Inspired by the biomechanics of the manta ray, researchers at North Carolina State University have developed an energy-efficient soft robot that can swim more than four times faster than previous soft swimming robots. The robots are called “butterfly robots” because their swimming motion resembles the way a person’s arms move when swimming the butterfly.

“To date, swimming soft robots have not been able to swim faster than a body length per second, but Marine animals— like manta rays — are able to swim much faster and much more efficiently,” says Jie Yin, corresponding author of a paper on the work and associate professor of mechanical and aerospace engineering at NC State. “We wanted to build on the biomechanics of these animals to see if we could develop faster, more energy-efficient soft robots. The prototypes we have developed work exceptionally well.”

Researchers have developed two types of butterfly robots. One was built specifically for speed and was able to reach average speeds of 3.74 body lengths per second. A second was designed to be very manoeuvrable, capable of making sharp turns to the right or to the left. This maneuverable prototype was capable of reaching speeds of 1.7 body lengths per second.

“Researchers who study aerodynamics and biomechanics use what is called a Strouhal number to assess the energetic efficiency flying, swimming animals,” says Yinding Chi, first author of the paper and recent NC State Ph.D. graduate. “Maximum propulsive efficiency occurs when an animal swims or flies with a Strouhal number between 0.2 and 0.4. Both of our butterfly robots had Strouhal numbers in this range.”

Butterfly robots derive their swimming power from their wings, which are “bistable”, meaning that the wings have two stable states. The wing is similar to a hair clip. A hair clip is stable until you apply a certain amount of energy (by bending it). When the amount of energy reaches critical pointthe hair clip snaps into a different shape, which is also stable.

In the butterfly robots, bistable wings inspired by hairpins are attached to a flexible silicone body. Users control the passage between the two stable states in the wings by pumping air into chambers inside the soft body. As these chambers inflate and deflate, the body bends up and down, forcing the wings to flap back and forth with it.

“Most previous attempts to develop flapping robots have focused on using motors to directly power the wings,” Yin says. “Our approach uses bistable wings which are driven passively by moving the central body. This is an important distinction, as it allows for a simplified design, which reduces weight.”

The faster butterfly robot has only one “drive unit” – the soft body – which controls its two wings. This makes it very fast, but difficult to turn left or right. The maneuverable butterfly robot basically has two drive units, which are connected side by side. This design allows users to manipulate the wings on both sides, or “flap” a single wing, allowing it to do sharp turns.

“This work is an exciting proof of concept, but it has limitations,” Yin says. “Obviously, the current prototypes are attached by thin tubes, which are what we use to pump air into the midbodies. We are currently working on developing a self-contained, untethered version.”

The article, “Snapping for high-speed and high-efficient, butterfly stroke-like soft swimmer,” will be published Nov. 18 in the open-access journal Scientists progress. The article was co-authored by Yaoye Hong, a Ph.D. student at NC State; and by Yao Zhao and Yanbin Li, postdoctoral researchers at NC State.