Moulds used to create the microfluidic MFC were fabricated via photolithography using laminated photoresists and graphite electrodes (Isomolded Graphite Plate, GraphiteStore.com Inc., USA). The membraneless MFC device contains electrodes that were positioned flush with the top PDMS wall in the popular side-by-side arrangement (Supplementary Materials, Figure ). Other details of the microfabrication and device design can be found in the Supplementary Materials (Section S1) and in the literature. The inoculum consisted of a subculture of G. sulfurreducens (wild type, strain PCA, ATCC® 51573) in a nutrient medium with 40 mM sodium fumarate as a soluble electron acceptor. The for two hours. To promote growth of the EAB after the inoculation, the same solution was used except that it was sterile and contained no sodium fumarate. A pH 7.0 sodium phosphate buffer containing 30 mM K3Fe(CN)6 (ferricyanide) was used as the catholyte solution to ensure that the cathode kinetics did not contribute to overshoot. At every stage, the catholyte formed a coflow with the anolyte (Qc=Qa=0.5 mL h -1 ). The laboratory temperature was 23 ± 0.5°C. Other details related to bacterial preparation are identical to those of previous studies. Following the experiment, scanning electron microscopy (SEM) was applied on the EAB. The pumping system was used to deliver a fixant before sample extraction. See the Supplementary Materials for more details on sample preparation and SEM measurements.