3.1 Investigation of Oxygen Mass Transfer Coefficient in 96-Well Microreactor
As previously discussed, kLa has been established as a critical parameter for predictive process scale- up [17,18] and optimal bioreactor performance. Figure 2 shows a comparison between the dissolved oxygen recovery curve (Figure 2A.) and estimated kLa values by equation (2) in a mixed microbioreactor versus an unmixed microbioreactor as measured by the dynamic gassing method (Figure 2B.). In this comparison, there was an increased rate of oxygen recovery following deoxygenation when mixed via Redbud Labs proprietary micro-posts than via diffusion alone (no mixing). This increased oxygen recovery corresponded to a calculated kLa value of 42.48 h-1 in the mixed condition when mixed at 7400 rpm versus 21.24 h-1 in the unmixed at a well volume of 150 µL.
To investigate the magnetic agitated micro-post’s capability to improve oxygen transfer in a 96-well microbioreactor, the kLa was determined in a single well at different magnetic rotational speeds (0-9000 rpm). For the unmixed control conditions, the observed kLa values ranged from 3.72 to 12.47 h-1 decreasing as culture volume increased, a variation which is consistent with variation seen in similar sized microtiter plates [5] and shake flasks [19].
In the 96 well plate, higher kLa values were obtained as the magnetic rotational speed was increased (Figure 2C) ranging from the unmixed kLa values to 89.28 h-1 (9000 rpm, 200 µL). At 3500 rpm the observed kLa values were 14.64 h -1 (150µL), 8.88 h -1(200µL), and 9.12 h -1 (300µL). At 6500 rpm the observed kLa values were 34.2 h-1(150µL), 33.6 h -1 (200µL), and 19.41 h-1 (300µL). At 9000 rpm the observed kLa values were 88.20 h -1 (150µL), 89.28 h-1 (200µL), and 37.20 h -1(300µL). This observed variation in kLa with increased rotational speed is comparable to the change observed with increased shaking frequency in comparable 200 µL microtiter plates [5,20]. Initially as the rotational speed increased, there is not a substantial increase in kLa. However, there is an observed critical rotational speed, beyond which kLa increases exponentially.