Membrane filters are constructed out of a wide range of synthetic materials, including cellulose acetate, regenerated cellulose, mixed esters (cellulose nitrate), polyamide (nylon), polycarbonate, polypropylene, polyvinylidene, polyethersulfone and polytetrafluoroethylene (Teflon). Because of their small pore-diameter, membrane filters tend to have a slower flow rate than depth filters and they are called surface filters.