Krill-Inspired Robots May One Day Navigate the Ocean

Scientists envision a future in which swarms of autonomous robots can deftly navigate the ocean to map areas where humans can't safely go, aid in complex search-and-rescue missions, and complete other tasks that currently aren't possible with existing oceanographic technology.

To facilitate this, researchers from Brown University have developed a new robotic platform inspired by small crustaceans called krill that have a unique swimming ability. The platform, called Pleobot, is currently composed of three articulated sections that replicate how krills swim, which they call "metachronal swimming."

Indeed, krill are known to have a particular skill in swimming, accelerating, braking, and turning that's aided by the number of legs they have in comparison to their body mass. However, though scientists have long known they could learn a lot about robotic navigation from the tiny creatures, it's not been easy to integrate this into technology, noted one of the researchers, Sara Oliveira Santos, a Ph.D. candidate at Brown’s School of Engineering.

“Experiments with organisms are challenging and unpredictable,” she said in a post on Brown's news page. “Our goal was to design a comprehensive tool to understand krill-like swimming, which meant including all the details that make krill such athletic swimmers.”

Pleobot—which emulates the legs of swimming krill and provides new insights on the fluid-structure interactions needed to sustain steady forward swimming—is the result of this research. "Pleobot allows us unparalleled resolution and control to investigate all the aspects of krill-like swimming that help it excel at maneuvering underwater," Santos said.

This, in turn, will also provide scientists with a foundation for building small, aquatic robots that can explore the ocean, giving them a technological basis for enabling these machines to navigate the underwater environment.

The Aquatic Robotic Platform

The work is a collaboration between Brown researchers in the lab of Assistant Professor of Engineering Monica Martinez Wilhelmus and scientists in the lab of Francisco Cuenca-Jimenez at the Universidad Nacional Autónoma de México.

The team wanted to understand how metachronal swimmers, like krill, can function in complex marine environments, performing massive vertical migrations of over 1,000 meters—equivalent to the height of three Empire State Buildings stacked atop each other—twice daily.

“We have snapshots of the mechanisms they use to swim efficiently, but we do not have comprehensive data,” said Nils Tack, a postdoctoral associate in the Wilhelmus lab.

The researchers “built and programmed a robot that emulates the essential movements of a krill's legs to produce specific motions as well as change the shape of the appendages," he said. They used this to study different configurations so they could take measurements and make comparisons between them that are impossible to achieve when using live animals.

The metachronal swimming technique of krills avails them with a unique maneuverability that they demonstrate by deploying their legs in a back-to-front, wave-like motion. This swimming technique also could be applied to swarms of robots that can be deployed in the Earth's oceans, the researchers said.

The team replicated this swimming technique—particularly the opening and closing motion of the fins—in Pleobot through control of two leg segments as well as passive control of Pleobot’s biramous fins, they said. The model is at 10 times the scale of krill and is composed primarily of 3D printable parts. They've also made the design open-source, which will allow other scientists to use and experiment with Pleobot in their own robotic designs.

Unlocking Biodesign Secrets

Researchers published a paper on their work in the journal, Scientific Reports, in which they share a key insight into the success of krill's swimming technique. Krill are heavier than water, which means they will start to sink if not constantly swimming. To avoid this, they still have to create some lift even while swimming forward to be able to remain at that same height in the water, Santos said.

How they do this is to generate lift through an effect of a low-pressure region at the back side of the swimming legs, the researchers discovered. This enhances the lift force of the moving legs.

“We were able to uncover that mechanism by using the robot,” said Yunxing Su, a postdoctoral associate in the lab.

Researchers hope to uncover more secrets of how krill swim and apply these to robotic designs in the future using the platform, they said. They also are currently advancing the platform design by integrating morphological characteristics of shrimp, such as flexibility and bristles around the appendages, into it.