an asymmetric membrane comprising a tubular polymer film in combination with a tubular braid on which the film is supported , requires the braid be macroporous and flexible , yet sufficiently strong to withstand continuous flexing , stretching and abrasion during use for microfiltration or ultrafiltration . the specifications for a braid of a long - lived membrane are provided . a membrane is formed by supporting a polymer film in which particles of calcined α - alumina are dispersed , on the defined tubular braid .