Miniaturization and Capture Efficiency

To implement our biofuel cell miniaturization approach, we applied our expertise in microfluidics. In basic terms, we designed a microfluidic chip that included a dome-like chamber over an electrode surface. The height of the dome was set to around 90 microns, which meant that the longest distance any bacterium could be from the nearest electrode would be no more than 90 microns. In a sense, the microfluidic chip constrained the geometry available to the bacteria to bring them close enough to the electrode to significantly improve the capture efficiency and thus the overall power output.

The figure on the left above shows the basic idea of a domed chamber (blue) defined inside a top elastomer component (drawn as a transparent box) positioned on top of a substrate (yellow) containing a metal electrode. The figure on the right shows the structure of the microfluidic component, made out of elastomer (black), with defined dome chamber and connecting microchannels (white). The chamber is prevented from collapse through the use of supporting columns (black squares) made of elastomer.

The electrode on the substrate was designed to have the shape of a fractal. This was done to utilize the fractal property that distance along wiring from the red central dot to any of the blue dots is the same (left figure above). This fractal can be copied and pasted through mirror imaging along both horizontal axes, allowing it to grow indefinitely and thus be able to cover a mathematically arbitrarily large area. This design was implemented in chrome-on-glass as shown on the microscope image on the right. The result was that the distribution of the bacteria inside the dome on this electrode substrate would not matter for the overall power output result, because all locations would be essentially equidistant from the electrode output.

The chrome electrode fractal was fabricated by etching chrome from a fused silica substrate using photolithography for masking and acid for etching. The elastomer component was separately produced by thermal curing and replication molding off molds made of photoresist defined photolithographically on a silicon wafer. The individual components were assembled as shown above. 

The resulting chips were shipped to our collaborators Dr. Meriah Arias-Thode’s group at NIWC-Pacific at San Diego. There they were loaded with different preps of benthic bacteria, buried in the sediment, and connected to electrical measurement circuitry to record the output power over time, as shown in the schematic above.

The experimental results are plotted in the figure above. Most importantly, the results show that 80 mW/m2 power density was achieved consistently. This is an increase of 4 times compared to previous power density measurements from macro-scale biofuel cell systems. Hence, these results confirmed our hypothesis about the importance of the distance between the individual bacterium and the nearest electrode. The results also confirmed the benefit of miniaturization of the biofuel cells.