Abstract: A method of manufacture of flat permeable membrane comprises the steps of mixing a polyolefin, an organic filler uniformly dispersible in the polyolefin in the molten state, and crystal seed forming agent for the polyolefin; discharging the resultant mixture in the molten state through a die; bringing one surface of the discharged molten membrane into contact with a cooling roll; placine the cooled and solidified flat membrane into contact with an extractant capable of dissolving and extracting the organic filler and capable of dissolving the polyolefin; and subjecting the cooled and solidified flat membrane in a fixed state in longitudinal and lateral directions to a heat treatment at a temperature 20.degree. to 50.degree. C. lower than the melting point of the polyolefin.

Abstract: In a membrane type artificial lung using as gas-exchange membranes porous hydrophobic membranes having a wall thickness in the range of 5 to 80 .mu.m, a porosity in the range of 20 to 80%, and a pore diameter in the range of 0.01 to 5 .mu.m, the minute pores of the membranes are blocked with minute particles of silica having a diameter smaller than the pore diameter and having a hydrophobic surface at least on the sides thereof intended for exposure to blood. Optionally, the gas-exchange membranes have the surfaces for exposure to blood coated with a biocompatible hydrophobic resin.