A tiny neural implant dubbed the Stentrode allows people with upper limb paralysis to regain motor function


A tiny device the size of a small paperclip has been shown to help patients with upper limb paralysis to text, email and even shop online in the first human trial.

The device, Stentrode™, has been implanted successfully in two patients, who both suffer from severe paralysis due to motor neuron disease (MND) – also known amyotrophic lateral sclerosis (ALS) – and neither had the ability to move their upper limbs.

Published in the Journal of NeuroInterventional Surgery, the results found the Stentrode™was able to wirelessly restore the transmission of brain impulses out of the body.

This enabled the patients to successfully complete daily tasks such as online banking, shopping and texting, which previously had not been available to them.

The Royal Melbourne Hospital’s Professor Peter Mitchell, Neurointervention Service Director and principal investigator on the trial, said the findings were promising and demonstrate the device can be safely implanted and used within the patients.

“This is the first time an operation of this kind has been done, so we couldn’t guarantee there wouldn’t be problems, but in both cases the surgery has gone better than we had hoped,” Professor Mitchell said.

Professor Mitchell implanted the device on the study participants through their blood vessels, next to the brain’s motor cortex, in a procedure involving a small ‘keyhole’ incision in the neck.

“The procedure isn’t easy, in each surgery there were differences depending on the patient’s anatomy, however in both cases the patients were able to leave the hospital only a few days later, which also demonstrates the quick recovery from the surgery,” Professor Mitchell said.

Neurointerventionalist and CEO of Synchron – the research commercial partner – Associate Professor Thomas Oxley, said this was a breakthrough moment for the field of brain-computer interfaces.

This shows the stentrode
The device pictured, Stentrode™, has been implanted successfully in two patients, who both suffer from severe paralysis due to motor neuron disease, enabling them to nabled the patients to successfully complete daily tasks such as online banking, shopping and texting. Credit: The researchers

“We are excited to report that we have delivered a fully implantable, take home, wireless technology that does not require open brain surgery, which functions to restore freedoms for people with severe disability,” Associate Professor Oxley, who is also co-head of the Vascular Bionics Laboratory at the University of Melbourne, said.

The two patients used the Stentrode™ to control the computer-based operating system, in combination with an eye-tracker for cursor navigation. This meant they did not need a mouse or keyboard.

They also undertook machine learning-assisted training to control multiple mouse click actions, including zoom and left click.

The first two patients achieved an average click accuracy of 92 per cent and 93 per cent, respectively, and typing speeds of 14 and 20 characters per minute with predictive text disabled.

University of Melbourne Associate Professor Nicholas Opie, co-head of the Vascular Bionics Laboratory at the University and founding chief technology officer of Synchron said the developments were exciting and the patients involved had a level of freedom restored in their lives.

“Observing the participants use the system to communicate and control a computer with their minds, independently and at home, is truly amazing,” Associate Professor Opie said.

“We are thankful to work with such fantastic participants, and my colleagues and I are honoured to make a difference in their lives. I hope others are inspired by their success.

“Over the last eight years we have drawn on some of the world’s leading medical and engineering minds to create an implant that enables people with paralysis to control external equipment with the power of thought. We are pleased to report that we have achieved this.”

The researchers caution that while it is some years away before the technology, capable of returning independence to complete everyday tasks is publicly available, the global, multidisciplinary team is working tirelessly to make this a reality.

The trial recently received a $AU1.48 million grant from the Australian commonwealth government to expand the trial to hospitals in New South Wales and Queensland, with hopes to enroll more patients.

About Stentrode™

Stentrode™ was developed by researchers from the University of Melbourne, the Royal Melbourne Hospital, the Florey Institute of Neuroscience and Mental Health, Monash University and the company Synchron Australia – the corporate vehicle established by Associate Professors Thomas Oxley (CEO) and Nicholas Opie (CTO) that aims to develop and commercialise neural bionics technology and products. It draws on some of the world’s leading medical and engineering minds.

Synchron is developing the Stentrode as an implantable interventional neuromodulation therapy for multiple proprietary targets in the brain and body – without the need for open surgery.

Deep brain stimulation is an FDA-approved treatment for several conditions, including Parkinson’s disease, essential tremor, epilepsy and OCD. The FDA has approved investigation into its use in depression, Alzheimer’s dementia, addiction and headache, however, traditional forms of deep brain stimulation require open brain surgery. 

Synchron is pioneering the field of interventional neuromodulation within the medical subspecialty of interventional neuroradiology, which is also known as interventional neurology or endovascular neurosurgery. This specialty focuses on the use of image-based catheter procedures to deliver technology into the brain and spine.

Synchron is developing the Stentrode as an implantable interventional neuromodulation therapy for several proprietary targets in the brain and body – without the need for open brain surgery. The Stentrode is designed to stimulate the nervous system from inside any blood vessel, providing potential access to a range of regions of the brain and body.

Losing the ability to move voluntarily can have devastating consequences for the independence and quality of life of a person. Stroke and spinal cord injury (SCI) are two important causes of paralysis which affect thousands of individuals around the world. Extraordinary efforts have been made in an attempt to mitigate the effects of paralysis.

In recent years, rehabilitation of voluntary movement has been enriched by the constant integration of new neurophysiological knowledge about the mechanisms behind motor function recovery. One central concept that has improved neurorehabilitation significantly is neuroplasticity, the ability of the central nervous system to reorganize itself during the acquisition, retention, and consolidation of motor skills [1].

In this document, we present one of the interventions that has flourished as a consequence of our increased understanding of the plasticity of the nervous system: functional electrical stimulation therapy or FEST. The document, which is not a systematic review, is intended to describe early work that played an important historical role in the development of this field, while providing a general understanding of the technology and applications that continue to be used today.

Readers interested in systematic reviews of functional electrical simulation (FES) are directed to other sources (e.g., [2–4]).


Stroke is the fifth cause of death in the United States and a leading cause of disability [5]. It is a localized death of brain tissue following an interruption of blood supply. A stroke caused by a ruptured blood vessel is often referred to as a hemorrhagic stroke, while one produced by a blockage of a blood vessel is an ischemic stroke.

The majority (87%) of strokes are ischemic [5]. The location and extent of the necrosis determine the effects of the stroke, which can affect behavior, emotion, communication, and voluntary movement, among other things. A common effect of stroke is hemiplegia in which the ability to move one side of the body is impaired.

This condition can range from mild, in which the decrease in motor function is barely noticeable, to severe, in which the ability to move is greatly impaired or completely lost. Recovery has been historically considered to peak at six months after stroke with decreasing probability of observing improvements afterward.

Spinal cord injury

Spinal cord injury (SCI) occurs when the spinal cord is damaged leading to a loss of sensory and/or motor function. The spinal cord is part of the central nervous system and it is composed of a major bundle of nerves that allow communication between the brain and the rest of the body (i.e., through peripheral nerves).

It also contains neuronal structures (grey matter) responsible for monosynaptic and polysynaptic reflexes, as well as for carrying out tasks such as bladder and bowel voiding, and locomotion. SCI often affects the body bilaterally and can be traumatic (e.g., resulting from a motor vehicle accident) or non-traumatic (e.g., due to a tumor).

SCI can be complete or incomplete according to the extent of the damage to the spinal cord. The level of SCI is important as sensory and/or motor impairment takes place below the level of injury, with higher lesions affecting a greater proportion of the body. In the context of voluntary motor function, a lesion of the lumbar and thoracic levels can result in paraplegia, which affects trunk and lower extremity function. An SCI at the cervical level can, in addition, affect the capacity to move the upper limbs, a condition known as tetraplegia.

Rehabilitation after stroke and SCI

Recovering voluntary motor function can improve the independence and quality of life after stroke and SCI. Therapy can focus on multiple aspects including, for example, increasing strength and range of motion. Recent interventions that integrate new knowledge on the neurological mechanisms behind recovery of movement have emerged.

Of particular importance has been the concept of neuroplasticity, the nervous system’s ability to modify its synaptic connectivity to reorganize itself and incorporate new motor abilities. This document describes functional electrical stimulation therapy (FEST), an intervention that takes advantage of neuroplasticity to restore the ability to perform voluntary movement after stroke and SCI.

Functional electrical stimulation

Electrical current can elicit a response in excitable cells including neurons. Devices that can deliver controlled discharges have made it possible to assist individuals with different medical conditions. Cochlear implants to restore hearing, phrenic pacemakers that assist respiration, systems to void the bladder, cardiac pacemakers to ensure cardiac function, and deep brain stimulation to control tremor due to Parkinson’s disease are examples of applications of electrical stimulation systems.

Neuromuscular stimulation

Neuromuscular stimulation (NMES) is one application of electrical stimulation used in rehabilitation of movement. In it, electrical stimulation produces contractions of paralyzed muscles that are still innervated [6]. NMES can increase the patients’ participation in voluntary activities by reducing impairment. For example, NMES can be used to increase muscle strength, improve shoulder subluxation (dislocation), reduce muscle tone, and produce movement.

Functional electrical stimulation

Functional electrical stimulation (FES) is a subtype of NMES in which the stimulation assists functional and purposeful movements. This is achieved by applying electrical stimulation to muscles that, when they contract, produce a movement that can be used functionally.

Examples of functional movements include lifting a book from a desk, bringing a bottle of water to the mouth, and holding a pen to write. The muscles, as well as the sequence in which they contract, are selected specifically to produce the desired movement. An FES system that facilitates a specific movement is often referred to as a neuroprosthesis or motor neuroprosthesis.

For example, a neuroprosthesis for grasping is an FES system that restores the ability to grasp objects. Other examples include neuroprostheses for standing, walking, reaching, as well as reaching and grasping, all of which will be described below.

Components of a neuroprosthesis

The basic components of a neuroprosthesis are an electrical stimulator, electrodes that deliver the stimulation, sensors for user or automatic control of the stimulation, and in some cases, an orthosis that provides additional assistance to perform the desired movement [7].

Electrical stimulator

The electrical stimulator is responsible for generating the electrical discharges that produce muscle contractions. Delivery of the stimulation is achieved through individual stimulation channels. A stimulation channel consists of a pair of electrodes (cathode and anode) used to deliver complex stimulation pulses (important characteristics of the stimulation pulses are described below).

A stimulator can have multiple stimulation channels, each of which can stimulate individual muscles using unique settings. A multichannel programmable stimulator, which allows specifying the sequence in which each channel is active, makes it possible to facilitate different functional movements [8].

Stimulation electrodes

Electrical stimulation can be delivered using electrodes with different levels of invasiveness; they can be completely or partially implanted, known as implanted and percutaneous electrodes, respectively, or can also be placed on the surface of the body (transcutaneous or noninvasive electrodes). Each type of electrode offers advantges and disadvantages with respect to their flexibility, stimulation specificity, usability, and cost. Table 1 displays a summary of the types of electrodes commonly used for stimulation.

Table 1 – Stimulation electrodes with different levels of invasiveness

TypeTypical currentAdvantagesDisadvantages
 Implanted25 mAHigh stimulation specificityRequire surgery
Suitable for long-term usePlacement cannot be modified after implantation
 Percutaneous25 mAHigh stimulation specificityRequire surgery
Suitable for short-term use
 Transcutaneous (surface)2 mA–120 mADo not require surgeryUnsuitable for stimulation of deep muscles
Easy to repositionOften require higher stimulation current

Implanted electrodes

As the name suggests, implanted electrodes are inserted surgically into the body. They can be placed in close proximity to targeted nerves resulting in high selectivity (i.e., it is easier to isolate specific muscles to stimulate). They are better suited for long-term use. One risk is that the surgery required to implant electrodes can increase the risk of infection [8].

Another consideration is the cost involved in delivering FES using implanted electrodes [9]. The electrical current necessary to produce a muscle contraction with implanted electrodes is in the range of 25 mA [8]. Theoretically speaking, once implanted, FES systems that use implanted electrodes require less time to don and doff compared to surface stimulation technology; despite full implantation of the electrodes there are often external components that the user must don as part of the complete neuroprosthesis (e.g., a controller interface).

However, recent advances with Bioness, MyndMove, and textile computing solutions (described below) challenge this long held premise; these new surface stimulation systems are at least as fast to don and doff as implanted systems. An important drawback of implanted electrodes is that, once implanted, their position cannot be readily changed.

Percutaneous electrodes

Percutaneous electrodes typically have the form of wires that penetrate the skin with a portion of them inserted in the body in close proximity to motor neurons [10, 11]. The electrodes are left in place temporarily while the stimulation is delivered; they are often used for short-term FES applications. Typical stimulation current amplitude used with percutaneous electrodes is also 25 mA.

Transcutaneous electrodes

Transcutaneous electrodes are placed on the surface of the body. They can be self-adhesive or can be secured to the skin with adhesive tape. During stimulation, the electrodes are placed over the nerve innervating the muscle to be stimulated. In addition to the fact that their use does not require surgery, transcutaneous electrodes can be repositioned immediately to ensure that the stimulation elicits the desired response (i.e., movement).

Changing the position of the electrodes (and hence their effect) may be of particular importance if the stimulation needs to be modified in response to the changing needs of an individual, as is often the case during neurorehabilitation. This makes them ideal for temporary use, such as when using electrical stimulation as part of a short-term rehabilitation intervention.

Also, the non-invasive nature of these electrodes makes it possible to use FES very early in the rehabilitation of patients who have had a stroke or an SCI, in which early intervention often leads to greater recovery [8]. The current used with transcutaneous electrodes (2–120 mA) is often greater than that necessary to produce muscle contractions with implanted ones.

One of the important limitations of surface electrodes is that they may be unsuitable for use with deep muscles (i.e., far below from the skin); stimulation of muscles that are deep may require greater intensity which in turn may result in simultaneous contraction of undesired muscles.

Sensors for allowing user control

In addition to the characteristics of the electrical stimulator (e.g., number of channels, and programmable or non-programmable), the user interface for the neuroprosthesis may offer an additional level of customization. Neuroprostheses may accept commercially available accessible switches and, in some cases, it may also be possible to incorporate other specialized sensors allowing individuals with different abilities to command the device.

For example, a goniometer can be used to trigger stimulation upon detection of wrist movements [12] and a potentiometer may allow the user to specify the grasp type to be produced through stimulation [13]. Specific examples are presented below.


The use of an orthosis can aid in the production of a specific movement when the stimulation alone is insufficient, by helping patients conserve energy or to prevent muscle fatigue. For example, an orthosis may be used to facilitate hand function [13] and provide stability while using electrical stimulation to facilitate walking [14–17].

Technical considerations of FES: general stimulation characteristics

In addition to the stimulator, electrodes, user interface/control and control strategies, the characteristics of the actual electrical pulses used ultimately play a central role in the effects of the stimulation.

Stimulation intensity: pulse amplitude and duration

The intensity of the stimulation is determined by three parameters: pulse amplitude, pulse duration and pulse frequency (Fig. 1). The pulse amplitude refers to the magnitude of the stimulation. It affects directly the type of nerve fibers that respond to the stimulation with large fibers in close proximity to the stimulation electrode being recruited first [6]. The pulse duration (pulse width) is the time in which the stimulation pulse is present.

Both parameters are inversely related so that an increase in pulse duration may require a pulse with lower amplitude to generate a response. Conversely, reducing the pulse duration may translate into the need to increase the amplitude of the stimulating pulse.

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Fig. 1
Functional electrical stimulation parameters. Pulse width, amplitude, and frequency define the muscles stimulated, force, and quality of the contraction

Stimulation frequency

The frequency of stimulation is the rate at which stimulation pulses are delivered and it affects the strength of the muscle contraction as well as its quality.

Each stimulation pulse with properly selected amplitude and duration produces a muscle twitch, characterized by a sharp increase in force followed by a slower return [18] to a relaxed state.

Quick application of subsequent stimulation pulses before the muscle is relaxed will produce additional muscle twitches. The force produced by each twitch is added so that the mean force of the contraction is greater than that produced by a single twitch.

Further increase in the pulse frequency results in a sustained contraction, in which no individual twitches are visible, and instead replaced by a smooth movement. This tetanic contraction is desired in FES applications. The minimum frequency required to induce fairly consistent contractions is between 16 and 20 Hz.

However, while tetanic contractions can be achieved with a minimum of 20 pulses per second [8], a pulse frequency of 40 Hz is often needed. Higher pulse frequencies generate stronger tetanic contractions; however, they can also result in faster muscle fatigue.

Pulse frequencies in the range from 20 to 50 Hz [6] are typical. For patients with SCI who often have problems with FES-induced muscle fatigue, in particular during early stages of FES use, pulse frequencies of 20–25 Hz are more common. In patients with stroke, for whom FES-induced muscle fatigue is not a major issue, we use a pulse frequency of 40 Hz.

Pulse shape

The pulses used for stimulation can be divided into monophasic and biphasic. In turn, biphasic pulses can be classified further into symmetric and asymmetric. It is believed that monophasic pulses can have a negative effect as they apply energy to the body that is never removed creating the potential to, among other things, damage the stimulated tissue.

Biphasic pulses, on the other hand, alternate the anode and cathode electrodes with each stimulation pulse, which is safer for the person receiving the stimulation. Symmetric biphasic pulses, as the name suggests, consist of two phases which are identical in duration and amplitude with their polarity as the only difference between them.

In contrast, asymmetric biphasic pulses also have two phases of opposite polarity but that are not identical in either amplitude and/or duration. However, in the case of balanced asymmetric biphasic pulses the amplitudes and durations of leading and trailing pulses are selected such that the total electrical charge delivered to the body during the leading pulse is identical to the total electrical charge removed from the body using the trailing pulse.

This ensures the long-term safety and integrity of the stimulated tissues, while making it possible to control exactly which electrode is used to deliver stimulation that generates contractions and the exact motor point; by selecting the amplitude of the leading pulse to be sufficiently high to generate a desired muscle contraction and by selecting the amplitude of the trailing pulse to be sufficiently low not to trigger muscle contraction, one can deliver stimulation only to desired motor points with precision. Figure 2 displays examples of the different pulses commonly used for electrical stimulation.

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Fig. 2
Examples of commonly used pulse shapes used for functional electrical stimulation
(Modified from [6])

reference link :https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7245767/

Source: University of Melbourne


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