Scientists learn how to control muscles with light

Researchers at MIT have developed a new approach that they hope will one day provide better muscle control while reducing fatigue. Instead of using electricity to stimulate muscles, they use light. In studies on mice, the researchers showed that this optogenetic technique dramatically reduced fatigue and allowed for more precise muscle control.

“We've found that by using light through optogenetics, we can control muscles in a more natural way. In terms of clinical applications, this type of interface could have very broad applications,” said Hugh Herr, MIT Professor of Media Arts and Sciences, co-director of the K. Lisa Yang Bionics Center, and associate member of MIT's McGovern Brain Institute.

Optogenetics is a method of genetically engineering cells to express light-sensitive proteins, allowing researchers to control the cells' activity by exposing them to light. This approach is not currently feasible in humans, but Her, MIT graduate student Guillermo Herrera Arcos, and their colleagues at the K. Lisa Yang Bionics Center are currently working on ways to safely and effectively deliver the light-sensitive proteins to human tissue.

Har was the senior author of the study. Science RoboticsHerrera Arcos is the lead author of the paper.

Optogenetic control

For decades, researchers have been investigating how to control muscles in the body using functional electrical stimulation (FES), which involves implanting electrodes that stimulate nerve fibers to cause muscles to contract. However, this stimulation tends to activate the entire muscle at once, which is not how the human body naturally controls muscle contractions.

“Humans have incredible control over the natural recruitment of muscles. As the signal strength increases, small motor units are recruited, then medium-sized motor units, then large motor units,” Herr says. “With FES, when you artificially apply electricity to a muscle, the largest units are recruited first. So as the signal increases, at first you don't have much force, but then all of a sudden you get too much force.”

This high force not only makes fine muscle control difficult, but it also quickly wears down your muscles within five to ten minutes.

The MIT team wanted to see if they could replace the entire interface with something else: instead of electrodes, they decided to try using optogenetic optical molecular machines to control muscle contractions.

The researchers used mice as an animal model to compare the muscle forces they could generate using a traditional FES approach with those they could generate using an optogenetics method. In their optogenetics study, they used mice genetically engineered to express a light-sensitive protein called channelrhodopsin 2. The researchers implanted a small light source near the tibial nerve, which controls the muscles in the lower legs.

The researchers measured muscle strength while gradually increasing the amount of light stimulation and found that, unlike FES stimulation, optogenetic control produced a steady and gradual increase in muscle contraction.

“By varying the light stimulation we deliver to the nerves, we can control muscle force in an almost linear, proportional manner, similar to how signals from the brain control muscles. This makes it easier to control muscles compared to electrical stimulation,” Herrera Arcos says.

Fatigue resistance

The researchers used the data from these experiments to create a mathematical model of optogenetic muscle control, which relates the amount of light input into the system to the muscle's output (the amount of force produced).

This mathematical model allowed the researchers to design a closed-loop controller. In this type of system, the controller sends a stimulation signal, the muscle contracts, and then a sensor detects the 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 reach the desired force.

Using this type of control, the researchers found that muscles could be stimulated for over an hour before becoming fatigued, whereas using FES stimulation the muscles became fatigued after just 15 minutes.

One hurdle researchers are trying to overcome now is how to safely deliver light-sensitive proteins to human tissue: A few years ago, Herr's lab reported that in rats, these proteins could trigger an immune response that inactivated the proteins, leading to muscle atrophy and cell death.

“A major goal of the K. Lisa Yang Bionics Center is to solve this problem,” Herr said. “A multipronged effort is underway to design new light-sensitive proteins and develop strategies to deliver them without eliciting an immune response.”

As a further step toward human patients, Herr's lab is also working on new sensors that can measure muscle strength and length, as well as new ways to implant the light source. If successful, the researchers hope that the strategy could also benefit people who have suffered strokes, amputations, or spinal cord injuries, or who have impaired limb control.

“This could lead to a minimally invasive strategy that could be game-changing in terms of clinical care for people suffering from limb diseases,” Herr said.