Design and Development of Functional Electrical Stimulation Circuit

Abstract

Functional electrical stimulator (FES) is a device that creates controlled electrical pulses with a specific waveform to contract or relax muscles when it’s placed in the correct position. The device is useful for patients with strokes or any spinal injury and they have lost or weakened the nerve control to their muscles. This study focuses on the design and development of a portable, single channel, biphasic and inexpensive FES circuit using Arduino Pro Mini [1]. A 7.4 V two-cell Li-ion battery feeds a 20 kHz PWM-driven DC–DC boost converter (L = 100 µH, C = 100 µF) that supplies up to 40 V to a MOSFET H-bridge. A resistive divider closes the voltage loop and holds the feedback signal within ±0.1 V of its set-point, while a 10 Ω shunt and an LM358 amplifier (28.5 mV/mA) monitor the load current. If the output reaches 40 V, the device stops stimulating for 2 s. Symmetric biphasic pulses were delivered at 30 Hz, with pulse widths of 224–1280 µs and a 150 µs interphase gap. The device operates entirely through the skin with no need of invasive methods using surface electrodes. The working frequency used is 30Hz, the pulse width is varied from 224-1280µs. Through experiments the output voltage was observed from 12.0-30.0 Vpp and PWM duty cycle from 19-62%. Beyond this level, the subjects began to experience mild discomfort. The experiments were performed on healthy subjects with the followed data. For participant 1 the voltage range was 12.0-22.6 Vpp and PWM duty cycle ranged from 19-49%. Respectively for participant 2 the voltage was 14.0-30.0 Vpp and PWM duty cycle from 23-62%. The reason for this variation can be different like ion concentration in muscles, sensitivity, skin thickness or subcutaneous fat layer. The FES system at the moment works as a voltage-controlled system but for future improvements multi-channel direct current controlled systems can be added. Functional electrical stimulation (FES) is a technique that uses low electrical currents to stimulate the nerves of the body muscles that has lost their senses such as in paralysis. This paper presents the design and development of a single-channel, biphasic FES device using an Arduino controller. The circuit consists of a DC-DC boost converter to generate high voltage, an H-bridge to produce biphasic pulses, a feedback mechanism for output control, and a pulse width modulation module. The device stimulates wrist flexion through surface electrodes on the Flexor Digitorum Superficialis (FDS) muscle. Experiments conducted on two healthy subjects demonstrate the ability to control pulse width (224 − 1280µs) and the voltage amplitude (up to 30V) to elicit muscle contractions. Mild wrist flexion is observed above 12 V. The portable Arduino-based device offers advantages such as cost-effectiveness, precise output regulation through feedback, and safety enhancements to prevent reverse current. Further clinical testing can affirm the therapeutic applicability for neuromuscular conditions. This FES device system can be enhanced to multi-channel capabilities and adaptive control algorithms.