According to the invention, a safety device suitable (S) for a pre-filled syringe containing a medicament comprises a chassis adapted to be affixed to the pre-filled syringe, a deformable flexible outer structure made from a material with a shape memory and a substantially cylindrical needle shield affixed or integral to the outer structure. The deformed flexible outer structure is capable of advancing the needle shield with respect to the chassis in a distal direction (D) into a position in which the needle shield projects from the chassis in the distal direction (D). A biasing force for advancing the needle shield originates from a shape memory effect that causes the deformed flexible outer structure to substantially return to its unstressed shape.