A unidirectional valve, wherein the valve comprises a valve body including a frame, a valve opening through the frame, and a valve seat extending from the frame and at least partially surrounding the valve opening. The valve further includes a valve flap having a first portion attached to the frame and...http://www.google.es/patents/US6883518?utm_source=gb-gplus-sharePatente US6883518 - Unidirectional respirator valve
