Researchers at the Massachusetts Institute of Technology in the United States have innovatively utilized optogenetic methods to more precisely control muscles and significantly reduce fatigue by stimulating them with light rather than electricity. The relevant paper was published in the new issue of Science Robotics. Stimulating muscle contraction through electric current can help restore limb function in paralyzed patients. But after years of research, this type of prosthesis was not widely used because it can cause rapid muscle fatigue and poor control. Photogenetics is a method of expressing photosensitive proteins based on genetically engineered cells. By shining light, we can control the activity inside cells. This method was previously not feasible in humans, but now researchers at the Massachusetts Institute of Technology have implanted a small light source near the tibial nerve in the calf muscles of mice to compare the muscle strength generated by traditional functional electrical stimulation methods with that generated by photogenetic methods. In the experiment, genetically modified mice were able to express a photosensitive protein called rhodopsin channel protein-2. Research has found that optogenetic control produces stable and gradually increasing muscle contractions, and light stimulation can almost proportionally control muscle strength, similar to the way the human brain signals control muscles. In addition, under the control of photogenetics, muscles only fatigue after receiving stimulation for over an hour, while using functional electrical stimulation for only 15 minutes causes muscle fatigue. Using this data, researchers created a mathematical model that links the amount of light entering the system with muscle output (how much force is generated), and designed a closed-loop controller. The current obstacle that still needs to be overcome is how to safely deliver photosensitive proteins to human tissues. If successful, it can benefit those with impaired limb control due to stroke, amputation, or spinal cord injury. (Lai Xin She)