For people with paralysis or amputations, neuroprosthetic systems that artificially stimulate muscle contraction with electrical current can help recover limb function. However, despite years of research, this type of prosthesis is not widespread due to the rapid onset of muscle fatigue and poor control. Now, MIT researchers have developed a new approach that they hope will offer better muscle control with less fatigue. An approach based on optogenetics: instead of using electricity to stimulate muscles, they used light.
In a study on mice, researchers demonstrated that this technique offers more precise muscle control, along with a dramatic reduction in fatigue. While not currently applicable to humans, this method could revolutionize the field of prosthetics and help people with compromised limb function.
Optogenetics: a light in the darkness of motor disability
The idea of controlling muscles with light may seem like science fiction, but it is actually based on a well-known principle in biology: optogenetics. This technique consists in genetically modifying cells to make them express light-sensitive proteins, thus allowing their activity to be controlled by exposing them to light pulses. So far, optogenetics has been used mostly to study the functioning of the brain, but MIT researchers had the intuition to apply it to movement control.
Their experiment on mice demonstrated that optical stimulation of muscles offers several advantages over traditional electrical stimulation. First of all, it allows a finer and more gradual control of the contraction force, similar to the natural one of the human body. While electrical stimulation tends to activate the entire muscle at once, causing sudden and imprecise movements, light allows the muscle fibers to be "recruited" progressively, obtaining a more fluid response proportional to the intensity of the stimulus.
But the real strength of optogenetics is resistance to fatigue. Electrically stimulated muscles exhaust quickly, in 5 to 10 minutes, making prolonged use of the prostheses difficult. With light, however, the researchers were able to maintain the stimulation for over an hour before observing signs of fatigue. A result that opens up exciting prospects for the recovery of motor functionality in people with disabilities.
From theory to practice: the challenges to face
Of course, the transition from mice to humans is neither obvious nor immediate. The main challenge is finding a safe and effective way to introduce light-sensitive proteins into human muscles. A few years ago, the laboratory of Hugh Herr (the author of the study that I link to you here) had reported that in rats these proteins can trigger an immune response that deactivates them and can lead to muscle atrophy and cell death. A significant obstacle, on which researchers are working hard.
“A key goal of the K. Lisa Yang Center for Bionics is to solve that problem,” Herr says. “A multi-pronged effort is underway to design new light-sensitive proteins, and strategies to deliver them, without triggering an immune response.”
Other steps necessary to reach human patients with optogenetics? The development of new sensors to measure muscle strength and length, as well as new ways to implant the light source. A significant engineering challenge, but one that researchers are determined to face. If successful, they hope their strategy could benefit people who have suffered strokes, limb amputations and spinal cord injuries, as well as others who have an impaired ability to control their limbs.
Beyond repair: towards strengthening the human body?
The implications of this research go far beyond the medical field. If the optogenetic technique were to prove effective and safe in humans, it could pave the way for a real enhancement of motor skills. Imagine being able to increase the strength, speed or endurance of your muscles with a simple injection of light-sensitive proteins. Or being able to control the movement of a robotic limb with thought, thanks to an optogenetic brain-machine interface. Scenarios that seem like science fiction today, but could become reality tomorrow.
As always, this also raises non-negligible ethical and social questions. Who will have access to these technologies? Will they be reserved only for medical purposes or will they also be available for "recreational" uses? What impact will they have on our conception of normality and disability? And how will the relationship between the biological body and technology evolve?
They are open questions, which require in-depth reflection and expanded debate. But one thing is certain: MIT research on optogenetic muscle stimulation offers us a fascinating glimpse into the future of prosthetics and human enhancement. A future where light could literally move the world, one muscle at a time.
