a hollow fiber membrane comprises a tubular macroporous support coated on its outer surface with a thin tubular asymmetric semipermeable film of polymer . the film , by itself , is non - self - supporting . the support itself is so flexible that it does not have a circular cross - section and collapses with finger pressure . such a tube of knitted or woven braid no greater than 2 . 5 mm o . d ., having interstitial voids which are non - uniformly shaped by the &# 34 ; ends &# 34 ; which are braided , is coated with a &# 34 ; dope &# 34 ; of polymer , then coagulated in a bath to form the film . yet , after coating , the braided membrane can withstand up to 1500 psig in hydraulic compression forces before a straight - line relationship between flux and pressure begins to flatten out . the voids are relatively much larger than pores in the film . these voids are small enough to inhibit substantial penetration of the dope which lies superficially supported on the upper portion of the braid . viewed in an elevational cross - sectional view looking down the longitudinal axis of the dope - coated braid , the dope extends over less than 33 % of the outer portion of the braid &# 39 ; s cross - sectional area . this thin coating of dope is made possible by a unique coating nozzle having sequential rounding and sizing orifices through which the braid is advanced by pulling it without unduly distorting the shapes of the voids in the braid . the shapes and sizes of the braid are thus retained except that the braid is distended into an essentially circular shape because of the tensile forces exerted by the coagulating dope . hollow fiber membranes made as disclosed herein are used to make fluid - fluid separations , the braid and film being tailored to the purpose at hand .