Biomedical Engineering Reference
In-Depth Information
1.9 Principle of membrane plasma separation.
For plasma separation, the blood plasma is separated from the cells by either
centrifugation or membrane filtration (Fig. 1.9). The pore size of the separation
membranes is in the range of less than 0.5m, thus small enough to hold back
the smallest blood cells: platelets.
Blood plasma is pumped through a filter very similar to a dialyser and plasma
migrates through the pores driven by an excess pressure on the blood side. This
excess pressure can be achieved by a pump or through gravity. The plasma can
then be discarded and replaced by saline or protein solutions or treated further.
The consecutive steps can be additional filtration steps as in cascade filtration or
adsorption processes. The blood cells are dispersed in saline or a protein solution
and returned to the patient.
1.3.2 Manufacture and resulting structure
Hydrophobic synthetic membranes
The production of hydrophobic synthetic plasma separation membranes is done
through a TIPS process and is very similar to that of blood oxygenation
membranes (see pages 10±12): the basic polymer (PE or PP) is dissolved in a hot
mixture of solvents. This solution is forced through a ring-shaped spinneret. After
exiting the spinneret, the solution is cooled, so that the polymer recrystallises.
The solvent aggregates to droplets, which are trapped in the polymer matrix.
After extraction of the solvent, the space where the solvent has been constitutes
the pores. These pores need to be larger than those for the blood oxygenation
membranes to allow a sufficient transport of plasma through the membrane wall.
The structure of these membranes consequently is very similar to that of
TIPS-produced blood oxygenation membranes (Fig. 1.10). The membrane has a
homogeneous, sponge-like structure with a constant pore size throughout the
whole membrane wall.
