Dialysis membrane having improved ability for removal of middle molecules

1. Method for producing a hydrophilic, water-wettable, semipermeable hollow-fibre membrane, comprising the following steps:

a. preparing a homogeneous spinning solution comprising 12 to 30 wt. % of a synthetic first polymer and, if necessary, other additives in a solvent system,

b. extruding the spinning solution through the annular slit of a hollow-fibre die to give a hollow fibre,

c. extruding an interior filler through the central opening of the hollow-fibre die, the interior filler being a coagulation medium for the synthetic first polymer and comprising a solvent and a non-solvent for the synthetic first polymer,

d. bringing the interior filler into contact with the inner surface of the hollow fibre to initiate coagulation in the interior of the hollow fibre and for formation of a separating layer on the inner surface of the hollow fibre and formation of the membrane structure,

e. passing the hollow fibre through a coagulation bath to complete the formation of the membrane structure if necessary and to fix the membrane structure,

f. extracting the hollow-fibre membrane thus formed, to remove the solvent system and soluble substances, and

g. drying the hollow-fibre membrane, characterized in that the interior filler contains a polyelectrolyte with negative fixed charges, wherein the steps of the method are to be carried out in such a way that a hollow-fibre membrane possessing an open-pored integrally asymmetric structure across its wall, a porous separating layer of thickness between 0.1 and 2 μm on its inner surface facing a lumen, and an open-pored supporting layer is obtained with an ultrafiltration rate in albumin solution in the range of 5 to 23.5 ml/(h·m 2 ·mmHg) and a maximum sieving coefficient for albumin of 0.005 combined with a sieving coefficient for cytochrome c that satisfies the following relation:

SCcc≧ 5·10 −5· UFR Alb 3 −0.004· UFR Alb 2 +0.1081· UFR Alb 0.25.

2. Method according to claim 1 , characterized in that the spinning solution also comprises 0.1 to 30 wt. % of a hydrophilic second polymer.

3. Method according to claim 1 , characterized in that the synthetic first polymer is a hydrophobic first polymer and the spinning solution also comprises 0.1 to 30 wt. % of a hydrophilic second polymer.

4. Method according to claim 3 , characterized in that an aromatic sulfone polymer such as polysulfone, polyethersulfone, polyphenylenesulfone or polyarylethersulfone, a polycarbonate, polyimide, polyetherimide, polyetherketone, polyphenylene sulfide, or a copolymer or mixture of these polymers is used as the hydrophobic first polymer.

5. Method according to claim 2 , characterized in that polyvinyl-pyrrolidone, polyethylene glycol, polyvinyl alcohol, polyglycol monoester, polysorbate, carboxymethylcellulose, or a copolymer of these polymers is used as the hydrophilic second polymer.

6. Method according to claim 1 , characterized in that the solvent system comprises a polar aprotic solvent.

7. Method according to claim 1 , characterized in that the polyelectrolyte is selected from the group of polyphosphoric acids, polysulfonic acids, or polycarboxylic acids.

8. Method according to claim 7 , characterized in that the polycarboxylic acids are homo- or copolymers of acrylic acid.

9. Method according to claim 1 , characterized in that the proportion by weight of the polyelectrolyte is 0.01 to 1 wt. % relative to the weight of the interior filler.