Thin permeable film of inert polymeric material cast in such a way from a mixture of solvents so that the size, number and shape (tortuosity) of the pores is controlled. Widely used in bacterial filtration – solution sterilising – in the medical field. In multiple units they are used industrially in beverage filtration and preparation of very pure water etc. Can be in cylindrical cartridge form. Fine grades used in ultrafiltration, reverse osmosis and dialysis.