Such systems usually comprise two hermetically closed vials, one containing an active product generally in the form of a lyophilized or otherwise dried powder or granulate and the second containing the liquid in which the active product is dissolved or suspended. The system further comprises a connecting piece for temporary connection of the two vials.

The active product is usually contained in a glass vial which is closed by an elastic stopper inserted in the neck of the vial. In the commonly used systems the stopper has in its axis a channel which is covered by a thin, per- forable airtight membrane, such as aluminum, which is glued or welded onto the top surface of the stopper.

The second vial containing the liquid is more often made of plastic material and is closed by a dropper with a tip having a diameter smaller than the said channel through the stopper. When being connected to the first vial the dropper perforates the membrane in such a way that airtightness is provided between the two vials.

The presently known stoppers have the disadvantage that they consist of two pieces namely the stopper as such and the membrane affixed to it. To produce such a stopper several consecutive manufacturing steps are necessary resulting in a relatively expensive product.

The aim of the present invention is to provide a stopper whose manufacture is less complicated and which is, thus, less expensive.

According to the invention this is achieved by a stopper consisting of one piece of elastic material which has a concentrical portion whose axial thickness is reduced to a membrane which has radially arranged cuts constituting defined breakage lines during perforation.

In the following a preferred embodiment of the invention is described with regard to the accompanying drawing.

The figure shows a top view and a cross-section along line A-A of a stopper 1 made of elastic material such as rubber. The stopper is generally cylindrical and has an upper flange-shaped portion 2 and a lower skirt-shaped portion 3. In its interior it is provided with a coaxial cavity 4 extending from the lower surface 5 up almost to the top surface 6 leaving a thin membrane-like layer 7 of the stopper material to keep the closing capability of the stopper.

The membrane-like layer 7 is provided with four radially arranged cuts 8 which meet in the stopper axis and which extend partially through the layer 7 without entirely perforating it to keep the airtightness of the stopper.

When during connection of a vial, which is closed by the stopper 1 with another vial having a dropper the membrane is perforated the cuts constitute predefined breakage lines which guarantee tight attachment of the membrane-like layer flaps to the dropper to keep airtightness during connec- tion. After removal of the other vial the flaps close the perforation to essentially retain tightness.

The stopper is particularly useful in connection with the dual vial connector described in PCT/EP 97/04660 and PCT/EP 04290 in which however the membrane and the sealing rings would be eliminated. This combination would be a preferable alternative for the user who due to habit or in view of manufacturing restrictions intends to remain with a stopper solution.