that the best diameter and interval of the fine pores are in subservient relationship. The relationship can be easily understood by assuming the relationship as the density of pores in the polymer layer. That is, when substantially circular pores are arranged at nearly equal intervals just like three patterns shown in <figref idref="f0003">Fig. 7</figref>, the density per unit area naturally depends on the diameter and the interval of the fine pores.

The relation between the density of pores and the thickness of the intimal thickening of a blood vessel caused by implantation of the stent into the blood vessel is shown in <figref idref="f0003">Fig. 8</figref>.

From <figref idref="f0003">Fig. 8</figref>, it is found that the preferable diameter and interval of fine pores are in relationship with the density of pores. It should be understood that, even with preferable density of pores, too small diameter of pores is not preferable because the growth of endothelial cells on the inner side of the stent must be poor and, on the other hand, too large diameter of pores is also not preferable because the strength of the polymer layer must be poor and ingression of endothelial tissues must be excessive.

The polymer layer may be coated with a biodegradable polymer (bioabsorbable polymer). Examples of such a biodegradable polymer include gelatine, poly-lactic acid, polyglycolic acid, caprolactone, lactic-glycol acid copolymer, polygioxisanone, and chitin.

The biodegradable polymer may contain a therapeutic drug such as an antiplatelet drug, an antithrombotic drug, a growth accelerator, a growth inhibitor, and an immune-suppressing drug. The therapeutic drug is discharged into living body according to the degradation of the biodegradable polymer and thus provide effects of inhibiting formation of thrombus, inhibiting growth of smooth muscle cells so as to prevent constriction, inhibiting growth of cancerous cells, or promoting the growth of endothelial cells so as to achieve early formation of endothelium lining.

Examples of such a therapeutic drug include heparin, low-molecular heparin, hirudin, argatroban, formacolin, vapiprost, prostamoline, prostakilin homolog, dextran, D-phe-pro-arg chloromethyl ketone, dipyridamole, platelet receptor antagonist of glycoprotein, recombinant hirudin, thrombin inhibitor, vascular heptyne, angiotensin-converting enzyme inhibitor, steroid, fibrocyte growth factor antagonist, fish oil, omega 3 fatty acid, histamine, antagonist, HMG-CoA reductase inhibitor, seramin, serotonin blocker, thioprotease inhibitor, triazolopyrimidine, interferon, vascular endothelial growth factor (VEGF), rapamycin, and FK506. Further, such a therapeutic drug may be a statin drug having a function of melting plaque such as mevalotin, fuluvastatin, or the like.

The polymer layer on the outer peripheral side of the stent may be coated with a lubricative substance in order to smooth the movement of the stent within a fine blood vessel in a human body. Examples of such a lubricative substance include low-molecular hydrophilic substances such as glycerin, biocompatible substances such as hyaluronic acid and gelatine, and synthetic hydrophilic polymers such as polyethylene glycol, polyacrylamide, polydimethyl acrylamide, and polyvinylpyrrolidone.

The stent of the invention in which the entire surface of the stent matrix is coated with the polymer layer wherein the polymer layer is closely attached to the entire surface of the stent matrix can be produced by the following process. [0039] The polymer solution may be a solution of polymer or a polymerized solution of monomer. As the polymer solution, a segmented polyurethane polymer solution prepared from an organic solvent such as dioxane or tetrahydrofuran may be used. As the polymerized solution of monomer, a deacetonated, dealcoholized, or deoximated silicon rubber of condensation hardening type may be used.

By adding a therapeutic drug into the biodegradable polymer solution, a coating containing the therapeutic drug is formed. By adjusting the kind and the molecular weight of the biodegradable polymer and the thickness of the coating, the time and period when the therapeutic drug is discharged into the body can be controlled.

As mentioned above, the fine pores are formed in the stent intermediate by laser or the like after the mold removal. There is no preference in whether the formation of the coating of a biodegradable polymer or a lubricative polymer or the formation of fine pores by laser machining is performed in first. However, the description has been made here with a method of performing the formation of fine pores by laser machining after the coating formation.

An embodiment of the present invention comprises the features of a process comprising: impregnating a mandrel in a polymer solution, pulling up the mandrel in the vertical direction so as to form an inner polymer layer, attaching a stent matrix to the mandrel having the inner polymer layer so that the stent matrix is overlaid onto the mandrel, then impregnating the mandrel with the stent matrix into the polymer solution, pulling up the mandrel with the stent matrix so as to form an outer polymer layer, and then pulling out the mandrel.

That is, a mandrel is impregnated into a polymer solution slowly not to entrap air bubbles, is then pulled up in the vertical direction, and is subjected to hardening treatment such as drying or ultraviolet irradiation, if necessary, so as to form an inner polymer layer. In case of using a solution of polymer as the polymer solution, the drying is suitable as the hardening treatment. In case of using a polymerized solution of monomer as the polymer solution, the ultraviolet irradiation or heat hardening is suitable as the hardening treatment.

Then, the stent matrix is attached to the mandrel having the inner polymer layer thus formed such that the stent matrix is overlaid onto the mandrel. The mandrel with the stent matrix attached is impregnated into the polymer solution slowly and is then pulled up in the vertical direction so as to form an outer polymer layer. After the outer polymer layer is subjected to hardening treatment, the mandrel is pulled out, thereby producing a stent intermediate. Since the inner polymer layer and the outer polymer layer of the stent intermediate thus produced generally protrude from the both ends of the stent matrix, the excess protruding portions of the polymer layers are cut off.

For forming a biodegradable polymer layer, the mandrel is impregnated into the biodegradable polymer solution, i.e. the mandrel is subjected to the same coating treatment as mentioned above before the formation of the inner polymer layer or after the formation of the outer polymer layer. By adding a therapeutic drug into the biodegradable polymer solution, a coating containing the therapeutic drug is formed. By adjusting the kind and the molecular weight of the biodegradable polymer and the thickness of the coating, the time and the period when the therapeutic drug is discharged into the body can be controlled. The same is true for the formation of a lubricative polymer layer.

The fine pores are formed by laser machining to penetrate the inner polymer layer and the outer polymer layer before or after pulling out the mandrel.

For pulling out the mandrel from the stent intermediate thus obtained, the mandrel is impregnated into an organic solvent which allows the polymer film to slightly swell preferably at a cubical expansion of 10% or less, thereby easily pulling out the mandrel from the stent intermediate. The kind of the organic solvent and the impregnating time depend on the material of the polymer film. For example, in case of using a segmented polyurethane resin for the polymer film, the mandrel is preferably impregnated into lower alcohol, preferably methanol or ethanol, particularly into methanol preferably for 1-30 hours, particularly for 5-20 hours. This facilitates the drawing of the mandrel. The reasons are considered, though remaining incompletely understood, that the polymer film slightly swells so that the adhesion between the polymer film and the mandrel is reduced and that the lower alcohol has affinities both to the metal and the polymer layer and low surface tension so that the lower alcohol enters into a boundary face between the metallic mandrel and the inner polymer layer so as to reduce adherence between the surface of the mandrel and the polymer layer and improve the sliding property.

In the stent of the invention produced in the aforementioned manner, as shown in <figref idref="f0001">Fig. 1</figref> as its sectional view, a polymer layer 12 closely adheres and coats the entire surfaces of stent struts 11 composing a mesh stent matrix. The inner surface A of the stent is a flat and smooth surface of the polymer layer 12. Since this stent has absolutely no exposed surface of the metallic stent matrix, the problem of causing allergic to metal, stimulus of tissues due to metal, and rust development is solved. The problem of formation of thrombus is also solved. Particularly, the inner surface is a flat and smooth surface without convexes and concaves, thereby solving the formation of thrombus on convexes and concaves. In addition, there is no problem of displacement between the polymer layer and the stent matrix before and after the expansion of the stent.

The coating thickness of the polymer layer represents the thickness of a part of the polymer layer 12 directly coating the stent strut 11, designated by "d" in <figref idref="f0001">Fig. 1</figref>.

Example 1

As the stent matrix, a mesh stent matrix 10 having a diameter of 4 mm, a length of 20 mm, and a thickness of 0.2 mm shown in <figref idref="f0001">Fig. 2</figref> was employed.

<figref idref="f0001">Fig. 3</figref> is a perspective view of the metallic stent matrix 10' after being expanded. The metallic stent matrix 10' in this state has a diameter of 8 mm, a length of 20 mm, and a thickness of 0.2 mm.

A stent was produced by coating the entire surfaces of the metallic stent matrix 10 with a segmented polyurethane polymer layer. As described concretely, a mandrel made of SUS316 was impregnated into a polyurethane solution to form a polyurethane layer for coating a cylindrical outer surface of the mandrel. The metallic stent matrix which was slightly expanded was overlaid on the polyurethane layer with significant pressure. The mandrel with the stent matrix was impregnated into the polyurethane solution to form a coating so that both the inner and outer peripheries of the stent matrix were coated. After laser machining, the portions of the films protruding from the both ends were cut out. The mandrel with the stent intermediate was impregnated into ethanol and the stent intermediate was pulled out from the mandrel.

The polyurethane solution was prepared by dissolving 10% by weight of Capdiomat (trade name) SPU: segmented polyurethane (available from Kontoron Cardiovascular Inc.) into a mixed solution of tetrahydrofuran and dioxane.

The polyurethane polymer layer thus formed was perforated by excimer laser to have pores having a diameter of 100 µm such that the pores were substantially equally arranged at intervals of 200 µm. Pores aligned along 24 lines in total were formed by repeating a process of turning the stent intermediate at 15° in the circumferential direction after forming pores in a line in the longitudinal direction,

An X-ray transmission image of the stent thus obtained was taken by an X-ray microscope system (Model 1072, available from Skyscan). The X-ray transmission image is <figref idref="f0004">Fig. 9</figref>. The coating thickness "d" was 25 µm. <figref idref="f0004">Fig. 9</figref> corresponds to a portion IX of the stent shown in <figref idref="f0002">Fig. 4</figref>, but shown as an enlarged image.

As shown in <figref idref="f0001">Fig. 1</figref>, the stent in which the entire surfaces of lattice-like struts 11 of the stent matrix are coated with the polyurethane polymer layer 12 with well adhesion. It is found that, even when the stent skeleton is moved according to the expansion of the stent matrix, the polyurethane polymer layer follows this movement, thus maintaining the positional relationship between the polymer layer and the stent. It is also found that the projecting structure of the stent struts as a factor of blocking bloodstream is laminated by the polymer film so as to have a flat and smooth surface.

Comparative Example 1

A coating of a polyurethane polymer film was formed only on the outer periphery of a stent matrix by the method described in <patcit id="pcit0013" dnum="JPH11299901A"><text>JP H11-299901A</text></patcit>. Fine pores were formed in the same manner as Example 1. An X-ray transmission image of the stent thus obtained was taken by the same way as Example 1. The X-ray transmission image is <figref idref="f0004">Fig. 10</figref>. It is found that, in the stent, the polymer film covers and is in contact with the outer periphery of the stent matrix by points (lines) as shown in <figref idref="f0002">Fig. 6</figref>, that is, the polymer film is fixed only by contact points. It is pointed out that the contact points are shifted by slide movement during the expansion of the stent.

These stents were grafted in carotid arteries of a rabbit. Observation was conducted after one month. The results are shown in Table 1, <figref idref="f0005">Figs. 11a, 11b</figref> and <figref idref="f0006">Fig. 12</figref>.

<figref idref="f0005">Fig. 11a</figref> shows Comparative Example 1 and <figref idref="f0005">Fig. 11b</figref> shows Example 1. Positioned outside the polymer layer is an existing intima and positioned inside the polymer layer is a neogenetic intima. As apparent from <figref idref="f0005">Figs. 11a, 11b</figref>, the thickness of the intimal thickening of Example 1 (<figref idref="f0005">Fig. 11b</figref>) is thinner than that of Comparative example (<figref idref="f0005">Fig. 11a</figref>). As shown in <figref idref="f0006">Fig. 12</figref>, since the stent struts project into blood stream in Comparative Example 1, thrombus may be easily formed around the struts so that platelet-derived growth factors and the like are discharged, resulting in intimal thickening. On the other hand, since the surface facing the bloodstream was a flat and smooth surface of the polymer layer in Example 1, the formation of thrombus was prevented. <tables id="tabl0001" num="0001"><table frame="all"><title>Table 1</title><tgroup cols="3"><colspec colnum="1" colname="col1" colwidth="41mm" /><colspec colnum="2" colname="col2" colwidth="50mm" /><colspec colnum="3" colname="col3" colwidth="76mm" /><thead><row><entry namest="col1" nameend="col3" align="left" valign="top">Comparison of intimal thickening</entry></row><row><entry align="center" valign="top" /><entry align="center" valign="top"><b>Thickness of intimal thickening (µm)</b></entry><entry align="center" valign="top"><b>Area ratio between neogenetic intima and existing intima (%)</b></entry></row></thead><tbody><row><entry align="center"><b>Comparative Example 1</b></entry><entry align="center"><b>304.6 (n=7)</b></entry><entry align="center"><b>302.9</b></entry></row><row><entry align="center"><b>Example 1</b></entry><entry align="center"><b>239.9 (n=7)</b></entry><entry align="center"><b>266.7</b></entry></row></tbody></tgroup></table></tables>

Comparative Example 2

A mixed aqueous solution containing 5% of photoreactable gelatine of spiron benzophenone series, 2.5% of heparin, and 0.1% of silver powder was prepared. The stent produced in Comparative Example 1 was placed horizontally statically and the mixed solution was dropped to the inner wall of the stent at an amount of 20 µL per 1cm². The dropped solution was stretched uniformly by a round bar made of PTFE and was exposed to light to fix a coating. This procedure was repeated twice. The stent of which inner wall was thus coated was expanded in air by a balloon catheter and, after that, was observed by an X-ray microscope. An X-ray transmission image taken of this stent is <figref idref="f0007">Fig. 13</figref>.

From the above, it is found that portions where the stent struts existed before expansion were coated with no agent and sliding phenomenon between the stent struts and the polymer film was caused by the expansion so that the portions coated with no agent were exposed outside. The fine pores formed in Comparative Example 1 moved or slid on the polymer layer so that the fine pores were positioned behind the radiopaque stent struts as shown by arrows X. As a result, the fine pores were not observed in the X-ray transmission image of <figref idref="f0007">Fig. 13</figref>. This means that the fine pores were blocked. Therefore, it is pointed out that, since the diameter and intervals of pores which are strictly designed are varied by the expansion of the stent matrix as shown in <figref idref="f0003">Fig. 8</figref>, the density of pores is also varied, thus bringing about a problem of intimal thickening.

According to the stent of the invention, since the polymer layer is closely attached to the entire surface of the stent matrix, excellent biocompatibility can be given to the stent, thereby preventing adverse effects to human tissues such as allergy and thrombus due to metal. In addition, the stent has no problem of displacement between the stent matrix and the polymer layer during the expansion of the stent.

A coating of the biodegradable polymer may be formed by first obtaining a stent intermediate using a liquid base resin material such as the segmented polyurethane polymer and, after releasing the stent intermediate from the mold, impregnating the stent intermediate into a liquid resin material of the biodegradable polymer.

In the process of producing a stent of the invention, as shown in <figref idref="f0008">Figs. 14a, 14b</figref>, a mandrel 31 is impregnated into a liquid resin material slowly not to entrap air bubbles, is then pulled up in the vertical direction, and is subjected to hardening treatment such as drying or ultraviolet irradiation, if necessary, so as to form an inner polymer film 32 as shown in <figref idref="f0008">Fig. 14c</figref>.

In case of using a solution of polymer as the liquid resin material, the drying is suitable as the hardening treatment. In case of using a monomer solution as the liquid resin material, the ultraviolet irradiation or heat hardening is suitable as the hardening treatment.

In the invention, the distribution of thickness of the inner polymer film 32 is made uniformly in the longitudinal direction of the mandrel 31 by gradually lowering the pulling-up speed of pulling up the mandrel. The pulling-up speed may be lowered in a linear fashion as indicated by a solid line "a" in <figref idref="f0008">Fig. 14f</figref> and may be lowered with decelerating speed as indicated by a double-dashed line "b". Alternatively, the pulling-up speed may be lowered such that degree of the decelerating speed may be gradually reduced as shown by dashed line C. Normally, it is preferable to take a deceleration pattern indicated by the solid line "a" or the double-dashed line "b". The deceleration is preferably continuously conducted. This does not mean exclusion of stepwise deceleration.

Then, as shown in <figref idref="f0008">Fig. 14d</figref>, a stent matrix 33 is attached to the mandrel 31 having the inner polymer layer thus formed such that the stent matrix 33 is overlaid onto the mandrel 31. As shown in <figref idref="f0008">Fig. 14e</figref>, the stent matrix 33 is impregnated into the liquid resin material slowly and is pulled up in the vertical direction so as to form an outer polymer film. The pulling-up speed of this time is also controlled to have a pattern being gradually lowered in the same manner as the case of the inner polymer film 32.

After the outer polymer film is subjected to hardening treatment, the mandrel 31 is pulled out, thereby producing a stent. Since the inner polymer film and the outer polymer film of the stent thus produced generally protrude from the both ends of the stent matrix 33, the excess protruding portions of the polymer films are cut off.

As mentioned above, the liquid resin material for forming the polymer films may be polymer solution or solution of a monomer. The polymer solution is preferable because polymerization is not required so that the formation of film is easy.

A therapeutic drug may be contained into the biodegradable polymer solution. By applying this biodegradable polymer, a coating layer of the biodegradable polymer containing the therapeutic drug can be formed. By adjusting the kind and the molecular weight of the biodegradable polymer and the thickness of the coating, the time and the period when the therapeutic drug is discharged into the body can be controlled.

After the outer polymer film is hardened, fine pores are formed to penetrate the inner polymer film 32 and the outer polymer film before or after pulling out the mandrel 31. The fine pores are formed by laser machining.

In the invention, for pulling out the mandrel from the stent thus obtained, the mandrel is impregnated into an organic solvent which allows the polymer film to slightly swell preferably at a cubical expansion of 10% or less, in the same manner as described in the first invention, thereby easily pulling out the mandrel from the stent. The kind of the organic solvent and the impregnating time depend on the material of the polymer film. For example, in case of using a segmented polyurethane resin for the polymer film, the mandrel is preferably impregnated into lower alcohol, preferably methanol or ethanol, particularly into methanol preferably for 1-30 hours, particularly for 5-20 hours. This facilitates the drawing of the mandrel.

Though only a single stent matrix 33 is overlaid onto the mandrel 31 as shown in <figref idref="f0008">Fig. 14d</figref> in the aforementioned embodiment, two or more stent matrixes may be overlaid onto the mandrel with being slightly spaced apart from each other. In such a stent, a portion between the stent matrixes can be flexibly bent.

Example 2

As the stent matrix, a mesh stent matrix 10 having a diameter of 4 mm, a length of 20 mm, and a thickness of 0.2 mm shown in <figref idref="f0001">Fig. 2</figref> was employed.

<figref idref="f0001">Fig. 3</figref> is a perspective view of the metallic stent matrix 10' after being expanded. The metallic stent matrix 10' in this state has a diameter of 8 mm, a length of 20 mm, and a thickness of 0.2 mm.

A stent was produced by coating the inner periphery and the outer periphery of the metallic stent matrix 10 with segmented polyurethane polymer films. As described concretely, a mandrel having a diameter of 3.8 mm and made of stainless steel was impregnated into a polyurethane solution, was then pulled up, and was dried so as to form a cylindrical coating of the polyurethane having 30 µm. The metallic stent matrix which was slightly expanded was overlaid on the coating with significant pressure. The mandrel with the stent matrix was impregnated into the polyurethane solution, was then pulled up, and was dried so as to form a coating having a thickness of 50 µm so that the inner and outer peripheries of the stent matrix were coated. After laser machining, the portions of the films protruding from the both ends were cut out. The mandrel with the stent was impregnated into methanol for 12 hours and the stent was pulled out from the mandrel.

The polyurethane solution was prepared by dissolving 10% by weight of segmented polyurethane, a trade name Capdiomat, (available from Kontoron Cardiovascular Inc.) into a mixed solution of tetrahydrofuran and dioxane.

The speed of pulling up the mandrel was lowered in a linear fashion such that the initial speed was 10 mm/minute and the last speed was 5 mm/minute. Sections cut in the radial direction by high stainless steel were taken by a microscope. As a result of measuring the thickness of the film at the sections, the thickness of the film at a section corresponding to an upper end of the mandrel during impregnation was 78.6 µm ± 4.3 µm and the thickness at a section corresponding to a lower end of the mandrel was 80.1 µm ± 2.4 µm. From this result, it was found that the film thus formed had a substantially uniform thickness.

The films were perforated by excimer laser to have pores having a diameter of 100 µm such that the pores were substantially equally arranged at intervals of 200 µm. Pores aligned along 24 lines in total were formed by repeating a process of turning the cylindrical polymer film at 15° in the circumferential direction after forming pores in a line in the longitudinal direction.

Comparative Example 3

A stent was produced in the same manner as Example 4 except that the speed of pulling up the mandrel was constant at 10 mm/minute. As a result of measuring the thickness of the film at radial sections in the same manner as Example 4, the thickness of the film at a section corresponding to an upper end of the mandrel during impregnation was 77.1 µm ± 3.1 µm and the thickness of the film at a section corresponding to a lower end of the mandrel was 89.3 µm ± 4.2 µm. From this result, it was found that thickness of the film thus formed was different in the vertical positional relationship. From this, it was noted that the thickness of the polymer film according to the present invention is uniform.

The metallic stent provided with polymer film covering of the third invention can prevent adverse effects to human tissues such as thrombus due to metal. Particularly, since the polymer films are attached to the inner periphery and the outer periphery of the stent matrix, excellent biocompatibility can be given to the stent. The present invention can achieve homogenization of thickness of the polymer film.