Mixing is a crucial bioreactor performance characteristic because it is responsible for minimizing gradients and maintaining
control within the cell culture environment. Good mixing in a bioreactor strives for sufficient fluid pumping and turnover
throughout the system to effectively create a single homogeneous environment which can be accurately monitored and controlled.
Good mixing should evenly distribute bioreactor contents, helping to minimize zones of uneven cell density, pH, temperature,
dissolved gases, and nutrient or waste concentrations, while minimizing the shear stress imparted on the cells by the fluid
dynamics or the mixing element itself.
Table II: Tip speed, Reynolds number, and ungassed power-per-unit volume values for the 3-L Mobius CellReady bioreactor process
Although no single parameter can guarantee comparable process performance between stirred tank bioreactor systems, choosing
an agitation rate that matches energy dissipation or power per unit volume (W/m3 ) is a common first approach (3, 4). The impeller design, fluid density, and agitation rate are considered in the power per
unit volume (Po/V) equation, Po/V = Np x ρ x n3
x d5 , where Po is ungassed power, V is liquid volume, Np is impeller power number, d is impeller diameter, n is impeller agitation rate
and ρ is fluid density. Using this scaling method, agitation rate is varied to maintain similar ungassed power-per-unit volume
Table III: Tip speed, Reynolds number, and ungassed power-per-unit volume values for the 50-L Mobius CellReady bioreactor
The highest shear zones in a stirred tank bioreactor are often described as existing within the impeller zone. Because the
outer edge of the impeller blades create shear as they rotate through the liquid, the impeller tip speed is often considered
during bioreactor comparisons. The impeller tip speed calculation, π x d x n, where d is impeller diameter and n is agitation
rate, however, does not take impeller design into consideration. Reynolds number (Re) can be considered when estimating fluid
conditions at a given agitation rate by providing a ratio of the inertial forces to the viscous forces where Re = (ρ x n x
d2 ) / µ. The fluid density is represented by ρ, n is agitation rate, d is impeller diameter, and µ is dynamic viscosity. The
system is considered fully turbulent when Re > 10,000 (5, 6). The Reynolds number calculation is based on the assumption of
a cylindrical tank with a centered rotating impeller and does not take impeller design into account. The Mobius CellReady
50-L and 200-L bioreactor process containers contain a bottom mounted, offset, pitched blade impeller and a baffle, which
together increase the turbulence and improve overall mixing.
Table IV: Tip speed, Reynolds number, and ungassed power-per-unit volume values for the 200-L Mobius CellReady bioreactor
Tables II, III, and IV outline various tip speed, Reynolds number and ungassed power-per-unit volume values calculated for a range of applicable
agitation rates for the 3-L, 50-L, and 200-L Mobius CellReady bioreactor process containers.
Mixing was evaluated for the 50-L and 200-L Mobius CellReady bioreactor process containers by observing conductivity probe
response curves measured at four locations (top, middle, bottom, and inserted in the probe port of the Mobius SensorReady
loop) within the system. The average of these four probes is considered the system average. A salt solution was introduced
at the liquid surface and the T95 mixing time was determined for each of the four probe locations as the time when conductivity profiles had reached 95% of
its final value. Each trial was performed in triplicate and results are shown in Figure 3. The Mobius CellReady 3-L bioreactor was evaluated in a similar manner, using just one conductivity probe inserted in the
head plate probe port.
Figure 3: System average mixing times of the family of Mobius CellReady bioreactor systems. Data points represent the system
average mixing time of n=15 individual trials. Error bars represent 1 standard error for each data point.
Results of these studies show that the Mobius CellReady 50-L and 200-L bioreactor process containers demonstrated similar
system average mixing times at equivalent power-per-unit volume. Mixing times for the 3-L bioreactor are significantly shorter
than mixing times observed in the larger bioreactor process containers for similar power per unit volume. This is to be expected
as larger liquid volumes result in longer fluid paths. To achieve a well mixed condition in the same amount of time, the fluid
velocity would have to increase for the condition in the larger volume tank. As a guide, fluid velocity is proportional to
the square root of power per unit volume in turbulent conditions (i.e., when Re > 10,000) (5, 6). As a result, an increase
in power-per-unit volume would be required to achieve similar mixing times at significantly larger scales. The system average
mixing times for the Mobius CellReady bioreactor process containers, as determined by the multipleprobe experimental methods
described above, do not dramatically increase with increasing power per unit volume across the agitation range tested. The
50-L and 200-L bioreactor process containers contain a single baffle, which coupled with the off-center, angled position of
the bottom mounted impeller, provides good mixing dynamics even at lower power inputs. Empirical results show the mixing time
to be 12–13 s from 2 to 15 W/m3 for the 3-L, 38–24 s from 5 to 30 W/m3 for the 50-L and 33–26 s from 1 to 25 W/m3 for the 200-L bioreactor process containers as displayed in Figure 3.
Table V: Comparison of operating parameters for 3-L, 50-L, and 200-L CellReady bioreactors.