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

OBJECTIVE: As neural interfaces demonstrate success in chronic applications, a novel class of reshaping electrodes with patterned regions of stiffness will enable application to a widening range of anatomical locations. Patterning stiff regions and flexible regions of the electrode enables nerve reshaping while accommodating anatomical constraints of various implant locations ranging from peripheral nerves to spinal and autonomic plexi. APPROACH: Introduced is a new composite electrode enabling patterning of regions of various electrode mechanical properties. The initial demonstration of the composite's capability is the composite flat interface nerve electrode (C-FINE). The C-FINE is constructed from a sandwich of patterned PEEK within layers of pliable silicone. The shape of the PEEK provides a desired pattern of stiffness: stiff across the width of the nerve to reshape the nerve, but flexible along its length to allow for bending with the nerve. This is particularly important in anatomical locations near joints or organs, and in constrained compartments. We tested pressure and volume design constraints in vitro to verify that the C-FINE can attain a safe cuff-to-nerve ratio (CNR) without impeding intraneural blood flow. We measured nerve function as well as nerve and axonal morphology following 3 month implantation of the C-FINE without wires on feline peripheral nerves in anatomically constrained areas near mobile joints and major blood vessels in both the hind and fore limbs. MAIN RESULTS: In vitro inflation tests showed effective CNRs (1.93 ± 0.06) that exceeded the industry safety standard of 1.5 at an internal pressure of 20 mmHg. This is less than the 30 mmHg shown to induce loss of conduction or compromise blood flow. Implanted cats showed no changes in physiology or electrophysiology. Behavioral signs were normal suggesting healthy nerves. Motor nerve conduction velocity and compound motor action potential did not change significantly between implant and explant (p > 0.15 for all measures). Axonal density and myelin sheath thickness was not significantly different within the electrode compared to sections greater than 2 cm proximal to implanted cuffs (p > 0.14 for all measures). SIGNIFICANCE: We present the design and verification of a novel nerve cuff electrode, the C-FINE. Laminar manufacturing processes allow C-FINE stiffness to be configured for specific applications. Here, the central region in the configuration tested is stiff to reshape or conform to the target nerve, while edges are highly flexible to bend along its length. The C-FINE occupies less volume than other NCEs, making it suitable for implantation in highly mobile locations near joints. Design constraints during simulated transient swelling were verified in vitro. Maintenance of nerve health in various challenging anatomical locations (sciatic and median/ulnar nerves) was verified in a chronic feline model in vivo.

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

OBJECTIVE: Electrical stimulation has been shown effective in restoring basic lower extremity motor function in individuals with paralysis. We tested the hypothesis that a flat interface nerve electrode (FINE) placed around the human tibial or common peroneal nerve above the knee can selectively activate each of the most important muscles these nerves innervate for use in a neuroprosthesis to control ankle motion. APPROACH: During intraoperative trials involving three subjects, an eight-contact FINE was placed around the tibial and/or common peroneal nerve, proximal to the popliteal fossa. The FINE's ability to selectively recruit muscles innervated by these nerves was assessed. Data were used to estimate the potential to restore active plantarflexion or dorsiflexion while balancing inversion and eversion using a biomechanical simulation. MAIN RESULTS: With minimal spillover to non-targets, at least three of the four targets in the tibial nerve, including two of the three muscles constituting the triceps surae, were independently and selectively recruited in all subjects. As acceptable levels of spillover increased, recruitment of the target muscles increased. Selective activation of muscles innervated by the peroneal nerve was more challenging. SIGNIFICANCE: Estimated joint moments suggest that plantarflexion sufficient for propulsion during stance phase of gait and dorsiflexion sufficient to prevent foot drop during swing can be achieved, accompanied by a small but tolerable inversion or eversion moment.

Selective stimulation of the human femoral nerve with a flat interface nerve electrode.

In humans, we tested the hypothesis that a flat interface nerve electrode (FINE) placed around the femoral nerve trunk can selectively stimulate each muscle the nerve innervates. In a series of intraoperative trials during routine vascular surgeries, an eight-contact FINE was placed around the femoral nerve between the inguinal ligament and the first nerve branching point. The capability of the FINE to selectively recruit muscles innervated by the femoral nerve was assessed with electromyograms (EMGs) of the twitch responses to electrical stimulation. At least four of the six muscles innervated by the femoral nerve were independently and selectively recruited in all subjects. Of these, at least one muscle was a hip flexor and at least two were knee extensors. Results from the intraoperative experiments were used to estimate the potential for the electrode to restore knee extension and hip flexion through functional electrical stimulation. Normalized EMGs and biomechanical simulations were used to estimate joint moments and functional efficacy. Estimated knee extension moments exceed the threshold required for the sit-to-stand transition.

Intraoperative evaluation of the spiral nerve cuff electrode on the femoral nerve trunk.

Evaluation of the Case Western Reserve University spiral nerve cuff electrode on the femoral nerve trunk was performed intraoperatively in four subjects undergoing femoral-popliteal bypass surgery. The threshold, nerve size and selective activation capabilities of the electrode were examined. The activation thresholds for the first muscle to be recruited were 6.3, 9, 10.6, and 37.4 nC with pulse amplitudes ranging from 0.3 to 1 mA. The femoral nerve was found to have an elliptical cross-section with a major axis average length of 9 mm (8-12 mm) and a minor axis length of 1.5 mm. In all four subjects selective activation of the sartorius was obtained. In two subjects, the rectus femoris could also be selectively activated and in one subject the vastus medialis was selectively activated. Each electrode had four independent contacts that were evaluated separately. Small air bubbles were formed in the space over some contacts, preventing stimulation. This occurred in one contact in each electrode, leaving three effective stimulation channels. This issue has been corrected for future studies.

Chronic stability and selectivity of four-contact spiral nerve-cuff electrodes in stimulating the human femoral nerve.

This study describes the stability and selectivity of four-contact spiral nerve-cuff electrodes implanted bilaterally on distal branches of the femoral nerves of a human volunteer with spinal cord injury as part of a neuroprosthesis for standing and transfers. Stimulation charge threshold, the minimum charge required to elicit a visible muscle contraction, was consistent and low (mean threshold charge at 63 weeks post-implantation: 23.3 +/- 8.5 nC) for all nerve-cuff electrode contacts over 63 weeks after implantation, indicating a stable interface with the peripheral nervous system. The ability of individual nerve-cuff electrode contacts to selectively stimulate separate components of the femoral nerve to activate individual heads of the quadriceps was assessed with fine-wire intramuscular electromyography while measuring isometric twitch knee extension moment. Six of eight electrode contacts could selectively activate one head of the quadriceps while selectively excluding others to produce maximum twitch responses of between 3.8 and 8.1 N m. The relationship between isometric twitch and tetanic knee extension moment was quantified, and selective twitch muscle responses scaled to between 15 and 35 N m in tetanic response to pulse trains with similar stimulation parameters. These results suggest that this nerve-cuff electrode can be an effective and chronically stable tool for selectively stimulating distal nerve branches in the lower extremities for neuroprosthetic applications.

Study of percolation and clustering in supercritical water-CO2 mixtures.

The microscopic structure of supercritical water-CO(2) mixture is investigated by neutron diffraction experiments exploiting the isotopic HD substitution. The investigated water reach mixtures are in the liquidlike region of the phase diagram, according to the behavior of the radial distribution functions, yet a reduction of the average number of hydrogen bonds, compared to equivalent states of pure water, is found. As a consequence, the average dimension of water clusters is reduced and the system stays below the percolation threshold. These results, along with the shift of the main peaks of the site-site radial distribution functions, suggest that the excess volume in these supercritical mixtures is likely associated with the CO(2) solvation shell.

Hybrid orthosis system with a variable hip coupling mechanism.

Existing reciprocating gait orthoses, to help restore gait to individuals with paraplegia, have a fixed 1:1 hip flexion/extension coupling ratios (FECR), limiting stride length and gait speed. The purpose of this study was to develop a hip reciprocating mechanism for the hybrid orthosis system that is capable of variable hip FECR. The design of the new variable hip reciprocating mechanism incorporates a hydraulic system which utilizes solenoid valves to control coupling between cylinders linked to each hip joint of the orthosis. A specific set of valves are pulsed to achieve continual variable hip coupling. It was shown that piston velocity was inversely proportional to pulse width and also dependent on pulsing frequency. Internal losses in the hydraulic hip reciprocating mechanism occur primarily in the cylinders. Feedback control will be achieved with a dual layer gait event detector consisting of a fuzzy inference system and a set of supervisory rules.

Pressure-induced formation of diblock copolymer "micelles" in supercritical fluids. A combined study by small angle scattering experiments and mean-field theory. I. The critical micellization density concept.

We developed a simple mean-field theory to describe polymer and AB diblock copolymer phase separation in supercritical (SC) fluids. The highly compressible SC fluid has been described by using a phenomenological hole theory, properly extended to consider the solvent/polymer/vacancy pseudoternary mixture. The model has been applied to describe the phase behavior of AB-diblock copolymers under the assumption of a strong solvent selectivity for just one copolymer chain. In our model the solvent selectivity is a strong function of the external pressure because in compressible fluids vacancies reduce the number of favorable solvent-polymer contacts. The combined effect of the pressure on the average solvent quality and selectivity for a single polymer chain makes the phase behavior of a diblock copolymer in SC fluids quite complex. Small angle neutron and x-ray scattering (SANS and SAXS) measurements have been performed on SC-CO2 solutions of different AB-diblock copolymers containing a perfluorinated chain. The data obtained over a wide range of pressure and temperature confirm our theoretical predictions.

Pressure-induced formation of diblock copolymer "micelles" in supercritical fluids. A combined study by small angle scattering experiments and mean-field theory. II. Kinetics of the unimer-aggregate transition.

We developed a simple time-dependent mean-field theory to describe the phase separation kinetics of either homopolymers or AB-diblock copolymers in supercritical (SC) fluids. The model, previously used to describe the phase behavior of AB-block copolymers under the assumption of strong solvent selectivity for just one copolymer chain, has been extended to study the kinetics of the phase separation process. Time resolved small angle x-ray scattering (TR-SAXS) measurements have been performed on different AB-diblock copolymers containing a perfluorinated chain and dissolved in SC-CO2. The data obtained over a wide range of pressure and temperature confirm our theoretical predictions. Particularly interesting is the presence of two relaxation frequencies for the homogeneous solution --> spherical aggregate transition, where the two relaxation processes depend on the depth of the pressure jump and on temperature. The whole phenomenon could be explained as an initial SC solvent/polymer phase separation followed by a slow reorientation process to form spherical aggregates driven by the copolymer solvophilic moiety.

The feasibility of a functional neuromuscular stimulation powered mechanical gait orthosis with coordinated joint locking.

The purpose of this study was to examine the feasibility of a hybrid orthosis for walking after spinal cord injury (SCI) that coordinates the locking and unlocking of knee and ankle joints of a reciprocating gait orthosis (RGO), while injecting propulsive forces and controlling unlocked joints with functional neuromuscular stimulation (FNS). The effectiveness of the hybrid system relative to gait stability and posture were determined in this simulation study. A three-dimensional computer model of a hybrid orthosis system (HOS) combining FNS with a RGO incorporating feedback control of muscle activation and coordinated joint locking was developed in Working Model 3D. The simulated hybrid orthosis system achieved gait speeds, stride lengths, and cadences of 0.51 +/- 0.03 m/s, 0.85 +/- 0.04 m, and 72 +/- 4 steps/min respectively, exceeding the performance of other hybrid systems. Forward trunk tilt was found to be necessary during initial step from standing and pro-swing, but posture and stability were significantly improved over FNS-only systems. The results of the model shows that a HOS that coordinates knee and ankle joint locking with electrical stimulation to the paralyzed muscles holds significant advantages over brace- and FNS-only walking systems in terms of enhanced trunk stability and posture.

SAXS investigation on aggregation phenomena in supercritical CO2.

Synchrotron Small-Angle X-Ray scattering (SAXS) measurements on aggregate formation of a Polyvinyl acetate- b-Perfluoro octyl acrylate (PVAc- b-PFOA) block copolymer in supercritical CO(2) are here reported. Experiments were carried out for a series of different thermodynamic conditions, changing the solvent density by profiling both the pressure at constant temperature and the temperature at constant pressure. This block copolymer and in general fluorocarbon-hydrocarbon di-blocks form aggregates depending on the value of CO(2) density. A sharp transition between monomers dissolved as random coils and micelles characterized by a solvophilic shell and a solvophobic core occurs when the CO(2) density reaches a critical value. Results of critical micellization density (CMD) derived from pressure and temperature ramps experiment along with the comparison with previous SANS results are here reported to give additional experimental support to the solvent density-driven aggregation process.

Selectivity of intramuscular stimulating electrodes in the lower limbs.

Intramuscular (IM) electrodes have been used safely and effectively for decades to activate paralyzed muscles in neuroprosthetic systems employing functional electrical stimulation (FES). However, the response to stimulation delivered by these and any type of electrode can be limited by a phenomenon known as spillover, in which the stimulus intended to produce a contraction in a particular muscle inadvertently activates another muscle, causes adverse sensation, or triggers undesired reflexes. The purpose of this retrospective study was to determine the selectivity of monopolar intramuscular stimulating electrodes implanted in the lower limbs of individuals with motor and sensory complete paraplegia secondary to spinal cord injury (SCI) and to catalog the most common electrode spillover patterns. The performance records of 602 electrodes from 10 subjects who participated in a program of standing and walking with FES in our laboratory over the past decade were examined. Sixty percent (358) of these electrodes were "stable" (i.e., stimulated responses were consistent during the first 6 months postimplant), and 32% of all stable electrodes (113) exhibited spillover as noted in clinical and laboratory records. Common spillover patterns for eight muscle groups were tabulated and analyzed in terms of their functional implications. The beneficial (activation of synergistic muscles) or deleterious (activation of compromising reflexes, antagonists, or adverse sensation) effects of spillover were highly context dependent, with several potentially useful spillover patterns in certain phases of gait becoming undesirable and limiting in others. Knowledge of the selectivity of intramuscular electrodes and the patterns of spillover they exhibit should guide surgeons and rehabilitationists installing lower-limb neuroprostheses during the implantation process and allow them to better predict the ultimate functional usefulness of the electrodes they choose.

Effects of stimulated hip extension moment and position on upper-limb support forces during FNS-induced standing--a technical note.

This study explores the effects of active hip extension moment produced by electrical stimulation on the support forces the arms must exert through an assistive device during quiet erect standing with functional neuromuscular stimulation (FNS) in individuals with spinal cord injuries (SCI). A static sagittal plane biomechanical model of human standing was developed to predict the effects of stimulated hip extension moment and sagittal plane hip angle on the arm support necessary to maintain an upright posture. Two individuals with complete thoracic SCI were then tested while they stood with continuous stimulation to the knee and trunk extensors. The steady-state active extension moment exerted at the hip was varied by activating different combinations of hip extensor muscles with continuous stimulation while steady-state support forces applied to the arms and feet during standing were measured. The steady-state support forces imposed on the arms during quiet standing decrease with increased stimulated hip extension moment and are highly dependent upon hip flexion angle, as predicted by the biomechanical simulations. Experimentally, the combination of gluteus maximus and semimembranosus stimulation produced three times more steady-state hip extension moment than did stimulation of the gluteus maximus and adductor magnus. This resulted in a ten-fold decrease in body weight supported on the arms. More vertical postures (smaller hip flexion angles) improve the effectiveness of the hip extensor muscles in reducing the support forces placed on the arms. A single Newton-meter of stimulated hip extension moment with the hips fixed at 5 degrees of flexion results in almost five times the reduction in arm support forces as with the hips at 20 degrees. To minimize the forces applied by the arms on an assistive device for support while standing with FNS, these preliminary results suggest that (1) efforts should be made to assume the most erect postures possible and (2) muscles and stimulation paradigms that maximize active hip extension moment should be chosen.

A reusable, self-adhesive electrode for intraoperative stimulation in the lower limbs.

A suction-based stimulating electrode was designed and fabricated to allow intraoperative testing of lower-limb muscles during routinely scheduled surgical procedures. The suction device can adhere to a small exposure of muscle surface with reproducible contact forces and can maintain its geometric relationship to the underlying tissue for sufficient time to grade the resulting muscle contraction before removal and repositioning. When operated with a 10-cc syringe, the device can generate between 0 and 23 N of contact force; correlation between measured contact forces and those analytically predicted was 0.989. Preliminary animal testing indicates that the reusable device maintains its position over the nerve entry point even during vigorous active contractions of the stimulated muscle. Thus, it may be a valuable useful tool for locating the optimal site for a permanent electrode for functional electrical stimulation (FES) applications, as well as an ideal means of providing accurate and repeatable stimulation in various locations.

Modeling the postural disturbances caused by upper extremity movements.

This paper describes the design, validation, and application of a dynamic, three-dimensional (3-D) model of the upper extremity for the purpose of estimating postural disturbances generated by movements of the arms. The model consists of two links representing the upper and lower arms, with the shoulder and elbow modeled as gimbal joints to allow three rotational degrees of freedom. With individualized segment inertial parameters based on anthropometric measurements, the model performs inverse dynamic analysis of recorded arm movements to calculate reaction forces and moments acting on the body at the shoulder in three dimensions. The method was validated by comparing the output of the model to estimates obtained from ground reaction loads during stereotypical and free form unilateral movements at various velocities and with different loads carried by human subjects while seated on biomechanical force platforms. The correlation between predicted and measured reaction forces and moments was very good under all conditions and across all subjects, with average rms errors less than 8% of measured peak-to-peak values. The model was then applied to bimanual activities representative of functional movements that would typically be performed while standing at a counter. The resulting estimates were consistent and adequate for the purpose of evaluating postural disturbances caused by upper extremity movements.

Surgical technique for installing an eight-channel neuroprosthesis for standing.

A standardized surgical procedure to implant an eight-channel functional neuromuscular stimulation system in the lower extremities for standing, exercise, and transfers for individuals with spinal cord injury has been developed. The implanted components include: (1) one eight-channel receiver-stimulator, (2) epimysial electrodes, (3) intramuscular electrodes, and (4) inline connectors. The development process included identifying the target muscle set for electrode placement and the corresponding surgical approaches, determining the stages of the surgical procedure, and assessing the effectiveness and stability of the implanted neuroprosthesis. The bilateral muscle set consists of the vastus lateralis, the gluteus maximus, the semimembranosus, and the erector spinae. Surgical approaches to the nerve entry points were developed through a series of cadaveric studies and intraoperative tests. Electrode placement is related to bony landmarks and based on standard orthopaedic approaches. The components of the neuroprosthesis are installed in one surgical session, with three stages. This procedure has been applied successfully in seven individuals, resulting in strong, isolated stimulated contractions adequate to raise and lower the body, maintain standing with a walker, and perform pivot transfers. The standardized surgical procedure is repeatable and teachable and will be used in upcoming multicenter clinical trials of the implanted neuroprosthesis.

Architecture of the rectus abdominis, quadratus lumborum, and erector spinae.

Quantitative descriptions of muscle architecture are needed to characterize the force-generating capabilities of muscles. This study reports the architecture of three major trunk muscles: the rectus abdominis, quadratus lumborum, and three columns of the erector spinae (spinalis thoracis, longissimus thoracis and iliocostalis lumborum). Musculotendon lengths, muscle lengths, fascicle lengths, sarcomere lengths, pennation angles, and muscle masses were measured in five cadavers. Optimal fascicle lengths (the fascicle length at which the muscle generates maximum force) and physiologic cross-sectional areas (the ratio of muscle volume to optimal fascicle length) were computed from these measurements. The rectus abdominis had the longest fascicles of the muscles studied, with a mean (S.D.) optimal fascicle length of 28.3 (4.2)cm. The three columns of the erector spinae had mean optimal fascicle lengths that ranged from 6.4 (0.6)cm in the spinalis thoracis to 14.2 (2.1)cm in the iliocostalis lumborum. The proximal portion of the quadratus lumborum had a mean optimal fascicle length of 8.5 (1.5)cm and the distal segment of this muscle had a mean optimal fascicle length of 5.6 (0.9)cm. The physiologic cross-sectional area of the rectus abdominis was 2.6 (0.9)cm(2), the combined physiologic cross-sectional area of the erector spinae was 11.6 (1.8)cm(2), and the physiologic cross-sectional area of the quadratus lumborum was 2.8 (0.5)cm(2). These data provide the basis for estimation of the force-generating potential of these muscles.

Preliminary performance of a surgically implanted neuroprosthesis for standing and transfers--where do we stand?

This paper describes the preliminary performance of a surgically implanted neuroprosthesis for standing and transfers after spinal cord injury (SCI) in an initial group of 12 volunteers with longstanding paralysis. The CWRU/VA standing neuroprosthesis consists of an 8-channel implanted receiver-stimulator, epimysial and surgically implanted intramuscular electrodes, and a programmable wearable external controller. After reconditioning exercise and rehabilitation with the system, most individuals with paraplegia or low tetraplegia were able to stand, transfer, and release one hand from a support device to manipulate objects in the environment or to perform swing-to ambulation in a walker. The effort and assistance required for transfers were reduced for users with mid-level tetraplegia, although the maneuvers were not independent. Neuroprosthesis users with tetraplegia and paraplegia alike benefited from the improvements in their general health derived from exercise, including reduced risk of decubiti and self-reported modulation of spasticity. Stimulated responses are stable and sufficiently strong for function, and implanted components are reliable with a 90% probability of epimysial electrode survival at 4 years post-implant. The techniques employed are repeatable and teachable, and suitable for multi-center clinical trial.