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.