## 5 Discussion and Conclusions  
The presented review shows that several modern approaches in biotechnology and synthetic biology can find applications in today's IoT-driven smart city concepts. Moreover, new possibilities  
Figure 5: Cyber-physical system (CPS) with hybrid living material (HLM) and two wireless sensor networks (WSNs) for process control. in biosensor and bioactuator developments and applications are arising. Thus, further application ideas have arisen with the appearance of results of new studies. However, all technologies have issues together with advantages. The issues that pertain to most of the biological systems discussed can be summarized as follows:  
* There are issues related to the complexity of biological systems. As shown in this paper, the biological object is sometimes a small element of the system, such as cells in cell-based biosensors, but processes inside this small cell are very complex and are influenced by many factors, which should be taken into account in the development of such systems;
* There is an environment dependence. Biological systems live in a narrow range of climate parameters, and that can be narrower for some species than common weather changes during the year;
* Interface issues persist. It is difficult to receive data concerning processes that take place inside the objects without their destruction. Synthetic biology yields some approaches that can help solve this issue, but its application requires additional sensors (optical, biochemical, etc. );
* There are legislation issues related to genetically engineered organisms, and overcoming these issues can provide a demonstration of the behavior predictability of systems with such organisms. All advantages that can be enjoyed by biotechnology applications for a city are difficult to identify because some of the above-mentioned technologies are still at the laboratory research level. Nevertheless, based on the presented data in this paper, some advantages can be defined:  
* Efficient data exchange between photobioreactors or urban vertical farming systems and a city's data centers can help to utilize data from all such installations for predictive mathematical models' development;
* Biofuel cells are the promising approach for WSN power supply, thus, WSN application can be expanded without the need for battery change maintenance or other energy supply systems;
* Novel biosensors can enhance both the number of different sensing molecules and the ability to analyze preliminary data. Then, there are more different parameters we can measure than more useful data for predictive models we can receive [126]. It is significant for surface water and rainwater quality monitoring. Additionally, synthetic biology approaches can help to develop cell-based biosensors which do not use electricity for measurements and processing inside the cell itself;
* Bioactuators can operate partially independently and generate a signal about its state almost because of its biological component activity and without electrical energy consumption. Thus, biotechnological approaches for sensing and actuation mentioned in this paper give the possibility to increase the amount of data without the energy consumption for sensing and actuation increasing. Several biotechnological approaches mentioned in this paper are already able to significantly change several aspects of a city in a positive way, producing food and different phototrophic microorganisms' products, treating air and water, and providing a fixation of carbon dioxide. Novel biosensors can significantly improve the capability of WSNs by adding a number of new measuring parameters in combination with MFCs, making them more versatile in terms of energy efficiency. Thus, it can be concluded that the integration of modern biotechnology approaches in today's cities can provide a significant positive influence on city developments.