Certain species of bacteria that exist in oxygen-deprived environments must find a way to breathe that doesn't involve oxygen. These microbes — which can be found deep within mines, at the bottom of lakes, and even in the human gut — have evolved a unique form of breathing that involves excreting and pumping out electrons; they actually produce electricity.
Scientists and engineers are exploring ways to harness these microbial power plants to run fuel cells and purify sewage water, among other uses. But pinning down a microbe's electrical properties has been a challenge: The cells are much smaller than mammalian cells and extremely difficult to grow in laboratory conditions.
A microfluidic technique was developed that can quickly process small samples of bacteria and gauge a specific property that's highly correlated with bacteria's ability to produce electricity. This property, known as polarizability, can be used to assess a bacteria's electrochemical activity in a safer, more efficient manner compared to current techniques.
Bacteria that produce electricity do so by generating electrons within their cells, then transferring those electrons across their cell membranes via tiny channels formed by surface proteins in a process known as extracellular electron transfer (EET). Existing techniques for probing bacteria's electrochemical activity involve growing large batches of cells and measuring the activity of EET proteins — a time-consuming process. Other techniques require rupturing a cell in order to purify and probe the proteins.
The new technique builds microfluidic chips etched with small channels, through which flow microliter samples of bacteria. Each channel is pinched in the middle to form an hourglass configuration. When a voltage is applied across a channel, the pinched section — about 100 times smaller than the rest of the channel — puts a squeeze on the electric field, making it 100 times stronger than the surrounding field. The gradient of the electric field creates a phenomenon known as dielectrophoresis, or a force that pushes the cell against its motion induced by the electric field. As a result, dielectrophoresis can repel a particle or stop it at different applied voltages, depending on that particle's surface properties.
Bacteria that are more electrochemically active tend to have a higher polarizability. Researchers can gauge electricity production by measuring the bacteria's polarizability — something that can be tracked easily, efficiently, and nondestructively using the microfluidic technique.