. . "The membrane may be made from a wide range of polymeric materials, for example cellulose, nitrocellulose and other cellulose derivatives such as cellulose esters; spun or woven polyamide fibres; polyvinylidene polymers; polycarbonate polymers; polysulfone polymers; polyalkylene polymers; acrylic or methacrylic acid polymers or co-polymers; polyamide polymers; polytetrafluorethylene polymers and th" . .