Injection device

1. An injection device comprising:

a housing adapted to receive a syringe having a discharge nozzle and a dispensing piston movable in the syringe to expel the contents of the syringe through the discharge nozzle;

a drive adapted on activation to act on the syringe to advance it from a retracted position in which the discharge nozzle is contained within the housing to an extended position in which the discharge nozzle extends from the housing;

a drive coupling for extending from the drive to the dispensing piston of the syringe so as to transfer movement of the drive to the piston;

wherein the drive coupling comprises a fixed-length drive coupling and an interchangeable drive coupling and;

an interchangeable release element adapted to actuate after a predetermined amount of movement of the dispensing piston a delay mechanism in the drive acting on the fixed-length drive coupling.

2. The injection device of claim 1 , wherein the interchangeable drive coupling comprises a rigid member configured to connect with the dispensing piston and with the fixed-length drive coupling.

3. The injection device of claim 2 , wherein the rigid member has a longitudinal axis.

4. The injection of claim 3 , wherein the quantity of the contents of the syringe which is expelled in use determines the length of the interchangeable drive coupling along its longitudinal axis.

5. The injection device of claim 4 , wherein the length of the interchangeable drive coupling is inversely proportional to the quantity of the contents of the syringe.

6. The injection device of claim 1 , wherein the interchangeable release element is a constriction adapted to act on at least one arm linking the fixed-length drive coupling to the drive, thereby releasing the drive from the fixed-length drive coupling.

7. The injection device of claim 6 , wherein the length of the constriction determines the predetermined amount of movement of the piston.

8. The injection device of claim 7 , wherein the length of the constriction is inversely proportional to the quantity of the contents of the syringe.

9. A method of manufacturing an injection device, comprising:

inserting a syringe having a piston into a first sub-assembly;

inserting an interchangeable drive coupling into the syringe to contact the piston;

providing a second sub-assembly comprising a drive and a fixed-length drive coupling;

connecting an interchangeable release element to the first sub-assembly, wherein the interchangeable release element is adapted to actuate after a predetermined amount of movement of the dispensing piston a delay mechanism in the drive acting on the fixed-length drive coupling;

assembling the first sub-assembly and second sub-assembly together,

wherein, on assembly, the fixed-length drive coupling and interchangeable drive coupling communicate in use to transfer movement of the drive to the piston to act on the syringe to advance it from a retracted position in which the discharge nozzle is contained within the housing to an extended position in which the discharge nozzle extends from the housing.

10. The method of claim 9 , wherein the interchangeable component has a longitudinal axis and comprises a rigid member configured to connect with the piston and with the drive coupling.

11. The method of claim 9 or claim 10 , further comprising selecting the interchangeable drive coupling in accordance with its length determined by the quantity of the contents of the syringe.

12. The method of claim 9 , further comprising selecting the interchangeable release element in accordance with its length determined by the quantity of the contents of the syringe.