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Walking on Water - Robotic Insect Inspired by Water Striders

A team of researchers have taken inspiration from water strider insects to develop a robot capable of leaping from the surface of water.

The team included researchers from the Seoul National University, Korea (SNU), the Harvard John A. Paulson School of Engineering and Applied Sciences, and Harvard's Wyss Institute for Biologically Inspired Engineering.

Water striders are small, semi-aquatic insects which jump off the water's surface to move themselves around. Jumping on water is a complex maneuver. However, the water striders easily float along the water's surface and use their legs to produce a sufficient amount of upward thrust to become airborne.

Although the idea of walking on water may sound strange, many tiny creatures use the surface tension of water to move from one place to another. Now, inspired by this natural water-based locomotion, the researchers have designed an innovative robotic insect capable of leaping from the water's surface. This latest development has shed new light on the natural mechanisms that enable these insects to leap from both fluid water and stiff ground with equal amount of height and power. The study results have appeared in the July issue of Science.

Jumping on Water: Robotic Water Strider

Harvard University |

"Water's surface needs to be pressed at the right speed for an adequate amount of time, up to a certain depth, in order to achieve jumping," stated Kyu Jin Cho, the study's co-senior author and Associate Professor in the Department of Mechanical and Aerospace Engineering and Director of the Biorobotics Laboratory at Seoul National University. "The water strider is capable of doing all these things flawlessly."

Water striders have legs with slightly curved tips, and use a rotational leg movement to jump off from the water's surface. This was discovered by Ho-Young Kim, the co-senior author, Director of SNU's Micro Fluid Mechanics Lab, and Professor in SNU's Department of Mechanical and Aerospace Engineering. Kim is also a former Wyss Institute Visiting Scholar who worked with Eunjin Yang, the study's co-first author and a graduate researcher at SNU's Micro Fluid Mechanics lab.

He collected these small insects and took videos of them to study the mechanism that allows these insects to float and jump off the water's surface. Following much trial and error, and by utilizing robotic models to test their theories, the researchers were finally able to interpret the mechanics of these semi-aquatic insects.

"If you apply as much force as quickly as possible on water, the limbs will break through the surface and you won't get anywhere," said Robert Wood, Ph.D., who is a co-author on the study, a Wyss Institute Core Faculty member, the Charles River Professor of Engineering and Applied Sciences at the Harvard Paulson School, and founder of the Harvard Microrobotics Lab.

By comparing their robotic insect prototypes to the studied water striders, the team found that the most suitable way to jump off from the water’s surface was to ensure that contact is maintained between the leg and water for as long as possible during the jump motion. The new robotic insect mimics these mechanics and can apply as much as 16 times its body weight on the surface of the water without falling beneath the surface.

"Using its legs to push down on water, the natural water strider exerts the maximum amount of force just below the threshold that would break the water's surface," stated Je-Sung Koh, Ph.D., the study's co-first author, who is now a Postdoctoral Fellow at the Harvard Paulson School and the Wyss Institute. A large number of organisms, including the water strider, can easily perform unique styles of movements such as floating, flying, jumping or swimming on water despite lacking any complicated cognitive skills.

This is due to their natural morphology. It is a form of embodied or physical intelligence, and we can learn from this kind of physical intelligence to build robots that are similarly capable of performing extreme maneuvers without highly-complex controls or artificial intelligence.

Cho, co-senior author

The researchers developed the robotic insect by leveraging a torque reversal catapult mechanism, which was inspired by the manner in which a flea moves and jumps, thus enabling this type of unique locomotion without using any intelligent control. Wood, Cho, and Koh first reported this locomotion in 2013 at the International Conference on Intelligent Robots and Systems.

To make the robotic insect to jump off the water’s surface, the catapult mechanism uses a burst of momentum combined with restricted upward thrust to push the robot from the water without actually breaking the water's surface. An automatic triggering mechanism was developed from actuators and composite materials, and this was used to trigger the lightweight catapult.

In order to create the body of the robotic insect, a technique called pop-up manufacturing was utilized to produce folded composite structures, which self-assemble in a similar way to the foldable parts that pop up in 3D books. This creative layering and folding process was developed by engineers at the Wyss Institute and the Harvard Paulson School, and helps in the rapid development of microrobots as well as a wide range of electromechanical instruments.

"The resulting robotic insects can achieve the same momentum and height that could be generated during a rapid jump on firm ground - but instead can do so on water - by spreading out the jumping thrust over a longer amount of time and in sustaining prolonged contact with the water's surface," said Wood.

"This international collaboration of biologists and roboticists has not only looked into nature to develop a novel, semi-aquatic bioinspired robot that performs an new extreme form of robotic locomotion, but has also provided us with new insights on the natural mechanics at play in water striders," said Wyss Institute Founding Director Donald Ingber, M.D., Ph.D.


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