Abstract: Researchers have developed a new approach to muscle control using light instead of electricity. This optogenetic technique enables more precise muscle control and significantly reduces fatigue in mice. Although not currently feasible in humans, this approach could revolutionize prosthetics and help people with impaired limb function.
Key facts:
- Optogenetic muscle stimulation offers more precise control than electrical stimulation.
- This method significantly reduces muscle fatigue compared to traditional approaches.
- Researchers are working on ways to safely deliver light-sensitive proteins into human tissue.
Source: MYTH
For people with paralysis or amputation, neuroprosthetic systems that artificially stimulate muscle contraction with electric current can help restore limb function. However, despite many years of research, this type of prosthesis is not widely used because it leads to rapid muscle fatigue and poorer control.
MIT researchers have developed a new approach that they hope could one day offer better muscle control with less fatigue. Instead of electricity to stimulate the muscles, they used light. In a mouse study, researchers showed that this optogenetic technique offers more precise muscle control, with a dramatic reduction in fatigue.
“It turns out that by using light, through optogenetics, we can control the muscles more naturally. In terms of clinical applications, this type of interface could have very broad utility,” says Hugh Herr, professor of media arts and sciences, co-director of the K. Lisa Yang Center for Bionics at MIT and an associate member of the MIT McGovern Institute for Brain Research.
Optogenetics is a method based on the genetic engineering of cells to express light-sensitive proteins, which allows researchers to control the activity of these cells by exposure to light. This approach is currently not feasible in humans, but Herr, MIT graduate student Guillermo Herrera-Arcos, and their colleagues at the K. Lisa Yang Center for Bionics are now working on ways to safely and efficiently deliver light-sensitive proteins to human tissue .
Herr is the senior author of the study, which appears today in the Scientific robotics. Herrera-Arcos is the main author of the paper.
Optogenetic control
For decades, researchers have been investigating the use of functional electrical stimulation (FES) to control muscles in the body. This method involves the implantation of electrodes that stimulate nerve fibers, causing muscle contraction. However, this stimulation tends to activate the entire muscle at once, which is not how the human body naturally controls muscle contraction.
“Humans have this incredible fidelity of control that is achieved through natural muscle recruitment, where small motor units are recruited, then medium-sized, then large motor units, in that order, as the strength of the signal increases,” says Herr. “With FES, when you artificially energize the muscle, the largest units are recruited first. So as you increase the signal, initially you don’t have any force, and then all of a sudden you get too much force.”
This high force not only makes it difficult to achieve fine muscle control, but also wears out the muscle quickly, within five or 10 minutes.
The MIT team wanted to see if they could replace the entire interface with something different. Instead of electrodes, they decided to try controlling 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 optogenetic method. For optogenetic research, they used mice that had already been genetically modified to express a light-sensitive protein called channelrhodopsin-2. They implanted a small light source near the tibial nerve, which controls the lower leg muscles.
The researchers measured muscle strength while gradually increasing the amount of light stimulation and found that, unlike FES stimulation, optogenetic control produced a steady, gradual increase in muscle contraction.
“As we vary the optical stimulation we deliver to the nerve, we can proportionally, in an almost linear fashion, control muscle strength. This is similar to the way signals from our brain control our muscles. This makes it easier to control the muscle compared to electrical stimulation,” says Herrera-Arcos.
Fatigue resistance
Using the 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 muscle output (how much force is generated).
This mathematical model allowed the researchers to design a closed-loop controller. In this type of system, the controller delivers a stimulating signal, and after the muscle contracts, a sensor can detect how much force the muscle is exerting. This information is sent back to the controller, which calculates whether, and how much, the light stimulation should be adjusted to achieve the desired force.
Using this type of control, the researchers found that muscles could be stimulated for over 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, Herr’s lab reported that in rats these proteins can trigger an immune response that deactivates the proteins and can lead to muscle atrophy and cell death.
“A key goal of the K. Lisa Yang Center for Bionics is to solve that problem,” says Herr. “A multifaceted effort is underway to design new light-sensitive proteins and strategies to deliver them, without triggering an immune response.”
As additional 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 incorporate light sources. If successful, the researchers hope their strategy could benefit people who have experienced stroke, limb amputation and spinal cord injury, as well as others who have a weakened ability to control their limbs.
“This could lead to a minimally invasive strategy that would change the game in terms of clinical care for people suffering from limb pathology,” says Herr.
Financing: The research was funded by the K. Lisa Yang Center for Bionics at MIT.
About this news from optogenetics and neuroscience research
Author: Melanie Grados
Source: MYTH
Contact: Melanie Grados – MYTH
Picture: Image credited to Neuroscience News
Original research: Closed access.
“Closed-loop optogenetic neuromodulation enables high-fidelity fatigue-resistant muscle control” Hugh Herr et al. Scientific robotics
Abstract
Closed-loop optogenetic neuromodulation enables fatigue-resistant high-fidelity muscle control
Closed-loop neuroprostheses show promise in restoring movement in people with neurological conditions.
However, conventional functional electrical stimulation (FES)-based activation strategies fail to accurately modulate muscle force and exhibit rapid fatigue due to their non-physiological recruitment mechanism.
Here, we present a closed-loop control framework that uses physiological force modulation under functional optogenetic stimulation (FOS) to enable high-fidelity muscle control over extended periods of time (>60 minutes) in vivo.
First, we revealed the force modulation characteristic of FOS, showing more physiological recruitment and significantly larger modulation ranges (>320%) compared to FES.
Second, we developed a neuromuscular model that accurately describes the highly nonlinear dynamics of optogenetically stimulated muscle.
Third, based on an optogenetic model, we demonstrated real-time muscle force control with improved performance and fatigue resistance compared to FES.
This work lays the groundwork for fatigue-resistant neuroprostheses and optogenetically controlled biohybrid robots with high-fidelity force modulation.