6.6.7

ACOUSTIC SETTLER

The use of acoustic settlers has been considered by many researchers since the early

1990s and they have been successfully applied to perfusion cultivations of bacteria

and mammalian cells [43,86,87]. Ultrasonic cell retention is achieved by formation

of a standing wave field. In the zones of maximum energy, the cells are aggregated

and retained like in a filter. The acoustic waves are generated by a piezoelectric

transducer and reflected from a reflector in the direction opposite to the propagation

of the wave [63,86]. As a result, cells are trapped at the nodal pressure planes of the

standing 3D wave, where they merge to form aggregates [86]. The resulting cell

agglomerates have such a high sinking velocity that they sediment well after the

energy field has been switched off. Afterwards, they are transferred back into the

bioreactor via the return flow [63]. To minimize the residence time of the cells in

the acoustic field periodic turn-offs (duty cycle) of the acoustic power and harvest

pump are used [86] (see Figure 6.10).

As retention depends strongly on the cell radius, the retention of viable cells can

be better than for dead cells, which can increase overall viability of the culture due

to the washout of dead cells. Most acoustic settlers operate at a fixed frequency

Acoustic settler

controller

Feed pump

Acoustic

settler

Balance

Weight control

Permeate pump

Spent medium

Feed

medium

FIGURE 6.10 Schematic illustration of an acoustic settler setup and acoustic filter chamber

for perfusion processes. The acoustic waves are generated by a piezoelectric transducer and

reflected from a reflector in the direction opposite to the propagation of the wave. As a result,

cells are trapped at the nodal pressure planes of the standing 3D wave where they merge to

form aggregates. Cell-free supernatant is removed in an intermittent way, allowing fresh

medium to be added to the bioreactor. Figure adapted from [ 65].

162

Bioprocessing of Viral Vaccines