towards the wall into the boundary layer. As the lower opening of the hydrocyclone

is extremely narrow, a complete discharge of the liquid is not possible and only the

boundary layer carrying heavier particles (e.g., cells) can escape. The rest of the

liquid reverses its vertical direction and flows in a strong vortex motion upwards

and out through the overflow pipe carrying the smaller particles (e.g., virions) [52].

As centrifugal and drag forces are proportional to particle-size and particle volume,

the separation performance could allow separation of viable and dead cells. Due to

their simple structure, hydrocyclones are easy to install, have a small footprint, are

in situ sterilizable, and easy to clean and re-use. Moreover, they would allow a

continuous harvest of virions and high perfusion rates while maintaining a good

retention efficiency at high flow rates. Major drawbacks are the high-pressure drops

and high shear stresses to which cells are submitted. However, several studies

demonstrated resistance of various mammalian cells to those pressure drops mainly

due to the extremely short RTs inside the hydrocyclone (0.03–0.1 s) [52]. As good

retention efficiency is only reached at high flow rates (typically exceeding 50–100 L/h),

larger bioreactors are necessary limiting their use in small-scale research applications.

To our knowledge, no research investigating the use of hydrocyclones for the pro-

duction of viruses has been published so far.

Feed pump

Hydrocy-

clone

Balance

Weight control

Permeate pump

Spent medium

Feed

medium

FIGURE 6.9 Schematic illustration of a hydrocyclone setup for perfusion processes.

Tangential injection of the feed results in the creation of a flow vortex in the conical section

of the hydrocyclone. Resulting centrifugal forces separate cells from the supernatant. Cell-

free supernatant is constantly removed through the upper part, cells settle down, and are then

transferred back into the bioreactor, fresh medium is added accordingly. Figure adapted from

[ 65].

Process intensification

161