Patent Publication Number: US-2022211524-A1

Title: Synthetic resin stent and stent delivery system

Description:
TECHNICAL FIELD 
     The present invention relates to a synthetic resin stent such as a biodegradable stent, and to a stent delivery system. 
     BACKGROUND ART 
     Stenotic diseases (such as tumors and inflammations) in natural tracts such as blood vessels and gastrointestinal tracts are heretofore treated by placing a stent at a stenotic site and dilating the stenotic site. Stents made of metal or synthetic resin are known, for example. Among these, when a metal stent is removed from the body, a surgical intervention is needed and imposes a significant burden on the patient. Therefore, use of a metal stent is limited to cases such as malignant tumors for which semi-permanent placement or surgical procedures are planned. Against such a background, a biodegradable stent as a synthetic resin stent has been proposed as a stent for use in cases where a metal stent cannot be used. 
     A synthetic resin stent is inferior to a metal stent, in self-expandability, restorability, adherence to the gastrointestinal tract such as the intestinal tract, and trackability to peristaltic movement of the gastrointestinal tract; therefore, required performance may not be achieved when a synthetic resin stent is manufactured in the same shape as a metal stent. On the other hand, for example, a stent formed by connecting biodegradable resin processed into a zigzag shape and covered with a membrane is disclosed (e.g., see Patent Document 1).
     Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2003-52834   

     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     However, since the stent disclosed in Patent Document 1 is covered with a membrane, it is difficult to reduce the diameter of the stent. Since the stent disclosed in Patent Document 1 is formed by connecting biodegradable resin processed into a zigzag shape, it is difficult to balance the functions of the stent, the ends of which essentially require trackability or restorability in relation to peristaltic movement of the gastrointestinal tract, and the central portion of which requires pressure strength. Therefore, a synthetic resin stent capable of achieving self-expandability, restorability, adherence to the gastrointestinal tract, and trackability to peristaltic movement of the gastrointestinal tract has been desired. 
     Accordingly, it is an object of the present invention to provide a synthetic resin stent and a stent delivery system capable of achieving self-expandability, restorability, adherence to the gastrointestinal tract, and trackability to peristaltic movement of the gastrointestinal tract. 
     Means for Solving the Problems 
     The present invention relates to a synthetic resin stent, including: a first stent including a first stent body formed of synthetic resin fiber into a tubular structure having a mesh, the first stent being deformable from a reduced diameter state to an expanded diameter state; and a second stent formed into a tubular structure having a mesh denser than the mesh of the first stent body, the second stent arranged so as to cover an outer periphery of the first stent body, and being deformable from the reduced diameter state to the expanded diameter state. 
     The first stent body is preferably formed by connecting a plurality of polygonal annular portions side by side in a longitudinal direction of the first stent, in a state in which the polygonal annular portions formed of synthetic resin fiber into a polygonal annular shape as viewed in the longitudinal direction are bent or curved so as to be convex in the longitudinal direction. 
     The first stent preferably further includes an end enlarged diameter portion connected to at least one end of the first stent body in the longitudinal direction, the end enlarged diameter portion being larger in diameter than the first stent body. 
     The first stent body is preferably connected side by side to the first stent in the longitudinal direction, in a state in which the plurality of polygonal annular portions formed of synthetic resin fiber into a polygonal annular shape as viewed in the longitudinal direction are bent or curved so as to be convex in the longitudinal direction; and the end enlarged diameter portion formed of synthetic resin fiber having a diameter smaller than that of the first stent body is preferably configured into a polygonal annular structure being more polygonal than the polygonal annular portions. 
     The first stent is preferably a stent including an end flare portion arranged at an end in an axial direction, the first stent being formed of wires; a plurality of peaks composed of tip corners protruding outward in the axial direction are preferably consecutively arranged in a circumferential direction, whereby the end flare portion is formed into an annular shape as viewed in the axial direction; an angle formed by two sides of each of the peaks is preferably 80° or less in a state in which the stent is placed inside the gastrointestinal tract; and the number of the plurality of peaks is preferably three to eleven. 
     The two sides composing each of the peaks are preferably composed of two sides of an end grid arranged at an end in the axial direction. 
     The first stent preferably includes a plurality of grids arranged side by side in the axial direction; and the end grid is preferably arranged at an end of the plurality of grids. 
     One side length of one of the two sides composing each of the peaks is preferably 16 mm to 22 mm. 
     Adjacent ones of the peaks of the end flare portion are preferably fixed at an intersection to the most end-side in the axial direction. 
     The second stent is preferably a synthetic resin stent including a first woven component portion being tubular and composed of a plurality of fibers woven into a mesh, and a second woven component portion composed of a plurality of fibers arranged so as to be woven into the first woven component portion and configured into an annular shape; the first woven component portion preferably includes a plurality of first fibers extending so as to be inclined at a predetermined angle with respect to the axial direction, a plurality of second fibers extending so as to intersect with the first fibers, and a plurality of first intersecting points configured with intersections of the plurality of first fibers and the plurality of second fibers; the second woven component portion preferably includes a plurality of wave-shaped third fibers arranged so as to be spaced apart in the axial direction, and a plurality of wave-shaped fourth fibers arranged so as to be spaced apart in the axial direction; and at least one first intersecting point of the plurality of first intersecting points is preferably arranged in intersecting regions surrounded by the third fibers and the fourth fibers. 
     The plurality of intersecting regions are preferably formed side by side in the circumferential direction of the first woven component portion; and the plurality of first intersecting points are preferably arranged side by side in the circumferential direction of the first woven component portion and arranged in the plurality of intersecting regions, respectively. 
     In a configuration in which the first intersecting points are arranged in the intersecting regions, respectively, the third fibers are preferably arranged in a state of being hookable by one or more of the first fibers, the second fibers and the fourth fibers, in relation to movement in a direction in which an overlapping portion of the third fibers and the fourth fibers shrinks in size; and the fourth fibers is preferably arranged in a state of being hookable by one or more of the first fibers, the second fibers and the third fibers, in relation to movement in a direction in which the overlapping portion of the third fibers and the fourth fibers shrinks in size. 
     A plurality of configurations are provided in which the first intersecting point is arranged in the intersecting region, in which the synthetic resin stent is preferably configured to partly include a configuration, in which the third fibers and the fourth fibers are arranged in a state of being mutually hookable, in relation to movement in a direction in which the overlapping portion of the third fibers and the fourth fibers shrinks in size, and arranged in a state of not being hookable by the first fibers and the second fibers when the third fibers and the fourth fibers move. 
     A loop having a loop shape is preferably formed at the top of the peaks of the wave-shaped third fibers and/or the wave-shaped fourth fibers, the loop arranged so as to surround any one or more of the first fibers, the second fibers, the third fibers and the fourth fibers. 
     The second woven component portion is preferably formed of synthetic resin fiber having an expansion force higher than the first woven component portion. 
     The second stent is preferably a synthetic resin stent, including a first woven component portion being tubular and composed of one more fibers configured into a mesh, and a second woven component portion arranged so as to be woven into the first woven component portion and composed of one or more fibers configured into an annular shape; the first woven component portion preferably includes a plurality of first fibers repeatedly bent so as to be inclined at a predetermined angle with respect to the axial direction and extending in the axial direction, a plurality of second fibers arranged to include a portion intersecting with the first fibers and repeatedly bent so as to be inclined at a predetermined angle with respect to the axial direction and extending in the axial direction, and a plurality of first intersecting regions configured with intersections of the plurality of first fibers and the plurality of second fibers; the second woven component portion preferably includes a plurality of third fibers repeatedly bent so as to be inclined at a predetermined angle with respect to the axial direction and extending in the circumferential direction, a plurality of fourth fibers arranged to include a portion intersecting with the third fibers and repeatedly bent so as to be inclined at a predetermined angle with respect to the axial direction and extending in the axial direction, and a plurality of second intersecting regions configured with intersections of the plurality of third fibers and the plurality of fourth fibers; and the first intersecting regions and the second intersecting regions are preferably arranged to at least partly overlap with each other. 
     In a configuration in which the first intersecting region is arranged to overlap with the second intersecting region, the first fibers are preferably arranged in a state of being hookable by one or more of the third fibers and the fourth fibers, in relation to movement in a direction in which an overlapping portion of the first fibers and the second fibers shrinks in size; and the second fibers are preferably arranged in a state of being hookable by one or more of the third fibers and the fourth fibers, in relation to movement in a direction in which an overlapping portion of the first fibers and the second fibers shrinks in size. 
     The present invention further relates to a stent delivery system for placing the synthetic resin stent in vivo, the system including: an outer tube that can interiorly load the first stent and the second stent, in which the second stent and the first stent are arranged side by side in this order from the distal end side; and a pushing member that is arranged inside the outer tube and can extrude the second stent and the first stent in this order from the distal end side of the outer tube. 
     Effects of the Invention 
     The present invention can provide a synthetic resin stent and a stent delivery system having self-expandability, restorability, adherence to the gastrointestinal tract, and trackability to peristaltic movement of the gastrointestinal tract. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view illustrating a biodegradable stent according to a first embodiment of the present invention; 
         FIG. 2  is a perspective view illustrating an inner stent of the first embodiment; 
         FIG. 3  is a perspective view illustrating an outer stent of the first embodiment; 
         FIG. 4  is a perspective view illustrating a biodegradable stent according to a second embodiment; 
         FIG. 5  is a perspective view illustrating an inner stent of the second embodiment; 
         FIG. 6  is a table illustrating values of tip angles and one side lengths corresponding to the pitch and number of peaks of an end flare portion of a stent manufactured using fiber having a fiber diameter of φ 0.4 mm in relation to the inner stent of the second embodiment, in which (a) is a table illustrating values in a placed state, and (b) is a table illustrating values in a state attached to a core rod; 
         FIG. 7  is a table illustrating migration of the inner stent of the second embodiment in relation to peristaltic movement, in a case in which the biodegradable stent is placed inside the intestinal tract, in a case in which the tip angle and the one side length are changed in the end flare portion of the stent using fiber having a fiber diameter of φ 0.4 mm; 
         FIG. 8  is a table illustrating migration of the inner stent of the second embodiment in relation to peristaltic movement, in a case in which the biodegradable stent is placed inside the intestinal tract, in a case in which the tip angle and the one side length are changed in the end flare portion of the stent using fiber having a fiber diameter of φ 0.5 mm; 
         FIG. 9  is a perspective view illustrating an outer stent of the second embodiment; 
         FIG. 10  is an enlarged view of the outer stent illustrated in  FIG. 9 ; 
         FIG. 11  is a view illustrating an outer stent according to a first variation of the second embodiment; 
         FIG. 12  is a view illustrating an outer stent according to a second variation of the second embodiment; 
         FIG. 13  is a view illustrating an outer stent according to a third variation of the second embodiment; 
         FIG. 14  is a perspective view illustrating a biodegradable stent according to a fourth variation of the second embodiment of the present invention; and 
         FIG. 15  is an enlarged view of the biodegradable stent illustrated in  FIG. 14 . 
     
    
    
     PREFERRED MODE FOR CARRYING OUT THE INVENTION 
     First Embodiment 
     Hereinafter, a first preferred embodiment of a synthetic resin stent of the present invention will be described with reference to the drawings.  FIG. 1  is a perspective view illustrating a biodegradable stent  1  according to the first embodiment of the present invention.  FIG. 2  is a perspective view illustrating an inner stent  2  of the first embodiment.  FIG. 3  is a perspective view illustrating an outer stent  5  of the first embodiment. In the present embodiment, a direction in which the biodegradable stent  1  extends in its entirety is referred to as a longitudinal direction X; a direction orthogonal to the longitudinal direction X is referred to a first direction D 1  which is the vertical direction in  FIG. 1 ; and a direction orthogonal to both of the longitudinal direction X and the first direction D 1  is referred to as a second direction D 2 . 
     The synthetic resin stent of the present embodiment is the biodegradable stent  1  composed of biodegradable fiber, and includes the inner stent  2  (first stent) and the outer stent  5  (second stent) being tubular as illustrated in  FIGS. 1 to 3 . The central part of the inner stent  2  in the longitudinal direction X is arranged inside the outer stent  5 . 
     The inner stent  2  extends in the longitudinal direction X and is deformable from a reduced diameter state to an expanded diameter state. The inner stent  2  includes an inner stent body  3  (first stent body) formed as extending in the longitudinal direction X in its entirety, and a pair of end flare portions  4  (end enlarged diameter portions). 
     As illustrated in  FIG. 2 , the inner stent body  3  is formed of synthetic resin fiber into a tubular structure having a mesh, configuring a plurality of polygonal annular portions  311 . The inner stent body  3  is configured by connecting the plurality of polygonal annular portions  311 . More specifically, the inner stent body  3  is configured by connecting a plurality of three-dimensional portions  31  each configured by the pair of polygonal annular portions  311 . 
     As illustrated in  FIG. 2 , the three-dimensional portion  31  is formed by connecting the pair of polygonal annular portions  311  in the longitudinal direction X. The polygonal annular portion  311  is formed by bending the annularly formed annular portion at a pair of bent portions  312  in the middle of the first direction D 1  and at a pair of bent portions  313  in the middle of the second direction D 2 . The bent portions  312  and  313  are configured with peaks and valleys of the polygonal annular portion  311 . 
     The polygonal annular portion  311  is formed into a square shape as viewed in the longitudinal direction X, or a substantially diamond shape in the present embodiment, for example. Both of the pair of bent portions  312  and the pair of bent portions  313  configuring the top of the polygonal annular portion  311  are bent into a substantially V-shape being convex toward one or the other side in the longitudinal direction X, as viewed in both of the first direction D 1  and the second direction D 2 . 
     One polygonal annular portion  311  is arranged such that the pair of bent portions  312  are bent so as to be convex toward one side in the longitudinal direction X in the middle of first direction D 1 , and the pair of bent portions  313  are bent so as to be convex toward the other side in the longitudinal direction in the middle of the second direction D 2 . Another polygonal annular portion  311  arranged adjacent to the one polygonal annular portion  311  is bent at the pair of bent portions  312  and the pair of bent portions  313  to the other side of the one polygonal annular portion  311 ; the pair of bent portions  312  are bent so as to be convex toward the other side in the longitudinal direction X in the middle of the first direction D 1 ; and the pair of bent portions  313  are bent so as to be convex toward one side in the longitudinal direction in the middle of second direction D 2 . 
     The three-dimensional portion  31  is formed by connecting the pair of bent portions  313  of the adjacent polygonal annular portions  311  with a tubular connecting portion  32  in the middle of the second direction D 2 , in a state in which mutual convex portions of the pair of bent portions  313  of the adjacent polygonal annular portions  311  face each other. The inner concave portion of the adjacent polygonal annular portions  311  is formed into a substantially diamond shape as viewed in the second direction D 2 . 
     The inner stent body  3  is configured by connecting the plurality of three-dimensional portions  31  side by side in the longitudinal direction X. The adjacent three-dimensional portions  31  are connected by connecting the pair of bent portions  312  of the adjacent polygonal annular portions  311  with the tubular connecting portion  32  in the middle of the first direction D 1 , in a state in which mutual convex portions of the pair of bent portions  312  of the adjacent polygonal annular portions  311  are arranged to face each other. The inner concave portion of the polygonal annular portions  311  of the adjacent three-dimensional portions  31  is formed into a substantially diamond shape as viewed in the first direction D 1 . 
     The inner stent body  3  described above is configured to have a mesh of a plurality of substantially diamond-shapes, with synthetic resin fiber configuring the plurality of polygonal annular portions  311 , by connecting the plurality of polygonal annular portions  311 . As described above, the inner stent body  3  is formed by connecting the plurality of polygonal annular portions  311  side by side in the longitudinal direction X of the inner stent body  3 , in a state in which the polygonal annular portions  311  formed of synthetic resin fiber into a polygonal annular shape as viewed in the longitudinal direction X are bent so as to be convex in the longitudinal direction X. 
     The inner stent body  3  is configured so as to be deformable between the reduced diameter state and the expanded diameter state. Here, the pair of adjacent polygonal annular portions  311  are connected such that the mutual convex portions made of synthetic resin fiber of the adjacent polygonal annular portions  311  are connected to each other with the tubular connecting portion  32  connecting the pair of bent portions  312 , and the mutual convex portions made of synthetic resin fiber of the adjacent polygonal annular portions  311  are connected to each other with the tubular connecting portion  32  connecting the pair of bent portions  313 . Therefore, the synthetic resin fiber forming the polygonal annular portion  311  is applied with a force to restore the bent portion of the tubular connecting portion  32  to a linear shape (the arrows in  FIG. 2 ). As a result, the synthetic resin fiber bent at the tubular connecting portion  32  is applied with a force to restore to the linear shape extending in the first direction D 1  and the second direction D 2  (direction intersecting with the longitudinal direction X of the inner stent body  3 ). Therefore, a radially expanding force is applied to the inner stent body  3 , whereby the inner stent body  3  can stably press a stenotic site. 
     The number of peaks and valleys of the polygonal annular portions  311  configuring the three-dimensional portions  31  of the inner stent body  3  is about four to eight in the case of the stent for the small intestine, for example. In the present embodiment, the polygonal annular portion  311  is formed of large-diameter fiber into a square shape as viewed in the longitudinal direction X of the inner stent body  3 , in which the number of peaks and valleys of the polygonal annular portions  311  is four, for example. 
     The shape of the inner stent body  3  is not limited in particular; for example, a structure is conceivable, in which synthetic resin fiber (fiber) is processed into a zigzag shape and connected in the longitudinal direction. In the case of the zigzag shape, the number of peaks and valleys of the zigzag shape is not limited in particular, but is preferably four to eight. 
     The pair of end flare portions  4  are connected at the ends of the inner stent body  3  in the longitudinal direction X, respectively, and formed into a zigzag shape of a polygonal annular structure having more peaks and valleys than the polygonal annular portion  311  of the inner stent body  3 . The pair of end flare portions  4  are arranged outside in the longitudinal direction X of the outer stent  5  so as to abut on a normal site. The end flare portions  4  extending in the first direction D 1  intersecting with the longitudinal direction X of the inner stent body  3  are formed larger in diameter than the inner stent body  3 , and deformable from the reduced diameter state to the expanded diameter state. The end flare portions  4  are alternately formed side by side in a direction in which the peaks and valleys intersect with the longitudinal direction X of the inner stent body  3 . 
     The end flare portions  4  are connected at the ends of the inner stent body  3  with the tubular connecting portions  32 , at the bent portions  412  which are convex toward the inner stent body  3  side. 
     Since the end flare portions  4  abut on a normal site, trackability or restorability in relation to peristaltic movement of the gastrointestinal tract such as the intestinal tract is important. Therefore, the end flare portion  4  is formed of synthetic resin fiber having a diameter smaller than that of the inner stent body  3 . The end flare portion  4  is configured into a polygonal annular structure being more polygonal than the polygonal annular portion  311  of the inner stent body  3  as viewed in the longitudinal direction X, and formed into a zigzag shape having a plurality of peaks and valleys configured with the plurality of bent portions  412  and  413  of the polygonal annular structure as viewed in the second direction D 2 . 
     The end flare portion  4  is formed into a zigzag shape of a polygonal annular structure having more peaks and valleys than the polygonal annular portion  311  of the inner stent body  3 . Assuming that the number of peaks and valleys of the inner stent body  3  is a, the number of peaks and valleys of the end flare portion  4  is preferably na (n: integer), and more preferably 2n. In the present embodiment, the number of peaks and valleys of the polygonal annular structure of the end flare portion  4  is eight, for example. The end flare portion  4  may be provided at the ends of the inner stent body  3  or may be provided at only one end thereof. 
     The material of the synthetic resin fiber configuring the inner stent body  3  and the end flare portion  4  is not limited in particular; however, a material having a high degree of restorability is preferable. Examples of the biodegradable resin may include homopolymer, copolymer, or blend polymer composed of L-lactic acid, D-lactic acid, DL-lactic acid, glycolic acid, s-caprolactone, or para-dioxanone. Non-biodegradable resin may also be used as long as the material has a high degree of restorability. In particular, for example, polydioxanone (PDO) is preferably used as a material of the fiber configuring the inner stent body  3  and the end flare portion  4 . 
     The synthetic resin fiber configuring the inner stent body  3  and the end flare portion  4  is not limited in particular, and may be monofilament yarn or multifilament yarn. From a perspective of enhancing the repulsive force against the pressure from the outer side of the inner stent body  3  in the radial direction at a stenotic site in vivo, the synthetic resin fiber configuring the inner stent body  3  is preferably monofilament yarn. The synthetic resin fiber configuring the inner stent body  3  and the end flare portion  4  may be twisted or may not be twisted. 
     The fiber diameter of the synthetic resin fiber configuring the inner stent body  3  and the end flare portion  4  described above is 0.05 mm to 0.7 mm, and preferably 0.4 mm to 0.6 mm, for example. The fiber diameter of the synthetic resin fiber configuring the end flare portion  4  is preferably the same as or smaller than the fiber diameter of the synthetic resin fiber configuring the inner stent body  3 . The size of the inner stent body  3  is not limited in particular; for example, the diameter is 10 mm to 25 mm and the length is 30 mm to 250 mm in the expanded diameter state. 
     As illustrated in  FIG. 3 , the outer stent  5  is formed into a tubular structure extending in the longitudinal direction X (predetermined direction) by braiding synthetic resin fiber. As illustrated in  FIG. 1 , the outer stent  5  is arranged so as to cover the outer periphery of the inner stent body  3  of the inner stent  2 . Since the synthetic resin fiber configuring the outer stent  5  has a diameter smaller than that of the inner stent body  3  of the inner stent  2 , the outer stent  5  has a mesh denser than the mesh of the inner stent body  3  of the inner stent  2 . The outer stent  5  is deformable from the reduced diameter state to the expanded diameter state. 
     The material of the synthetic resin fiber configuring the outer stent  5  is not limited in particular; however, a material having a high degree of rigidity is preferable. Examples of the biodegradable resin may include homopolymer, copolymer or blend polymer composed of L-lactic acid, D-lactic acid, DL-lactic acid, glycolic acid, s-caprolactone or para-dioxanone. Non-biodegradable resin may also be used as long as the material has a high degree of rigidity. In particular, for example, poly-L-lactic acid (PLLA) is preferably used as a material of the fiber configuring the outer stent  5 . In the present embodiment, the fiber configuring the outer stent  5  is formed from poly-L-lactic acid (PLLA), for example. 
     The shape of the outer stent  5  is not limited in particular; for example, the shape has a structure by braiding synthetic resin fiber. The end of the outer stent  5  is not limited in particular; and the end preferably has a shape with a high degree of self-expandability. The outer stent  5  may not have self-expandability, restorability, or trackability in relation to peristaltic movement. 
     In the case of placing the biodegradable stent  1  inside the gastrointestinal tract, the outer stent  5  in the compressed state is placed inside the gastrointestinal tract. As a result, the outer stent  5  is expanded and placed inside the gastrointestinal tract. Thereafter, the inner stent  2  in the compressed state is placed inside the outer stent  5 . Therefore, the inner stent  2  and the outer stent  5  are placed so as to overlap with each other, in a state in which the inner stent body  3  of the inner stent  2  is arranged inside the outer stent  5 . 
     More specifically, the biodegradable stent  1  is placed inside the gastrointestinal tract by using the stent delivery system. The stent delivery system of the present embodiment includes an outer sheath member (outer tube) (not illustrated) to be inserted into the gastrointestinal tract, and a pushing member (not illustrated). The inner stent  2  and the outer stent  5  can be loaded into the outer sheath member. The outer stent  5  and the inner stent  2  in the compressed state are linearly arranged side by side in this order from the distal end side to the proximal end side inside the outer sheath member. The outer stent  5  and the inner stent  2  in this state are extruded in this order from the distal end side of the outer sheath member by the pushing member arranged to the proximal end side inside the outer sheath member. As a result, the outer stent  5  is firstly extruded from the distal end side of the outer sheath member, then expanded inside the gastrointestinal tract, and placed inside the gastrointestinal tract. Thereafter, the pushing member further pushes the inner stent  2 , whereby the inner stent  2  is extruded from the distal end side of the outer sheath member, expanded inside the outer stent  5 , and placed inside the outer stent  5 . 
     As described above, in the case of placing the biodegradable stent  1 , the outer stent  5  is placed first, then the inner stent  2  is placed inside the outer stent  5 , whereby the inner stent body  3  of the inner stent  2  can press the outer stent  5  toward the gastrointestinal tract side, and the end flare portions  4  arranged outside the ends of the outer stent  5  in the longitudinal direction X can press the gastrointestinal tract. 
     Here, in relation to the biodegradable stent  1  of the present invention, a description will be provided on reasons why the outer stent  5  formed of synthetic resin fiber (small-diameter fiber) having a diameter smaller than that of the inner stent body  3  into a tubular structure having a dense mesh is arranged so as to cover the outer periphery of the inner stent body  3  formed of synthetic resin fiber having a large diameter (large-diameter fiber) into a tubular structure having a sparse mesh. 
     In the case of configuring a stent of synthetic resin such as biodegradable resin, fiber having a relatively small diameter is used, from a perspective of ensuring loadability into a delivery sheath. In this case, sufficient strength of the stent thus manufactured cannot be ensured. 
     Therefore, in the present invention, the stent  1  is configured by arranging the outer stent  5  formed of synthetic resin fiber (small-diameter fiber) having a diameter smaller than that of the inner stent body  3  into a tubular structure having a mesh denser than the mesh of the inner stent body  3 , so as to cover the outer periphery of the inner stent body  3  formed of synthetic resin fiber having a large diameter (large-diameter fiber) into a tubular structure having a sparse mesh. 
     As a result, the inner stent body  3  formed of large-diameter fiber with a sparse mesh is arranged inside the outer stent  5  formed of small-diameter fiber with a dense mesh, whereby the inner stent body  3  presses the outer stent  5  from the inside of the outer stent  5 . Therefore, the pressing force of the inner stent body  3  from the inside of the outer stent  5  can reinforce the strength of the outer stent  5  and ensure the strength of the biodegradable stent  1  in its entirety. The inner stent  2  formed of large-diameter fiber is configured with a sparse mesh, whereby loadability into a delivery sheath can be ensured. 
     The end flare portion  4  is arranged at the ends of the inner stent body  3  (portions abutting at a normal site), where trackability or restorability in relation to peristaltic movement of the gastrointestinal tract is important. The end flare portion  4  is formed into a polygonal annular structure having more peaks and valleys than the polygonal annular portion  311  of the inner stent body  3 , by using synthetic resin fiber having a small diameter (small-diameter fiber), whereby the end flare portion  4  is easily deformable to increase the area of adhesion to the gastrointestinal tract, therefore the trackability or restorability in relation to peristaltic movement of the gastrointestinal tract can be improved. Even though the end flare portion  4  is configured into a polygonal annular structure having more peaks and valleys than the polygonal annular portion  311  of the inner stent body  3 , since the end flare portion  4  is formed of synthetic resin fiber having a small diameter (small-diameter fiber), loadability into a delivery sheath can be ensured. 
     An example of manufacturing the biodegradable stent  1  will be briefly described. In the present manufacture example, the inner stent body  3  of the inner stent  2  was manufactured as follows: polydioxanone (PDO) having a fiber diameter of 0.5 mm was processed into a square shape (zigzag shape) (number of peaks and valleys: four) to manufacture the polygonal annular portion  311 ; and six consecutive polygonal annular portions  311  bent in different directions in the longitudinal direction X were alternately arranged in the longitudinal direction X to manufacture three consecutive three-dimensional portions  31 , whereby the inner stent body  3  was manufactured. In this manner, the inner stent body  3  having a core rod diameter of 20 mm was manufactured. The end flare portion  4  of the inner stent  2  was manufactured into a zigzag shape having a polygonal annular structure (number of peaks and valleys: eight) with the same material and fiber diameter as the inner stent body  3 , and was connected at the ends of the inner stent body  3 . The outer stent  5  was manufactured by braiding PLLA fibers (0.25 mm and 0.3 mm). The inner stent  2  was arranged inside the outer stent  5 , and the outer stent  5  was placed as overlapping the outer side of the inner stent  2 , whereby the biodegradable stent  1  was manufactured. 
     According to biodegradable stent  1  of the present embodiment described above, the following effects can be achieved. 
     (1a) The biodegradable stent  1  is configured to include: the inner stent  2  including the inner stent body  3  formed of synthetic resin fiber into a tubular structure having a mesh, the inner stent  2  being deformable from the reduced diameter state to the expanded diameter state; and the outer stent  5  formed into a tubular structure having a mesh denser than the mesh of the inner stent body  3 , arranged so as to cover the outer periphery of the inner stent body  3 , and deformable from the reduced diameter state to the expanded diameter state. Therefore, the inner stent body  3  formed with a sparse mesh is arranged inside the outer stent  5  formed with a dense mesh, whereby the pressing force of the inner stent body  3  from the inside of the outer stent  5  can reinforce the strength of the outer stent  5  and ensure strength of the biodegradable stent  1  in its entirety. As a result, the gastrointestinal tract can be pressed in the state in which the strength of the biodegradable stent  1  in its entirety is ensured by the pressing force of the inner stent body  3  from the inside of the outer stent  5 , whereby the biodegradable stent  1  can be prevented from moving while preventing stenosis. Therefore, the biodegradable stent  1  capable of exerting self-expandability, restorability, adherence to the gastrointestinal tract, and trackability to peristaltic movement can be achieved. 
     (1b) The inner stent body  3  is formed by connecting the plurality of polygonal annular portions  311  side by side in the longitudinal direction X of the inner stent body  3 , in a state in which the polygonal annular portions  311  formed of synthetic resin fiber into a polygonal annular shape as viewed in the longitudinal direction X are bent or curved so as to be convex in the longitudinal direction X. As a result, a simple configuration can achieve the biodegradable stent  1  capable of exerting self-expandability, restorability, adherence to the gastrointestinal tract, and trackability to peristaltic movement. The synthetic resin fiber forming the polygonal annular portion  311  is applied with a force to restore the bent portion of the tubular connecting portion  32  to a linear shape (the arrows in  FIG. 2 ). As a result, the synthetic resin fiber bent at the tubular connecting portion  32  is applied with a force to restore to the linear shape extending in the direction intersecting with the longitudinal direction X of the inner stent body  3 . Therefore, the inner stent body  3  can stably press a stenotic site. 
     (1c) The inner stent  2  further includes the end flare portions  4  connected at the ends of the inner stent body  3  in the longitudinal direction X and having a size larger in diameter than the inner stent body  3 . As a result, the end flare portion  4  can stably press the gastrointestinal tract. Thus, the end flare portions  4  arranged at the ends of the inner stent body  3  in the longitudinal direction X allow the biodegradable stent  1  to be stably held in the gastrointestinal tract. 
     (1d) The end flare portion  4  formed of synthetic resin fiber having a diameter smaller than that of the inner stent body  3  is configured into a polygonal annular structure being more polygonal than the polygonal annular portion  311  of the inner stent body  3 . As a result, the end flare portion  4  is easily deformable to increase the area of adhesion to the gastrointestinal tract, whereby the trackability or restorability in relation to gastrointestinal motility can be improved. The end flare portion  4  is formed of synthetic resin fiber having a small diameter (small-diameter fiber), whereby loadability into a delivery sheath can be ensured. 
     (1e) The stent delivery system for placing the biodegradable stent  1  in vivo includes: the outer sheath member capable of interiorly loading the inner stent  2  and the outer stent  5 , in which the outer stent  5  and the inner stent  2  are arranged side by side in this order from the distal end side; and the pushing member arranged inside the outer sheath member and capable of extruding the outer stent  5  and the inner stent  2  in this order from the distal end side of the outer sheath member. As a result, the pushing member extrudes the outer stent  5  and the inner stent  2  in this order, whereby the biodegradable stent  1  can be easily placed inside the gastrointestinal tract. 
     Second Embodiment 
     Next, a second embodiment of the present invention will be described.  FIG. 4  is a perspective view illustrating a biodegradable stent  10  according to the second embodiment. The biodegradable stent  10  of the second embodiment mainly differs in configuration of an inner stent  6  and an outer stent  11 . 
     As illustrated in  FIG. 4 , the synthetic resin stent of the second embodiment is the biodegradable stent  10  composed of biodegradable fiber, and includes the inner stent  6  (first stent) and the outer stent  11  (second stent) arranged on the outer side of the inner stent  6 . The central portion of the inner stent  6  in the longitudinal direction X (axial direction) is arranged inside the outer stent  11 . The structures described in the first embodiment are omitted from description of the second embodiment. Similar to the biodegradable stent  1  of the first embodiment, the biodegradable stent  10  of the second embodiment is configured to include: the inner stent  6  including the inner stent body  71  formed of synthetic resin fiber into a tubular structure having a mesh, and deformable from the reduced diameter state to the expanded diameter state; and the outer stent  11  formed of synthetic resin fiber having a diameter smaller that of than the inner stent body  71  into a tubular structure having a mesh denser than the mesh of the inner stent body  71 , the outer stent  11  arranged so as to cover the outer periphery of the inner stent body  3  and deformable from the reduced diameter state to the expanded diameter state. 
     The inner stent  6  will be described.  FIG. 5  is a perspective view illustrating the inner stent  6  of the second embodiment. As illustrated in  FIG. 5 , the inner stent  6  of the present embodiment is a biodegradable stent composed of biodegradable fiber, formed as extending in the longitudinal direction X (axial direction) and deformable from the reduced diameter state to the expanded diameter state.  FIG. 5  illustrates the inner stent  6  in a natural condition. The inner stent  6  can deform from the natural state illustrated in  FIG. 5  to the reduced diameter state; and when the inner stent  6  in the reduced diameter state is placed inside the gastrointestinal tract, the inner stent  6  deforms from the reduced diameter state to the expanded diameter state, depending on the size of the gastrointestinal tract. 
     The inner stent  6  is woven into a mesh of a plurality of fibers  60  (wires) and formed into a tubular structure, and includes a multitude of grids  61  formed of the fibers  60  on the outer periphery and configured with argyle voids arranged in an orderly fashion. The plurality of grids  61  are arranged side by side in the axial direction and arranged side by side in the circumferential direction. The mesh of the inner stent  6  becomes sparse in the axial direction when the inner stent  6  is in the reduced diameter state, and becomes dense in the axial direction when the inner stent  6  is in the expanded diameter state. In the present embodiment, intersecting points between the woven fibers  60  are fixed. Examples of a fixing method may include an adhesion method or an ultrasonic welding method. At least, adjacent peaks  721  of an end flare portion  72  to be described later are fixed at an intersecting point (intersection) to the most end-side in the axial direction. In the present embodiment, the inner stent  6  is fixed at all of the intersecting points at which the woven fibers  60  intersect. As a result, compressive strength of the inner stent  6  can be improved. The intersecting point at which the adjacent peaks  721  of the end flare portion  72  are fixed may be fixed by intersecting with two linear fibers  60  or may be fixed by fixing apices of the bent portions composed of two bent fibers  60 . 
     The inner stent  6  includes the inner stent body  71  (first stent body) formed as extending in the longitudinal direction X in its entirety; and the end flare portion  72  arranged at one end of the inner stent  6  in the longitudinal direction X (axial direction). The inner stent  6  is placed inside the gastrointestinal tract, in a state in which a portion of the inner stent body  71  is arranged inside the outer stent  11 , and the end flare portion  72  is not covered by the outer stent  11 . 
     The inner stent body  71  is formed into a tubular structure having a mesh with the plurality of grids  61 . The plurality of grids  61  are arranged side by side in the axial direction and arranged side by side in the circumferential direction. In the present embodiment, the plurality of grids  61  are configured with argyle voids, respectively. 
     In the present embodiment, the end flare portion  72  is only provided at one end side of the inner stent  6  in the longitudinal direction. Since the end flare portion  72  abuts at a normal site of the gastrointestinal tract, trackability or restorability in relation to peristaltic movement of the gastrointestinal tract such as the intestinal tract is important, and it is important that post-placement migration due to peristaltic movement of the gastrointestinal tract can be suppressed. In the present embodiment, the end flare portion  72  is only arranged at one end of the inner stent  6  in the axial direction, but is not limited thereto, and may be arranged at the ends of the inner stent  6  in the axial direction. 
     The plurality of peaks  721  composed of tip corners protruding outward in the axial direction are consecutively arranged at one end of the inner stent  6  in the circumferential direction, whereby the end flare portion  72  is formed into an annular shape as viewed in the axial direction. More specifically, the plurality of peaks  721  are each formed by bending the fiber  60  at the end of the inner stent  6 . The number of the plurality of peaks  721  is preferably three to eleven, for example. In the present embodiment, the number of the plurality of peaks  721  is three, for example. 
     Two sides  721   a  and  721   a  of the peak  721  are composed of two sides  611   a  and  611   a  of the end grid  611  arranged at the end in the axial direction, among the plurality of grids  61  arranged side by side in the axial direction. 
     The end flare portion  72  is formed such that a tip angle θ (angle) formed by the two sides  611   a  and  611   a  of the end grid  611  configuring the two sides  721   a  and  721   a  of the peak  721  is 80° or less in the state in which the inner stent  6  is placed inside the gastrointestinal tract. The reason why the tip angle θ formed by the two sides  721   a  and  721   a  of the peak  721  is set to 80° or less is based on the result of the evaluation test described later. As described later in the result of the evaluation test, in the case in which the tip angle θ formed by the two sides  721   a  and  721   a  of the peak  721  is 80° or less, post-placement migration of the inner stent  6  due to peristaltic movement of the gastrointestinal tract is 1 cm or less, after repeating movement simulating peristaltic movement 10 times. 
     In the present embodiment, the end flare portion  72  is configured such that the tip angle θ formed by the two sides  721   a  and  721   a  of the peak  721  is 80° or less in the state in which the inner stent  6  is placed inside the gastrointestinal tract, and the plurality of peaks  721  are consecutively arranged in the circumferential direction such that the number of the plurality of peaks  721  is three to eleven. The shape of the end flare portion  72  configured as described above is determined such that: the number of the peaks  721  arranged in the circumferential direction and the tip angle θ formed by the two sides  721   a  and  721   a  of the peak  721  determine the density in the circumferential direction and the protrusion length of the tip corners of the peaks  721 ; and the plurality of peaks  721  composed of the tip corners protruding outward in the axial direction are consecutively arranged in the circumferential direction in a relatively sparse state. In the present embodiment, the end flare portion  72  is configured such that the ends of the inner stent  6  are woven sparser than the inner stent body  71 . As described later in the result of the evaluation test, post-placement migration of the inner stent  6  due to peristaltic movement of the gastrointestinal tract is 1 cm or less; therefore, the end flare portion  72  thus configured can achieve trackability to peristaltic movement of the gastrointestinal tract, and can suppress post-placement migration due to peristaltic movement of the gastrointestinal tract. 
     As illustrated in  FIG. 5 , in the present embodiment, an axial distance (amplitude) between the apex of the peak  721  and the apex of the valley  722  of the end flare portion  72  is referred to as a pitch P 1 . An axial distance (amplitude) between the peaks protruding in the axial direction of the grid  61  of the inner stent body  71  is referred to as a pitch P 2 . The length of one side  721   a  among the two sides  721   a  and  721   a  configuring the peak  721  of the end flare portion  72  is referred to as one side length L. One side length L of one side  721   a  of the peak  721  is the length from the apex of the peak  721  to the apex of the valley  722 . 
     In the inner stent  6  of the present embodiment, the pitch P 1  of the end flare portion  72  is formed large, and the pitch P 2  of the inner stent body  71  is formed smaller than the pitch P 1  of the end flare portion  72 . For example, the ratio of the pitch P 1  of the end flare portion  72  to the pitch P 2  of the inner stent body  71  (pitch P 1 /pitch P 2 ) is preferably 3.3 to 7.0. As a result, the inner stent body  71  and the end flare portion  72  are formed without sharp change in pitch. The ratio of the pitch P 1  of the end flare portion  72  to the pitch P 2  of the inner stent body  71  (pitch P 1 /pitch P 2 ) falls within a range of 3.3 to 7.0, whereby the trackability to peristaltic movement of the gastrointestinal tract can be improved. The end flare portion  72  is arranged at a normal site of the gastrointestinal tract, and the inner stent body  71  is arranged at a stenotic site of the gastrointestinal tract. 
     The reason why the end flare portion  72  is arranged at a normal site and the inner stent body  71  is arranged at a stenotic site will be described. In the case in which the pitch P 1  of the stent is large, the peaks  721  become parallel to the gastrointestinal tract in the axial direction even at a site of the gastrointestinal tract having a larger diameter, whereby post-placement migration of the stent due to peristaltic movement of the gastrointestinal tract can be suppressed. In the case in which the pitch P 1  of the stent is large, the mesh becomes sparse; therefore, the compressive strength decreases. Therefore, by increasing the pitch P 1 , post-placement migration of the stent due to peristaltic movement of the gastrointestinal tract can be suppressed, even at a site of the gastrointestinal tract having a large diameter. Here, the stent placed at a normal site is not required to have high compressive strength. Therefore, the stent having the large pitch P 1  is preferably arranged at a normal site, in which the diameter of the gastrointestinal tract is large so that high compressive strength is not required. 
     On the other hand, in the case in which the pitch P 2  of the stent is small, when the diameter of the gastrointestinal tract for placing the stent is small, the peaks  721  become close to parallel to the gastrointestinal tract in the axial direction, whereby post-placement migration of the stent due to peristaltic movement of the gastrointestinal tract can be suppressed. In the case in which the pitch P 2  of the stent is small, the mesh becomes dense; therefore, compressive strength increases. Here, the stent placed at a stenotic site is required to have high compressive strength. Therefore, the stent having the small pitch P 2  is preferably arranged at a stenotic site, in which the diameter of the gastrointestinal tract is small so that high compressive strength is required. 
     One side length L of one side  721   a  of the two sides  721   a  and  721   a  configuring the peak  721  of the end flare portion  72  preferably falls within a range of 16 mm to 22 mm, for example, based on the result of the evaluation test described later. In the case in which one side length L of one side  721   a  of the peak  721  of the end flare portion  72  falls within a range of 16 mm to 22 mm, for example, post-placement migration of the inner stent  6  due to peristaltic movement of the gastrointestinal tract is 1 cm or less, as described later in the result of the evaluation test, whereby trackability to peristaltic movement of the gastrointestinal tract can be achieved, and post-placement migration due to peristaltic movement of the gastrointestinal tract can be suppressed. 
     The material of the synthetic resin fibers  60  configuring the inner stent body  71  and the end flare portion  72  is not limited in particular; however, a material having a high degree of restorability is preferable. Examples of the biodegradable resin may include homopolymer, copolymer, or blend polymer composed of L-lactic acid, D-lactic acid, DL-lactic acid, glycolic acid, s-caprolactone, para-dioxanone, or trimethylene carbonate. Non-biodegradable resin may also be used as long as the material has a high degree of restorability. In particular, for example, polydioxanone (PDO) is preferably used as a material of the fiber configuring the inner stent body  71  and the end flare portion  72 . 
     In the present embodiment, the inner stent  6  may be configured by connecting the axially extending fiber of a zigzag shape in the circumferential direction, or may be configured by weaving a single fiber, for example. 
     The synthetic resin fiber configuring the inner stent body  71  and the end flare portion  72  is not limited in particular, and may be monofilament yarn or multifilament yarn. From a perspective of enhancing the repulsive force against the pressure from the outer side in the radial direction of the inner stent body  71  at a stenotic site in vivo, the synthetic resin fiber configuring the inner stent body  71  is preferably monofilament yarn. 
     The fiber diameter (diameter) of the synthetic resin fiber configuring the inner stent body  71  and the end flare portion  72  is 0.05 mm to 0.7 mm, and preferably 0.4 mm to 0.6 mm, for example. The diametrical size of the inner stent body  71  is not limited in particular; however, for example, the diameter is 10 mm to 25 mm and the length is 30 mm to 250 mm in the expanded diameter state. 
     In the case in which the diameter of the abutting gastrointestinal tract is φ 16 mm to φ 20 mm, the pitch P 1  of the end flare portion  72  is preferably 12 mm or more. In the case in which the diameter of the abutting gastrointestinal tract is about φ 12 mm, the pitch P 2  of the inner stent body  71  is preferably 3 mm or more. 
     The diameter of the inner stent body  71  of the inner stent  6  is formed slightly larger in diameter (e.g., by about 10 to 20%) than the target gastrointestinal tract. In the case in which the inner stent  6  is placed inside the gastrointestinal tract, the inner stent  6  in the reduced diameter state is placed inside the target gastrointestinal tract and expanded after placement. 
     For example, in the case in which the inner stent  6  is placed inside the gastrointestinal tract having a diameter of φ 16 mm, the diameter of the inner stent body  71  of the inner stent  6  to be placed is preferably 17 mm to 32 mm in a natural state, for example. Three to eleven of the peaks  721  are preferably consecutively arranged in the circumferential direction of the end flare portion  72  of the inner stent  6  to be placed, in which one side length L of one side  721   a  among the two sides  721   a  and  721   a  configuring the peak  721  is preferably 16 mm to 22 mm. In the end flare portion  72 , the angle formed by the two sides  721   a  and  721   a  configuring the peak  721  is preferably 38.5° to 147.3° in a natural state, for example, and the angle formed by the two sides  721   a  and  721   a  configuring the peak  721  is preferably 36.2° to 78.4° after placing the inner stent  6  inside the gastrointestinal tract, for example. 
     Here, referring to  FIGS. 6 to 8 , the result of the evaluation test using the inner stent  6  of the present embodiment will be described. In the present evaluation test, the inner stent  6  illustrated in  FIG. 5  including the inner stent body  71  and the end flare portion  72  arranged at one end side was manufactured using monofilaments of polydioxanone (PDO) having a fiber diameter of 0.4 mm or 0.5 mm, by changing each parameter (number of peaks, pitch P 1 , tip angle θ, and one side length L). The end flare portion  72  was formed by consecutively arranging the plurality of peaks  721  side by side in the circumferential direction at the end of the inner stent  6 . 
     For example, in the present evaluation test, the inner stent  6  was manufactured using fiber having a fiber diameter of 0.4 mm, such that the parameters (number of peaks, pitch P 1 , tip angle θ, and one side length L) will be as illustrated in (a) of  FIG. 6 , in the state of being placed inside the intestinal tract of φ 16 mm. Here, the diameter of the inner stent  6  is reduced when placed inside the intestinal tract; therefore, the inner stent  6  attached to a core rod of φ 20 mm is manufactured, and the inner stent  6  thus manufactured is placed in the intestinal tract of φ 16 mm. 
     For example, as illustrated in the upper table of (a) of  FIG. 6 , in the case of manufacturing the inner stent  6  having three peaks in which the pitch P 1  of the end flare portion  72  in the placed state is 7.0 mm, 8.7 mm, 11.0 mm, 11.8 mm, 13.5 mm, 16.3 mm, 19.0 mm, or 22.0 mm, the inner stent  6  having the pitch P 1  smaller than the pitch P 1  when attached to the core rod of φ 20 mm needs to be manufactured; therefore, as illustrated in (b) of  FIG. 6 , the inner stent  6  having the pitch P 1  of 3.0 mm, 6.0 mm, 9.0 mm, 10.0 mm, 12.0 mm, 15.0 mm, 18.0 mm, or 21.0 mm will be manufactured. As illustrated in the lower table of (a) of  FIG. 6 , in the case of manufacturing the biodegradable stent having four peaks in which the pitch P 1  of the end flare portion  72  in the placed state is 5.6 mm, 7.7 mm, 10.1 mm, 11.0 mm, 12.8 mm, 15.7 mm, 18.7 mm, or 21.5 mm, the inner stent  6  having the pitch P 1  smaller than the pitch P 1  when attached to the core rod of φ 20 mm needs to be manufactured; therefore, as illustrated in (b) of  FIG. 6 , the inner stent  6  having the pitch P 1  of 3.0 mm, 6.0 mm, 9.0 mm, 10.0 mm, 12.0 mm, 15.0 mm, 18.0 mm, or 21.0 mm will be manufactured. 
     In this case, the pitch P 1  and the number of peaks can be used to calculate a theoretical value of “tip angle” as illustrated in (a) and (b) of  FIG. 6 . One side length is the same length in the placed state and in the case of manufacturing a stent attached to a core rod. 
     As described above, in the present evaluation test, in the case of manufacturing the inner stent  6  using fiber having a fiber diameter of 0.4 mm, the parameters (number of peaks, pitch P 1 , tip angle θ, and one side length L) of the stent placed inside the intestinal tract of φ 16 mm were calculated to provide the theoretical values of the parameters (number of peaks, pitch P 1 , tip angle θ, and one side length L) of the stent attached to the core rod of φ 20 mm. The fiber was attached to the core rod of φ 20 mm to manufacture the inner stent  6  before placement so as to provide the values (theoretical values) of the parameters (number of peaks, pitch P 1 , tip angle θ, and one side length L) of the stent attached to the core rod of φ 20 mm. 
     The present evaluation test was conducted using the stent manufactured as described above. As a testing method, the inner stent  6  manufactured by changing the parameters (number of peaks, pitch P 1 , tip angle θ, and one side length L) was placed inside a polyethylene tube having an inner diameter of 16 mm or 20 mm, and movement simulating peristaltic movement was applied to the polyethylene tube. The movement simulating peristaltic movement was repeated 10 times for the inner stent  6  placed inside the gastrointestinal tract, in which the polyethylene tube was moved in one direction and returned, then migration of the stent from the initial position was measured. 
     An evaluation result illustrated in  FIGS. 7 and 8  will be described.  FIGS. 7 and 8  illustrate the evaluation results, in which the inner stent  6  was manufactured using fiber having a fiber diameter of φ 0.4 mm or φ 0.5 mm, having three or four peaks  721  of the end flare portion  72 , by changing the one side length L of one side  721   a  among the two sides  721   a  and  721   a  of the end flare portion  72  as well as the tip angle θ, then the inner stent  6  thus manufactured was used to measure a distance in post-placement migration of the end flare portion  72 , and the results are listed in the tables. The values in the blank cells of  FIGS. 7 and 8  were not actually measured but estimated from the values in cells neighboring the blank cells. 
     First, the evaluation result illustrated in  FIG. 7  will be described. An evaluation result  1  illustrated in  FIG. 7  is the evaluation result using fiber having a fiber diameter of φ 0.4 mm. 
     As illustrated in (a) of  FIG. 7 , in the case in which the number of the peaks was three, the one side length L 1  of one side  721   a  of the peak  721  of the end flare portion  72  was in the range of 14.5 mm to 23.5 mm, and the tip angle θ formed by the two sides  721   a  configuring the peak  721  was in the range of 46.4° to 78.4°, the post-placement migration of the inner stent  6  was 1 cm or less. Therefore, the evaluation result thus obtained shows that, in the case in which the number of the peaks  721  is three, the one side length L 1  of one side  721   a  of the peak  721  of the end flare portion  72  is preferably in the range of 14.5 mm to 23.5 mm, and the tip angle θ formed by the sides  721   a  and  721   a  configuring the peak  721  is preferably 78.4° or less. 
     As illustrated in (b) of  FIG. 7 , in the case in which the number of the peaks was four, the one side length L 1  of one side  721   a  of the peak  721  of the end flare portion  72  was in the range of 16.9 mm to 22.4 mm, and the tip angle θ formed by the two sides  721   a  configuring the peak  721  was in the range of 36.2° to 61.4°, the post-placement migration of the inner stent  6  was 1 cm or less. Therefore, the evaluation result thus obtained shows that, in the case in which the number of the peaks is four, the one side length L 1  of one side  721   a  of the peak  721  of the end flare portion  72  is preferably in the range of 16.9 mm to 22.4 mm, and the tip angle θ formed by the sides  721   a  and  721   a  configuring the peak  721  is preferably 61.4° or less. 
     Next, an evaluation result  2  illustrated in  FIG. 8  will be described.  FIG. 8  illustrates the evaluation result  2  using fiber having a fiber diameter of φ 0.5 mm. 
     As illustrated in (a) of  FIG. 8 , in the case in which the number of the peaks was three, the one side length L 1  of one edge  721   a  of the peak  721  of the end flare portion  72  was in the range of 18.3 mm to 23.5 mm, and the tip angle θ formed by the two sides  721   a  configuring the peak  721  was in the range of 46.4° to 67.1°, the post-placement migration of the inner stent  6  was 1 cm or less. Therefore, the evaluation result thus obtained shows that, in the case in which the number of the peaks is three, the one side length L 1  of one side  721   a  of the peak  721  of the end flare portion  72  is preferably in the range of 18.3 mm to 23.5 mm, and the tip angle θ formed by the two sides  721   a  and  721   a  configuring the peak  721  is preferably 67.1° or less. 
     As illustrated in (b) of  FIG. 8 , in the case in which the number of the peaks was four, the one side length L 1  of one side  721   a  of the peak  721  of the end flare portion  72  was in the range of 12.7 mm to 22.4 mm, and the tip angle θ formed by the two sides  721   a  configuring the peak  721  was in the range of 36.2° to 65.9°, the post-placement migration of the inner stent  6  was 1 cm or less. Therefore, the evaluation result thus obtained shows that, in the case in which the number of the peaks is three, the one side length L 1  of one side  721   a  of the peak  721  of the end flare portion  72  is preferably in the range of 12.7 mm to 22.4 mm, and the tip angle θ formed by the sides  721   a  and  721   a  configuring the peak  721  is preferably 65.9° or less. 
     The evaluation result  2  thus obtained shows that the tip angle θ formed by the two sides  721   a  and  721   a  of the peak  721  of the end flare portion  72  is preferably 78.4° or less; therefore, the tip angle θ is preferably 80° or less, for example. The evaluation result  2  thus obtained shows that the one side length L of one side  721   a  among the two sides  721   a  and  721   a  configuring the peak  721  of the end flare portion  72  is preferably in the range of 16.9 mm to 22.4 mm; therefore, the one side length L is preferably in the range of 16 mm to 22 mm, for example. 
     According to inner stent  6  of the present embodiment described above, the following effects can be achieved. 
     (2a) The inner stent  6  is configured to include the end flare portion  72  arranged at the end in the axial direction; the end flare portion  72  is formed into an annular shape as viewed in the axial direction by consecutively arranging the plurality of peaks  721  in the circumferential direction, the peaks  721  consisting of the tip corners protruding outward in the axial direction; the tip angle θ formed by the two sides  721   a  and  721   a  configuring the peak  721  is 80° or less in the state in which the inner stent  6  is placed inside the gastrointestinal tract; and the number of the plurality of peaks  721  is three to eleven. As a result, as shown in the result of the evaluation test, the post-placement migration of the inner stent  6  due to peristaltic movement of the gastrointestinal tract is 1 cm or less; therefore, trackability to peristaltic movement of the gastrointestinal tract can be achieved, and post-placement migration of the inner stent  6  due to peristaltic movement of the gastrointestinal tract can be suppressed. 
     (2b) The two sides  721   a  and  721   a  configuring the peak  721  of the end flare portion  72  are configured with the two sides  611   a  and  611   a  of the end grid  611  arranged at the end in the axial direction. As a result, the end grid  611  is formed into a grid shape to have compressive strength; therefore, the compressive strength of the peaks  721  can be enhanced. Therefore, post-placement migration of the inner stent  6  due to peristaltic movement of the gastrointestinal tract can be further suppressed, in the state in which the compressive strength of the end flare portion  72  is ensured. 
     (2c) The inner stent  6  includes the plurality of grids  61  arranged side by side in the axial direction, and the end grid  611  is arranged at the end of the plurality of grids  61 . As a result, compressive strength of the inner stent  6  in its entirety can be ensured, while post-placement migration of the inner stent  6  due to peristaltic movement of the gastrointestinal tract can be further suppressed at the end flare portion  72 . 
     (2d) The one side length of one side  721   a  of the two sides  721   a  and  721   a  configuring the peak  721  is 16 mm to 22 mm. As a result, as shown in the result of the evaluation test, the post-placement migration of the inner stent  6  due to peristaltic movement of the gastrointestinal tract is 1 cm or less; therefore, trackability to peristaltic movement of the gastrointestinal tract can be achieved, and post-placement migration of the inner stent  6  due to peristaltic movement of the gastrointestinal tract can be further suppressed. 
     (2e) Adjacent peaks  721  of the end flare portion  72  are fixed at the intersecting point to the most end-side in the axial direction. As a result, post-placement migration of the inner stent  6  due to peristaltic movement of the gastrointestinal tract can be further suppressed, in the state in which the compressive strength of the end flare portion  72  is ensured. 
     In relation to the inner stent  6 , in the present embodiment, for example, the number of the peaks  721  of the synthetic resin stent  6  is three or four, but is not limited thereto. The number of the peaks  721  of the synthetic resin stent  6  is preferably three to eight in the case of applying the synthetic resin stent  6  to a stent for the small intestine, for example, and preferably from three to eleven in the case of applying the synthetic resin stent  6  to a stent for the esophagus, for example. 
     Next, the outer stent  11  will be described.  FIG. 9  is a perspective view illustrating the outer stent  11  of the second embodiment.  FIG. 10  is an enlarged view of the outer stent  11  illustrated in  FIG. 9 . The outer stent  11  of the present embodiment is a biodegradable stent composed of biodegradable fiber, and includes a meshed tubular portion  12  (first woven component portion) and a wavily woven portion  13  (second woven component portion) arranged so as to be woven in the meshed tubular portion  12 , as illustrated in  FIGS. 9 and 10 . 
     The meshed tubular portion  12  is woven into a mesh of a plurality of fibers  120  and configured into a tubular structure, and includes a multitude of argyle voids formed of the fibers  120  on the outer periphery and arranged in an orderly fashion. The mesh of the meshed tubular portion  12  becomes sparse in the axial direction when the outer stent  11  is in the reduced diameter state, and becomes dense in the axial direction when the outer stent  11  is in the expanded diameter state. 
     In the present embodiment, as illustrated in  FIG. 10 , the plurality of fibers  120  configuring the meshed tubular portion  12  includes a plurality of first fibers  121  and a plurality of second fibers  122 . As viewed from the side, the meshed tubular portion  12  includes a multitude of argyle voids formed of the first fibers  121  and the second fibers  122 , and includes a plurality of first intersecting points  123  configured by intersections of the plurality of first fibers  121  and the plurality of second fibers  122 . 
     The plurality of first fibers  121  are formed of synthetic resin fiber extending so as to be inclined at a predetermined angle with respect to the axial direction. In the present embodiment, as illustrated in  FIG. 10 , the plurality of first fibers  121  are arranged so as to be inclined and extending from the upper right side to the lower left side. 
     The plurality of second fibers  122  are formed of synthetic resin fiber extending so as to intersect with the plurality of first fibers  121 . In the present embodiment, as illustrated in  FIG. 10 , the plurality of second fibers  122  are arranged so as to be inclined and extending from the upper left side to the lower right side. 
     The material of the first fibers  121  and the second fiber  122  is not limited; however, a material having a high degree of rigidity is preferable. Examples of the biodegradable resin may include homopolymer, copolymer, or blend polymer composed of L-lactic acid, D-lactic acid, DL-lactic acid, glycolic acid, s-caprolactone, or para-dioxanone. Non-biodegradable resin may also be used as long as the material has a high degree of rigidity. In particular, for example, polylactic acid (PLA) or poly-L-lactic acid (PLLA) is preferably used as the material of the fiber configuring the first fibers  121  and the second fiber  122 . In the present embodiment, the first fibers  121  and the second fiber  122  are formed from polylactic acid (PLA), for example. 
     In the case of using biodegradable fiber as the fiber  120 , the diameter thereof is preferably 0.1 mm to 0.4 mm. When the diameter of the biodegradable fiber  120  is less than 0.1 mm, the strength of the outer stent  11  tends to decrease. When the diameter of the biodegradable fiber  120  exceeds 0.4 mm, the diameter increases in the reduced diameter state, so that it tends to be difficult to load the outer stent  11  into a fine tubular member such as a delivery system. The upper limit of the diameter of the biodegradable fiber  120  is further preferably 0.3 mm, from a perspective of loading into a delivery system having a small inner diameter. The lower limit of the diameter of the biodegradable fiber  120  is more preferably 0.2 mm, from a perspective of maintaining high strength. In the present embodiment, biodegradable fiber having a diameter of 0.2 mm and biodegradable fiber having a diameter of 0.3 mm are used as the fibers  120 . 
     As illustrated in  FIG. 9 , the plurality of annularly formed wave-shaped fibers  130  of the wavily woven portion  13  are arranged so as to be woven in the meshed tubular portion  12 . In the present embodiment, the plurality of fibers  130  configuring the wavily woven portion  13  include: a plurality of third fibers  131  arranged so as to be spaced apart in the axial direction; and a plurality of fourth fibers  132  arranged so as to be spaced apart in the axial direction. The wavily woven portion  13  includes a plurality of second intersecting points  133  formed by intersections of the plurality of third fibers  131  and the plurality of fourth fibers  132 . 
     As illustrated in  FIG. 10 , the third fibers  131  and the fourth fibers  132  are formed into a wave shape extending in the circumferential direction of the meshed tubular portion  12 , in which peaks and valleys consecutively alternate. The third fibers  131  and the fourth fibers  132  are arranged such that the mutual convex portions face each other and the facing convex portions partly overlap with each other. 
     More specifically, the third fibers  131  and the fourth fibers  132  are formed into a wave shape having peaks convex toward the first direction D 1  side and peaks convex toward the second direction D 2  side, in which mutual peaks partly overlap with each other and intersect at two second intersecting points  133 , as viewed from the side. The wavily woven portion  13  includes an intersecting region  134  as viewed from the side. The intersecting region  134  is a region surrounded by the third fiber  131  and the fourth fiber  132  between the two adjacent second intersecting points  133  among the plurality of second intersecting points  133 , where the mutual convex portions of the third fiber  131  and the fourth fiber  132  overlap with each other. The plurality of intersecting regions  134  are formed side by side in the circumferential direction of the meshed tubular portion  12  being tubular. 
     The material of the synthetic resin fiber configuring the third fiber  131  and the fourth fiber  132  is not limited in particular; however, a material having a high degree of restorability is preferable. Examples of the biodegradable resin may include homopolymer, copolymer, or blend polymer composed of L-lactic acid, D-lactic acid, DL-lactic acid, glycolic acid, s-caprolactone, or para-dioxanone. Non-biodegradable resin may also be used as long as the material has a high degree of restorability. For example, polydioxanone (PDO) is preferably used as the material of the third fiber  131  and the fourth fiber  132 . 
     In the case of using biodegradable fiber as the fiber  130 , the diameter thereof is preferably 0.1 mm to 0.4 mm. In the present embodiment, the biodegradable fiber having a diameter of 0.15 mm to 0.22 mm is used as the fiber  130 . 
     The first intersecting points  123  of the meshed tubular portion  12  are arranged in the plurality of intersecting regions  134  of the wavily woven portion  13 , respectively, as the meshed tubular portion  12  is viewed from the side. The plurality of first intersecting points  123  are arranged in the plurality of intersecting regions  134 , respectively, and arranged side by side in the circumferential direction of the meshed tubular portion  12  being tubular. The portion where the first intersecting point  123  of the meshed tubular portion  12  is arranged in the intersecting region  134  of the wavily woven portion  13  configures a first hooking portion  141 . The outer stent  11  of the present embodiment includes the plurality of first hooking portions  141 , in which a row of the plurality of first hooking portions  141  arranged side by side in the circumferential direction is formed throughout the axial direction. 
     In the first hooking portion  141 , the third fiber  131  is arranged in the state of being hookable by one or more of the first fibers  121 , the second fibers  122 , and the fourth fibers  132 , in relation to movement in a direction in which the mutually overlapping convex portion of the third fiber  131  and the fourth fiber  132  shrinks in size. The fourth fiber  132  is arranged in the state of being hookable by one or more of the first fibers  121 , the second fibers  122 , and the third fibers  131 , in relation to movement in a direction in which the mutually overlapping convex portion of the third fiber  131  and the fourth fiber  132  shrinks in size. 
     In the present embodiment, the third fiber  131  and the fourth fiber  132  configuring the wavily woven portion  13  are formed of synthetic resin fiber having an expansion force higher than the first fibers  121  and the second fiber  122  configuring the meshed tubular portion  12 ; therefore, the bent portion thereof has a property of returning to a straight line. At least part of the third fiber  131  and the fourth fiber  132  is arranged so as to be woven in the meshed tubular portion  12 , can apply a force to increase the diameter of the outer stent  11 , and can deform the meshed tubular portion  12  from the reduced diameter state to the expanded diameter state. 
     The configuration of the first hooking portion  141  will be described.  FIG. 10  is a view, in which the radial direction of the tubular outer stent  11  in  FIG. 9  is rearranged along the direction perpendicular to the paper (direction penetrating the paper) of  FIG. 10 . Therefore, the inside of the outer stent  11  in the radial direction is the backside in the vertical direction of the paper of  FIG. 10 ; and the outer side of the outer stent  11  in the radial direction is the frontside in the vertical direction of the paper of  FIG. 10 . 
     As illustrated in  FIG. 10 , the first fibers  121  and the second fibers  122  of the meshed tubular portion  12  decussate at the first intersecting point  123  in the first hooking portion  141 . 
     The third fiber  131  is arranged on the frontside or backside in  FIG. 10  with respect to the fourth fiber  132  (outer side or inner side of the outer stent  11  in the radial direction) at both of the two second intersecting points  133 . As a result, the third fiber  131  and the fourth fiber  132  of the outer stent  11  are arranged in the state of not being mutually hookable, in relation to mutual movement toward the first direction D 1  or the second direction D 2 . 
     The first intersecting point  123  of the first fiber  121  and the second fiber  122  is arranged in the intersecting region  134  surrounded by the third fiber  131  and the fourth fiber  132 , in the overlapping convex portion between the third fiber  131  and the fourth fiber  132  of the wavily woven portion  13 . 
     As illustrated in  FIG. 10 , the first fiber  121  is arranged so as to be inclined and extending from the upper right side to the lower left side in the intersecting region  134 . From the upper right side toward the lower left side, the first fiber  121  passes the frontside of one of the third fiber  131  and the fourth fiber  132 , intersects with the second fiber  122  at the first intersecting point  123 , and passes the backside of the other one of the third fiber  131  and the fourth fiber  132 . As illustrated in  FIG. 10 , the second fiber  122  is arranged so as to be inclined and extending from the upper left side to the lower right side in the intersecting region  134 . The second fiber  122  passes the frontside of one of the third fiber  131  and the fourth fiber  132 , intersects with the first fiber  121  at the first intersecting point  123 , and passes the backside of the other one of the third fiber  131  and the fourth fiber  132 . 
     The first fiber  121 , the second fiber  122 , the third fiber  131 , and the fourth fiber  132  are arranged as described above, whereby, in the first hooking portion  141 , one of the third fiber  131  and the fourth fiber  132  having a peak convex toward the first direction D 1  is arranged in the state of being hookable by the first fiber  121  and the second fiber  122 , in relation to movement in a direction in which the mutually overlapping convex portion of the third fiber  131  and the fourth fiber  122  shrinks in size; and the other one of the third fiber  131  and the fourth fiber  132  having a peak convex toward the second direction D 2  opposite to the first direction D 1  is arranged in the state of being hookable by the first fiber  121  and the second fiber  122 , in relation to movement in a direction in which the mutually overlapping convex portion of the third fiber  131  and the fourth fiber  132  shrinks in size. 
     The outer stent  11  as described above may be manufactured by weaving the wavily woven portion  13  and then weaving the meshed tubular portion  12 , or conversely, by weaving the meshed tubular portion  12  and then weaving the wavily woven portion  13 . In the case of manufacturing the outer stent  11 , for example, a tubular tool with a plurality of pins standing on a circumferential surface at a predetermined interval may be used, the fiber is hooked by the plurality of pins to weave the wavily woven portion  13 , and then the fiber of the meshed tubular portion  12  passes through the intersecting region of the wavily woven portion  13 , whereby the outer stent  11  can be manufactured. 
     The meshed tubular portion  12  of the outer stent  11  configured as described above is woven into a tubular structure with the first fiber  121  and the second fiber  122  inclined with respect to the axial direction, whereby the shape of the stent is maintained in the tubular structure. The wave-shaped wavily woven portion  13  is woven into the meshed tubular portion  12 , and the wavily woven portion  13  (third fiber  131  and fourth fiber  132 ) is formed of synthetic resin fiber having an expansion force higher than the meshed tubular portion  12  (first fiber  121  and second fiber  122 ), and the bent portion thereof has a property of returning to a straight line. Therefore, the wavily woven portion  13  is woven into a wave shape so as to circle the meshed tubular portion  12  in the circumferential direction, whereby the wavily woven portion  13  can apply a force to increase the diameter of the outer stent  11 , and can enhance the expansion force. Thus, the expansion force of the outer stent  11  in the radial direction can be strengthened to achieve self-expandability. Adherence to the wall of the gastrointestinal tract can be increased, and trackability to gastrointestinal motility can be achieved. 
     The outer stent  11  is formed into a tubular structure extending in the longitudinal direction X (predetermined direction) by braiding synthetic resin fiber. The outer stent  11  is arranged so as to cover the outer periphery of the central part of the inner stent  6  in the longitudinal direction X. The outer stent  11  is formed of synthetic resin fiber having a diameter smaller than that of the inner stent  6 , and has a mesh denser than the mesh of the inner stent  6 . The outer stent  11  is deformable from the reduced diameter state to the expanded diameter state. 
     In the case of placing the biodegradable stent  10  inside the gastrointestinal tract, the outer stent  11  in the reduced diameter state is placed inside the gastrointestinal tract, and then expanded after placement. As a result, the outer stent  11  is expanded and placed inside the gastrointestinal tract. Thereafter, the inner stent  6  in the reduced diameter state is placed inside the outer stent  11  and expanded after placement. As a result, the inner stent  6  and the outer stent  11  are placed so as to overlap with each other, in the state in which the central side of the inner stent  6  is arranged inside the outer stent  11 . 
     According to the outer stent  11  of the second embodiment described above, the following effects can be achieved. 
     (3a) The outer stent  11  is configured to include the meshed tubular portion  12  being tubular and composed of the plurality of fibers  121  and  122  woven into a mesh, and the wavily woven portion  13  composed of the plurality of fibers  131  and  132  annularly formed and woven into the meshed tubular portion  12 ; the meshed tubular portion  12  is configured to include the plurality of first fibers  121  extending so as to be inclined at a predetermined angle with respect to the axial direction, the plurality of second fibers  122  extending so as to intersect with the first fibers  121 , and the plurality of first intersecting points  123  formed at intersections of the plurality of first fibers  121  and the plurality of second fibers  122 ; the wavily woven portion  13  is configured to include the plurality of wave-shaped third fibers  131  arranged so as to be spaced apart in the axial direction, and the plurality of wave-shaped fourth fibers  132  arranged so as to be spaced apart in the axial direction; and the at least one of the first intersecting points  123  is arranged in the intersecting region  134  surrounded by the third fibers  131  and the fourth fibers  132 . 
     As a result, the meshed tubular portion  12  of the outer stent  11  is woven into a tubular structure with the first fibers  121  and the second fibers  122  inclined with respect to the axial direction, whereby the shape of the stent is maintained in the tubular structure. The wave-shaped wavily woven portion  13  is woven into the meshed tubular portion  12 , and the first intersecting point  123  of the meshed tubular portion  12  is arranged in the intersecting region  134  of the wavily woven portion  13 , whereby the wavily woven portion  13  can apply a force to increase the diameter and can enhance the expansion force in the radial direction. Thus, the expansion force of the outer stent  11  in the radial direction can be strengthened to achieve self-expandability. Adherence to the wall of the gastrointestinal tract can be increased, and trackability to gastrointestinal motility can be achieved. Therefore, the stent can ensure loadability into a fine tubular member such as a delivery system, in which migration of the stent is unlikely to occur after placement at the affected site of the natural tracts. 
     (3b) The plurality of intersecting regions  134  are formed side by side in the circumferential direction of the meshed tubular portion  12  being tubular; and the plurality of first intersecting points  123  are formed side by side in the circumferential direction of the meshed tubular portion  12  being tubular, and arranged in the plurality of intersecting regions  134 , respectively. As a result, the wavily woven portion  13  can be woven along the circumferential direction, whereby the expansion force of the outer stent  11  in the radial direction can be further strengthened. 
     (3c) When the first intersecting point  123  of the meshed tubular portion  12  is configured so as to be arranged in the intersecting region  134  of the wavily woven portion  13 , the third fiber  131  is arranged in the state of being hookable by one or more of the first fiber  121 , the second fiber  122 , and the fourth fiber  132 , in relation to movement in a direction in which the overlapping portion of the third fiber  131  and the fourth fiber  132  shrinks in size; and the fourth fiber  132  is arranged in the state of being hookable by one or more of the first fiber  121 , the second fiber  122 , and the third fiber  131 , in relation to movement in a direction in which the overlapping portion of the third fiber  131  and the fourth fiber  132  shrinks in size. As a result, any of these fibers is hooked by the third fiber  131  and the fourth fiber  132  of the wavily woven portion  13 , whereby displacement of the first intersecting point  123  can be prevented. 
     (3d) The wavily woven portion  13  (third fiber  131  and fourth fiber  132 ) is formed of synthetic resin fiber having an expansion force higher than the meshed tubular portion  12  (first fiber  121  and second fiber  122 ). As a result, the meshed tubular portion  12  being tubular is formed of the first fiber  121  and the second fiber  122 , and the third fiber  131  and fourth fiber  132  can expand the first fiber  121  and the second fiber  122  in the radial direction; therefore, the expansion force in the radial direction can be further enhanced. 
     According to the biodegradable stent  10  of the second embodiment described above, similar to the effects of (1a) of the first embodiment, the following effects (4a) can be achieved. 
     (4a) The biodegradable stent  10  is configured to include the inner stent  6  including the inner stent body  71  formed of synthetic resin fiber into a tubular structure having a mesh, and being deformable from the reduced diameter state to the expanded diameter state; and the outer stent  11  formed into a tubular structure having a mesh denser than the mesh of the inner stent body  71 , arranged so as to cover the outer periphery of the inner stent body  71 , and being deformable from the reduced diameter state to the expanded diameter state. Therefore, the inner stent body  71  formed with a sparse mesh is arranged inside the outer stent  11  formed with a dense mesh, whereby the pressing force of the inner stent body  71  from the inside of the outer stent  11  can reinforce the strength of the outer stent  11 , and the strength of the biodegradable stent  10  in its entirety can be ensured. As a result, the gastrointestinal tract can be pressed in the state in which the strength of the biodegradable stent  10  in its entirety is ensured by the pressing force of the inner stent body  71  from the inside of the outer stent  11 , whereby the biodegradable stent  10  can be prevented from moving while preventing stenosis. Therefore, the biodegradable stent  10 , which can exert self-expandability, restorability, adherence to the gastrointestinal tract, and trackability to peristaltic movement, can be achieved. 
     &lt;First Variation of Second Embodiment&gt; 
     An outer stent  11 A of a first variation of the second embodiment will be described.  FIG. 11  is a view illustrating the outer stent  11 A according to the first variation of the second embodiment. As illustrated in  FIG. 11 , the outer stent  11 A of the first variation of the second embodiment is configured to include a first hooking portion  141  (left side in  FIG. 11 ) and a second hooking portion  142  (right side in  FIG. 11 ). The plurality of first hooking portions  141  and the plurality of second hooking portions  142  are spirally and alternately arranged in the circumferential direction in the outer stent  11 A. 
     Since the configuration of the first hooking portion  141  illustrated in  FIG. 11  is similar to that of the first hooking portion  141  described in the second embodiment, description thereof is omitted. 
     The configuration of the second hooking portion  142  will be described. As illustrated in  FIG. 11 , the first fiber  121  and the second fiber  122  of the meshed tubular portion  12  decussate at the first intersecting point  123  in the second hooking portion  142  as well, similar to the first hooking portion  141  of the second embodiment. 
     In the second hooking portion  142 , the third fiber  131  is arranged frontside of the fourth fiber  132  at one of the two second intersecting points  133  (left side in  FIG. 11 ) and arranged backside of the fourth fiber  132  at the other one of the second intersecting points  133  (right side in  FIG. 11 ). As a result, the third fiber  131  and the fourth fiber  132  are arranged in the state of being mutually hookable, in relation to movement in a direction in which the mutually overlapping convex portion of the third fiber  131  and the fourth fiber  132  shrinks in size. 
     The first intersecting point  123  of the first fiber  121  and the second fiber  122  is arranged in the intersecting region  134  surrounded by the third fiber  131  and the fourth fiber  132  of the wavily woven portion  13 . 
     As illustrated in  FIG. 11 , the first fiber  121  is arranged so as to be inclined and extending from the upper right side to the lower left side in the intersecting region  134 , and passes the backside of the fourth fiber  132 , intersects with the second fiber  122  at the first intersecting point  123 , and passes the backside of the third fiber  131 , from the upper right side to the lower left side. As illustrated in  FIG. 11 , the second fiber  122  is arranged so as to be inclined and extending from the upper left side to the lower right side in the intersecting region  134 , passes the backside of the fourth fiber  132 , intersects with the first fiber  121  at the first intersecting point  123 , and passes the backside of the third fiber  131 , from the upper left side to the lower right side. In other words, in the second hooking portion  142 , the first fiber  121  and the second fiber  122  in their entirety are arranged backside of the third fiber  131  and the fourth fiber  132 , whereby the third fiber  131  and the fourth fiber  132  in their entirety are arranged frontside of the first fiber  121  and the second fiber  122 . 
     The first fiber  121 , the second fiber  122 , the third fiber  131 , and the fourth fiber  132  are arranged as above, whereby, in the second hooking portion  142 , the third fiber  131  and the fourth fiber  132  are arranged in the state of being mutually hookable, in relation to movement in a direction in which the mutually overlapping convex portion of the third fiber  131  and the fourth fiber  132  shrinks in size; the third fiber  131  and the fourth fiber  132  in their entirety are arranged frontside of the first fiber  121  and the second fiber  122 ; and the third fiber  131  and the fourth fiber  132  are arranged in the state of not being hookable by the first fiber  121  and the second fiber  122 , in relation to movement of the third fiber  131  and the fourth fiber  132  in the first direction D 1  or the second direction D 2  direction. 
     According to the outer stent  11 A of the first variation of the second embodiment described above, in addition to the effects (3a) to (3d) described above, the following effects can be achieved. (3e) The plurality of configurations are provided, in which the first intersecting point  123  of the meshed tubular portion  12  is arranged in the intersecting region  134  of the wavily woven portion  13 ; and the third fiber  131  and fourth fiber  132  are arranged in the state of being mutually hookable, in relation to movement in a direction in which the overlapping portion of the third fiber  131  and fourth fiber  132  shrinks in size, and also arranged in the state of not being hookable by the first fiber  121  and the second fiber  122  when the third fiber  131  and fourth fiber  132  move. As a result, the third fiber  131  and/or the fourth fiber  132  are/is arranged in the state of not being hookable by the first fiber  121  and the second fiber  122 , whereby the length in the axial direction is unlikely to be restricted when the outer stent  11 A is extended; therefore, loadability into a delivery system can be improved. 
     &lt;Second Variation of Second Embodiment&gt; 
     An outer stent  11 B of a second variation of the second embodiment will be described.  FIG. 12  is a view illustrating the outer stent  11 B according to the second variation of the second embodiment. As compared with the outer stent  11  of the second embodiment, in the case of the outer stent  11 B of the second variation of the second embodiment, the rows of the plurality of first hooking portions  141  arranged in the circumferential direction are arranged so as to be spaced one row apart from each other, instead of being packed in the axial direction of the outer stent  11 A, in the wave-shaped wavily woven portion  13  (second woven component portion) woven into the meshed tubular portion  12  (first woven component portion). 
     As illustrated in  FIG. 12 , the wavily woven portion  13  is configured to include a row in which the plurality of first hooking portion  141  are arranged side by side in the circumferential direction, and a row without the first hooking portions  141 , in the axial direction of the meshed tubular portion  12  having the plurality of first intersecting points  123 . The first intersecting point  123  is arranged in the intersecting region  134  of the wavily woven portion  13  in the first hooking portion  141 , in the row in which the plurality of first hooking portions  141  are arranged side by side in the circumferential direction. The plurality of first intersecting points  123  are arranged side by side in the circumferential direction, in the row without the first hooking portions  141 . 
     According to the outer stent  11 B of the second variation of the second embodiment described above, in addition to the effects (3a) to (3e) described above, the following effects can be achieved. 
     (3f) The rows of the plurality of first hooking portions  141  arranged in the circumferential direction are arranged so as to be spaced one row apart from each other in the axial direction. As a result, the rows without the first hooking portions  141  are provided, instead of packing the first hooking portions  141  in the axial direction of the outer stent  11 B, whereby the length in the axial direction is unlikely to be restricted by the third fiber  131  and the fourth fiber  132  when the outer stent  11 B is extended; therefore, loadability into a delivery system can be improved. 
     &lt;Third Variation of Second Embodiment&gt; 
     An outer stent  11 C of the third variation of the second embodiment will be described.  FIG. 13  is a view illustrating the outer stent  11 C according to the third variation of the second embodiment. 
     As illustrated in  FIG. 13 , the outer stent  11 C of the third variation of the second embodiment includes a plurality of loops  135  formed at the top of the peaks of the wave-shaped fourth fiber  132 . The loop  135  is formed into a loop shape surrounding the first fiber  121 , the second fiber  122 , the third fiber  131 , and the fourth fiber  132 , in which the first intersecting point  123  of the meshed tubular portion  12  is arranged in the intersecting region  134  of the wavily woven portion  13 . 
     The plurality of loops  135  may be consecutively provided at the top of the peaks of the wave-shaped fourth fiber  132 , or may be intermittently provided at the top of the plurality of peaks of the wave-shaped fourth fiber  132 . The loops  135  may not be configured to surround all of the first fiber  121 , the second fiber  122 , the third fiber  131 , and the fourth fiber  132 , or may be configured to surround only part of the first fiber  121 , the second fiber  122 , the third fiber  131 , and the fourth fiber  132 . 
     According to the outer stent  11 C of the third variation of the second embodiment described above, in addition to the effects (3a) to (3f) described above, the following effects can be achieved. 
     (3g) The loops  135  are provided at the top of the peaks of the wave-shaped third fiber  131  and/or the wave-shaped fourth fiber  132 . Here, for example, in the case without the loops  135  at the top of the peaks of the wavily woven portion  13 , only an expansion force in the radial direction is applied to the wave-shaped third fiber  131  and the wave-shaped fourth fiber  132  of the wavily woven portion  13 ; therefore, it is difficult to control the diametrical size of the outer stent  11 C. In contrast, in the present invention, the loops  135  can apply a contraction force in the radial direction; therefore, the diametrical size of the outer stent  11 C can be controlled. 
     &lt;Fourth Variation of Second Embodiment&gt; 
     Referring to  FIGS. 14 and 15 , an outer stent  110  according to a fourth variation of the second embodiment will be described.  FIG. 14  is a perspective view illustrating the outer stent  110  according to the fourth variation of the second embodiment of the present invention.  FIG. 15  is an enlarged view of the outer stent  110  illustrated in  FIG. 14 . In the outer stent  110  illustrated in  FIG. 15 , one side in the axial direction is referred to as a first direction D 1 , and the other side in the axial direction is referred to as a second direction D 2 . In the outer stent  110 , one side in the circumferential direction is referred to as a third direction D 3  (left side of  FIG. 15 ), and the other side in the circumferential direction is referred to as a fourth direction D 4  (right side of  FIG. 15 ). 
     As illustrated in  FIGS. 14 and 15 , the synthetic resin stent of the present embodiment is the outer stent  110  composed of biodegradable fiber, and includes a first bent woven portion  200  (first woven component portion) and a second bent woven portion  300  (second woven component portion) arranged so as to be woven in the first bent woven portion  200 . 
     The first bent woven portion  200  is formed into a mesh, in which a plurality of fibers  220  repeatedly bent so as to extend in the axial direction are arranged in the circumferential direction and formed into a tubular structure. In the present embodiment, as illustrated in  FIG. 15 , the fiber  220  configuring the first bent woven portion  200  is configured with a plurality of first fibers  221  and a plurality of second fibers  222 . 
     The plurality of first fibers  221  are formed of synthetic resin fiber repeatedly bent so as to be inclined at a predetermined angle with respect to the axial direction and extending in the axial direction. The plurality of first fibers  221  are repeatedly bent and extend in the axial direction so as to shuttle in a predetermined range of width in the circumferential direction of the first bent woven portion  200 . 
     The plurality of second fibers  222  are arranged to include a portion intersecting with the plurality of first fibers  221 , and formed of synthetic resin fiber repeatedly bent so as to shuttle and extending in the axial direction. The plurality of second fibers  222  are repeatedly bent and extend in the axial direction so as to shuttle in a predetermined range of width in the circumferential direction of the first bent woven portion  200 . 
     In the present embodiment, the plurality of first fibers  221  and the plurality of second fibers  222  are composed of a single fiber, folding back at the upper and lower ends of the first bent woven portion  200  in the axial direction. The plurality of first fibers  221  and the plurality of second fibers  222  are part of a single fiber. In other words, the first fiber  221  and the second fiber  222  are alternately arranged in the circumferential direction of the first bent woven portion  200 . The first bent woven portion  200  may be configured with a plurality of fibers. 
     More specifically, as illustrated in  FIG. 15 , the first fiber  221  and the second fiber  222  both include a plurality of bent portions including peaks convex toward the third direction D 3  side and peaks convex toward the fourth direction D 4  side. As viewed from the side, the first fiber  221  and the second fiber  222  are arranged to have bent portions overlapping with each other and intersect in the first intersecting region  223 . The first fiber  221  and the second fiber  222  are formed such that the region surrounded by the first fiber  221  and the second fiber  222  is an opening having a substantially diamond shape in the first intersecting region  223 . 
     In the first intersecting region  223 , the first fiber  221  and the second fiber  222  may be arranged so as to overlap with each other as viewed from the side, and may be hooked by each other or may not be hooked by each other. In the present embodiment, the bent portions of the first fiber  221  and the second fiber  222  are hooked by each other in the upper end portion and the lower end portion of the outer stent  110  in the axial direction, and are not hooked by each other in portions excluding the upper end portion and the lower end portion of the outer stent  110  in the axial direction. The first intersecting regions  223  are arranged side by side in both the axial direction and the circumferential direction of the first bent woven portion  200 . 
     The first intersecting region  223  is formed such that the region surrounded by the first fiber  221  and the second fiber  222  is an opening. However, the size of the opening of the first intersecting region  223  is not limited. In the first intersecting region  223 , the first fiber  221  and the second fiber  222  may pull each other in the overlapping portion in a direction to shrink in size, so that the first fiber  221  and the second fiber  222  may be hooked by each other, whereby the region surrounded by the first  221  and the second fiber  222  may not be an opening. 
     The material of the first fiber  221  and the second fiber  222  is not limited in particular; however, a material having a high degree of rigidity is preferable. Examples of the biodegradable resin may include homopolymer, copolymer, or blend polymer composed of L-lactic acid, D-lactic acid, DL-lactic acid, glycolic acid, s-caprolactone, or para-dioxanone. Non-biodegradable resin may also be used as long as the material has a high degree of rigidity. In particular, for example, polylactic acid (PLA) or poly-L-lactic acid (PLLA) is preferably used as the material of the fiber composing the first fiber  221  and the second fiber  222 . In the present embodiment, the first fibers  221  and the second fiber  222  are formed from polylactic acid (PLA), for example. 
     In the case of using biodegradable fiber as the fiber  220 , the diameter thereof is preferably 0.1 mm to 0.4 mm. When the diameter of the biodegradable fiber  220  is less than 0.1 mm, the strength of the outer stent  110  tends to decrease. When the diameter of the biodegradable fiber  20  exceeds 0.4 mm, the diameter in the reduced diameter state increases, so that it tends to be difficult to load the outer stent  110  into a fine tubular member such as a delivery system. The upper limit of the diameter of the biodegradable fiber  20  is further preferably 0.3 mm, from a perspective of loading into a delivery system having a small inner diameter. The lower limit of the diameter of the biodegradable fiber  220  is more preferably 0.2 mm, from a perspective of maintaining high strength. In the present embodiment, biodegradable fiber having a diameter of 0.2 mm and biodegradable fiber having a diameter of 0.3 mm are used as the fibers  220 . 
     As illustrated in  FIGS. 14 and 15 , the second bent woven portion  300  is arranged so as to be woven into the first bent woven portion  200 , in which a plurality of circularly configured fibers  230  repeatedly bent so as to extend in the circumferential direction are arranged side by side in the axial direction. In the present embodiment, as illustrated in  FIG. 15 , the fiber  230  configuring the second bent woven portion  300  is configured with a plurality of third fibers  231  and a plurality of fourth fibers  232 . 
     The plurality of third fibers  231  are formed of synthetic resin fiber repeatedly bent so as to be inclined at a predetermined angle with respect to the axial direction and extending in the circumferential direction. The plurality of third fibers  231  are repeatedly bent and extend in the circumferential direction so as to shuttle in a predetermined range of width in the axial direction of the second bent woven portion  300 . 
     The plurality of fourth fibers  232  are arranged to include a portion intersecting with the plurality of third fibers  231 , and formed of synthetic resin fiber repeatedly bent so as to extend in the circumferential direction. The plurality of fourth fibers  232  are repeatedly bent and extend in the circumferential direction so as to shuttle in a predetermined range of width in the axial direction of the second bent woven portion  300 . 
     More specifically, the third fiber  231  and the fourth fiber  232  both include a plurality of bent portions including peaks convex toward the first direction D 1  side and peaks convex toward the second direction D 2  side. As viewed from the side, the third fiber  231  and the fourth fiber  232  are arranged to have bent portions overlapping with each other and intersect in the second intersecting region  233 . In the second intersecting region  233 , the third fiber  231  and the fourth fiber  232  may be arranged so as to overlap with each other as viewed from the side, and may be hooked by each other or may not be hooked by each other. In the present embodiment, the bent portions of the third fiber  231  and the fourth fiber  232  are hooked by each other in the second intersecting region  233 , and the region surrounded by the third fiber  231  and the fourth fiber  232  is not an opening. The second intersecting regions  233  are arranged side by side in the axial direction and the circumferential direction of the second bent woven portion  300 . 
     The bent portions of the third fiber  231  and the fourth fiber  232  are hooked by each other, and the region surrounded by the third  231  and the fourth fiber  232  is not an opening in the second intersecting region  233 , which is not limited, however. The region surrounded by the third  231  and the fourth fiber  232  may be an opening in the second intersecting region  233 , and the size of the opening in the second intersecting region  233  is not limited. 
     Of the plurality of third fibers  231  and the plurality of fourth fibers  232 , the third fiber  231  or the fourth fiber  232  arranged at the upper end or the lower end of the outer stent  110  is arranged so as to be wound around the first fiber  221  or the second fiber  222  of the first bent woven portion  200 . 
     The material of the synthetic resin fiber configuring the third fiber  231  and the fourth fiber  232  is not limited in particular; however, a material having a high degree of restorability is preferable. Examples of the biodegradable resin may include homopolymer, copolymer, or blend polymer composed of L-lactic acid, D-lactic acid, DL-lactic acid, glycolic acid, s-caprolactone, or para-dioxanone. Non-biodegradable resin may also be used as long as the material has a high degree of restorability. For example, polydioxanone (PDO) is preferably used as the material of the third fiber  231  and the fourth fiber  232 . 
     In the case of using biodegradable fiber as the fiber  230 , the diameter thereof is preferably 0.1 mm to 0.4 mm. In the present embodiment, the biodegradable fiber having a diameter of 0.15 mm to 0.22 mm is used as the fiber  230 . 
     The first intersecting region  223  of the first bent woven portion  200  and the second intersecting region  233  of the second bent woven portion  300  are arranged so as to be at least partially overlapping with each other, as the outer stent  110  is viewed from the side. The at least partially overlapping portion of the second intersecting region  233  of the second bent woven portion  300  and the second intersecting region  233  of the first bent woven portion  200  configures a hooking portion  241 . The outer stent  110  of the present embodiment includes a plurality of hooking portions  241 , in which a row of the hooking portions  241  arranged in the circumferential direction is formed throughout the axial direction. 
     The configuration of the hooking portion  241  will be described.  FIG. 15  is a view, in which the radial direction of the tubular outer stent of  110  in  FIG. 14  is rearranged along the direction perpendicular to the paper (direction penetrating the paper) of  FIG. 15 . Therefore, the inside of the outer stent  110  in the radial direction is the backside in the vertical direction of the paper of  FIG. 15 ; and the outer side of the outer stent  110  in the radial direction is the frontside in the vertical direction of the paper of  FIG. 15 . Some parts of the upper and lower ends of the outer stent  110  in the axial direction are woven to prevent the fibers from coming apart. Here, the hooking portion  241  in the present embodiment will be described for the portions excluding the upper and lower ends of the outer stent  110  in the axial direction. 
     As illustrated in  FIG. 15 , the second intersecting region  233  of the third fiber  231  and the fourth fiber  232  is arranged in the first intersecting region  223  surrounded by the first fiber  221  and the second fiber  222 , in the hooking portion  241 . 
     The first fiber  221  and the second fiber  222  of the first bent woven portion  200  are arranged to include an opening in the first intersecting region  223  in the hooking portion  241 , in which a bent portion convex toward the third-direction D 3  side and a bent portion convex toward the fourth-direction D 4  side overlap with each other. 
     The third fiber  231  and the fourth fiber  232  of the second bent woven portion  300  are arranged in the state in which a bent portion convex toward the first direction D 1  side and a bent portion convex toward the second direction D 2  side are hooked by each other, in the second intersecting region  233  in the hooking portion  241 . 
     As illustrated in  FIG. 15 , the bent portion convex toward the third direction D 3  side in the first fiber  221  and the second fiber  222  of the first bent woven portion  200  passes the backside of the third fiber  231  and the fourth fiber  232  on the apex side of the convex portion (third direction D 3  side), and passes the frontside of the third fiber  231  and the fourth fiber  232  on the opening side of the convex portion (fourth direction D 4  side), in the hooking portion  241 . The bent portion convex toward the fourth direction D 4  side in the first fiber  221  and the second fiber  222  passes the frontside of the third fiber  231  and the fourth fiber  232  on the apex side of the convex portion (fourth direction D 4  side), and passes the frontside of the third fiber  231  and the fourth fiber  232  on the opening side of the convex portion (third direction D 3  side). The bent portion convex toward the third direction D 3  side in the first fiber  221  and the second fiber  222  passes the frontside of the bent portion convex toward the fourth direction D 4  side. 
     The first fiber  221 , the second fiber  222 , the third fiber  231 , and the fourth fiber  232  are arranged as above, whereby, in the hooking portion  241 , the first fiber  221  is arranged in the state of being hookable by the third fiber  231  and the fourth fiber  232 , in relation to movement in a direction in which the mutually overlapping convex portion of the first fiber  221  and the second fiber  222  shrinks in size; and the second fiber  222  is arranged in the state of being hookable by the third fiber  231  and the fourth fiber  232 , in relation to movement in a direction in which the mutually overlapping convex portion of the first fiber  221  and the second fiber  222  shrinks in size. 
     The outer stent  110  as described above may be manufactured by forming the second bent woven portion  300  and then forming the first bent woven portion  200 , or conversely, by forming the first bent woven portion  200  and then forming the second bent woven portion  300 . 
     The outer stent  110  configured as described above is formed into a tubular structure with the first fiber  221  and the second fiber  222  repeatedly bent so as to be inclined with respect to the axial direction and extending in the axial direction in the first bent woven portion  200 , whereby the shape of the stent is maintained in the tubular structure. The second bent woven portion  300  is woven into the first bent woven portion  200 , the second bent woven portion  300  (third fiber  231  and fourth fiber  232 ) is formed of synthetic resin fiber having a higher expansion force than the first bent woven portion  200  (first fiber  221  and second fiber  222 ), and the bent portion thereof has a property of returning to a straight line. Therefore, the second bent woven portion  300  is woven and repeatedly bent so as to circle the first bent woven portion  200  in the circumferential direction, whereby the second  300  can apply a force to increase the diameter of the outer stent  110 , and can enhance the expansion force. Thus, the expansion force of the outer stent  110  in the radial direction can be strengthened to achieve self-expandability. Adherence to the wall of the gastrointestinal tract can be increased, and trackability to gastrointestinal motility can be achieved. 
     According to the outer stent  110  of the fourth variation of the second embodiment described above, the following effects can be achieved. 
     (4a) The outer stent  110  is configured to include the tubular first bent woven portion  200  composed of one or more fibers formed into a mesh, and the second bent woven portion  300  composed of one or more fibers annularly arranged so as to be woven into the first bent woven portion  200 ; the first bent woven portion  200  is configured to include the plurality of first fibers  221  repeatedly bent so as to be inclined at a predetermined angle with respect to the axial direction and extending in the axial direction, the plurality of second fiber  222  repeatedly bent so as to be inclined at a predetermined angle with respect to the axial direction and extending in the axial direction and arranged to include a portion intersecting with the first fiber  221 , and the plurality of first intersecting regions  223  configured by intersections of the plurality of first fibers  221  and the plurality of second fibers  222 ; the second bent woven portion  300  is configured to include the plurality of third fibers  231  repeatedly bent so as to be inclined at a predetermined angle with respect to the axial direction and extending in the circumferential direction; the plurality of fourth fiber  232  repeatedly bent so as to be inclined at a predetermined angle with respect to the axial direction and extending in the axial direction and arranged to include a portion intersecting with the third fiber  231 , and the plurality of second intersecting regions  233  configured by intersections of the plurality of third fibers  231  and the plurality of fourth fibers  232 ; and the first intersecting region  223  and the second intersecting region  233  are arranged so as to be at least partially overlapping with each other. 
     As a result, the first fiber  221  and the second fiber  222  repeatedly bent so as to be inclined at a predetermined angle with respect to the axial direction and extending in the axial direction maintains the first bent woven portion  200  in the tubular structure, whereby the tubular structure of the outer stent  110  is maintained. The second bent woven portion  300  is woven into the first bent woven portion  200 , and the first intersecting region  223  of the first bent woven portion  200  and the second intersecting region  233  of the second bent woven portion  300  are arranged to at least partially overlap with each other, whereby the second bent woven portion  300  can apply a force to increase the diameter, and can enhance the expansion force in the radial direction. Thus, the expansion force of the outer stent  110  in the radial direction can be strengthened to achieve self-expandability. Adherence to the wall of the gastrointestinal tract can be increased, and trackability to gastrointestinal motility can be achieved. Therefore, the stent can ensure loadability into a fine tubular member such as a delivery system, in which migration of the stent is unlikely to occur after placement at the affected site of the natural tracts. 
     (4b) In the configuration in which the first intersecting region  223  of the first bent woven portion  200  and the second intersecting region  233  of the second bent woven portion  300  are arranged to overlap each other, the first fiber  221  is arranged in the state of being hookable by one or more of the third fiber  231  and the fourth fiber  232 , in relation to movement in a direction in which the overlapping portion of the first fiber  221  and the second fiber  222  shrinks in size; and the second fiber  222  is arranged in the state of being hookable by one or more of the third fiber  231  and the fourth fiber  232 , in relation to movement in a direction in which the overlapping portion of the first fiber  221  and the second fiber  222  shrinks in size. As a result, the first fiber  221  and the second fiber  222  of the first bent woven portion  200  are hooked by any one of the third fiber  231  and the fourth fiber  232 , whereby displacement of the first intersecting region  223  and the second intersecting region  233  can be prevented. 
     A manufacture example and an example of the biodegradable stent of the first to third variations of the second embodiment will be briefly described. In the present manufacture example, the outer stent  11 A of the first variation of the second embodiment (see  FIG. 11 ) and the outer stent  11 B of the second variation of the second embodiment (see  FIG. 12 ) are manufactured by braiding six PLA fibers (three fibers having a fiber diameter of 0.2 mm, and three fibers having a fiber diameter of 0.3 mm), and using PDO fiber having a fiber diameter of 0.15 mm to 0.22 mm to manufacture a wave shape. The outer stent  11 C of the third variation of the second embodiment (see  FIG. 13 ) is manufactured by braiding six PLA fibers (having a diameter of 0.2 mm) and using PDO fiber to manufacture a wave shape with a fiber diameter of 0.30 mm to 0.349 mm, and a loop shape with a fiber diameter of 0.15 mm to 0.22 mm. The outer stents  11 A,  11 B and  11 C are formed by weaving and winding the PLA fiber around the PDO fiber having a wave shape, whereby the shape of the stents is unlikely to collapse. 
     The outer stents  11 A,  11 B and  11 C are manufactured under the conditions described above, whereby a stent loadable into a delivery system for the small intestine (φ 2.8 mm) can be achieved. In the case of use in other gastrointestinal tracts, the diameter of the delivery system increases and the fiber diameter can increase as well, whereby a stent having further higher strength can be expected to be manufactured. The fiber diameter as well as the stent diameter and length may be arbitrary. 
     The outer stents  11 A and  11 C thus prepared were used to conduct the following experiment. A tool manufactured in-house to simulate peristaltic movement was used to conduct a migration test on the stents. The tool for use in the present test simulates peristaltic movement, in which the stent was placed inside a tube mimicking the intestinal tract, and the tube was squeezed 10 times with a tool having a hole-diameter of 10 mm, assuming that the intestinal tract shrinks to φ 10 mm due to peristaltic movement. 
     In an intestinal tract model in which the diameter of the intestinal tract expands to φ 17 mm and shrinks to φ 10 mm during peristaltic movement, the outer stent  11 A of the second embodiment (stent having a length of 55 mm) moved by 35 mm. In an intestinal tract model in which the diameter of the intestinal tract expands to φ 12 mm and shrinks to φ 10 mm during peristaltic movement, the outer stent  11 C of the third variation of the second embodiment (stent having a length of 36 mm) moved by 10 mm. In the same intestinal tract models, a metallic stent (stent having a length of 110 mm) moved by 40 mm, suggesting that the outer stents  11 A and  11 C of the first and third variations of the second embodiment have trackability to the intestinal tract. 
     The preferred embodiments of the synthetic resin stent of the present invention have been described above; however, the present invention is not limited to the embodiments and can be modified as appropriate. 
     For example, the biodegradable stent composed of biodegradable fiber has been used as a synthetic resin stent in the embodiments, which are not limited. In other words, nonbiodegradable synthetic resin fiber may be used to compose a stent. 
     The plurality of first hooking portions  141  are provided to the outer stent  11  in its entirety, in the second embodiment. The first hooking portions  141  and the second hooking portions  142  are alternately provided, in the third variation of the second embodiment. 
     The plurality of first hooking portions  141  are arranged side by side in the circumferential direction, in the second variation of the second embodiment. However, the present invention is not limited to the embodiments, and the first hooking portions  141  and/or the second hooking portions  142  may not be provided to the biodegradable stent in its entirety, or may be provided to part of the biodegradable stent. 
     In the second embodiment, the loop  135  is provided at the top of the peaks of the wave-shaped fourth fiber  132 , which is not limited, and may be provided at the top of the peaks at the wave-shaped third fiber  131 . 
     The preferred embodiments of the synthetic resin stent and the stent delivery system of the present invention have been described above; however, the present invention is not limited to the embodiments and can be modified as appropriate. 
     For example, the biodegradable stent composed of biodegradable fiber is used as a synthetic resin stent in the embodiments, which are not limited. In other words, nonbiodegradable synthetic resin fiber may be used to compose a stent. 
     The biodegradable stent  1  is configured by combining the inner stent  2  and the outer stent  5  in the first embodiment, and the biodegradable stent  10  is configured by combining the inner stent  6  and the outer stent  11  in the second embodiment; however, the present invention is not limited thereto. The biodegradable stent may be configured by combining the inner stent  2  of the first embodiment and the outer stent  11  of the second embodiment, or may be configured by combining the inner stent  6  of the second embodiment and the outer stent  5  of the first embodiment. 
     EXPLANATION OF REFERENCE NUMERALS 
     
         
         
           
               1 ,  10 : biodegradable stent (synthetic resin stent) 
               2 : inner stent (first stent) 
               3 : inner stent body (first stent body) 
               4 : end flare portion (end enlarged diameter portion) 
               5 : outer stent (second stent) 
               6 : inner stent (first stent) 
               11 ,  11   a ,  11   b ,  11   c ,  110 : outer stent (second stent) 
               32 : tubular connecting portion (connecting portion) 
               311 : polygonal annular portion 
               200 : first bent woven portion (first woven component portion) 
               300 : second bent woven portion (second woven component portion) 
               221 : first fiber 
               222 : second fiber 
               223 : first intersecting region 
               231 : third fiber 
               232 : fourth fiber 
               233 : second intersecting region 
               241 : hooking portion