This is Pleobot, a krill-inspired robot for underwater exploration

What makes krill super swimmers?

Krill are exceptional aquatic athletes that excel in swimming, braking, turning, and accelerating. The swimming technique, called metachronal swimming, involves the coordinated movement of multiple appendages or legs sequentially, creating a wave-like motion that propels the organism forward.

Thanks to the metachronal swimming technique, krill can function in diverse and complex oceanic habitats and make large vertical migrations of over 1,000 meters twice daily. Additionally, it results in remarkable maneuverability.

"Experiments with organisms are challenging and unpredictable. Pleobot allows us unparalleled resolution and control to investigate all the aspects of krill-like swimming that help it excel at maneuvering underwater. 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," said Santos in a press release.

Building Pleobot

The construction of Pleobot involved researchers combining fluid mechanics, biology, and mechatronics. Pleobot was built at a scale ten times larger than the krill, typically the size of a paperclip. 

It primarily consists of 3D printable parts with active and passive actuation of the joints to create natural kinematics. They utilized force and fluid flow measurements and biological data to establish the relationship between the flow around the appendage and thrust.

Inspired by the krill, the team built Pleobot to have three movable sections, replication metachronal swimming. The researchers can control the two leg segments of Pleobot and have passive control of its biramous fins, which replicate the opening and closing motion observed in the fins of krill.

The study unveils a previously unknown mechanism of krill swimming. It explains how krill generate lift while swimming forward to prevent sinking since they are slightly heavier than water. Even when not swimming, krill need to create a lift to maintain their position in the water.

"We were able to uncover that mechanism by using the robot. We identified an important effect of a low-pressure region at the back side of the swimming legs that contributes to the lift force enhancement during the power stroke of the moving legs," said Yunxing Su, a postdoctoral associate in the lab and co-author of the study, in the press release.

According to the researchers, Pleobot has the potential to exploit millions of years of evolution in krill to produce superior ocean navigation robots. 

Pleobot offers valuable insights into the fluid-structure interactions required for steady forward swimming in krill.

"Krill aggregations are an excellent example of swarms in nature: they are composed of organisms with a streamlined body, traveling up to one kilometer each way, with excellent underwater maneuverability. This study is the starting point of our long-term research aim of developing the next generation of autonomous underwater sensing vehicles. Understanding fluid-structure interactions at the appendage level will allow us to make informed decisions about future designs," said Monica M. Wilhelmus in the press release, who runs the Wilhelmus Lab at Brown. 

The findings of the study are published in Scientific Reports.

Study abstract:

Metachronal propulsion is widespread in aquatic swarming organisms to achieve performance and maneuverability at intermediate Reynolds numbers. Studying only live organisms limits our understanding of the mechanisms driving these abilities. Thus, we present the design, manufacture, and validation of the Pleobot—a unique krill-inspired robotic swimming appendage constituting the first platform to study metachronal propulsion comprehensively. We combine a multi-link 3D printed mechanism with active and passive actuation of the joints to generate natural kinematics. Using force and fluid flow measurements in parallel with biological data, we show the link between the flow around the appendage and thrust. Further, we provide the first account of a leading-edge suction effect contributing to lift during the power stroke. The repeatability and modularity of the Pleobot enable the independent manipulation of particular motions and traits to test hypotheses central to understanding the relationship between form and function. Lastly, we outline future directions for the Pleobot, including adapting morphological features. We foresee a broad appeal to a wide array of scientific disciplines, from fundamental studies in ecology, biology, and engineering, to developing new bio-inspired platforms for studying oceans across the solar system.