Saturday, July 24, 2010

Robot eats sewage for energy: researchers develop synthetic gut

Hot on the heels of the first synthetic cell comes a slightly lower-brow advance: a synthetic gut. The basic function that it provides could be the key to freedom for self-sustaining robots.

In the bid to create such autonomous robots, researchers turned to biomass as an energy source. By being able to feed themselves, robots could be set to work for long periods without human intervention.

Such food-munching robots have been demonstrated in the past, often generating power with the help of microbial fuel cells (MFCs) - bio-electrochemical devices that enlist cultures of bacteria to break down food to generate power. Until now, though, no one had tackled the messy but inevitable issue of finding a way to evacuate the waste these bugs produce.

What was needed was an artificial gut, says Chris Melhuish, director of the Bristol Robotics Lab in the UK. He has spent three years with Ioannis Ieropoulos and colleagues working up the concept. The result: Ecobot III.

"Diarrhoea-bot would be more appropriate," Melhuish admits. "It's not exactly knocking out rabbit pellets." Even so, he says, it marks the first demonstration of a biomass-powered robot that can operate unaided for some time.

Previous incarnations of Ecobot showed that it is possible to generate enough power for the robot to exhibit certain basic, yet intelligent behaviours, such as moving towards a light source. Human intervention was needed to clean up after meals, though.

Now, by redesigning the robot to include a digestive tract, Ecobot III has shown that it can survive for up to seven days, feeding and "watering" itself unaided. It obediently expels its waste into a litter tray once every 24 hours.

The key to getting this gut to work, says Ieropoulos, is a recycling system that relies on a gravity-fed peristaltic pump which, like the human colon, applies waves of pressure to squeeze unwanted matter out of a tube.

At the start of the digestive process the robot feeds itself by moving into contact with a dispenser. This pumps a nutrient-rich solution of partially processed sewage into its "mouth" where it is distributed into 48 separate MFCs within the robot. This fluid is a concoction of minerals, salts, yeast extracts and other nutrients. As unappetising as this mixture sounds, for the culture of microbes in the robot's stomach it is ambrosia itself.

At the heart of the process is a reduction-oxidation reaction that takes place in the anode chambers of each of the robot's MFCs. As the bacteria metabolise the organic matter, hydrogen atoms are given off. The hydrogen's electrons migrate to the electrode, generating a current, while hydrogen ions pass through a proton-exchange membrane into the cathode chamber of the cell, which contains water. Here, oxygen dissolved in that water combines with the protons to produce additional water. Because this supply of water gradually evaporates, the robot also needs regular drinks, which it gets from a separate spout.

The cells are arranged in a stack of two tiers of 24 (see picture), designed to allow gravity to direct any heavy undigested matter to accumulate in a central trough. The contents are repeatedly re-circulated from the trough into the robot's feeder tanks to extract as much energy as possible, before being excreted.

Getting rid of this waste not only prevents fuel cells from filling up and becoming clogged, but also removes any acidic waste products from the digester that might poison the bacteria, says Ieropoulos.

As things stand, the fuel cells are capable of extracting a mere 1 per cent of the chemical energy available in its food, despite the recycling process. The system uses off-the-shelf components, so modifying the anodes to have a larger surface area upon which bacteria can attach themselves, should help extract far more energy, says Ieropoulos.

Robert Finkelstein who is heading the Energetically Autonomous Tactile Robot (EATR) project at the US's military research agency DARPA, thinks MFC technology is the wrong choice. It is inefficient and too slow to convert energy, he says.

EATR will derive its energy from burning biomass rather than eating it. Using a novel combustion engine, developed by Cyclone Power Technology of Pompano Beach, Florida, the hope is that when EATR is assembled and tested later this month it will generate enough energy to roll 160 kilometres on 60 kilograms of biomass. In terms of the calorific value of the fuel, that's better than the average car, says Finkelstein.

One of the advantages of MFCs, though, is that they can consume almost anything, including waste water, a substance that isn't easily burned, says Ieropoulos. The bacteria in Ecobot III's gut are made up of hundreds of different species, allowing it to adapt to different foodstuffs. One of the ideas the group is playing with, and the reason they are using waste water as food, is to see if these fuel cells could be used as part of a filtration system to clean up sewage water.

The work will be presented at the Artificial Life conference in Odense, Denmark, next month. The next step is to explore how the robot will cope with a heartier meal, namely flies.

The carnivorous-robot fearing public need not worry, says Melhuish. Much of the energy generated from flies will go into powering the robot's digestive system. With an average speed of about 21 centimetres a day, it is unlikely to catch you, he says.