Fig. 2. Integrated biofilm devices using mesh electrodes.

a Schematic and actual photo (bottom) of a biofilm device. Scale bar, 0.5 mm. b Representative open-circuit voltage V_o (top) and short-circuit current I_sc (bottom) from a device placed on a water surface (schematics). The device size was 5 × 5 mm². c V_o (black) and I_sc (red) from devices with respect to different porosities in the mesh electrodes. The pore size was kept 100 µm and the device sizes were 25 mm². d V_o (black) and I_sc (red) from devices with fixed porosity of 0.4 but varying pore sizes (from 1000 to 20 µm) in the mesh electrodes. The device sizes were 25 mm². At the pore size of 20 µm, the estimated optimal energy density (V_o·I_sc/4) was ~0.84 µW/cm² in the active biofilm region. The previous device structure (Fig. 1bi) yielded higher density, probably because the biofilm-tissue paper interface improves water transport. e V_o (black) and I_sc (red) from devices of different areas, with the porosity and pore size in the mesh electrodes, kept 0.4 and 100 µm. f Measured V_o from devices connected in series (upper inset) using a “buckle” design in electrodes (bottom schematic). All the tests were performed in the ambient environment (RH~50%). All the error bars are standard deviations.