Patent ID: 7198615

Claim:
A drive mechanism for use in an injection device having a main housing, whereby in said drive mechanism a second piston is successively moved in relation to a first end of a medicament cartridge containing a medicament selectively to drive a bung closing a first end of the medicament cartridge into the medicament cartridge to expel medicament through a delivery member located at a second end of the medicament cartridge, wherein said drive mechanism comprises a means defining a passageway, a first piston which is fixed to said housing and the second piston, in which the passageway has a first end and a second end, the first piston closing the first end of the passageway and a first end of the second piston closing the second end of the passageway to define a chamber in the passageway, a non-compressible medium comprising a fluid being contained within the chamber such that a first relative movement in a first direction at the first end of the passageway results in a second relative movement in a direction opposite the first direction at the second end of the passageway.