On a table in a lab in west London sits a plastic chamber two feet tall, one foot wide, and about three inches thick. It’s divided vertically in half by a thin polymer film: on one side roils a solution of either ferricyanide or buffered oxygen; on the other, nitrogen bubbles through a broth of organic chemicals. And swimming in that nutrient broth are trillions of single-celled microbes, noshing.
What electrochemist Peter Bennetto and his colleagues at King’s College have created in their little plastic chamber of microbes is a battery--a living battery. Harnessed properly, Bennetto says, the energy released by these bugs could one day power everything from wristwatches and automobiles to Third World villages. The potential, he claims, is enormous.
Looking at the latest prototype--a cell the size and shape of a Jeep’s outboard gasoline can, with a meager half-volt of power--an observer might be forgiven some skepticism. But though the device may seem primitive, it’s the product of more than 200 years of speculation and research. The late-eighteenth-century Italian physician
Luigi Galvani, who made frogs’ legs dance by passing charges through them, was the first to assert the intimate relationship between electricity and living things. A century after Galvani, biologists were beginning to agree that electricity plays a key role in respiration--not merely breathing but rather the larger process by which cells make use of oxygen. Scientists understood that respiration yields a crop of free electrons, which oxygen molecules ultimately absorb in the creation of water. But those same scientists had no conceptual tools with which to explain electricity’s role in metabolic processes. In 1910, for example, English botanist Michael Potter plunged a platinum electrode into a solution containing either baker’s yeast or Escherichia coli (a common bacterium in the human gut), put the other electrode into an organism-free solution, and registered a current. Potter demonstrated that when his mixtures were drained of electricity, they could recharge themselves, indicating that the current was indeed originating with the organisms. However, Potter and his contemporaries were unable to explain how and why electricity is generated during digestion, and thus could offer no real explanation for the experiment’s results. The scientific community, says Bennetto, was not overawed.
In the decades since Potter, however, researchers have fully examined and described the electrochemical steps by which complex molecules--in particular, carbohydrates--are disassembled by organisms into usable forms and parts. At the cellular level, this breakdown is called catabolism; at the atomic level, where electrons--the raw stuff of electricity--are lost and gained, it’s known as the reduction-oxidation, or redox, reaction. Carbohydrates--sugar, starch, cellulose--are the raw ore of an organism’s energy, rich with the electrons needed to power the reactions that sustain life. As a carbohydrate fuel molecule is taken apart, it loses many of these electrons; it is, in the chemical parlance, oxidized. Normally the electrons are seized by compounds that are constantly being created and consumed. These intermediate substances are said to be reduced--they absorb and hold the loose electrons until the electrons reach their ultimate destination, which is usually oxygen or pyruvic acid.
It is this very process that the King’s College device is designed to exploit. In each of the plastic chamber’s two sides hangs a screen of carbon cloth into which wires are sewn. As the bugs living in the nutrient broth do their metabolic stuff, the electrons they liberate flow out of the broth, up the wires in the carbon cloth, through an external circuit, and back through the carbon cloth’s wires into the other side of the chamber, where they’re absorbed by the oxygen or the ferricyanide solution. (The nitrogen in the broth is there to scour away any oxygen that strays over to the broth side, where it could grab the electrons and stop the current before it even gets started.) If the process sounds familiar, that’s because it is. It’s the conventional setup for an electrochemical cell, says John Stirling, the team’s biochemist.
Take a typical flashlight battery, for example. At the flick of a switch, molecules at the battery’s negative terminal--Bennetto’s organic soup terminal--are broken down, freeing electrons. Their movement through an external circuit and toward the positive terminal--Bennetto’s oxygen or ferricyanide terminal--creates a current. Once they arrive, the electrons are absorbed by oxygen as it combines with hydrogen to become water.
As Bennetto explains, the chief difference between a manufactured battery and a living one is one of potential. A bug digesting its lunch turns a larger proportion of fuel into energy than do typical zinc- or lead-based batteries. Indeed, the quantity of energy a microbe can drain from a given amount of fuel can approach that of lithium--long considered an almost ideal, but rather expensive, battery material. In one of the team’s trials, a bug called Proteus vulgaris surrendered to an electrode fully half the electrons it loosed in catabolizing glucose. The yield would have been higher but for the fact that these bugs do not normally degrade the glucose right down to carbon dioxide and water, Bennetto explains. They’re lazy beasts. E. coli, a more industrious bug, converts more than 90 percent of a sucrose meal into carbon dioxide and water, sparking an electrical yield approaching the theoretical maximum, he says.
Bennetto sees the bugs that populate his fuel cells as living bags packed with enzymes capable of performing a whole series of catabolic reactions. And these bags are fairly cheap. A volume of one cubic centimeter of solution, which is smaller than a sugar cube, notes Bennetto, may contain 100 billion organisms that expose a reactive surface area of five square meters.
The harvest from the team’s four-liter device, however, is a sobering reminder of the distance between theoretical maximums and today’s technologies. It makes about two-tenths of a watt, maybe half a volt, Stirling says. That seems singularly unimpressive, particularly when compared with the microbially powered 35-volt battery put together by Barnett Cohen, a biochemist at Cambridge University, in 1931. But Cohen’s battery required the connection of scores of feeble fuel cells; it was such an unwieldy setup that his colleagues dismissed the device as little more than a parlor trick. This is only one cell, notes Stirling. Our purpose in building this was just to demonstrate that it could function.
To get it to function, the researchers had to clear a few hurdles first, not the least of which involved figuring out how best to collect the electrons on which the device is based. Normally the electrons freed by digestion of a carbohydrate molecule are not truly free--they are always being held, either by the intermediaries or by the final electron recipient.
By the 1970s a few researchers had succeeded in tapping catabolism’s energy but were able to capture only about 1 percent of the energy stored in the carbohydrate feedstock. What they needed to do, they soon realized, was to reach into the microbes somehow, snatch the electrons away from the intermediaries, and ferry them to the wires before they could be delivered to the final recipient.
The stumbling block was the bugs’ thick skin, a double layer of fatty substances called lipids, behind which the catabolic process is carried on. The researchers had to find a way to insinuate some chemical probe into this membrane to sweep up the freed electrons. Bennetto and his cohorts cracked the problem in 1980, when they hit on the idea of spiking the microbes’ nutrient solution with a group of chemical marauders they call mediators--substances able to pierce the cell’s lipid walls like molecular needles, draw off those loose electrons, and spirit them away to an electrode. In effect, we’re replacing oxygen with mediators in the early stages of the catabolic reaction, Stirling says. We’re interposing mediators between the electrons and the oxygen molecules so we can gather the electrons to make electricity.
These chemicals had to fulfill some fairly stringent requirements. First of all, they had to be electron hungry. Usually a molecule’s atoms complement one another and balance out electrically. But sometimes there’s a mismatch, and the molecule constantly gropes for--or tries to get rid of--extra electrons. So Bennetto and Stirling began their hunt by combing through the compounds that other researchers have found particularly adept at collecting and transporting electrons. They also wanted chemicals that would dissolve into their organic soup without breaking down. Once they’d narrowed their choices to a couple of dozen compounds, they tried to find those that were lipophilic--fat loving. Lipophilic compounds have a molecular structure that allows them to embed themselves in a microbe’s membrane. From that select group, the researchers identified the mediators best able to give up their hijacked electrons readily to an electrode.
If added in the right concentration, the mediators allow the chemists to harvest enough electrons to spark a current while leaving the microbes enough to sustain their necessary biological processes. Thus a living battery could theoretically run forever, given enough food. Bennetto feels obliged to point out, however, that although the reaction could continue for a very long time, it would probably not proceed forever. We all get old, he notes--microbes as well as humans. Still, when the team tested the procedure with a strain of E. coli swimming in a lactose broth, the bugs not only generated an electric current but sustained it for more than three months. At that point the research team got tired of watching and turned off the equipment. We didn’t expect the experiment to go on for so long, Bennetto says. It could have gone on longer if we’d let it, but we got fed up. We’d proved that the principle of microbial generation of electricity was on a sound scientific footing. We had other things to do.
For one thing, they needed to find the best foods to feed their bugs. Over the years they’ve developed a list that matches a spectrum of carbohydrates--from glucose to table sugar to molasses--to the microbes that will digest them most efficiently. That gives us a whole range of microbiological tricks, Bennetto adds. For every naturally occurring carbon compound--and some other things as well--there’s a microbe that will eat the stuff.
But like everything else that eats, microbes give off waste. And because fuel cells have no sewer systems, the waste remains in the solution. Some of those waste products--particularly hydrogen ions-- accumulate, so the solution gradually becomes acidic, Bennetto says. Living things don’t do well in acid, and microbes are no exception. In addition, the microbes don’t always eat all their dinner. Some of those leftover things can physically stick to the electrode and grunge up the works, he notes.
The battery’s problems don’t end there. The group is always on the lookout for new and improved mediators--many of their most effective compounds tend to decay when exposed to light. It’s well within the range of chemical synthesis to design things that are much better, Bennetto notes. In related areas, such as light-sensitive devices and sensors, people are synthesizing new compounds all the time that mediate electron transfer.
Yet even as they test these newly concocted compounds, Bennetto and Stirling are considering scrapping mediators altogether. If they could only connect the electrodes directly to the microbes themselves, they say, they could gather the electrons from their source, without the need for any intermediary. An efficient way to do that would be to create a microscopically fine mesh of electrode material that would allow intimate physical contact between the material and the bacteria, Stirling muses. In such a system, you’d no longer need soluble mediators.
It’s a question of implanting something into the bacterium, adds Bennetto, perhaps a matter of genetically modifying microorganisms to have a suitable receptor site. It may sound ambitious now, but it’s well within the range of what might be done within a very few years.
Their battery’s woes don’t worry the team. Because our device is still experimental, we haven’t tried to solve all these problems, Bennetto says with a shrug. We’ve merely anticipated them. Instead, their eyes are firmly fixed on the future. One possibility is a refrigerator-size fuel cell capable of churning out a steady kilowatt of power, enough to meet a sizable share of a typical household’s demand. Once those devices prove viable, bigger biobatteries are only a matter of scale. Our estimates suggest that a room-size reactor, containing a million liters of liquid and ten tons of microorganisms, could produce a megawatt of power from 200 kilograms of carbohydrates per hour, Bennetto says. He points out that tanks and operations of this dimension are often used in wastewater treatment facilities in breweries--although they consume energy, while his device would produce it.
Bennetto is convinced that such reactors could bring cheap electricity to places too remote or too poor to be served by conventional electrical generating plants. In Third World sugar-producing nations such as the Philippines and Cuba, for example, molasses and other processing waste from refineries would become ready-to-use feedstock. Similarly, microbial energy cells could serve as a kind of bioreactor to consume waste products that now pollute. Bennetto notes that the dairy industry often has no place to put the lactose-bearing whey that remains after raw milk is processed. Currently, in New Zealand and parts of Europe, that waste is dumped in the sea or sprayed over forests. A better use is to ‘burn’ the carbohydrate component in a large-scale bioreactor based on the microbial fuel cell, he contends. Materials that are otherwise unusable and would perhaps require expensive treatment could be disposed of easily while generating useful power. Indeed, researchers already know about microbes that can break down wastes as diverse as coffee grounds and phenol.
The research team even envisions bug-powered cars. Bennetto calculates that a comfortable-size electric vehicle could travel as far as 15 miles on two pounds of sugar in a concentrated solution; 13 gallons of the stuff could send a car some 600 miles. While he admits that the energy to be gleaned from catabolism is less than half that derived from the best current engine fuels, he argues that the efficiency of carbohydrate-fueled cells is potentially much greater than that of gasoline-burning engines and compares favorably with the power systems in today’s electric cars. Since the biofuel cell is regenerative--the bugs reproduce constantly--no recharging is necessary, Bennetto says. Occasionally the biotech service station would do a ‘bug change’ rather than an oil change. And, unlike oil, sugar is cheap and replenishable. From an economic viewpoint, sugar power may even prove to be a tough competitor to rechargeable batteries or solar cells, he adds.
For the immediate future of bug power, however, the team knows it’s probably best to think small. The real uses might be in miniature batteries for wristwatches and that sort of thing, Stirling says. It’s surprising, but that kind of device could contain enough feedstock and bacteria to last quite a long time. One of the group’s early prototypes was only an eighth of an inch square but supplied enough power to run a digital clock for a day. Less than a tenth of a gram of carbohydrate would power a quartz analog watch for a year, Bennetto says. Even a single small cell containing less than a tenth of a gram of microorganisms is capable of driving a small motor.
Though they’re more than willing to speculate on the future of bug power, Bennetto and Stirling quickly lose interest in the minutiae involved in getting there--figuring out how miniaturization can be achieved, working out a more efficient electrode, concocting a more stable mediator. We’ve established that you can get energy from bugs, Stirling says. Beyond that, you’re just talking about engineering. We’re in this for the science.
