MIT scientists manage to control mice muscles using light

A Massachusetts Institute of Technology (MIT) team led by Mexican researcher Guillermo Herrera-Arcos, an expert in biomechatronics, and biophysical engineer Hugh Herr, presented this week in the journal Science Robotics a new method that uses light instead of using electricity to activate muscles.

In a study conducted on mice, scientists demonstrated that the optogenetic method provides more precise muscle control and a dramatic reduction in fatigue compared to electrical stimulation.

“Stimulation from electronic devices tends to activate the entire muscle at once, causing fatigue and loss of control within 5 to 10 minutes of movement,” Herrera-Arcos tells SINC.

According to this expert, “with optogenetics, control of motor neurons and muscles is similar to how our brains do it. On the other hand, with electrical stimulation, this control is not natural, so it is inaccurate and causes muscle pain. Our system can perform this control with high precision and without fatigue for a long time.”

Optogenetics involves genetically engineering cells to express light-sensitive proteins, allowing the activity of those cells to be directed by exposing them to light.

Not yet applicable in humans

The technique is not yet applicable to humans, but Herrera-Arcos and Hugh Herr, along with researchers at MIT’s C. Lisa Young Center for Bionics, are now working on ways to safely and effectively introduce light-sensitive proteins into human tissue.

“By using light through optogenetics, muscles can be controlled in a more natural way. In future clinical applications, this type of interface could have very wide applications,” emphasizes Herr, who received the Princess of Wales Award for Science and Research in 2016. Technical research for the development of high-tech bionic prosthetics, which he himself uses.

Hugh Herr (left) believes the use of optogenetics could lead to the development of a minimally invasive strategy that would be a game-changer for the clinical care of people with limb pathologies. /Steph Stevens

In recent years, great advances have been made in the field of electrical stimulation of the spinal cord. Among the most advanced groups is neuroscientist Grégoire Courtin of the École Polytechnique Fédérale de Lausanne (Switzerland). Last week in Nature Medicine, the team presented results using a non-invasive electrical spinal cord stimulation device that improved arm and hand function in 43 people with tetraplegia.

Curtin’s group has previously made paralyzed people walk again using electronic spinal cord implants. Despite advances in the field, MIT researchers insist that the use of this type of electronic device is not widespread because it causes rapid muscle wasting and poor control.

However, Herrera-Arcos acknowledges to SINC that fatigue has not yet been widely studied in recent studies, such as those conducted by Curtin’s team.

Optical molecular machines

In the new work, MIT technologists attempted to control muscle contraction using optical molecular machines through optogenetics rather than using electrodes.

To do this, they used mice as an animal model and compared the amount of muscle force they could generate using the traditional method of electrical stimulation with the force generated by their optogenetic system.

The rodents were genetically modified to express a light-sensitive protein called channelrhodopsin-2. They had a small light source implanted near the tibial nerve, which controls the muscles in the lower leg.

Muscle strength was then measured while gradually increasing the amount of light stimulation. They found that, unlike electrical stimulation, optogenetic control caused a constant and gradual increase in muscle contraction.

Herrera-Arcos reiterates that by varying the optical stimulation of the nerve, they controlled the strength of the muscle in a proportional, almost linear manner. “It’s similar to how brain signals control muscles. This is why they are easier to control than electrical stimulation.”

Fatigue resistance

Using data from the experiments, the researchers created a mathematical model of optogenetic muscle control capable of relating the amount of light entering the system to the muscle’s power output (how much force is generated).

The mathematical model allowed the design of a closed-loop controller. In this type of system, the controller emits a stimulating signal and when the muscle contracts, the sensor detects the force it exerts. Then calculate whether and to what extent the light stimulation needs to be adjusted to achieve the desired strength.

They noticed that muscles could be stimulated for over an hour before they fatigued, whereas with electrical stimulation the muscles weakened after just 15 minutes.

Researchers found that muscles can be stimulated for more than an hour before they become fatigued, whereas with electrical stimulation, muscles weaken within 15 minutes.

One of the obstacles these experts are trying to overcome is how to safely introduce light-sensitive proteins into human tissue.

Herrera-Arcos explains to SINC that “these proteins are already in human clinical trials to restore vision in patients with diseases such as retinitis pigmentosa. However, he adds, when they are applied to systems other than the visual system, such as, for example, in the peripheral muscle control system, an immune response to the protein occurs.

That, he says, “is now our main goal: to make the light-sensitive protein compatible with the peripheral nervous system. In addition, we are working on integrating this stimulation technology with tissue sensory systems to restore movement in complex situations such as walking and running.”

Returning sensations

As for the possible benefits of his technique when perfected, the Mexican researcher points out that “in many conditions in which paralysis occurs, the muscles are healthy, but they do not receive a signal about when and how they should move.” “

He adds that with his system, “signals that would normally come from the brain will be replaced by light impulses that will tell the muscles when and how to move.”

Using optogenetics, the MIT team’s goal is to “restore muscle function in people with paralysis and those who have undergone amputation. In the latter, this could be used to stimulate muscles in response to movements of the prosthesis and return sensation,” says Herrera-Arcos.

“This could lead to the development of a minimally invasive strategy that will be a game-changer when it comes to the clinical treatment of people suffering from limb pathologies,” Herr concludes.

Link:

Herrera-Arcos, Mister el-al. “Loop-loop optogenetic neuromodulation enables highly precise, fatigue-resistant muscle control.” Scientific Robotics (2024)