# A pH Monitoring Algorithm for Orifice Plate Culture Medium
## 1 Introduction  
Cell therapy transplants or imports normal or bioengineered human cells into patients. The newly imported cells can replace the damaged cells for the purpose of treating diseases. Along with the rapid development of cell engineering, stem cell biology, immunology, molecular technology, and tissue engineering technology, cell therapy technology is becoming more and more mature. While cell culture techniques are fundamental to cell therapy, previous techniques were based on artificial culture. Recent developments in cell therapy have heightened the need for automated cell culture in vitro. For example, the culture of IPS cells (induced pluripotent stem cells). Ensuring that high-quality cells are stable and prepared with culture methods amenable to scaling is the key to achieving a wide range of clinical applications in the future. Automated cell culture technology can solve problems such as apoptosis or degeneration of cells due to mistakes resulting from the external environment, technical operations, and so on, to which artificial culture is subjected. Therefore, automatic cell culture using cellular automata may play a vital role in development [1; 2]. One of the important parameters of cell culture is the pH of the culture medium. The optimum growth pH of most animal cells is 7.2-7.4 [3; 4; 5; 6; 7; 8]. In the process of cell culture, the metabolites produced by cell growth will cause a change in medium pH. Therefore, the culture environment needs to contain about 5% CO\({}_{2}\), which acts as a buffer with NaHCO\({}_{3}\) and hydroxyethyl piperazine ethylthiosulfonic acid (HEPES) in the culture medium. Its pH is 7.2-7.4, and the color is displayed by the phenol red indicator. However, with the growth of cells, their metabolites accumulate to a certain amount, and CO\({}_{2}\) can no longer play a buffer role. The pH will decrease to 6.5-6.6, and the color will turn yellow, which seriously affects the growth of cells [9; 10]. Therefore, it is necessary to monitor the change in pH. When the pH drops to a set value, fresh medium must be restored or an alkali supplement operation is required. Due to the small amount of stem cells in vitro culture, well plates are generally used for culturing as seed cells. Consequently, the change in pH is faster and not easy to observe. The traditional method for measuring the pH of the cell culture medium is to observe the color change of phenol red in the cell culture medium with the naked eye [11], with pH change countermeasures relying on subsequent liquid change or alkali replenishment as informed by training experience. However, traditional methods present changes in the color of the culture fluid as a yellow hue at pH 6.8-7.2. It is difficult for the naked eye to judge the specific value of pH, which can only be determined empirically. It is easy to manually judge where situations such as failure to change the culture solution in time or conduct alkali supplementation occurred, which adversely affects the cells. For the costly stem cell culture process, the cell damage caused by pH determination mistakes can also cause economic losses. Therefore, the method of manually determining pH not only affects the cell culture quality but also demands too much of the operator. Some current cell automated culture devices involve methods of pH determination based on manual experience. In addition, there are parts of automated culture equipment for direct determination of pH by invasive sensors and also some by sampling assays. This may present the possibility of cell contamination, which is also not suitable for automated culture equipment for stem cells. Our method takes advantage of the chromogenic properties of phenol red for pH and machine vision technology to achieve intelligent pH monitoring of cell culture fluids. This method can not only improve the stability of cell culture quality but also produce savings in human resources.