A portable, programmable, multichannel stimulator with high compliance voltage for noninvasive neural stimulation of motor and sensory nerves in humans

Abstract

Most neural stimulators do not have a high enough compliance voltage to pass current through the skin. The few stimulators that meet the high compliance voltage necessary for transcutaneous stimulation are typically large benchtop units that are not portable, and the stimulation waveforms cannot be readily customized. To address this, we present the design and validation of a portable, programmable, multichannel, noninvasive neural stimulator that can generate three custom bipolar waveforms at ± 150 V with microsecond temporal resolution. The design is low-cost, open-source, and validated on the benchtop and with a healthy population to demonstrate its functionality for sensory and motor stimulation. Sensory stimulation included electrocutaneous stimulation targeting cutaneous mechanoreceptors at the surface of the skin and transcutaneous nerve stimulation targeting the median nerve at the wrist. Both electrocutaneous stimulation on the hand and transcutaneous stimulation at the wrist can elicit isolated tactile percepts on the hand but changes in pulse frequency are more discriminable for electrocutaneous stimulation. Also, neuromuscular electrical stimulation of the flexor digiti profundus is evoked by applying electrical stimulation directly above the muscle in the forearm and to the median and ulnar nerves in the upper arm. Muscle and nerve stimulation evoked similar grip forces and force rise times, but nerve stimulation had a significantly slower fatigue rate. The development and validation of this noninvasive stimulator and direct comparison of common sensory and motor stimulation targets in a human population constitute an important step towards more widespread use and accessibility of neural stimulation for education and research.

Case report: Ultrasound-guided median nerve electrical stimulation on functional recovery of hemiplegic upper limb after stroke

“…The nerve fibers activated by non-invasive nerve stimulation are dependent on the location of the stimulus and the magnitude of the current (13,(22)(23)(24). Non-invasive stimulation approaches require a high compliance voltage to drive current through the high impedance of the skin to activate sufficient muscle fibers (25). Generally, the non-invasive electrical stimulation was set at a frequency of 4-50 Hz, intensity of 2-10 mA, and duration of 20-30 min/day, 5 days/week for 4-6 weeks (18,(26)(27)(28)(29)(30)(31).…”

“…At typically higher currents and longer pulse durations, electrical stimulation is often associated with a sharp noxious sensation, which is uncomfortable or even painful to the recipient, limiting user adoption of electrical stimulation systems. Therefore, there is a need to innovate a new electrical stimulation approach that could precisely direct the current to the underlying neural structure while activating multiple fibers using a low-voltage, comfortable current (25).…”

A Finite Element Approach for Forearm Neuromodulation: Impact of Electrode Size on the Current Density

“…The electrode arrangements are a promising technology that is widely used for transcutaneous electrical stimulation (TES). It has been approved that the dynamic adaptation of electrode size and position helps to simplify the use of electrical stimulation systems and to increase their clinical efficacy [8]- [10]. However, it is still unclear how the electrode size affects the current distribution of the target muscle.…”

A Sensory Feedback Neural Stimulator Prototype for Both Implantable and Wearable Applications

“…Adding the third PCB to the implantable system, it is possible to adapt the system to a completely external and wearable scenario. The voltage compliance of the implantable system (14 V) is insufficient for the wearable scenario: as discussed in [38,39], high voltage compliance is necessary to adapt to variations in electrode-skin impedance, allowing the device to stimulate loads over 10 kΩ with currents of 5 mA. So, for external stimulation (i.e., TENS), it is mandatory to have a higher voltage compliance, which is achieved by adding the third board.…”