Cell-capturing filter

The present application is a continuation of International application No. PCT/JP2018/005513, filed Feb. 16, 2018, which claims priority to Japanese Patent Application No. 2017-046432, filed Mar. 10, 2017, the entire contents of each of which are incorporated herein by reference.

The present invention relates to a cell-capturing filter that filters out cells.

A filter including a metallic porous film is known to date as a cell-capturing filter that filters out cells in a fluid to capture the cells (see, for example, PTL 1).

Such an existing cell-capturing filter includes a metallic porous film including multiple through holes that extend through flat main surfaces opposite to each other. A fluid containing cells, which are to be filtered out, is fed from one main surface of the metallic porous film and caused to pass through the through holes so that the cells can be captured on the main surface.

PTL 1: Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2010-520446

To analyze cells captured on one main surface of the metallic porous film of such an existing cell-capturing filter, the metallic porous film needs to be moved to an analyzer. However, scarcely any fluid such as a culture solution is left on the main surface of the metallic porous film. As such, the captured cells may be dried and damaged so that they are incapable of being analyzed.

The present invention aims to address the above existing problem and to provide a cell-capturing filter capable of improving the handleability of cells captured by a metallic porous film.

A cell-capturing filter according to an aspect of the present invention includes a metallic porous film having a plurality of through holes that extend through a first main surface and a second main surface, which are opposite to each other. The metallic porous film includes a filtering portion, in which the plurality of through holes are formed, and a frame portion, which is disposed to surround an outer periphery of the filtering portion. In the filtering portion, the metallic porous film at a center of the filtering portion has a film thickness smaller than a film thickness of the metallic porous film at a portion closer to the frame portion than the center of the filtering portion.

The present invention can provide a cell-capturing filter capable of improving the handleability of cells captured by a metallic porous film.

FIG. 1 is a schematic diagram of a structure of a filter according to an embodiment of the present invention.

FIG. 2 is an enlarged view of a portion of a metallic porous film of the filter illustrated in FIG. 1 .

FIG. 3 is an enlarged view of a portion of the metallic porous film of the filter illustrated in FIG. 1 .

FIG. 4(A) is a schematic plan view of the structure of the metallic porous film of the filter illustrated in FIG. 1 , and FIG. 4(B) is a schematic sectional view of the filter illustrated in FIG. 1 .

FIG. 5 is a flowchart of a method for manufacturing a filter according to an embodiment of the present invention.

FIGS. 6(A) to 6(F) are schematic sectional views of processes in the method for manufacturing a filter illustrated in FIG. 5 .

FIG. 7 is a plan view of a resist image for concurrently manufacturing multiple metallic porous films with the method for manufacturing a filter according to a modification example of the present invention.

A cell-capturing filter that filters out cells according to an aspect of the present invention includes a metallic porous film having a plurality of through holes that extend through a first main surface and a second main surface, which are opposite to each other. The metallic porous film includes a filtering portion, in which the plurality of through holes are formed, and a frame portion, which is disposed to surround an outer periphery of the filtering portion. In the filtering portion, the metallic porous film at a center of the filtering portion has a film thickness smaller than a film thickness of the metallic porous film at a portion closer to the frame portion than the center of the filtering portion.

In this structure, the metallic porous film at a center of the filtering portion has a film thickness smaller than a film thickness of the metallic porous film at a portion closer to the frame portion than the center of the filtering portion. Thus, at least one of the first main surface and the second main surface can partially have a concave surface in the filtering portion. When the surface having the concave surface is used to capture cells, the concave surface is more likely to allow a fluid puddle to be formed thereon with the effect of the surface tension of the fluid, so that the captured cells are prevented from being dried. This structure thus improves the handleability of cells captured by the metallic porous film.

In the filtering portion, the metallic porous film may have a film thickness increasing continuously or stepwise from the center of the filtering portion toward the frame portion.

In this structure, a smooth concave surface can be formed on the first main surface or the second main surface of the metallic porous film. The smooth concave surface thus formed is more likely to allow a fluid puddle to be formed thereon with the effect of the surface tension of a fluid exerted on the concave surface. Thus, the handleability of cells captured by the metallic porous film can be improved. The surface tension of a fluid can be controlled by changing, in accordance with the viscosity of the fluid, the surface state of the metallic porous film, the shape of the through holes, or the opening area.

The second main surface of the metallic porous film may be a flat surface, and the first main surface at the filtering portion may be a concave surface having a portion closer to the center of the filtering portion lowered and a portion closer to the frame portion raised.

To use a fluid puddle formed using the concave surface, cells may be captured on the first main surface having the concave surface. On the other hand, to facilitate removal of the fluid without allowing a fluid puddle to be formed, cells may be captured on the second main surface having a flat surface. By thus differently using the first main surface and the second main surface for different purposes, the handleability of the cell-capturing filter can be improved.

In a direction parallel to the second main surface of the metallic porous film, the frame portion has a width greater than a distance between adjacent through holes of the plurality of through holes in the filtering portion.

Making the width of the frame portion different from the distance between adjacent through holes enables varying the film thickness of the filtering portion through, for example, plating using a resist image. This structure can thus provide a cell-capturing filter that can improve the handleability of cells captured by the metallic porous film.

Embodiments of the present invention are described below with reference to the appended drawings. The drawings illustrate components in an exaggerated manner for ease of illustration.

Entire Structure of Filter

FIG. 1 is a schematic diagram of a cell-capturing filter 1 (hereinafter referred to as a filter 1 ) according to a first embodiment of the present invention. FIG. 2 is an enlarged view of a portion of the filter 1 . FIG. 3 is an enlarged plan view of a portion of the filter 1 near the periphery. X and Y directions in FIG. 2 are directions parallel to the surface of the filter 1 and orthogonal to each other. A Z direction is a thickness direction of the filter 1 and perpendicular to the X direction and the Y direction.

As illustrated in FIG. 1 and FIG. 3 , the filter 1 includes a metallic porous film 10 including multiple through holes 11 that extend through in the thickness direction. The metallic porous film 10 includes a filtering portion R 1 , which is an area in which the multiple through holes 11 are formed, and a frame portion R 2 , which is an area surrounding the filtering portion R 1 . In the present first embodiment, the filtering portion R 1 is a circular area. The frame portion R 2 is an annular area, which is an area that holds the filtering portion R 1 at the periphery. The frame portion R 2 has no through hole 11 . The filter 1 may include a member that clamps the frame portion R 2 of the metallic porous film 10 to enhance the handleability of the filter 1 . The present first embodiment describes, as an example, a case where the filtering portion R 1 and the frame portion R 2 are integrated. However, the filtering portion R 1 and the frame portion R 2 may be separate from each other and the frame portion R 2 may hold the filtering portion R 1 . The shape of the filtering portion R 1 is not limited to a circle and may be another shape such as a regular square, a rectangle, or an ellipse.

The metallic porous film 10 allows a fluid containing an object intended to be filtered out to pass through the through holes 11 to separate the object intended to be filtered out from the fluid. In the description, the “object intended to be filtered out” refers to an object that is to be filtered out by the metallic porous film 10 . Examples of the object intended to be filtered out include biologically derived substances and PM2.5. Here, “biologically derived substances” refer to biologically derived substances such as cells (eukaryote), bacteria (true bacteria), and viruses. Examples of cells (eukaryote) include induced pluripotent stem cells (iPS cells), embryonic stem (ES) cells, stem cells, mesenchymal stem cells, mononuclear cells, single cells, cell masses, floating cells, adherent cells, nerve cells, white blood cells, regeneration medicine cells, self-cells, cancer cells, circulating cancer cells (CTC), HL-60 cells, HeLa cells, and fungi. Examples of bacteria (true bacteria) include colon bacilli and tubercle bacilli.

As illustrated in FIG. 1 , the metallic porous film 10 is a circular metal mesh. As illustrated in FIG. 2 , the metallic porous film 10 has a structure that includes a first main surface PS 1 and a second main surface PS 2 , which are opposite to each other, and multiple through holes 11 , which extend through both main surfaces. The multiple through holes 11 are arranged at regular intervals in the first main surface PS 1 and the second main surface PS 2 in the filtering portion R 1 of the metallic porous film 10 . The metallic porous film 10 contains, for example, nickel as a main component. The metallic porous film 10 may contain any metal as a main component, for example, gold, silver, copper, nickel, stainless steel, palladium, titanium, or an alloy of any of these. Particularly, from the view point of biocompatibility with biologically derived substances, gold, nickel, stainless steel, or titanium is preferable as the main component of the metallic porous film 10 to capture biologically derived substances.

As illustrated in FIGS. 2 and 3 , the metallic porous film 10 has a plate-like structure (lattice structure) in which the multiple through holes 11 are arranged in a matrix at regular intervals. The multiple through holes 11 have a square shape when viewed from the first main surface PS 1 of the metallic porous film 10 , that is, viewed in the Z direction. The multiple through holes 11 are equidistantly arranged in two arrangement directions parallel to the sides of the squares, that is, in the X direction and the Y direction in FIG. 2 . The shape of the through holes 11 is not limited to a regular square, and may be, for example, a rectangle, a circle, or an ellipse. The arrangement of the holes is not limited to the tetragonal lattice arrangement. For example, as long as being arranged in a quadrangle, the holes may be arranged in a rectangular arrangement in which the intervals in the two arrangement directions are not equal to each other, in a triangular lattice arrangement, or in a quasi-periodic arrangement.

The shape or size of the through holes 11 is appropriately designed in accordance with the size or shape of an object intended to be filtered out. In the present first embodiment, each through hole 11 is, for example, a regular square when viewed from the first main surface PS 1 of the metallic porous film 10 , that is, viewed in the Z direction, and is designed to have a length of greater than or equal to 0.1 μm to smaller than or equal to 50 μm and a width of greater than or equal to 0.1 μm to smaller than or equal to 50 μm. Each interval between the through holes 11 is, for example, greater than once the through hole 11 and smaller than or equal to ten times of the through hole 11 , more preferably, smaller than or equal to three times of the through hole 11 . Alternatively, each through hole 11 preferably has an aperture ratio of higher than or equal to 10%. In the direction parallel to the second main surface PS 2 of the metallic porous film 10 , the frame portion R 2 has a width greater than a distance between the through holes 11 adjacent to each other in the filtering portion R 1 (distance between the edges of the through holes 11 ).

FIGS. 4(A) and 4(B) are schematic diagrams of a structure of the filter 1 according to the present first embodiment. FIG. 4(A) is a schematic plan view of the filter 1 , and FIG. 4(B) is a schematic sectional view of the filter 1 .

As illustrated in FIG. 4(B) , instead of having a uniform film thickness, the metallic porous film 10 of the filter 1 has its film thickness at a center portion smaller than that at the periphery. Specifically, in the filtering portion R 1 of the metallic porous film 10 , a center of the filtering portion R 1 has a film thickness smaller than the film thickness of the metallic porous film 10 at a portion closer to the frame portion R 2 than the center of the filtering portion R 1 .

As illustrated in FIG. 4(A) , the entirety of the filtering portion R 1 is a circular area having a radius D 3 with respect to a center P 1 . In the filtering portion R 1 , imaginary circles C 1 and C 2 respectively having radii D 1 and D 2 with respect to the center P 1 are drawn. Here, the radius D 1 is one third of the radius D 3 , and the radius D 2 is two thirds of the radius D 3 . In the filtering portion R 1 , the circular area encircled with the imaginary circle C 1 is defined as a central area R 11 , an annular area interposed between the imaginary circle C 1 and the imaginary circle C 2 is defined as an intermediate area R 12 , and an annular area on the outer side of the imaginary circle C 2 is defined as a peripheral area R 13 . When an imaginary straight line L passing the center P 1 of the filtering portion R 1 is drawn in a plan view of the filtering portion R 1 , the points of intersection (first position) between the imaginary straight line L and the imaginary circle C 1 are denoted with P 2 , and the points of intersection (second position) between the imaginary straight line L and the imaginary circle C 2 are denoted with P 3 .

FIG. 4(B) is a schematic sectional view of the imaginary straight line L in the metallic porous film 10 illustrated in FIG. 4(A) . As illustrated in FIG. 4(B) , the filtering portion R 1 of the metallic porous film 10 has a film thickness in the central area R 11 , which is away from the frame portion R 2 , smaller than the film thickness in the peripheral area R 13 , which is closer to the frame portion R 2 than the central area R 11 . In addition, in the filtering portion R 1 , the intermediate area R 12 located between the central area R 11 and the peripheral area R 13 has a film thickness greater than the film thickness of the central area R 11 and smaller than the film thickness of the peripheral area R 13 . Specifically, these film thicknesses satisfy the relationship of T 1 <T 2 <T 3 , where the film thickness at the center P 1 is denoted with T 1 , the film thickness at the points of intersection P 2 is denoted with T 2 , and the film thickness at the points of intersection P 3 is denoted with T 3 . Specifically, in the filtering portion R 1 of the metallic porous film 10 , the film thickness is determined to increase radially (that is, toward the frame portion R 2 ) from the center P 1 of the filtering portion R 1 . The film thickness may increase continuously or stepwise.

As illustrated in FIG. 4(B) , the second main surface PS 2 of the metallic porous film 10 is a flat surface. The surface in the first main surface PS 1 corresponding to the filtering portion R 1 is a concave surface having a portion closer to the frame portion R 2 higher than a portion closer to the center P 1 of the filtering portion R 1 . The frame portion R 2 of the metallic porous film 10 has a film thickness T 0 that is approximately uniform. The film thickness T 0 is greater than or equal to the film thickness T 3 at the points of intersection P 3 in the peripheral area R 13 .

The metallic porous film 10 according to the present first embodiment has, for example, a diameter of 8 mm (contour of the frame portion R 2 ), a width of the frame portion R 2 of 1 mm, and an interval between adjacent through holes 11 of greater than or equal to 1 μm to smaller than or equal to 500 μm. The filtering portion R 1 has a diameter of 6 mm. In the filtering portion R 1 , the film thickness T 3 at the points of intersection P 3 closer to the frame portion R 2 is 1.1 μm, and the film thickness T 1 at the center P 1 is 0.8 μm.

The description of the areas R 11 , R 12 , and R 13 illustrated in FIGS. 4(A) and 4(B) is provided to describe an increase of the film thickness from the center P 1 of the filtering portion R 1 toward the frame portion R 2 . Thus, the structure is not limited the case where the radius D 1 of the imaginary circle C 1 is one third of the radius D 3 , and the radius D 2 of the imaginary circle C 2 is two thirds of the radius D 3 . For example, the description may be similarly provided using an imaginary circle in which the respective radii have the relationship of D 1 <D 2 <D 3 .

Method for Manufacturing Filter

Subsequently, an example of a method for manufacturing the filter 1 according to the present first embodiment is described. FIG. 5 illustrates a flowchart of a method for manufacturing the filter 1 , and FIG. 6(A) to FIG. 6(F) illustrate sections of the filter 1 in manufacturing processes in the manufacturing method.

In step S 21 (forming of a copper thin film) in FIG. 5 , a metal film is formed. As illustrated in FIG. 6(A) , a metal film is formed on a substrate 31 made of, for example, silicon. Copper is used as an example of a metal forming the metal film, and a copper thin film 32 is formed on the substrate 31 . The copper thin film 32 can be formed by, for example, vapor deposition or sputtering. The surface film has higher quality when formed by sputtering than when formed by vapor deposition. This copper thin film 32 is used as a feeder electrode during plating, described below. An intermediate layer (for example, Ti film) may be formed to secure adhesion between the substrate 31 and the copper thin film 32 .

As illustrated in FIG. 6(B) , subsequently, a resist film 33 is formed on the copper thin film 32 (step S 22 , forming of a resist film). Specifically, a resist is applied to the copper thin film 32 by, for example, spin coating, and dried to form a resist film 33 . The resist film 33 has a thickness of, for example, approximately 2 μm.

Subsequently, as illustrated in FIG. 6(C) , the resist film 33 is exposed to light and subjected to development to form a resist image 36 including groove portions 34 and 35 , by removing portions corresponding to the metallic porous film 10 from the resist film 33 (step S 23 , exposure and development (forming of a resist image)). The groove portions 34 are groove portions formed in the filtering portion R 1 of the metallic porous film 10 , and the groove portion 35 is a groove portion formed in the frame portion R 2 . The opening width W 1 of the groove portions 34 corresponds to the distance between the adjacent through holes 11 in the metallic porous film 10 , and the opening width W 2 of the groove portion 35 corresponds to the width of the frame portion R 2 . Specifically, the opening width W 1 of the groove portions 34 is smaller than the opening width W 2 of the groove portion 35 . The copper thin film 32 is exposed at the bottom of the groove portions 34 and 35 .

Subsequently, as illustrated in FIG. 6(D) , in the resist image 36 , a metallic porous film 10 is formed in the groove portions 34 and 35 by depositing metal, as a main material, in the groove portions 34 and 35 (step S 24 , plating). The metallic porous film 10 is formed by electroplating using, for example, the copper thin film 32 as a feeder electrode. Nickel is an example of a deposited metal.

As described above, in the resist image 36 , the opening width W 1 of the groove portions 34 is smaller than the opening width W 2 of the groove portion 35 . In the resist image 36 , the groove portions 34 are densely formed in the center portion, and the groove portion 35 is annularly formed to surround these groove portions 34 . When the resist image 36 in this state is plated, a plating solution smoothly enters the groove portion 35 having a large opening width, whereas the plating solution enters with difficulty the groove portions 34 having a small opening width. Even within the area where the groove portions 34 are formed, the plating solution enters the groove portions 34 in a portion corresponding to the central area R 11 of the filtering portion R 1 , in which the groove portions 34 are surrounded by other groove portions 34 , with greater difficulty than the portion corresponding to the peripheral area R 13 located closer to the frame portion R 2 . This is probably because protrusions of the resist image 36 defining the groove portions 34 are densely arranged to hinder the plating solution from entering the groove portions 34 . As the plating rate is increased further, the smoothness of the plating solution in entering the groove portions varies more significantly.

After plating in this manner, the plating solution that has entered the groove portions 34 and 35 forms a precipitate, and forms a film with the precipitated metal having a film thickness proportional to the amount of the plating solution that has entered the groove portions 34 and 35 . As illustrated in FIG. 6(D) , the film thickness T 0 of the metallic porous film 10 formed in the groove portion 35 is the maximum. In the groove portions 34 , the film thickness T 3 at the boundary (points of intersection P 3 ) between the peripheral area R 13 and the intermediate area R 12 , the film thickness T 2 at the boundary (points of intersection P 2 ) between the intermediate area R 12 and the central area R 11 , and the film thickness T 1 at the center P 1 of the central area R 11 descend in this order.

Subsequently, as illustrated in FIG. 6(E) , the workpiece is immersed in a solvent (for example, acetone) to dissolve the resist image 36 to detach the metallic porous film 10 from the copper thin film 32 (step S 25 , removal of a resist image).

Subsequently, as illustrated in FIG. 6(F) , the copper thin film 32 and the substrate 31 are removed, (step S 26 , removal of the substrate and the copper thin film). Specifically, the copper thin film 32 is removed by etching using an etchant to detach the metallic porous film 10 from the substrate 31 .

This procedure forms the metallic porous film 10 including a filtering portion R 1 including the multiple through holes 11 , and a frame portion R 2 around the filtering portion R 1 . The first main surface PS 1 forms a concave surface with its film thickness descending in order of the film thickness T 0 of the frame portion R 2 of the metallic porous film 10 , the film thickness T 3 at the points of intersection P 3 of the filtering portion R 1 , the film thickness T 2 at the points of intersection P 2 , and the film thickness T 1 at the center P 1 . On the other hand, the second main surface PS 2 of the metallic porous film 10 , which has been in contact with the copper thin film 32 , is a flat surface. This manufacturing method manufactures a filter 1 including the metallic porous film 10 having a first main surface formed as a concave surface and a second main surface formed as a flat surface.

The method for manufacturing the filter 1 is not limited to the case of manufacturing a single metallic porous film 10 from a single substrate 31 . For example, as illustrated in FIG. 7 , a resist image 136 , which is a group of resist images 36 corresponding to multiple metallic porous films 10 , may be formed on a single substrate 131 to concurrently manufacture the multiple metallic porous films 10 .

In the filter 1 according to the present first embodiment, the first main surface PS 1 in the filtering portion R 1 of the metallic porous film 10 is formed into a concave surface so that the film thickness T 1 at the center P 1 of the filtering portion R 1 is smaller than the film thicknesses T 2 and T 3 located at the positions closer to the frame portion R 2 than the center P 1 . This structure facilitates formation of a fluid puddle on the concave surface with the effect of the surface tension of the fluid when cells are captured on the first main surface PS 1 having the concave surface. For example, handling the captured cells while being left on the concave surface in the culture solution serving as a liquid pool enables a process such as an analysis while the cells are prevented from being dried. Thus, the handleability of the cells captured by the metallic porous film 10 can be improved.

In the filtering portion R 1 , the film thickness of the metallic porous film 10 increases continuously or stepwise from the center P 1 of the filtering portion R 1 toward the frame portion R 1 to form a smooth concave surface on the metallic porous film 10 . This structure enables formation of a fluid puddle on the concave surface with the effect of the surface tension, and further improves the handleability of cells captured by the metallic porous film 10 .

The second main surface PS 2 of the metallic porous film is a flat surface, and the first main surface PS 1 is a concave surface. Thus, for example, to use a fluid puddle formed using the concave surface, cells may be captured on the first main surface PS 1 , which forms a concave surface. On the other hand, to facilitate removal of the fluid without forming a fluid puddle, cells can be captured on the second main surface PS 2 , which is a flat surface. The first main surface PS 1 and the second main surface PS 2 are thus differently used for difference purposes to improve the handleability of the cell-capturing filter.

In the metallic porous film 10 , the width of the frame portion R 2 is greater than the distance between adjacent through holes 11 in the filtering portion R 1 . Making the width of the frame portion R 2 different from the distance between adjacent through holes 11 enables varying the smoothness with which the plating solution enters the groove portions 34 and 35 during, for example, plating using the resist image 36 . This structure can vary the film thickness so that the filtering portion R 1 has a concave surface. This structure can thus provide the filter 1 that can improve the handleability of cells captured by the metallic porous film 10 .

The above embodiment describes the case, as an example, where a concave surface is formed to have a film thickness that increases continuously or stepwise in the filtering portion R 1 of the metallic porous film 10 from the center P 1 of the filtering portion R 1 toward the frame portion R 2 . However, the present invention is not limited to the above case. For example, a portion having a film thickness increasing from the frame portion R 2 toward the center P 1 of the filtering portion R 1 may be included partially in the filtering portion R 1 . Even such a structure can have the effects of the present invention as long as the average film thickness in the central area R 11 in the filtering portion R 1 as a whole is smaller than the average film thickness in the peripheral area R 13 .

An example of a method for manufacturing a filter according to the present invention is described below.

In this example, a filter 1 was fabricated by performing steps S 21 to S 26 in FIG. 5 .

Firstly, in step S 21 , a copper thin film was formed on the upper surface of a silicon substrate using a sputtering device. The copper thin film was formed by using an argon gas as a sputtering gas, at a degree of vacuum in the sputtering device of 5.0×10 −4 Pa, and with an application of an electric power of DC 500 W. Sputtering was performed for 27 minutes to form the copper thin film.

Subsequently, in step 22 , a resist film having a predetermined film thickness was formed on the copper thin film using a spin coater. Specifically, a resist agent was applied onto the copper thin film, and the solvent was left in an atmosphere of nitrogen at 130° C. to volatilize and then cooled to form a resist film. The resist film having a film thickness of 2 μm was formed with the spin coater at the number of revolutions of 1130 rpm using a novolak resin and an organic solvent for the resist agent.

Subsequently, in step S 23 , the resist film was exposed to light and subjected to development to form a resist image in which a groove portion corresponding to a metallic porous film is formed on the resist film. The resist film was exposed to light by being irradiated with a light beam with an energy density of 2500 J/m 2 containing a wavelength of 365 nm for 0.25 seconds. Thereafter, the portion of the resist film exposed to light was brought into contact with an alkaline solution to be removed to form a groove portion. In the resist image, the groove portions in a portion corresponding to the frame portion of the metallic porous film have an opening width of 0.8 mm, and the groove portion in a portion corresponding to the filtering portion has an opening width of 0.8 μm.

Subsequently, in step S 24 , a metallic porous film formed of a plated film containing nickel as a main component was formed by electroplating in the groove portions of the resist image using the previously formed copper thin film as a feeder electrode. Firstly, as preprocessing, the substrate having a copper thin film and a resist image formed thereon was immersed in a dilute sulfuric acid for 60 seconds to activate the surface of the copper thin film exposed at the bottom of the groove portion of the resist image. Thereafter, while being shaken in a sulfamic acid nickel plating solution (liquid temperature of 55° C., and pH 4.0), the substrate was subjected to electroplating while receiving a voltage using the copper thin film as a feeder electrode. The speed of plating was 0.5 μm/min.

Thereafter, in step S 25 , the substrate was immersed in the solvent to dissolve the resist image and to detach the resist image from the copper thin film. In an acetone solution used as a solvent, the substrate received ultrasonic waves for 15 minutes to accelerate dissolution and separation of the resist image.

Subsequently, in step S 26 , the copper thin film and the substrate were removed. Specifically, the copper thin film was etched by being immersed for 48 hours in the environment of 25° C. in a solution, serving as an etchant, formed by mixing a 60% hydrogen peroxide solution, acetic acid, and pure water at a volume ratio of 1:1:20. Thus, the copper thin film and the substrate were removed to complete a filter including a metallic porous film according to the example.

In measurements of the metallic porous film according to the example, the contour of the frame portion R 2 (that is, the diameter of the metallic porous film) was 8 mm, the diameter of the filtering portion R 1 was 6 mm, the film thickness of the center P 1 of the filtering portion R 1 was 0.7 μm, and the film thickness at the boundary (points of intersection P 3 ) between the peripheral area R 13 and the intermediate area R 12 was 1.2 μm.

Any of the above-described embodiments may be combined appropriately to exert their effects.

In the present invention, at least one of the first main surface and the second main surface in the filtering portion of the filter partially includes a concave surface. When cells are to be captured on the concave surface, the concave surface is more likely to allow a fluid puddle formed thereon with the effect of the surface tension of a fluid. This structure can thus prevent captured cells from being dried, and thus improves the handleability of cells captured by the metallic porous film and the handleability of the metallic porous film. The present invention is thus applicable to various different technical fields in which such a filter is usable, for example, the field of chemical analysis, drug design or drug development, clinical examination, public health management, or environmental measurement.

1 filter

10 metallic porous film

11 through hole

31 substrate

32 copper thin film

33 resist film

34 , 35 groove portion

36 resist image

PS 1 first main surface

PS 2 second main surface

R 1 filtering portion

R 2 frame portion

R 11 central area

R 12 intermediate area

R 13 peripheral area

C 1 , C 2 imaginary circle

D 1 , D 2 , D 3 radius

L imaginary straight line

P 1 center

P 2 , P 3 point of intersection

T 0 , T 1 , T 2 , T 3 film thickness

W 1 , W 2 opening width