Physicists and biologists have teamed up to build a living transistor — an unusual kind of bacteria that produces long stringy filaments outside its body that conduct electrons better than some metals.
Scientists describe these filaments as a "living nanowire" that could be a big step forward in merging biological systems and electronic devices, leading to tiny organic super-batteries or biological superconductors made for a fraction of the cost of existing silicon-based chips.
The finding was reported in this month's issue Nature Nanotechnology by a team at the University of Massachusetts at Amherst.
The strings of nanowires, called pili, allow the bacteria to get rid of electrons that are a byproduct of its digestive process. Humans and animals get rid of electrons through breathing, according to Mark Tuominen, professor of physics at UMass and lead author of the paper. But bacteria living in anaerobic zones don't have oxygen to carry the electrons. Each one of these spaghetti-like strands is 10 to 20 times longer than the bacteria itself, according to Tuominen.
The single-celled Geobacter sulfurreducens bacteria was discovered in the 1980s in the oxygen-starved mud beneath the Potomac River as well as the beaches of Nantucket Island. For his experiment, Tuominen and colleague Derek Lovely, a UMass microbiologist, created a tiny electrode and grew a biofilm of bacteria around it. They were then able to measure its conductivity by passing a current of electricity through the electrode.
"These nanowire networks showed the same properties of metal networks," Tuominen said. "We didn't think nature could make something similar to metal. This is the first time this has been discovered and this is very exciting for us."
Other experts in the field agree that the discovery is a big deal. Stuart Lindsay, director of the Center for Single Molecule Biophysics and the Biodesign Institute at Arizona State University, called it "remarkable."
"It's a story that's been developing over years. These measurements are beautiful. They tell you the proteins are behaving like nanoscale wires. That is astounding."
Lindsay predicted that biological engineers could use the results to build biological batteries more powerful than chemical ones. He notes that existing batteries are constrained by the amount of surface area on their electrodes. To generate a bigger charge, you need a bigger battery. But the microscopic pili have a huge amount of surface area that could be harnessed to create electricity.
Tuominen adds that he believes these conducting bacteria could also be used underwater, where the performance of typical electronic materials is unsuitable.
"Water-based sensors and energy storage or energy conversion devices are two possibilities, since this is a material that clearly functions best in water," Tuominen said. "A big plus is that these materials do not require any rare or expensive feedstock materials. They will be cheap to produce because they are nature designed and nature made."