## 3 Biotechnological Approaches for Sensing and Actuation  
### Biotechnology for Energy Harvesting  
Installing a WSN or a network of Wi-Fi transmitters in streets and gardens requires solving the issue of power supply. Many approaches can be used, and one of them is based on the application of MFCs [18]. MFCs are able to harvest energy by consuming organic compounds (substrate) in rainwater or water in soils to generate electricity [30; 31]. In such biofuel cells (BFCs), microorganisms are used as biocatalysts. For environmental use, MFCs with microbial communities as biocatalysts can provide more stability and reliability during long-time operation [32; 33]. Another interesting approach is the integration of MFCs with higher plant or phototrophic microorganisms--phototrophic microbial fuel cells (PMFCs) [34; 35]. Depending on the plants and microorganisms, a cathode or anode can be integrated with a plant. PMFCs can be a more stable and reliable energy source in comparison with MFCs under the same conditions, but further studies are needed [34; 36]. Streets and parks can be a good option for further PMFC testing. The MFC and PMFC power outputs strongly depend on the materials of the electrodes, the concentration of organic compounds, the bacterial biofilm in the electrode, and many other factors [32; 37]. It is possible to await an unstable generation around several mW-cm-2 (mW per electrode surface) at achievable voltage values such as 0.5 V [18]. An overview of some MFCs and PMFCs is shown in Table 1, where it is shown that different types of MFC can demonstrate very close power output. In all MFCs, power generation strongly relates to the concentration of consumable organic compounds, the number of active cells on the electrode, and the speed of the biochemical reactions that take place in this system. Therefore, power output cannot be unlimited with the increase in the concentration of organic compounds [38]. Regarding unstable energy generation, it is necessary to use a power control system with energy storage in the battery. Therefore, MFC and PMFC applications require such a system for an effective sensor power supply [18; 39]. This leads to the development of combined power sources that can include MFCs and, for example, small solar cells. Here, the possibilities of application of biofuel cells for low-power sensor networks in rain gardens and green streets will be discussed. MFCs can be both a main and additional power source for increasing the time between battery changes [10]. Experiments such as [18] show the possibility of using MFCs for powering sensors and providing periodical data transmission via low-power Bluetooth radio. It is necessary to mention that the MFCs used in this study can be improved via new electrode materials and more effective cell immobilization methods [33, 52, 53]. Thus, microbial MFCs can be used as a power source for sensor networks. This leads to the possible application of MFCs as local power sources for low-power wide-area networks (LPWANs) installed in streets and parks. A LPWAN is a very effective approach for installing energy-efficient sensor networks in smart cities [54, 55]. Figure 2 illustrates possible variants of microbial MFC applications for an LPWAN component power supply. The most obvious method is applying MFCs to increase the operation live-time of wireless sensors (Figure 2A). To decrease the cost of the sensor network, MFCs can be used for the most significant sensors or for sensors whose operation stability is critical. MFCs can be used for a sensor's transmitter power  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**N** & **Type and Overview** & **Biocatalysts** & **Power Output** & **Reference** \\ \hline
**1** & MFC with microfluidic system of chambers & _Gobuctor_-enriched mixed bacterial biofilm & 4.7 \(\mu\)W/cm\({}^{2}\) & [40] \\ \hline
**2** & MFC with different strains of yeasts. Glucose and xylose were used in substrates & Several strains of yeasts. Most _Kluyeromyces marzianus_ & 2.5-3.32 mW with xylose.