Researchers grew skeletal muscle cells that contract under light, then steered a two-finned robot through a water maze by flashing one side or the other.

Engineers at MIT have built a robot that swims using living muscle as its motor. The device, about the size of a stick of gum, is coated with a layer of skeletal muscle cells thinner than a human hair, according to MIT News. Shine light on the muscle, and it contracts — flapping the robot's fins.

By flashing light on one fin or the other, researchers steered the robot through a simple watery maze. The work, led by Ritu Raman, an associate professor of mechanical engineering, was published in Advanced Functional Materials.

The appeal of these "living machines" is practical as well as striking. Muscle-powered robots could one day work in delicate places, such as wetlands, reefs or inside the body, where rigid metal machines might cause damage.

A Stick-of-Gum Body with Two Muscle Fins

The robot's skeleton is a soft gel film about half a millimeter thick, split into two fins. Each fin carries its own muscle layer, so the two can be triggered independently. Light on both fins drives the robot forward, while pulsing one side turns it. "You can think of the robot as having two independent muscles," Raman said in the research summary.

At top speed, the robot covers about four body lengths per minute. That is slow next to an Olympic swimmer, who can manage up to 65 body lengths per minute, but similar to a leisurely cow shark. "It takes a lot of force to move through water versus air," Raman said, explaining why swimming is a harder test than earlier crawling designs.

The thin design is the key advance. Earlier biohybrid robots from the same lab used bulky blocks of lab-grown muscle. A flat sheet uses fewer cells and spreads force across a wider area, which the team says makes it more efficient.

Growing Muscle That Lines Up and Pulls

The cells are skeletal muscle cells genetically engineered to contract when they are illuminated, a technique that lets researchers trigger them with light instead of electrical wires. Muscle generates the most force when its fibers line up in the same direction, so the team had to coax the cells into tidy rows.

First author Maheera Bawa and colleagues grew the cells on a gel called gelatin methacrylate, or GelMA. They had earlier used fibrin, a softer material that shriveled under the muscle's pull. Stiffer GelMA held its shape and produced better-aligned, stronger muscle. Square-bottomed grooves cut into the gel guided the cells into parallel fibers.

Before assembly, the researchers "exercised" the tissue with repeated light pulses, strengthening it much as training builds muscle in an athlete. The work was partly funded by the Office of Naval Research.

Timing the light is how the team controls the robot. Pulsing at set intervals sets the rhythm of each fin stroke, and switching which fin receives light changes direction. In the maze test, a researcher moved the light source by hand, and the robot followed it, turning corners as its fins were triggered in sequence.

Slow Swimmer, Big Engineering Step

The researchers see environmental monitoring as one early use. Small, soft robots could move through fragile aquatic habitats to collect data without tearing plants or disturbing animals. Over the longer term, delicate tasks in unpredictable settings are a goal, and Forbes has reported speculation about medical uses such as microsurgery.

Living tissue also has theoretical advantages. Muscle can repair itself and grow stronger with use, and thin designs could lower manufacturing costs. Those benefits have not been demonstrated in this robot.

Raman described the robot as strong for its size, and the team sees the thin-film design as a template that could be adapted to other body shapes.

The test conditions were limited. The robot swam in a lab dish with a researcher moving a light source by hand. It has not been tested in open water, and MIT did not report how long the muscle stays alive and functional. Raman's team says its next goal is a better body design for faster swimming.

Ethical and Practical Limits of Living Machines

Biohybrid robots raise questions that ordinary machines do not. The muscle needs nutrients, the right temperature, and sterile conditions to survive, which makes field use difficult today. Any robot released into a lake or reef would also need a plan for what happens to engineered cells when the device fails.

Muscle cells alone have no nerves or brain, so these robots are not animals. Still, open questions will grow as biohybrid systems become more complex, including where cells come from, how engineered tissue is contained, and how it is disposed of.

For now, the MIT robot is a laboratory demonstration — not a field-ready tool. It shows that a sheet of muscle thinner than a hair can generate enough force to swim and be steered, a step toward soft machines that work alongside nature rather than against it. Practical devices would need to survive outside the lab, swim faster and carry sensors, milestones the team has not yet reached.

The broader field of biohybrid robotics is still young. Each step, from crawling to swimming to steering, has come from careful lab work rather than field trials. The MIT result is a meaningful engineering advance, and readers should expect years of development before living machines leave the lab in any useful form.

What Readers Want to Know

What did MIT build? A soft, gum-sized robot with two fins powered by a thin layer of living skeletal muscle.

How is the muscle triggered? The cells are genetically engineered to contract when light shines on them.

How fast does it swim? About four body lengths per minute at top speed.

What could it be used for? Researchers point to environmental monitoring in fragile waters, with longer-term interest in delicate tasks.

Is the robot alive? It uses living muscle cells, but it has no nerves or brain and is not an animal.

Can it work outside the lab yet? No. It has been tested only in a lab dish with a hand-moved light.