Swimming Soft Robots Inspired by Biomechanics of Manta Ray

Butterfly Bot" is the fastest swimming soft Robot ever, inspired by manta ray Biomechanics.

Researchers used the biomechanics of the manta ray to create an energy-efficient soft robot that can swim more than four times faster than previous swimming soft robots. The soft robots were developed at North Carolina State University (NC State) and are known as "butterfly bots".

As their swimming motion is similar to that of a person swimming a butterfly stroke. "To date, swimming soft robots have not been able to swim faster than one body length per second, but marine animals - such as manta rays canto swim much faster, and much more efficiently," says Jie Yin, an associate professor of mechanical and aerospace engineering at NC State and the paper's corresponding author. "We wanted to use these animals' biomechanics to see if we could develop faster, more energy-efficient soft robots." The prototypes we've created perform admirably." The researchers created two types of butterfly bots. One was designed for speed and could reach average speeds of 3.74 body lengths per second.

A second was created to be highly maneuverable, with the ability to make sharp turns to the right or left. This agile prototype was capable of reaching speeds of 1.7 body lengths per second. "Researchers studying aerodynamics and biomechanics use something called a Strouhal number to assess the energy efficiency of flying and swimming animals," says Yinding Chi, the paper's first author, and a recent NC State Ph.D. graduate. "When an animal swims or flies with a Strouhal number between 0.2 and 0.4, it is said to be at peak propulsive efficiency." Our butterfly bots both had Strouhal numbers in this range." The butterfly bots' swimming power is derived from their "bistable" wings, which have two stable states. The wing resembles a snap hair clip. A hair clip is stable until a certain amount of energy is applied to it. When the amount of energy reaches a critical point, the hair clip snaps into a different, stable shape.

North Carolina State University researchers recently developed a fast and efficient soft robotic swimmer that swims in the butterfly-stroke style used by humans. It has a high average swimming speed of 3.74 body length per second, which is nearly five times faster than the fastest similar soft swimmers, as well as a high power efficiency with a low energy cost. The small-sized swimmer is lightweight, weighing only 2.8 grams. It has a soft body and two flapping wings that are bistable. The bistable hair clips inspired the design of the wing. The soft body can bend up and down due to pressurized pneumatic air, which drives the fast flapping of the wings via snapping, as seen in the Venus flytrap.

The butterfly bots have bistable wings inspired by hair clips that are attached to a soft silicone body. Pumping air into chambers inside the soft body allows users to control the switch between the two stable states in the wings. The body bends up and down as those chambers inflate and deflate, causing the wings to snap back and forth with it. "Most previous attempts to develop flapping robots focused on using motors to directly power the wings," Yin says. "Our approach employs bistable wings that are driven passively by moving the central body." This is a significant distinction because it allows for a simpler design, which reduces weight."

The soft body of the faster butterfly bot serves as its "drive unit," controlling both of its wings. This makes it very fast, but also makes turning left or right difficult. The butterfly bot's maneuverability is based on two drive units that are connected side by side. This design allows users to manipulate the wings on both sides or only one wing, allowing it to make sharp turns. "This work is an exciting proof of concept," Yin says, "but it has limitations." "The current prototypes are tethered by slender tubing, which is what we use to pump air into the central bodies." We're currently working on an untethered, self-contained version." The paper, titled "Snapping for high-speed and high-efficiency, butterfly stroke-like soft swimmers," will be published in the open-access journal Science Advances on November 18.