Asymmetrical microporous hollow fiber for hemodialysis

An asymmetric microporous wettable hollow fiber, consisting essentially of an inner barrier layer and an outer foam-like supporting structure said fiber comprising a hydrophobic first organic polymer in an amount equal to 90 to 99% by weight and 10 to 1% by weight of polyvinyl pyrrolidone which is produced by the following steps:

• (a) wet spinning a polymer solution made up of a solvent, of 12 to 20% by weight of the first said polymer and of 2 to 10% by weight of the polyvinyl pyrrolidone, said solution having a viscosity of 500 to 3,000 cps, through a ring duct of a spinnerette having an external ring duct and an internal hollow core,
• (b) simultaneously passing through said hollow internal core a precipitant solution comprising an aprotic solvent in conjunction with at least 25% by weight of a nonsolvent which acts in an outward direction on the polymer solution after issuing from the spinneret
• (c) casting into an aqueous washing bath, said spinnerette and the upper surface of said washing bath being separated by an air gap, said air gap being so provided that full precipitation of components will have occurred before the precipitated polymer solution enters said washing bath thereby,
• (d) dissolving out and washing away a substantial portion of the polyvinyl pyrrolidone and of the said solvent, to form a fibre having a high clearance rate according to DIN 58352, of 200-290 ml/min for urea and 200-250 ml/min for creatinine and phosphate, at a blood flow rate of 300 ml/min., for fibres having 1.25 m.sup.2 of active surface.