Injection device

An injection device for receiving a two-compartment syringe having a front end quipped with a septum that can be pierced by a double-ended needle, a cylindrical side wall, a slidable front wall and a slidable rear wall that are liquid-tightly and slidably fitted in the side wall, and a bypass which is defined in the internal surface of the side wall between the front end and the slidable front wall and has a longitudinal length sufficient to stride the thickness of the slidable front wall, the two-compartment syringe containing a first pharmaceutical component in the front space defined between the front end and the slidable front wall and a second, liquid pharmaceutical component in the rear space defined between the slidable front wall and the slidable rear wall;

• mixing, in the two-compartment syringe, the first pharmaceutical component and the second pharmaceutical component;
• and then conducting injection of thus prepared injectable liquid, the injection device comprising
• a front casing designed to enclose and hold the two-compartment syringe with a double-ended needle attached to the front end of the two-compartment syringe,
• a rear casing designed to receive the two-compartment syringe held by the front casing being inserted in a rear-end first manner, and to lock and hold the front casing, with the two-compartment syringe inserted in the rear casing,
• a piston rod placed slidably in the longitudinal direction in the rear casing and designed to push from behind and carry forward the slidable rear wall, wherein the piston rod can be pushed forwardly from behind the rear casing, and
• a spring provided in association with the piston rod in the region between the front end of the piston rod and rear part of the rear casing so that, when the front end of the piston rod is moved backward, the spring may be compressed and then push the piston rod to a forward position again,
• wherein the spring has sufficient length and strength such that, when the two-compartment syringe held by the front casing is inserted in the rear casing and the front casing is locked by the rear casing, with the septum pierced by a double-ended needle to put the front space into communication with the outside, the spring is once compressed, by the piston rod being moved backward due to the pressure applied by the slidable rear wall of the two-compartment syringe, and then, by its repulsive force, forwardly drives the slidable rear wall together with the second pharmaceutical component and the slidable front wall, and, after the rear end of the bypass is exposed behind the slidable front wall, push in the slidable rear wall until it abuts on the slidable front wall, thereby transferring the second pharmaceutical component into the front space.