Engineers design soft and versatile ‘skeletons’ for muscle-powered robots

For these reasons, engineers are exploring ways to power robots with natural muscles. They’ve demonstrated a handful of “biohybrid” robots that use muscle-based actuators to power artificial skeletons that walk, swim, pump, and grip. But for each bot, there is a very different construct, and no general blueprint for find out how to get probably the most out of muscles for any given robot design.

Now, MIT engineers have developed a spring-like device that could possibly be used as a basic skeleton-like module for nearly any muscle-bound bot. The brand new spring, or “flexure,” is designed to get probably the most work out of any attached muscle tissues. Like a leg press that is fit with just the best amount of weight, the device maximizes the quantity of movement that a muscle can naturally produce.

The researchers found that after they fit a hoop of muscle tissue onto the device, very similar to a rubber band stretched around two posts, the muscle pulled on the spring, reliably and repeatedly, and stretched it five times more, compared with other previous device designs.

The team sees the flexure design as a brand new constructing block that will be combined with other flexures to construct any configuration of artificial skeletons. Engineers can then fit the skeletons with muscle tissues to power their movements.

“These flexures are like a skeleton that individuals can now use to show muscle actuation into multiple degrees of freedom of motion in a really predictable way,” says Ritu Raman, the Brit and Alex d’Arbeloff Profession Development Professor in Engineering Design at MIT. “We’re giving roboticists a brand new algorithm to make powerful and precise muscle-powered robots that do interesting things.”

Raman and her colleagues report the main points of the brand new flexure design in a paper appearing within the journal Advanced Intelligent Systems. The study’s MIT co-authors include Naomi Lynch ’12, SM ’23; undergraduate Tara Sheehan; graduate students Nicolas Castro, Laura Rosado, and Brandon Rios; and professor of mechanical engineering Martin Culpepper.

Muscle pull

When left alone in a petri dish in favorable conditions, muscle tissue will contract by itself but in directions that aren’t entirely predictable or of much use.

“If muscle just isn’t attached to anything, it can move loads, but with huge variability, where it’s just flailing around in liquid,” Raman says.

To get a muscle to work like a mechanical actuator, engineers typically attach a band of muscle tissue between two small, flexible posts. Because the muscle band naturally contracts, it might probably bend the posts and pull them together, producing some movement that will ideally power a part of a robotic skeleton. But in these designs, muscles have produced limited movement, mainly since the tissues are so variable in how they contact the posts. Depending on where the muscles are placed on the posts, and the way much of the muscle surface is touching the post, the muscles may achieve pulling the posts together but at other times may wobble around in uncontrollable ways.

Raman’s group looked to design a skeleton that focuses and maximizes a muscle’s contractions no matter exactly where and the way it’s placed on a skeleton, to generate probably the most movement in a predictable, reliable way.

“The query is: How can we design a skeleton that the majority efficiently uses the force the muscle is generating?” Raman says.

The researchers first considered the multiple directions that a muscle can naturally move. They reasoned that if a muscle is to drag two posts together along a particular direction, the posts needs to be connected to a spring that only allows them to maneuver in that direction when pulled.

“We’d like a tool that may be very soft and versatile in a single direction, and really stiff in all other directions, in order that when a muscle contracts, all that force gets efficiently converted into motion in a single direction,” Raman says.

Soft flex

Because it seems, Raman found many such devices in Professor Martin Culpepper’s lab. Culpepper’s group at MIT makes a speciality of the design and fabrication of machine elements similar to miniature actuators, bearings, and other mechanisms, that will be built into machines and systems to enable ultraprecise movement, measurement, and control, for a wide selection of applications. Among the many group’s precision machined elements are flexures — spring-like devices, often constituted of parallel beams, that may flex and stretch with nanometer precision.

“Depending on how thin and much apart the beams are, you may change how stiff the spring appears to be,” Raman says.

She and Culpepper teamed as much as design a flexure specifically tailored with a configuration and stiffness to enable muscle tissue to naturally contract and maximally stretch the spring. The team designed the device’s configuration and dimensions based on quite a few calculations they carried out to relate a muscle’s natural forces with a flexure’s stiffness and degree of movement.

The flexure they ultimately designed is 1/100 the stiffness of muscle tissue itself. The device resembles a miniature, accordion-like structure, the corners of that are pinned to an underlying base by a small post, which sits near a neighboring post that’s fit directly onto the bottom. Raman then wrapped a band of muscle across the two corner posts (the team molded the bands from live muscle fibers that they grew from mouse cells), and measured how close the posts were pulled together because the muscle band contracted.

The team found that the flexure’s configuration enabled the muscle band to contract mostly along the direction between the 2 posts. This focused contraction allowed the muscle to drag the posts much closer together — five times closer — compared with previous muscle actuator designs.

“The flexure is a skeleton that we designed to be very soft and versatile in a single direction, and really stiff in all other directions,” Raman says. “When the muscle contracts, all of the force is converted into movement in that direction. It’s an enormous magnification.”

The team found they may use the device to exactly measure muscle performance and endurance. After they varied the frequency of muscle contractions (for example, stimulating the bands to contract once versus 4 times per second), they observed that the muscles “grew drained” at higher frequencies, and didn’t generate as much pull.

“Taking a look at how quickly our muscles get drained, and the way we are able to exercise them to have high-endurance responses — that is what we are able to uncover with this platform,” Raman says.

The researchers are actually adapting and mixing flexures to construct precise, articulated, and reliable robots, powered by natural muscles.

“An example of a robot we try to construct in the longer term is a surgical robot that may perform minimally invasive procedures contained in the body,” Raman says. “Technically, muscles can power robots of any size, but we’re particularly excited in making small robots, as that is where biological actuators excel by way of strength, efficiency, and flexibility.”