Introduction: Functional electrical stimulation (FES) is a technique that has been successfully employed in rehabilitation treatment to mitigate problems after spinal cord injury (SCI). One of the most relevant modules in a typical FES system is the power or output amplifier stage, which is responsible for the application of voltage or current pulses of proper intensity to the biological tissue, applied noninvasively via electrodes, placed on the skin surface or inside the muscular tissue, closer to the nervous fibers. The goals of this paper are to describe and discuss about the main power output designs usually employed in transcutaneous functional electrical stimulators as well as safety precautions taken to protect patients. Methods: A systematic review investigated the circuits of papers published in IEEE Xplore and ScienceDirect databases from 2000 to 2016. The query terms were "((FES or Functional electric stimulator) and (circuit or design))" with 274 papers retrieved from IEEE Xplore and 29 from ScienceDirect. After the application of exclusion criteria the amount of papers decreased to 9 and 2 from IEEE Xplore and ScienceDirect, respectively. One paper was inserted in the results as a technological contribution to the field. Therefore, 12 papers presented power stage circuits suitable to stimulate great muscles. Discussion: The retrieved results presented relevant circuits with different electronic strategies and circuit components. Some of them considered patient safety strategies or aimed to preserve muscle homeostasis such as biphasic current application, which prevents charge accumulation in stimulated tissues as well as circuits that dealt with electrical impedance variation to keep the electrode-tissue interface within an electrochemical safe regime. The investigation revealed a predominance of design strategies using operational amplifiers in power circuits, current outputs, and safety methods to reduce risks of electrical hazards and discomfort to the individual submitted to FES application.

Abstract-An Electrical Stimulator (ES) can promote physiological benefits during rehabilitation of patients with spinal cord injury. One of the main circuits in an ES system is its output stage. This paper outlines a transformer-based output circuit topology and shows the results obtained during the development of the ES biphasic output. The current in the circuit is controlled by a voltage-to-current converter whose input is driven by amplitude modulated pulses generated by a microcontroller. Darlington configuration transistor switching is responsible for regulating the current flow in the transformer's primary coil. The pulse width, frequency and amplitude can be set accordingly. Battery-supplied power enhances patient safety by avoiding use of high voltage sources. Tests with the transformer-based prototype indicated efficiency on pulse generation, transfer of data packets between stages, and amplification of signals. The circuit provided output currents of 200 mA and a 200 V on a 1 kΩ load. The pulse frequency varied from 500 to 5000 Hz, and pulse width from 30μs to 400μs. The burst frequency ranged from 10 to 100 Hz. The designed system represents a useful and versatile tool for functional and therapeutic applications since it has resources for patient safety and can be altered and expanded.

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