# Novel Applications of Microbial Fuel Cells in Sensors and Biosensors
## 2 MFC Power Supply for Sensors and Biosensors  
Most of the commercial sensor and biosensor devices are conventionally powered by electrochemical batteries (e.g., lithium batteries) with a limited and relatively short lifetime that have to be periodically recharged or replaced [50]. Finding efficient and self-renewable energy sources to produce enough power for remote devices, where batteries replacement is not feasible or convenient, is nowadays a strongly pursued challenge [51]. Within that context, self-renewable MFCs appear as a long-term alternative power supply for remote monitoring sensors and biosensors, as schematically illustrated in Figure 3. Replacing conventional batteries with MFC power supplies in remote sensors, considerably lowers operational costs and environmental risks [51; 52]. Figure 3: Schematic diagram of a remote sensor powered by MFC. A key factor for the MFC energetic efficiency is a suitable feedstock, either organic or inorganic, for which extensive research has been implemented [16; 32; 90]. Nevertheless, and despite all the advantages, the most challenging hindrance for further expanding the applicability of MFCs in sensors are the low power outputs intrinsically associated with them, along with unstable electric potentials generated below the working potential of conventional electronic components in sensors [53]. Accordingly, regardless of the feedstock employed, working principles of MFCs as energy sources, as well as power requirements and detailed performance of power management systems for remote sensors, have been investigated in order to provide rational solutions to overcome the aforementioned drawbacks of MFCs [32; 51; 52; 54]. As a result, one of the approaches to increase the power output of MFCs relates to the connection of several MFCs in series or in parallel [55]. As a case in point, [PERSON] et al. recently developed a remote biosensor based on MFCs placed in parallel, for an alert system to prevent the use of a harmful water stream [56]. The sensor was designed with a cathode-shared integrating four MFCs, based on the principle that only the bioanode would be used as the sensing and transducing element. The sensor showed an immediate voltage decrease when exposed to a pH variation from 6 to 4. The compact design of the cathode-shared MFC sensor array assured the detection credibility, and even more interestingly, the number of integrated MFC sensors was alterable, based on the monitoring requirement. In addition, it has been demonstrated that, with the appropriate energy management system, an MFC is able to operate sensors and integrated telemetry systems for the wireless transmission of remote signals. However, one of the usage problems for these devices is the need of a high-power supply. In the same vein, neither the parallel or series design from [PERSON] et al. could increase the open circuit voltage (OCV) of several anaerobic fluidized bed microbial fuel cells compared to the sum of OCV of the individual cells [20]. Beyond the approach based on parallel or series design, a more feasible strategy behind the enhanced operability of MFCs seems to be the use of ultra-capacitors (among other electronic devices and configurations) to store the low energy generated by the MFC in order to be delivered only when the level of output energy is adequate to provide enough power to operate the essential functions of the remote device in question [32; 51; 52]. Moreover, DC/DC converters have been successfully employed to increase and stabilize the electric potentials of MFCs, as schematically illustrated in Figure 4.  
This technology has recently been applied for the design of a new type of floating microbial fuel cell (f-MFC) for the power supply of a remote environmental sensor and data transmission [57]. In detail, ten operating MFCs generated a cell potential in the range of 100-800 mV. Power production peaked around 3-3.5 mW, recorded after about 20-30 days of start-up period.