The present invention is directed to a multi-compartment medical device for segregated storage and on demand mixing of at least two components and expression of the resulting mixture from the device. The mixing device comprises a syringe having an open end and outlet and containing a first component; and a cap with a fixed outer body and an expandable chamber contained therein, wherein said cap is removably attached to the outlet of the syringe and a cross-section of the cap is larger than a syringe cross-section and the cap is shorter than the syringe.