Patients with spinal cord injuries are cut off from the world. The communication network between the brain and body is severed, paralyzing the limbs and causing internal organs to lose function. They cannot contract the bladder to pass urine, and the digestive tract cannot push food along. Scientists have found a new way to reconnect paralyzed patients with the world.
A research team led by Hugh Herr, a professor at the Massachusetts Institute of Technology (MIT) Media Lab, said in Nature Communications on the 31st (local time) that animal experiments showed that they could restore organ motility in paralyzed patients by creating a myoneural actuator (MNA), a biological motor that uses living muscle controlled by a computer.
Unlike before, signals from the brain are not sent to implanted mechanical devices in the body; instead, a biological motor composed of real muscle and nerves is operated by a computer. As a result, body parts disconnected from the brain can naturally regain function without mechanical assistance, and senses such as hunger and touch can also be restored. The team said it is the first achievement to develop a living implant device that revives the function of paralyzed organs.
◇Developing a biological engine from living muscles and neurons
Herr is a seminal figure in biomedical engineering. At 17 in 1982, he lost both legs in a rock-climbing accident and has used prosthetic legs ever since. Seeking to develop better robotic legs, he earned a master's in mechanical engineering at MIT and a Ph.D. in biophysics at Harvard University. Herr has developed robotic limbs that move according to a patient's intent and can even feel touch. Wearing robotic legs, he returned to climbing.
Herr, along with postdoctoral fellow Guillermo Herrera-Arcos and Song Hyeon-geun, took on a new challenge to restore organ function in paralyzed patients. Although mechanical devices can be implanted in organs to contract them artificially, making them small and safe enough to be placed inside the body is not easy. Herr's team proposed a biological motor as an alternative to machines.
Song Hyeon-geun said, "We created a motor that restores organ movement by repurposing existing muscle." The nerve that carries computer signals was replaced with the sensory nerve originally present in the organ, instead of the motor nerve whose connection with the brain had already been severed. Over time, the ends of the sensory nerves connected with the muscle fibers of the biological motor.
Here is how the biological motor works. The computer calculates signals to contract the organ for digestion or excretion. When those signals are sent to the biological motor, acetylcholine, a signaling molecule, is released at the terminals of the sensory nerves, causing the associated muscle to contract. The organ muscle then contracts as well, expelling urine and pushing food forward. The researchers attached the biological motor to part of a mouse's small intestine and confirmed that the organ contracted and relaxed in sync with computer commands.
The challenge is to prevent signal interference. Herrera-Arcos said, "For the actuator to control an organ automatically, it must not be under brain control." If a motor nerve is connected to the biological motor before the communication network between the brain and organ is fully restored, the brain could directly control it and conflict with computer commands.
The team blocked signals coming from the brain so the biological motor would receive only commands from the computer via sensory nerves. They explained that sensory nerves, which receive signals from across the body and relay them to the brain, are better suited for the biological motor. In effect, the organ operates solely under the computer and the biological motor. At the same time, they prevented signals sent by the computer to the biological motor from traveling to the brain through the sensory nerves. Otherwise, the computer signals could cause pain or abnormal sensations for the patient.
◇Feeling hunger and sensing virtual worlds as if real
The sensory nerves used in the biological motor also helped reduce muscle fatigue. Axons, the cable-like fibers that deliver signals from motor nerves to muscles, vary in size. As a result, the thickest axons fire first, quickly tiring the muscle. In contrast, sensory nerves have axons of similar size, distributing signals evenly across muscle fibers. That helps prevent fatigue. In experiments, fatigue resistance improved by 260%, the team said.
The biological motor does more than restore organ function. Sensory nerves can not only deliver computer commands but also relay the sensation of organ contraction back to the brain. That allows a person to feel food moving through or clearing from the intestines. A person could again sense fullness or hunger, or the bladder filling and emptying.
By the same principle, the sense of touch for objects in Virtual Reality (VR) can be felt as real. Until now, when a user wearing VR goggles touched an object in view, sensors on a glove detected it and generated corresponding vibrations to simulate touch. But because it is a mechanical device, it has been difficult to perceive tactile sensations like real hands and feet do. Using a biological motor changes that.
The researchers attached a myoneural actuator to a mouse's calf muscle and contracted it using the same method as with organs. This means the remaining muscles of patients with amputated arms or legs can be controlled in the same way. If an avatar lifts a rock in Virtual Reality (VR), the computer contracts the muscles of the biological motors in the remaining limbs to match the action. This allows a person to feel tactile sensations in the virtual world as real, because the brain receives signals via the sensory nerves of the biological motor that their muscles are actually exerting force. It is not mechanical vibration mimicking touch, but true proprioceptive sensation from living tissue.
◇Restoring touch for robotic limbs is possible
With further advances, robotic limbs could be felt and controlled in real time as if they were part of the body. In 2024, Herr and Song's team reported results enabling a patient with an amputated leg to control a robotic leg 100% with the brain. That was thanks to linking brain signals to the robotic leg. The MIT team connected the motor nerves of the remaining leg muscles to the robotic leg so that brain signals translated into robotic movements. That enabled more natural motion.
If a biological motor with sensory nerves is additionally mounted on the remaining leg muscles, the touch sensed by robotic limbs could likely be felt as real. While sensors at the ends of robotic limbs can already relay contact to the brain, using a biological motor appears to enable more natural sensation.
Song, a co–first author of this paper, graduated from Gyeonggi Science High School and Nagoya University's Department of Electrical and Electronic Engineering, and earned a master's degree from the University of Tokyo. Song received a Ph.D. at MIT in biomedical engineering. Song is now preparing a robotics startup. Song said many challenges remain before the results can be commercialized.
Song said, "The core of this study is that we recontrolled paralyzed muscle through sensory nerves and converted living muscle into an actuator that can be driven by an external computer," adding, "The current research is at the stage of verifying feasibility through animal experiments, and applying it to humans will require long-term system optimization and assessments of safety and efficiency."
References
Nature Communications (2026), DOI: https://doi.org/10.1038/s41467-026-70626-6
Nature Medicine (2024), DOI: https://doi.org/10.1038/s41591-024-02994-9