Key points

An injury to the spinal cord can disrupt communications between the brain and body, leading to a loss of control over otherwise intact neuromuscular systems. By taking advantage of these intact neuromuscular systems, several neuroprostheses have been developed to restore functions through functional electrical stimulation (FES) of the central and peripheral nervous system. Neuroprostheses using FES to control the paralyzed muscles may prevent many secondary medical complications and improve functional independence by providing a means to exercise and negotiate physical barriers. Improvements in multiple body systems and functions have been reported through the use of FES, and they are discussed in this article. These devices range in complexity and include components such as power supplies (which may be completely external to the body or implanted and recharged with radio frequency waves), a control circuit (ie, the brains of the device), lead wires, connectors, external braces, and sensors. This article describes the basic properties of the electrodes, the current FES system being developed in research and in clinical practice, and the future of these devices.

The basic properties of electrodes for nerve stimulation

In neuroprostheses, electrodes are the interface between the external circuitry and the tissue, delivering a charge that stimulates the nerves connected to the muscles of interest. This charge perturbs the resting potential of the neuron (typically around −65 mV); if this value is raised beyond a threshold, membrane depolarization occurs. This depolarization results in an influx of Na ⁺ ions, initiating an action potential that can travel spatially down the length of an axon. A coordinated group of action potentials can lead to a muscle contraction. By targeting nerves rather than the muscle fibers themselves (which can also be stimulated electrically), substantially smaller charge densities may be used, consuming less power and avoiding tissue damage.

Provided that the neuromuscular system is intact, stimulation may be achieved at a variety of locations (from the origin of the neuron in the spinal cord to the peripheral nerve and to the skin above the muscle) using various types of electrodes. The simplest configuration uses large (of the order of square centimeters) electrodes placed on the surface of the skin. The electrodes are easily replaced; however, achieving accurate and precise positioning is challenging, and charge is distributed over a large area. A more invasive approach is to implant needlelike electrodes percutaneously into the muscle of interest. This method is considered a precursor to fully implanted systems, although subcutaneous electrodes themselves can remain functional for years. When electrodes are fully implanted in close proximity to the nerve, even more precise targeting can be achieved using even smaller current densities, which are less likely to damage the tissue.

Electrodes have been designed to wrap around individual nerves, with a range of geometries, including spiral, helical, and rectangular. To selectively address smaller groups of axons within a nerve and to reach areas that are not readily accessible from the surface, intrafascicular electrodes may be inserted into the nerve itself. Pools of neurons may also be stimulated directly in the spinal cord in intraspinal microstimulation (ISMS). Although implanted devices offer superior targeting, the obvious drawback is the invasiveness of the insertion process and the potential risk of infection, although this has not been reported as a significant issue.

In FES, the electrode typically acts as a conductor, delivering electrical charge from a power supply to the tissue. Charge transfer occurs when voltage applied between the active electrode and a second electrode (called the reference electrode) generates an electric field, which in turn forces electrical charge to flow. In systems in which multiple stimulation channels are used, a single reference electrode may be used. When a voltage is applied, the energy can drive several unwanted chemical reactions. To avoid generating H ₂ gas from water, the voltage generated between the electrodes must not exceed the amount required to electrolyze water (−0.6 V to −0.8 V depending on electrode type ). The amount of charge that can be delivered within these limits depends on the impedance of the material, which should be low to maximize the current delivered. To balance the charge injected to stimulate the neurons and prevent the electrochemical decomposition of tissue, a secondary pulse of opposite polarity should be included in the stimulation profile (ie, a biphasic pulse should be applied). The electrodes themselves must be selected to be resistant to corrosion under physiologic conditions, even under an applied voltage. Common electrode materials for implanted devices include corrosion-resistant stainless steel and noble metals such as PtIr or Pt (which have highly stable atomic configurations and therefore are resistant to chemical processes such as corrosion or oxidation). Other metals (including silver, iron, and copper) are known to elicit dramatic inflammatory response in vivo and should be avoided.

The time-dependent failure of neural interfaces in vivo is an impediment to long-term use, particularly for recording electrodes and stimulating electrodes, which inject small currents into small target areas. The principal cause of failure of these devices is the encapsulation, which occurs as a part of the foreign body response, insulating the electrodes from their surroundings. To avoid scar formation initiated by mechanical mismatch between stiff electrodes and soft tissues, there is an increasing interest in fabricating electrodes and arrays from soft (low modulus) materials such as silicone elastomer. Beyond this, several strategies have been undertaken to modify the surface properties of electrodes to improve the interactions that take place with surrounding tissue and reduce glial scar formation. When developing new electrodes, arrays, and coatings, in vitro testing may be used initially to screen the cellular response, but they must be tested in vivo following the standard ISO 10993.

The basic properties of electrodes for nerve stimulation

In neuroprostheses, electrodes are the interface between the external circuitry and the tissue, delivering a charge that stimulates the nerves connected to the muscles of interest. This charge perturbs the resting potential of the neuron (typically around −65 mV); if this value is raised beyond a threshold, membrane depolarization occurs. This depolarization results in an influx of Na ⁺ ions, initiating an action potential that can travel spatially down the length of an axon. A coordinated group of action potentials can lead to a muscle contraction. By targeting nerves rather than the muscle fibers themselves (which can also be stimulated electrically), substantially smaller charge densities may be used, consuming less power and avoiding tissue damage.

Provided that the neuromuscular system is intact, stimulation may be achieved at a variety of locations (from the origin of the neuron in the spinal cord to the peripheral nerve and to the skin above the muscle) using various types of electrodes. The simplest configuration uses large (of the order of square centimeters) electrodes placed on the surface of the skin. The electrodes are easily replaced; however, achieving accurate and precise positioning is challenging, and charge is distributed over a large area. A more invasive approach is to implant needlelike electrodes percutaneously into the muscle of interest. This method is considered a precursor to fully implanted systems, although subcutaneous electrodes themselves can remain functional for years. When electrodes are fully implanted in close proximity to the nerve, even more precise targeting can be achieved using even smaller current densities, which are less likely to damage the tissue.

Electrodes have been designed to wrap around individual nerves, with a range of geometries, including spiral, helical, and rectangular. To selectively address smaller groups of axons within a nerve and to reach areas that are not readily accessible from the surface, intrafascicular electrodes may be inserted into the nerve itself. Pools of neurons may also be stimulated directly in the spinal cord in intraspinal microstimulation (ISMS). Although implanted devices offer superior targeting, the obvious drawback is the invasiveness of the insertion process and the potential risk of infection, although this has not been reported as a significant issue.

In FES, the electrode typically acts as a conductor, delivering electrical charge from a power supply to the tissue. Charge transfer occurs when voltage applied between the active electrode and a second electrode (called the reference electrode) generates an electric field, which in turn forces electrical charge to flow. In systems in which multiple stimulation channels are used, a single reference electrode may be used. When a voltage is applied, the energy can drive several unwanted chemical reactions. To avoid generating H ₂ gas from water, the voltage generated between the electrodes must not exceed the amount required to electrolyze water (−0.6 V to −0.8 V depending on electrode type ). The amount of charge that can be delivered within these limits depends on the impedance of the material, which should be low to maximize the current delivered. To balance the charge injected to stimulate the neurons and prevent the electrochemical decomposition of tissue, a secondary pulse of opposite polarity should be included in the stimulation profile (ie, a biphasic pulse should be applied). The electrodes themselves must be selected to be resistant to corrosion under physiologic conditions, even under an applied voltage. Common electrode materials for implanted devices include corrosion-resistant stainless steel and noble metals such as PtIr or Pt (which have highly stable atomic configurations and therefore are resistant to chemical processes such as corrosion or oxidation). Other metals (including silver, iron, and copper) are known to elicit dramatic inflammatory response in vivo and should be avoided.

The time-dependent failure of neural interfaces in vivo is an impediment to long-term use, particularly for recording electrodes and stimulating electrodes, which inject small currents into small target areas. The principal cause of failure of these devices is the encapsulation, which occurs as a part of the foreign body response, insulating the electrodes from their surroundings. To avoid scar formation initiated by mechanical mismatch between stiff electrodes and soft tissues, there is an increasing interest in fabricating electrodes and arrays from soft (low modulus) materials such as silicone elastomer. Beyond this, several strategies have been undertaken to modify the surface properties of electrodes to improve the interactions that take place with surrounding tissue and reduce glial scar formation. When developing new electrodes, arrays, and coatings, in vitro testing may be used initially to screen the cellular response, but they must be tested in vivo following the standard ISO 10993.

Upper extremity functional restoration with FES

For persons with cervical-level SCI, restoration of hand function is their top priority. Neuroprostheses using FES provide the most promising method for significant gain in hand and arm function for this population. Muscle contractions can be orchestrated to produce coordinated grasp opening and closing; thumb opening, closing, and positioning; wrist extension and flexion; forearm pronation; and elbow extension for persons with C5-C6–level SCI. Neuroprostheses can be coupled with tendon transfers to maximize function. The objectives of these neuroprostheses are to reduce the need to rely on assistance from others; the need for adaptive equipment, braces, or other orthotic devices; and the time it takes to perform tasks. Neuroprostheses make use of the patient’s own paralyzed musculature to provide the power for grasp and the patient’s voluntary musculature to control the grasp. Typically, persons with SCI use the neuroprosthesis for eating, personal hygiene, writing, and office tasks.

Neuroprostheses have been clinically implemented and investigated using systems based on surface electrodes, percutaneous electrodes, and implanted devices. Surface and percutaneous systems have potential application in muscle conditioning and in short-term research or clinical applications. Implanted systems are generally used for long-term functional enhancement.

All existing upper extremity neuroprosthetic systems consist of (1) a stimulator that activates the muscles of the forearm and hand and (2) an input transducer and control unit. The control signal for grasp is derived from an action that the user has retained voluntary control over, which can include joint movement, muscle activity, respiration, or voice control. A coordinated stimulation pattern is developed so that the muscles are activated in a sequence that produces a functional grasp pattern as the user typically has control over grasp opening and closing but does not have direct control over the activation of each muscle.

Surface stimulation of the forearm and hand can be used to exercise and to produce functional movements. Nathan developed a splint that incorporates surface electrodes for grasp. This system is commercially available (NESS H200, Bioness, Valencia, CA, USA) and is primarily intended for therapeutic applications after stroke or SCI, such as building muscle strength, preventing joint contractures, and improving tissue viability. Popovic and colleagues have developed a surface stimulation system called the ETHZ-ParaCare neuroprosthesis. This system is capable of 4 channels of stimulation and can be interfaced with a variety of control inputs. Early functional results indicate that subjects can use the system to perform a variety of activities of daily living (ADL) in the home.

Implanted FES systems have been used for long-term functional enhancement for persons with cervical SCI. The largest clinical trial of an upper extremity neuroprosthesis was the Freehand trial, initiated by the Cleveland Functional Electrical Stimulation Center in 1992. The Freehand neuroprosthesis used an implanted 8-channel receiver-stimulator, and control of grasp opening and closing was achieved through graded elevation of the user’s contralateral shoulder. Using the neuroprosthesis, 100% of the participants ( n = 28) improved in independence in at least 1 task, and 78% were less dependent in at least 3 tasks. More than 90% were satisfied with the neuroprosthesis. The Freehand system was transferred to industry (NeuroControl Corp, Elyria, OH, USA) and was implemented successfully in more than 200 patients with SCI using neuroprostheses. Despite the clinical success, the company exited the SCI market in 2001 and no longer markets the Freehand System.

A second-generation implanted neuroprosthesis has been developed, improving on the features of the Freehand System. This system, called the Implanted Stimulator Telemeter Twelve-channel System (IST-12), has 12 stimulation channels and 2 channels of myoelectric signal recording acquisition. To date, 12 subjects with SCI have been implanted with the IST-12 system, including 3 subjects with systems for restoring movement in both hands. Subjects successfully use the processed myoelectric signal from a wrist extensor for proportional control of grasp opening and closing. Every subject has demonstrated improvement in at least 2 activities and as many as 11 activities. Most commonly, improvement was demonstrated in eating with a fork and writing with a pen. Other tasks in which subjects showed improvement included office tasks, using a cell phone, getting money out of a wallet, and embroidery, as illustrated in Fig. 1 .

Functional activities performed using the IST-12 myoelectrically controlled neuroprosthesis. From left to right: eating with a fork, holding a pen to write, holding a cup, needle embroidery, holding a tennis racquet.

Lower extremity functional restoration with FES

In persons with SCI, the inability to stand or step significantly limits the performance of many ADL such as washing dishes at a counter or reaching items on high shelves. For persons with thoracic-level complete SCI, stimulated contractions of the lower extremity muscles can enable standing and stepping, increase personal mobility, and improve general health and quality of life. In persons with incomplete injuries, walking performance can be improved.

Eight channels of continuous stimulation to the knee, hip, and trunk extensors can power the sit-to-stand transition and support the body vertically against collapse ( Fig. 2 ). Stimulation to the hip ab/adductors and ankle plantar/dorsiflexors has been included in experimental systems for sensor-based control of standing balance in the coronal and sagittal planes. Existing neuroprostheses for lower extremity function use maximal levels of constant stimulation at the hips and knees. Recipients of a neuroprosthesis with epimysial and intramuscular electrodes that continuously activated the vasti, gluteals, hamstrings, and lumbar erector spinae exhibited mean and median standing times of 10 and 3 minutes, respectively. This time is sufficient for facilitating transfers to high surfaces, performing swing-to gait for short distances in wheelchair-inaccessible environments, and participating in other social, work, and personal activities. Some implant recipients in a phase II clinical trial of the system were able to stand for more than 20 minutes, and all were able to release 1 hand from a walker or assistive device to reach objects overhead ( Fig. 3 ). On average, 90% of body weight was placed on the legs, reducing requirements on the arms to only light touch to maintain balance. System performance and patterns of usage were maintained after discharge for at least 1 year of follow-up. Although there were no discernible interactions between injury level, degree of preserved sensation, or time postinjury and system performance, outcomes seem to be inversely proportional to height and weight, implying that body mass index may be an important clinical factor for determining expectations. Long-term use of neuroprostheses for standing was safe and effective and had no adverse physiologic effects.

Implant recipient (C7 AIS C) standing with FES to the knee, hip, and trunk extensors, and hip/trunk ab/adductors. Multicontact cuff electrodes on the femoral nerves selectively activate the uniarticular heads of the quadriceps (vastus lateralis, intermedius, and medialis).

Eight-channel implant recipient (T9 AIS A) releases one hand for overhead reaching activities while standing with the neuroprosthesis.

Stepping of up to 100 m has also been achieved after paralysis with simple preprogrammed patterns of open-loop stimulation delivered from the surface or via 8- and 16-channel implanted pulse generators. Once initiated by the user, stepping motions can cycle continuously while the appropriate adjustments are made with the upper body until the pattern is stopped. Alternatively, the stimulation for sequential steps can be triggered from successive depressions of ring- or walker-mounted switches or automatically from body-mounted sensors, such as inclinometers, accelerometers, gyroscopes, or foot or heel switches. The largest potential impact of stimulation may be for people with motor incomplete injuries ( Fig. 4 ) who require activation of a small number of muscles during the gait cycle to become household or community ambulators. In such cases, gait training with stimulation can have a therapeutic effect in terms of improved voluntary strength, walking speed, stride length, and cadence even after completion of aggressive conventional therapies. Interactive use of stimulation to assist gait resulted consistently in an additional 20% improvement in walking speed and 6-minute distance, as well as a more than 3-fold increase in maximum walking distance, illustrating a significant neuroprosthetic effect. Walking with stimulation was also more dynamic as evidenced by decreased time spent in the double support phases of gait. The electromyographic activity of muscles under volitional control has also been exploited as a command source to control stimulation in persons with incomplete injuries, which has the potential to coordinate stimulated contractions with voluntary motor function, and in so doing reinforce voluntary movement patterns and provide a mechanism to continuously modulate walking speed and cadence.

Subject with incomplete SCI (C5 AIS D) walking with an 8-channel implanted receiver stimulator for activation of hip flexors and ankle dorsiflexors.

Surface FES to the lower extremity muscles with intact innervation has allowed cycling movement that simulates exercise training, leading to increase in oxygen consumption during exercise, muscle mass and strength, and quality of life in persons with chronic SCI.

Trunk control and posture with FES

After SCI, trunk muscles can oftentimes not provide the necessary forces to adequately control trunk posture because of a lack of innervation and/or muscle atrophy, significantly limiting their performance during ADL and even leading to secondary health complications such as reduced respiratory capacity. To compensate for insufficient muscle control during sitting, persons with SCI usually tilt their pelvis further backward to increase stability in the anterior direction. When reaching, they oftentimes use one arm thrown over the back of their chair to provide the external forces necessary to keep the trunk from bending forward uncontrollably. Compensational sitting arrangements can, however, lead to kyphosis and pressure ulcers (PUs) that arise from asymmetric trunk orientation and infrequent weight redistribution. It is therefore not surprising that persons with SCI have prioritized the recovery of trunk control over the recovery of walking function and other essential functional abilities.

Bracing devices such as corsets are perhaps the most common items for stabilizing the trunk after SCI. To improve reaching and wheelchair propulsion, some persons with SCI use chest straps. In the general case of reaching from a wheelchair during ADL, chest straps or other restraints are highly undesirable as they hinder free and spontaneous movement, decrease available trunk range of motion, and draw undue attention to themselves. Other studies have shown that the large forces exerted on the abdomen by a fabric corset might cause abnormal increases in the intra-abdominal pressure, potentially leading to disturbance of the viscera.

Stiffening the paralyzed trunk and hip extensors with continuous FES has a multitude of benefits: it can correct kyphotic seated postures, normalize lateral vertebral alignment, improve ventilation and respiratory volumes, and alter interface pressures. It can also expand bimanual workspace, statically stabilize the torso ( Fig. 5 ), increase the forces that can be exerted on objects with the upper extremities, return users to erect sitting from a fully forward-flexed posture, and improve manual wheelchair propulsion efficiency at comfortable speeds. Independent bed turning and wheelchair transfers can also be facilitated by more rigidly coupling the pelvis to the shoulders when the paralyzed core trunk muscles are continuously activated with stimulation to stiffen the torso. In addition, activating the quadratus lumborum with surface or implanted electrodes has been shown to enhance mediolateral stability and assist with attaining side leaning postures, whereas coactivation with the abdominal muscles can further stiffen the trunk while seated or assist in attaining forward leaning postures. Some of the required muscles to achieve these clinical outcomes can be accessed via surface stimulation; however, strong and isolated contractions are robustly and repeatably achieved by exciting the T12-L2 spinal nerves associated with the lumbar erector spinae and other muscles ( Fig. 6 ) using intramuscular electrodes and surgically implanted pulse generators. The strategy of continuously activating the core trunk and hip muscles only substitutes one statically stable posture for another. Upper extremity effort is still required to stabilize the body during transitions between nonstimulated and stimulated postures and to maintain balance or restore erect sitting when exposed to internal or external perturbations.

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