At 42 days, the voltage across the 30 kΩ external resistor had decreased and continued to drop, reaching 50% of its original value at t=50 days. Because the laminar flow in the microchannels promotes strong concentration gradients, we used simulations to investigate the possibility that the voltage loss occurred because some portions of the electrode experienced reduced activity due to local nutrient limitations (Supplementary Materials, Section S3). A simulation of the acetate concentration above the anode showed that acetate was the most depleted near the downstream portions of the anode due to continuous consumption during the liquid transit across the upstream portions. Nevertheless, the concentration never fell below 9 mM at any position on the anode. As this is well above the concentration threshold between first-and zero-order kinetics (3-5 mM), we conclude that access to substrate molecules was not a factor in the decreased cell voltage. A similar simulation showed that ferricyanide concentrations remained above 25.5 mM everywhere on the cathode, which is sufficient to avoid bottlenecks in reduction kinetics. It is also doubtful that the observed voltage loss was due to EAB erosion because the shear stress was only (0.5 mPa) at the flow rates until this point, which is well below the values used in previous work. The microfluidic MFC was also designed to prevent O2 contamination and ferricyanide crossover to the anolyte flow stream, but we do not rule out accidental contamination during syringe replacement or from slow O2 leaks through faults in the device gas-protection layer, fluid connects or even through the porous graphite electrodes. Other possibilities include biological contamination and metabolically inactive outer layers, as recently reported. We exploited the ability of the microfluidic MFC to apply a large range of shear stresses to investigate whether the outer layers comprised of older cells were the cause of the poor MFC performance by testing this hypothesis. At t=50 days, we applied a shear stress of 7.5 Pa via three consecutive 3-second flow pulses (Qa=Qc=250 mL min -1 ) to detach the EAB outer layers. Although no published data are available on the critical shear stress threshold for erosion of G.