Muscles can be controlled by light, MIT study shows

Researchers Develop Novel Technique Using Optogenetics, Paving the Way for Advanced Prosthetics and Biohybrid Robots

CAMBRIDGE, Mass. — Imagine being able to control your muscles with the precision of a finely tuned instrument, each movement executed with perfect timing and just the right amount of force. For individuals with neurological conditions or paralysis, such exquisite control might seem like an impossible dream. But now, a groundbreaking study by researchers at MIT’s K. Lisa Yang Center for Bionics and published in Science Robotics is bringing that dream closer to reality. The secret? Harnessing the power of light to stimulate genetically engineered muscle fibers in a technique called optogenetics.

Traditionally, scientists have used electrical stimulation to artificially activate muscles in individuals with impaired motor function. While functional electrical stimulation (FES) has enabled significant advancements in prosthetic limbs and rehabilitation, it comes with some major drawbacks. Electrical signals tend to fatigue muscles quickly and lack the fine-tuned control needed for dexterous tasks like grasping objects or playing an instrument.

That brings us to optogenetics, a cutting-edge field that uses light to control genetically modified cells. By inserting light-sensitive proteins called opsins into specific neurons, scientists can selectively activate or inhibit those cells simply by shining light on them. While optogenetics has revolutionized neuroscience research, its applications in controlling muscle have been relatively unexplored – until now.

The MIT research team, led by Guillermo Herrera-Arcos, hypothesized that using light to stimulate muscles through the peripheral nerves could provide more naturalistic, fatigue-resistant control compared to electrical stimulation. To test this idea, they turned to a special strain of mice genetically engineered to express the light-sensitive protein channelrhodopsin-2 (ChR2) in their motor neurons.

Using an innovative light-based stimulation platform that can precisely target specific nerves, the researchers put their theory to the test. They found that by modulating the pulse width (duration) and frequency of the light signals, they could achieve graded control of muscle force with remarkable accuracy. Optogenetic stimulation not only generated higher maximum forces than electrical stimulation but also enabled a more linear, step-wise increase in force compared to the “all or nothing” activation typical of FES.

“With FES, when you artificially blast the muscle with electricity, the largest units are recruited first. So, as you increase signal, you get no force at the beginning, and then suddenly you get too much force,” says Hugh Herr, a professor of media arts and sciences, co-director of the K. Lisa Yang Center for Bionics at MIT, in a media release.

Person in wheelchair
Harnessing the power of light to stimulate genetically engineered muscle fibers may soon give individuals with neurological conditions or paralysis new hope. (© saelim – stock.adobe.com)

So, what makes optogenetics more effective than electricity for muscle control? The key lies in how the nervous system naturally recruits motor units (the functional pairing of a motor neuron and the muscle fibers it controls). Normally, our brain activates smaller motor units first for delicate, low-force tasks and then progressively recruits larger, more powerful motor units as force requirements increase. This orderly recruitment allows for a wide dynamic range of force generation.

In contrast, electrical stimulation tends to activate the largest, most fatigable motor units first, resulting in rapid fatigue and poor force resolution. Optogenetics, by virtue of directly stimulating motor neurons, can tap into the natural size-based recruitment order. This not only reduces fatigue but also enables more precise, incremental control of force.

To capture this enhanced naturalistic control, the researchers developed a computational model that could predict the complex, non-linear relationships between light stimulation and muscle force production. By incorporating biophysical factors like opsin dynamics, muscle-tendon properties, and motor unit recruitment, their model paves the way for designing closed-loop optogenetic control systems.

The potential applications for this technology are vast. Individuals with paralysis or neurological conditions could regain the ability to perform tasks requiring fine motor skills. Prosthetic limbs could be imbued with more lifelike, fatigue-resistant movements. Optogenetically-controlled muscles could even power a new generation of agile, biohybrid robots.

But the benefits don’t stop there. The researchers found that their optogenetic stimulation platform could evoke controlled muscle contractions for over an hour — a remarkable improvement over the mere minutes típica of FES before fatigue sets in. In a heartening nod to clinical feasibility, the team calculated that their light-based control system could theoretically enable the mouse muscle to perform over 2,000 walking cycles, covering 465 meters – no small feat for a creature that could fit in the palm of your hand.

Of course, there’s still a long road ahead before optogenetic muscle control becomes a clinical reality for humans. Challenges include developing safe, reliable methods to deliver opsins to human peripheral nerves, engineering wireless and implantable optical stimulation devices, and fine-tuning the technology for individual patients’ needs. But armed with this groundbreaking proof-of-concept, researchers now have a clearer path forward.

“This could lead to a minimally invasive strategy that would change the game in terms of clinical care for persons suffering from limb pathology,” Herr concludes.

StudyFinds Editor-in-Chief Steve Fink contributed to this report.

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