Embodiments of the invention are directed to systems, methods and devices for sustained medical infusion with controlled rate injection of a fluid into a body. Such a system may include a first separate reusable unit, a second separate depletable unit a third separate disposable unit having a cannula, and may include a fourth separate remote control unit. Emission of appropriate instructions from the fourth unit, when the first unit, the second unit, and the third unit are coupled together in associative operation and disposed on the skin, power is supplied to an engine for generating motion to a fluid transfer system, and when the cannula is inserted in the body, fluid is transferred from the reservoir to the body, via the tube and the cannula.