Patent ID: 8561841

Claim:
A pump unit for dispensing a predetermined volume of fluid comprising: a stepper motor in threaded engagement with a threaded drive rod, the stepper motor rotatable in one direction by a number of steps to advance the threaded drive rod; a cartridge mount coupled to the stepper motor, the cartridge mount having a locking means with an aperture sized to pass the threaded drive rod; a disposable cartridge having a first end adapted to engage the locking means for securing the disposable cartridge to the cartridge mount, the disposable cartridge having a chamber sealed with a plunger slidable in the chamber, the disposable cartridge having an outlet nozzle at a second end for dispensing contents of the chamber when the plunger is pushed by the threaded drive rod, and the plunger including an expandable plunger top of resilient material deformable to an expanded state when the plunger is pushed by a piston connected to the threaded drive rod, the expandable plunger top returning to a relaxed state when the piston is withdrawn from the plunger by a predetermined distance.