It might not make for a good marketing angle, but you never can tell: There are certain bacteria that produce minute amounts of electricity and could be used as the electrolyte in batteries. These natural bacteria species—which live in oxygen-deprived environments such as deep within mines, at the bottom of lakes, and even in the human gut—have developed a form of “breathing” where they excrete electrons (we call it electricity) in a process formally called extracellular electron transfer (EET). The immediate practical problem is how to quickly and effectively determine if the bacteria under test (to possibly be known as the “BUT”?) is the type that produces electricity.
To address this question, a team at MIT has combined a microfluidic channel, electric field, and advanced techniques to assess the bacteria’s polarizability, which is an electrochemical characteristic that’s closely correlated with electricity production. They report on the process in their highly detailed, intensive paper published in AAAS Science Advances, “Microfluidic dielectrophoresis illuminates the relationship between microbial cell envelope polarizability and electrochemical activity,”along with their exhaustive Supplementary Materials.
The MIT team’s approach differs significantly from existing time-consuming techniques that probe the bacteria’s electrochemical activity. Such methods involve growing large batches of cells and measuring the activity of EET proteins, or rupturing cells to purify and probe their proteins.
A physical element of the project is the fabrication of a microfluidic chip etched with small channels that are “pinched” in their middle down to 1/100th the size of the main channel. Microliter-volume samples of bacteria are then driven through the channel in a technique known as “three-dimensional insulator-based dielectrophoresis (3DiDEP)” (see References below).
1. This simplified rendering shows how the microfluidic technique quickly sorts bacteria to determine their potential ability to generate electricity. (Credit: Qianru Wang, MIT)
A voltage is applied across a channel to create an electric field, and the field strength across the orifice in the pinched section is correspondingly larger than it is in the main channel by the ratio of the cross-sectional areas (Fig. 1). The resulting field gradient results in a dielectrophoresis force that pushes the cell against its motion induced by the electric field, and can stop and even repel a particle depending on the voltage level. The relationship is a function of the particle’s surface properties among other factors. (It’s analogous to attracting, stopping, or even repelling a stream of electrons by using an electrified grid or electric field, a well-known and long-used physics phenomenon for measuring the energy of an electron.)
Electrochemical Activity and Polarizability
While dielectrophoresis has been used to quickly sort bacteria according to more general properties, including size and species, the MIT team’s goal was to use the same technique to evaluate the much more subtle electrochemical activity by seeing if there was a correlation between electrochemical activity and higher polarizability.
To do this, they increased the voltage across the microchannel from 0 to 80 V, then observed bacteria behavior as the electric field propelled the bacteria to the pinched part where the stronger field would “trap” them (Fig. 2). By measuring the trapping voltage for each cell and measuring the cell size, and by using a COMSOL-based simulation and calculation model, they were able to determine the relative ease with which a cell could form an electric dipole as a response to the electric flied.
2. A 3DiDEP microfluidic device with an array of multiple microchannels; a potential difference increasing linearly at 1 V/sec was applied across the channel (a) (Credit: Qianru Wang, MIT). A magnified view of the microchannel highlighting the constricted area (b). Schematic of the 3DiDEP trapping principle, where bacteria near the constriction are immobilized when the DEP force (proportional to the electric field gradient) is balanced by drag forces due to the background electroosmotic flow and electrophoresis (c). The magnitude distribution of the x component of the field gradient is shown in the background color scale (dark red indicates higher values).
Team leader Qianru Wang, a postdoctoral student in MIT’s Department of Mechanical Engineering, noted that, “We have the necessary evidence to see that there’s a strong correlation between polarizability and electrochemical activity. In fact, polarizability might be something we could use as a proxy to select microorganisms with high electrochemical activity.”
The team also found that bacteria that were more electrochemically active tended to have a higher polarizability across all species of bacteria tested.
Advances in micro-dimensioned channels are key to the physical aspects of the experiment. The 3DiDEP device was fabricated by CNC micromachining a piece of polymethyl methacrylate (PMMA) sheet and bonding it to another blank PMMA chip. The final channel was 1-cm long with a 50-mm-long, 50- × 50-mm cross-section constricted zone in the center (area of 2500 mm2). The two main channels have 500 × 500 mm cross-section dimensions that yield the desired ×100 constriction ratio. (Reference 4 shows details of fabrication of another 3DiDEP device.)
This research was supported in part by the National Science Foundation, and the Institute for Collaborative Biotechnologies, through a grant from the U.S. Army. Who knows, perhaps there will be a specialty battery sold as “powered by bacteria” as its claim to uniqueness and environmental friendliness?
- Qianru Wang, MIT Master’s Thesis, “High Constriction Ratio Continuous Insulator Based Dielectrophoretic Particle Sorting”
- AES Electrophoresis Society, “Three Dimensional Insulator Based Dielectrophoresis (3D-iDEP)”
- National Center for Biotechnology Information, “3D Insulator-based dielectrophoresis using DC-biased, AC electric fields for selective bacterial trapping”
- Research Gate, “Photographs of microfluidic chip during fabrication”