For people with amputations, neuroprosthetic systems that artificially stimulate muscle contraction with functional electrical stimulation (FES) can help them regain limb function. However, this type of prosthesis is not widely used because it leads to rapid muscle fatigue and poor control. Researchers at MIT have developed an approach using light instead of electricity that they hope could offer better muscle control with less fatigue.
In a study in mice, the researchers showed that the technique offered more precise muscle control and a dramatic decrease in fatigue.
“It turns out that by using light, through optogenetics, one can control muscle more naturally. In terms of clinical application, this type of interface could have very broad utility,” said Hugh Herr, PhD, a professor of media arts and sciences, codirector of the K. Lisa Yang Center for Bionics at MIT, and an associate member of MIT’s McGovern Institute for Brain Research. “This could lead to a minimally invasive strategy that would change the game in terms of clinical care for persons suffering from limb pathology.”
Optogenetics is based on genetically engineering cells to express light-sensitive proteins, which allowed researchers to control activity of those cells by exposing them to light. The approach is not currently feasible in humans, but Herr, MIT graduate student Guillermo Herrera-Arcos, and their colleagues are working on ways to deliver light-sensitive proteins safely and effectively into human tissue.
For decades, researchers have explored the use of FES to control muscles by implanting electrodes that stimulate nerve fibers, causing a muscle to contract. However, this stimulation tends to activate the entire muscle at once, which is not the way that the human body naturally controls muscle contraction.
“Humans have this incredible control fidelity that is achieved by a natural recruitment of the muscle, where small motor units, then moderate-sized, then large motor units are recruited, in that order, as signal strength is increased,” Herr said. “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.”
Using mice, the researchers compared the amount of muscle force they could generate using the traditional FES approach with forces generated by the optogenetic method. The mice had already been genetically engineered to express a light-sensitive protein called channelrhodopsin-2, and the researchers implanted a small light source near the tibial nerve, which controls muscles of the lower leg.
The researchers measured muscle force as they gradually increased the amount of light stimulation, and found that, unlike FES stimulation, optogenetic control produced a steady, gradual increase in contraction of the muscle.
“As we change the optical stimulation that we deliver to the nerve, we can proportionally, in an almost linear way, control the force of the muscle. This is similar to how the signals from our brain control our muscles. Because of this, it becomes easier to control the muscle compared with electrical stimulation,” said Herrera-Arcos.
Using data from the experiments, the researchers created a mathematical model of optogenetic muscle control. The model related the amount of light going into the system to the output of the muscle (how much force is generated).
The model allowed the researchers to design a closed-loop controller to deliver a stimulatory 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 if, and how much, the light stimulation needs to be adjusted to reach the desired force.
Using this type of control, the researchers found that muscles could be stimulated for more than an hour before fatiguing, while muscles became fatigued after only 15 minutes using FES stimulation.
One hurdle the researchers are now working to overcome is how to safely deliver light-sensitive proteins into human tissue. As additional steps toward reaching human patients, Herr’s lab is also working on sensors that can be used to measure muscle force and length, as well as new ways to implant the light source. If successful, the researchers hope their strategy could benefit people who have experienced strokes, limb amputation, and spinal cord injuries, as well as others who have impaired ability to control their limbs.
Editor’s note: This story was adapted from materials provided by MIT.
To watch a video about the research, visit the MIT Media Lab on YouTube.
The open-access study, “Closed-loop optogenetic neuromodulation enables high-fidelity fatigue-resistant muscle control” was published in Science Robotics.
