US Patent 7815598 Auto-pen for a two-chamber ampoule

An injection device for use with a two-chamber ampoule, wherein the device includes a front casing part with which the two-chamber ampoule can be associated, a rear casing part, a threaded drive accommodated by the rear casing part, a piston rod which, during a blending movement, is moved by the threaded drive toward a dispensing end of the device and slaves at least a closing piston of the two-chamber ampoule in the process, wherein a spring element presses the piston rod toward the dispensing end during a delivery movement.