This is a continuation of application Ser. No. 551,127 filed Nov. 14, 1983, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a flat sheet, tubular, or hollow fiber membrane. The membrane is suitable for use, for example, in so-called simultaneous hemodialysis/hemofiltration. The invention also relates to a process for producing a membrane. 
     Examples of known hemodialysis membranes include the membranes described in, for example, U.S. Pat. Nos. 4,069,151; 4,075,108 and 4,308,145, while examples of known hemofiltration membranes include the membrane described in, for example, European Patent Publication No. 0 046 816. 
     Membranes which are suitable for so-called simultaneous hemodialysis/hemofiltration are also desirable. To be suitable for use in simultaneous hemodialysis/hemofiltration, a membrane should not only possess high hydraulic permeability, but also should simultaneously have high diffusive as well as high convective permeabilities to solutes to be separated. In other words, such a membrane should as much as possible have the membrane characteristics of a hemodialysis membrane and a hemofiltration membrane at one and the same time. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided a flat sheet, tubular, or hollow fiber membrane which is particularly, though not exclusively, adapted for use in connection with so-called simultaneous hemodialysis/hemofiltration. The membrane has a hydraulic permeability to water (ultrafiltration rate) of from about 10 to about 100 ml/m 2  /h/mmHg, preferably from about 30 to about 50 ml/m 2  /h/mmHg, a diffusive permeability to chloride (Cl - ) of more than about 10 cm/sec×10 -4 , preferably more than about 12 cm/sec×10 -4 , a diffusive permeability to vitamin B 12  of more than about 2 cm/sec×10 -4 , preferably more than about 3 cm/sec×10 -4 , and a diffusive permeability to inulin of more than about 0.5 cm/sec×10 -4 , preferably more than 1.0 cm/sec×10 -4 . These characteristics in themselves describe the advantages of the membrane of the invention. Because of these characteristics, the membrane according to the present invention is not only suitable for use in so-called simultaneous hemodialysis/hemofiltration, but also for use in normal hemodialysis as well as in normal hemofiltration. Generally speaking, the membrane of the invention may be used whenever it is intended to separate a solute from a solution, irrespective of whether this separation is carried out by diffusive or convective solute transport through the membrane. 
     The membrane of the invention can be produced by a process in which a polymer solution containing a high molecular weight swelling agent is cast, extruded or spun to form a flat sheet, tube or hollow fiber material. The flat sheet, tube, or hollow fiber material is then exposed to partial evaporation of solvent prior to gelling. The exposed flat sheet, tube or hollow fiber material is gelled and subsequently washed and dried. 
     DETAILED DESCRIPTION OF THE INVENTION 
     As mentioned above, the membrane of the invention has a collection of characteristics as follows: a hydraulic permeability to water (ultrafiltration rate) of from about 10 to about 100 ml/m 2  /h/mmHg, preferably from about 30 to about 50 ml/m 2  /h/mmHg, a diffusive permeability to chloride (Cl - ) of more than about 10 cm/sec×10 -4 , preferably more than about 12 cm/sec×10 -4 , a diffusive permeability to vitamin B 12  of more than about 2 cm/sec×10 -4 , preferably more than about 3 cm/sec×10 -4 , and a diffusive permeability to inulin of more than about 0.5 cm/sec×10 -4 , preferably more than 1.0 cm/sec×10 -4 . 
     The cut-off value of the membrane can be set at any predetermined value depending on the intended use of the membrane. When the membrane is intended for use in extracorporeal treatment of blood, it is, however, desired to set the cut-off such that toxic uremic substances (metabolites) will freely pass through the membrane, while materials such as albumin (M w  =68,000 Daltons) are retained. A preferred cut-off value is about 50,000 Daltons. 
     Suitable polymers for preparing the membrane of the invention include polycarbonate block copolymers such as polyether-polycarbonate block copolymers and organopolysiloxane-polycarbonate block copolymers. Further examples of suitable polymers for the present membrane include polyacrylonitriles as well as modified polyacrylonitriles such as sulfonated polyacrylonitrile. 
     The most preferred polymer is a polyether-polycarbonate block copolymer. Preferably, the polyether-polycarbonate block copolymer is bisphenol A/polyalkylene oxide polycondensate containing from about 5 to about 35% by weight of polyalkylene oxide, preferably about 20%, and having the general formula: ##STR1## wherein R represents --CH 2  CH 2  --, ##STR2## or combinations of --CH 2  --CH═CH-- and --CH═CH--, preferably --CH 2  CH 2  --, wherein m and p are integers selected such that the bisphenol A carbonate unit is about 95 to about 65% of the weight of the recurring unit and the alkylene oxide unit is about 5 to 35% of the weight of the recurring unit, and wherein n is an integer of from about 10 to about 220, preferably about 182. The molecular weight of the polymer is typically from about 50,000 to about 750,000, preferably from about 200,000 to about 500,000. 
     The thickness of the membrane of the invention may vary within wide limits and is usually within the range of from about 10 to about 60 microns. A preferred thickness is from about 25 to about 45 microns. 
     In the process of the invention, a polymer solution containing a high molecular weight swelling agent is cast, extruded or spun to form a flat sheet, tube or hollow fiber membrane material. The casting, extruding and spinning of polymer solutions is conventional and therefore will not be described here. The flat sheet, tube or hollow fiber material is then exposed to partial evaporation of solvent prior to gelling. Typically, from about 5 to about 30% by weight of solvent is evaporated. The exposed flat sheet, tube or hollow fiber is then gelled and subsequently washed and dried in conventional manner. The use of a high molecular weight swelling agent in the polymer solution forms a highly water and solute permeable porous structure, while the partial evaporation of solvent prior to gelling the membrane forms a denser layer which restricts the permeability of the membrane to solutes. These steps thus are very important in providing the advantageous characteristics of the membrane of the invention as discussed above. 
     By the term &#34;high molecular weight&#34; swelling agent in accordance with the present invention, we mean swelling agents having a molecular weight within a range of from about 1,000 to about 20,000 Daltons. A preferred molecular weight range is from about 3,000 to about 15,000 Daltons. 
     Suitable high molecular weight swelling agents for use in accordance with the present invention include any of the known swelling agents which have a high molecular weight, as defined above, which is soluble in the polymer solution, and which is removable in the gelation bath. Examples of high molecular weight swelling agents suitable for use in the process of this invention are polyethylene glycols, preferably polyethylene glycol having a molecular weight of 8,000 Daltons, and polypropylene oxide-polyethylene oxide block copolymers, such as Pluronic F 68 (BASF Wyandotte). Further examples of suitable swelling agents include dextran, inulin and polyvinylpyrrolidone. 
     The high molecular weight swelling agents are in general used in the process of the invention in amounts ranging from about 1 to about 8% by weight in the polymer solution (casting or spinning solution). Preferably, they are used in amounts of from about 2 to about 5% by weight. 
     Suitable polymers for use in the process of the invention include those mentioned above in connection with the membrane, namely, polycarbonbate block copolymers such as polyether-polycarbonate block copolymers and organopolysiloxane-polycarbonate block copolymers. Further examples of suitable polymers include polyacrylonitriles and modified polyacrylonitriles such as sulfonated polyacrylonitrile. Again, the most preferred polymer is a polyether-polycarbonate block copolymer, preferably, a bisphenol A/polyalkylene oxide polycondensate containing from about 5 to about 35% by weight of polyalkylene oxide, preferably about 20%, and having the general formula ##STR3## wherein R represents --CH 2  CH 2  --, ##STR4## or combinations of --CH 2  --CH═CH-- and --CH═CH--, preferably --CH 2  CH 2  --, wherein m and p are integers selected such that the bisphenol A carbonate unit is about 95 to about 65% of the weight of the recurring unit and the alkylene oxide unit is about 5 to about 35% of the weight of the recurring unit, and wherein n represents an integer of from about 10 to about 220, preferably about 182. 
     The polymer solution is generally formed to have a viscosity within the range of from about 4,000 to about 50,000 cPs. A preferred viscosity range is from about 18,000 to about 25,000 cPs. 
    
    
     The following examples are intended to illustrate, but not to limit, the present invention. 
     EXAMPLE 1 
     A filtered polymer solution is prepared from 85.7% by weight of dioxolane, 12.25% by weight of polyether-polycarbonate block copolymer [prepared by reacting phosgene with a comonomer mixture of bisphenol A (80 weight percent) and polyethylene glycol (Carbowax) 8000 (20 weight percent)], and 2.05% by weight of polyethylene glycol (M w  =3,400). The polymer solution had a viscosity of 18,000 cPs and was extruded via a doctor blade onto a moving substrate. After evaporation of about 20% of the dioxolane, the extruded membrane was precipitated with a water bath, removed from the substrate, washed free of dioxolane and polyethylene glycol, stabilized in an ethanol/glycerol/water mixture, and dried with air. 
     The resultant membrane had the properties listed in Table 1 below: 
     
                       TABLE 1______________________________________Ultrafiltration rate: 21 ml/m.sup.2 /h/mmHgDiffusive permeability to chloride:                 13.0 × 10.sup.-4 cm/secDiffusive permeability to B.sub.12 :                 5.0 × 10.sup.-4 cm/secDiffusive permeability to inulin:                 0.8 × 10.sup.-4 cm/secThickness:            28 microns______________________________________ 
    
     EXAMPLE 2 
     A filtered polymer solution is prepared from 84.3% by weight of dioxolane, 12.1% by weight of polyether-polycarbonate block copolymer [prepared by reacting phosgene with a comonomer mixture of bisphenol A (80 weight percent) and polyethylene glycol (Carbowax) 8000 (20 weight percent)], and 3.6% by weight of polyethylene glycol (M w  =8,000). The polymer solution had a viscosity of 20,000 cPs and was extruded via a doctor blade onto a moving substrate. After evaporation of about 25% of the dioxolane, the membrane was precipitated in a water bath, removed from the substrate, washed free from dioxolane and polyethylene glycol, stabilized in a glycerol/water mixture, and dried in warm air. 
     The resultant membrane had the properties listed in Table 2 below: 
     
                       TABLE 2______________________________________Ultrafiltration rate: 60 ml/m.sup.2 /h/mmHgDiffusive permeability to chloride:                 13.8 × 10.sup.-4 cm/secDiffusive permeability to B.sub.12 :                 5.2 × 10.sup.-4 cm/secDiffusive permeability to inulin:                 1.7 × 10.sup.-4 cm/secThickness:            45 microns______________________________________ 
    
     EXAMPLE 3 
     A filtered polymer solution is prepared from 86.6% by weight of dioxolane, 12.2% by weight of polyether-polycarbonate block copolymer [prepared by reacting phosgene with a comonomer mixture of bisphenol A (80 percent weight) and polyethylene glycol (Carbowax) 8000 (20 weight percent)], and 1.2% by weight of Pluronic F 68 (BASF Wyandotte). The polymer solution had a viscosity of 22,000 cPs and was extruded via a doctor blade onto a moving substrate. After evaporation of about 20% of the dioxolane, the membrane was precipitated in a water bath, removed from the substrate, washed free of dioxolane and Pluronic F 68, stabilized in an ethanol/glycerol/water mixture, and dried with air. 
     The resultant membrane had the properties listed in Table 3 below: 
     
                       TABLE 3______________________________________Ultrafiltration rate: 13 ml/m.sup.2 /h/mmHgDiffusive permeability to chloride:                 12.1 × 10.sup.-4 cm/secDiffusive permeability to B.sub.12 :                 4.6 × 10.sup.-4 cm/secDiffusive permeability to inulin:                 1.7 × 10.sup.-4 cm/secThickness:            28 microns______________________________________ 
    
     EXAMPLE 4 
     In Table 4 below the effect of molecular weight of polyethylene glycol (PEG), used as swelling agent, on ultrafiltration rate of a membrane obtained in the process according to the present invention is illustrated: 
     
                       TABLE 4______________________________________Swelling Agent/          Ultrafiltration RateMolecular Weight          (ml/m.sup.2 /h/mmHg)______________________________________PEG 1540        9PEG 3400       21PEG 8000       50PEG 18500      12PEG 20000       9______________________________________ 
    
     Table 4 shows the effect of equal amounts of polyethylene glycols of various molecular weights added to the polymer solution upon the ultrafiltration rate of the membrane obtained. A maximum can be observed for polyethylene glycol having a molecular weight of 8,000 Daltons. 
     It will be understood that the embodiments described herein are merely exemplary and that a person skilled in the art may make many variations and modifications without departing from the spirit and scope of the invention. All such modifications and variations are intended to be included within the scope of the invention as defined in the appended claims.
A membrane for hemodialysis is prepared by casting, extruding or spinning a solution of a polyether-polycarbonate block copolymer and a swelling agent having a molecular weight of about 1,000 to about 20,000 Daltons, then partially evaporating the solvent, gelling and washing. The membrane has a top cut-off value less than about 68,000 Daltons, and hence is substantially impermeable to albumin, but nonetheless provides high permeability to water and to other small molecules.