In order to gain a better understanding of Chinese hamster ovary (CHO) cell genome instability, the ambr® 250 bioreactor platform was adopted as the highly-controlled environment for this project. Development of the baseline and stress conditions for the genome instability studies has highlighted the need for control of critical process parameters, including the Proportional, Integral, and Derivative (PID) control loops. Process parameters or factors that are considered scale-independent, dissolved oxygen (DO) and pH, were determined to be sensitive to the PID values for the ambr® 250 process. Unlike larger bioreactor control loops that can use cascade settings to control DO utilizing agitation, air, and oxygen concurrently, the ambr® 250 requires individual levels of control that are used sequentially to change a particular process parameter. For example, the Level 1 for DO control could have PID values for the gas sparge (Air) 0.5 mL/min to 2.5 mL/min (P = 0.1, I = 500, D = 0). Once the DO drops below the setpoint, Level 2 would be called. The Level 2 default increases the Stir rate from 300 rpm to 600 rpm (P = 3.5, I = 250, D = 0). The consequence of having multiple levels is that each level needs to have the PID values tuned individually. For mammalian cell cultures Sartorius provides with the ambr® 250 platform with three default DO Levels and a set of upper and lower pH PID values. However, even with minor adjustments these PID values resulted in high variability in DO, were too slow to respond to antifoam additions at high-cell densities, and allowed the pH to drift away from the setpoint that resulted in lactate accumulation. After consulting with industrial ambr® 250 users and Sartorius field specialists, and tuning the PID for each level, DO and pH control loops PID values were developed that minimized uncontrolled oscillation and over- and under-shooting, as well. These improved PID control values can support CHO cell growth, reproducibly to over 47 million cells/mL in a fed-batch process. This work highlights that PID control loop values can profoundly affect culture outcomes, yet are often neglected and rarely mentioned in publications.

PID control: Forgotten process parameter

ACS Fall 2020 Virtual Meeting

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