An injector for delivery of a pharmaceutical composition. The injector includes a cylinder extending along a longitudinal axis, the cylinder having an inner wall, an outer wall, and an outlet at an outlet end opposite an actuating end, a piston assembly comprising a compressible section between an actuating piston element and a passive piston element, which piston elements abut the inner wall of the cylinder at abutting interfaces thereby sealing the compressible section when the piston assembly is inserted in the cylinder, the compressible section comprising a compressible fluid and a resilient frame linking the actuating piston element to the passive piston element. The injector is suited for pre-filled injectors allowing long term storage of protein based pharmaceutical compositions.
