<p>OF THE DISCLOSE: An unidirectional fluid flow membrane in the form of a sheet of porous plastics material comprising a multitude of interconnected pores that together define passages suitable to allow a gas such as air to flow. The sheet of plastics material has one face heat-treated to partially close the passages to an extent sufficient to prevent a liquid such as water from flowing across the sheet toward the other face, whereby this sheet acts as a check-valve with respect to the liquid only. Owing to its unidirectional structure, this membrane can advantageously be used as overpressure valve in a life- or sea- jacket or as purge valve or exhaust diaphragm in an underwater mask, snorkel or regulator.</p>
