While Slugbot never digested its prey, it laid the groundwork for future bots powered by biomass.
In 2004 researchers unveiled Ecobot II. About the size of a dessert plate, the device could operate for 12 days on a diet of eight flies.
"The flies [were] given as a whole insect to each of the fuel cells on board the robot," said Ioannis Ieropoulos, who co-developed Ecobot II as part of his Ph.D. research.
With its capacitors charged, the bot could roll 3 to 6 inches (8 to 16 centimeters) an hour, moving toward light while recording temperature. It sent data via a radio transmitter.
While hardly a speedster, Ecobot II was the first robot powered by biomass that could sense its world, process it, act in it, and communicate, Melhuish says.
The scientist sees analogs in the autonomously powered robots of the future.
"If you really do want robots that are going to monitor fences, [oceans], pollution levels, or carbon dioxideall of those thingswhat you need are very, very cheap little robots," he said.
"Now our robots are quite big. But in 20 to 30 years time, they could be quite minuscule."
Whether microbial fuel-cell technology can advance enough to power those robots, however, is unclear.
Stuart Wilkinson, a mechanical engineer at the University of South Florida in Tampa, developed the world's first biomass-powered robot, a toy-train-like bot nicknamed Chew-Chew that ran on sugar cubes.
He says the major drawback of MFCs is that it takes a big fuel cell to produce a small amount of power.
Most AA batteries, for example, produce far more power than a single MFC.
"MFCs are capable of running low-power electronics, but are not well suited to power-hungry motors needed for machine motion," Wilkinson said in an email interview.
He added that scientists "need to develop MFC technology further before it can be of much practical use for robot propulsion."
Ieropoulos, Ecobot II's co-developer, agrees that MFCs need a power boost.
He and his colleagues are exploring ways to improve the materials used in MFCs and to maintain resident microbes at their peak.
To date, the Bristol team has hand-fed its bots.
But if the researchers are going to realize their vision of autonomously powered robots, then the machines will need to start gathering their own food.
When Ecobot II debuted in 2004, Melhuish suggested one way that it might lure and capture its fly food-source: a combination fly-trap/suction pump baited with pheromones.
Whether the accessory will appear in Ecobot III is anyone's guess. The BRL team remains tight-lipped about their current project, preferring to finish their work in secret before discussing it publicly.
Melhuish will say this, however: "What we've got to do is develop a better digestion system . There are many, many problems that have to be overcome, and waste removal is one of them."
Will the future bring robot restrooms? Watch this space.
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