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JESUS BUGS: So called due to their water-walking skills, water striders even mate on water [bottom]. Their mechanical counterpart, a STRIDE bug [top], is no pushover either.
METIN SITTI & MARKUS GAYDA

As if signing books and performing surgery on patients were not enough, robots can now walk on water, too, thanks to engineers at Carnegie Mellon University (CMU). What started as a class project three years ago ended up as insectlike mechanical robots with four to sixteen legs. The "bugs," two to six inches long and weighing a few grams, can scoot over water, reports IEEE Transactions on Robotics. Called STRIDE, for surface tension based robotic insect dynamic explorer, the robots use water's surface tension to amble on their spindly legs exactly like water striders, the insects that motivated the challenge.

Surface tension, the force that produces the lovely teardrop shape of dripping water, also resists surface ruptures. That is why, for example, a thin sewing needle can be placed on a slip of tissue paper on water, and the needle will stay afloat even after the tissue becomes waterlogged and sinks. This is very different from the buoyant force that keeps ships floating, because unlike a ship, the needle is in fact on average denser than water and will sink if gently nudged downward. This experiment works better if the needle has a hydrophobic—or water repellent—coating such as Teflon (like the surface of nonstick pans).

CMU's water-walking robots use the force of surface tension on Teflon-coated legs a few hundred microns thick and two to four inches in length to keep themselves above water. To move and turn, they use "a sculling motion, [where] during the front stroke the leg is in the air, and during the reverse stroke the leg is pushing water [but] never breaking the water surface," says principal investigator Metin Sitti. This is exactly how water-striders move, although their peak speeds are around five feet per second, compared with several inches a second for these robots.

What limits their speed? Although the robots pump their legs much faster than the insects (40 times a second , as opposed to no more than 10 for the insect), the strokes are much shorter and, moreover, the robots are a hundred times heavier than the insects. "If you scale down the system, you become much more power-efficient and agile," Sitti says about the insects' superior speed.

But a higher speed isn't the only goal; a single water-strider leg can support 15 times the body weight of the insect, whereas a mosquito leg on water can support 23 times its body weight — compared with only about half a body weight per leg for a robotic bug. Furthermore, insects routinely sink below water's surface due to rain or turbulence, but always manage to resurface . Both of these impressive abilities are due to microscopic hairs on their legs, which, in addition to making their legs more hydrophobic, also trap air that acts like a buoyancy cushion or life jacket. Sitti is trying to mimic that feature using hair from geckos' feet on his robots' legs; microfabrication of synthetic hairs may come later. He is also trying to use non-Teflon coatings that are more hydrophobic, like that used on Gore-Tex boots.

These bugs "are really cheap to manufacture on a large scale, because they're really simple," Sitti explains, which makes them prime targets for several applications. Besides their educational and entertainment value, these robots can be used for environmental monitoring of still-water ecosystems like ponds and marshes. They can walk on water as shallow as a tenth of an inch, and carry sensors that transmit information back to the shore, functioning as minimally invasive probes, and opening up a whole new form of bugging.