Patent ID: 8083730

Claim:
An implantable medical device for delivering a liquid therapeutic substance to a delivery site within a patient, the device comprising: a housing including a stationary wall, having an inner face and an outer face, and maintaining a reservoir defining an internal region for containing the therapeutic substance, the reservoir including: an inlet, an outlet, a base wall that is movable relative to the stationary wall, wherein a volume of the internal region is a function of a distance between the base wall and the stationary wall; and a volume sensor assembly for generating information indicative of a current volume of the internal region, the assembly including: a cap defining an internal passage extending from an open end to a closed end, the cap being mounted to the stationary wall such that the open end of the passage is open relative to the inner face and the internal region of the reservoir, a shaft comprised of a non-ferromagnetic material having a first end and a second end, wherein the first end is attached to the base wall opposite the cap, a target comprised of a ferromagnetic material assembled to the second end of the shaft and movably arranged within the passage, circuitry associated with the cap and adapted to detect a longitudinal position of the target within the passage and generate information indicative of the longitudinal position of the target relative to a dimension of the cap; wherein the longitudinal position of the target relative to the dimension of the cap is representative of the volume of the internal region.