Biomedical Engineering Reference
In-Depth Information
1.10 SEMs of a TIPS produced hydrophobic plasma separation membrane.
To be applicable for plasma separation, a hydrophobic membrane needs to be
hydrophilised before use, so that blood plasma can enter and penetrate the pores.
This is either done in the place of use or the plasma filter comes ready
hydrophilised. Such hydrophilisation can be achieved, for example, by rinsing
the membrane with a liquid such as alcohol, that completely penetrates the pores
easily. This liquid is then replaced by water or saline and completely washed out
of the pores. Alternatively, solutions containing surfactants can be used instead
of the alcohol. The plasma separator must not be dried completely after this
hydrophilisation step, so that the pores keep being wetted by the water or saline.
Once dried completely, the membrane regains its hydrophobic properties and
needs to be re-hydrophilised. To prevent complete drying, some low volatile
hydrophilic components can be added, such as glycerol.
Hydrophobic synthetic membranes dominated membrane plasma separation
for many years, but have today increasingly been replaced by hydrophilic
synthetic membranes more and more.
Synthetic hydrophilic membranes
Synthetic hydrophilic plasma separation membranes are very similar in com-
position and production to dialysis membranes. The majority consist of a blend
of polysulphone (PSu) or polyethersulphone (PES) and polyvinylpyrrolidone
(PVP) and are produced according to the same schemes as synthetic dialysis
membranes (reference to dialysis chapter). Other materials include polyvinyl-
chloride (PVC), polymethylmethacrylate (PMMA) and polyvinylalcohol (PVA).
As the polymers PSu and PES have the largest industrial meaning, the following
will focus on these two materials. The membrane formation as such is achieved
by a solvent-induced phase separation process (SIPS) (Fig. 1.11).
For this, the basic polymers PSu + PVP or PES + PVP are dissolved in a
mixture of solvents. Various solvent systems are used by the different manufac-
turers. As for all productions of capillaries, this polymer solution is pumped
through a ring-shaped spinneret, at the same time a core liquid is pumped
