Selective activation of the human tibial and common peroneal nerves with a flat interface nerve electrode

Clinical outcomes of peripheral nerve interfaces for rehabilitation in paralysis and amputation: a literature review

Peripheral nerve interfaces (PNIs) are electrical systems designed to integrate with peripheral nerves in patients, such as following central nervous system (CNS) injuries to augment or replace CNS control and restore function. We review the literature for clinical trials and studies containing clinical outcome measures to explore the utility of human applications of PNIs. We discuss the various types of electrodes currently used for PNI systems and their functionalities and limitations. We discuss important design characteristics of PNI systems, including biocompatibility, resolution and specificity, efficacy, and longevity, to highlight their importance in the current and future development of PNIs. The clinical outcomes of PNI systems are also discussed. Finally, we review relevant PNI clinical trials that were conducted, up to the present date, to restore the sensory and motor function of upper or lower limbs in amputees, spinal cord injury patients, or intact individuals and describe their significant findings. This review highlights the current progress in the field of PNIs and serves as a foundation for future development and application of PNI systems.

Implanted Electrodes for Functional Electrical Stimulation to Restore Upper and Lower Extremity Function: History and Future Directions

Functional electrical stimulation (FES) to activate nerves and muscles in paralyzed extremities has considerable promise to improve outcome after neurological disease or injury, especially in individuals who have upper motor nerve dysfunction due to central nervous system pathology. Because technology has improved, a wide variety of methods for providing electrical stimulation to create functional movements have been developed, including muscle stimulating electrodes, nerve stimulating electrodes, and hybrid constructs. However, in spite of decades of success in experimental settings with clear functional improvements for individuals with paralysis, the technology has not yet reached widespread clinical translation. In this review, we outline the history of FES techniques and approaches and describe future directions in evolution of the technology.

Fractal Microelectrodes for More Energy-Efficient Cervical Vagus Nerve Stimulation

Vagus nerve stimulation (VNS) has the potential to treat various peripheral dysfunctions, but the traditional cuff electrodes for VNS are susceptible to off-target effects. Microelectrodes may enable highly selective VNS that can mitigate off-target effects, but they suffer from the increased impedance. Recent studies on microelectrodes with non-Euclidean geometries have reported higher energy efficiency in neural stimulation applications. These previous studies use electrodes with mm/cm-scale dimensions, mostly targeted for myelinated fibers. This study evaluates fractal microelectrodes for VNS in a rodent model (N = 3). A thin-film device with fractal and circle microelectrodes is fabricated to compare their neural stimulation performance on the same radial coordinate of the nerve. The results show that fractal microelectrodes can activate C-fibers with up to 52% less energy (p = 0.012) compared to circle microelectrodes. To the best of the knowledge, this work is the first to demonstrate a geometric advantage of fractal microelectrodes for VNS in vivo.

Restoration of sensory information via bionic hands

Individuals who have lost the use of their hands because of amputation or spinal cord injury can use prosthetic hands to restore their independence. A dexterous prosthesis requires the acquisition of control signals that drive the movements of the robotic hand, and the transmission of sensory signals to convey information to the user about the consequences of these movements. In this Review, we describe non-invasive and invasive technologies for conveying artificial sensory feedback through bionic hands, and evaluate the technologies' long-term prospects.

Fascicles split or merge every ∼560 microns within the human cervical vagus nerve

Objective.Vagus nerve stimulation (VNS) is Food and Drug Administration-approved for epilepsy, depression, and obesity, and stroke rehabilitation; however, the morphological anatomy of the vagus nerve targeted by stimulatation is poorly understood. Here, we used microCT to quantify the fascicular structure and neuroanatomy of human cervical vagus nerves (cVNs).Approach.We collected eight mid-cVN specimens from five fixed cadavers (three left nerves, five right nerves). Analysis focused on the 'surgical window': 5 cm of length, centered around the VNS implant location. Tissue was stained with osmium tetroxide, embedded in paraffin, and imaged on a microCT scanner. We visualized and quantified the merging and splitting of fascicles, and report a morphometric analysis of fascicles: count, diameter, and area.Main results.In our sample of human cVNs, a fascicle split or merge event was observed every ∼560µm (17.8 ± 6.1 events cm-1). Mean morphological outcomes included: fascicle count (6.6 ± 2.8 fascicles; range 1-15), fascicle diameter (514 ± 142µm; range 147-1360µm), and total cross-sectional fascicular area (1.32 ± 0.41 mm2; range 0.58-2.27 mm).Significance.The high degree of fascicular splitting and merging, along with wide range in key fascicular morphological parameters across humans may help to explain the clinical heterogeneity in patient responses to VNS. These data will enable modeling and experimental efforts to determine the clinical effect size of such variation. These data will also enable efforts to design improved VNS electrodes.

Examining the in vivo functionality of the Magnetically Aligned Regenerative Tissue-Engineered Electronic Nerve Interface (MARTEENI)

OBJECTIVE

Although neural-enabled prostheses have been used to restore some lost functionality in clinical trials, they have faced difficulty in achieving high degree of freedom, natural use compared to healthy limbs. This study investigated the in vivo functionality of a flexible and scalable regenerative peripheral-nerve interface suspended within a microchannel-embedded, tissue-engineered hydrogel (the Magnetically Aligned Regenerative Tissue-Engineered Electronic Nerve Interface, MARTEENI) as a potential approach to improving current issues in peripheral nerve interfaces.

APPROACH

Assembled MARTEENI devices were implanted in the gaps of severed sciatic nerves in Lewis rats. Both acute and chronic electrophysiology were recorded, and channel-isolated activity was examined. In terminal experiments, evoked activity during paw compression and stimulus response curves generated from proximal nerve stimulation were examined. Electrochemical impedance spectroscopy was performed to assess the complex impedance of recording sites during chronic data collection. Features of the foreign-body response in non-functional implants were examined using immunohistological methods.

MAIN RESULTS

Channel-isolated activity was observed in acute, chronic, and terminal experiments and showed a typically biphasic morphology with peak-to-peak amplitudes varying between 50 to 500 µV. For chronic experiments, electrophysiology was observed for 77 days post-implant. Within the templated hydrogel, regenerating axons formed minifascicles that varied in both size and axon count and were also found to surround device threads. No axons were found to penetrate the foreign-body response. Together these results suggest the MARTEENI is a promising approach for interfacing with peripheral nerves.

SIGNIFICANCE

Findings demonstrate a high likelihood that observed electrophysiological activity recorded from implanted MARTEENIs originated from neural tissue. The variation in minifascicle size seen histologically suggests that amplitude distributions observed in functional MARTEENIs may be due to a combination of individual axon and mini-compound action potentials. This study provided an assessment of a functional MARTEENI in an in vivo animal model for the first time.

Wireless microelectrode arrays for selective and chronically stable peripheral nerve stimulation for hindlimb movement

Objective. Maximizing the stability of implanted neural interfaces will be critical to developing effective treatments for neurological and neuromuscular disorders. Our research aims to develop a stable neural interface using wireless communication and intrafascicular microelectrodes to provide highly selective stimulation of neural tissue.Approach. We implanted a wireless floating microelectrode array into the left sciatic nerve of six rats. Over a 38 week implantation period, we recorded stimulation thresholds and movements evoked at each implanted electrode. We also tracked each animal's response to sensory stimuli and performance on two different walking tasks.Main results. Presence of the microelectrode array inside the sciatic nerve did not cause any obvious motor or sensory deficits in the hindlimb. Visible movement in the hindlimb was evoked by stimulating the sciatic nerve with currents as low as 4.1µA. Thresholds for most of the 96 electrodes we implanted were below 20µA, and predictable recruitment of plantar flexion and dorsiflexion was achieved by stimulating rat sciatic nerve with the intrafascicular microelectrode array. Further, motor recruitment patterns for each electrode did not change significantly throughout the study.Significance. Incorporating wireless communication and a low-profile neural interface facilitated highly stable motor recruitment thresholds and fine motor control in the hindlimb throughout an extensive 9.5 month assessment in rodent peripheral nerve. Results of this study indicate that use of the wireless device tested here could be extended to other applications requiring selective neural stimulation and chronic implantation.

Selective neural stimulation methods improve cycling exercise performance after spinal cord injury: a case series

BACKGROUND

Exercise after paralysis can help prevent secondary health complications, but achieving adequate exercise volumes and intensities is difficult with loss of motor control. Existing electrical stimulation-driven cycling systems involve the paralyzed musculature but result in rapid force decline and muscle fatigue, limiting their effectiveness. This study explores the effects of selective stimulation patterns delivered through multi-contact nerve cuff electrodes on functional exercise output, with the goal of increasing work performed and power maintained within each bout of exercise.

METHODS

Three people with spinal cord injury and implanted stimulation systems performed cycling trials using conventional (S-Max), low overlap (S-Low), low duty cycle (C-Max), and/or combined low overlap and low duty cycle (C-Low) stimulation patterns. Outcome measures include total work (W), end power (Pend), power fluctuation indices (PFI), charge accumulation (Q), and efficiency (η). Mann-Whitney tests were used for statistical comparisons of W and Pend between a selective pattern and S-Max. Welch's ANOVAs were used to evaluate differences in PFIs among all patterns tested within a participant (n ≥ 90 per stimulation condition).

RESULTS

At least one selective pattern significantly (p < 0.05) increased W and Pend over S-Max in each participant. All selective patterns also reduced Q and increased η compared with S-Max for all participants. C-Max significantly (p < 0.01) increased PFI, indicating a decrease in ride smoothness with low duty cycle patterns.

CONCLUSIONS

Selective stimulation patterns can increase work performed and power sustained by paralyzed muscles prior to fatigue with increased stimulation efficiency. While still effective, low duty cycle patterns can cause inconsistent power outputs each pedal stroke, but this can be managed by utilizing optimized stimulation levels. Increasing work and sustained power each exercise session has the potential to ultimately improve the physiological benefits of stimulation-driven exercise.

Recent Developments in Prosthesis Sensors, Texture Recognition, and Sensory Stimulation for Upper Limb Prostheses

Current developments being made in upper limb prostheses are focused on replacing lost sensory information to the amputees. Providing sensory stimulation from the prosthesis can directly improve control over the prosthetic and provide a sense of body ownership. The focus of this review article is on recent developments while including foundational knowledge for some of the critical concepts in neural prostheses. Reported concepts follow the flow of information from sensors to signal processing, with emphasis on texture recognition, and then to sensory stimulation strategies that reestablish the lost sensory feedback loop. Prosthetic sensors are used to detect the physical environment, converting pressure, force, and position into electrical signals. The electrical signals can then be processed in an effort to identify the surrounding environment using distinctive characteristics such as stiffness and texture. In order for the amputee to use this information in a natural manner, there must be real-time sensory stimulation, perception, and motor control of the prosthesis. Although truly complete sensory replacement has not yet been realized, some basic percepts can be partially restored, allowing progress towards a more realistic prosthesis with natural sensations.

Chronic nerve health following implantation of femoral nerve cuff electrodes

BACKGROUND

Peripheral nerve stimulation with implanted nerve cuff electrodes can restore standing, stepping and other functions to individuals with spinal cord injury (SCI). We performed the first study to evaluate the clinical electrodiagnostic changes due to electrode implantation acutely, chronic presence on the nerve peri- and post-operatively, and long-term delivery of electrical stimulation.

METHODS

A man with bilateral lower extremity paralysis secondary to cervical SCI sustained 5 years prior to enrollment received an implanted standing neuroprosthesis including composite flat interface nerve electrodes (C-FINEs) electrodes implanted around the proximal femoral nerves near the inguinal ligaments. Electromyography quantified neurophysiology preoperatively, intraoperatively, and through 1 year postoperatively. Stimulation charge thresholds, evoked knee extension moments, and weight distribution during standing quantified neuroprosthesis function over the same interval.

RESULTS

Femoral compound motor unit action potentials increased 31% in amplitude and 34% in area while evoked knee extension moments increased significantly (p < 0.01) by 79% over 1 year of rehabilitation with standing and quadriceps exercises. Charge thresholds were low and stable, averaging 19.7 nC ± 6.2 (SEM). Changes in saphenous nerve action potentials and needle electromyography suggested minor nerve irritation perioperatively.

CONCLUSIONS

This is the first human trial reporting acute and chronic neurophysiologic changes due to application of and stimulation through nerve cuff electrodes. Electrodiagnostics indicated preserved nerve health with strengthened responses following stimulated exercise. Temporary electrodiagnostic changes suggest minor nerve irritation only intra- and peri-operatively, not continuing chronically nor impacting function. These outcomes follow implantation of a neuroprosthesis enabling standing and demonstrate the ability to safely implant electrodes on the proximal femoral nerve close to the inguinal ligament. We demonstrate the electrodiagnostic findings that can be expected from implanting nerve cuff electrodes and their time-course for resolution, potentially applicable to prostheses modulating other peripheral nerves and functions.

TRIAL REGISTRATION

ClinicalTrials.gov NCT01923662 , retrospectively registered August 15, 2013.

Selective Nerve Cuff Stimulation Strategies for Prolonging Muscle Output

Neural stimulation systems are often limited by rapid muscle fatigue. Selective nerve cuff electrodes can target independent yet synergistic motor unit pools (MUPs), which can be used in duty-cycle reducing stimulation paradigms to prolong joint moment output.

OBJECTIVE

This study investigates waveform parameters within moment-prolonging paradigms and determines strategies for their optimal implementation.

METHODS

Composite flat-interface nerve cuff electrodes (C-FINEs) were chronically implanted on feline proximal sciatic nerves. Cyclic stimulation tests determined effects of stimulation period and duty cycle in different MUP types. Ideal parameters were then used in duty-cycle reducing carousel stimulation. Time to 50% reduction in moment (T50), moment overshoot, and moment ripple were determined for constant, open-loop carousel, and moment feedback-controlled closed-loop carousel stimulation.

RESULTS

A stimulation period of 1 s best maintained joint moment for all MUPs. Low (25%) duty cycles consistently improved joint moment maintenance, though allowable duty cycle varied among MUPs by gross muscle and fiber type. Both open- and closed-loop carousel stimulation significantly increased T50 over constant stimulation. Closed-loop carousel significantly decreased moment overshoot over the other conditions, and significantly decreased moment ripple compared with open-loop stimulation.

CONCLUSION

Selectivity-enabled carousel stimulation prolongs joint moment over conventional constant stimulation. Appropriate waveform parameters can be quickly determined for individual MUPs and stimulation can be controlled for additional performance improvements with this paradigm.

SIGNIFICANCE

Providing prolonged, stable joint moment and muscle output to recipients of motor neuroprostheses will improve clinical outcomes, increase independence, and positively impact quality of life.

Sum of phase-shifted sinusoids stimulation prolongs paralyzed muscle output

Neuroprostheses that activate musculature of the lower extremities can enable standing and movement after paralysis. Current systems are functionally limited by rapid muscle fatigue induced by conventional, non-varying stimulus waveforms. Previous work has shown that sum of phase-shifted sinusoids (SOPS) stimulation, which selectively modulates activation of individual motor unit pools (MUPs) to lower the duty cycle of each while maintaining a high net muscle output, improves joint moment maintenance but introduces greater instability over conventional stimulation. In this case study, implementation of SOPS stimulation with a real-time feedback controller successfully decreased joint moment instability and further prolonged joint moment output with increased stimulation efficiency over open-loop approaches in one participant with spinal cord injury. These findings demonstrate the potential for closed-loop SOPS to improve functionality of neuroprosthetic systems.

Image-guided focused ultrasound modulates electrically evoked motor neuronal activity in the mouse peripheral nervous system in vivo

OBJECTIVE

Focused ultrasound (FUS) has recently been demonstrated capable of exciting motor neuronal activity. However, comprehensive understanding of elucidated excitatory and inhibitory effects is required to better assess FUS-mediated modulation. In this study, we demonstrate that image-guided FUS can selectively modulate motor neuron activity in the mouse sciatic nerve in vivo and attribute motor responses to thermal effects.

APPROACH

FUS was applied on the sciatic nerve of anesthetized mice in vivo through the intact skin and muscle using ultrasound imaging for targeting. Both excitatory and inhibitory effects were recorded using electromyography (EMG) along with muscle response of the hind limb. The effects of FUS modulation versus heating by invasive alternative heating source (AHS) on electrically evoked EMG responses in the sciatic nerve in vivo were also investigated. The safety and reversibility of the technique were validated using histology and EMG recovery.

MAIN RESULTS

The FUS was capable of eliciting motor neuronal activity comparable to electrical stimulation ES, and facilitating motor neuronal response on electrically evoked potentials with temperature elevation up to 11.5 °C  ±  0.3 °C (PRF  ⩽  40 Hz). On the other hand, FUS-induced temperature elevations above 15.1 °C  ±  1.6 °C (PRF  ⩾  100 Hz) resulted in the suppression of electrically-evoked motor neuronal activity along with a decrease in EMG latency and area under the curve (AUC), which was validated against the invasive AHS with temperature elevation of 18.1 °C  ±  8.5 °C. Histological findings along with EMG responses after FUS modulation demonstrated a reversible or irreversible modulation.

SIGNIFICANCE

The findings reported herein indicate that image-guided FUS (PRF  ⩽  100 Hz) induces safe and controllable modulation of involuntarily evoked motor neuron activity in vivo.

Implanted High Density Cuff Electrodes Functionally Activate Human Tibial and Peroneal Motor Units Without Chronic Detriment to Peripheral Nerve Health

OBJECTIVE

Peripheral nerve stimulation via multi-contact nerve cuff electrodes (NCEs) has proved effective in restoring function to individuals with lower-extremity paralysis. This study investigates clinical measures of nerve health over one year post-implantation of a composite flat-interface nerve electrode (C-FINE) on the tibial and peroneal nerves above the knee in a human volunteer. This represents the first deployment of a novel NCE on new neural targets in a uniquely challenging location prone to prolonged externally applied forces, making acute and chronic postoperative observation critical.

MATERIALS AND METHODS

A 27-year-old man with an incomplete spinal cord injury (AIS C) at the C3 to C4 level received eight-contact C-FINEs bilaterally on the tibial and peroneal nerves, proximal to the knee. Access to four contacts per cuff exhibiting the most desirable responses was externalized via temporary percutaneous leads. Percutaneous leads were later removed, with contacts generating the best dorsiflexion (two of four) and plantar flexion (one of four) reconnected to a permanently implanted pulse generator. For 13 months post-implantation, nerve health and cuff performance were assessed through motor nerve conduction velocity (MNCV) studies, clinical needle electromyography, compound motor action potential (CMAP), sensory nerve action potential (SNAP), stimulation-evoked tetanic moment collection, and lower-limb circumference measurements.

RESULTS

Tibial and peroneal MNCVs remained stable bilaterally above 40 m/sec, with CMAPs increased or stable after six months. SNAPs remained stable across all measurements. CMAP initial charge thresholds remained below 50 nC, with minimal changes to muscle recruitment order in three of four externalized contacts per cuff. Peak tetanic moments remained stable, with bilateral increases in thigh and calf circumferences of 5% and 14% over one year.

CONCLUSIONS

Above-knee tibial and peroneal NCEs can restore stimulated ankle-joint function without chronic nerve health detriments. Alongside previous femoral nerve data, this study demonstrates the ability of NCEs to enhance lower-extremity function with limited neuromuscular impact.

Intraoperative Responses May Predict Chronic Performance of Composite Flat Interface Nerve Electrodes on Human Femoral Nerves

Peripheral nerve cuff electrodes (NCEs) in motor system neuroprostheses can generate strong muscle contractions and enhance surgical efficiency by accessing multiple muscles from a single proximal location. Predicting chronic performance of high contact density NCEs based on intraoperative observations would facilitate implantation at locations that maximize selective recruitment, immediate connection of optimal contacts to implanted pulse generators (IPGs) with limited output channels, and initiation of postoperative rehabilitation as soon as possible after surgery. However, the stability of NCE intraoperative recruitment to predict chronic performance has not been documented. Here we report the first-in-human application of a specific NCE, the composite flat interface nerve electrode (C-FINE), at a new and anatomically challenging location on the femoral nerve close to the inguinal ligaments. EMG and moment recruitment curves were recorded for each of the 8 contacts in 2 C-FINE intraoperatively, perioperatively, and chronically for 6 months. Intraoperative measurements predicted chronic outcomes for 87.5% of contacts with 14/16 recruiting the same muscles at 6 months as intraoperatively. In both 8-contact C-FINEs, 3 contacts elicited hip flexion and 5 selectively generated knee extension, 3 of which activated independent motor unit populations each sufficient to support standing. Recruitment order stabilized in less than 3 weeks and did not change thereafter. While confirmation of these results will be required with future studies and implant locations, this suggests that remobilization and stimulated exercise may be initiated 3 weeks after surgery with little risk of altering performance.

Implantable Neural Interfaces and Wearable Tactile Systems for Bidirectional Neuroprosthetics Systems

Neuroprosthetics and neuromodulation represent a promising field for several related applications in the central and peripheral nervous system, such as the treatment of neurological disorders, the control of external robotic devices, and the restoration of lost tactile functions. These actions are allowed by the neural interface, a miniaturized implantable device that most commonly exploits electrical energy to fulfill these operations. A neural interface must be biocompatible, stable over time, low invasive, and highly selective; the challenge is to develop a safe, compact, and reliable tool for clinical applications. In case of anatomical impairments, neuroprosthetics is bound to the need of exploring the surrounding environment by fast-responsive and highly sensitive artificial tactile sensors that mimic the natural sense of touch. Tactile sensors and neural interfaces are closely interconnected since the readouts from the first are required to convey information to the neural implantable apparatus. The role of these devices is pivotal hence technical improvements are essential to ensure a secure system to be eventually adopted in daily life. This review highlights the fundamental criteria for the design and microfabrication of neural interfaces and artificial tactile sensors, their use in clinical applications, and future enhancements for the release of a second generation of devices.

A review for the peripheral nerve interface designer

Informational density and relative accessibility of the peripheral nervous system make it an attractive site for therapeutic intervention. Electrode-based electrophysiological interfaces with peripheral nerves have been under development since the 1960s and, for several applications, have seen widespread clinical implementation. However, many applications require a combination of neural target resolution and stability which has thus far eluded existing peripheral nerve interfaces (PNIs). With the goal of aiding PNI designers in development of devices that meet the demands of next-generation applications, this review seeks to collect and present practical considerations and best practices which emerge from the literature, including both lessons learned during early PNI development and recent ideas. Fundamental and practical principles guiding PNI design are reviewed, followed by an updated and critical account of existing PNI designs and strategies. Finally, a brief survey of in vitro and in vivo PNI characterization methods is presented.

Distributed stimulation increases force elicited with functional electrical stimulation

OBJECTIVE

The maximum muscle forces that can be evoked using functional electrical stimulation (FES) are relatively modest. The reason for this weakness is not fully understood but could be partly related to the widespread distribution of motor nerve branches within muscle. As such, a single stimulating electrode (as is conventionally used) may be incapable of activating the entire array of motor axons supplying a muscle. Therefore, the objective of this study was to determine whether stimulating a muscle with more than one source of current could boost force above that achievable with a single source.

APPROACH

We compared the maximum isometric forces that could be evoked in the anterior deltoid of anesthetized monkeys using one or two intramuscular electrodes. We also evaluated whether temporally interleaved stimulation between two electrodes might reduce fatigue during prolonged activity compared to synchronized stimulation through two electrodes.

MAIN RESULTS

We found that dual electrode stimulation consistently produced greater force (~50% greater on average) than maximal stimulation with single electrodes. No differences, however, were found in the fatigue responses using interleaved versus synchronized stimulation.

SIGNIFICANCE

It seems reasonable to consider using multi-electrode stimulation to augment the force-generating capacity of muscles and thereby increase the utility of FES systems.

A translational framework for peripheral nerve stimulating electrodes: Reviewing the journey from concept to clinic

The purpose of this review article is to describe the underlying methodology for successfully translating novel interfaces for electrical modulation of the peripheral nervous system (PNS) from basic design concepts to clinical applications and chronic human use. Despite advances in technologies to communicate directly with the nervous system, the pathway to clinical translation for most neural interfaces is not clear. FDA guidelines provide information on necessary evidence which should be generated and submitted to allow the agency evaluate safety and efficacy of a new medical device. However, a knowledge gap exists on translating neural interfaces from pre-clinical studies into the clinical domain. Our article is intended to inform the field on some of the key considerations for such a transition process specific to neural interfaces that may not be already covered by FDA guidances. This framework focuses on non-penetrating peripheral nerve stimulating electrodes that have been proven effective for motor and sensory neural prostheses and successfully transitioned from pre-clinical through first-in-human and chronic clinical deployment. We discuss the challenges of moving these neural interfaces along the translational continuum and ultimately through FDA approval for human feasibility studies. Specifically, we describe a translational process involving: quantitative human anatomy, neural modeling and simulation, acute intraoperative testing and verification, clinical demonstration with temporary percutaneous access, and finally chronic clinical deployment and functional performance. To clarify and demonstrate the importance of each step of this translational framework, we present case studies from electrodes developed at Case Western Reserve University (CWRU), specifically the spiral cuff, the Flat Interface Nerve Electrode (FINE), and the Composite FINE (C-FINE). In addition, we demonstrate that success along this translational pathway can be further expedited by: appropriate selection of well-characterized materials, validation of fabrication and sterilization protocols, well-implemented quality control measures, and quantification of impact on neural structure, health, and function. The issues and approaches identified in this review for the peripheral nervous system may also serve to accelerate the dissemination of any new neural interface into clinical practice, and consequently advance the performance, utility, and clinical value of new neural prostheses or neuromodulation systems.

Real-Time Prosthetic Digit Actuation by Optical Read-out of Activity-Dependent Calcium Signals in an Ex Vivo Peripheral Nerve

Improved neural interfacing strategies are needed for the full articulation of advanced prostheses. To address limitations of existing control interface designs, the work of our laboratory has presented an optical approach to reading activity from individual nerve fibers using activity-dependent calcium transients. Here, we demonstrate the feasibility of such signals to control prosthesis actuation by using the axonal fluorescence signal in an ex vivo mouse nerve to drive a prosthetic digit in real-time. Additionally, signals of varying action potential frequency are streamed post hoc to the prosthesis, showing graded motor output and the potential for proportional neural control. This proof-of-concept work is a novel demonstration of the functional use of activity-dependent optical read-out in the nerve.

Tissue-Engineered Peripheral Nerve Interfaces

Research on neural interfaces has historically concentrated on development of systems for the brain; however, there is increasing interest in peripheral nerve interfaces (PNIs) that could provide benefit when peripheral nerve function is compromised, such as for amputees. Efforts focus on designing scalable and high-performance sensory and motor peripheral nervous system interfaces. Current PNIs face several design challenges such as undersampling of signals from the thousands of axons, nerve-fiber selectivity, and device-tissue integration. To improve PNIs, several researchers have turned to tissue engineering. Peripheral nerve tissue engineering has focused on designing regeneration scaffolds that mimic normal nerve extracellular matrix composition, provide advanced microarchitecture to stimulate cell migration, and have mechanical properties like the native nerve. By combining PNIs with tissue engineering, the goal is to promote natural axon regeneration into the devices to facilitate close contact with electrodes; in contrast, traditional PNIs rely on insertion or placement of electrodes into or around existing nerves, or do not utilize materials to actively facilitate axon regeneration. This review presents the state-of-the-art of PNIs and nerve tissue engineering, highlights recent approaches to combine neural-interface technology and tissue engineering, and addresses the remaining challenges with foreign-body response.

Motor-commands decoding using peripheral nerve signals: a review

During the last few decades, substantial scientific and technological efforts have been focused on the development of neuroprostheses. The major emphasis has been on techniques for connecting the human nervous system with a robotic prosthesis via natural-feeling interfaces. The peripheral nerves provide access to highly processed and segregated neural command signals from the brain that can in principle be used to determine user intent and control muscles. If these signals could be used, they might allow near-natural and intuitive control of prosthetic limbs with multiple degrees of freedom. This review summarizes the history of neuroprosthetic interfaces and their ability to record from and stimulate peripheral nerves. We also discuss the types of interfaces available and their applications, the kinds of peripheral nerve signals that are used, and the algorithms used to decode them. Finally, we explore the prospects for future development in this area.

Optical read-out and modulation of peripheral nerve activity

Numerous clinical and research applications necessitate the ability to interface with peripheral nerve fibers to read and control relevant neural pathways. Visceral organ modulation and rehabilitative prosthesis are two areas which could benefit greatly from improved neural interfacing approaches. Therapeutic neural interfacing, or ‘bioelectronic medicine’, has potential to affect a broad range of disorders given that all the major organs of the viscera are neurally innervated. However, a better understanding of the neural pathways that underlie function and a means to precisely interface with these fibers are required. Existing peripheral nerve interfaces, consisting primarily of electrode-based designs, are unsuited for highly specific (individual axon) communication and/or are invasive to the tissue. Our laboratory has explored an optogenetic approach by which optically sensitive reporters and actuators are targeted to specific cell (axon) types. The nature of such an approach is laid out in this short perspective, along with associated technologies and challenges.

A novel flexible cuff-like microelectrode for dual purpose, acute and chronic electrical interfacing with the mouse cervical vagus nerve

OBJECTIVE

Neural reflexes regulate immune responses and homeostasis. Advances in bioelectronic medicine indicate that electrical stimulation of the vagus nerve can be used to treat inflammatory disease, yet the understanding of neural signals that regulate inflammation is incomplete. Current interfaces with the vagus nerve do not permit effective chronic stimulation or recording in mouse models, which is vital to studying the molecular and neurophysiological mechanisms that control inflammation homeostasis in health and disease. We developed an implantable, dual purpose, multi-channel, flexible 'microelectrode' array, for recording and stimulation of the mouse vagus nerve.

APPROACH

The array was microfabricated on an 8 µm layer of highly biocompatible parylene configured with 16 sites. The microelectrode was evaluated by studying the recording and stimulation performance. Mice were chronically implanted with devices for up to 12 weeks.

MAIN RESULTS

Using the microelectrode in vivo, high fidelity signals were recorded during physiological challenges (e.g potassium chloride and interleukin-1β), and electrical stimulation of the vagus nerve produced the expected significant reduction of blood levels of tumor necrosis factor (TNF) in endotoxemia. Inflammatory cell infiltration at the microelectrode 12 weeks of implantation was limited according to radial distribution analysis of inflammatory cells.

SIGNIFICANCE

This novel device provides an important step towards a viable chronic interface for cervical vagus nerve stimulation and recording in mice.

Toward Bioelectronic Medicine-Neuromodulation of Small Peripheral Nerves Using Flexible Neural Clip

Neural modulation technology and the capability to affect organ function have spawned the new field of bioelectronic medicine. Therapeutic interventions depend on wireless bioelectronic neural interfaces that can conformally and easily attach to small (few hundred micrometers) nerves located deep in the body without neural damage. Besides size, factors like flexibility and compliance to attach and adapt to visceral nerves associated moving organs are of paramount importance and have not been previously addressed. This study proposes a novel flexible neural clip (FNC) that can be used to interface with a variety of different peripheral nerves. To illustrate the flexibility of the design, this study stimulates the pelvic nerve, the vagus nerve, and branches of the sciatic nerve and evaluates the feasibility of the design in modulating the function of each of these nerves. It is found that this FNC allows fine-tuning of physiological processes such as micturition, heart rate, and muscle contractions. Furthermore, this study also tests the ability of wirelessly powered FNC to enable remote modulation of visceral pelvic nerves located deep in the body. These results show that the FNC can be used with a range of different nerves, providing one of the critical pieces in the field of bioelectronics medicines.

Selectivity and Longevity of Peripheral-Nerve and Machine Interfaces: A Review

For those individuals with upper-extremity amputation, a daily normal living activity is no longer possible or it requires additional effort and time. With the aim of restoring their sensory and motor functions, theoretical and technological investigations have been carried out in the field of neuroprosthetic systems. For transmission of sensory feedback, several interfacing modalities including indirect (non-invasive), direct-to-peripheral-nerve (invasive), and cortical stimulation have been applied. Peripheral nerve interfaces demonstrate an edge over the cortical interfaces due to the sensitivity in attaining cortical brain signals. The peripheral nerve interfaces are highly dependent on interface designs and are required to be biocompatible with the nerves to achieve prolonged stability and longevity. Another criterion is the selection of nerves that allows minimal invasiveness and damages as well as high selectivity for a large number of nerve fascicles. In this paper, we review the nerve-machine interface modalities noted above with more focus on peripheral nerve interfaces, which are responsible for provision of sensory feedback. The invasive interfaces for recording and stimulation of electro-neurographic signals include intra-fascicular, regenerative-type interfaces that provide multiple contact channels to a group of axons inside the nerve and the extra-neural-cuff-type interfaces that enable interaction with many axons around the periphery of the nerve. Section Current Prosthetic Technology summarizes the advancements made to date in the field of neuroprosthetics toward the achievement of a bidirectional nerve-machine interface with more focus on sensory feedback. In the Discussion section, the authors propose a hybrid interface technique for achieving better selectivity and long-term stability using the available nerve interfacing techniques.

Optical Read-out of Neural Activity in Mammalian Peripheral Axons: Calcium Signaling at Nodes of Ranvier

Current neural interface technologies have serious limitations for advanced prosthetic and therapeutic applications due primarily to their lack of specificity in neural communication. An optogenetic approach has the potential to provide single cell/axon resolution in a minimally invasive manner by optical interrogation of light-sensitive reporters and actuators. Given the aim of reading neural activity in the peripheral nervous system, this work has investigated an activity-dependent signaling mechanism in the peripheral nerve. We demonstrate action potential evoked calcium signals in mammalian tibial nerve axons using an in vitro mouse model with a dextran-conjugated fluorescent calcium indicator. Spatial and temporal dynamics of the signal are presented, including characterization of frequency-modulated amplitude. Pharmacological experiments implicate T-type CaV channels and sodium-calcium exchanger (NCX) as predominant mechanisms of calcium influx. This work shows the potential of using calcium-associated optical signals for neural activity read-out in peripheral nerve axons.

The design of and chronic tissue response to a composite nerve electrode with patterned stiffness

OBJECTIVE

As neural interfaces demonstrate success in chronic applications, a novel class of reshaping electrodes with patterned regions of stiffness will enable application to a widening range of anatomical locations. Patterning stiff regions and flexible regions of the electrode enables nerve reshaping while accommodating anatomical constraints of various implant locations ranging from peripheral nerves to spinal and autonomic plexi.

APPROACH

Introduced is a new composite electrode enabling patterning of regions of various electrode mechanical properties. The initial demonstration of the composite's capability is the composite flat interface nerve electrode (C-FINE). The C-FINE is constructed from a sandwich of patterned PEEK within layers of pliable silicone. The shape of the PEEK provides a desired pattern of stiffness: stiff across the width of the nerve to reshape the nerve, but flexible along its length to allow for bending with the nerve. This is particularly important in anatomical locations near joints or organs, and in constrained compartments. We tested pressure and volume design constraints in vitro to verify that the C-FINE can attain a safe cuff-to-nerve ratio (CNR) without impeding intraneural blood flow. We measured nerve function as well as nerve and axonal morphology following 3 month implantation of the C-FINE without wires on feline peripheral nerves in anatomically constrained areas near mobile joints and major blood vessels in both the hind and fore limbs.

MAIN RESULTS

In vitro inflation tests showed effective CNRs (1.93  ±  0.06) that exceeded the industry safety standard of 1.5 at an internal pressure of 20 mmHg. This is less than the 30 mmHg shown to induce loss of conduction or compromise blood flow. Implanted cats showed no changes in physiology or electrophysiology. Behavioral signs were normal suggesting healthy nerves. Motor nerve conduction velocity and compound motor action potential did not change significantly between implant and explant (p  >  0.15 for all measures). Axonal density and myelin sheath thickness was not significantly different within the electrode compared to sections greater than 2 cm proximal to implanted cuffs (p  >  0.14 for all measures).

SIGNIFICANCE

We present the design and verification of a novel nerve cuff electrode, the C-FINE. Laminar manufacturing processes allow C-FINE stiffness to be configured for specific applications. Here, the central region in the configuration tested is stiff to reshape or conform to the target nerve, while edges are highly flexible to bend along its length. The C-FINE occupies less volume than other NCEs, making it suitable for implantation in highly mobile locations near joints. Design constraints during simulated transient swelling were verified in vitro. Maintenance of nerve health in various challenging anatomical locations (sciatic and median/ulnar nerves) was verified in a chronic feline model in vivo.

Restoring standing capabilities with feedback control of functional neuromuscular stimulation following spinal cord injury

This paper reviews the field of feedback control for neuroprosthesis systems that restore advanced standing function to individuals with spinal cord injury. Investigations into closed-loop control of standing by functional neuromuscular stimulation (FNS) have spanned three decades. The ultimate goal for FNS standing control systems is to facilitate hands free standing and enabling the user to perform manual functions at self-selected leaning positions. However, most clinical systems for home usage currently only provide basic upright standing using preprogrammed stimulation patterns. To date, online modulation of stimulation to produce advanced standing functions such as balance against postural disturbances or the ability to assume leaning postures have been limited to simulation and laboratory investigations. While great technological advances have been made in biomechanical sensing and interfaces for neuromuscular stimulation, further progress is still required for finer motor control by FNS. Another major challenge is the development of sophisticated control schemes that produce the necessary postural adjustments, adapt against accelerating muscle fatigue, and consider volitional actions of the intact upper-body of the user. Model-based development for novel control schemes are proven and sensible approaches to prototype and test the basic operating efficacy of potentially complex and multi-faceted control systems. The major considerations for further innovation of such systems are summarized in this paper prior to describing the evolution of closed-loop FNS control of standing from previous works. Finally, necessary emerging technologies to for implementing FNS feedback control systems for standing are identified. These technological advancements include novel electrodes that more completely and selectively activate paralyzed musculature and implantable sensors and stimulation modules for flexible neuroprosthesis system deployment.

Interleaved neuromuscular electrical stimulation: Motor unit recruitment overlap

INTRODUCTION

In this study, we quantified the "overlap" between motor units recruited by single pulses of neuromuscular electrical stimulation (NMES) delivered over the tibialis anterior muscle (mNMES) and the common peroneal nerve (nNMES). We then quantified the torque produced when pulses were alternated between the mNMES and nNMES sites at 40 Hz ("interleaved" NMES; iNMES).

METHODS

Overlap was assessed by comparing torque produced by twitches evoked by mNMES, nNMES, and both delivered together, over a range of stimulus intensities. Trains of iNMES were delivered at the intensity that produced the lowest overlap.

RESULTS

Overlap was lowest (5%) when twitches evoked by both mNMES and nNMES produced 10% peak twitch torque. iNMES delivered at this intensity generated 25% of maximal voluntary dorsiflexion torque (11 Nm).

DISCUSSION

Low intensity iNMES leads to low overlap and produces torque that is functionally relevant to evoke dorsiflexion during walking. Muscle Nerve 55: 490-499, 2017.

Electrically stimulated signals from a long-term Regenerative Peripheral Nerve Interface

Despite modern technological advances, the most widely available prostheses provide little functional recovery beyond basic grasping. Although sophisticated upper extremity prostheses are available, optimal prosthetic interfaces which give patients high-fidelity control of these artificial limbs are limited. We have developed a novel Regenerative Peripheral Nerve Interface (RPNI), which consists of a unit of free muscle that has been neurotized by a transected peripheral nerve. In conjunction with a biocompatible electrode on the muscle surface, the RPNI facilitates signal transduction from a residual peripheral nerve to a neuroprosthetic limb. The purpose of this study was to explore signal quality and reliability in an RPNI following an extended period of implantation. Following a 14-month maturation period, electromyographic signal generation was evaluated via electrical stimulation of the innervating nerve. The long-term RPNI was viable and healthy, as demonstrated by evoked compound muscle action potentials as well as histological tissue analysis. Signals exceeding 4 mV were successfully acquired and amplitudes were consistent across multiple repetitions of applied stimuli. There were no evident signs of muscle denervation, significant scar tissue, or muscle necrosis. This study provides further evidence that after a maturation period exceeding 1 year, reliable and consistent signals can still be acquired from an RPNI.

Microstimulation of the lumbar DRG recruits primary afferent neurons in localized regions of lower limb

Patterned microstimulation of the dorsal root ganglion (DRG) has been proposed as a method for delivering tactile and proprioceptive feedback to amputees. Previous studies demonstrated that large- and medium-diameter afferent neurons could be recruited separately, even several months after implantation. However, those studies did not examine the anatomical localization of sensory fibers recruited by microstimulation in the DRG. Achieving precise recruitment with respect to both modality and receptive field locations will likely be crucial to create a viable sensory neuroprosthesis. In this study, penetrating microelectrode arrays were implanted in the L5, L6, and L7 DRG of four isoflurane-anesthetized cats instrumented with nerve cuff electrodes around the proximal and distal branches of the sciatic and femoral nerves. A binary search was used to find the recruitment threshold for evoking a response in each nerve cuff. The selectivity of DRG stimulation was characterized by the ability to recruit individual distal branches to the exclusion of all others at threshold; 84.7% (n = 201) of the stimulation electrodes recruited a single nerve branch, with 9 of the 15 instrumented nerves recruited selectively. The median stimulation threshold was 0.68 nC/phase, and the median dynamic range (increase in charge while stimulation remained selective) was 0.36 nC/phase. These results demonstrate the ability of DRG microstimulation to achieve selective recruitment of the major nerve branches of the hindlimb, suggesting that this approach could be used to drive sensory input from localized regions of the limb. This sensory input might be useful for restoring tactile and proprioceptive feedback to a lower-limb amputee.

Implantable neurotechnologies: electrical stimulation and applications

Neural stimulation using injected electrical charge is widely used both in functional therapies and as an experimental tool for neuroscience applications. Electrical pulses can induce excitation of targeted neural pathways that aid in the treatment of neural disorders or dysfunction of the central and peripheral nervous system. In this review, we summarize the recent trends in the field of electrical stimulation for therapeutic interventions of nervous system disorders, such as for the restoration of brain, eye, ear, spinal cord, nerve and muscle function. Neural prosthetic applications are discussed, and functional electrical stimulation parameters for treating such disorders are reviewed. Important considerations for implantable packaging and enhancing device reliability are also discussed. Neural stimulators are expected to play a profound role in implantable neural devices that treat disorders and help restore functions in injured or disabled nervous system.

Implantable neurotechnologies: bidirectional neural interfaces--applications and VLSI circuit implementations

A bidirectional neural interface is a device that transfers information into and out of the nervous system. This class of devices has potential to improve treatment and therapy in several patient populations. Progress in very large-scale integration has advanced the design of complex integrated circuits. System-on-chip devices are capable of recording neural electrical activity and altering natural activity with electrical stimulation. Often, these devices include wireless powering and telemetry functions. This review presents the state of the art of bidirectional circuits as applied to neuroprosthetic, neurorepair, and neurotherapeutic systems.

The Evolution of Neuroprosthetic Interfaces

The ideal neuroprosthetic interface permits high-quality neural recording and stimulation of the nervous system while reliably providing clinical benefits over chronic periods. Although current technologies have made notable strides in this direction, significant improvements must be made to better achieve these design goals and satisfy clinical needs. This article provides an overview of the state of neuroprosthetic interfaces, starting with the design and placement of these interfaces before exploring the stimulation and recording platforms yielded from contemporary research. Finally, we outline emerging research trends in an effort to explore the potential next generation of neuroprosthetic interfaces.

Neural interfaces for somatosensory feedback: bringing life to a prosthesis

PURPOSE OF REVIEW

When an individual loses a limb, he/she loses touch with the world and with the people around him/her. Somatosensation is critical to the feeling of connection and control of one's own body. Decades of attempts to replace lost somatosensation by sensory substitutions have been ineffective outside of the laboratory. This review discusses important recent results demonstrating chronic somatosensory restoration through direct peripheral nerve stimulation.

RECENT FINDINGS

Stimulation of peripheral nerves results in somatosensory perception on the phantom limb. Sensations are localized to several independent and functionally relevant locations, such as the fingertips, thenar eminence, ulnar border and dorsal surface. Patterns in stimulation intensity change the perception experience by the user, opening new dimensions on neuromodulation.

SUMMARY

Neural interfaces with sophisticated stimulation paradigms create a user's perception of his/her hand to touch and manipulate objects. The pattern of intensity and frequency of stimulation is critical to the quality and intensity of perceived sensation. Restoring feeling has allowed the individuals to, 'feel [my] hand for the first time since the accident,' and 'feel [my] wife touch my hand'. Individuals using a prosthetic hand with sensation can pull cherries and grapes from the stem, open water bottles and move objects without destroying these objects - all while audio and visually deprived. After regaining sensation, phantom pain is eliminated in individuals that had frequent, sometimes debilitating, pain following limb loss. With over 5 subject-years of experience, this work is leading the evolution of a new era in prostheses. Somatosensory prosthetics as a standard procedure to augment and restore somatosensation are now within our reach.

Update in facial nerve paralysis: tissue engineering and new technologies

PURPOSE OF REVIEW

To present the recent advances in the treatment of facial paralysis, emphasizing the emerging technologies. This review will summarize the current state of the art in the management of facial paralysis and discuss the advances in nerve regeneration, facial reanimation, and use of novel biomaterials. This review includes surgical innovations in reinnervation and reanimation as well as progress with bioelectrical interfaces.

RECENT FINDINGS

The last decade has witnessed major advances in the understanding of nerve injury and approaches for management. Key innovations include strategies to accelerate nerve regeneration, provide tissue-engineered constructs that may replace nonfunctional nerves, approaches to influence axonal guidance, limiting of donor-site morbidity, and optimization of functional outcomes. Approaches to muscle transfer continue to evolve, and new technologies allow for electrical nerve stimulation and use of artificial tissues.

SUMMARY

The fields of biomedical engineering and facial reanimation increasingly intersect, with innovative surgical approaches complementing a growing array of tissue engineering tools. The goal of treatment remains the predictable restoration of natural facial movement, with acceptable morbidity and long-term stability. Advances in bioelectrical interfaces and nanotechnology hold promise for widening the window for successful treatment intervention and for restoring both lost neural inputs and muscle function.

Stability and selectivity of a chronic, multi-contact cuff electrode for sensory stimulation in human amputees

OBJECTIVE

Stability and selectivity are important when restoring long-term, functional sensory feedback in individuals with limb-loss. Our objective is to demonstrate a chronic, clinical neural stimulation system for providing selective sensory response in two upper-limb amputees.

APPROACH

Multi-contact cuff electrodes were implanted in the median, ulnar, and radial nerves of the upper-limb.

MAIN RESULTS

Nerve stimulation produced a selective sensory response on 19 of 20 contacts and 16 of 16 contacts in subjects 1 and 2, respectively. Stimulation elicited multiple, distinct percept areas on the phantom and residual limb. Consistent threshold, impedance, and percept areas have demonstrated that the neural interface is stable for the duration of this on-going, chronic study.

SIGNIFICANCE

We have achieved selective nerve response from multi-contact cuff electrodes by demonstrating characteristic percept areas and thresholds for each contact. Selective sensory response remains consistent in two upper-limb amputees for 1 and 2 years, the longest multi-contact sensory feedback system to date. Our approach demonstrates selectivity and stability can be achieved through an extraneural interface, which can provide sensory feedback to amputees.