The present invention concerns an attachable membrane device for providing a piercable liquid barrier membrane to the surface of a pierceable fluid container, such as an infusion bag. The attachable membrane device have a fastening part comprising a fastening surface adapted to be attached to the surface of the pierceable fluid container and thereby provide a liquid tight seal between the fastening surface and the surface of the pierceable fluid container. Additionally, the attachable membrane device is further provided with a barrier membrane part, adapted to provide a liquid tight seal after penetration by a piercing member. Connection means is arranged to the attachable membrane device. The invention simplifies the injection or withdrawal of a fluid in a pierceable fluid container.