Millistak+HC AppGd_textC.qxd - Millipore

How Are Millistak+ HC
Filters Used?
The typical downstream recovery
process for mammalian cell derived
biologics consists of multiple clarification
steps to prepare the liquid-borne
product for either sterile filtration or
direct chromatographic purification.
In the first stage or primary separation
step, the bulk of the cell mass is
removed from the broth—depending
on the volume and type of fermentation
process, different clarification
methods are used, as shown in
Figure 3.
Batch Processing
In large-scale (> 1,000 liter) batch
and fed-batch fermentations, the bulk
of the cell mass is ordinarily separated
by centrifugation or tangential flow
microfiltration. For small batch fermentations
(< 1,000 liter), cartridge depth
filters (NFF) are often used in the pore
size range of 3–5 µm.
Perfused Bioreactor
In contrast to batch processes, perfused
bioreactors are harvested continuously
over extended operating periods
lasting several weeks or months.
For primary clarification, the flowcontrolled
product stream from the
perfusion bioreactor will typically pass
through either a membrane separator
(hollow fiber TFF) or a gravity settler,
with the cell mass recycled to the
fermenter. The crude product is pooled
and then batch-processed for secondary
clarification and final sterile filtration.
A Single Step for Clarification
and Prefiltration
The effluent of these various primary
clarification steps is generally unsuitable
for either sterile filtration or
chromatographic processing. The level
of suspended matter in all cases
(which varies by method) will cause
rapid downstream fouling, with significant
cost impact. The turbidity ranges
noted in the process schematic in
Figure 3 are indicative of the particle
loads present. Even in the case of
TFF microfiltration—typically executed
with a 0.45 – 0.65 µm membrane—
particle loading in the permeate does
not allow for direct sterile filtration at
any reasonable capacity or cost.
Further clarification is required, for
which disposable NFF filters are
normally used (depth or surface types).
The challenge confronting the process
engineer is to arrive at an integrated
clarification and sterile filtration train that
will minimize overall production costs.
In many cases, secondary clarification
of tissue culture fluid involves several
(two or three) serial filtration steps. The
need for multiple separations arises from
the broad range of particle sizes and
Millistak+ HC Filters Application Guide 3