Abstract: A drive mechanism for delivery of infusion medium in an implantable medical device includes an inlet for receiving the infusion medium and a piston channel for communication of infusion medium received by the inlet. The piston channel has a distal end and a proximal end. The proximal end is closer to the inlet than the distal end. The drive mechanism further includes a coil surrounding the piston channel and a piston located within the piston channel and moveable axially within the piston channel to drive infusion medium out of the distal end of the piston channel. The mechanism also includes an armature operably coupled to the piston and disposed adjacent the coil. The armature has first and second opposing major surfaces and a plurality of vents extending through the armature from the first major surface to the second major surface. The plurality of vents cumulatively occupy between about 20% and about 40% of the total surface area of the first major surface.