Patent Publication Number: US-11045587-B2

Title: Membrane for immunoisolation, chamber for transplantation, and device for transplantation

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of PCT International Application No. PCT/JP2017/040338 filed on Nov. 9, 2017, which claims priority under 35 U.S.C § 119 (a) to Japanese Patent Application No. 2016-220200 filed on Nov. 11, 2016, the entire content of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a membrane for immunoisolation. The present invention further relates to a chamber for transplantation which has the membrane for immunoisolation, and a device for transplantation. 
     2. Description of the Related Art 
     Immunoisolation is one of methods for preventing immune reactions in a recipient during transplantation of biological components such as cells, tissues, or organs. A membrane for immunoisolation is a selectively permeable membrane which allows water, oxygen, glucose, or the like to permeate, and which, at the same time, performs immunoisolation by inhibiting permeation of immune cells and the like involved in an immune rejection. For example, while preventing an immune rejection, it is possible to achieve a purpose of transplantation by a device for transplantation utilizing a membrane for immunoisolation which allows physiologically active substances to permeate therethrough, for transplantation of cells secreting the physiologically active substances. 
     Membranes made from polymers have been used as a membrane for immunoisolation. For example, JP1994-507412A (JP-H06-507412A) discloses that it is possible to use polyacrylonitrile-polyvinyl chloride copolymer (PAN/PVC), polysulfone, and the like to form an immuno-blocking vehicle suitable for insulin release, and that it is possible to use polyvinylpyrrolidone as a hydrophilic or hydrophobic additive which affects membrane permeability. Polytetrafluoroethylene (PTFE) is used for a membrane for immunoisolation in TheraCyte™, which is a commercially available immunoisolation device capable of transplantation. 
     SUMMARY OF THE INVENTION 
     In the membrane for immunoisolation disclosed in JP1994-507412A (JP-H06-507412A) or a commercially available membrane for immunoisolation, a synthetic polymer is used. A membrane for immunoisolation is intended to be used in a device for transplantation and to function in a living body, and thus is required to be usable for a long period of time without imparting a burden on a recipient. An object of the present invention is to provide a membrane for immunoisolation having a high bioaffinity. 
     As a result of investigations using various synthetic polymers, the inventors of the present invention have found that even membranes made from the same polymer have different levels of a bioaffinity depending on the distribution of elements in the membrane, and therefore have completed the present invention based on these findings. 
     That is, the present invention provides the following &lt;1&gt; to &lt;20&gt;. 
     &lt;1&gt; A membrane for immunoisolation, comprising: 
     a porous membrane that contains a polymer, 
     in which Formulas (I) and (II) are satisfied for at least one surface of the porous membrane,
 
 B/A≤ 0.7  (I)
 
 A≥ 0.015  (II)
 
     in the formula, A represents a ratio of an N element to a C element on a surface of the membrane, and B represents a ratio of the N element to the C element at a depth of 30 nm from the surface of the membrane. 
     &lt;2&gt; The membrane for immunoisolation according to &lt;1&gt;, in which Formulas (I) and (II) are satisfied for both surfaces of the porous membrane. 
     &lt;3&gt; The membrane for immunoisolation according to &lt;1&gt; or &lt;2&gt;, which is made from the porous membrane. 
     &lt;4&gt; The membrane for immunoisolation according to any one of &lt;1&gt; to &lt;3&gt;, in which the porous membrane contains 0.05% to 8.0% by mass of a nitrogen-containing compound with respect to a mass of the porous membrane. 
     &lt;5&gt; The membrane for immunoisolation according to &lt;4&gt;, in which the nitrogen-containing compound is a polyvinylpyrrolidone. 
     &lt;6&gt; The membrane for immunoisolation according to any one of &lt;1&gt; to &lt;5&gt;, in which the porous membrane contains a polysulfone. 
     &lt;7&gt; The membrane for immunoisolation according to any one of &lt;1&gt; to &lt;6&gt;, in which the porous membrane has a thickness of 10 μm to 250 μm. 
     &lt;8&gt; The membrane for immunoisolation according to any one of &lt;1&gt; to &lt;7&gt;, in which the porous membrane has a minimum pore diameter of 0.02 μm to 1.5 μm. 
     &lt;9&gt; The membrane for immunoisolation according to any one of &lt;1&gt; to &lt;7&gt;, in which the porous membrane has a minimum pore diameter of 0.02 μm to 1.3 μm. 
     &lt;10&gt; The membrane for immunoisolation according to any one of &lt;1&gt; to &lt;9&gt;, in which the porous membrane has a layered compact portion where a pore diameter is the smallest within the membrane, and the pore diameter continuously increases in a thickness direction from the compact portion toward at least one surface of the porous membrane. 
     &lt;11&gt; The membrane for immunoisolation according to &lt;10&gt;, in which a thickness of the compact portion is 0.5 μm to 30 μm. 
     &lt;12&gt; The membrane for immunoisolation according to &lt;10&gt; or &lt;11&gt;, in which the compact portion is located between any one surface X of the porous membrane and a portion at a distance of one-third the thickness of the porous membrane from the surface X. 
     &lt;13&gt; The membrane for immunoisolation according to &lt;12&gt;, in which Formulas (I) and (II) are satisfied for the surface X. 
     &lt;14&gt; A chamber for transplantation for enclosing a biological constituent therein, the chamber comprising: 
     the membrane for immunoisolation according to any one of &lt;1&gt; to &lt;13&gt; on at least a part of a surface forming an inside and an outside of the chamber for transplantation. 
     &lt;15&gt; The chamber for transplantation according to &lt;14&gt;, in which the surface satisfying Formulas (I) and (II) is on the inside. 
     &lt;16&gt; A chamber for transplantation for enclosing a biological constituent therein, the chamber comprising: 
     the membrane for immunoisolation according to &lt;12&gt; or &lt;13&gt; on at least a part of a surface forming an inside and an outside of the chamber for transplantation, 
     in which the surface X of the porous membrane is on the inside. 
     &lt;17&gt; The chamber for transplantation according to any one of &lt;14&gt; to &lt;16&gt;, in which the biological constituent is a cell. 
     &lt;18&gt; A device for transplantation, comprising the chamber for transplantation according to any one of &lt;14&gt; to &lt;17&gt; enclosing the biological constituent therein. 
     &lt;19&gt; The device for transplantation according to &lt;18&gt;, in which the biological constituent releases a physiologically active substance. 
     &lt;20&gt; The device for transplantation according to &lt;19&gt;, in which the physiologically active substance is insulin. 
     According to the present invention, it is possible to provide a membrane for immunoisolation which has a high bioaffinity. A device for transplantation including chamber for transplantation, which has the membrane for immunoisolation of the present invention, enclosing a biological constituent therein, can be used for a long period of time without receiving an immune rejection after transplantation into a recipient. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows results of an XPS measurement of a porous membrane  3  produced in Examples. (1) shows results regarding a surface side exposed to air during manufacture, (2) shows results regarding a back surface thereof, and (3) shows an N/C compositional ratio of a surface X and the back surface calculated from (1) and (2). 
         FIG. 2  shows results of an XPS measurement of a porous membrane  7  produced in Examples. (1) shows results regarding a surface side exposed to air during manufacture, (2) shows results regarding a back surface thereof, and (3) shows an N/C compositional ratio of a surface X and the back surface calculated from (1) and (2). 
         FIG. 3  shows graphs showing results of TOF-SIMS analysis on the porous membrane  3  and the porous membrane  7  produced in Examples. 
         FIG. 4  is a view showing an SEM image of a cross section of the porous membrane  7  produced in Examples. 
         FIG. 5  is a graph showing distribution of an average pore diameter in a thickness direction of the porous membrane  7  produced in Examples. 
         FIG. 6  shows images of histologically stained sections of the porous membranes  3  and  7 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, the present invention will be described in detail. 
     In the present specification, “to” is used to refer to a meaning including numerical values denoted before and after “to” as a lower limit value and an upper limit value. 
     &lt;Membrane for Immunoisolation&gt; 
     In the present specification, a membrane for immunoisolation refers to a membrane used for immunoisolation. 
     Immunoisolation is a method for preventing an immune rejection. In general, immunoisolation is one of a method for preventing an immune rejection by a recipient in a case of transplantation. Here, the immune rejection is a rejection by a recipient with respect to a biological constituent to be transplanted. A biological constituent is isolated from an immune rejection by a recipient due to immunoisolation. Examples of immune rejections include reactions due to cellular immune responses and reactions due to humoral immune responses. 
     The membrane for immunoisolation is a selectively permeable membrane that allows nutrients such as oxygen, water, and glucose to permeate therethrough, and inhibits permeation of immune cells and the like involved in an immune rejection. Examples of immune cells include macrophages, dendritic cells, neutrophils, eosinophils, basophils, natural killer cells, various T cells, B cells, and other lymphocytes. 
     Depending on the application, the membrane for immunoisolation of the embodiment of the present invention preferably inhibits permeation of high-molecular-weight proteins such as immunoglobulins (IgM, IgG, and the like) and complements, and preferably allows a relatively low-molecular-weight physiologically active substances such as insulin to permeate therethrough. 
     The selective permselectivity of the membrane for immunoisolation may be adjusted according to the application. The membrane for immunoisolation of the embodiment of the present invention may be a selectively permeable membrane which blocks a substance having a molecular weight such as 500 kDa or more, 100 kDa or more, 80 kDa or more, or 50 kDa or more. For example, it is preferable that the membrane for immunoisolation be capable of inhibiting permeation of the smallest IgG (molecular weight of about 160 kDa) among antibodies. In addition, the membrane for immunoisolation of the embodiment of the present invention may be a selectively permeable membrane which blocks a substance having a diameter such as 500 nm or more, 100 nm or more, 50 nm or more, or 10 nm or more, as a sphere size. 
     The membrane for immunoisolation of the embodiment of the present invention includes a porous membrane containing a polymer. The membrane for immunoisolation of the embodiment of the present invention may formed of only the porous membrane or may contain other layers. Examples of other layers include a hydrogel membrane. 
     It is preferable that at least one surface of the membrane for immunoisolation of the embodiment of the present invention be the porous membrane. In particular, it is preferable that a surface of the porous membrane satisfying Formulas (I) and (II) be a surface of the membrane for immunoisolation. The membrane for immunoisolation of the embodiment of the present invention may have a peelable protective film on the surface thereof for transportation or the like. In this case, the porous membrane may be disposed in an inside of the protective film. 
     A thickness of the membrane for immunoisolation of the embodiment of the present invention is not particularly limited, but may be 10 μm to 500 μm, is preferably 20 μm to 300 μm, and is more preferably 30 μm to 250 μm. In particular, the thickness of the membrane for immunoisolation of the embodiment of the present invention is more preferably 10 μm to 200 μm, is even more preferably 10 μm to 100 μm, and is particularly preferably 10 μm to 50 μm. 
     [Porous Membrane] 
     (Elemental Distribution of Porous Membrane) 
     Formulas (I) and (II) are satisfied for at least one surface of the porous membrane.
 
 B/A≤ 0.7  (I)
 
 A≥ 0.015  (II)
 
     In the formula, A represents a ratio of an N element (nitrogen atom) to a C element (carbon atom) on a surface of the membrane, and B represents a ratio of the N element to the C element at a depth of 30 nm from the same surface. 
     Formula (II) shows that a certain amount or more of N element is present on at least one surface of the porous membrane, and Formula (I) shows that an N element in the porous membrane is localized at a depth of less than 30 nm of the surface. An N element is preferably derived from a nitrogen-containing compound. In addition, it is preferable that a nitrogen-containing compound be polyvinylpyrrolidone. 
     The inventors of the present invention have found that a bioaffinity of the porous membrane becomes high by Formula (I) and Formula (II) satisfying for the surface thereof. In particular, the porous membrane has a high bioaffinity on the surface side satisfying Formulas (I) and (II). 
     In the porous membrane, either one of surfaces may satisfy Formulas (I) and (II), or both surfaces may satisfy Formulas (I) and (II), but it is preferable that both surfaces satisfy Formulas (I) and (II). In a case where either one of surfaces satisfies Formulas (I) and (II), the surface thereof may be in an inside or an outside of a chamber for transplantation to be described later, but the surface is preferably in the inside thereof. 
     In the present specification, a ratio (A value) of N element to C element on the membrane surface and a ratio (B value) of N element to C element at a depth of 30 nm from the surface are obtained by calculating using XPS measurement results. The XPS measurement is X-ray photoelectron spectroscopy, which is a method for irradiating a membrane surface with X-rays, measuring kinetic energy of photoelectrons emitted from the membrane surface, and analyzing a composition of elements constituting the membrane surface. Under conditions using a monochromated Al-Kα, ray described in Examples, the A value is calculated from results at the start of sputtering, and the B value is calculated from time results, which are calculated that the ray is at 30 nm from the surface of the membrane measured from a sputtering rate. 
     B/A may be 0.02 or more, is preferably 0.03 or more, and is more preferably 0.05 or more. 
     A is preferably 0.050 or more, and is more preferably 0.080 or more. In addition, A may be 0.20 or less, is preferably 0.15 or less, and is more preferably 0.10 or less. 
     B may be 0.001 to 0.10, is preferably 0.002 to 0.08, and is more preferably 0.003 to 0.07. 
     (Structure of Porous Membrane) 
     The porous membrane is a membrane having a plurality of pores. Pores can be confirmed by, for example, captured images of a scanning electron microscope (SEM) or captured images of a transmission electron microscope (TEM) of a cross section of the membrane. 
     A pore diameter may be measured from a photograph of a cross section of the membrane obtained by an electron microscope. The porous membrane can be cut with a microtome or the like, and it is possible to obtain a photograph of a cross section of the porous membrane as a section of a thin membrane which a cross section can be observed. 
     A pore diameter of the porous membrane is preferably 0.02 μm to 25 μm, is more preferably 0.2 μm to 23 μm, and is even more preferably 0.4 μm to 21 μm. A value is a value obtained as an average pore diameter of a section to be described later. Pores having a smaller pore diameter or a larger pore diameter than this value may be present. The pore diameter of the porous membrane may or may not change in a thickness direction. In a case where a pore diameter changes in the thickness direction, that is, in a case where the porous membrane has a pore diameter distribution, it is possible to improve the life of the membrane for immunoisolation. The reason is that, by using the membrane having substantially different pore diameters, effects are obtained as though multistage filtration would be carried out, and therefore a deterioration in the membrane can be prevented. 
     A thickness of the porous membrane is not particularly limited, but may be 10 μm to 250 μm, is preferably 20 μm to 220 μm, and is more preferably 30 μm to 200 μm. In particular, the thickness of the porous membrane is more preferably 10 μm to 200 μm, is even more preferably 10 μm to 100 μm, and is particularly preferably 10 μm to 50 μm. By setting the thickness of the porous membrane to 10 μm or more, it is possible to obtain membrane hardness at which the chamber for transplantation does not break in a recipient. In addition, by setting the thickness of the porous membrane to 250 μm or less, it is possible to obtain the chamber for transplantation having stiffness to the extent that a recipient is not uncomfortable. 
     In the membrane for immunoisolation of the embodiment of the present invention, it is preferable that the porous membrane have, within the membrane, a layered compact portion where a pore diameter becomes smallest, and a pore diameter continuously increase in a thickness direction from this compact portion toward at least one surface of the porous membrane. The pore diameter is determined by an average pore diameter of a section or a parting line which will be described later. The pore diameter means a diameter of pores. 
     The surface of the membrane means a main surface (a front surface or a back surface showing an area of the membrane), and does not mean a surface in the thickness direction of an end of the membrane. The surface of the porous membrane may be an interface with another layer. In the membrane for immunoisolation of the embodiment of the present invention, it is preferable that the porous membrane have almost the same structure in an intra-membrane direction (a direction parallel to the membrane surface) with respect to pore diameters, and in the entire area with respect to pore diameter distributions (a difference in pore diameters in the thickness direction). 
     A pore diameter may be measured from a photograph of a cross section of the membrane obtained by an electron microscope. The porous membrane can be cut with a microtome or the like, and it is possible to obtain a photograph of a cross section of the porous membrane as a section of a thin membrane which a cross section can be observed. 
     In the present specification, in a case of comparing pore diameters in the thickness direction of the membrane (particularly in a case of comparing pore diameters in the thickness direction of the membrane having a thickness greater than 100 μm), comparison is performed by diving an SEM image of the cross section of the membrane in the thickness direction of the membrane. The number of divisions can be appropriately selected from a thickness of the membrane. The number of divisions is at least 5 or more, and for example, in a case of a membrane having a thickness of 200 μm, the membrane is divided into 20 parts from a surface X to be described later. A size of a division width means a size of a width in the thickness direction of the membrane, and does not mean a width size in a photograph. In the comparison of pore diameters in the thickness direction of the membrane, pore diameters are compared as an average pore diameter of each section. An average pore diameter of each section may be, for example, an average value of 50 pores in each section of a cross-sectional view of the membrane. The cross-sectional view of the membrane in this case may be obtained with a width of 80 μm (a distance of 80 μm in a direction parallel to a surface), for example. In this case, for a section in which pores are large and therefore only less than 50 pores can be measured, it is sufficient that an average pore diameter be an average pore diameter obtained by measuring the available number of pores that can be counted in that section. In addition, in this case, in a case where pores are too large to fit in this section, a size of pore is measured over another section. 
     The layered compact portion having the smallest pore diameter refers to a layered portion of the porous membrane corresponding to a section having the smallest average pore diameter among sections of the membrane cross section. The compact portion may be formed of a portion corresponding to one section, or may be formed of portions corresponding to a plurality of sections having an average pore diameter 1.1 times or less an average pore diameter of the section having the smallest average pore diameter such as 2 or 3. 
     The comparison of pore diameters in the thickness direction of the membrane (in particular, the comparison of pore diameters in the thickness direction of the membrane having a thickness of 100 μm or less) may be performed by comparing pore diameters in 19 parting lines in a case where an SEM image of the membrane cross section is divided into 20 in the thickness direction of the membrane. 50 or more consecutive pores that intersect or are in contact with the parting line are selected, each of pore diameters are measured, and an average value is calculated as an average pore diameter. Here, as the pore diameter, not a length of a portion where the selected pore intersects the parting line, but a diameter is used, the diameter being calculated using an area, which is obtained by calculating an area of pores calculated from an SEM image of the membrane cross section by image processing, as an area of a true circle. In this case, for a parting line in which pores are large and therefore only up to 50 pores can be selected, an average pore diameter is assumed to an average pore diameter obtained by measuring 50 pores by broadening the field of view of an SEM image for obtaining the membrane cross section. Pore diameters in the thickness direction of the membrane are compared by comparing the obtained average pore diameter for each parting line. 
     Even in a case where pore diameters in the thickness direction of the membrane are compared using a parting line, a structure of the porous membrane can be determined as in the case of using the division as described above. 
     The layered compact portion where the pore diameter is the smallest in a case where the comparison of pore diameters in the thickness direction of the membrane is performed using a parting line, refers to a layered portion of the porous membrane including a parting line with the smallest average pore diameter among parting lines in photograph of the membrane cross section described above. The compact portion may include two or more parting lines. For example, in a case where two or more parting lines, which have an average pore diameter 1.1 times or less the minimum average pore diameter, are consecutive, the compact portion is assumed to include two or more consecutive parting lines. A thickness of the compact portion is a product of the number of parting lines included in the compact portion and one twentieth of the thickness of the membrane. 
     A thickness of the compact portion may be 0.5 μm to 50 μm, and is preferably 0.5 μm to 30 μm. In the present specification, an average pore diameter of the compact portion is denoted as the minimum pore diameter of the porous membrane. The minimum pore diameter of the porous membrane is preferably 0.02 μm to 1.5 μm, and is more preferably 0.02 μm to 1.3 μm. The reason is that the minimum pore diameter of such a porous membrane can inhibit permeation of at least normal cells. Here, an average pore diameter of the compact portion is regarded to be an average pore diameter measured according to ASTM F316-80. 
     It is preferable that the porous membrane have a compact portion within the membrane. The phrase “within the membrane” means that the compact portion is not in contact with the surface of the membrane. The phrase “having the compact portion within the membrane” means that the compact portion is not the closest section with respect to any surface of the membrane. By using the porous membrane having a structure having the compact portion within the membrane, permeability of a substance intended to permeate is unlikely to lower compared to a case of using a porous membrane having the same compact portion, which is in contact with the surface thereof. Although not bound by any theory, it is perceived that protein adsorption is less likely to occur due to the presence of the compact portion within the membrane. 
     It is preferable that the compact portion be biased to one of the front surface side than a central portion in thickness of the porous membrane. Specifically, the compact portion preferably located at a distance of less than half the thickness of the porous membrane from the surface of one of the porous membranes, more preferably located within a distance of two-fifths, even more preferably located within a distance of one-third, and still even more preferably located within a distance of one-fourth. This distance may be determined from the photograph of the membrane cross section described above. In the present specification, the surface of the porous membrane closer to the compact portion is referred to as a “surface X.” The surface X is preferably a surface satisfying Formulas (I) and (II). 
     In the porous membrane, it is preferable that a pore diameter continuously increase in the thickness direction from the compact portion toward at least one of the surfaces. In the porous membrane, the pore diameter may continuously increase in the thickness direction toward the surface X from the compact portion, the pore diameter may continuously increase in the thickness direction toward the surface opposite to the surface X from the compact portion, and the pore diameter may continuously increase in the thickness direction toward any surface of the porous membrane from the compact portion. Among them, it is preferable that the pore diameter continuously increase in the thickness direction toward at least the surface opposite to the surface X from the compact portion, and it is preferable that the pore diameter continuously increase in the thickness direction toward any surface of the porous membrane from the compact portion. The sentence “the pore diameter continuously increases in the thickness direction” means that a difference in average pore diameters between the sections adjacent in the thickness direction increases by 50% or less of a difference between maximum average pore diameters (maximum pore diameter) and minimum average pore diameters (minimum pore diameter), preferably by 40% or less, and more preferably by 30% or less. The phrase “continuously increasing” essentially means that a pore diameter increases uniformly without decreasing, but a decreasing portion may occur accidentally. For example, in a case of combining two sections from the surface, in a case where an average value of a combination increases uniformly (uniformly decreases toward the compact portion from the surface), it can be determined that “the pore diameter continuously increases in the thickness direction toward the surface of the membrane from the compact portion.” 
     A structure of the porous membrane in which a pore diameter continuously increases in the thickness direction can be realized by, for example, a manufacturing method to be described later. 
     A maximum pore diameter of the porous membrane is preferably more than 1.5 μm and 25 μm or less, is more preferably 1.8 μm to 23 μm, and is even more preferably 2.0 μm to 21 μm. In the present specification, an average pore diameter of the section having a maximum average pore diameter among sections of the membrane cross section is used to refer to a maximum pore diameter of the porous membrane. 
     A ratio of an average pore diameter of the compact portion to the maximum pore diameter of the porous membrane (a ratio of a minimum pore diameter to a maximum pore diameter of the porous membrane, which is a value obtained by dividing the maximum pore diameter by the minimum pore diameter, an “anisotropy ratio” in the present specification) is preferably 3 or more, is more preferably 4 or more, and is even more preferably 5 or more. The reason is that an average pore diameter except for that of the compact portion increases to increase substance permeability of the porous membrane. In addition, the anisotropy ratio is preferably 25 or less, and is more preferably 20 or less. The reason is that effects, as though multistage filtration would be carried out, can be efficiently obtained within a range where an anisotropy ratio is 25 or less. 
     It is preferable that a section with a maximum average pore diameter be a section closest to any surface of the membrane or a section in contact with that section. 
     In the section closest to any surface of the membrane, it is preferable that an average pore diameter be more than 0.05 μm and 25 μm or less, be more preferably more than 0.08 and 23 μm or less, and be even more preferably more than 0.5 μm and 21 μm or less. In addition, a ratio of an average pore diameter of the compact portion to an average pore diameter of the section closest to any surface of the membrane is preferably 1.2 to 20, is more preferably 1.5 to 15, and is even more preferably 2 to 13. 
     (Composition of Porous Membrane) 
     The porous membrane contains a polymer. It is preferable that the porous membrane be essentially composed of a polymer. 
     The polymer forming the porous membrane is preferably biocompatible. Here, the term “biocompatible” means that the polymer has non-toxic and non-allergenic properties, but does not have properties such that that the polymer is encapsulated in a living body. 
     The number average molecular weight (Mn) of the polymer is preferably 1,000 to 10,000,000, and is more preferably 5,000 to 1,000,000. 
     Examples of polymers include thermoplastic or thermosetting polymers. Thermoplastic polymers are preferred. Specific examples of the polymer include polysulfone, cellulose acylate such as cellulose acetate, nitrocellulose, sulfonated polysulfone, polyethersulfone, polyvinylidene fluoride, polyacrylonitrile, styrene-acrylonitrile copolymer, styrene-butadiene copolymer, saponified ethylene-vinyl acetate copolymer, polyvinyl alcohol, polycarbonate, an organosiloxane-polycarbonate copolymer, a polyester carbonate, an organopolysiloxane, a polyphenylene oxide, a polyamide, a polyimide, polyamideimide, polybenzimidazole, ethylene vinyl alcohol copolymer, polytetrafluoroethylene (PTFE), and the like. From the viewpoints of solubility, optical physical properties, electrical physical properties, strength, elasticity, and the like, polymers may be homopolymers, copolymers, polymer blends, or polymer alloys. 
     Among them, polysulfone and cellulose acylate are preferable, and polysulfone is more preferable. 
     It is preferable that the porous membrane further contain a nitrogen-containing compound. The nitrogen-containing compound is preferably contained by an amount of 0.05 to 8.0% by mass, more preferably 0.1 to 5.0% by mass, and even more preferably 0.2 to 4.0% by mass, with respect to a mass of the porous membrane. Examples of nitrogen-containing compounds include polyvinylpyrrolidone (referred to as “PVP” in the present specification in some cases), polypropylacrylamide, chitin, chitosan, polyacrylamide, polyamine, polylysine, and the like. 
     The porous membrane may contain other components than the polymer as an additive. 
     Examples of additives include metal salts of inorganic acids such as sodium chloride, lithium chloride, sodium nitrate, potassium nitrate, sodium sulfate, and zinc chloride; metal salts of organic acids such as sodium acetate and sodium formate; other polymers such as polyethylene glycol; high polymer electrolytes such as sodium polystyrene sulfonate and polyvinyl benzyl trimethyl ammonium chloride; ionic surfactants such as sodium dioctyl sulfosuccinate and sodium alkyl sodium taurate; and the like. The additive may act as a swelling agent for a porous structure. 
     As an additive, it is preferable to use a metal salt. The porous membrane containing polysulfone or polyethersulfone preferably contains lithium chloride. 
     The porous membrane is preferably a membrane formed from a single composition as a single layer, and preferably not has a laminated structure of a plurality of layers. By forming the porous membrane from one composition as a single layer, it is possible to manufacture the membrane for immunoisolation at low costs by a simple procedure. 
     (Method for Manufacturing Porous Membrane) 
     A method for manufacturing the porous membrane is not particularly limited as long as the method can form the porous membrane having the above structure, and any general methods for forming a polymer membrane can be used. Examples of methods for forming a polymer membrane include a stretching method, a casting method, and the like, and a casting method is preferable. 
     In general methods for forming a polymer membrane, a porous membrane satisfying Formulas (I) and (II) can be formed by performing a treatment such that a compound containing a nitrogen atom in a composition for forming a membrane is unevenly distributed on the surface. A solution containing a compound containing a nitrogen atom is applied to a surface of a porous membrane formed by general methods for forming a polymer membrane, or a surface of a commercially available porous membrane, so that Formulas (I) and (II) are satisfied. The compound containing a nitrogen atom is preferably a nitrogen-containing compound described above. 
     For manufacture of the porous membrane, it is particularly preferable to use a casting method. In the casting method, it is possible to produce a porous membrane having the above-mentioned structure (pore diameter distribution) by adjusting the type and amount of a solvent used in a stock solution for forming a membrane, and a drying method after casting. 
     Manufacture of a porous membrane by a casting method can be carried out by a method including, for example, the following (1) to (4) in this order. 
     (1) A stock solution for forming a membrane, which contains a polymer, if necessary an additive and, if necessary a solvent, is flow-cast on a support while being in a dissolved state. 
     (2) The surface of the flow-cast liquid membrane is exposed to temperature-controlled humid air. 
     (3) The membrane obtained after being exposed to temperature-controlled humid air is immersed in a coagulation liquid. 
     (4) A support is peeled off if necessary. 
     A temperature of temperature-controlled humid air may be 4° C. to 60° C., and is preferably 10° C. to 40° C. A relative humidity of the temperature-controlled humid air may be 15% to 100%, and is preferably 25% to 95%. The temperature-controlled humid air may be applied at a wind speed of 0.1 m/s to 10 m/s for 0.1 seconds to 30 seconds, preferably 1 second to 10 seconds. 
     The elemental distribution of the porous membrane, especially the distribution of an N element, can be controlled by a moisture concentration contained in the temperature-controlled humid air, a time to apply the temperature-controlled humid air, a temperature of a coagulation liquid, an immersion time, a temperature of diethylene glycol during washing which will be described later, a speed of a porous manufacture line, and the like. The distribution of the N element can also be controlled by an amount of moisture contained in a stock solution for forming a membrane. 
     In addition, an average pore diameter and position of the compact portion can also be controlled by a moisture concentration contained in the temperature-controlled humid air and a time of applying the temperature-controlled humid air. An average pore diameter of the compact portion can also be controlled by an amount of moisture contained in a stock solution for forming a membrane. 
     By applying the temperature-controlled humid air to the surface of the liquid membrane as described above, it is possible to cause coacervation from the surface of the liquid membrane toward the inside of the membrane by controlling evaporation of a solvent. By immersing the membrane in a coagulation liquid containing a solvent having low solubility of the polymer but compatible with the solvent of the polymer in this state, the above-mentioned coacervation phase is fixed as fine pores, and pores other than the fine pores can also be formed. 
     A temperature of the coagulation liquid may be −10° C. to 80° C. in a process of immersing the membrane in the coagulation liquid. By changing a temperature during this period, it is possible to control a size of a pore diameter up to a support surface side by adjusting a time from the formation of the coacervation phase on the support surface side to the solidification from the compact portion. In a case where a temperature of the coagulation liquid is raised, the formation of the coacervation phase becomes faster and a time for solidification becomes longer, and therefore the pore diameter toward the support surface side tends to become large. On the other hand, in a case where a temperature of the coagulation liquid is lowered, the formation of the coacervation phase becomes slower and a time for solidification becomes shorter, and therefore the pore diameter toward the support surface side is unlikely to become large. 
     As the support, a plastic film or a glass plate may be used. Examples of materials of the plastic film include polyester such as polyethylene terephthalate (PET), polycarbonate, acrylic resin, epoxy resin, polyurethane, polyamide, polyolefin, a cellulose derivative, silicone, and the like. As the support, a glass plate or PET is preferable, and PET is more preferable. 
     The stock solution for forming a membrane may contain a solvent. A solvent having high solubility of the polymer to be used (hereinafter referred to as “favorable solvent”) may be used depending on a polymer to be used. As a favorable solvent, it is preferable that the solvent be quickly substituted with the coagulation liquid in a case where the membrane is immersed in the coagulation liquid. Examples of solvents include N-methyl-2-pyrrolidone, dioxane, tetrahydrofuran, dimethylformamide, dimethylacetamide, or a mixed solvent thereof in a case where the polymer is polysulfone and the like; dioxane, N-methyl-2-pyrrolidone, dimethylformamide, dimethylacetamide, dimethylsulfoxide, or a mixed solvent thereof in a case where the polymer is polyacrylonitrile and the like; dimethylformamide, dimethylacetamide, or a mixed solvent thereof in a case where the polymer is polyamide and the like; acetone, dioxane, tetrahydrofuran, N-methyl-2-pyrrolidone, or a mixed solvent thereof in a case where the polymer is cellulose acetate and the like. Among them, N-methyl-2-pyrrolidone is preferably used. 
     In addition to a favorable solvent, the stock solution for forming a membrane preferably use a solvent (hereinafter referred to as “non-solvent”) in which the solubility of the polymer is low but is compatible with the solvent of the polymer. Examples of non-solvents include water, cellosolves, methanol, ethanol, propanol, acetone, tetrahydrofuran, polyethylene glycol, glycerin, and the like. Among these, it is preferable to use water. 
     A concentration of the polymer as the stock solution for forming a membrane may be 5% by mass to 35% by mass, is preferably 10% by mass to 30% by mass. By setting the concentration thereof to 35 mass % or less, sufficient permeability (for example, water permeability) can be imparted to the obtained porous membrane. By setting the concentration thereof to 5 mass % or more, the formation of a porous membrane which selectively allows substances to permeate can be secured. An amount of additive to be added is not particularly limited as long as the homogeneity of the stock solution for forming a membrane is not lost by the addition, but is 0.5% by volume to 10% by volume respect to a general solvent. In a case where the stock solution for forming a membrane contains a non-solvent and a favorable solvent, a ratio of the non-solvent to the favorable solvent is not particularly limited as long as a mixed solution can be maintained in a homogeneous state, but is preferably 1.0% by mass to 50% by mass, is more preferably 2.0% by mass to 30% by mass, and is even more preferably 3.0% by mass to 10% by mass. 
     In addition, in the stock solution for forming a membrane for manufacturing a porous membrane containing a polymer selected from the group consisting of polysulfone and polyethersulfone, and containing polyvinylpyrrolidone, polyvinylpyrrolidone is preferably contained by an amount of 50% by mass to 120% by mass, and more preferably by an amount of 80% by mass to 110% by mass, with respect to a total mass of polysulfone and polyethersulfone. By using such a stock solution for forming a membrane, a porous membrane containing about 0.05% to 8.0 mass % of polyvinylpyrrolidone is obtained. The reason why an amount of polyvinylpyrrolidone is reduced is that most parts of polyvinylpyrrolidone are removed in a washing step. 
     Furthermore, in a case where the stock solution for forming a membrane contains lithium chloride as an additive, lithium chloride is preferably contained by an amount of 5% by mass to 20% by mass, and more preferably by 10% by mass to 15% by mass, with respect to the total mass of polysulfone and polyethersulfone. 
     As the coagulation liquid, it is preferable to use a solvent having a low solubility of the polymer used. Examples of such solvents include water, alcohols such as methanol, ethanol, and butanol; glycols such as ethylene glycol and diethylene glycol; aliphatic hydrocarbons such as ether, n-hexane, and n-heptan; glycerol such as glycerin; and the like. Examples of preferred coagulation liquids include water, alcohols, or a mixture of two or more of these. Among these, it is preferable to use water. 
     After immersion in the coagulation liquid, it is also preferable to perform washing with a solvent different from the coagulation liquid that has been used. Washing can be carried out by immersing in a solvent. Diethylene glycol is preferable as a washing solvent. Distribution of an N element in the porous membrane can be adjusted by adjusting either or both of a temperature and an immersion time of diethylene glycol in which a film is immersed by using diethylene glycol as a washing solvent. In particular, in a case where polyvinylpyrrolidone is used as the stock solution for forming a membrane of the porous membrane, a residual amount of polyvinylpyrrolidone on the membrane can be controlled. After washing with diethylene glycol, furthermore, the membrane may be washed with water. 
     As the stock solution for forming a membrane of the porous membrane, the stock solution for forming a membrane, which is obtained by dissolving polysulfone and polyvinylpyrrolidone in N-methyl-2-pyrrolidone and adding water, is preferable. 
     Regarding a method for manufacturing the porous membrane, reference can be made to JP1992-349927A (JP-H04-349927A), JP1992-068966B (JP-H04-068966B), JP1992-351645A (JP-H04-351645A), JP2010-235808A, and the like. 
     [Other Layers] 
     The membrane for immunoisolation of the embodiment of the present invention may contain layers other than the porous membrane. Examples of other layers include a hydrogel membrane. As a hydrogel membrane, a biocompatible hydrogel membrane is preferable. Examples thereof include an alginic acid gel membrane, an agarose gel membrane, a polyisopropyl acrylamide membrane, a membrane containing cellulose, a membrane containing a cellulose derivative (for example, methyl cellulose), a polyvinyl alcohol membrane, or the like. The hydrogel membrane is preferably an alginic acid gel membrane. Specific examples of alginic acid gel membranes include a polyion complex membrane of alginic acid-poly-L-lysine-alginic acid. 
     &lt;Use of Membrane for Immunoisolation&gt; 
     The membrane for immunoisolation can be used to prevent an immune rejection. Specifically, the membrane for immunoisolation can be used to prevent an immune rejection by a recipient with respect to a transplanted biological constituent. That is, the membrane for immunoisolation can be used for protecting biological constituents from an immune system of a recipient. In the present specification, a recipient means a living body to which transplantation is performed. A recipient is preferably a mammal, and is more preferably a human. 
     [Biological Constituent] 
     The biological constituent means a structure body derived from a living body. Examples of living bodies include viruses, bacteria, yeasts, fungal cells, insects, plants, mammals, and the like. It is preferable that a living body be generally a mammal. Examples of mammals include bovines, swine, sheep, cats, dogs, humans, and the like. The biological constituent is preferably a structure body derived from any of mammals. 
     Examples of biological constituents include organs, tissues, cells, and the like. Among these, cells are preferred as biological constituents. As cells, a single cell may be used or a plurality of cells may be used. It is preferable that a plurality of cells be used. A plurality of cells may be separated from each other or may be an aggregate. 
     The biological constituent may be obtained directly from a living body. In addition, particularly in a case where the biological constituent is a cell, the biological constituent may be directly obtained from a living body, or may be obtained by differentiation-induction of cells such as embryonic stem cells (ES cell), induced pluripotent stem cells (iPS cell), and mesenchymal stem cells. The cell may be a progenitor cell. 
     As a biological constituent, as one aspect, it is preferable to release a physiologically active substance. Examples of physiologically active substances include various hormones, various cytokines, various enzymes, and various other biologic factors in a living body. More specific examples include insulin, dopamine, factor VIII, and the like. 
     Here, insulin is a polypeptide (molecular weight of about 6000) in which an A chain of 21 amino acid residues and a B chain of 30 amino acid residues are linked via a disulfide bond. In insulin in a living body of a mammal is secreted from 13 cells in pancreatic islets of Langerhans. In a case of using insulin-secreting cells as the biological constituent in the present invention, insulin secreted may be human-type insulin or other mammalian-type (for example, porcine-type) insulin. Insulin may be insulin produced by a genetic recombination method. As a method for obtaining genetically modified insulin, for example, the description of Kadowaki Takashita: Diabetes Navigator (refer to 270-271, Takeo Tao, Yoshikazu Oka “Insulin Preparations of Present and Future,” Medical Review, 2002) can be referred to. Various types of insulin analogues (refer to, for example, H. C. Lee, J. W. Yoon, et al., Nature, 408, 483-488, 2000) may be used. 
     The biological constituent is preferably an insulin-secreting cell. Insulin-secreting cells are cells that can secrete insulin in response to changes in blood glucose level. The insulin-secreting cells are not particularly limited. Examples thereof include pancreatic β cells present in pancreatic islets of Langerhans. Pancreatic β cells may be human pancreatic β cells, or may be pancreatic β cells such as pigs and mice. For a method for extracting pancreatic β cells from a pig, reference can be made to the description in JP2007-195573A. In addition, the insulin-secreting cells may be cells derived from human stem cells (refer to, for example, Junichi Miyazaki, Regenerative Medicine, Vol. 1, No. 2, pp. 57-61, 2002), or cells induced from small intestinal epithelial stem cells (refer to, for example, Fumikomi Mineko et al., Regenerative Medicine, Volume 1, No. 2, pp. 63 to 68, 2002), or insulin-secretory cells into which a gene encoding insulin has been incorporated (refer to, for example, H. C. Lee, J. W. Yoon, et al., Nature, 408, pp. 483-488, 2000). Furthermore, the insulin-secreting cells may be pancreatic islets of Langerhans (refer to, for example, Horiyama, Kazumori Inoue, Regenerative Medicine, Volume 1, No. 2, pp. 69 to 77, 2002). 
     [Chamber for Transplantation] 
     The membrane for immunoisolation of the embodiment of the present invention can be used as a constructional element of a chamber for transplantation enclosing the biological constituent therein. The chamber for transplantation can be used as a container for enclosing the biological constituent in a case of transplanting the biological constituent into a recipient. The membrane for immunoisolation is disposed on at least a part of the surface forming the inside and the outside of the chamber for transplantation (a boundary that separates the inside and the outside of the chamber for transplantation). By disposing in such a manner, it is possible to protect the biological constituent enclosed in the chamber for transplantation from immune cells and the like present outside, and to introduce nutrients such as water, oxygen, and glucose into the inside of the chamber for transplantation from the outside. 
     The membrane for immunoisolation may be disposed on the entire surface forming the inside and the outside of the chamber for transplantation, and may be disposed a part of the surface corresponding to an area of, for example, 1% to 99%, 5% to 90%, 10% to 80%, 20% to 70% %, 30% to 60%, 40% to 50%, or the like with respect to the entire area. A surface on which the membrane for immunoisolation is disposed may be one continuous portion or may be divided into two or more portions. In a case where the membrane for immunoisolation is not disposed on the entire surface of the boundary forming the inside and the outside of the chamber for transplantation, it is sufficient that remaining portions are formed of a material such as an impermeable membrane not allowing permeation of nutrients such as oxygen, water, and glucose, in addition to cells and the like. 
     A shape of the chamber for transplantation is not limited, and may be a pouched-like shape, a bag shape, a tube shape, a microcapsule shape, or a drum shape. For example, a drum-shaped chamber for transplantation can be formed by adhering the membrane for immunoisolation to the top and bottom of a silicone ring. A shape of the chamber for transplantation is preferably a shape capable of preventing movement of the chamber for transplantation within a recipient in a case where the chamber for transplantation is used as a device for transplantation to be described later. Specific examples of shapes of the chamber for transplantation include a cylindrical shape, a disk-like shape, a rectangular shape, an egg shape, a star shape, a circular shape, and the like. The chamber for transplantation may be in a form of a sheet, a strand, a spiral, or the like. The chamber for transplantation may be a chamber for transplantation for enclosing the biological constituent and becomes the above-described shape only in a case where the chamber for transplantation used as a device for transplantation to be described later. 
     The chamber for transplantation may contain a biocompatible plastic or the like for maintaining the shape and strength as a container. For example, the boundary between the inside and the outside of the chamber for transplantation may be made from the membrane for immunoisolation and a biocompatible plastic that does not correspond to the membrane for immunoisolation. In addition, in the chamber for transplantation in which the membrane for immunoisolation is disposed on the entire surface forming substantially the inside and the outside, a biocompatible plastic having a net-like structure may be further disposed on the outside of the surface forming the inside and the outside, from the viewpoint of strength. 
     In the chamber for transplantation, it is preferable that the surface satisfying Formulas (I) and (II) of the porous membrane be disposed on the inside thereof. The reason is that, by disposing the surface on a side that is in contact with the biological constituent, it is possible to reduce irritation with respect to the biological constituent. In addition, in the chamber for transplantation, it is preferable that the surface X of the porous membrane be on the inside thereof. That is, it is preferable that the membrane for immunoisolation be disposed so that the compact portion of the porous membrane in the membrane for immunoisolation is closer to the inside of the chamber for transplantation. By setting the surface X in the inside of the chamber for transplantation, it is possible to make permeability of physiologically active substances higher. 
     The chamber for transplantation may have a joint portion at which the membranes for immunoisolation face each other to be joined. A portion of the membrane for immunoisolation that is being joined is not particularly limited, but is preferably an end portion of the membrane for immunoisolation. In particular, it is preferable that end portions be joined to each other. It is preferable that all of outer peripheries except an injection port and the like to be described later be joined to each other between the membranes for immunoisolation. For example, the chamber for transplantation preferably has a configuration in which two membranes for immunoisolation face each other and outer peripheries thereof are joined, or a structure in which one membrane for immunoisolation having a line symmetric structure is folded into two and facing outer peripheries are joined. Joining can be performed by adhesion using an adhesive, fusion welding, and the like. 
     In addition, the chamber for transplantation may include an injection port or the like for injecting the biological constituent or the like into the chamber for transplantation. As the injection port, a tube communicating with the inside of the chamber for transplantation may be provided. 
     The tube may be a tube containing a thermoplastic resin such as polyethylene, polyurethane, or polyvinyl chloride. 
     In addition, the chamber for transplantation may have a structure-reinforcing material which is for protecting internal biological constituents and the like. The structure-reinforcing material may be provided inside or outside the chamber for transplantation. 
     The structure-reinforcing material may be mesh, nets, nonwoven fabrics, woven fabrics, metals, resins, or the like. 
     [Device for Transplantation] 
     The device for transplantation is a complex including at least the chamber for transplantation and the biological constituent. In the device for transplantation, the chamber for transplantation encloses the biological constituent therein. 
     In the device for transplantation, the chamber for transplantation may enclose only the biological constituent therein, or may enclose the biological constituent, and constituents or components other than the biological constituent therein. For example, the biological constituent may be enclosed in the chamber for transplantation together with a hydrogel, and preferably in a state of being enclosed in the hydrogel. In addition, the device for transplantation may contain pH buffers, inorganic salts, organic solvents, proteins such as albumin, or peptides. 
     The device for transplantation may contain only one biological constituent or may contain two or more biological constituents. For example, the device for transplantation may contain only a biological constituent which releases physiologically active substances for the purpose of transplantation, or which serves other functions of transplantation; or may further contain a biological constituent assisting functions of these biological constituents. 
     The device for transplantation may be, for example, a device to be transplanted intraperitoneally or subcutaneously. In addition, the device for transplantation may be a blood-vessel-connecting device. For example, in a case where insulin-secreting cells are used as the biological constituent, insulin secretion corresponding to a change in blood glucose level becomes possible by performing transplantation such that blood and the membrane for immunoisolation come into direct contact with each other. 
     Regarding the device for transplantation and chamber for transplantation, the description of Protein Nucleic Acid Enzyme, Vol. 45, pp. 2307 to 2312, (Okawara Hisako, 2000), JP2009-522269A, JP1994-507412A (JP-H06-507412A), and the like can be referred to. 
     EXAMPLES 
     Characteristics of the present invention will be described in more detail with reference to the following examples and comparative examples. The materials, amounts used, proportions, treatment details, treatment procedures, and the like disclosed in the following Examples can be modified as appropriate as long as the gist of the invention is maintained. Therefore, the scope of the invention should not be limitedly interpreted by the specific examples described below. 
     &lt;Production of Porous Membrane&gt; 
     [Porous Membranes  1  to  7 ] 
     15 parts by mass of polysulfone (P3500 manufactured by Solvay), 15 parts by mass of polyvinylpyrrolidone, 2 parts by mass of lithium chloride, and 1.2 parts by mass of water were dissolved in 66.8 parts by mass of N-methyl-2-pyrrolidone. Therefore, a mixture for forming a membrane was obtained. This mixture was flow-cast on a surface of a PET film by a thickness of 200 μm. The flow-cast membrane surface was exposed to air adjusted to 25° C. and absolute humidity 7.8 g/kg air, at 2 m/sec for 5 seconds. Immediately thereafter, the exposed membrane surface was immersed in a coagulation liquid tank filled with water. PET was peeled off, and therefore a porous membrane was obtained. A temperature of a coagulation liquid was 25° C. 
     Thereafter, at 2 m/sec, the immersed membrane surface was put into a diethylene glycol bath for 120 seconds, and then was thoroughly washed with pure water. A temperature of the diethylene glycol bath was changed to be within a range between 4° C. to 120° C., and therefore porous membranes  1  to  7  each having a structure shown in Table 1 were obtained. A thickness of each membrane after drying was 195 to 205 μm. 
     [Porous Membrane  11 ] 
     A porous membrane  11  was obtained in the same procedure as the manufacture of the porous membrane  1  except that immersion in a diethylene glycol bath (4° C.) was performed at a rate of 4 m/sec for 60 seconds. A thickness of the membrane after drying was 200 μm. 
     [Porous Membrane  12 ] 
     The porous membrane  1  was put into a diethylene glycol bath (room temperature), ultrasonic washing was carried out overnight, and then washing thoroughly with pure water was carried out. Therefore, a porous membrane  12  was obtained. A thickness of the membrane after drying was 200 μm. 
     [Porous Membrane  8 ] 
     PTFE filter (0.8 μm, T080A047A) manufactured by Advantech Co., Ltd. was pasted on a 10 cm square aluminum plate. 3 mL of an aqueous solution of 0.5% polyvinylpyrrolidone (PVP 40 SIGMA-ALDRICH) was added dropwise onto this filter, and spin-coated at 1000 rpm for 60 seconds. Thereafter, the mixture was dried, and therefore a porous membrane  8  was obtained. A thickness of the membrane after drying was 36 μm. 
     [Porous Membrane  13 ] 
     PTFE filter (0.8 μm, T080A047A) manufactured by Advantech Co., Ltd. was used as it was. 
     &lt;Structure Analysis of Porous Membrane&gt; 
     (Analysis Method of Elemental Distribution in Thickness Direction) 
     XPS measurement in the thickness direction of the porous membrane was performed from both surfaces of the obtained porous membrane under the following conditions. 
     Apparatus: Versa Probe II manufactures by Ulvac-PHI 
     X-ray source: Monochromated Al-Kα ray (25 W, beam diameter of 100 μm) 
     X-ray irradiation range: 300×300 μm 2    
     Photoelectron take-off angle: 45° 
     Sputtering condition: Ar-GCIB (10 kV) is used, sputtering area of 2 mm square (polystyrene (PS) sputtering rate of 60 nm/min). 
       FIG. 1  and  FIG. 2  show results (1) of a surface side exposed to air at the time of manufacture, results (2) of a back surface thereof (a PET side at the time of manufacture), and N/C compositional ratios (3) of front and back surfaces calculated from these results, all of which were obtained for the porous membrane  3  and the porous membrane  7 , respectively. 
     In any examples, it was found that an N element was halved at a sputtering time of 0.1 seconds. In these results, a sputtering time of 0.5 seconds corresponds to a depth of 30 nm from a sputtering rate. 
     Using the results of XPS measurement, a ratio (A value) of an N element to a C element on a side exposed to air at the time of manufacturing the porous membrane, or on a surface on a side on which PVP was spin-coated was obtained from a value at the start of sputtering. In addition, a ratio (B value) of the N element to the C element at the depth of 30 nm (sputtering time of 0.5 seconds) from the same surface of the porous membrane was obtained. Therefore, a B/A value was calculated. The A value, the B value, and the B/A value are shown in Table 1. 
     (Method of Analyzing PVP on Surface) 
     TOF-SIMS analysis was performed on the surface of the obtained porous membrane using TOF-SIMS V manufactured by ION-TOF. Analysis was carried out with Bunching mode (high mass resolution) using Bi 3+  ion gun. A measurement area was 200 μm square, a surface resolution was 128×128 pixels, and the number of integration times was 32. 
       FIG. 3  shows graphs of results of measuring C 4 H 6 NO −  (mass of 84) and SO 2   −  (mass of 64) in the porous membrane  3  and the porous membrane  7 . 
     (Quantitative Determination of PVP Amount in Bulk) 
     About 15 mg of the obtained porous membrane was weighed and dissolved in 0.5 mL of a DMF-d7 solution (DMF being N,N-dimethylformamide, internal standard: about 2000 ppm dichloromethane). Therefore, a measurement sample was obtained. 50 μg, 300 μg, and 3000 μg of PVP were weighed and dissolved in 0.5 mL of a DMF-d7 solution (internal standard: about 2000 ppm dichloromethane). Therefore, a calibration curve sample was obtained. 
     AVANCE 600 manufactured by Bruker Biospin Co., Ltd. was used as measurement equipment. Pulse program was zg 30. A measurement wait time was 2 seconds. The number of integration times was 0.3 k. 
     (Distribution of Pore Diameter) 
     Each porous membrane obtained was impregnated with methanol and frozen in liquid nitrogen. Sections for cross section observation were cut out from the frozen porous membrane with a microtome (EM UC6 manufactured by Leica), and subjected to SEM imaging (SU8030 type FE-SEM manufactured by Hitachi High-Technologies Corporation). SEM imaging was performed at 3000 times. A photograph of a cross section of the porous membrane  7  is shown in  FIG. 4 . In  FIG. 4 , an upper side is a side exposed to air at the time of manufacture, and a lower side is a PET film side at the time of manufacture. An SEM image photograph of a cross section of each porous membrane was divided into 20 in the thickness direction from the upper side, pores of each obtained section were traced with a digitizer, and therefore an average pore diameter of 50 pores in each section was obtained. However, for a section in which pores were large and therefore only less than 50 pores could be measured, only available number of pores that could be counted in that section was measured. The obtained average pore diameter of each section was plotted in order from one surface to the other surface, and therefore distribution of the average pore diameter in the thickness direction of the membrane was obtained. Measurement results of the porous membrane  7  are shown in  FIG. 5 . A section having the smallest average pore diameter was used as a compact portion, and the average pore diameter of this portion was measured by ASTM F316-80 method. Measured values are shown in Table 1. 
     &lt;Evaluation of Porous Membrane&gt; 
     (Insulin Permeability) 
     A hole having a diameter of 1.0 cm was made in the center of one wall surface of a container made from vinyl chloride which has a size of 2.0 cm in length, 1.0 cm in width, and 2.0 cm in height, and the periphery of the hole was covered with a silicone sheet (50°, thickness of 1 mm) made from Tigers polymer. A porous membrane cut into 1.5 cm×2.0 cm was placed so as to cover the silicone sheet. Other same container and a silicone sheet were prepared and fixed with a clip such that the holes were aligned. 
     4.0 mL of a medium (medium for pancreatic islet culture, Cosmobio, PNIM3) containing 0.1 unit of insulin (Wako Pure Chemical Industries, Ltd., Insulin Humane Recombinant, 097-06474) was put into one container (supply side). 4.0 mL of the same medium but not containing insulin was put into the other container (permeation side). A surface X of the porous membrane was set to either the supply side or the permeation side as shown in Table 1. After 240 minutes, the media on the supply side and permeation side were collected. An amount of insulin was quantitatively determined with Insulin ELISA (80-INSRT-E01 manufactured by ALPCO), and evaluated according to the following criteria. The results are shown in Table 1. 
     An insulin concentration on the permeation side was 95% or more compared to the supply side after 240 minutes: AA 
     An insulin concentration on the permeation side was 70% or more compared to the supply side after 240 minutes: A 
     An insulin concentration on the permeation side was 45% or more compared to the supply side after 240 minutes: B 
     An insulin concentration on the permeation side was less than 45% compared to the supply side after 240 minutes: C 
     (Cell Irritating Properties) 
     Whether or not the porous membrane produced above caused an inflammatory reaction in a living body was checked by the following procedure, and therefore cell irritating properties (bioaffinity) of the porous membrane were evaluated as follows. 
     A 2 cm square porous membrane was implanted subcutaneously at the back of an SD rat (Sprague-Dawley rat) and sutured. After breeding for 1 week, the same portion was excised. Sections histologically stained with hematoxylin/eosin (HE) were produced. An image of a cross section of the site where the porous membrane was implanted was captured.  FIG. 6  shows each image of the histologically stained sections including the porous membrane  7  and the porous membrane  3 . As can be seen from  FIG. 6 , in the porous membrane  7  and the porous membrane  3 , inflammatory cells were not observed and a tissue reaction was relaxed. 
     A: No inflammatory cells were observed. 
     C: Multinucleated giant cells were observed on a surface of the membrane. 
     The results are shown in Table 1. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
             
            
               
                   
                 Example 1 
                 Example 2 
                 Example 3 
                 Example 4 
                 Example 5 
                 Example 6 
                 Example 7 
               
               
                   
               
               
                 Porous membrane 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
               
               
                 Polymer 
                 PSf 
                 PSf 
                 PSf 
                 PSf 
                 PSf 
                 PSf 
                 PSf 
               
               
                 A value 
                 0.082 
                 0.082 
                 0.082 
                 0.082 
                 0.082 
                 0.054 
                 0.054 
               
               
                 B value 
                 0.056 
                 0.03 
                 0.009 
                 0.005 
                 0.003 
                 0.02 
                 0.004 
               
               
                 B/A 
                 0.683 
                 0.366 
                 0.110 
                 0.061 
                 0.037 
                 0.370 
                 0.074 
               
               
                 Amount of PVP 
                 7.45 
                 3.85 
                 1.28 
                 0.15 
                 0.09 
                 2.53 
                 0.26 
               
               
                 contained in bulk 
               
               
                 (% by mass) 
               
               
                 Average pore 
                 0.8 
                 0.8 
                 0.8 
                 0.8 
                 0.8 
                 0.45 
                 0.45 
               
               
                 diameter of 
               
               
                 compact portion 
               
               
                 (μm) 
               
               
                 Direction of 
                 Supply side 
                 Supply side 
                 Supply side 
                 Supply side 
                 Supply side 
                 Supply side 
                 Supply side 
               
               
                 surface x 
               
               
                 Insulin 
                 AA 
                 AA 
                 AA 
                 AA 
                 B 
                 A 
                 A 
               
               
                 permeability 
               
               
                 Cell irritating 
                 A 
                 A 
                 A 
                 A 
                 A 
                 A 
                 A 
               
               
                 properties 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 Comparative 
                 Comparative 
                   
                 Comparative 
               
               
                   
                 Example 8 
                 Example 9 
                 Example 1 
                 Example 2 
                 Example 10 
                 Example 3 
               
               
                   
               
               
                 Porous membrane 
                 2 
                 3 
                 11 
                 12 
                 8 
                 13 
               
               
                 Polymer 
                 PSf 
                 PSf 
                 PSf 
                 PSf 
                 PTFE 
                 PTFE 
               
               
                 A value 
                 0.082 
                 0.082 
                 0.082 
                 0.012 
                 0.078 
                 0 
               
               
                 B value 
                 0.03 
                 0.009 
                 0.061 
                 0.005 
                 0.044 
                 0 
               
               
                 B/A 
                 0.366 
                 0.110 
                 0.744 
                 0.417 
                 0.564 
                 — 
               
               
                 Amount of PVP 
                 1.95 
                 0.82 
                 8.30 
                 0.03 
                 4.02 
                 0.00 
               
               
                 contained in bulk 
               
               
                 (% by mass) 
               
               
                 Average pore diameter 
                 0.8 
                 0.8 
                 0.8 
                 0.8 
                 0.8 
                 0.8 
               
               
                 of compact portion 
               
               
                 (μm) 
               
               
                 Direction of surface x 
                 Permeation 
                 Permeation 
                 Supply side 
                 Supply side 
                 Supply side 
                 Supply side 
               
               
                   
                 side 
                 side 
               
               
                 Insulin permeability 
                 A 
                 A 
                 C 
                 C 
                 AA 
                 C 
               
               
                 Cell irritating 
                 A 
                 A 
                 A 
                 C 
                 A 
                 C 
               
               
                 properties 
               
               
                   
               
            
           
         
       
     
     EXPLANATION OF REFERENCES 
     
         
         
           
               1 : porous membrane