Our muscles are nature’s perfect actuators – devices that convert energy into movement. Due to their size, muscle fibers are stronger and more precise than most synthetic actuators. They can even heal damage and grow stronger with exercise.
For these reasons, engineers are exploring ways to power robots with natural muscles. They demonstrated several “biohybrid” robots that exploit muscle-based actuators to power artificial skeletons that walk, swim, pump and grasp. However, each bot is built completely differently, and there is no general blueprint for maximum muscle utilization for any given robot design.
Now, engineers at MIT have developed a spring-like device that can be used as the basic skeleton-like module for almost any muscle bot. The novel spring or “flex” is designed to make the most of the attached muscle tissues. Much like a weighted leg press, the device maximizes the range of motion a muscle can naturally produce.
The researchers found that when they fit a ring of muscle tissue to the device, much like a rubber band stretched around two posts, the muscle reliably and repeatedly compresses the spring and stretches it five times more compared to other previous device designs.
The team sees the adaptable element design as a novel building block that can be combined with other adaptable elements to build any configuration of artificial skeletons. Engineers can then equip the skeletons with muscle tissue to enhance their movements.
“These flexions are like a framework that people can now use to translate muscle activation into multiple degrees of freedom of movement in a very predictable way,” says Ritu Raman, the British and Alex d’Arbeloff Professor of Career Development in Engineering Design at MIT. “We are giving roboticists a new set of rules for creating powerful and precise muscle-powered robots that do interesting things.”
Raman and her colleagues describe the details of the novel adaptable design in: newspaper published today in the journal MIT study co-authors include Naomi Lynch ’12, SM ’23; student Tara Sheehan; graduates Nicolas Castro, Laura Rosado and Brandon Rios; and mechanical engineering professor Martin Culpepper.
Muscle strain
Muscle tissue left alone in a Petri dish under favorable conditions contracts on its own, but in directions that are not entirely predictable or useful.
“If the muscle isn’t attached to anything, it will move a lot, but with a lot of variability, and it will just thrash around in the fluid,” Raman says.
To make a muscle act as a mechanical actuator, engineers typically attach a strand of muscle tissue between two miniature, adaptable posts. As the muscle group naturally contracts, it can bend the posts and pull them together, creating movement that would ideally power part of the robot’s skeleton. However, in these designs, the muscles have restricted movement, mainly because the tissues contact the posts in a very variable manner. Depending on where the muscles are placed on the posts and how much of the muscle surface is touching the post, the muscles may succeed in pulling the posts together, but at other times they may wobble uncontrollably.
Raman’s group wanted to design a skeleton that would focus and maximize muscle contractions regardless of where and how it was placed on the skeleton, in order to generate as much movement as possible in a predictable and reliable manner.
“The question is: How do we design the skeleton to most effectively utilize the force produced by the muscle?” – says Ramana.
Scientists first considered the many directions in which a muscle could naturally move. They concluded that if a muscle was to pull two posts together in a specific direction, the posts should be connected to a spring that would allow them to move in that direction only when pulled.
“We need a device that is very soft and flexible in one direction and very stiff in all other directions, so that when the muscle contracts, all of the force is effectively converted into movement in one direction,” Raman says.
Tender flex
As it turns out, Raman found many such devices in Professor Martin Culpepper’s laboratory. Culpepper’s group at MIT specializes in the design and manufacture of machine components such as miniature actuators, bearings, and other mechanisms that can be incorporated into machines and systems to enable ultra-precise motion, measurement, and control in a wide range of applications. Among the precisely machined components in this group are elastic elements — spring-like devices, often made of parallel beams, that can bend and stretch with nanometer precision.
“Depending on how thin and how far apart the beams are, you can change the stiffness of the spring,” says Raman.
She and Culpepper teamed up to design a custom flex element with a configuration and stiffness that allows muscle tissue to naturally contract and maximize the spring’s extension. The team designed the configuration and dimensions of the device based on numerous calculations they performed to relate natural muscle forces to flexion stiffness and degree of movement.
Ultimately, the flexion they design is 1/100 of the stiffness of the muscle tissue itself. The device resembles a miniature accordion-like structure, the corners of which are attached to the base by a miniature post that is located near an adjacent post that is attached directly to the base. Raman then wrapped a strand of muscle around two corner posts (the team formed the strands from living muscle fibers they grew from mouse cells) and measured how closely the posts were pulled together as the muscle strand contracted.
The team found that the flexion configuration allowed the muscle band to contract primarily in the direction between the two posts. This focused contraction allowed the muscle to pull the posts much closer together – five times closer – compared to previous muscle actuator designs.
“The bend is a framework that we designed to be very soft and flexible in one direction and very stiff in all other directions,” says Raman. “When a muscle contracts, all of the force is converted into movement in that direction. This is a huge increase.”
The team found that they could exploit this device to precisely measure muscle performance and endurance. When they changed the frequency of muscle contractions (for example, by stimulating the bands to contract once instead of four times per second), they observed that the muscles “tired” at higher frequencies and did not generate as much tension.
“By looking at how quickly our muscles fatigue and how we can train them to achieve high endurance, that’s what we can discover with this platform,” says Raman.
Scientists are now adapting and combining adaptable components to build precise, articulated and reliable robots powered by natural muscles.
“An example of a robot we are trying to build in the future is a surgical robot that can perform minimally invasive procedures inside the body,” Raman says. “Technically, muscles can power robots of any size, but we’re particularly excited about creating small robots because that’s where biological actuators excel in terms of strength, efficiency, and adaptability.”