Genetically engineered muscle cells have been made to flex in response to light, a step toward using such light-sensitive tissue to build highly articulated robots. Scientists at MIT and the University of Pennsylvania say their "bio-integrated" approach may one day enable robotic animals to move with the strength and flexibility of their living counterparts. "With bio-inspired designs, biology is a metaphor, and robotics is the tool to make it happen," said MIT engineering professor Harry Asada, co-author of a paper about the work. "With bio-integrated designs, biology provides the materials, not just the metaphor. This is a new direction we're pushing in biorobotics." Asada and MIT postdoc Mahmut Selman Sakar collaborated with biological and mechanical engineering professor Roger Kamm to develop the new approach. They used skeletal muscle — a stronger, more powerful tissue than cardiac or smooth muscle — which needs external stimuli to flex. Although electrodes have been used in the lab to act as neurons to excite muscles by stimulating them with small amounts of current, the technique is unwieldy and likely would bog down a small robot. So, instead, Asada and colleagues turned to the field of optogenetics, in which genetically modified neurons respond to short laser pulses. Experiments started in the brain, but recent projects have included stimulating cardiac cells to twitch. Asada’s team wanted to do the same with skeletal muscle cells. The researchers cultured such cells, or myoblasts, genetically modifying them to express a light-activated protein, then fused myoblasts into long muscle fibers. When they shone 20-ms pulses of blue light into the dish, they found that the genetically altered fibers responded in spatially specific ways: Small beams of light that were shone on just one fiber caused only that fiber to contract, while larger beams covering multiple fibers stimulated all those fibers to contract. The group is the first to successfully stimulate skeletal muscle using light, providing a new, “wireless” way to control muscles. Going a step further, Asada grew muscle fibers with a mixture of hydrogel to form a 3-D muscle tissue, and again stimulated the tissue with light. He found that the muscle responded in much the same way as individual muscle fibers, bending and twisting in areas exposed to beams of light. The strength of the engineered tissue was tested using a small micromechanical chip — designed by Christopher Chen at Penn — that contains multiple wells, each housing two flexible posts. The group attached muscle strips to each post, then stimulated the tissue with light. As the muscle contracts, it pulls the posts inward; because the stiffness of each post is known, the muscle’s force can be calculated using each post’s bent angle. The device also serves as a training center for engineered muscle, providing a workout of sorts to strengthen the tissue. “Like bedridden people, its muscle tone goes down very quickly without exercise,” Asada said. Because the tissue exhibits a wide range of motions, the group is working toward using it in highly articulated, flexible robots. One potential robotic device may involve endoscopy. Asada said a robot made of light-sensitive muscle may be small and nimble enough to navigate tight spaces — even within the body’s vasculature. Although it will be some time before such a device can be engineered, the group’s results are a promising start, he said. “We can put 10 degrees of freedom in a limited space, less than one millimeter,” Asada said. “There’s no actuator that can do that kind of job right now.” The light-activated muscle may have multiple applications in robotics, medical devices, navigation and locomotion, said Rashid Bashir, a professor of electrical and computer engineering and bioengineering at the University of Illinois at Urbana-Champaign. Exploring these applications would mean the researchers would first have to address a few hurdles. “Development of ways to increase the forces of contraction and being able to scale up the size of the muscle fibers would be very useful for future applications,” he said. In the meantime, a more immediate application for both the engineered muscles and microchip could be to screen drugs for motor-related diseases, Asada said. Scientists may grow light-sensitive muscle strips in multiple wells, and monitor their reaction — and the force of their contractions — in response to various drugs. The other authors on the paper are Devin Neal, Yinqing Li and Ron Weiss from MIT, and Thomas Boudou and Michael Borochin from Penn. The research was supported by the National Science Foundation and the National Institutes of Health, among others. It will appear in Lab on a Chip. For more information, visit: web.mit.edu