Summary: Scientists have developed a new approach to controlling muscles 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 individuals 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.
- Scientists are working on ways to safely deliver light-sensitive proteins into human tissue.
Source: HAVE
For people with paralysis or amputation, neuroprosthetic systems that artificially stimulate muscle contraction with electrical 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 poor control.
MIT researchers have developed a new approach that they hope could one day offer better muscle control with less fatigue. Instead of using electricity to stimulate the muscles, they used light. In a mouse study, researchers showed that this optogenetic technique offers more precise muscle control along with a dramatic reduction in fatigue.
“It turns out that light can be used to control muscles more naturally through optogenetics. From a clinical application perspective, this type of interface could have very broad uses,” says Hugh Herr, professor of media arts and sciences, co-director of MIT’s K. Lisa Yang Center for Bionics and an associate member of MIT’s. McGovern Institute for Brain Research.
Optogenetics is a method based on genetically engineering cells to express light-sensitive proteins, allowing researchers to control the activity of these cells by exposing them 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 lead author of the study, which appears today in Scientific robotics. Herrera-Arcos is the lead author of the paper.
Optogenetic control
For decades, researchers have explored 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 the muscle 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 this incredible fidelity of control that is achieved by the natural recruitment of the muscle, where small motor units are recruited, then medium and large motor units, in that order as the strength of the signal increases,” says Herr. “With FES, when you artificially break down a muscle with electricity, the largest units are recruited first. So as you increase the signal, you have no power at the beginning and then all of a sudden you get too much power.”
This great 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 this entire interface with something else. 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 the optogenetic studies, they used mice that had already been genetically engineered to express a light-sensitive protein called channelrhodopsin-2. They implanted a small light source near the tibial nerve that controls the lower leg muscles.
The researchers measured muscle strength as they gradually increased the amount of light stimulation and found that, unlike FES stimulation, optogenetic control produced a steady, gradual increase in muscle contractions.
“As we change the optical stimulation we deliver to the nerve, we can proportionally, in an almost linear fashion, control the strength of the muscle. This is similar to how signals from our brain control our muscles. Because of this, it’s easier to control the muscle compared to electrical stimulation,” says Herrera-Arcos.
Fatigue resistance
Using data from these experiments, the researchers created a mathematical model of optogenetic muscle control. This model relates the amount of light that goes into the system to the output of the muscle (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 stimulation signal, and after the muscle contracts, the 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 needs to be adjusted to achieve the desired strength.
Using this type of control, the researchers found that muscles can be stimulated for more than an hour before fatigue, while muscles fatigue after just 15 minutes with FES stimulation.
One of the hurdles scientists 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 could trigger an immune response that inactivated the proteins and could also lead to muscle atrophy and cell death.
“A key goal of the K. Lisa Yang Center for Bionics is to solve this problem,” says Herr. “Multi-pronged efforts are underway to design new light-sensitive proteins and strategies to deliver them without triggering an immune response.”
As next 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 could benefit people who have experienced stroke, limb amputation and spinal cord injury, as well as others who have impaired ability to control their limbs.
“This could lead to a minimally invasive strategy that would be a game-changer in terms of clinical care for people with limb pathology,” says Herr.
Funding: The research was funded by the K. Lisa Yang Center for Bionics at MIT.
About these news from optogenetics and neuroscience research
Author: Melanie Grados
Source: HAVE
Contact: Melanie Grados – MIT
Picture: Image is credited to Neuroscience News
Original Research: Closed access.
“Closed-Loop Optogenetic Neuromodulation Enables Fatigue-Resistant High-Fidelity Muscle Control” by Hugh Herr et al. Scientific robotics
Abstract
Closed-loop optogenetic neuromodulation enables high-fidelity, fatigue-resistant muscle control
Closed-loop neuroprostheses show promise in restoring movement in individuals with neurological disease.
However, conventional functional electrical stimulation (FES)-based activation strategies cannot accurately modulate muscle force and exhibit rapid fatigue due to their non-physiological recruitment mechanism.
Here, we present a closed-loop control framework that utilizes physiological force modulation under functional optogenetic stimulation (FOS) to enable high-fidelity muscle control over extended periods (>60 min) in vivo.
First, we revealed the force modulation characteristic of FOS, which shows a more physiological recruitment and a significantly higher range of modulation (>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 foundation for fatigue-resistant neuroprostheses and optogenetically controlled biohybrid robots with high-fidelity force modulation.