Engineers at the Massachusetts Institute of Technology (MIT) have created a paper-thin swimming robot powered by living muscle cells that contract in response to light. This biohybrid robot, approximately the size of a stick of gum, represents a new approach to designing small, efficient machines that integrate biological components. The research, published in the journal Advanced Functional Materials on September 28, 2026, describes a device capable of swimming and steering through water by leveraging a single layer of muscle cells.The robot's structure consists of a flexible gel film that forms two fins. Each fin is coated with a layer of live muscle cells derived from mice, engineered to twitch when exposed to flashes of light. This muscle layer is exceptionally thin, measuring less than 15 micrometers, which is roughly one-fifth the thickness of a human hair. By directing light onto one fin or the other, researchers can induce muscle contractions, causing the fin to flap and propel the robot. Varying the intervals and direction of the light allows for control over the robot's speed and direction.In experiments, the team demonstrated the robot's ability to swim and swivel through a simple watery maze. The robot achieved a maximum speed of about four times its body length per minute. While this pace is slower than an Olympic swimmer, it is comparable to the speed of some marine life, such as the cow shark. Ritu Raman, an associate professor of mechanical engineering at MIT and a study author, noted that the robot is "quite strong, given its size," considering the force required to move through water.The development builds on earlier work from Raman's laboratory, which last year created an iris-inspired artificial muscle disk. That previous design, while demonstrating multi-directional force generation, produced smaller movements. For the swimming robot, the engineers focused on optimizing the gel "skeleton" to maximize muscle force. They found that a gelatin methacrylate (GelMA) film, stamped with microscopic square-bottomed grooves, provided better support for the muscle cells. These grooves guided the developing muscle cells to grow in a uniform direction, forming aligned muscle fibers that could contract more effectively. The team also incorporated a "training" regimen, stimulating the muscle with light for 15 minutes daily, which reportedly increased fin displacement fourfold compared to unstimulated cells.Existing biohybrid robots often rely on bulkier, three-dimensional muscle structures that require millions of cells. The MIT team's design, using a single, thin layer of muscle cells, offers a potentially more efficient and cost-effective method for building such robots. The researchers envision that biohybrid robots powered by living muscle could eventually perform delicate tasks, such as environmental monitoring or exploring fragile or unpredictable environments where traditional hardware might be unsuitable. Living tissue offers advantages such as softness, responsiveness to surroundings, and the capacity for self-healing. The next objective for the researchers is to further optimize the robot's body design to achieve faster swimming speeds.