# Bioreactors: Applications and Innovations for a Sustainable and Healthy Future--A Critical Review
## 5 Advances in Electrical Sensors in bioreactors  
Electrochemical sensors play a crucial role in monitoring bioprocesses, classified into several categories such as potentiometric, conductometric, voltammetric, and amperometric. Each type serves unique functions: potentiometric sensors detect changes in electrical  
\begin{table}
\begin{tabular}{l c c} \hline \hline \multicolumn{1}{c}{**Application**} & **Mechanism** & **References** \\ \hline Cell proliferation & Measurement of optical density in the cell culture environment & [56] \\ Algae proliferation and reduction of QA & Measurement using dual modulation LED kinetic fluorescence & [58] \\ (quinone acceptor in photosystem II) & Digital microscopic scanning in cell cultures using a & \\ Cellular imaging & miniaturized automatic microscope device & [37; 42] \\ Surface affinity of molecules or cells & Using surface plasmon resonance to measure wavelength shifts & [42; 44] \\ OD measurement by Infrared spectroscopy & Miniaturized probe-type infrared spectrometry sensor & [37] \\ Fermentation process analysis & Smartphone-based optical fiber sensor with colorimetric & [44] \\  & analysis of images captured by the camera & \\ \hline \hline \end{tabular}
\end{table}
Table 4: Optical sensor models under development and their applications.  
potential, conductometric sensors measure variations in conductivity, voltammetric sensors track alterations in charge transport under variable potential, and amperometric sensors measure charge transport while maintaining a constant potential. Similar to optical sensors, they are highly valued for their rapid response, wide measurement range, cost-effectiveness, and widespread adoption across various industries. Current applications include monitoring pH levels, dissolved oxygen (DO), and quantifying concentrations of glucose and glutamate [44; 61; 62].  
Advancements in electronics and semiconductor technologies have facilitated the development of compact, portable electrochemical sensors that operate efficiently with minimal energy consumption. Innovations in chemical detection methods, biomodification techniques, and manufacturing processes have significantly enhanced the sensitivity, selectivity, and compatibility of these sensors with biological systems. Emerging models of electrical sensors are exploring sophisticated applications, including real-time analysis of cell growth dynamics in culture plates and assessment of cell adhesion behaviors on diverse surfaces [36; 39; 63]. These assessments utilize changes in electrical conductivity in the growth medium, microfluidic dynamics between interconnected microplates, and fluctuations in electrical potentials indicative of biochemical reactions. Figure 4 shows an example of an electrical sensor developed for monitoring cell growth [63; 64; 65].  
The integration of electrochemical sensors into bioreactors and laboratory setups has simplified bioprocess monitoring by offering immediate and accurate measurements. For example, sensors designed to monitor cell growth use enzymatic oxidation processes to transfer electrons from specific substrates to an electrode interface [38; 41; 42]. This capability ensures precise monitoring of cellular metabolic activities in real time, crucial for optimizing biotechnological processes. Electrochemical sensors for measuring glucose and glutamate represent significant advancements in bioprocess monitoring, enabling real-time and precise monitoring of cellular metabolism. These sensors utilize enzymatic oxidation processes to transfer electrons from the measured substrate to an electrode and are designed  
Figure 4: Schematic of an online and offline monitoring system.  
for immediate use, easily integrated into shake flasks or disposable bioreactors. Figure 5 shows an example of an electrical sensor developed for monitoring cell growth [38; 65; 66].