6 is empty the length of linear spring extends to its original length whereby piston 15 is moved forward causing pump chamber 7 to be emptied in this way medications of pump chambers 6 7 are delivered in an sequential manner in its simplest form a collapsible reservoir to be used in a medication delivery device according to the present invention is made from sheet material which is folded and welded thus forming a closed bag if this type of collapsible reservoir is employed it is normally necessary to attach some sort of coupling unit to the reservoir although the sheet material for a simple reservoir can be chosen from a wide range of materials the preferred materials are thermoplastics or laminates containing at least one layer of thermoplastic material the sheet material should fulfill a number of different demands if employed for production of a reservoir most important is that the reservoir should have excellent barrier properties and be compatible with the medication to be stored in the reservoir additionally the material should be processable i e if welding is chosen as the preferred process of joining the sheet material the material should be weldable additionally the material should be able to withstand the mechanical loads to which it will be subjected during processing transport and.