Popular articles on robotics usually focus on high-tech features like manipulators or vision or bipedality, but the people who actually have to build them worry a lot more about power. There are very few attractive options for machines that are supposed to run around untethered, especially if they have to do so outdoors. Batteries run out of charge, solar power arrays are cumbersome, internal combustion engines need to be refueled, and electrical outlets are few and far between.
This fact of life explains the unusual chain of associations Chris Campbell catalyzed with his exploding beer bottles. In 1996 Campbell was a graduate student in mechanical engineering at the University of South Florida and an enthusiastic amateur brewmaster. One day he forgot to vent some of his bottles properly and they blew up. Later that day he mentioned his experience to a roboticist named Stuart Wilkinson. "Aha!" the professor recalls thinking.
What he saw was that brewery energetics could be applied to solve the robot power problem. You could feed sugars to yeast, producing carbon dioxide (and alcohol), and then use the pressure of the CO2 to turn wheels, moving the robot along. "We called it the flatulence engine," Wilkinson says with a grin, referring to a comparable process that takes place during animal digestion.
The point and virtue of the flatulence engine was that sugar exists naturally, in all vegetation, and vegetation is abundant, accessible, free and available 24 hours a day, rain or shine (at least outside of cities). A robot that could live off the land would have no more power worries than a goat. One obvious application would be gardening robots—mowers, trimmers, weeders—that would draw their power from digesting their own work product. Farmers might become entirely energy-independent, running all their machinery directly from a fraction of their crops. Mobile sensor platforms could be dispatched to wilderness areas for forest management, environmental monitoring and ecological research. There is a long line of people who might be interested in oceangoing machines that could feed themselves by filtering algae or plankton from the water like baleen whales or that could sit on the bottom and live like clams. A bit further out, the machines might become the antibodies of an ecological immune system—equipped to detect and consume ecological invaders such as kudzu or Asian cheatgrass. Military applications would include robot snipers that could live in the woods, feeding on leaves and shrubs.
Rather quickly, however, Campbell and Wilkinson were forced to conclude that while the flatulence engine was a breakthrough conceptually, it was not really practical. A useful robot would almost certainly have to have computers, a radio, navigation and warning lights, probably cameras, and possibly many other types of sensors. All these devices run off electricity, and that the flatulence engine, being mechanical, could not provide. (Mechanical energy can be converted to electrical, but the inefficiencies involved are depressing.)
Still, the idea focused his research, and soon Wilkinson stumbled on the work of a crusading British electrochemist named H. Peter Bennetto. Bennetto, now retired from King’s College in London, had become an authority on biologically produced electricity.
As a result of metabolism, organisms generate a kind of electricity chemically, by making ions, or electrically charged atoms, inside cells. These ions then organize electron flows throughout the cell, bringing power to wherever it is needed. Bennetto discovered a chemical that could penetrate cell walls (in this case, those of microbes), absorb some of these electrons and then exit. Once outside the cell, the chemicals could be stimulated to shed the diverted electrons, creating an electrical gradient or potential. Possible applications include sustainable power plants in third world nations, harvesting electricity from bacterial cultures fed with local vegetation, and creating power from waste products at sugar refineries or dairy farms.
The discovery of Bennetto’s microbial fuel cell (as the device is called) made Wilkinson confident enough to commit his work, for the moment, to the study and design of gastrobots—robots with stomachs. Today, Wilkinson has a simplified test device—basically a train with three 7- by 11-inch cars that is fed on refined sugar—running around a track in his lab. He calls it the gastronome.
The engineer says he gets a flow of mail from citizens worried that he is throwing away our leverage over robots, handing them the freedom they need to become the dominant species. It’s bad enough that they will be able to feed themselves; suppose someday they develop a taste for meat? The energy content of animal protein and fat is far higher than that of vegetable matter. Suppose gastrobots took over the factories—which would no doubt be automated—that make them? Where would we be then? Driven into the dark corners of the world by tireless, implacable, flesh-eating machines. At least one really bad movie could be made about this idea.
There actually is a team in Bristol, England, developing a gastrobot that may someday be powered by meat, specifically that of slow-moving garden pests. They have already designed a mobile unit—the "slugbot"—that identifies and captures slugs. The device was designed to bring slugs back to a central digester, which would in turn power the robot.
Neither the slugbot nor the gastronome is heading outside anytime soon, however. Gastrobotics is a brand-new idea in mechanical engineering, and it raises a long list of issues. Primary, for the slugbot, is the problem of breaking down animal protein and fat: an important part of the puzzle for "flesh-eating robots" that hasn’t been solved yet. It seems that these robots (or rather the microbes that would power them) need to have a balanced diet. "For them to digest all that meat, they need a bit of starch, or the odd leaf," says Chris Melhuish, one of the slugbot’s creators, and part of a team that includes fellow researchers David McFarland and John Greenman.
But even for the vegetarian robot, an internal environment has to be designed that will keep the digesting microbes happy for months on end. The machines will need to distinguish between foods that will keep their microbes well and those that will kill them. Also, bacterial colonies yield more power if they are fed continuously rather than at different times during the day, which suggests that gastrobots will need precise senses of hunger and satiation. Wilkinson’s gastronome now "consumes" pure sugar, creating very little waste, but once more complex carbohydrates (or meat) are involved, robots will have to have some mechanism for expelling waste. Ways must then be found to prevent the electron-transporting chemical from being excreted when the gastrobot relieves itself, so the machines will need a kidney.
Still, it does seem inevitable. The gastrobot combines two strong trends in modern technology: finding ways to harvest power from local sources (solar, wind, geothermal) and borrowing design ideas from biology. Today, engineers are building submarines that swim like fish, airplanes that get about by flapping their wings, and land vehicles that move by walking. One promising approach in artificial intelligence is neural nets, originally based on explicit analogies to the brain. The nature of progress in robotics and automation can be understood as de-domesticating machines; allowing them to operate on their own, eventually even to have original ideas. Gastrobots are another illustration of this deep trend toward blurring the ancient distinction between biology and engineering. Perhaps someday we will have wild machines roaming the landscape, self-sufficient, fully autonomous, answerable to no master but themselves and fate.