Low extractables.
Media filters must not only retain contaminants, but they must also be chemically and biologically compatible with the cell
culture media. This means that the filter must be constructed of components that are proven to be safe and that the materials
that extract from the filter during normal operation have been quantified and characterized. Filter manufacturers will typically
provide flushing recommendations for their particular filter products and also have standard specifications for many contaminants,
including (but not limited to): total organic carbon (TOC), oxidizable substances, toxic compounds, particulates, and fibers.
Some manufacturers also offer application-specific testing of extractables.
Low nonspecific binding.
Potential interactions between filters and culture media must be assessed carefully to ensure no inhibition of cell growth
or protein expression. Cell cultures are highly sensitive to growth media composition; hence, the materials of construction
for a sterilizing filter must be proven inert. Membranes used in media sterilization operations should have low nonspecific
binding to ensure that key media ingredients are not removed during the filtration process.
Physical robustness.
Physical robustness is important in all filtration applications, but it is critical in media sterilization applications because
of the stress which the filters undergo before, during, and after use. A typical media filtration process consists of the
following steps:
- Filter installation and wet-out
- Pre-use filter integrity test
- Steam-in-place
- Media filtration
- Filter flush
- Post-use filter integrity test
- Clean-in-place
- Filter disposal.
Many of these steps result in physical, thermal, and/or chemical stress on the membrane and other filter components. Nonetheless,
every component must retain its functionality for the media sterilization operation to be considered successful.
High permeability.
The term "permeability" refers to the flux rate achieved through a filter, normalized with respect to differential pressure.
Permeability is typically reported with units of liters per square meter per hour per psi. Permeability is important because
most media sterilization processes have relatively short batch times (1-2 h) and filters with low permeability may drive the
required filter area to be larger than required based on filter capacity alone. (This results in a filter train that is underutilized
with respect to capacity.) Media sterilization filters with high permeability result in a system that maximizes the filter
throughput.
High capacity.
The term "capacity" refers to the volume of feed that can be processed by a given membrane area before the membrane's resistance
to flow becomes unacceptably high. Sterile media filters are expected to have high capacity (thousands of liters per square
meter of membrane area). This is an important characteristic for media sterilization filters because of process economics,
ease-of-use considerations, and the minimization of nonspecific binding. For sterilizing-grade filters that are designed
for cell culture media sterilization, high capacity is achieved by the addition of an on-board prefilter layer—typically having
a pore size rating of 0.4 - 0.8 µm.
Survey of media sterilization filters.
GE Healthcare (West Borough, MA) performed a study to evaluate the most common filters that are designed for media sterilization.
In this study, five commercially-available cell culture media were prepared from dry powder per the manufacturer instructions.
The tested media are as follows:
- DIFCO Miller LB broth for E. coli
- DIFCO YPD broth for yeast
- Invitrogen RPMI-1640 basal media supplemented with Sigma Type IV soy peptone
- Hyclone SFM4CHO utility
- SAFC Biosciences EX-CELL 302.
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