Safety is a critical consideration when designing an electrical neural stimulator, given the direct contact with neural tissue. This paper presents the design of a charge balancing system suitable for frequencies up to the kilohertz domain, to be used as an add-on system for stimulators over a wide range of frequencies, also covering nerve conduction blocking. It operates independently of the stimulator timing by continuously sensing the offset voltage, and applying a corrective current to the electrode, using the offset compensation technique. To ensure its stand-alone capability, the system is battery-powered, and includes a safety and start-up circuit. Electrical measurements verified the functionality of the circuit, demonstrating a residual offset of only 0.7 mV for 1 V biphasic pulses at 50 kHz. When tested for 20 kHz biphasic pulse at a 5 V amplitude, the offset was measured at -11.6 mV, which is still within the (commonly used) ±50 mV safety window.@en

During the last few decades, electrical neural stimulators have successfully been employed as a means of treatment for a wide range of neurological disorders. By targeting the peripheral and central nervous systems, electrical neurostimulators activate/inhibit neural activity by manipulating the stimulus- induced electric field arising at the targeted area through diverse electrode configurations. Aiming at reducing the overall size of implanted stimulator systems, these are being designed to be wirelessly powered and batteryless. Technologies for wireless power transfer to implants are mainly based on inductive coupling and, more recently, ultrasonic waves. To increase the power efficiency and thereby minimize the power consumption of the implant, we have previously proposed a stimulation technique that alters the electric field at the tissue by delivering charge in small packets in a very rapid manner (e.g. 1 Mpps). This charge is consequently accumulated by the tissue’s integrating nature.1 This approach removes the need to ensure continuous accurate control of the stimulus current amplitude, resulting in power savings. To the same end, in an ultrasonically powered system, unnecessary power-conversion blocks can be eliminated, as the incoming ultrasonic wave is harvested, converted into an electrical signal, rectified, and then directly used for stimulation.2 Inspired by the above, this work focuses on providing a discrete-component multi-channel neural stimulator, powered wirelessly through ultrasound (US), to activate and inhibit neural activity. The use of mostly commercially available discrete components will ensure reproducibility by other research labs and help provide a platform technology that can be used as an experimental tool in a variety of applications.3 The US pressure wave obtained at the receiving US transducer will be converted into electrical energy and used both for powering the system as well as to shape the charge packets of the eventual stimulus pulse. In this manner, the need for DC-DC up-conversion, typically needed for neuromodulation, will be eliminated.4 A charge-balancing technique, matching the fast pulse repetition rates (PRR) required for inhibition of neural activity (typically ≥ 5 kHz), will ensure the safety of the implant, while the injected charge will be constantly monitored and adapted for safe and efficacious activation/inhibition.@en

In neuromodulation applications, conventional current mode stimulation is often preferred over its voltage mode equivalent due to its good control of the injected charge. However, it comes at the cost of less energy-efficient output stages. To increase energy efficiency, recent studies have explored non-rectangular stimuli. The current work highlights the importance of an adaptive supply for an output stage with programmable non-rectangular stimuli and accordingly proposes a system-level architecture for multi-channel stimulators. In the proposed architecture, a multi-output DC/DC Converter (DDC) allows each channel to choose among the available supply levels (i.e., DDC outputs) independently and based on its instant voltage/current requirement. A system-level analysis is carried out in Matlab to calculate the possible energy savings of this solution, compared to the conventional approach with a fixed supply. The energy savings have been simulated for a variety of supply levels and waveform amplitudes, suggesting energy savings of up to 83% when employing 6 DDC outputs and the lowest current amplitude explored (250A), and as high as 26% for a full-scale amplitude (4 mA).

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Spinal Cord Injury (SCI) is characterized by a disruption of the spinal pathways connecting the brain to the rest of the body. This can result in an impairment or complete loss of sensory and/or motor functions, depending on the level and severity of the injury. The most common approach of attempting motor recovery using neuromodulation has been through epidural spinal cord electrical stimulation (eSCS), with or without rehabilitation and robotic assistance¹ , ². Dorsal Root Ganglion (DRG) stimulation is a relatively new neuromodulation treatment, which has already been established as a safe and effective treatment for chronic pain. However, its application for motor recovery after SCI has remained unexplored territory. A recent study by Soloukey et al.³ showed the first promising results using L4-level DRG-stimulation to evoke strong and reproducible motor responses in the upper leg muscles of patients with motor complete SCI. The current work presents the development of a prototype stimulator intended for use in early clinical trials for the purpose of further exploring the effects of multi-level DRG-stimulation on motor recovery after SCI. The prototype stimulator is created using commercially available components, focussing primarily on the safety considerations for the use of the prototype in a clinical research environment. The prototype allows for access to multiple leads in parallel to facilitate faster, sequential stimulation, catering for a maximum of 16 leads at once. The final system is equipped with a microcontroller for the programming of the stimulation parameters, a Bluetooth module for the communication with external components (Graphical User Interface (GUI) on a local PC) and tailored connections to the stimulating DRG-leads. The safety and well-being of the patient is set as a first priority, leading to a tailored design and development of the stimulator accordingly. Parameters like the patient's comfort when connected to the device, the safety of the patient and the medical personnel with regards to powering the device through the mains, and protection from any current or DC voltage leakage, are discussed. The prototype is currently undergoing safety evaluation and the complete system design together with these results will be presented during the conference.@en

This paper presents the development of a Dorsal Root Ganglion (DRG) stimulator system intended for use in early clinical trials for motor recovery after Spinal Cord Injury (SCI). It allows for independent control of multisite/multilevel bilateral (on both sides of the spinal cord) stimulation, it can supply a high output current of 25.4mA, and has the ability to program pulse sequences similar to actual locomotion patterns. These characteristics ultimately provide the required versatility for examining the effects of DRG stimulation on locomotion recovery, which is lacking in currently available commercial systems. The device is created using commercially available components to make the design reproducible by other research labs and to facilitate the critical approval procedure for use in a clinical research environment. Throughout the design phase, essential considerations regarding the safety of the participating patient, as well as of the medical personnel involved, were taken into account and these are analyzed and demonstrated in this paper. Such considerations are very rarely discussed in scientific literature and the authors consider that, apart from the design of the system itself, this discussion is a critical contribution of this paper.

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