a hollow fiber membrane is made by covering a tubular supporting structure with a membrane dope and converting the membrane dope into a solid porous membrane wall . optionally , a textile reinforcing structure in the form of a circular knit may be added around the supporting structure before it is covered in dope . the reinforcing structure thereby becomes embedded in the membrane wall . the supporting structure may be soluble in a non - solvent of the membrane wall , for example water , and may be removed from the membrane . alternatively , the supporting structure may be porous . a porous supporting structure may be made by a non - woven textile process , a sintering process within an extrusion machine , or by extruding a polymer mixed with a second component . the second component may be a soluble solid or liquid , a super - critical gas , or a second polymer that does not react with the first polymer .