Fully implanted battery-free high power platform for chronic spinal and muscular functional electrical stimulation

Electrical stimulation of the nervous system is a successful approach for treating various clinical disorders, including restoring hearing, electroanalgesia, and managing motor disorders like Parkinson's. This approach often involves replacing lost neural signals or interrupting dysfunctional neural circuits. However, there's growing interest in using electrical stimulation to retrain or rehabilitate neural circuits by enhancing neuroplasticity or neuromodulation, potentially accelerating rehabilitation or achieving outcomes beyond current methods. A fundamental challenge is providing sufficient power for the necessary currents and voltages, especially for long-term use. Large batteries require recharging/replacement, creating patient barriers. In small animals, battery size is limited, and frequent replacements disrupt behavior. External wired connections increase infection risk, cause scar tissue, displace electrodes, limit activity, and are incompatible with MRI. Thus, batteries and wired connections hinder evaluation of continuous stimulation protocols lasting days or weeks. Wireless systems using near-field power transfer offer potential, but current systems are limited in harvested power and voltage. The maximum power point voltage depends on the inductance ratio and coupling between transmitter and receiver antennas, creating a tradeoff between operational voltage and power harvesting. Therefore, technologies to increase power transfer at desired voltages for chronic functional electrical stimulation are crucial. Additional challenges include the stiffness of standard neural interface devices, limiting body motion and causing discomfort. Recent advances in flexible circuits and biocompatible materials allow for conformable interfaces and wireless power/control. Many functional electrical stimulation applications involve complex protocols across multiple locations, requiring precise temporal relationships and regulated stimulation levels. Existing battery-free systems use passive, unregulated voltage control or simple voltage programming, while devices capable of controlled currents require expensive application-specific integrated circuits (ASICs). Applications may target areas with vastly different current/voltage requirements (e.g., intraspinal vs. epidural spinal vs. muscle stimulation). Therefore, a need exists for a system capable of delivering both low and high ranges of currents/voltages simultaneously, supporting multichannel electrical stimulation of various neuromuscular sites.

The authors reviewed existing literature on electrical stimulation therapies, highlighting the limitations of current power delivery methods, particularly for chronic applications. The challenges associated with battery-powered systems, wired connections, and the limitations of existing wireless power transfer technologies were discussed. The literature review also covered previous attempts to create fully implantable, battery-free systems, noting their limitations in power output and voltage compliance. The development of flexible electronics and biocompatible materials was addressed as enabling technologies for long-term implants. Existing systems for delivering complex stimulation protocols were examined, emphasizing the cost and limitations of application-specific integrated circuits (ASICs). Finally, the diverse current and voltage requirements for different stimulation sites (spinal cord, muscle) were reviewed, emphasizing the need for a system capable of handling this range.

This study presents a fully implanted, battery-free device for functional electrical stimulation (FES). The device leverages a soft, biocompatible, flexible design for implantation in highly mobile areas (rat back and hind leg), enabling stimulation in freely moving subjects without affecting mobility or behavior. The device features eight channels for both spinal and muscle FES. It uses a monopolar configuration with a common return electrode. Mechanical flexibility is maximized through separate rigid islands for integrated circuits (ICs) connected by strain-isolating serpentine traces. The device uses a monolithic, dual-sided flexible circuit board. Wireless power is delivered via an instrumentalized cage using amplitude shift keying (ASK) modulation of radio frequency (RF) power. To improve power harvesting efficiency, the device uses a co-planar antenna combining a passive resonator and receiver. A center-tapped antenna design creates positive and negative voltages, and a precision current driver enables a small circuit design. A power management system includes voltage protection and a step-down converter. The low-power microcontroller allows programming via ASK modulation. Telemetry is transmitted via infrared (IR) communication, allowing monitoring of supply voltage, electrode voltage, and electrode impedance. The resonant antenna design, embedding a passive resonator, significantly improves power transfer efficiency and enables high voltage compliance. The device is optimized for a treadmill enclosure using a 2-turn transmitter antenna. Simulations (Ansys Electronics Desktop) demonstrate coupling enhancement and misalignment insensitivity of the 3-antenna system (transmitter, receiver, resonator). The study characterized power harvesting across a range of RF power levels and angular misalignments, showing significantly improved efficiency (500%) compared to previous designs. Thermal simulations (ANSYS) and benchtop experiments assessed the thermal impact on surrounding tissues, showing minimal temperature increase. The device uses RF ASK and IR modulation for two-way communication. The microcontroller controls stimulation parameters independently for each channel using an 8-bit DAC and multiplexer. The device can be used with various electrode types (spinal epidural and intramuscular electrodes). Accelerated lifetime testing evaluated electrode impedance changes under continuous stimulation, demonstrating good chronic performance. The device's material choices (polyimide, gold-plated copper, Parylene-C, silicone) and mechanical design (rigid islands, serpentine interconnects) are optimized for long-term stability and biocompatibility. Finite element simulations (ANSYS) and benchtop experiments characterized the mechanical properties of the interconnects, showing resilience to physiological strain. Surgical procedures are detailed, involving electrode placement, device implantation, and post-operative care. X-ray and µCT imaging were used to monitor device placement and stability. Behavioral analyses (DeepLabCut) assessed the impact of the device on spontaneous activity and locomotion. Stimulation-evoked movements were monitored in anesthetized and freely behaving animals, quantifying responses to varying stimulation parameters. Chronic device performance was assessed over 6 weeks, evaluating recruitment curves and stimulation thresholds.

The study successfully developed a fully implantable, battery-free FES device capable of delivering high power (over 300mW) and high voltage (±20V). The device exhibited significant improvements in power transfer efficiency (500% increase) compared to previous systems, primarily attributed to the novel passive resonator-optimized power transfer design. The device demonstrated reliable multichannel (8-channel) operation with precise current control (40 µA to 4.7 mA) and versatile biphasic stimulation capabilities. The device remained functional for over 6 weeks in both intact and spinal cord-injured rats, showcasing robust chronic performance. In vivo experiments demonstrated the device's ability to evoke graded and controlled movements by stimulating spinal and muscle sites independently. The flexible design minimized impact on animal behavior and locomotion, with spontaneous activity returning to normal levels within two weeks of implantation. The device showed minimal thermal impact on surrounding tissues during operation. Comprehensive analysis of electrode impedance revealed acceptable stability over time, even with prolonged stimulation. The flexible design and strain-isolating interconnects ensured device integrity during animal movement, with minimal electrode displacement over one month. Finally, the device successfully facilitated functional limb movements in spinal cord-injured rats during treadmill locomotion, demonstrating the potential for rehabilitation applications.

This study successfully addresses the limitations of existing FES technologies by developing a fully implantable, battery-free device capable of delivering high power and voltage for chronic stimulation. The high power transfer efficiency (500% improvement), achieved through the novel passive resonator design, enables complex stimulation protocols crucial for neuromodulation and functional restoration. The device's ability to consistently evoke graded movements over several weeks highlights its suitability for chronic studies. The minimal impact on animal behavior showcases the device's biocompatibility and the advantages of the flexible, fully implanted design. The results demonstrate the device's suitability for preclinical research focusing on neural circuit rehabilitation. The design’s scalability and relatively low cost using off-the-shelf components suggest broad applicability within the neuroscience community. Future research could explore the device's application in different animal models and investigate its potential for clinical translation. Addressing challenges related to power delivery in larger animals and humans will require further development, such as incorporating wearable systems for directed power and communication.

This research presents a significant advancement in FES technology by introducing a fully implantable, battery-free, high-power device with exceptional chronic stability. The device's design overcomes key limitations of existing systems, enabling the study of complex stimulation protocols for neural circuit rehabilitation in freely moving animals. Future work will focus on translation to larger animal models and ultimately, human applications.

While the device demonstrated impressive chronic performance (6 weeks), longer-term studies are needed to fully assess its longevity. The study primarily focused on rodent models; further investigation is required to determine its efficacy and adaptability in larger animals and humans. The current design's power delivery relies on an instrumentalized cage; future iterations should explore alternative power delivery methods to enhance versatility. Although the device demonstrated minimal impact on behavior, individual variations in animal responses to implantation cannot be entirely ruled out.