A one-part reconstitution device is described, for temporarily separating two substances into two compartments within a cartridge or syringe and allowing for mixture of the two substances within the cartridge or syringe. In various embodiments, the one-part reconstitution device may include an outer, contact portion configured to contact an inner surface of the cartridge or syringe, and an inner, deflection portion positioned within the contact portion. The deflection portion may be movable from a locked position, in which the deflection portion causes the contact portion to exert a first radial force against the inner surface, to an unlocked position, in which the deflection portion causes the contact portion to exert a second radial force against the inner surface. The second radial force may be less than the first radial force, and the contact portion and the deflection portion may comprise one part.