Researchers at MIT have created a new optogenetic technique that could aid individuals with amputation or paralysis in regaining control of their limbs. This technique provides more precise control over muscle contraction and also significantly reduces muscle fatigue. For those with paralysis or amputation, neuroprosthetic systems that use electrical current to artificially stimulate muscle contraction can help restore limb function. However, these systems are not widely used due to the rapid muscle fatigue and poor control they often lead to.Researchers have developed a new approach for controlling muscle movement in mice using light, known as optogenetics. This method offers more precise muscle control and reduces fatigue compared to traditional methods using electricity. Hugh Herr, a professor at MIT, believes that this technique could have wide-ranging applications in clinical settings.The Institute for Brain Research is working on a method called optogenetics, which involves genetically engineering cells to express light-sensitive proteins. This allows researchers to control the activity of those cells by exposing them to light. While this method is not currently feasible in humans, the team at the K. Lisa Yang Center for Bionics, led by MIT graduate student Guillermo Herrera-Arcos, is researching ways to safely and effectively deliver light-sensitive proteins into human tissue. The study, authored by Herr and Herrera-Arcos, is published in Science Robotics.The use of functional electrical stimulation (FES) has been a focus of research for many years as a way to control muscle activity in the body. This involves placing electrodes that stimulate nerve fibers to make a muscle contract. However, this method often results in the entire muscle being activated at once, which is not how the human body naturally controls muscle movement. “Humans have a remarkable ability to precisely control muscle activity, achieved by naturally recruiting small, then moderate-sized, and finally large motor units,” said one expert.
According to Herr, as the signal strength increases, the largest muscle units are recruited first when using FES. This can result in a sudden increase in force after initially getting no force at the beginning.
This sudden increase in force not only makes it difficult to achieve precise muscle control, but it also leads to rapid muscle fatigue within a short period of time, approximately five or 10 minutes.
The team from MIT aimed to explore the possibility of replacing the existing interface with a different approach. Instead of relying on electrodes, they opted to experiment with controlling muscle contraction using optical molecular machines through optogenetics.
<p rnrnThe researchers conducted a study using mice to compare the muscle force generated by the traditional FES approach with the force generated by their optogenetic method. They used mice that had been genetically modified to express a light-sensitive protein called channelrhodopsin-2 for the optogenetic studies. A small light source was implanted near the tibial nerve, which controls the muscles of the lower leg.
As they gradually increased the amount of light stimulation, the researchers measured muscle force and found that the optogenetic control produced a steady, gradual increase in force, unlike the FES stimulation.
When changing the optical stimulation delivered to the nerve, it has a proportional and almost linear control over the muscle’s force. This is similar to how our brain’s signals control our muscles, making it easier to control the muscle than with electrical stimulation,” Herrera-Arcos explains.
Fatigue Resistance
From the experiments’ data, the researchers developed a mathematical model of optogenetic muscle control, which correlates the light input to the muscle’s output.The researchers have developed a mathematical model to control the stimulation of muscles using light. This model allows for the design of a closed-loop controller, where the controller delivers a stimulatory signal to the muscle, and a sensor detects the force exerted by the muscle after it contracts. The information about the force is sent back to the controller, which then calculates the necessary adjustments to the light stimulation in order to achieve the desired force.
By using this control system, the researchers observed that muscles could be stimulated for over an hour before experiencing fatigue, whereas muscles stimulated with FES fatigue within 15 minutes.One challenge that the researchers are currently trying to overcome is finding a way to safely introduce light-sensitive proteins into human tissue. A few years back, Herr’s laboratory found that these proteins can cause an immune response in rats, which deactivates the proteins and could potentially result in muscle atrophy and cell death.
Herr explains, “An important goal of the K. Lisa Yang Center for Bionics is to address this issue. We are currently taking a multi-faceted approach to develop new light-sensitive proteins and delivery methods that do not trigger an immune response.”
In addition to this, Herr’s lab is also working on new strategies to eventually be able to use this technology on human patients.The researchers are working on developing sensors that can measure muscle force and length, as well as finding new ways to implant the light source. They believe that if successful, their approach could help people who have had strokes, limb amputations, spinal cord injuries, and others with limited ability to control their limbs.
Professor Herr believes that this approach could revolutionize clinical care for people with limb pathology, offering a minimally invasive solution.
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