Neuroprosthetic systems, which artificially stimulate muscle contraction using electrical current, offer a glimmer of hope for paralyzed or amputee people. Despite decades of research, these devices remain little used due to rapid muscle fatigue and the limited control they provide.
A new approach for better muscle control
MIT researchers have developed an innovative method to improve muscle control while reducing fatigue. Rather than using electricity to stimulate muscles, they opted for light. A study carried out on mice demonstrated that this optogenetic technique allows more precise muscle control and a significant reduction in fatigue.
“ It turns out that by using light, via optogenetics, we can control muscles in a more natural way. In terms of clinical application, this type of interface could have very broad utility. says Hugh Herr, professor of media sciences and the arts, co-director of the K. Lisa Yang Center for Bionics at MIT, and associate member of the McGovern Institute for Brain Research at MIT.
The principle of optogenetics
Optogenetics relies on genetically engineering cells to express light-sensitive proteins, allowing researchers to control the activity of these cells by exposing them to light. Although this approach is not yet feasible in humans, Hugh Herr, MIT graduate student Guillermo Herrera-Arcos, and their colleagues at the K. Lisa Yang Center for Bionics are currently working on ways to deliver these proteins safe and effective in human tissues.
Herr is the lead author of the study, published today in Science Robotics. Herrera-Arcos is the main author.
Optogenetic control
For decades, researchers have explored the use of functional electrical stimulation (FES) to control the body’s muscles. This method involves the implantation of electrodes that stimulate nerve fibers, causing a muscle to contract. However, this stimulation tends to activate the entire muscle at once, which is not the natural way the human body controls muscle contraction.
“ Humans possess incredible control fidelity, achieved by natural muscle recruitment, where small motor units, then medium-sized units, then large motor units are recruited, in that order, as signal strength increases. », explains Hugh Herr. “ With FES, when you artificially stimulate the muscle with electricity, the largest units are recruited first. So as you increase the signal you get no strength at first and then suddenly you get too much strength. »
This great force not only makes fine control of the muscle more difficult, but it also exhausts the muscle quickly, within five to ten minutes.
Fatigue resistance
Using data from these experiments, the researchers created a mathematical model of optogenetic muscle control. This model relates the amount of light entering the system to the force generated by the muscle.
This mathematical model allowed researchers to design a closed-loop controller. In this type of system, the controller delivers a stimulation signal, and after the muscle contracts, a sensor detects the force exerted by the muscle. This information is sent back to the controller, which calculates whether, and to what extent, the light stimulation needs to be adjusted to achieve the desired strength.
Using this type of control, researchers found that muscles could be stimulated for more than an hour before becoming fatigued, while muscles became fatigued after just 15 minutes of FES stimulation.
Challenges and perspectives
One obstacle that researchers are working to overcome is how to safely deliver the light-sensitive proteins into human tissues. A few years ago, Hugh Herr’s lab reported that in rats, these proteins can trigger an immune response that inactivates the proteins and can also lead to muscle atrophy and cell death.
“ A key goal of the K. Lisa Yang Center for Bionics is to solve this problem », indicated Hugh Herr. “ A multipronged effort is underway to design new light-sensitive proteins and strategies to deliver them, without triggering an immune response. »
To reach human patients, Herr’s lab is also working on new sensors that can be used to measure muscle strength and length, as well as new methods for implanting the light Source. If these efforts are successful, the researchers hope their strategy could benefit people who have suffered strokes, limb amputations, spinal cord injuries, as well as others with impaired limb control ability.
“ This could lead to a game-changing, minimally invasive strategy in clinical care for people with limb conditions. », concludes Hugh Herr.
Illustration caption: MIT researchers have developed a way to help amputees or paralyzed people regain control of their limbs. Instead of using electricity to stimulate muscles, they used light. Here, Guillermo Herrera-Arcos observes the light emanating from an optical neurostimulator. Credit: Steph Stevens
Article: “Closed-loop optogenetic neuromodulation enables high-fidelity fatigue-resistant muscle control” – DOI: https://www.science.org/doi/10.1126/scirobotics.adi8995
