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A miniaturized biological solar-cell device. [Image: Seokheun “Sean” Choi / Binghamton University, SUNY, N.Y., USA]

Seokheun Choi and Lin Liu from Binghamton University, State University of New York, USA, report creating a miniaturized biological solar cell (micro-BSC) that they say can generate more power for a longer period “than any existing cell of its kind.” The new device captures electricity generated by cyanobacterial photosynthesis and respiration, and could be a step toward realizing low-cost, self-sustaining clean power sources for point-of-care devices in remote locations or low-resource settings (Lab Chip, doi: 10.1039/c7lc00941k).

Sustaining and increasing micro-BSC output

The team’s new micro-BSC rests on several years of earlier efforts to create such a platform, which is built on microelectromechanical-system (MEMS) and microfluidic technology. The devices work by using biological organisms (usually blue-green algae, or cyanobacteria) as power plants that can produce light-driven electricity. Earlier generations of the device, however, have had trouble sustaining high power densities for any meaningful time, and “did not have any practical application,” according to the researchers.

To boost sustainability and power in their latest device, Choi and Liu made a number of mods in two of the micro-BSC’s seven layers  to allow for increased gas exchange, which keeps the bacteria alive longer, and to maximize biofilm formation, which improves electron capture thereby increasing power production.

In one modification, Choi and Liu added to their design a polydimethylsiloxane (PDMS) membrane that allows gases that cyanobacteria need for photosynthesis and respiration—for example, oxygen, nitrogen and carbon dioxide—to flow into and out of the micro-BSC. The researchers’ previous design had included an enclosed “headspace” in a gas-permeable polycarbonate membrane to facilitate gas exchange. However, the gases in this headspace would eventually run out, and the bacteria would suffocate. The new PDMS membrane significantly extends the lifespan of the bacteria, increasing the micro-BSC’s sustainability from 10 days to nearly 20 days.  

The researchers also redesigned the device’s 3-D anode to beef up power generation. Previously, Choi and Liu used conducive carbon cloth in the anode as a sort of 3-D scaffold for the cyano-bacteria to adhere to and form a biofilm. In their newest design, they found that modifying the carbon cloth with poly(3, 4-ethylene dioxythiophene) : polystyrene sulfonate (PEDOT : PSS) allowed the photosynthetic bacteria to burrow deeper into the 3-D anode structure without completely blocking the spaces between fibers.

This change led to a more densely packed biofilm with high porosity, allowing for efficient nutrient and waste transport through the anode. By maximizing biofilm formation, Choi and Liu were able to increase the rate of bacterial extracellular electron transfer, resulting in an increase in maximum power density from 2.7 µW cm–2 to 43.8 µW cm–2.

Testing micro-BSC performance

The researchers tested micro-BSC performance using an open-circuit configuration to measure open-circuit voltage (OCV). These initial runs showed a higher OCV when the device was exposed to light compared to OCV levels in the dark, thereby confirming electrical output from bacterial photosynthesis and respiration. Specifically, they observed a sustained power density of 18.6 µW cm–2 with light exposure (generated by bacterial photosynthesis and respiration) and 11.4 µW cm–2 without light (generated only by bacterial respiration).

According to Choi and Liu, their micro-BSC could offer a practical and sustainable power source for point-of-care diagnostics “that work independently and self-sustainably in challenging field conditions.”