Electronic chips, those thumbnail cities of transistors etched into silicon, have become so smart that sometimes it seems as if they can do everything except get up and stroll around the room. Thanks to Johannes Smits, pretty soon they’ll be doing that, too.
While other scientists mull over ways to pack more and more circuits onto a chip, Smits, a materials scientist at Boston University, has been trying to give chips the one component he thinks they really lack: legs. Last year he figured out a way to do it; the basic idea is to carve the legs right into the chip as part of the same process that lays down the transistors. Now, patent in hand, he is well on his way toward a prototype of the first walking chip.
Why would anybody want a walking chip, you ask? Smits beams at the question. You could equip them with radiation sensors and turn them loose at night in hospital radiation-therapy rooms to track down and dispose of radioactive dust particles, he says. You could build in tiny video cameras to explore the inside of contaminated pipes. You could distribute armies of them in fields to hunt down crop-eating insects. Smits finds it hard to stop once he gets started.
The key to Smits’s scheme for an ultratiny robot lies in his concept of gearless, jointless piezoelectric legs. Piezoelectric materials, such as quartz or zinc oxide, expand or contract when they are exposed to a voltage, as their molecules twist around like compass needles to align their internal charges with the electric field. As a result, such materials can be fashioned into tiny mechanical devices that extend or curl in response to a tiny electric jolt. It’s a beautiful effect, says Smits, who has been working with piezoelectrics for 22 years.
In 1988 Smits attended a conference at which one speaker mentioned the possibility of chip-size robots. I was sure there was some way I could do this with piezoelectricity, he recalls. The way he found was to mount a V-shaped foot between the tips of two parallel piezoelectric beams--the leg--which would be carved into the chip and would project from its sides. The foot could be moved up and down and back and forth by the two beams, which would be controlled by electrical impulses from the chip.
Then there was the question of how many legs to mount on the chip. After studying the gaits of his four cats, as well as that of an occasional raccoon, Smits was finally swayed by the sight of an ant in his backyard pushing a crumb of food many times its own weight. With four legs, you have to worry about where the center of gravity is when you lift two of them, he explains, getting down on his hands and knees to demonstrate how easy it is for a four-legged creature to tip over. So I settled on six legs. Smits’s fifth-of-an-inch-long robot would walk as an ant does, always lifting the middle leg on one side together with the front and back legs on the other.
Smits has since worked out a fairly complete design for his robot, including a solar cell for power and a microphone that would allow a human controller to send signals to the ant’s microprocessor brain via blasts of sound. The microprocessor itself would store the algorithm that tells the robot how to walk and overcome obstacles, but the human would tell it where to walk and what to grab with its pincers. Best of all, the entire design--brain, solar cell, microphone, legs, and all--could be built into the chip in one process.
Or at least it could in theory. In practice, Smits found the task of carving out the piezoelectric leg beams a daunting one. No one had ever built a piezoelectric device small enough and flexible enough to mimic an ant leg. Last year, after two years of experimentation, Smits finally came up with a 200-step chip-etching process that neatly produces zinc-oxide beams that are thinner than a human hair and capable of bending 90 degrees in response to an electric jolt. That was the hard part, he says.
Smits thinks he can build a complete prototype of the robot in two years. In addition to all its other applications, he notes, an ant with a camera and a microphone would make an effective spy. And if you were to throw in a microscopic syringe filled with a deadly toxin . . . well, you get the idea.