Abstract:
A filtration filter comprises a porous film including a central film portion having a plurality of through holes and an outer edge portion adjacent to the central film portion. The porous film portion lies in a flat plane in the absence of a force being applied to the central portion of the central film portion. A frame holds the outer edge portion of the porous film in such a manner that when a force is applied to the central film portion in a first direction, the central film portion moves in the first direction relative to the flat plane and the outer edge portion moves in a second direction, opposite to the first direction, relative to the flat plane. In this way stresses applied to the porous film during a filtering operation are reduced.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a continuation of International application No. PCT/JP2016/066559, filed on Jun. 3, 2016, which claims priority to Japanese Patent Application No. 2015-124772, filed on Jun. 22, 2015, the entire contents of each of which are incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a filtration filter that filters a filtration object in fluid. 
       BACKGROUND ART 
       [0003]    A cell trapping system has recently been disclosed as a usage example of a filter that filters a filtration object in fluid (see, for example, International Publication No. 2015/019889). In this cell trapping system, a filter for trapping cells is fitted in a tensioned state between a lid member and a storage member. Cells are trapped by causing fluid containing the cells to pass through the filter while the filer is in the tensioned state. 
       SUMMARY OF INVENTION 
       [0004]    Because the filter of the forgoing cell trapping system is held in the tensioned state, when the fluid passes through the filter, the filter is broken by stress applied thereto. 
         [0005]    The present invention solves this problem by preferably holding the filter in a non-tensioned state and reducing the forces applied to the filter. 
         [0006]    In accordance with a preferred embodiment of the invention, the filter includes a porous film including a central film portion having a plurality of through holes and an outer edge portion adjacent to the central film portion. The porous film portion lies in a flat plane in the absence of a force being applied to the central portion of the central film portion. A frame holds the outer edge portion of the porous film in such a manner that when a force is applied to the central film portion in a first direction, the central film portion moves in the first direction relative to the flat plane and the outer edge portion moves in a second direction, opposite to the first direction, relative to the flat plane. 
         [0007]    The porous film is preferably made of metal. The outer edge portion can have a holding hole and the frame can have a first projection extending through the holding hole. The diameter and/or shape of the holding hole allows the outer edge portion to move in the second direction. The diameter of the first projection is preferably larger than a diameter of the holding hole and has a conical shape. 
         [0008]    In an alternative embodiment, the frame has a recess that receives the outer edge portion. A dimension of the recess in a direction perpendicular to the flat plane is larger than a thickness of the outer edge portion in the direction perpendicular to the flat plane. The recess defines a fulcrum about which the porous film pivots when the force is applied to the central film portion. The frame preferably prevents the porous film from moving along the plane when it is bent by the force applied to the central film portion. 
         [0009]    The recess preferably has first and second opposed surfaces which are spaced apart from one another in a direction perpendicular to the flat plane. The central film portion is preferably circular in shape and has a center. The length of the first and second opposed surfaces, as measured along a direction parallel to the flat plane, can be the same or different. 
         [0010]    In one embodiment, the first and second flat surfaces terminate at first and second circular openings, respectively, the first circular opening being larger than the second circular opening. 
         [0011]    In another embodiment, the frame includes one or more support surfaces located at respective positions spaced from flat plane, each support surface limiting the amount that the central film portion can move in response to the application of an external force to the central film portion in the direction of the support surfaces. The one more support surfaces can include a plurality of projections. 
         [0012]    In this embodiment, central film portion is preferably circular in shape and has a center. Each of the respective projections is a respective distance from the center of the central film portion. Each of the respective projections is also spaced from the flat plane by a respective distance as measured in a direction perpendicular to the flat plane. The distance that a respective projection is spaced from the flat plane decreases as a function of the distance of the respective projection from the center of the central film portion such that respective projections which are closer to the center of the central film portion are spaced further away from the flat plane that respective projections which are further from the center of the central film portion. 
         [0013]    It is possible for the one or more support surfaces to come into contact with a crosspiece of the central film portion when a fluid to be filtered passes through the central film portion. 
         [0014]    According to the present invention, it is possible to provide a filter that can suppress breakage of the filter by relaxing the stress applied to the filter. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0015]      FIG. 1  is a schematic structural view of a filtration filter according to a first embodiment of the present invention. 
           [0016]      FIG. 2  is a cross-sectional view of the filtration filter, taken along line A-A of  FIG. 1 . 
           [0017]      FIG. 3  is a schematic view of a part of a film portion in a porous film according to the first embodiment of the present invention. 
           [0018]      FIG. 4  is a schematic view of the part of the film portion of  FIG. 3 , when viewed from the thickness direction. 
           [0019]      FIG. 5  illustrates the motion of the filtration filter according to the first embodiment of the present invention during passage of liquid. 
           [0020]      FIG. 6  illustrates the motion of a modification of the filtration filter according to the first embodiment of the present invention during passage of the liquid. 
           [0021]      FIG. 7  is a schematic structural view of a filtration filter according to a second embodiment of the present invention. 
           [0022]      FIG. 8  illustrates the filtration filter according to the second embodiment of the present invention during passage of liquid. 
           [0023]      FIG. 9  is a schematic structural view of a filtration filter according to a third embodiment of the present invention. 
           [0024]      FIG. 10  is a schematic structural view of a frame in the third embodiment of the present invention. 
           [0025]      FIG. 11  illustrates the filtration filter according to the third embodiment of the present invention during passage of liquid. 
           [0026]      FIG. 12  is a schematic structural view of another filtration filter according to the third embodiment of the present invention. 
           [0027]      FIG. 13  is a schematic structural view of another frame in the third embodiment of the present invention. 
           [0028]      FIG. 14A  illustrates a modification of a support part in the third embodiment of the present invention. 
           [0029]      FIG. 14B  illustrates a modification of the support part in the third embodiment of the present invention. 
           [0030]      FIG. 14C  illustrates a modification of the support part in the third embodiment of the present invention. 
           [0031]      FIG. 14D  illustrates a modification of the support part in the third embodiment of the present invention. 
           [0032]      FIG. 14E  illustrates a modification of the support part in the third embodiment of the present invention. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0033]    Embodiments of the present invention will be described below with reference to the accompanying drawings. In the drawings, elements are exaggeratedly illustrated for easy explanation. 
       First Embodiment 
     [Overall Structure] 
       [0034]      FIG. 1  is a schematic view of a filtration filter  100 A according to a first embodiment of the present invention.  FIG. 2  is a cross-sectional view of the filtration filter  100 A, taken along line A-A of  FIG. 1 . As illustrated in  FIGS. 1 and 2 , the filtration filter  100 A includes a porous film  10  that separates a filtration object contained in fluid which is passed through the porous film, and a frame  20  that holds an edge portion of the porous film  10 . The porous film  10  is composed of a film portion  12  having a plurality of through holes  11 , and an edge portion  13  adjacent to the film portion  12 . In the first embodiment, the edge portion  13  ( FIG. 2 ) of the porous film  10  is held by a recess  21  of the frame  20  so that it moves in the thickness direction of the porous film  10 . 
         [0035]    Fluid containing a filtration object is passed through the porous film  10  and the filtration filter  100 A separates the filtration object from the fluid. In this description, the term “filtration object” refers to an object to be filtered by the porous film  10 . In the first embodiment, a biological substance is preferably used as the filtration object, and liquid is preferably used as the fluid. 
         [0036]    In this description, the term “biological substance” refers to a substance derived from organisms, for example, a cell (eukaryote), a bacterium ( eubacterium ), and a virus. Examples of the cell (eukaryote) include an ovum, a sperm, an induced pluripotent stem cell (iPS cell), an ES cell, a stem cell, a mesenchymal stem cell, a mononuclear cell, a single cell, a cell mass, a floating cell, an adherent cell, a nerve cell, a white blood cell, a lymphocyte, a regeneration medical cell, a self-cell, a cancer cell, a circulating tumor cell (CTC), HL-60, HELA, and germs. Examples of the bacterium ( eubacterium ) include a gram-positive bacterium, a gram-negative bacterium, an  escherichia coli , and a  tubercle bacillus . Examples of the virus include a DNA virus, an RNA virus, a rotavirus, an (avian) influenza virus, a yellow fever virus, a dengue fever virus, an encephalitis virus, a hemorrhagic fever virus, and an immunodeficiency virus. In the first embodiment, the filtration filter  100 A is excellent in separating, especially, an induced pluripotent stem cell (iPS cell), an ES cell, a stem cell, and a circulating tumor cell (CTC) from the liquid. 
       &lt;Porous Film&gt; 
       [0037]    The porous film  10  is a porous film that separates a biological substance from a fluid. The porous film  10  is preferably a metallic thin film composed of a film portion  12  having a plurality of through holes  11  and an edge portion  13  adjacent to the film portion  12 . As illustrated in  FIG. 1 , in the first embodiment, the porous film  10  is formed by a circular metal mesh and includes a pair of opposed principal surfaces and a plurality of through holes  11  penetrating both principal surfaces of the film portion  12 . The plurality of through holes  11  may be periodically arranged all over the principal surfaces of the film portion  12 . For example, the porous film  10  may be made of Ni, the dimensions of the porous film  10  may be 6 mm in diameter and 1.2 μm in thickness. The thickness of the porous film  10  is preferably within the range of 0.5 to 100 μm. The void ratio of the porous film is preferably within the range of 10% to 90%. The void ratio (i.e., the ratio of the area of the holes to the area of the principal surface (including the holes) of the porous film. is more preferably within the range of 20% to 50%. This structure can allow the porous film  10  to be easily bent when the fluid containing the filtration object passes through the porous film  10  and can restrict the porous film  10  from being bent when the fluid does not pass through the porous film  10 . The material of the porous film  10  may be gold, silver, copper, platinum, iron, nickel, chromium, stainless steel, palladium, titanium, or an alloy of these materials. In particular, when the biological substance is trapped, gold, nickel, stainless steel, or titanium is preferably used from the viewpoint of biocompatibility with the biological substance. The material of the porous film  10  may be an elastic material having a Young&#39;s modulus of 1 GP or more. 
         [0038]      FIG. 3  is a schematic structural view of a part of the film portion  12  formed by a two-dimensional periodic structure. In  FIG. 3 , the X, Y, and Z directions respectively represent the longitudinal direction, the lateral direction, and the thickness direction of the structure.  FIG. 4  illustrates the part of the film portion  12  shown in  FIG. 3 , when viewed from the Z-direction. As illustrated in  FIGS. 3 and 4 , the film portion  12  may be a plate-shaped structure (i.e., a flat structure) in which a plurality of through holes  11  are arranged at regular intervals and in a matrix. The film portion  12  is a plate-shaped structure in which a plurality of square through holes  11  are provided when viewed from the Z-direction on the principal surface side. The plural through holes  11  are provided at regular intervals in two arrangement directions parallel to the sides of squares, that is, in the X-direction and the Y-direction in  FIG. 3 . The shape of the through holes  11  is not limited to the square shape, but may be, for example, a rectangular shape, a circular shape, or an elliptic shape. Also, the arrangement of the holes is not limited to the square lattice arrangement, but may be, for example, a rectangular arrangement in which the intervals are not equal between the two arrangement directions as long as the arrangement is the quadrangular arrangement, or may be, for example, a triangular lattice arrangement or a quasiperiodic arrangement. 
         [0039]    The shape and dimensions of the through holes  11  in the film portion  12  are appropriately designed according to the size and shape of the biological (or other) substance to be filtered. For example, the through holes  11  are square when viewed from the principal surface side of the film portion  12 , that is, viewed from the Z-direction, and are designed to be within the range of 0.1 to 500 μm in length and within the range of 0.1 to 500 μm in width. The interval between the through holes  11  is, for example, within the range of 1 to 10 times of the size of the through holes  11 , and more preferably 3 times or less of the size of the through holes  11 . Alternatively, the aperture ratio is preferably 10% or more. 
       &lt;Frame&gt; 
       [0040]    The frame  20  holds the edge portion  13  of the porous film  10 . As illustrated in  FIG. 1 , in the first embodiment, the frame  20  is formed by an annular member. As illustrated in  FIG. 2 , the frame  20  has a recess  21  opening toward the porous film  10  to hold the edge portion  13  of the porous film  10 . The dimension of the recess  21  in the thickness direction of the frame  20  is designed to be larger than the thickness of the edge portion  13 . Also, the dimension of the recess  21  in the thickness direction of the frame  20  is designed so that a distal end of the edge portion  13  of the porous film  10  comes into contact with an upper inner wall of the recess  21  and a part of a lower surface of the edge portion  13  comes into contact with a lower inner wall of the recess  21  when the fluid passes through the porous film  10 . By thus designing the dimensions of the recess  21 , the upper inner wall and the lower inner wall of the recess  21  can hold the edge portion  13  of the porous film  10  when the film portion  12  is bent. 
         [0041]    For example, the dimension of the recess  21  in the thickness direction of the frame  20  is designed to be larger than 100% and smaller than or equal to 500% of the thickness of the edge portion  13 . More preferably, the dimension of the recess  21  in the thickness direction of the frame  20  is designed to be within the range of 200% to 400% of the thickness of the edge portion  13 . This can form a gap that permits the edge portion  13  of the porous film  10  to move in the thickness direction of the porous film  10  while at the same time preventing the porous film  10  from coming out of the frame  20 . In this way, the frame  20  holds the edge portion  13  of the porous film  10  in a state in which the porous film  10  is not fixed and no tension is applied thereto. The frame  20  does not always need to hold the entire circumference of the edge portion  13  of the porous film  10 , and for example, may hold two opposed portions of the edge portion  13  or hold a plurality of spaced, preferably equally spaced, portions of the edge portion  13 . 
         [0042]    In the first embodiment, the frame  20  has first and second frames (these frames are shown combined as a single from in  FIGS. 1 and 2 ). In the frame  20  of the first embodiment, after the porous film  10  is inserted into the first frame, the second frame is fitted to the first frame. This allows the porous film  10  to be held inside the frame  20 . 
       [Motion of Filtration Filter During Passage of Liquid] 
       [0043]      FIG. 5  illustrates the motion of the filtration filter  100 A during passage of liquid through the filtration filter  100 A. A white arrow denoted by reference numeral  50  in  FIG. 5  shows the flow of the liquid from the upstream side to the downstream side of the filtration filter  100 A. As illustrated in  FIG. 5 , in the filtration filter  100 A, when liquid containing a biological substance passes through the porous film  10  in the direction  50 , stress in the direction  50  is applied to a center portion of the film portion  12 . Because the pivot (fulcrum) defined at the bottom inner edges of the recess  21 , the edge portion  13  of the porous film  10  moves in a direction opposite from the direction  50  inside the recess  21 . Specifically, the edge portion  13  of the porous film  10  is curved (bent) and raised in the direction opposite from the direction  50  at an angle to the center portion of the film portion  12 . Since the edge portion  13  of the porous film  10  can thus move in the thickness direction of the porous film  10  without being fixed, the center portion of the porous film  10  can be bent in the direction  50  opposite from the direction in which the edge portion  13  moves. 
         [0044]    The motion of the edge portion  13  of the porous film  10  is restricted by the upper wall surfaces of the recess  21 . Specifically, when the edge portion  13  of the porous film  10  is raised in the direction opposite from the direction  50 , the distal end of the edge portion  13  of the porous film  10  comes into contact with an upper inner wall of the recess  21 , and another part of the edge portion  13  comes into contact with an inner edge of the lower inner wall of the recess  21 . That is, the distal end of the edge portion  13  is supported by the upper inner wall of the recess  21 , whereas the part of the lower surface of the edge portion  13  is supported by the lower inner wall of the recess  21 . For this reason, the edge portion  13  of the porous film  10  does not come out of the recess  21 , but is held by the frame  20 . The amount that the film portion  12  can bend depends on the dimension of the recess  21  in the thickness direction of the frame  20 . Specifically, the bending amount depends on the dimension of the gap between the upper wall of the recess  21  and the edge portion  13 . As this gap increases, the amount the edge portion  13  of the porous film  10  can move when the liquid to be filtered passes through the film portion  10  increases. during passage of the liquid increases. Hence, the bending amount of the film portion  12  increases. 
         [0045]    In this way, when the liquid passes through the filtration filter  100 A, the film portion  12  is bent in the direction  50  in which the liquid flows. This can reduce the force of the liquid in the direction perpendicular to the film portion  12 , and relax the stress applied to the film portion  12 . That is, in the filtration filter  100 A, the force in the direction  50 , in which the liquid flows, can be released from the film portion  12  and the stress applied to the film portion  12  can be relaxed by bending the film portion  12 . Also, when the liquid does not pass through the filtration filter  100 A, the film portion  12  is held in an unbent state by the frame  20 , as illustrated in  FIG. 2 . 
         [0046]    As described above, in the filtration filter  100 A, when the liquid containing the biological substance passes through the porous film  10 , the biological substance is separated from the liquid while the film portion  12  is bent and the stress applied to the film portion  12  is reduced. 
       [Effects] 
       [0047]    According to the filtration filter  100 A of the first embodiment, the following effects can be achieved. 
         [0048]    In the filtration filter  100 A, the recess  21  of the frame  20  holds the edge portion  13  of the porous film  10  so that the edge portion  13  moves in the thickness direction of the porous film  10 . That is, the frame  20  does not fix the porous film  10 , but holds the porous film  10  under no tension. According to this structure, the film portion  12  can be bent in the direction  50  in which the liquid flows when the liquid passes through the filtration filter  100 A. As a result, when the liquid passes through the filtration filter  100 A, the stress applied to the film portion  12  can be relaxed by bending the film portion  12 , and this can suppress breakage of the film portion  12 . 
         [0049]    In the filtration filter  100 A, the dimension of the recess  21  in the thickness direction of the frame  20  is designed to be larger than the thickness of the edge portion  13  of the porous film  10 . This structure can form, inside the recess  21 , the gap that permits the edge portion  13  of the porous film  10  to move in the thickness direction of the porous film  10 . Since the edge portion  13  of the porous film  10  moves in the thickness direction of the porous film  10  inside this gap, the film portion  12  can be moved in the direction opposite from the direction in which the edge portion  13  moves. This allows the film portion  12  to be bent reliably. Also, in the filtration filter  100 A, the bending amount of the film portion  12  can be controlled by adjusting the dimension of the recess  21  in the thickness direction of the frame  20 . 
         [0050]    In the filtration filter  100 A, when the liquid does not pass through the porous film  10 , the film portion  12  is held in an unbent state. In this way, in the filtration filter  100 A, the film portion  12  is bent when the liquid passes. This can enhance handleability of the user. For example, when the filtration filter  100 A is mounted in a filtration device, the user can mount the filtration filter  100 A while holding only the frame  20 . At this time, the film portion  12  is held in an unbent state. For this reason, the filtration filter  100 A can suppress the user from erroneously touching the film portion  12  and can reduce the risk of soiling the film portion  12 , compared with the filtration filter in which the film portion  12  is always bent. 
         [0051]    The porous film  10  is preferably made of metal. This structure can further suppress breakage of the porous film  10 . Also, when the liquid passes through the film portion  12 , since the through holes  11  hardly deform, the biological substance can be suppressed from passing through the film portion  12  owing to deformation of the through holes  11 . 
         [0052]    While the terms “filtration object” and “fluid” have been respectively described as, for example, the biological substance and the liquid in the first embodiment, they are not limited thereto. The fluid may be gas. The filtration object may be, for example, particulate matter (PM 10, SPM, or PM 2.5). 
         [0053]    While the metallic thin film is used as the porous film  10  in the first embodiment, the porous film  10  is not limited thereto. For example, the porous film  10  may be a film formed, for example, by a membrane, filter paper, or nonwoven fabric. 
         [0054]      FIG. 6  illustrates the motion of a modification of the filtration filter  100 A according to the first embodiment. As illustrated in  FIG. 6 , a frame  20   a  may be structured in a manner such that the area of a lower inner wall of a recess  21  is larger than the area of an upper inner wall of the recess  21 . This structure increases the area in which a lower surface of an edge portion  13  of a porous film  10  is in contact with the lower inner wall of the recess  21 . Hence, when a film portion  12  is bent, the lower surface of the edge portion  13  is easily supported by the lower inner wall of the recess  21 . 
         [0055]    While the frame  20  has two frames in the first embodiment, the invention is not so limited. For example, the frame  20  may have two or more frames. Alternatively, the frame  20  may be formed by a single component. 
       Second Embodiment 
     [Overall Structure] 
       [0056]    A filtration filter according to a second embodiment of the present invention will be described with reference to  FIG. 7 . 
         [0057]      FIG. 7  illustrates a schematic structure of a filtration filter  100 B according to a second embodiment. In the second embodiment, differences from the first embodiment will be mainly described. In the second embodiment, structures identical or equivalent to those of the first embodiment are denoted by the same reference numerals. Also, in the second embodiment, descriptions overlapping with those of the first embodiment are skipped. 
         [0058]    As illustrated in  FIG. 7 , the filtration filter  100 B of the second embodiment is different from the filtration filter  100 A of the first embodiment in the structure for holding a porous film  10 . Specifically, an edge portion  13  of the porous film  10  has a holding hole  14 , and a frame  20  has a projection  22  instead of the recess  21 . 
       &lt;Holding Hole&gt; 
       [0059]    The holding hole  14  is a hole in which the projection  22  is to be inserted, and is provided in the edge portion  13  of the porous film  10 . The holding hole  14  communicates between two opposed principal surfaces of the porous film  10 . The holding hole  14  preferably has a circular shape, when viewed from the principal surface side of the porous film  10 . The diameter of the holding hole  14  is designed to be larger than the diameter of the projection  22 . In the second embodiment, for example, a plurality of holding holes  14  are equally spaced on the concentric edge portion  13  of the porous film  10 , when viewed from the principal surface side of the porous film  10  formed by a circular metal mesh. 
       &lt;Projection&gt; 
       [0060]    The projection  22  holds the edge portion  13  of the porous film  10  by being inserted in the holding hole  14  of the porous film  10 . The projection  22  projects from an upper surface of the frame  20  in the thickness direction of the frame  20 . In the second embodiment, for example, a plurality of projections  22  are provided at positions corresponding to the holding holes  14  of the porous film  10 . 
         [0061]    The projections  22  may be, for example, conical pins. The diameter of the projections  22  is designed to be smaller than the diameter of the holding holes  14 . That is, the diameter of the holding holes  14  is designed to be larger than the diameter of the projections  22 , and for example, the diameter of the holding holes  14  is designed to be larger than 100% and smaller than or equal to 200% of the diameter of the projections  22 . Thus, when the projections  22  are inserted in the holding holes  14 , gaps that allow the edge portion  13  of the porous film  10  to move in the thickness direction of the porous film  10  can be formed between inner walls of the holding holes  14  and the projections  22 . Also, the height of the projections  22  in the thickness direction of the frame  20  is designed at such a height that the holding holes  14  do not come out of the projections  22 . The height of the projections  22  is appropriately determined according to the dimensions such as the diameter of the holding holes  14  and the diameter of the porous film  10 . 
       [Motion of Filtration Filter During Passage of Liquid] 
       [0062]      FIG. 8  illustrates the motion of the filtration filter  100 B when a liquid passes there through. A white arrow denoted by reference numeral  50  in  FIG. 8  shows the flow of liquid from the upstream side to the downstream side of the filtration filter. As illustrated in  FIG. 8 , in the filtration filter  100 B, when liquid containing a biological substance passes through the porous film  10  in the direction  50 , stress in the direction  50  is applied to a center portion of the film portion  12 . At this time, the edge portion  13  of the porous film  10  moves in a direction opposite from the direction  50  along outer walls of the projections  22  inserted in the holding holes  14 . Specifically, the edge portion  13  of the porous film  10  is curved and raised in the direction opposite from the direction  50  at an angle to the center portion of the film portion  12 . Since the edge portion  13  of the porous film  10  can thus move in the thickness direction of the porous film  10  without being fixed, the center portion of the film portion  12  can be bent in the direction  50 . 
         [0063]    When the edge portion  13  of the porous film  10  moves in the direction opposite from the direction  50 , the inner walls of the holding holes  14  are caught by outer walls of the projections  22  and the edge portion  13  of the porous film  10  is held with an angle by the frame  20 . In this way, the projections  22  function as fall-preventing members for the porous film  10  while restricting the movement of the edge portion  13 . The bending amount of the film portion  12  depends on the diameter of the holding holes  14 . Specifically, the bending amount depends on the dimensions of the gaps between the inner walls of the holding holes  14  and the outer walls of the projections  22 . When the gaps increase, the moving amount of the edge portion  13  of the porous film  10  during passage of the liquid increases. Hence, the bending amount of the film portion  12  increases. 
       [Effects] 
       [0064]    According to the filtration filter  100 B of the second embodiment, the following effects can be achieved. 
         [0065]    In the filtration filter  100 B, the frame  20  holds the porous film  10  with the projections  22  thereof inserted in the holding holes  14  provided in the edge portion  13  of the porous film  10 . Also, since the diameter of the holding holes  14  is larger than the diameter of the projections  22 , when the projections  22  are inserted in the holding holes  14 , gaps can be formed between the inner walls of the holding holes  14  and the projections  22 . Accordingly, when the liquid passes through the filtration filter  100 B, the edge portion  13  of the porous film  10  is moved with an angle in the thickness direction of the porous film  10  and this can bend the film portion  12  in the direction  50  in which the liquid flows. As a result, when the liquid passes through the filtration filter  100 B, the stress applied to the film portion  12  can be relaxed by bending of the film portion  12  and this can suppress breakage of the film portion  12 . 
         [0066]    In the filtration filter  100 B, the bending amount of the film portion  12  can be controlled by adjusting the diameter of the holding holes  14  and the diameter of the projections  22 . 
         [0067]    While the holding holes  14  have a circular shape when viewed from the principal surface side of the porous film  10  in the second embodiment, the shape is not limited thereto. It is only necessary that the holding holes  14  should have such a shape to permit insertion of the projections  22 . The holding holes  14  may have an arbitrary shape such as a triangular shape, a quadrangular shape, a trapezoidal shape, or an elliptic shape. Alternatively, the holding holes  14  may be slots extending toward the center portion of the porous film  10 . When the holding holes  14  are formed as slots extending toward the center portion of the porous film  10 , the movement of the edge portion  13  of the porous film  10  in the directions other than the thickness direction, for example, the movement in the circumferential direction of the porous film  10  can be restricted. 
         [0068]    While the projections  22  are conical pins, for example, in the second embodiment, the shape is not limited thereto. The projections  22  can have any shape as long as they can hold the porous film  10  by being inserted in the holding holes  14  of the porous film  10  while permitting the edge portion  13  of the porous film  10  to move in the thickness direction of the porous film  10 . For example, the projections  22  may be shaped like a triangular prism, a quadrangular prism, or a circular column. 
         [0069]    While the projections  22  are provided on the upper surface of the frame  20  in the second embodiment, the structure is not limited thereto. For example, the projections  22  may be provided inside the recess  21  of the first embodiment. According to this structure, since the recess  21  and the projections  22  can hold the edge portion  13  of the porous film  10 , the porous film  10  can be held while more reliably bending the film portion  12 . 
       Third Embodiment 
     [Overall Structure] 
       [0070]    A filtration filter according to a third embodiment of the present invention will be described with reference to  FIGS. 9 and 10 . 
         [0071]      FIG. 9  illustrates a schematic structure of a filtration filter  100 C according to the third embodiment.  FIG. 10  illustrates a schematic structure of a frame  20  in the third embodiment. In  FIG. 10 , the porous film  10  is not illustrated for easy explanation. 
         [0072]    Differences of the third embodiment from the first embodiment will be mainly described. In the third embodiment, structures identical or equivalent to those of the first embodiment are denoted by the same reference numerals. Also, in the third embodiment, descriptions overlapping with those of the first embodiment will be skipped. 
         [0073]    As illustrated in  FIGS. 9 and 10 , the filtration filter  100 C of the third embodiment is different from the filtration filter  100 A of the first embodiment in having a support part  30 . The support part  30  supports a bent film portion  12  when liquid passes through the filtration filter  100 C. As illustrated in  FIGS. 9 and 10 , the support part  30  is disposed at a position spaced from the principal surface of the film portion  12  in the thickness direction. Specifically, the support part  30  is provided on an inner wall of a frame  20 , and is disposed on the downstream side of the film portion  12  relative to the flow of fluid through the filtration filter. The support part  30  has two plate-shaped members, and is disposed so that the members intersect at a position corresponding to a center portion of the film portion  12 . On a surface of the support part  30  on the side of the film portion  12 , a plurality of projections  31   a ,  31   b ,  31   c ,  31   d , and  31   e  are provided. 
         [0074]    As illustrated in  FIG. 10 , the projection  31   a  is disposed at the position corresponding to the center portion of the film portion  12 , that is, at the position where the two plate-shaped members intersect. The projections  31   b ,  31   c ,  31   d , and  31   e  are disposed at positions at a predetermined distance from the projection  31   a . The plurality of projections  31   a  to  31   e  are arranged to be in contact with the film portion  12  along the shape of the bent film portion  12 . Specifically, as illustrated in  FIG. 9 , the plurality of projections  31   a  to  31   e  are designed so that the distance between distal ends of the projections  31   a  to  31   e  and the lower principal surface of the film portion  12  decreases from the center portion toward the outer side portion of the film portion  12 . That is, the height of the projection  31   a  in the thickness direction of the film portion  12  is designed to be smaller than the height of the projections  31   b ,  31   c ,  31   d , and  31   e . The plurality of projections  31   a  to  31   e  may be conical pins. 
       [Motion of Filtration Filter During Passage of Liquid] 
       [0075]      FIG. 11  illustrates the motion of the filtration filter  100 C while a liquid (more generally a fluid) is passing through there through. In  FIG. 11 , a white arrow denoted by reference numeral  50  in  FIG. 11  shows the flow of liquid from the upstream side to the downstream side of the filtration filter. As illustrated in  FIG. 11 , when liquid containing a biological substance passes through the film portion  12  of the filtration filter  100 C in the direction  50 , stress in the direction  50  is applied to the center portion of the film portion  12 , and the center portion of the film portion  12  is bent in the direction  50 . The bent film portion  12  comes into contact with the distal ends of the plurality of projections  31   a  to  31   e  in the support part  30 . In this way, the support part  30  suppresses excessive bending of the film portion  12 . 
         [0076]    The plurality of projections  31   a  to  31   e  of the support part  30  preferably come into contact with a crosspiece of the film portion  12  when the fluid passes through the film portion  12 . The crosspiece of the film portion  12  refers to a portion of the film portion  12  where the through holes  11  are not provided. According to this structure, the plurality of projections  31   a  to  31   e  of the support part  30  come into contact with the crosspiece of the film portion  12  and this can restrict bending of the film portion  12  without hindering the flow of the fluid. 
       [Effects] 
       [0077]    According to the filtration filter  100 C of the third embodiment, the following effects can be achieved. 
         [0078]    In the filtration filter  100 C, the frame  20  is provided with the support part  30  that supports the bent film portion  12  when the liquid passes there through. Also, in the support part  30 , the plurality of projections  31   a  to  31   e  projecting toward the film portion  12  are arranged to come into contact with the bent film portion  12 . This structure restricts excessive bending of the film portion  12  during passage of the liquid. As a result, stress concentration can be restricted from being caused by excessive bending of the film portion  12 . Further, since the plurality of projections  31   a  to  31   e  are provided on the surface of the support part  30 , the support part  30  can support the film portion  12  while dispersing the stress applied to the film portion  12 . 
         [0079]    In the filtration filter  100 C, the plurality of projections  31   a  to  31   e  are designed so that the distance between the distal ends of the projections and the principal surface of the film portion  12  decreases from the center portion toward the outer side portion of the film portion  12 . According to this structure, the plurality of projections  31   a  to  31   e  come into contact with the film portion  12  along the shape of the film portion  12  bent when the liquid passes. For this reason, the projections  31   a  to  31   e  can more equally disperse the stress applied to the film portion  12 . As a result, the filtration filter  100 C can reliably suppress breakage of the film portion  12 . 
         [0080]    By using the conical pins as the plurality of projections  31   a  to  31   e , the film portion  12  can be supported without hindering the flow of the liquid. 
         [0081]    While the support part  30  is added to the structure of the first embodiment in the structure of the third embodiment, the structure is not so limited.  FIG. 12  illustrates a schematic structure of another filtration filter according to the third embodiment. For example, as illustrated in  FIG. 12 , the support part  30  may be added to the structure of the second embodiment. 
         [0082]    While the support part  30  has the plurality of projections  31   a  to  31   e  in the third embodiment, the structure is not so limited. The number of projections can be set at an arbitrary number, for example, according to the size of the film portion  12 . For example, the support part  30  may be structured to have no projection, to have only one projection, or to have more than five projections.  FIG. 13  illustrates a schematic structure of another frame  20  in the third embodiment. As illustrated in  FIG. 13 , for example, the support part  30  may have one projection  31   a  at a position corresponding to the center portion of the film portion  12 . By reducing the number of projections on the support part  30 , the flow of the liquid can be suppressed from being hindered by the projections. 
         [0083]    While the illustrated projections  31   a  to  31   e  are shaped like conical pins, the structure is not so limited. The support part  30  can have any shape that can come into contact with the film portion  12  bent during passage of the liquid.  FIGS. 14A to 14E  illustrate modifications of the support part in the third embodiment. As illustrated in  FIGS. 14A to 14E , the support part  30  may be a circular support part  32 , an acute triangular support part  33 , a rectangular support part  34 , a square support part  35 , or an inverse-T shaped support part  36 , in a cross section taken along the direction in which the fluid flows. To firmly support the film portion  12 , a support part having a large surface area in contact with the principal surface of the film portion  12 , for example, the support part  32  and the support part  35  respectively having the circular cross section and the square cross section illustrated in  FIGS. 14A and 14D  may be used. To suppress the support part from hindering the flow of the fluid, a support part having a small surface area in contact with the principal surface of the film portion  12 , for example, the support part  33 , the support part  34 , and the support part  36  respectively having the acute triangular cross section, the rectangular cross section, and the inverse-T shaped cross section illustrated in  FIGS. 14B, 14C, and 14E  may be used. 
         [0084]    While the present invention has been sufficiently described in conjunction with the preferred embodiments with reference to the accompanying drawings, various modifications and alterations are obvious to those skilled in the art. It should be understood that such modifications and alterations are included in the present invention without departing from the scope of the present invention described in the accompanying claims. 
       INDUSTRIAL APPLICABILITY 
       [0085]    The present invention relates to the filtration filter, and is excellent in suppressing breakage of the filtration filter when the fluid passes therethrough. For example, the invention can be used for medical diagnosis by taking out cells from a biospecimen and used for environmental measures by trapping PM 2.5 existing in the air. 
       REFERENCE SIGNS LIST 
       [0000]    
       
         
           
               10 : porous film 
               11 : through hole 
               12 : film portion 
               13 : edge portion 
               14 : holding hole 
               20 : frame 
               21 : recess 
               22 : projection 
               30 : support part 
               31 : projection 
               32 ,  33 ,  34 ,  35 ,  36 : support part 
               50 : direction 
               100 A,  100 B,  100 C: filtration filter