Osmosis is the diffusion of a substance through a selectively permeable membrane. The process of reverse osmosis uses thin film composite membranes (TFC or TFM). These semipermeable membranes are manufactured primarily for use in water purification and desalination systems. There are also uses in chemical applications, such as, batteries and fuel cells. Medical applications include using semipermeable membrane is dialysis tubing. Thin film composite material is a molecular sieve constructed using film from two or more layered materials.
Membranes used in reverse osmosis are usually made from polyimide; however, there are other types like cellulose ester membrane (CEM), charge mosaic membrane (CMM), bipolar membrane (BPM), anion exchange membrane (AEM) alkali anion exchange membrane (AAEM) and proton exchange membrane (PEM).