Optogenetic techniques have enabled noninvasive control of miniature biological robots, or biobots, powered by muscle cells, bringing such bots one step closer to application in health, sensing and the environment. A research team from the University of Illinois had previously demonstrated biobots that were activated with an electrical field. But electricity can cause adverse side effects to a biological environment and does not allow for selective stimulation of distinct regions of muscle to steer the biobot, said research professor Rashid Bashir. Muscle-powered walking biobots respond to light and have a modular design. Courtesy of Ritu Raman, University of Illinois. Now the team has reported a modular, light-controlled skeletal muscle-powered bioactuator that can generate up to 300 μN of active tension force in response to a noninvasive optical stimulus. When coupled to the 3D-printed flexible biobot skeleton, the actuators drove directional locomotion at a rate of 310 μm/s (1.3 body lengths/min) and 2D rotational steering at 2°/s in a precisely targeted and controllable manner. The researchers began by growing rings of muscle tissue from a mouse cell line containing an added gene, such that a certain wavelength of blue light stimulates the muscle to contract, a technique called optogenetics. The rings were looped around posts on 3D-printed flexible backbones, ranging from about 7 mm to 2 cm in length. "The skeletal muscle rings we engineer are shaped like rings or rubber bands because we want them to be modular," said graduate student Ritu Raman. "This means we can treat them as building blocks that can be combined with any 3D-printed skeleton to make biobots for a variety of different applications." In addition to the modular design, the thin muscle rings had the advantages of allowing light and nutrients to diffuse into the tissue from all sides, in contrast with earlier biobot designs, which used a thick strip of muscle tissue grown around the skeleton. The researchers tried skeletons of a variety of sizes and shapes to find which configurations generated the most net motion. They also exercised the muscle rings daily, triggering the muscle with a flashing light, to make them stronger so that the bots moved farther with each contraction. "This is a much more flexible design," Bashir said. "With the rings, we can connect any two joints or hinges on the 3D-printed skeleton. We can have multiple legs and multiple rings. With the light, we can control which direction things move. People can now use this to build higher-order systems." The research was published in the Proceedings of the National Academy of Sciences (doi: 10.1073/pnas.1516139113).