criteria by which the success of the
operation will be measured. The
primary goals for a successful protein
processing are:
. High product yield
. High product quality (or activity)
. High product purity
. Controlled bioburden and endotoxin
In addition, the process should
scale up accurately, enable
straightforward validation, and be
robust during continued use at
industrial scale. Finally, the economic
objectives for the process must be met
and any constraints such as process
time, unit operation size, or buffer use
must be observed. Discussion on how
the process design impacts yield,
quality, bioburden, scalability,
robustness, and economics begins on
page 19.
Choosing the Right Equipment
The primary components of a TFF
process are the membrane material
and the membrane module format.
Choosing the most appropriate
components early in the development
process, with consideration for the
requirements of the process, increases
the success and robustness of the final
step.
Membranes
Millipore provides ultrafiltration
membranes in several different
materials to suit a wide range of
applications. The different membrane
materials offer alternatives in fouling
characteristics and chemical
compatibility. Each of the membrane
materials is available in a number of
NMWLs. Two of the most common
materials for ultrafiltration membranes
are regenerated cellulose and
polyethersulfone.
Millipore's Ultracel. membrane is
regenerated cellulose. The Ultracel PL
family, standard regenerated
cellulose, is available in NMWLs from
1 to 300 kD. The Ultracel PLC
composite regenerated cellulose
ranges in NMWLs from 5 to 1000 kD.
Ultracel PLC membranes are cast on a
microporous polyethylene substrate
and have superior resistance to
reverse pressure versus the PL series.
All regenerated cellulose
membranes are very hydrophilic,
exhibiting low fouling and ultra-low
protein adsorption. They are more
compatible with organic solvents than
are the polyethersulfone-based
membranes, but are less tolerant to
extreme pH’s. Ultracel membranes
are recommended for use in all
applications where harsh pH
conditions are not needed and
especially when protein loading is
low (<20 g/m2) or the feedstock is
highly fouling.
Traditional polyethersulfone
membranes tend to adsorb protein as
well as other biological components,
leading to membrane fouling and
lowered flux. Millipore’s Biomax.
membrane is polyethersulfone-based,
but has been hydrophilically modified
to be more resistant to fouling. The
Biomax membrane line ranges in
NMWLs from 5 to 1000 kD. Biomax
membranes operate over a wide
temperature range and are highly
stable at pH’s from 1 to 14, but have
limited solvent compatibility. The use
of Biomax membrane is recommended
for applications where very harsh pH
conditions are required for processing
or cleaning. In order to minimize
adsorption losses maintain moderately
high protein loading ( >20 g/m2).
10 kD Biomax. Traditional PES 10 10 kD Ultracel. PLC
Scanning electron micrograph images of cross sections of different membranes.
Table 1 shows the magnitude of
protein losses due to adsorption on
several UF membrane materials. These
losses were measured at Millipore
with model protein feedstocks. In
addition, the percentage yield loss
due to adsorption is shown for two
theoretical processes. The “Low Protein
Case” is a process in which 1000 L
of 0.1 g/L solution is concentrated to
2 g/L on a 10 m2 unit. The “High
Protein Case” is a process in which
1000 L of 10 g/L solution is concentrated to 200 g/L on a 20 m2 unit.
The unique construction of both the
Biomax and Ultracel PLC product lines
makes these membranes free of voids
and defects and well-attached to the
substrate. The membranes are rugged,
have very high integrity, and have
excellent retention characteristics. A
membrane from either the Biomax or
Ultracel PLC family should be the first
choice when developing a process.
Since NMWLs for UF membranes
do not indicate absolute
retention/sieving ratings, some rules
of thumb are useful in determining
what membrane rating is applicable
For a batch UF and constant-
volume DF process, where retention
remains constant throughout the
process, this loss is calculated as:
Product Loss (%) = 100* (1 – e (R-1)[lnVCF + N])
The relationship is plotted in figure 6
