Water-Strider Article

After four months of work, Carnegie Mellon graduate student Steve Suhr and assistant professor Dr. Metin Sitti have created a water-strider robot that can walk on water. The journey began when Dr. Sitti, head of the NanoRobotics Lab, asked the students of his interdisciplinary Micro/Nano-Robotics class to think outside the box: ?try to understand a water strider: how it really stays on water, and swims on water, and design your own robot,? he challenged. The course?s first homework assignment had been part of Dr. Sitti?s curriculum for three years, including one teaching at University of California at Berkeley before he arrived in Pittsburgh.
Although the proposal was meant to be the starting point of the course, first-year graduate student Steve Suhr pursued the robot as his final project. Suhr had originally been exposed to the challenges of walking on water by noticing the basilisk lizard, a reptile referred to as the ?Jesus lizard? due to its unique ability to run across the surface of water. ?Maybe someone can make a robot out of that,? Suhr remembered thinking. His interest was further piqued by watching a video of the simple Robostrider made by MIT researchers. Its motion was the result of a wound-up elastic thread and was not continuous.
Having worked with robots that can swim and fly, Dr. Sitti had been interested in the water-strider for some time, and knew that its details were what made it a truly tough system to master. Though the project began with three students, Suhr continued the project after the school year and spent the summer in the NanoRobotics Lab, with Sitti advising him and providing motivation. Suhr also received assistance from other members of the lab. ?Everyone was very helpful,? he said.
The robot is a scant .96 g (its biological inspiration weighs
.1 g) and its body is 1 cubic cm. Six stainless steel legs make up the bulk of its size, stretching the robot?s length to two inches. A polymer of hydrophobic molecules coats each leg, taking advantage of water?s high surface tension to keep the strider standing. Following the example set by the real insect, only four of the robot?s legs are actually on the water, with the remaining two serving as oars to push it across the surface. The movement is the result of a pair of piezoelectric actuators that form a T-shaped plate fastened to the body of the strider. As electricity flows through the actuators, they bend to create a combination of horizontal and vertical swaying that propels the strider.
At first, standard copper electrical wire was used to create the robot. Copper does not float well on water, and a lighter and more hydrophobic material was found in stainless steel. Creating the strider was not without its challenges, however. One major obstacle was faced in perfecting the abnormal movements: unlike a simple back-and-forth movement, the trajectory of the water-strider?s motions is elliptical. The nature of the movements was only recently realized by MIT professor John Bush, who placed real striders on water surfaces marked with dye and watched their movements with high-speed cameras. It was shown that the legs of the strider never broke the surface of the water and moved by a special form of rowing called vortex sculling.
Both Suhr and Sitti emphasized that the project began more for fun than for study. Now that the original goal has been met, work will continue to perfect the robot for real-world applications. Masters student Yun Seong and doctorate student Sang Jun began working on the project this semester. It is expected that the group will continue to work on problems related to this research. As the robot gains in weight and size, it will become more complicated to keep it afloat because its ability to stand on water will become a question of buoyancy rather than surface tension. It is also hoped that the strider will eventually run on batteries.
One application of this research that Sitti and Suhr envision would be a strider working to protect water supplies. The strider would skim across the surface and monitor for toxic materials or pollutants. Cleaning tools could be added to have the robot clean waste from water after a contaminant spill. It could serve as an educational device in science museums, a tool of exploration, or even as an inexpensive toy.
The strider is a single example of the many unique projects being developed in the field of micro or nanorobotics. Dr. Sitti looks forward to adapting swimming robots for use in the human body; the tiny robots will navigate their way through urine or cerebral fluid to inspect for signs of disease. He sees a possibility of attaching non-pathogenic bacteria to the robot to propel it, rather than using batteries. ?If you really want to go to the miniature scale ... you have to use biological systems in your robot,? Sitti said. He points out that the smallest actuator in nature can be likened to the power that results from a bacteria?s metabolism of simple sugar.
Sitti sees the strider as a step in the right direction for microrobotics. Although the dimensions of the robot are on the centimeter scale, he envisions a future with increasingly smaller robots. ?Smaller is better,? he emphasized.