higher crossflow rate would be
needed, which would expose the
protein to more passes through the
pump.
Bioburden Control
The ability to reliably control
bioburden levels in a process stream is
very important in protein processing.
An outbreak of bioburden in an
upstream process step may raise
quality concerns but be tolerable,
while in a downstream or final process
step it may cause failure of the entire
batch. While the risk of bioburden
contamination is not necessarily
greater in a TFF step than in other
processing steps, there are ways that
the risk can be minimized during TFF
processing.
The method used to clean and
sanitize a TFF unit operation between
uses is obviously important in
controlling bioburden (as well as
ensuring removal of endotoxin). The
chemicals that are chosen must be not
only effective for cleaning and
sanitization, but also compatible with
the membrane and piping materials
and able to be rinsed out to
acceptably low levels before further
processing. Typical chemicals used to
clean TFF systems include sodium
hydroxide and sodium hypochlorite.
The Millipore Maintenance Procedures
provide appropriate cleaning
concentrations, times, and
temperatures for specific membrane
systems.
The total cycle time for a TFF
process can affect bioburden loads.
Since TFF processes are typically run
under sanitized but non-sterile
conditions, extended processing times
increase the potential for higher
bioburden loads.
Scaleup and Scaledown
Once a process has been developed
at lab bench scale, it must be
translated to industrial scale and
validated, and this can present
unanticipated surprises and
challenges. Often, there is little
opportunity to perform intermediate-
scale runs due to time and material
constraints. In addition, material from
the initial industrial-scale runs is usually
required for time-sensitive clinical or
marketed supply. Therefore, accurate
and dependable scaleup is critical for
the success of a process.
Pellicon 2 Industrial Scale
The simplest way to ensure
accurate and predictable translation of
product yield and purity from bench to
industrial scale is to use linear scale
techniques. To linearly scale a TFF
process, all fluid dynamic and
membrane module parameters must be
kept constant between scales of
operation. Fluid dynamic parameters,
which are set by the user, include the
ratio of feed volume to membrane
area (at constant feed concentration),
feed rate per membrane area, filtrate
flux, and retentate and filtrate
pressures. Membrane module
parameters are inherent in the specific
filters that are chosen, and include
membrane material and pore size,
turbulence promoter, channel height,
channel length, and feed and filtrate
flow geometries.
Linear scaling is also used to
reduce the scale of a process. This
can be very useful for process
validation, where it is sometimes not
economically feasible to perform all of
the required validation at full operating
scale. Validation performed at small
scale will only be acceptable if it can
be shown to produce results that are
equivalent to industrial scale. In
addition, linear scaledown is used to
Pellicon 2 Mini Holder
troubleshoot or develop improvements to
a process once it has been implemented into a manufacturing line.
Robustness
Once implemented at large scale, a
process must be robust if it is to be
successful. A robust TFF process will
perform well within the lot-to-lot
feedstock and membrane variations
that it encounters. While developing a
process, it can be very useful to test
performance at the extremes of these
variations, if possible. For instance,
determining flux of the most fouling lot
of feedstock using membranes having
the lowest acceptable permeability
would allow the membrane area to be
sized such that the maximum process
time was not unacceptable.
Alternatively, determining retention of
the product in the least fouling lot of
feedstock using membranes having the
highest acceptable permeability would
ensure that the chosen membrane
cutoff would not lead to significant
product loss. A robust TFF unit
operation should also be able to
withstand some variations in operation
without catastrophic failure.
Consideration of membrane and
module characteristics such as
susceptibility to reverse transmembrane
pressure, chemical compatibility, or
