An anonymous reader writes: Harvard researchers report in the journal Science this week that they've built a "bio-inspired swimming robot that mimics a ray fish [and] can be guided by light." The robot's body consists of "a cast elastomer body with a skeleton of gold, along with a single layer of carefully aligned muscle fibers harvested from neonatal rat hearts." The fibers were genetically modified to respond to pulses of blue light and structured along the body of the robot such that contractions result in a repetitive undulating motion, propelling the robot forward. "About 200,000 live rat heart cells form the muscle layer that powers the robot, which has a body 16.3 mm long and weighs just over 10 grams," reports IEEE. "At full tilt, it can swim at a speed of 3.2 mm/s, which isn't bad for such a tiny thing." You can watch the video that shows the robot being led through a 250 mm long obstacle course.