summary: Researchers have developed a new way to control muscles using light instead of electricity. This optogenetic technology allows for more precise muscle control and significantly reduces fatigue in mice. Although this approach is not currently possible in humans, it could revolutionize prosthetics and help individuals with poor limb function.
Key facts:
- Optical muscle stimulation provides more precise control than electrical stimulation.
- This method significantly reduces muscle fatigue compared to traditional methods.
- Researchers are working to find ways to safely deliver light-sensitive proteins into human tissue.
source: Massachusetts Institute of Technology
For people with paralysis or amputation, neuroprosthetic systems that artificially stimulate muscle contraction with electrical current can help them regain limb function. However, despite many years of research, this type of prosthesis is not widely used because it leads to rapid muscle fatigue and poor control.
Researchers from MIT have developed a new approach that they hope will one day provide better muscle control with less fatigue. Instead of using electricity to stimulate the muscles, they used light. In a study conducted on mice, researchers showed that this optogenetic technique provides more precise muscle control, along with a significant reduction in fatigue.
“It turns out that by using light, through optogenetics, one can control muscles more naturally,” says Hugh Herr, professor of media arts and sciences, co-director of the K. Lisa Young Center for Bioelectronics at MIT, and an associate faculty member at MIT. “In terms of clinical application, this type of interface could have very broad utility.”
Optogenetics is an approach that relies on genetically engineering cells to express light-sensitive proteins, allowing researchers to control the activity of those cells by exposing them to light. This approach is not currently possible in humans, but Hare, MIT graduate student Guillermo Herrera Arcos and their colleagues at the K. Lisa Yang Bioelectronics is now working on ways to safely and effectively deliver light-sensitive proteins into human tissue.
Hare is lead author of the study, which appears today in Scientific robots. Herrera Arcos is the lead author of this paper.
Visual control
For decades, researchers have been exploring the use of functional electrical stimulation (FES) to control the body’s muscles. This method involves implanting electrodes that stimulate nerve fibres, causing muscles to contract. However, this stimulation tends to activate the entire muscle at once, which is not how the human body naturally controls muscle contraction.
“Humans have amazing precision of control that is achieved through natural muscle recruitment, where small, then medium-sized, then large motor units are recruited, in that order, with increasing signal strength,” Hare says. “With FES, when you artificially blast the muscle with electricity, the largest units are recruited first. So, as the signal increases, you get no force at first, and then suddenly you get a lot of force.
This large force not only makes it difficult to achieve precise muscle control, but it also wears out the muscles quickly, within five or 10 minutes.
The MIT team wanted to see if they could replace this entire interface with something different. Instead of electrodes, they decided to try to control muscle contraction using optical molecular machines via optogenetics.
Using mice as an animal model, the researchers compared the amount of muscle force they could generate using the traditional FES approach with the forces generated by their optical method. For the optogenetics studies, they used mice that were already genetically modified to express a light-sensitive protein called Channelrhodopsin-2. They implanted a small light source near the tibial nerve, which controls the muscles of the lower leg.
The researchers measured muscle force while gradually increasing the amount of optical stimulation and found that, unlike FES stimulation, optogenetic control produced a steady and gradual increase in muscle contraction.
“As we vary the visual stimulation we deliver to the nerve, we can proportionally control, in an almost linear manner, the muscle force. This is similar to how signals from our brain control our muscles. For this reason, muscle control becomes easier compared to electrical stimulation.”
Fatigue resistance
Using data from those experiments, the researchers created a mathematical model of genetic muscle control. This model links the amount of light entering the system to muscle output (the amount of force generated).
This mathematical model allowed the researchers to design a closed-loop controller. In this type of system, a controller delivers a stimulating signal, and after the muscle contracts, a sensor can detect the amount of force exerted by the muscle. This information is sent back to the control unit, which calculates whether and by how much the light stimulation needs to be adjusted to reach the desired strength.
Using this type of control, researchers found that muscles could be stimulated for more than an hour before fatigue, while muscles became fatigued after only 15 minutes using FES stimulation.
One hurdle researchers are now working to overcome is how to safely deliver light-sensitive proteins into human tissue. Several years ago, Hare’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.
“The main goal of the K. Lisa Yang Electronics Center is to solve this problem,” Hare says. “Multifaceted efforts are underway to design new light-sensitive proteins, and strategies to deliver them, without eliciting an immune response.”
As further steps toward reaching 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 ways to implant a light source. If successful, the researchers hope their strategy will benefit people who have suffered strokes, amputations, and spinal cord injuries, as well as others with poor control of their limbs.
“This could lead to a minimally invasive strategy that will be a game-changer for the clinical care of people with limb disease,” Hare says.
Financing: The research was funded by the K. Lisa Yang Center for Bioelectronics at MIT.
About optogenetics and neuroscience research news
author: Melanie Grados
source: Massachusetts Institute of Technology
communication: Melanie Grados – Massachusetts Institute of Technology
picture: Image credited to Neuroscience News
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“Closed-loop optogenetic neuromodulation enables muscle control with high precision and resistance to fatigue“By Hugh Hare et al. Scientific robots
a summary
Closed-loop optogenetic neuromodulation enables muscle control with high precision and resistance to fatigue
Closed-loop neural prosthetics show promise in restoring movement in individuals with neurological conditions.
However, traditional activation strategies based on functional electrical stimulation (FES) fail to accurately modulate muscle force and exhibit rapid fatigue due to non-physiological recruitment mechanism.
Here, we present a closed-loop control framework that leverages physiological force modulation under functional optogenetic stimulation (FOS) to enable high-precision muscle control for long periods of time (>60 min) in vivo.
We first explored the force modulation property of FOS, which shows more physiological recruitment and significantly higher modulation ranges (>320%) compared to FES.
Second, we developed a neuromuscular model that accurately describes the highly nonlinear dynamics of visually stimulated muscles.
Third, on the basis of the optogenetic model, we demonstrated real-time muscle force control with improved performance and fatigue resistance compared with FES.
This work lays the foundation for fatigue-resistant neural prosthetics and optogenetic-controlled biohybrid robots with high-precision force modulation.
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