Biomedical Engineering Reference
In-Depth Information
will tend to remove the cells from the membrane wall and support flow through
the wall. In particular towards the exit of the plasma separator, where a con-
siderable fraction of plasma has already been removed and the haematocrit rises
to up to 70%, parts of the membrane surface may be lost to removal of plasma
due to increased blocking.
Most plasma separators have active lengths between 200 and 250mm, which
seems to be the optimum. A longer device will need higher operating pressures,
in particular due to the concentration of blood cells and the rising viscosity of the
blood over the length of the separator. Higher operating pressures will also mean
higher transmembrane pressures at least by the blood inflow into the device and
can result in a higher tendency towards haemolysis. Shorter devices may result in
too low a pressure gradient, which means that fewer plasma can be pressed off.
1.3.6 Alternative technologies
A direct alternative to plasma separation with membranes is the centrifugation of
blood. This method is currently far more popular than plasma separation with
membranes. It is used widely in blood and plasma donation centres. The patient's
or donator's blood is pumped into a centrifuge, where the plasma is simply
centrifuged off. Both fractions can be handled as after a membrane separation:
cells are returned to the patient or donor and the plasma is treated further.
This process is technically established almost everywhere in the world. It is a
proven technology and fairly simple to operate. However, it needs to rely on an
elaborate infrastructure with energy supply to the centrifuge and sterilisation of
the reusable parts of the machine. A membrane plasma separation is independent
of such infrastructure and can also be operated in any country. Another down-
side of centrifuged plasma is that it may contain cell debris generated during the
procedure. Such debris is rarely generated during membrane plasma separation,
and additionally any particulates would be completely retained by any plasma
separation membrane.
Another option would be removal of the target molecules from whole blood,
without separation into cells and plasma.
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This, however, is a very difficult
task. It demands highly selective adsorbents to the target molecule to be
removed, which needs to avoid any triggering of immune response or other bio-
incompatibilities. Today, only a few methods for direct removal from whole
blood exist, such as DALI or Liposorber for removal of lipoproteins.
1.4 Economic considerations
1.4.1 Blood oxygenation
In the era of ever-growing economic pressure, including in the healthcare sector,
any treatment and component must be put under scrutiny for its economy.
