Neural interfaces for regenerated nerve stimulation and recording

Compliant peripheral nerve interfaces

Peripheral nerve interfaces (PNIs) record and/or modulate neural activity of nerves, which are responsible for conducting sensory-motor information to and from the central nervous system, and for regulating the activity of inner organs. PNIs are used both in neuroscience research and in therapeutical applications such as precise closed-loop control of neuroprosthetic limbs, treatment of neuropathic pain and restoration of vital functions (e.g. breathing and bladder management). Implantable interfaces represent an attractive solution to directly access peripheral nerves and provide enhanced selectivity both in recording and in stimulation, compared to their non-invasive counterparts. Nevertheless, the long-term functionality of implantable PNIs is limited by tissue damage, which occurs at the implant-tissue interface, and is thus highly dependent on material properties, biocompatibility and implant design. Current research focuses on the development of mechanically compliant PNIs, which adapt to the anatomy and dynamic movements of nerves in the body thereby limiting foreign body response. In this paper, we review recent progress in the development of flexible and implantable PNIs, highlighting promising solutions related to materials selection and their associated fabrication methods, and integrated functions. We report on the variety of available interface designs (intraneural, extraneural and regenerative) and different modulation techniques (electrical, optical, chemical) emphasizing the main challenges associated with integrating such systems on compliant substrates.

Cell guidance on nanogratings: a computational model of the interplay between PC12 growth cones and nanostructures

Background

Recently, the effects of nanogratings have been investigated on PC12 with respect to cell polarity, neuronal differentiation, migration, maturation of focal adhesions and alignment of neurites.

Methodology/Principal Findings

A synergistic procedure was used to study the mechanism of alignment of PC12 neurites with respect to the main direction of nanogratings. Finite Element simulations were used to qualitatively assess the distribution of stresses at the interface between non-spread growth cones and filopodia, and to study their dependence on filopodial length and orientation. After modelling all adhesions under non-spread growth cone and filopodial protrusions, the values of local stress maxima resulted from the length of filopodia. Since the stress was assumed to be the main triggering cause leading to the increase and stabilization of filopodia, the position of the local maxima was directly related to the orientation of neurites. An analytic closed form equation was then written to quantitatively assess the average ridge width needed to achieve a given neuritic alignment (R² = 0.96), and the alignment course, when the ridge depth varied (R² = 0.97). A computational framework was implemented within an improved free Java environment (CX3D) and in silico simulations were carried out to reproduce and predict biological experiments. No significant differences were found between biological experiments and in silico simulations (alignment, p = 0.3571; tortuosity, p = 0.2236) with a standard level of confidence (95%).

Conclusions/Significance

A mechanism involved in filopodial sensing of nanogratings is proposed and modelled through a synergistic use of FE models, theoretical equations and in silico simulations. This approach shows the importance of the neuritic terminal geometry, and the key role of the distribution of the adhesion constraints for the cell/substrate coupling process. Finally, the effects of the geometry of nanogratings were explicitly considered in cell/surface interactions thanks to the analytic framework presented in this work.

Modality-specific axonal regeneration: toward selective regenerative neural interfaces

Regenerative peripheral nerve interfaces have been proposed as viable alternatives for the natural control of robotic prosthetic devices. However, sensory and motor axons at the neural interface are of mixed sub-modality types, which difficult the specific recording from motor axons and the eliciting of precise sensory modalities through selective stimulation. Here we evaluated the possibility of using type specific neurotrophins to preferentially entice the regeneration of defined axonal populations from transected peripheral nerves into separate compartments. Segregation of mixed sensory fibers from dorsal root ganglion neurons was evaluated in vitro by compartmentalized diffusion delivery of nerve growth factor (NGF) and neurotrophin-3 (NT-3), to preferentially entice the growth of TrkA+ nociceptive and TrkC+ proprioceptive subsets of sensory neurons, respectively. The average axon length in the NGF channel increased 2.5-fold compared to that in saline or NT-3, whereas the number of branches increased threefold in the NT-3 channels. These results were confirmed using a 3D “Y”-shaped in vitro assay showing that the arm containing NGF was able to entice a fivefold increase in axonal length of unbranched fibers. To address if such segregation can be enticed in vivo, a “Y”-shaped tubing was used to allow regeneration of the transected adult rat sciatic nerve into separate compartments filled with either NFG or NT-3. A significant increase in the number of CGRP+ pain fibers were attracted toward the sural nerve, while N-52+ large-diameter axons were observed in the tibial and NT-3 compartments. This study demonstrates the guided enrichment of sensory axons in specific regenerative chambers, and supports the notion that neurotrophic factors can be used to segregate sensory and perhaps motor axons in separate peripheral interfaces.

Control of neural interfacing in peripheral nerves through regenerative molecular guidance

Regenerative peripheral nerve interfaces have been proposed as viable alternatives for the natural control of robotic prosthetic devices. However, random axonal pathfinding during peripheral nerve regeneration leads to mixed populations of sensory and motor neurons at the electrode interfaces preventing the precise identification of the modality nature of the recorded action potentials; motor or specific sensory sub-modalities. This study present evidence that supports the notion that type-specific neurotrophins can be used to preferentially entice and segregate the growth of defined axonal populations from transected peripheral nerves. Segregation of mixed sensory fibers from dorsal root ganglion neurons was evaluated in vitro by compartmentalized diffusion delivery of nerve growth factor (NGF) and neurotrophin-3 (NT-3), to preferentially entice the growth of trkA+ nociceptive and trkC+ proprioceptive subsets of sensory neurons, respectively.

Early interfaced neural activity from chronic amputated nerves

Direct interfacing of transected peripheral nerves with advanced robotic prosthetic devices has been proposed as a strategy for achieving natural motor control and sensory perception of such bionic substitutes, thus fully functionally replacing missing limbs in amputees. Multi-electrode arrays placed in the brain and peripheral nerves have been used successfully to convey neural control of prosthetic devices to the user. However, reactive gliosis, micro hemorrhages, axonopathy and excessive inflammation currently limit their long-term use. Here we demonstrate that enticement of peripheral nerve regeneration through a non-obstructive multi-electrode array, after either acute or chronic nerve amputation, offers a viable alternative to obtain early neural recordings and to enhance long-term interfacing of nerve activity. Non-restrictive electrode arrays placed in the path of regenerating nerve fibers allowed the recording of action potentials as early as 8 days post-implantation with high signal-to-noise ratio, as long as 3 months in some animals, and with minimal inflammation at the nerve tissue-metal electrode interface. Our findings suggest that regenerative multi-electrode arrays of open design allow early and stable interfacing of neural activity from amputated peripheral nerves and might contribute towards conveying full neural control and sensory feedback to users of robotic prosthetic devices.

On the use of longitudinal intrafascicular peripheral interfaces for the control of cybernetic hand prostheses in amputees

Significant strides have been recently made to develop highly sensorized cybernetic prostheses aimed at restoring sensorimotor limb functions to those who have lost them because of a traumatic event (amputation). In these cases, one of the main goals is to create a bidirectional link between the artificial devices (e.g., robotic hands, arms, or legs) and the nervous system. Several human-machine interfaces (HMIs) are currently used to this aim. Among them, interfaces with the peripheral nervous system and in particular longitudinal intrafascicular electrodes can be a promising solution able to improve the current situation. In this paper, the potentials and limits of the use of this interface to control robotic devices are presented. Specific information is provided on: 1) the neurophysiological bases for the use peripheral nerve interfaces; 2) a comparison of the potentials of the different peripheral neural interfaces; 3) the possibility of extracting and appropriately interpreting the neural code for motor commands and of delivering sensory feedback by stimulating afferent fibers by using longitudinal intrafascicular electrodes; 4) a preliminary comparative analysis of the performance of this approach with the ones of others HMIs; 5) the open issues which have to be addressed for a chronic usability of this approach.

BioMEMS -Advancing the Frontiers of Medicine

Biological and medical application of micro-electro-mechanical-systems (MEMS) is currently seen as an area of high potential impact. Integration of biology and microtechnology has resulted in the development of a number of platforms for improving biomedical and pharmaceutical technologies. This review provides a general overview of the applications and the opportunities presented by MEMS in medicine by classifying these platforms according to their applications in the medical field.

Improved long-term recording of nerve signal by modified intrafascicular electrodes in rabbits

Methods for long-term recording of peripheral nerve activity via intrafascicular electrodes have not been optimized. We compared the long-term functionality of custom-made 95%Pt/5%Ir intrafascicular electrodes containing a proximal spring-like structure to that of conventional straight electrodes. The modified electrode was implanted into the sciatic nerve fascicle of a random hind limb in 14 rabbits for 9 months. A conventional electrode was implanted in the opposite hind limb as a control. Orthodromic and antidromic nerve potentials were sampled and analyzed monthly. Latency, amplitude, and nerve conduction velocity of electrical signals were generally similar within the modified group and straight control group at different time intervals (P > 0.05). However, at the conclusion of the study period, the modified electrode group had a greater number of functioning electrodes (P < 0.05) and a greater total functioning electrode time (P = 0.006). Intrafascicular electrodes with a spring-like structure demonstrated superior potential for long-term electrophysiological monitoring over straight electrodes.

Residual motor signal in long-term human severed peripheral nerves and feasibility of neural signal-controlled artificial limb

PURPOSE

The residual motor pathways after amputation have not been fully elucidated. We sampled potentials from peripheral nerve stumps with intrafascicular electrodes to study residual motor transmission and explore the feasibility of nerve signal-controlled artificial limbs.

METHODS

Six intrafascicular electrodes were inserted into the ulnar, radial, and median nerves in the stump of an amputee. An electrode was placed outside the fascicle as a reference. Potentials from 4 of the 6 electrodes per trial were monitored using a 4-channel electromyogram machine, and 32 groups of electrophysiologic tests were conducted under volitional control. Actions included finger extension and flexion, forearm pronation and supination, and wrist extension and flexion. Each action was carried out with light, intermediate, and full efforts. Then, 2 of 6 electrodes randomly chosen per trial were interfaced to a nerve signal-controlled artificial limb. Finger extension and flexion of the prosthesis were tested under volitional control.

RESULTS

The volitional motor nerve potentials uniquely associated with the missing limb were recorded successfully with intrafascicular electrodes. The signal amplitude from the radial nerve was 5.5 microV +/- 0.8 (mean +/- SD), which was greater than the amplitudes from the ulnar (2.5 microV +/- 0.4) and median (2.2 microV +/- 0.3) nerves. Under volitional control of the subject, finger extension of the artificial limb was triggered by the radial nerve signal, but the remaining actions were unsuccessful.

CONCLUSIONS

The long-term amputee was able to generate motor neuron activity related to phantom limb movement. Intrafascicular electrodes can be used to monitor residual motor nerve activity in the stump, and the amplitude may predict successful control of artificial limbs.

Brain-Controlled Interfaces: Movement Restoration with Neural Prosthetics

Brain-controlled interfaces are devices that capture brain transmissions involved in a subject's intention to act, with the potential to restore communication and movement to those who are immobilized. Current devices record electrical activity from the scalp, on the surface of the brain, and within the cerebral cortex. These signals are being translated to command signals driving prosthetic limbs and computer displays. Somatosensory feedback is being added to this control as generated behaviors become more complex. New technology to engineer the tissue-electrode interface, electrode design, and extraction algorithms to transform the recorded signal to movement will help translate exciting laboratory demonstrations to patient practice in the near future.

A critical review of interfaces with the peripheral nervous system for the control of neuroprostheses and hybrid bionic systems

Considerable scientific and technological efforts have been devoted to develop neuroprostheses and hybrid bionic systems that link the human nervous system with electronic or robotic prostheses, with the main aim of restoring motor and sensory functions in disabled patients. A number of neuroprostheses use interfaces with peripheral nerves or muscles for neuromuscular stimulation and signal recording. Herein, we provide a critical overview of the peripheral interfaces available and trace their use from research to clinical application in controlling artificial and robotic prostheses. The first section reviews the different types of non-invasive and invasive electrodes, which include surface and muscular electrodes that can record EMG signals from and stimulate the underlying or implanted muscles. Extraneural electrodes, such as cuff and epineurial electrodes, provide simultaneous interface with many axons in the nerve, whereas intrafascicular, penetrating, and regenerative electrodes may contact small groups of axons within a nerve fascicle. Biological, technological, and material science issues are also reviewed relative to the problems of electrode design and tissue injury. The last section reviews different strategies for the use of information recorded from peripheral interfaces and the current state of control neuroprostheses and hybrid bionic systems.

The measurement of compound neural action potential in sciatic nerve using microelectrode array

As a method of observing regeneration of damaged nerves, research is being conducted on analyzing the electric signals of nerve fibers that are damaged and regenerating by implanting a microelectrode array between those nerves. Microelectrode arrays possess high impedance and a unique phase characteristic according to their structural features, thus it requires a phase linearity test and an impedance test to prevent neural signal distortion. Therefore, this paper analyzes the features of microelectrode array and designs a bioamplifier. We also measured signals from sciatic nerves in rats with microelectrode array.

The measurement of compound neural action potential in sciatic nerve using microelectrode array

As a method of observing regeneration of damaged nerves, research is being conducted on analyzing the electric signals of nerve fibers that are damaged and regenerating by implanting a microelectrode array between those nerves. Microelectrode arrays possess high impedance and a unique phase characteristic according to their structural features,thus it requires a phase linearity test and an impedance test to prevent neural signal distortion.Therefore, this paper analyzes the features of microelectrode array and designs a bioamplifier. We also measured signals from sciatic nerves in rats with microelectrode array.

Effects of Short-Term Training on Sensory and Motor Function in Severed Nerves of Long-Term Human Amputees

Much has been studied and written about plastic changes in the CNS of humans triggered by events such as limb amputation. However, little is known about the extent to which the original pathways retain residual function after peripheral amputation. Our earlier, acute study on long-term amputees indicated that central pathways associated with amputated peripheral nerves retain at least some sensory and motor function. The purpose of the present study was to determine if these functional connections would be strengthened or improved with experience and training over several days time. To do this, electrodes were implanted within fascicles of severed nerves of long-term human amputees to evaluate the changes in electrically evoked sensations and volitional motor neuron activity associated with attempted phantom limb movements. Nerve stimulation consistently resulted in discrete, unitary, graded sensations of touch/pressure and joint-position sense. There was no significant change in the values of stimulation parameters required to produce these sensations over time. Similarly, while the amputees were able to improve volitional control of motor neuron activity, the rate and pattern of change was similar to that seen with practice in normal individuals on motor tasks. We conclude that the central plasticity seen after amputation is most likely primarily due to unmasking, rather than replacement, of existing synaptic connections. These results also have implications for neural control of prosthetic limbs.

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

PURPOSE

It is not known whether motor and sensory pathways associated with a missing or denervated limb remain functionally intact over periods of many months or years after amputation or chronic peripheral nerve transection injury. We examined the extent to which activity on chronically severed motor nerve fibers could be controlled by human amputees and whether distally referred tactile and proprioceptive sensations could be induced by stimulation of sensory axons in the nerve stumps.

METHODS

Amputees undergoing elective stump procedures were invited to participate in this study. Longitudinal intrafascicular electrodes were threaded percutaneously and implanted in severed nerves of human amputees. The electrodes were interfaced to an amplifier and stimulator system controlled by a laptop computer. Electrophysiologic tests were conducted for 2 consecutive days after recovery from the surgery.

RESULTS

It was possible to record volitional motor nerve activity uniquely associated with missing limb movements. Electrical stimulation through the implanted electrodes elicited discrete, unitary, graded sensations of touch, joint movement, and position, referring to the missing limb.

CONCLUSIONS

These findings indicate that both central and peripheral motor and somatosensory pathways retain significant residual connectivity and function for many years after limb amputation. This implies that peripheral nerve interfaces could be used to provide amputees with prosthetic limbs that have more natural feel and control than is possible with current myoelectric and body-powered control systems.