Biohybrid robots work by combining biological components like muscles, plant material, and even fungi with non-biological materials. While we are pretty good at making the non-biological parts work, we’ve always had a problem with keeping the organic components alive and well. This is why machines driven by biological muscles have always been rather small and simple—up to a couple centimeters long and typically with only a single actuating joint.
“Scaling up biohybrid robots has been difficult due to the weak contractile force of lab-grown muscles, the risk of necrosis in thick muscle tissues, and the challenge of integrating biological actuators with artificial structures,” says Shoji Takeuchi, a professor at the Tokyo University, Japan. Takeuchi led a research team that built a full-size, 18 centimeter-long biohybrid human-like hand with all five fingers driven by lab-grown human muscles.
Keeping the muscles alive
Out of all the roadblocks that keep us from building large-scale biohybrid robots, necrosis has probably been the most difficult to overcome. Growing muscles in a lab usually means a liquid medium to supply nutrients and oxygen to muscle cells seeded on petri dishes or applied to gel scaffoldings. Since these cultured muscles are small and ideally flat, nutrients and oxygen from the medium can easily reach every cell in the growing culture.
When we try to make the muscles thicker and therefore more powerful, cells buried deeper in those thicker structures are cut off from nutrients and oxygen, so they die, undergoing necrosis. In living organisms, this problem is solved by the vascular network. But building artificial vascular networks in lab-grown muscles is still something we can’t do very well. So, Takeuchi and his team had to find their way around the necrosis problem. Their solution was sushi rolling.
The team started by growing thin, flat muscle fibers arranged side by side on a petri dish. This gave all the cells access to nutrients and oxygen, so the muscles turned out robust and healthy. Once all the fibers were grown, Takeuchi and his colleagues rolled them into tubes called MuMuTAs (multiple muscle tissue actuators) like they were preparing sushi rolls. “MuMuTAs were created by culturing thin muscle sheets and rolling them into cylindrical bundles to optimize contractility while maintaining oxygen diffusion,” Takeuchi explains.
Movement in the MuMuTAs was triggered by delivering electrical signals through electrodes attached at both ends. These muscle sushi rolls could bend or rotate, depending on which fibers were contracted. Their contractile force was regulated by modulating the applied voltage.
Once the team made MuMuTAs work, they used five of them to actuate multi-jointed fingers in a robotic hand.
Rock-paper-scissors
The hand, suspended in a liquid medium, was 3D-printed out of plastic. Each finger had three joints and was actuated by a cable connected to a MuMuTA, five of which were located in the forearm. The MuMuTAs were installed in glass containers to limit the diffusion of the electric field, allowing each of them to be actuated separately. They were anchored to the plastic structure on the back end and connected to cables to drive the fingers at the front.
By selectively contracting the MuMuTAs, the hand could do various gestures, like the ones used in the rock, paper, scissors game, or manipulate objects like a pipette. All this was possible because MuMuTAs were strong compared to typical lab-grown muscle systems, with each generating 8 mN of contractile force which is more or less enough to lift a small paperclip. On top of that, Takeuchi’s sushi rolling idea boosted the muscles’ longevity, since MuMuTAs could be unrolled after use to provide oxygen and nutrition to the cells.
The team bumped into some limitations, though. The first issue was that the fingers could only actuate in one direction—the muscles driving them contracted in response to electrical signals, but they only returned to their original position due to the buoyancy of the material. (We mostly use a second set of muscles to return joints to their original position.) Takeuchi suggested one option to solve that would be using an elastic material in the joints, which would make the fingers bounce back faster. The other option he mentioned was adding five more antagonistic MuMuTAs to achieve bidirectional movement—the solution used by the real human hand.
The second problem was that MuMuTAs and the entire hand they actuated couldn’t work without the liquid suspension. “To transition to a dry environment, future developments will need to incorporate artificial nutrient delivery systems and protective scaffolds to maintain tissue viability outside the liquid medium,” Takeuchi says.
But perhaps the most obvious issue with using biological muscles in robots remained unresolved: After making gestures and manipulating objects for about 10 minutes, the biohybrid hand got tired.
Biohybrid robots’ gym
The team noticed the signs of fatigue in MuMuTAs when they were testing how much force the muscles would generate in response to higher voltages. When Takeuchi and his colleagues pushed the MuMuTAs really hard, they saw the contractile force the muscles could generate dropped after a handful of tests. Things went back to normal once the hand rested in its medium for about an hour. This happened even though the muscles grown in petri dishes never worked nearly as hard as they would if they ended up in an actual human being. So, overall, they were rather weak.
The contractile force per unit area Takeuchi achieved in his lab-grown muscles was 0.7 mN per square millimeter, which is not bad compared to other lab-grown muscles. They looked quite feeble, however, compared to living muscles that could generate roughly 6 mN per square millimeter. Takeuchi thinks the solution could be the one we all should be using: exercising.
“Just like natural muscles, engineered muscles may benefit from exercise, where repeated contractions enhance endurance and contractile strength over time,” Takeuchi suggests. Another option Takeuchi’s team proposed in the paper was using chemical growth factors, though. So, we can get them juiced to the gills instead.
Takeuchi’s work on the biohybrid robotic hand is published in Science Robotics: http://dx.doi.org/10.1126/scirobotics.adr5512