Our muscles are nature's perfect actuators – devices that transform energy into movement. For their size, muscle fibers are stronger and more precise than most synthetic actuators. You can also heal damage and become stronger through exercise.
For this reason, engineers are exploring ways to power robots using natural muscles. They demonstrated a handful of “biohybrid” robots that use muscle-based actuators to power an artificial skeleton that walks, swims, pumps and grasps. However, every bot has a very different build, and there is no general blueprint for how to best utilize muscle for a particular robot design.
Now MIT engineers have developed a spring-like device that can be used as the basic skeletal module for almost any muscle-linked robot. The new spring, or “flexure,” is designed to get the most out of the attached muscle tissue. It is a device that maximizes the amount of movement that muscles can naturally produce, like a leg press that fits the body with appropriate weight.
The researchers found that when they fitted the device with a ring of muscle tissue, like a rubber band stretched around two poles, the muscles reliably and repeatedly pulled on the spring, resulting in a stretch five times greater than with other previous device designs. .
The team sees the flexure design as a new building block that can be combined with other flexures to build any configuration of an artificial skeleton. Engineers can then fit muscle tissue to the skeleton to power movement.
“These bends are now like a framework that people can use to translate muscle action into different degrees of freedom of motion in a very predictable way,” says Ritu Raman, the Brit and Alex d'Arbeloff Career Development Professor in Engineering Design at MIT. “We are giving roboticists a new set of rules for building powerful, precise muscle robots that do interesting things.”
Raman and her colleagues report details of their new bending design in a paper published in the journal Advanced intelligent system. MIT co-authors on the study include Naomi Lynch ’12, SM ’23; undergraduate student Tara Sheehan; graduate students Nicolas Castro, Laura Rosado, and Brandon Rios; Martin Culpepper is a professor of mechanical engineering.
muscle pulling
When left in a Petri dish under favorable conditions, muscle tissue contracts on its own, but in completely unpredictable or underused directions.
“If the muscle wasn’t attached to anything, it would move around a lot, but the amount of wobble around in a liquid is very variable,” says Raman.
To make a muscle act like a mechanical actuator, engineers typically attach a band of muscle tissue between two small, flexible pillars. As the muscle bands naturally contract, the columns can be bent and pulled together, creating movements that could ideally power parts of a robotic skeleton. However, muscle movement is limited in this design. This is primarily due to the wide variation in the way tissues contact the column. Depending on where the muscles are placed on the pole and how much of the muscle's surface touches the pole, the muscles may succeed in pulling the pole together, but sometimes they may also shake it about in an uncontrollable way.
Raman's group sought to design a skeleton that would focus and maximize muscle contractions, regardless of the exact location and location of the skeleton, to produce maximum movement in a predictable and reliable way.
“The question is how to design a skeleton that most efficiently uses the force generated by the muscles.” Raman says:
The researchers first considered the different directions in which muscles could naturally move. They reasoned that for a muscle to pull two poles together along a particular direction, it must be connected to a spring that allows the poles to move in that direction only when pulled.
“We need a device that is very soft and flexible in one direction and very rigid in all other directions, so that when the muscle contracts, all the force is efficiently converted into one-directional movement,” says Raman.
soft flex
As it turns out, Raman discovered a number of such devices in Professor Martin Culpepper's lab. MIT's Culpepper group specializes in the design and fabrication of mechanical elements, such as miniature actuators, bearings, and other mechanisms, that can be embedded in machines and systems to enable ultra-precision movement, measurement, and control in a variety of applications. Among the group's precision-machined elements is Flexure. Flexures are spring-like devices that are often made of parallel beams and can bend and stretch with nanometer precision.
“Depending on how thin and far apart the beams are, you can change how stiff the spring looks,” Raman says.
She and Culpepper teamed up to design a flexure specifically tailored to its composition and stiffness to allow muscle tissue to naturally contract and spring to maximum length. The team designed the device's configuration and dimensions based on numerous calculations they performed to relate the muscle's natural forces to its flexion stiffness and degree of movement.
The flexion they ultimately engineered is 1/100th the stiffness of the muscle tissue itself. The device resembles a miniature accordion-like structure, the corners of which are secured to the main base by small posts located near neighboring posts that fit directly into the base. Raman then wrapped a muscle band around two corner pillars (the team molded the bands from living muscle fibers grown in mouse cells) and measured how close the pillars were pulled when the muscle bands contracted.
The team found that the configuration of the flexion allows the muscle band to contract mostly along the direction between the two poles. These concentrated contractions allowed the muscles to pull the poles much closer together (five times closer) compared to previous muscle actuator designs.
“Flexure is a framework designed to be very soft and flexible in one direction and very rigid in all other directions,” says Raman. “When a muscle contracts, all that force is converted into movement in that direction. That’s a huge magnification.”
The team found that the device could be used to accurately measure muscle performance and endurance. When they varied the frequency of muscle contractions (for example, stimulating the band at one contraction and four contractions per second), they observed that the muscles “got tired” at higher frequencies and did not produce as much pulling force.
“Looking at how quickly our muscles tire and how we can exercise them to achieve a high endurance response is something we can uncover through this platform,” says Raman.
Researchers are now applying and combining bends to create precise, articulated and reliable robots powered by natural muscles.
“An example of a robot we might want to create in the future is a surgical robot that can perform minimally invasive surgeries inside the body,” Raman said. “Technically, muscles can power robots of any size, but biological actuators are of particular interest for building small robots because they excel in strength, efficiency and adaptability.”