Residual function in peripheral nerve stumps of amputees: implications for neural control of artificial limbs

Phantom limb syndrome: from pathogenesis to treatment. A narrative review

Phantom Limb Syndrome (PLS) can be defined as the disabling or painful sensation of the presence of a body part that is no longer present after its amputation. Anatomical changes involved in Phantom Limb Syndrome, occurring at peripheral, spinal and brain levels and include the formation of neuromas and scars, dorsal horn sensitization and plasticity, short-term and long-term modifications at molecular and topographical levels. The molecular reorganization processes of Phantom Limb Syndrome include NMDA receptors hyperactivation in the dorsal horn of the spinal column leading to inflammatory mechanisms both at a peripheral and central level. At the brain level, a central role has been recognized for sodium channels, BDNF and adenosine triphosphate receptors. In the paper we discuss current available pharmacological options with a final overview on non-pharmacological options in the pipeline.

New insights into muscle activity associated with phantom hand movements in transhumeral amputees

Introduction

Muscle activity patterns in the residual arm are systematically present during phantom hand movements (PHM) in transhumeral amputees. However, their characteristics have not been directly investigated yet, leaving their neurophysiological origin poorly understood. This study pioneers a neurophysiological perspective in examining PHM-related muscle activity patterns by characterizing and comparing them with those in the arm, forearm, and hand muscles of control participants executing intact hand movements (IHM) of similar types.

Methods

To enable rigorous comparison, we developed meta-variables independent of electrode placement, quantifying the phasic profile of recorded surface EMG signals and the specificity of their patterns across electrode sites and movement types.

Results

Similar to the forearm and hand muscles during IHM, each signal recorded from the residual upper arm during PHM displays a phasic profile, synchronized with the onset and offset of each movement repetition. Furthermore, the PHM-related patterns of phasic muscle activity are specific not only to the type of movement but also to the electrode site, even within the same upper arm muscle, while these muscles exhibit homogeneous activities in intact arms.

Discussion

Our results suggest the existence of peripheral reorganization, eventually leading to the emergence of independently controlled muscular sub-volumes. This reorganization potentially occurs through the sprouting of severed axons and the recapture of muscle fibers in the residual limb. Further research is imperative to comprehend this mechanism and its relationship with PHM, holding significant implications for the rehabilitation process and myoelectric prosthesis control.

Neuroanatomical correlates of peripersonal space: bridging the gap between perception, action, emotion and social cognition

Peripersonal space (PPS) is a construct referring to the portion of space immediately surrounding our bodies, where most of the interactions between the subject and the environment, including other individuals, take place. Decades of animal and human neuroscience research have revealed that the brain holds a separate representation of this region of space: this distinct spatial representation has evolved to ensure proper relevance to stimuli that are close to the body and prompt an appropriate behavioral response. The neural underpinnings of such construct have been thoroughly investigated by different generations of studies involving anatomical and electrophysiological investigations in animal models, and, recently, neuroimaging experiments in human subjects. Here, we provide a comprehensive anatomical overview of the anatomical circuitry underlying PPS representation in the human brain. Gathering evidence from multiple areas of research, we identified cortical and subcortical regions that are involved in specific aspects of PPS encoding.We show how these regions are part of segregated, yet integrated functional networks within the brain, which are in turn involved in higher-order integration of information. This wide-scale circuitry accounts for the relevance of PPS encoding in multiple brain functions, including not only motor planning and visuospatial attention but also emotional and social cognitive aspects. A complete characterization of these circuits may clarify the derangements of PPS representation observed in different neurological and neuropsychiatric diseases.

A hybrid sensory feedback system for thermal nociceptive warning and protection in prosthetic hand

Background

Advanced prosthetic hands may embed nanosensors and microelectronics in their cosmetic skin. Heat influx may cause damage to these delicate structures. Protecting the integrity of the prosthetic hand becomes critical and necessary to ensure sustainable function. This study aims to mimic the sensorimotor control strategy of the human hand in perceiving nociceptive stimuli and triggering self-protective mechanisms and to investigate how similar neuromorphic mechanisms implemented in prosthetic hand can allow amputees to both volitionally release a hot object upon a nociceptive warning and achieve reinforced release via a bionic withdrawal reflex.

Methods

A steady-state temperature prediction algorithm was proposed to shorten the long response time of a thermosensitive temperature sensor. A hybrid sensory strategy for transmitting force and a nociceptive temperature warning using transcutaneous electrical nerve stimulation based on evoked tactile sensations was designed to reconstruct the nociceptive sensory loop for amputees. A bionic withdrawal reflex using neuromorphic muscle control technology was used so that the prosthetic hand reflexively opened when a harmful temperature was detected. Four able-bodied subjects and two forearm amputees randomly grasped a tube at the different temperatures based on these strategies.

Results

The average prediction error of temperature prediction algorithm was 8.30 ± 6.00%. The average success rate of six subjects in perceiving force and nociceptive temperature warnings was 86.90 and 94.30%, respectively. Under the reinforcement control mode in Test 2, the median reaction time of all subjects was 1.39 s, which was significantly faster than the median reaction time of 1.93 s in Test 1, in which two able-bodied subjects and two amputees participated. Results demonstrated the effectiveness of the integration of nociceptive sensory strategy and withdrawal reflex control strategy in a closed loop and also showed that amputees restored the warning of nociceptive sensation while also being able to withdraw from thermal danger through both voluntary and reflexive protection.

Conclusion

This study demonstrated that it is feasible to restore the sensorimotor ability of amputees to warn and react against thermal nociceptive stimuli. Results further showed that the voluntary release and withdrawal reflex can work together to reinforce heat protection. Nevertheless, fusing voluntary and reflex functions for prosthetic performance in activities of daily living awaits a more cogent strategy in sensorimotor control.

Mapping the hotspots and future trends of electrical stimulation for peripheral nerve injury: A bibliometric analysis from 2002 to 2023

Peripheral nerve injuries often result in severe personal and social burden, and even with surgical treatment, patients continue to have poor clinical outcomes. Over the past two decades, electrical stimulation has been shown to promote axonal regeneration and alleviate refractory neuropathic pain. The aim of this study was to analyse this field using a bibliometric approach. Literature was searched through Web of Science Core Collection (WOSCC) for the years 2002-2023. Literature analysis included: (1) Describing publication trends in the field. (2) Exploring collaborative network relationships. (3) Finding research advances and research hotspots in the field. (4) Summarizing research trends in the field. With the number of studies in this field still increasing, a total of 693 publications were included in the analysis. This field of research is interdisciplinary in nature. Research hotspots include peripheral nerve regeneration, the treatment of neuropathic pain, materials for nerve injury repair, and the restoration of sensory function in patients with peripheral nerve injury. Correspondingly, the development of nerve conduits and systems for peripheral nerve electrical stimulation, clinical trials of peripheral nerve electrical stimulation, and tactile recovery and movement for amputees have shown significant promise as future research trends in this field.

Using Compound Neural Action Potentials for Functional Validation of a High-Density Intraneural Interface: A Preliminary Study

Compound nerve action potentials (CNAPs) were used as a metric to assess the stimulation performance of a novel high-density, transverse, intrafascicular electrode in rat models. We show characteristic CNAPs recorded from distally implanted cuff electrodes. Evaluation of the CNAPs as a function of stimulus current and calculation of recruitment plots were used to obtain a qualitative approximation of the neural interface's placement and orientation inside the nerve. This method avoids elaborate surgeries required for the implantation of EMG electrodes and thus minimizes surgical complications and may accelerate the healing process of the implanted subject.

Selective chronic recording in small nerve fascicles of sciatic nerve with carbon nanotube yarns in rats

Objective. The primary challenge faced in the field of neural rehabilitation engineering is the limited advancement in nerve interface technology, which currently fails to match the mechanical properties of small-diameter nerve fascicles. Novel developments are necessary to enable long-term, chronic recording from a multitude of small fascicles, allowing for the recovery of motor intent and sensory signals.Approach. In this study, we analyze the chronic recording capabilities of carbon nanotube yarn electrodes in the peripheral somatic nervous system. The electrodes were surgically implanted in the sciatic nerve's three individual fascicles in rats, enabling the recording of neural activity during gait. Signal-to-noise ratio (SNR) and information theory were employed to analyze the data, demonstrating the superior recording capabilities of the electrodes. Flat interface nerve electrode and thin-film longitudinal intrafascicular electrode electrodes were used as a references to assess the results from SNR and information theory analysis.Main results. The electrodes exhibited the ability to record chronic signals with SNRs reaching as high as 15 dB, providing 12 bits of information for the sciatic nerve, a significant improvement over previous methods. Furthermore, the study revealed that the SNR and information content of the neural signals remained consistent over a period of 12 weeks across three different fascicles, indicating the stability of the interface. The signals recorded from these electrodes were also analyzed for selectivity using information theory metrics, which showed an information sharing of approximately 1.4 bits across the fascicles.Significance. The ability to safely and reliably record from multiple fascicles of different nerves simultaneously over extended periods of time holds substantial implications for the field of neural and rehabilitation engineering. This advancement addresses the limitation of current nerve interface technologies and opens up new possibilities for enhancing neural rehabilitation and control.

Sensory Restoration for Improved Motor Control of Prostheses

Somatosensory neuroprostheses are devices with the potential to restore the senses of touch and movement from prosthetic limbs for people with limb amputation or paralysis. By electrically stimulating the peripheral or central nervous system, these devices evoke sensations that appear to emanate from the missing or insensate limb, and when paired with sensors on the prosthesis, they can improve the functionality and embodiment of the prosthesis. There have been major advances in the design of these systems over the past decade, although several important steps remain before they can achieve widespread clinical adoption outside the lab setting. Here, we provide a brief overview of somatosensory neuroprostheses and explores these hurdles and potential next steps towards clinical translation.

A highly integrated bionic hand with neural control and feedback for use in daily life

Restoration of sensorimotor function after amputation has remained challenging because of the lack of human-machine interfaces that provide reliable control, feedback, and attachment. Here, we present the clinical implementation of a transradial neuromusculoskeletal prosthesis-a bionic hand connected directly to the user's nervous and skeletal systems. In one person with unilateral below-elbow amputation, titanium implants were placed intramedullary in the radius and ulna bones, and electromuscular constructs were created surgically by transferring the severed nerves to free muscle grafts. The native muscles, free muscle grafts, and ulnar nerve were implanted with electrodes. Percutaneous extensions from the titanium implants provided direct skeletal attachment and bidirectional communication between the implanted electrodes and a prosthetic hand. Operation of the bionic hand in daily life resulted in improved prosthetic function, reduced postamputation, and increased quality of life. Sensations elicited via direct neural stimulation were consistently perceived on the phantom hand throughout the study. To date, the patient continues using the prosthesis in daily life. The functionality of conventional artificial limbs is hindered by discomfort and limited and unreliable control. Neuromusculoskeletal interfaces can overcome these hurdles and provide the means for the everyday use of a prosthesis with reliable neural control fixated into the skeleton.

Reassessing referral of touch following peripheral deafferentation: The role of contextual bias

Some amputees have been famously reported to perceive facial touch as arising from their phantom hand. These referred sensations have since been replicated across multiple neurological disorders and were classically interpreted as a perceptual correlate of cortical plasticity. Common to all these and related studies is that participants might have been influenced in their self-reports by the experimental design or related contextual biases. Here, we investigated whether referred sensations reports might be confounded by demand characteristics (e.g., compliance, expectation, suggestion). Unilateral upper-limb amputees (N = 18), congenital one-handers (N = 19), and two-handers (N = 22) were repeatedly stimulated with computer-controlled vibrations on 10 body-parts and asked to report the occurrence of any concurrent sensations on their hand(s). To further manipulate expectations, we gave participants the suggestion that some of these vibrations had a higher probability to evoke referred sensations. We also assessed similarity between (phantom) hand and face representation in primary somatosensory cortex (S1), using functional Magnetic Resonance Imaging (fMRI) multivariate representational similarity analysis. We replicated robust reports of referred sensations in amputees towards their phantom hand; however, the frequency and distribution of reported referred sensations were similar across groups. Moreover, referred sensations were evoked by stimulation of multiple body-parts and similarly reported on both the intact and phantom hand in amputees. Face-to-phantom-hand representational similarity was not different in amputees' missing hand region, compared with controls. These findings weaken the interpretation of referred sensations as a perceptual correlate of S1 plasticity and reveal the need to account for contextual biases when evaluating anomalous perceptual phenomena.

Phantom limb therapy improves cortical efficiency of the sensorimotor network in a targeted muscle reinnervation amputee: a case report

Targeted muscle reinnervation (TMR) surgery involves the coaptation of amputated nerves to nearby motor nerve branches with the purpose of reclosing the neuromuscular loop in order to reduce phantom limb pain. The purpose of this case study was to create a phantom limb therapy protocol for an amputee after undergoing TMR surgery, where the four main nerves of his right arm were reinnervated into the chest muscles. The goal of this phantom limb therapy was to further strengthen these newly formed neuromuscular closed loops. The case participant (male, 21- years of age, height = 5'8″ and weight = 134 lbs) presented 1- year after a trans-humeral amputation of the right arm along with TMR surgery and participated in phantom limb therapy for 3 months. Data collections for the subject occurred every 2 weeks for 3 months. During the data collections, the subject performed various movements of the phantom and intact limb specific to each reinnervated nerve and a gross manual dexterity task (Box and Block Test) while measuring brain activity and recording qualitative feedback from the subject. The results demonstrated that phantom limb therapy produced significant changes of cortical activity, reduced fatigue, fluctuation in phantom pain, improved limb synchronization, increased sensory sensation, and decreased correlation strength between intra-hemispheric and inter-hemispheric channels. These results suggest an overall improved cortical efficiency of the sensorimotor network. These results add to the growing knowledge of cortical reorganization after TMR surgery, which is becoming more common to aid in the recovery after amputation. More importantly, the results of this study suggest that the phantom limb therapy may have accelerated the decoupling process, which provides direct clinical benefits to the patient such as reduced fatigue and improved limb synchronization.

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.

Extra-neural signals from severed nerves enable intrinsic hand movements in transhumeral amputations

Robotic prostheses controlled by myoelectric signals can restore limited but important hand function in individuals with upper limb amputation. The lack of individual finger control highlights the yet insurmountable gap to fully replacing a biological hand. Implanted electrodes around severed nerves have been used to elicit sensations perceived as arising from the missing limb, but using such extra-neural electrodes to record motor signals that allow for the decoding of phantom movements has remained elusive. Here, we showed the feasibility of using signals from non-penetrating neural electrodes to decode intrinsic hand and finger movements in individuals with above-elbow amputations. We found that information recorded with extra-neural electrodes alone was enough to decode phantom hand and individual finger movements, and as expected, the addition of myoelectric signals reduced classification errors both in offline and in real-time decoding.

Preliminary Evaluation of the Effect of Mechanotactile Feedback Location on Myoelectric Prosthesis Performance Using a Sensorized Prosthetic Hand

A commonly cited reason for the high abandonment rate of myoelectric prostheses is a lack of grip force sensory feedback. Researchers have attempted to restore grip force sensory feedback by stimulating the residual limb’s skin surface in response to the prosthetic hand’s measured grip force. Recent work has focused on restoring natural feedback to the missing digits directly through invasive surgical procedures. However, the functional benefit of utilizing somatotopically matching feedback has not been evaluated. In this paper, we propose an experimental protocol centered on a fragile object grasp and lift task using a sensorized myoelectric prosthesis to evaluate sensory feedback techniques. We formalized a suite of outcome measures related to task success, timing, and strategy. A pilot study (n = 3) evaluating the effect of utilizing a somatotopically accurate feedback stimulation location in able-bodied participants was conducted to evaluate the feasibility of the standardized platform, and to inform future studies on the role of feedback stimulation location in prosthesis use. Large between-participant effect sizes were observed in all outcome measures, indicating that the feedback location likely plays a role in myoelectric prosthesis performance. The success rate decreased, and task timing and task focus metrics increased, when using somatotopically-matched feedback compared to non-somatotopically-matched feedback. These results were used to conduct a power analysis, revealing that a sample size of n = 8 would be sufficient to achieve significance in all outcome measures.

A Custom Interface for the Joint Operation of Med Associates and Tucker-Davis Technologies Hardware in a Rodent Behavioral Model

BACKGROUND

Rodent behavioral models with an electrophysiological component may require the joint operation of hardware from Med Associates, Inc. (St. Albans, VT) and Tucker-Davis Technologies (TDT; Alachua, FL). Although these manufacturers do produce supplemental hardware for interfacing with each other, investing in such hardware may be untenable for research groups with limited funds who wish to use equipment already in their possession.

NEW METHOD

We designed a printed circuit board (PCB) in KiCad and had it fabricated by Advanced Circuits (Aurora, CO), with components sourced from Digi-Key (Thief River Falls, MN). The PCB provided 8 channels of bidirectional communication for the transmission of signals between Med Asssociates' SG-716B SmartCtrl connection panel and TDT's RZ5D base station. This setup enabled the coordinated operation of programs running separately on each set of hardware.

RESULTS

The custom-built PCB facilitated the joint operation of Med Associates and TDT hardware in a go/no-go detection task involving rats with electrical implants in their sciatic nerves.

COMPARISON WITH EXISTING METHODS

Conventional methods for interfacing Med Associates and Tucker-Davis Technologies rely on the purchase of pre-built hardware whose costs can add up to thousands of dollars. The present method offers a viable alternative that is easily implemented and considerably less expensive (below $200).

CONCLUSION

The present approach provides an inexpensive yet effective alternative to far more costly interfacing solutions offered by Med Associates and Tucker-Davis Technologies.

Sensorimotor integration within the primary motor cortex by selective nerve fascicle stimulation

KEY POINTS

Cortical integration of sensory inputs is crucial for dexterous movement. Short-latency somatosensory afferent inhibition of motor cortical output is typically produced by peripheral whole-nerve stimulation. We exploited intraneural multichannel electrodes used to provide sensory feedback for prosthesis control to assess whether and how selective intraneural sensory stimulation affects sensorimotor cortical circuits in humans. The activation of the primary somatosensory cortex (S1) was explored by recording scalp somatosensory evoked potentials. Sensorimotor integration was tested by measuring the inhibitory effect of the afferent stimulation on the output of the primary motor cortex (M1) generated by transcranial magnetic stimulation. We demonstrate in humans that selective intraneural sensory stimulation elicits a measurable activation of S1 and that it inhibits the output of M1 at the same time range of whole-nerve superficial stimulation.

ABSTRACT

The integration of sensory inputs in the motor cortex is crucial for dexterous movement. We recently demonstrated that a closed-loop control based on the feedback provided through intraneural multi-channel electrodes implanted in the median and ulnar nerves of a participant with upper limb amputation improved manipulation skills and increased prosthesis embodiment. Here we assessed, in the same participant, whether and how selective intraneural sensory stimulation also elicits a measurable cortical activation and affects sensorimotor cortical circuits. After estimating the activation of the primary somatosensory cortex evoked by intraneural stimulation, sensorimotor integration was investigated by testing the inhibition of primary motor cortex (M1) output to transcranial magnetic stimulation, after both intraneural and perineural stimulation. Selective sensory intraneural stimulation evoked a low-amplitude, 16 ms-latency, parietal response in the same area of the earliest component evoked by whole-nerve stimulation, compatible with fast-conducting afferent fiber activation. For the first time, we show that the same intraneural stimulation was also capable of decreasing M1 output, at the same time range of the short-latency afferent inhibition effect of whole-nerve superficial stimulation. The inhibition generated by the stimulation of channels activating only sensory fibers was stronger than the one due to intraneural or perineural stimulation of channels activating mixed fibers. We demonstrate in a human subject that the cortical sensorimotor integration inhibiting M1 output previously described after the experimental whole-nerve stimulation is present also with a more ecological selective sensory fiber stimulation. Abstract Figure: Double-sided filament electrodes (ds-FILE), bearing 16 active sites, and perineural Cuff electrodes were implanted in the median and ulnar nerve of the arm in a hand amputee (upper left panel, single nerve represented). Selectivity of stimulation (1), evoked activity in the somatosensory cortex (2), and sensorimotor integration (3) were investigated. TMS: transcranial magnetic stimulation. This article is protected by copyright. All rights reserved.

Sensory Percepts Elicited by Chronic Macro-Sieve Electrode Stimulation of the Rat Sciatic Nerve

Objective: Intuitive control of conventional prostheses is hampered by their inability to provide the real-time tactile and proprioceptive feedback of natural sensory pathways. The macro-sieve electrode (MSE) is a candidate interface to amputees’ truncated peripheral nerves for introducing sensory feedback from external sensors to facilitate prosthetic control. Its unique geometry enables selective control of the complete nerve cross-section by current steering. Unlike previously studied interfaces that target intact nerve, the MSE’s implantation requires transection and subsequent regeneration of the target nerve. Therefore, a key determinant of the MSE’s suitability for this task is whether it can elicit sensory percepts at low current levels in the face of altered morphology and caliber distribution inherent to axon regeneration. The present in vivo study describes a combined rat sciatic nerve and behavioral model developed to answer this question.

Approach: Rats learned a go/no-go detection task using auditory stimuli and then underwent surgery to implant the MSE in the sciatic nerve. After healing, they were trained with monopolar electrical stimuli with one multi-channel and eight single-channel stimulus configurations. Psychometric curves derived by the method of constant stimuli (MCS) were used to calculate 50% detection thresholds and associated psychometric slopes. Thresholds and slopes were calculated at two time points 3 weeks apart.

Main Results: For the multi-channel stimulus configuration, the average current required for stimulus detection was 19.37 μA (3.87 nC) per channel. Single-channel thresholds for leads located near the nerve’s center were, on average, half those of leads located near the periphery (54.92 μA vs. 110.71 μA, or 10.98 nC vs. 22.14 nC). Longitudinally, 3 of 5 leads’ thresholds decreased or remained stable over the 3-week span. The remaining two leads’ thresholds increased by 70–74%, possibly due to scarring or device failure.

Significance: This work represents an important first step in establishing the MSE’s viability as a sensory feedback interface. It further lays the groundwork for future experiments that will extend this model to the study of other devices, stimulus parameters, and task paradigms.

Advanced Bioelectrical Signal Processing Methods: Past, Present, and Future Approach-Part III: Other Biosignals

Analysis of biomedical signals is a very challenging task involving implementation of various advanced signal processing methods. This area is rapidly developing. This paper is a Part III paper, where the most popular and efficient digital signal processing methods are presented. This paper covers the following bioelectrical signals and their processing methods: electromyography (EMG), electroneurography (ENG), electrogastrography (EGG), electrooculography (EOG), electroretinography (ERG), and electrohysterography (EHG).

Sensory feedback for limb prostheses in amputees

Commercial prosthetic devices currently do not provide natural sensory information on the interaction with objects or movements. The subsequent disadvantages include unphysiological walking with a prosthetic leg and difficulty in controlling the force exerted with a prosthetic hand, thus creating health issues. Restoring natural sensory feedback from the prosthesis to amputees is an unmet clinical need. An optimal device should be able to elicit natural sensations of touch or proprioception, by delivering the complex signals to the nervous system that would be produced by skin, muscles and joints receptors. This Review covers the various neurotechnological approaches that have been proposed for the development of the optimal sensory feedback restoration device for arm and leg amputees.

A brain-computer interface that evokes tactile sensations improves robotic arm control

Prosthetic arms controlled by a brain-computer interface can enable people with tetraplegia to perform functional movements. However, vision provides limited feedback because information about grasping objects is best relayed through tactile feedback. We supplemented vision with tactile percepts evoked using a bidirectional brain-computer interface that records neural activity from the motor cortex and generates tactile sensations through intracortical microstimulation of the somatosensory cortex. This enabled a person with tetraplegia to substantially improve performance with a robotic limb; trial times on a clinical upper-limb assessment were reduced by half, from a median time of 20.9 to 10.2 seconds. Faster times were primarily due to less time spent attempting to grasp objects, revealing that mimicking known biological control principles results in task performance that is closer to able-bodied human abilities.

Foreign Body Reaction to Implanted Biomaterials and Its Impact in Nerve Neuroprosthetics

The implantation of any foreign material into the body leads to the development of an inflammatory and fibrotic process-the foreign body reaction (FBR). Upon implantation into a tissue, cells of the immune system become attracted to the foreign material and attempt to degrade it. If this degradation fails, fibroblasts envelop the material and form a physical barrier to isolate it from the rest of the body. Long-term implantation of medical devices faces a great challenge presented by FBR, as the cellular response disrupts the interface between implant and its target tissue. This is particularly true for nerve neuroprosthetic implants-devices implanted into nerves to address conditions such as sensory loss, muscle paralysis, chronic pain, and epilepsy. Nerve neuroprosthetics rely on tight interfacing between nerve tissue and electrodes to detect the tiny electrical signals carried by axons, and/or electrically stimulate small subsets of axons within a nerve. Moreover, as advances in microfabrication drive the field to increasingly miniaturized nerve implants, the need for a stable, intimate implant-tissue interface is likely to quickly become a limiting factor for the development of new neuroprosthetic implant technologies. Here, we provide an overview of the material-cell interactions leading to the development of FBR. We review current nerve neuroprosthetic technologies (cuff, penetrating, and regenerative interfaces) and how long-term function of these is limited by FBR. Finally, we discuss how material properties (such as stiffness and size), pharmacological therapies, or use of biodegradable materials may be exploited to minimize FBR to nerve neuroprosthetic implants and improve their long-term stability.

Surgical and technological advances in the management of upper limb amputees

Upper limb amputations, ranging from transhumeral to partial hand, can be devastating for patients, their families, and society. Modern paradigm shifts have focused on reconstructive options after upper extremity limb loss, rather than considering the amputation an ablative procedure. Surgical advancements such as targeted muscle reinnervation and regenerative peripheral nerve interface, in combination with technological development of modern prosthetics, have expanded options for patients after amputation. In the near future, advances such as osseointegration, implantable myoelectric sensors, and implantable nerve cuffs may become more widely used and may expand the options for prosthetic integration, myoelectric signal detection, and restoration of sensation. This review summarizes the current advancements in surgical techniques and prosthetics for upper limb amputees. Cite this article: Bone Joint J 2021;103-B(3):430-439.

A cutaneous mechanoneural interface for neuroprosthetic feedback

Amputation destroys sensory end organs and does not provide an anatomical interface for cutaneous neuroprosthetic feedback. Here, we report the design and a biomechanical and electrophysiological evaluation of the cutaneous mechanoneural interface consisting of an afferent neural system that comprises a muscle actuator coupled to a natively pedicled skin flap in a cuff-like architecture. Muscle is actuated through electrical stimulation to induce strains or oscillatory vibrations on the skin flap that are proportional to a desired contact duration or contact pressure. In rat hindlimbs, the mechanoneural interface elicited native dermal mechanotransducers to generate at least four levels of graded contact and eight distinct vibratory afferents that were not significantly different from analogous mechanical stimulation of intact skin. The application of different patterns of electrical stimulation independently engaged slowly adapting and rapidly adapting mechanotransducers, and recreated an array of cutaneous sensations. The cutaneous mechanoneural interface can be integrated with current prosthetic technologies for tactile feedback.

Discriminability of multiple cutaneous and proprioceptive hand percepts evoked by intraneural stimulation with Utah slanted electrode arrays in human amputees

BACKGROUND

Electrical stimulation of residual afferent nerve fibers can evoke sensations from a missing limb after amputation, and bionic arms endowed with artificial sensory feedback have been shown to confer functional and psychological benefits. Here we explore the extent to which artificial sensations can be discriminated based on location, quality, and intensity.

METHODS

We implanted Utah Slanted Electrode Arrays (USEAs) in the arm nerves of three transradial amputees and delivered electrical stimulation via different electrodes and frequencies to produce sensations on the missing hand with various locations, qualities, and intensities. Participants performed blind discrimination trials to discriminate among these artificial sensations.

RESULTS

Participants successfully discriminated cutaneous and proprioceptive sensations ranging in location, quality and intensity. Performance was significantly greater than chance for all discrimination tasks, including discrimination among up to ten different cutaneous location-intensity combinations (15/30 successes, p < 0.0001) and seven different proprioceptive location-intensity combinations (21/40 successes, p < 0.0001). Variations in the site of stimulation within the nerve, via electrode selection, enabled discrimination among up to five locations and qualities (35/35 successes, p < 0.0001). Variations in the stimulation frequency enabled discrimination among four different intensities at the same location (13/20 successes, p < 0.0005). One participant also discriminated among individual stimulation of two different USEA electrodes, simultaneous stimulation on both electrodes, and interleaved stimulation on both electrodes (20/24 successes, p < 0.0001).

CONCLUSION

Electrode location, stimulation frequency, and stimulation pattern can be modulated to evoke functionally discriminable sensations with a range of locations, qualities, and intensities. This rich source of artificial sensory feedback may enhance functional performance and embodiment of bionic arms endowed with a sense of touch.

Compensation Strategies for Bioelectric Signal Changes in Chronic Selective Nerve Cuff Recordings: A Simulation Study

Peripheral nerve interfaces (PNIs) allow us to extract motor, sensory, and autonomic information from the nervous system and use it as control signals in neuroprosthetic and neuromodulation applications. Recent efforts have aimed to improve the recording selectivity of PNIs, including by using spatiotemporal patterns from multi-contact nerve cuff electrodes as input to a convolutional neural network (CNN). Before such a methodology can be translated to humans, its performance in chronic implantation scenarios must be evaluated. In this simulation study, approaches were evaluated for maintaining selective recording performance in the presence of two chronic implantation challenges: the growth of encapsulation tissue and rotation of the nerve cuff electrode. Performance over time was examined in three conditions: training the CNN at baseline only, supervised re-training with explicitly labeled data at periodic intervals, and a semi-supervised self-learning approach. This study demonstrated that a selective recording algorithm trained at baseline will likely fail over time due to changes in signal characteristics resulting from the chronic challenges. Results further showed that periodically recalibrating the selective recording algorithm could maintain its performance over time, and that a self-learning approach has the potential to reduce the frequency of recalibration.

High Throughput Ultrasonic Multi-implant Readout Using a Machine-Learning Assisted CDMA Receiver

Untethered, wireless peripheral nerve recording for prosthetic control requires multi-implant communications at high data rates. This work presents a multiple-access ultrasonic uplink data communication channel comprised of 4 free-floating implants and a single-element external transducer. Using code-division multiple access (CDMA), overall channel data rates of up to 784 kbps were measured, and a machine-learning assisted decoder improved BER by >100x. Compared with prior art, this work incorporates the largest number of implants at the highest data rate and spectral efficiency reported.

Targeted Muscle Reinnervation and Osseointegration for Pain Relief and Prosthetic Arm Control in a Woman with Bilateral Proximal Upper Limb Amputation

BACKGROUND

Bilateral proximal upper limb loss is a dramatic life-changing event. Replacement of the lost function with prosthetic arms, including multiple mechatronic joints, has remained a challenge from the control, comfort, and pain management perspectives. Targeted muscle reinnervation (TMR) is a peripheral nerve surgical procedure proposed to improve the intuitive control of the prosthetic arm and for neuroma and phantom pain management. Moreover, osseointegrated percutaneous implants (OPIs) allow for direct skeletal attachment of the prosthetic arm, ensuring freedom of movement to the patient's residual articulations.

CASE DESCRIPTION

We have reported the first combined application of TMR and an OPI to treat a 24-year-old woman with a bilateral amputation at the shoulder level on the right side and at the very proximal transhumeral level on the left side. TMR was performed bilaterally in a single day, accounting for the peculiar patient's anatomy, as preparatory stage to placement of the OPI, and considering the future availability of implantable electromyographic sensors. The 2 OPI surgeries on the left side were completed after 8.5 months, and prosthetic treatment was completed 17 months after TMR.

CONCLUSIONS

The use of TMR resolved the phantom pain bilaterally and the right-side neuroma pain. It had also substantially reduced the left side neuroma pain. The actual prosthetic control result was intuitive, although partially different from expectations. At 2 years after TMR, the patient reported improvement in essential activities of daily living, with a remarkable preference for the OPI prosthesis. Only 1 suspected case of superficial infection was noted, which had resolved. Overall, this combined treatment required a highly competent multidisciplinary team and exceptional commitment by the patient and her family.

Neural feedback strategies to improve grasping coordination in neuromusculoskeletal prostheses

Conventional prosthetic arms suffer from poor controllability and lack of sensory feedback. Owing to the absence of tactile sensory information, prosthetic users must rely on incidental visual and auditory cues. In this study, we investigated the effect of providing tactile perception on motor coordination during routine grasping and grasping under uncertainty. Three transhumeral amputees were implanted with an osseointegrated percutaneous implant system for direct skeletal attachment and bidirectional communication with implanted neuromuscular electrodes. This neuromusculoskeletal prosthesis is a novel concept of artificial limb replacement that allows to extract control signals from electrodes implanted on viable muscle tissue, and to stimulate severed afferent nerve fibers to provide somatosensory feedback. Subjects received tactile feedback using three biologically inspired stimulation paradigms while performing a pick and lift test. The grasped object was instrumented to record grasping and lifting forces and its weight was either constant or unexpectedly changed in between trials. The results were also compared to the no-feedback control condition. Our findings confirm, in line with the neuroscientific literature, that somatosensory feedback is necessary for motor coordination during grasping. Our results also indicate that feedback is more relevant under uncertainty, and its effectiveness can be influenced by the selected neuromodulation paradigm and arguably also the prior experience of the prosthesis user.

Evoking haptic sensations in the foot through high-density transcutaneous electrical nerve stimulations

OBJECTIVE

Evoking haptic sensation on upper limb amputees via peripheral nerve stimulation has been investigated intensively in the past decade, but related studies involving lower limb amputees are limited. This study aimed to evaluate the feasibility of using non-invasive transcutaneous electrical nerve stimulation to evoke haptic sensation along the phantom limb of the amputated foot of transtibial amputees.

APPROACH

A high-density electrode grid (4 × 4) was placed over the skin surface above the distal branching of the sciatic, tibial, and common peroneal nerves. We hypothesized that electrical stimulation delivered to distinct electrode pairs created unique electric fields, which can activate selective sets of sensory axons innervating different skin regions of the foot. Five transtibial amputee subjects (three unilateral and two bilateral) and one able-bodied subject were tested by scanning all possible electrode pair combinations.

MAIN RESULTS

All subjects reported various haptic percepts at distinct regions along the foot with each corresponding to specific electrode pairs. These results demonstrated the capability of our non-invasive nerve stimulation method to evoke haptic sensations in the foot of transtibial amputees and the able-bodied subject.

SIGNIFICANCE

The outcomes contribute important knowledge and evidence regarding missing tactile sensation in the foot of lower limb amputees and might also facilitate future development of strategies to manage phantom pain and enhance embodiment of prosthetic legs in the future.

The future of upper extremity rehabilitation robotics: research and practice

The loss of upper limb motor function can have a devastating effect on people's lives. To restore upper limb control and functionality, researchers and clinicians have developed interfaces to interact directly with the human body's motor system. In this invited review, we aim to provide details on the peripheral nerve interfaces and brain-machine interfaces that have been developed in the past 30 years for upper extremity control, and we highlight the challenges that still remain to transition the technology into the clinical market. The findings show that peripheral nerve interfaces and brain-machine interfaces have many similar characteristics that enable them to be concurrently developed. Decoding neural information from both interfaces may lead to novel physiological models that may one day fully restore upper limb motor function for a growing patient population.

Modelling the effects of ephaptic coupling on selectivity and response patterns during artificial stimulation of peripheral nerves

Artificial electrical stimulation of peripheral nerves for sensory feedback restoration can greatly benefit from computational models for simulation-based neural implant design in order to reduce the trial-and-error approach usually taken, thus potentially significantly reducing research and development costs and time. To this end, we built a computational model of a peripheral nerve trunk in which the interstitial space between the fibers and the tissues was modelled using a resistor network, thus enabling distance-dependent ephaptic coupling between myelinated axons and between fascicles as well. We used the model to simulate a) the stimulation of a nerve trunk model with a cuff electrode, and b) the propagation of action potentials along the axons. Results were used to investigate the effect of ephaptic interactions on recruitment and selectivity stemming from artificial (i.e., neural implant) stimulation and on the relative timing between action potentials during propagation. Ephaptic coupling was found to increase the number of fibers that are activated by artificial stimulation, thus reducing the artificial currents required for axonal recruitment, and it was found to reduce and shift the range of optimal stimulation amplitudes for maximum inter-fascicular selectivity. During propagation, while fibers of similar diameters tended to lock their action potentials and reduce their conduction velocities, as expected from previous knowledge on bundles of identical axons, the presence of many other fibers of different diameters was found to make their interactions weaker and unstable.

Electrochemical characteristics of ultramicro-dimensioned SIROF electrodes for neural stimulation and recording

OBJECTIVE

With ever increasing applications of neural recording and stimulation, the necessity for developing neural interfaces with higher selectivity and lower invasiveness is inevitable. Reducing the electrode size is one approach to achieving such goals. In this study, we investigated the effect of electrode geometric surface area (GSA), from 20 μm² to 1960 μm², on the electrochemical impedance and charge-injection properties of sputtered iridium oxide (SIROF) coated electrodes in response to current-pulsing typical of neural stimulation. These data were used to assess the electrochemical properties of ultra-small SIROF electrodes (GSA  <  200 μm²) for stimulation and recording applications.

APPROACH

SIROF charge storage capacities (CSC), impedance, and charge-injection characteristics during current-pulsing of planar, circular electrodes were evaluated in an inorganic model of interstitial fluid (model-ISF).

MAIN RESULTS

SIROF electrodes as small as 20 μm² could provide 1.3 nC/phase (200 μs pulse width, 0.6 V versus Ag|AgCl interpulse bias) of charge during current pulsing. The 1 kHz impedance of all electrodes used in this study were below 1 MΩ, which is suitable for neural recording.

SIGNIFICANCE

Ultra-small SIROF electrodes are capable of charge injection in buffered saline at levels above some reported thresholds for neural stimulation with microelectrodes.

Neural Implants Without Electronics: A Proof-of-Concept Study on a Human Skin Model

Objective: Chronic neural implants require energy and signal supply. The objective of this work was to evaluate a multichannel transcutaneous coupling approach in an ex vivo split-concept study, which minimizes the invasiveness of such an implant by externalizing the processing electronics. Methods: Herein, the experimental work focused on the transcutaneous energy and signal transmission. The performance was discussed with widely evaluated concepts of neural interfaces in the literature. Results: The performance of the transcutaneous coupling approach increased with higher channel count and higher electrode pitches. Electrical crosstalk among channels was present, but acceptable for the stimulation of peripheral nerves. Conclusions: Transcutaneous coupling with extracorporeal transmitting arrays and subcutaneous counterparts provide a promising alternative to the inductive concept particularly when a fully integration of the system in a prosthetic shaft is intended. The relocation of the electronics can potentially prevent pressure sores, improve accessibility for maintenance and increase lifetime of the implant.

Hindlimb motor responses evoked by microstimulation of the lumbar dorsal root ganglia during quiet standing

Somatosensory afferent pathways have been a target for neural prostheses that seek to restore sensory feedback from amputated limbs and to recruit muscles paralyzed by neurological injury. These pathways supply inputs to spinal reflex circuits that are necessary for coordinating muscle activity in the lower limb. The dorsal root ganglia (DRG) is a potential site for accessing sensory neurons because DRG microstimulation selectively recruits major nerve branches of the cat hindlimb. Previous DRG microstimulation experiments have been performed in anesthetized animals, but effects on muscle recruitment and behavior in awake animals have not been examined.

OBJECTIVE

The objective of the current study was to measure the effects of DRG microstimulation on evoking changes in hindlimb muscle activity during quiet standing.

APPROACH

In this study, 32-channel penetrating microelectrode arrays were implanted chronically in the left L6 and L7 DRG of four cats. During each week of testing, one DRG electrode was selected to deliver microstimulation pulse-trains during quiet standing. Electromyographic (EMG) signals were recorded from intramuscular electrodes in ten hindlimb muscles, and ground-reaction forces (GRF) were measured under the foot of the implanted limb.

MAIN RESULTS

DRG Microstimulation evoked a mix of excitatory and inhibitory responses across muscles. Response rates were highest when microstimulation was applied on the L7 array, producing more excitatory than inhibitory responses. Response rates for the L6 array were lower, and the composition of responses was more evenly balanced between excitation and inhibition. On approximately one third of testing weeks, microstimulation induced a transient unloading of the hindlimb as indicated by a decrease in GRF. Reciprocal inhibition at the knee was a prevalent response pattern across testing days which contributed to the unloading force on this subset of testing weeks.

SIGNIFICANCE

Results show that single-channel microstimulation in the lumbar DRG evokes stereotyped patterns of muscle recruitment in awake animals, demonstrating that even limited sensory input can elicit hindlimb behavior. These findings imply that DRG microstimulation may have utility in neural prosthetic applications aimed at restoring somatosensory feedback and promoting motor function after neurological injury.

Selectivity of afferent microstimulation at the DRG using epineural and penetrating electrode arrays

OBJECTIVE

We have shown previously that microstimulation of the lumbar dorsal root ganglia (L5-L7 DRG) using penetrating microelectrodes, selectively recruits distal branches of the sciatic and femoral nerves in an acute preparation. However, a variety of challenges limit the clinical translatability of DRG microstimulation via penetrating electrodes. For clinical translation of a DRG somatosensory neural interface, electrodes placed on the epineural surface of the DRG may be a viable path forward. The goal of this study was to evaluate the recruitment properties of epineural electrodes and compare their performance with that of penetrating electrodes. Here, we compare the number of selectively recruited distal nerve branches and the threshold stimulus intensities between penetrating and epineural electrode arrays.

APPROACH

Antidromically propagating action potentials were recorded from multiple distal branches of the femoral and sciatic nerves in response to epineural stimulation on 11 ganglia in four cats to quantify the selectivity of DRG stimulation. Compound action potentials (CAPs) were recorded using nerve cuff electrodes implanted around up to nine distal branches of the femoral and sciatic nerve trunks. We also tested stimulation selectivity with penetrating microelectrode arrays implanted into ten ganglia in four cats. A binary search was carried out to identify the minimum stimulus intensity that evoked a response at any of the distal cuffs, as well as whether the threshold response selectively occurred in only a single distal nerve branch.

MAIN RESULTS

Stimulation evoked activity in just a single peripheral nerve through 67% of epineural electrodes (35/52) and through 79% of the penetrating microelectrodes (240/308). The recruitment threshold (median  =  9.67 nC/phase) and dynamic range of epineural stimulation (median  =  1.01 nC/phase) were significantly higher than penetrating stimulation (0.90 nC/phase and 0.36 nC/phase, respectively). However, the pattern of peripheral nerves recruited for each DRG were similar for stimulation through epineural and penetrating electrodes.

SIGNIFICANCE

Despite higher recruitment thresholds, epineural stimulation provides comparable selectivity and superior dynamic range to penetrating electrodes. These results suggest that it may be possible to achieve a highly selective neural interface with the DRG without penetrating the epineurium.

Neuromusculoskeletal Modeling-Based Prostheses for Recovery After Spinal Cord Injury

Concurrent stimulation and reinforcement of motor and sensory pathways has been proposed as an effective approach to restoring function after developmental or acquired neurotrauma. This can be achieved by applying multimodal rehabilitation regimens, such as thought-controlled exoskeletons or epidural electrical stimulation to recover motor pattern generation in individuals with spinal cord injury (SCI). However, the human neuromusculoskeletal (NMS) system has often been oversimplified in designing rehabilitative and assistive devices. As a result, the neuromechanics of the muscles is seldom considered when modeling the relationship between electrical stimulation, mechanical assistance from exoskeletons, and final joint movement. A powerful way to enhance current neurorehabilitation is to develop the next generation prostheses incorporating personalized NMS models of patients. This strategy will enable an individual voluntary interfacing with multiple electromechanical rehabilitation devices targeting key afferent and efferent systems for functional improvement. This narrative review discusses how real-time NMS models can be integrated with finite element (FE) of musculoskeletal tissues and interface multiple assistive and robotic devices with individuals with SCI to promote neural restoration. In particular, the utility of NMS models for optimizing muscle stimulation patterns, tracking functional improvement, monitoring safety, and providing augmented feedback during exercise-based rehabilitation are discussed.

Fascicle specific targeting for selective peripheral nerve stimulation

OBJECTIVE

Electrical stimulation is a blunt tool for evoking neural activity. Neurons are naturally activated asynchronously and non-uniformly, whereas stimulation drives simultaneous activity within a population of cells. These differences in activation pattern can result in unintended side effects, including muddled sensory percepts and undesirable muscle contractions. These effects can be mitigated by the placement of electrodes in close approximation to nerve fibers and careful selection of the neural interface's location. This work describes the benefits of placing electrodes within specific fascicles of peripheral nerve to form selective neural interfaces for bidirectional neuroprosthetic devices.

APPROACH

Chronic electrodes were targeted to individual fascicles of the ulnar and median nerves in the forearm of four human subjects. During the surgical implant procedure, fascicles were dissected from each nerve, and functional testing was used to identify the relative composition of sensory and motor fibers within each. FAST-LIFE arrays, composed of longitudinal intrafascicular arrays and fascicular cuff electrodes, were implanted in each fascicle. The location, quality, and stimulation parameters associated with sensations evoked by electrical stimulation on these electrodes were characterized throughout the 90-180 d implant period.

MAIN RESULTS

FAST-LIFE arrays enable selective and chronic electrical stimulation of individual peripheral nerve fascicles. The quality of sensations evoked by stimulation in each fascicle is predictable and distinct; subjects reported tactile and cutaneous sensations during stimulation of sensory fascicles and deeper proprioceptive sensations during stimulation of motor fascicles. Stimulation thresholds and strength-duration time constants were typically higher within sensory fascicles.

SIGNIFICANCE

Highly selective, stable neural interfaces can be created by placing electrodes within and around single fascicles of peripheral nerves. This method enables targeting electrodes to nerve fibers that innervate a specific body region or have specific functions. Fascicle-specific interfacing techniques have broad potential to maximize the therapeutic effects of electrical stimulation in many neuromodulation applications. (Clinical Trial ID NCT02994160.).

Methodology for creating a chronic osseointegrated neural interface for prosthetic control in rabbits

BACKGROUND

Chronic stability and high degrees of selectivity are both essential but somewhat juxtaposed components for creating an implantable bi-directional PNI capable of controlling of a prosthetic limb. While the more invasive implantable electrode arrays provide greater specificity, they are less stable over time due to compliance mismatch with the dynamic soft tissue environment in which the interface is created.

NEW METHOD

This paper takes the surgical approach of transposing nerves into bone to create neural interface within the medullary canal of long bones, an osseointegrated neural interface, to provide greater stability for implantable electrodes. In this context, we describe the surgical model for transfemoral amputation with transposition of the sciatic nerve into the medullary canal in rabbits. We investigate the capacity to create a neural interface within the medullary canal histolomorphologically. In a separate proof of concept experiment, we quantify the chronic physiological capacity of transposed nerves to conduct compound nerve action potentials evoked via an Osseointegrated Neural Interface.

COMPARISON WITH EXISTING METHOD(S)

The rabbit serves as an important animal model for both amputation neuroma and osseointegration research, but is underutilized for the exploration neural interfacing in an amputation setting.

RESULTS

Our findings demonstrate that transposed nerves remain stable over 12 weeks. Creating a neural interface within the medullary canal is possible and does not impede nerve regeneration or physiological capacity.

CONCLUSIONS

This article represents the first evidence that an Osseointegrated Neural Interface can be surgically created, capable of chronic stimulation/recording from amputated nerves required for future prosthetic control.

A rat model for assessing the long-term safety and performance of peripheral nerve electrode arrays

BACKGROUND

High-resolution peripheral nerve interfaces (PNIs) can provide amputees with intuitive motor control and sensory feedback. Current PNIs are limited by early device failure and suboptimal long-term stability. The present study aims to incorporate functional assessment into an in vivo test platform to assess the long-term safety and performance of PNIs for recording and stimulation.

NEW METHODS

Utah electrode arrays (EA) were implanted in the rat sciatic nerve along with electromyography wires in the gastrocnemius and tibialis anterior. Cranial EEG screws were implanted in the somatosensory cortex for 12 weeks. Spontaneous neural activity was recorded using the implanted EA and stimulation-induced activity was monitored throughout the experiment. The impedance of each electrode was measured, and nerve function tests were conducted throughout the EA lifetime. Post-hoc safety assessments included scanning electron microscopy (SEM) of the EA and nerve histomorphometric analysis.

RESULTS

EA recordings were stable, and stimulation with EA elicited somatosensory evoked potentials and muscle contractions. Motor and sensory function tests indicated minimal deficits. Histomorphometric analysis indicated changes in nerve microstructure. SEM indicated EA-tip fracture, while lead wire breakage primarily caused device failure.

COMPARISON WITH EXISTING METHODS

We improved our prior platform with the addition of functional assessments of sensory pathways, a robust EMG array design to increase device longevity, and quantitative analysis of nerve microstructure.

CONCLUSION

We present a test platform for long-term assessment of peripheral nerve interfaces for stimulation and recording. Using this platform, we demonstrate recording and stimulation with minimal impact on nerve function, while EA lead wire breakage and tip fracture could limit long-term device use.

Human motor decoding from neural signals: a review

Many people suffer from movement disability due to amputation or neurological diseases. Fortunately, with modern neurotechnology now it is possible to intercept motor control signals at various points along the neural transduction pathway and use that to drive external devices for communication or control. Here we will review the latest developments in human motor decoding. We reviewed the various strategies to decode motor intention from human and their respective advantages and challenges. Neural control signals can be intercepted at various points in the neural signal transduction pathway, including the brain (electroencephalography, electrocorticography, intracortical recordings), the nerves (peripheral nerve recordings) and the muscles (electromyography). We systematically discussed the sites of signal acquisition, available neural features, signal processing techniques and decoding algorithms in each of these potential interception points. Examples of applications and the current state-of-the-art performance were also reviewed. Although great strides have been made in human motor decoding, we are still far away from achieving naturalistic and dexterous control like our native limbs. Concerted efforts from material scientists, electrical engineers, and healthcare professionals are needed to further advance the field and make the technology widely available in clinical use.

Highly Flexible Precisely Braided Multielectrode Probes and Combinatorics for Future Neuroprostheses

The braided multielectrode probe (BMEP) is an ultrafine microwire bundle interwoven into a precise tubular braided structure, which is designed to be used as an invasive neural probe consisting of multiple microelectrodes for electrophysiological neural recording and stimulation. Significant advantages of BMEPs include highly flexible mechanical properties leading to decreased immune responses after chronic implantation in neural tissue and dense recording/stimulation sites (24 channels) within the 100-200 μm diameter. In addition, because BMEPs can be manufactured using various materials in any size and shape without length limitations, they could be expanded to applications in deep central nervous system (CNS) regions as well as peripheral nervous system (PNS) in larger animals and humans. Finally, the 3D topology of wires supports combinatoric rearrangements of wires within braids, and potential neural yield increases. With the newly developed next generation micro braiding machine, we can manufacture more precise and complex microbraid structures. In this article, we describe the new machine and methods, and tests of simulated combinatoric separation methods. We propose various promising BMEP designs and the potential modifications to these designs to create probes suitable for various applications for future neuroprostheses.

Bionic hand as artificial organ: Current status and future perspectives

Even though the hand comprises only 1% of our body weight, about 30% of our central nervous systems (CNS) capacity is related to its control. The loss of a hand thus presents not only the loss of the most important tool allowing us to interact with our environment, but also leaves a dramatic sensory-motor deficit that challenges our CNS. Reconstruction of hand function is therefore not only an essential part of restoring body integrity and functional wholeness but also closes the loop of our neural circuits diminishing phantom sensation and neural pain. If biology fails to restore meaningful function, today we can resort to complex mechatronic replacement that have functional capabilities that in some respects even outperform biological alternatives, such as hand transplantation. As with replantation and transplantations, the challenge of bionic replacement is connecting the target with the CNS to achieve natural and intuitive control. In recent years, we have developed a number of strategies to improve neural interfacing, signal extraction, interpretation and stable mechanical attachment that are important parts of our current research. This work gives an overview of recent advances in bionic reconstruction, surgical refinements over technological interfacing, skeletal fixation, and modern rehabilitation tools that allow quick integration of prosthetic replacement.

Microneurography as a tool to develop decoding algorithms for peripheral neuro-controlled hand prostheses

BACKGROUND

The usability of dexterous hand prostheses is still hampered by the lack of natural and effective control strategies. A decoding strategy based on the processing of descending efferent neural signals recorded using peripheral neural interfaces could be a solution to such limitation. Unfortunately, this choice is still restrained by the reduced knowledge of the dynamics of human efferent signals recorded from the nerves and associated to hand movements.

FINDINGS

To address this issue, in this work we acquired neural efferent activities from healthy subjects performing hand-related tasks using ultrasound-guided microneurography, a minimally invasive technique, which employs needles, inserted percutaneously, to record from nerve fibers. These signals allowed us to identify neural features correlated with force and velocity of finger movements that were used to decode motor intentions. We developed computational models, which confirmed the potential translatability of these results showing how these neural features hold in absence of feedback and when implantable intrafascicular recording, rather than microneurography, is performed.

CONCLUSIONS

Our results are a proof of principle that microneurography could be used as a useful tool to assist the development of more effective hand prostheses.

An ASIC for Recording and Stimulation in Stacked Microchannel Neural Interfaces

This paper presents an active microchannel neural interface (MNI) using seven stacked application specific integrated circuits (ASICs). The approach provides a solution to the present problem of interconnect density in three-dimensional (3-D) MNIs. The 4 mm² ASIC is implemented in 0.35 μm high-voltage CMOS technology. Each ASIC is the base for seven microchannels each with three electrodes in a pseudo-tripolar arrangement. Multiplexing allows stimulating or recording from any one of 49 channels, across seven ASICs. Connections to the ASICs are made with a five-line parallel bus. Current controlled biphasic stimulation from 5 to 500 μA has been demonstrated with switching between channels and ASICs. The high-voltage technology gives a compliance of 40 V for stimulation, appropriate for the high impedances within microchannels. High frequency biphasic stimulation, up to 40 kHz is achieved, suitable for reversible high frequency nerve blockades. Recording has been demonstrated with mV level signals; common-mode inputs are differentially distorted and limit the CMRR to 40 dB. The ASIC has been used in vitro in conjunction with an oversize (2 mm diameter) microchannel in phosphate buffered saline, demonstrating attenuation of interference from outside the microchannel and tripolar recording of signals from within the microchannel. By using five-lines for 49 active microchannels the device overcomes limitations when connecting many electrodes in a 3-D miniaturized nerve interface.

Closed-Loop Continuous Hand Control via Chronic Recording of Regenerative Peripheral Nerve Interfaces

Loss of the upper limb imposes a devastating interruption to everyday life. Full restoration of natural arm control requires the ability to simultaneously control multiple degrees of freedom of the prosthetic arm and maintain that control over an extended period of time. Current clinically available myoelectric prostheses do not provide simultaneous control or consistency for transradial amputees. To address this issue, we have implemented a standard Kalman filter for continuous hand control using intramuscular electromyography (EMG) from both regenerative peripheral nerve interfaces (RPNI) and an intact muscle within non-human primates. Seven RPNIs and one intact muscle were implanted with indwelling bipolar intramuscular electrodes in two rhesus macaques. Following recuperations, function-specific EMG signals were recorded and then fed through the Kalman filter during a hand-movement behavioral task to continuously predict the monkey's finger position. We were able to reconstruct continuous finger movement offline with an average correlation of and a root mean squared error (RMSE) of 0.12 between actual and predicted position from two macaques. This finger movement prediction was also performed in real time to enable closed-loop neural control of a virtual hand. Compared with physical hand control, neural control performance was slightly slower but maintained an average target hit success rate of 96.70%. Recalibration longevity measurements maintained consistent average correlation over time but had a significant change in RMSE ( ). Additionally, extracted single units varied in amplitude by a factor of +18.65% and -25.85% compared with its mean. This is the first demonstration of chronic indwelling electrodes being used for continuous position control via the Kalman filter. Combining these analyses with our novel peripheral nerve interface, we believe that this demonstrates an important step in providing patients with more naturalistic control of their prosthetic limbs.