Engineers
have developed a microfluidic technique that can quickly process small samples
of bacteria and gauge a specific property that's highly correlated with bacteria's
ability to produce electricity. They say that 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, or EET.
Existing
techniques for probing bacteria's electrochemical activity involve growing
large batches of cells and measuring the activity of EET proteins -- a
meticulous, time-consuming process.
Researchers
have been building microfluidic chips etched with small channels,
through which they 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 in its tracks at different
applied voltages, depending on that particle's surface properties.
The
team flowed very small, microliter samples of each bacterial strain through the
hourglass-shaped microfluidic channel and slowly amped up the voltage across
the channel, one volt per second, from 0 to 80 volts. Through an imaging
technique known as particle image velocimetry, they observed that the resulting
electric field propelled bacterial cells through the channel until they
approached the pinched section, where the much stronger field acted to push
back on the bacteria via dielectrophoresis and trap them in place.
Some
bacteria were trapped at lower applied voltages, and others at higher voltages.
Wang took note of the "trapping voltage" for each bacterial cell,
measured their cell sizes, and then used a computer simulation to calculate a
cell's polarizability -- how easy it is for a cell to form electric dipoles in
response to an external electric field.
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