Biomedical Engineering Reference
In-Depth Information
clogging as the turbulent flow of the feed across the filter removes deposited
materials, something that is not possible in dead-end filtration.
Filtration Media
Filter media generally comprises layers of solid materials in a network or
mesh with voids, pores, and channels that allow the passage of liquid but
retain larger particles, larger than the size of the openings, which may be in
nanometers.
Depth filters use their entire depth to retain particulate on the basis of
sieving compounded by adsorption effects unlike retentive filters where the
filtered material is concentrated on the surface. The depth filter media domi-
nate prefiltration and clarification applications because of the high solid
mass that is generally required to be removed at this stage.
Sieving or size exclusion have more uniform pore sizes throughout the bed
and are thus used to remove selective size of particles; these filters, mostly
membrane types, are ideal as sterilizing filters, for example, the commonly
use 0.22 μm filter to sterilize liquid. While the main mechanism of their
operation is sieving, the chemical nature of these membranes makes them a
good base to adsorb organic substances.
Depth filter are made of fibers that are spread out on a substrate to make
a mesh just like making paper; special additives such as activated carbon,
ceramic fibers, and other such specific components are embedded with the
help of a binder to form the filter.
Sheet filters are also made like paper using milled cellulose fibers and
may contain diatomaceous earth or perlite along with a binder to strengthen
the filter.
One of the world's largest suppliers of these filters in biopharmaceutical
manufacturing is Pall (
Table 10.1
).
Because of their thickness, the sheet filters provide a slow filtration option
yet are extensively used for prefiltration
Microglass fibers are also used to filter media; these are nonwoven spun
fibers of borosilicate glass whose web is strengthened by a binder allowing
for a 3D structure of asymmetric voids as small as 0.2 μm to act as sterilizing
filters.
Polypropylene and polyester fibers are also used by spinning from polymer
melt and bonded by the polymer itself giving better chemical compatibility
as no binder is added to them. These are always the preferred filters over
polyamide and cellulose filters. The convention method of their manufacture
leaves pore sizes 20-50 μm making them unsuitable for sterile filtration; a
special blown process is used to reduce the pore size in the range of 5-50 μm;
further spinning is needed to reduce the size further to 1-10 μm range.
