Even if you haven’t seen The Matrix or its sequels, you most likely know the basic premise of the movie: It’s the distant future, and intelligent machines rule the world, having learned to harness an omnipresent and previously underutilized source of electrical power—humans. The machines “grow” people in vast industrial farms and siphon off the small current of electricity generated by the bodies. You, me—we are battery.
Thankfully, civilization hasn’t advanced quite so far. But there’s a scrap of truth to this scenario: Living cells and their multicellular conglomerates (people) do generate a slight electric current. Only now are scientists figuring out how to plug into this resource. The advantages are tantalizing.
A biological fuel cell could generate electricity directly, without the polluting by-products associated with fossil-fuel combustion, and at safe body temperatures. A biological battery also holds the promise of being very small in size, a boon in an age of ever-shrinking electronic devices.
“If you compare the fuel we use with the devices we have, we’re very limited,” says Leonard Tender, an electrochemist at the Naval Research Laboratory in Washington, D.C. “We can look at how microbes do it and use them as a model.”
Whether they choose to eat sugar, sunlight, or filet mignon, cells ultimately derive their energy by shuffling electrons, the negatively charged particles that flitter in atoms and molecules. Some molecules, with minor prompting, will readily give up an electron or two.
Likewise, other molecules are greedy for stray electrons and will relinquish some energy when they get them. Cells ply the middle ground between supply and demand. Aided by specialized enzymes, a yeast bacterium cracks open a sugar molecule, harvests a couple of electrons, then introduces them to nearby oxygen molecules. The reaction generates water, some carbon dioxide, and a little burst of energy, which the bacterium pockets. It’s a subatomic economy, running on a currency of electrons.
An electric current, meanwhile, is nothing more than a steady flow of electrons. A man-made battery is a miniature electron factory, powered by chemicals that react within the battery’s walls. Batteries are handy because their electrons flow through an electrode (that nub at the top of a dry-cell battery) and from there are easily channeled into MP3 players, flashlights, toys, smoke detectors, and so on.
If microbes could be convinced to do the same—to donate their electrons to an electrode rather than to random panhandling molecules—they, too, could supply us with electricity.
One persistent obstacle has been the membrane surrounding the cell. Most microbes conduct electron transactions deep in their own recesses, where the enzymes are; as a consequence, the organisms tend to seek out soluble electron receptors like oxygen that can slip inside the cell membrane, nab electrons, and slip out again.
A man-made electrode is not soluble; it can’t make microbial house calls. Chemical mediators, which ferry electrons back and forth between the cell interior and the electrode, can do the trick, but they tend to be expensive and inefficient, and require frequent replenishing.
Shelley Minteer, a biomolecular chemist at Saint Louis University, has taken a different approach: Rather than pry into a microbe’s internal mechanics, she has pried the mechanics out of the microbe. Using enzymes extracted from Escherichia coli bacteria, her team built an enzymatic fuel cell two years ago that runs on ethanol and can be attached directly to a circuit.
“It’s a microbial fuel cell without the cell wall,” Minteer says. The enzymes convert ethanol into vinegar, generating a stream of electricity. Actually, she says, it’s more of a trickle, and her fuel cell isn’t exactly miniature: It would take a 20-square-inch sheet of enzymes to power something like a cell phone. “We have a little fan powered off our fuel cell that just spins around,” Minteer says. “But we don’t have my air conditioner running off it or anything.”
Sugar, the main fuel of living cells, is present in everything from alcohol to sewage, lawn clippings to body fluids. If scientists can learn to tap it, this represents an almost unlimited energy source: Bacteria fueled by a cup of sugar could run a 60-watt lightbulb for 17 hours.
Adam Heller, of the University of Texas at Austin, has pushed this concept into the microscopic realm. He has created a battery consisting of two microfibers laid side by side, each coated with an enzyme—one (on the anode) that nabs electrons from glucose, another (on the cathode) that passes the electrons to oxygen. A quarter-inch-long device can generate 600 nanowatts of energy, enough to power a microelectronic circuit.
With typical batteries, the most difficult component to miniaturize is the outer casing; by avoiding one altogether, Heller has created the smallest battery known. Potentially, it could sit in a person’s bloodstream and run off the glucose there—a perfect power source for an implanted medical glucose monitor. A similar premise underlies one of Heller’s prior inventions: a small device, ideal for diabetics, that instantly indicates blood-sugar levels with an electrical signal.
Membranes do have their virtues, of course: They protect the inner workings of gazillions of living cells that are already out there generating electricity. Ideally, a biobattery would draw on this energy while leaving the cells intact.
Last October Derek Lovley, a microbiologist at the University of Massachusetts at Amherst, announced just such a breakthrough involving Rhodoferax ferrireducens, a common strain of bacteria that lives in underwater mud. Unlike most microbes, which trade their harvested electrons to oxygen molecules, R. ferrireducens trades with earth minerals like iron oxides.
Although iron oxides are insoluble, the microbe somehow manages to push electrons outside its cell membrane to where the iron oxide is situated. “Most life-forms have it pretty easy,” Lovley says. “Most oxidize oxygen inside the cell. These iron reducers had to devise this strategy. It’s an ingenious way of making a living.”
In the laboratory, it was a relatively simple step for Lovley to dupe the enterprising microbes into passing their electron booty to an even more attractive mineral—an electrode made of unpolished graphite. Immersed in a small tank of sugar water, the microbes settled on the electrode and got busy, extracting electrons from the sugar and dropping them directly on the electrode, which fed an external circuit: Voilà, electricity. R. ferrireducens is highly efficient, passing a full 80 percent of the available electrons to the electrode. (Other microbes turn over less than 50 percent.)
The concept holds tremendous promise, but only if it can be sped up: The output from Lovley’s tiny chain gang is just vigorous enough to light up a single Christmas-tree bulb. That might run a battery charger, or maybe low-use spacecraft electronics, but the process wants refinement.
Lovley is eager to study the R. ferrireducens genome to better understand how the microbe exports electrons and to see if it could be genetically modified to pick up the pace a bit. Meanwhile, his colleagues are investigating ways to make the battery more compact—at present, it’s about the size of a 10-gallon aquarium. If they succeed, R. ferrireducens will be able to do some serious dirty work: It could generate electricity from sewage, for instance, and break down the waste in the process.
Lest one should worry, the microbes do enjoy working for The Man. “We know they benefit because over time they form a colony on the electrode,” notes Lovley’s collaborator Leonard Tender, of the Naval Research Laboratory. Lovley adds, “You can stick one of these electrodes into the mud and it will be naturally colonized by these kinds of organisms.”
That preference is so strong that Tender has discovered he can draw a low but steady current simply by sticking an electrode into the seafloor mud where colonies of microbes live. In effect, Tender has plugged into the biggest battery on the planet: the planet itself. The current isn’t much (about 30 milliwatts from an electrode the size of a manhole cover), but it flows indefinitely, and for free—ideal, he suggests, for running low-power seafloor devices, such as ocean-monitoring instruments.
“You can picture the seafloor as a huge, ready-made fuel cell,” Tender says. “For me, that’s exciting.” Clearly it’s exciting for the microbes too, or they wouldn’t be doing it. Just hope they don’t spend their energy watching Matrix reruns.
