Biomedical Engineering Reference
In-Depth Information
where TMP is the transmembrane pressure; p
i
is the blood pressure at inlet; p
o
is
the blood pressure at outlet; and p
f
is the pressure of filtrate.
Too high a transmembrane pressure may result in blood stress and damage and
thus pressure must be monitored closely and kept at (ideally) about 50mmHg, but
not higher than 150mmHg, where haemolysis may occur. This haemolysis is
caused by damage to the red blood cells in the rather large pores. At high pressure,
the red cells are pressed into the pores and rupture. It is therefore advisable to start a
plasma separation procedure at zero plasma flow, to allow a certain protein
deposition on the membrane walls (see Section 1.3.5) to protect the blood cells.
1.3.4 Membrane make-up
Plasma separators are produced from membrane bundles. As the separation
principle is simply to press the plasma out through the capillary wall, the
separator architecture and thus bundle makeup can be rather basic. Blood is
flowing inside the lumen of the membranes and the plasma simply runs off on
the outside of the capillaries. The packing density of the bundle must thus not be
too high to allow for an unhindered flow of plasma. Further flow optimisation on
the outside of the capillaries, such as that employed, for example, by undulation
or spacer yarns for dialysers, is unnecessary and even counterproductive.
In the manufacture of the membrane bundles, the bundles are therefore simply
collected to multifilament strands. This can be achieved by winding the
capillaries on a large wheel or by laying them on a table in parallel fashion. These
strands are then cut to the length needed for the final device and wrapped into a
foil to aid the insertion of the bundle into the housing of the plasma separator.
1.3.5 Medical device considerations
To ensure a proper functioning of the final separator, several issues need to be
addressed:
· high porosity (both surface and volume) for sufficient plasma flow;
· low fouling (i.e. protein adsorption and blocking of the capillary);
· biocompatibility (low complement activation, no haemolysis);
· sterilisation possible by ETO, gamma irradiation, steam.
A finished plasma separator contains about 0.2±0.9m
2
of membrane surface and
is operated at blood flows of 50±200ml/min. The transmembrane pressure is
kept low at typically less than 50mmHg and, depending on the device, does not
exceed 150mmHg.
During the plasma separation, blood cells and larger proteins will accumulate
by the membrane wall, as the plasma exits through the wall and the larger
components of blood are retained. This so-called cell concentration polarisation
will hinder the plasma flow across the membrane wall. An increasing blood flow
