Devices for preparing a liquid solution are disclosed. The device includes a receptacle defining a cavity for receiving at least one active substance to be dissolved, at least one inlet leading into the cavity for feeding at least one solvent into the cavity, and at least one outlet leading out of the cavity for discharging the liquid solution including the at least one active substance and the at least one solvent from the cavity. The cavity or the receptacle may include a flexible outer shell for enclosing the cavity and an inner, preferably columnar, supporting element for stretching the flexible outer shell between at least two points, the supporting element comprising the at least one inlet at its first end and the at least one outlet at its second end and being enclosed by the flexible outer shell to provide an axially rigid cartridge that can be used in currently customary dialysis machines comprising cartridge connection systems, which can be produced at low cost.
