Biomedical Engineering Reference
In-Depth Information
schemes for cellulosic dialysis membranes. Membranes for plasma separation
are produced from cellulose acetate. For this, cellulose acetate is dissolved, e.g.
in an acetic acid solution, and spun through a ring nozzle into a precipitation
bath that can have only a low concentration of acetic acid. To keep the capillary
open, a bore liquid is injected through an opening in the centre of the ring
nozzle into the lumen of the capillary. The capillary is washed in several steps
with water: ideally the final washing step brings the membrane into contact
with a pore stabilising agent such as glycerol. It is finally wound to bundles and
then dried.
1.3.3 Characterisation of plasma separation membranes
Plasma separation membranes are much more open than dialysis membranes.
Table 1.3 gives an overview of plasma separation and plasma fractionation
membrane classification according to their sieving coefficients. The sieving
coefficient SC is the fraction of a given molecule that can pass the membrane:
SC
2C
f
C
o
C
i
where SC is the sieving coefficient; C
f
is the concentration in the filtered
plasma; C
o
is the concentration in the blood at outlet; and C
i
is the concentration
in the blood at inlet.
Sieving coefficients of a plasma separation membrane for virtually all plasma
proteins (e.g. albumin, 66 kDa; IgG, 150 kDa; IgM, 970 kDa; -lipoprotein,
2400 kDa) should be in the range of 0.95 to 1.00 to allow for an efficient
removal in particular of the larger proteins. A plasma fractionation membrane
should hold back these larger molecules to a considerable portion, e.g. with an
SC for IgM of 0.2 or similar. The IgM-containing fraction can then be discarded,
and the smaller proteins like albumin returned to the patient.
Another important value to characterise a plasma separation membrane is the
transmembrane pressure (TMP) during operation:
TMP
p
i
p
o
2
ÿp
f
Table 1.3 Typical sieving coefficients of separation membranes
Beta-2 microglobulin
Albumin
IgM
Low flux dialysis membrane
0.00
High flux dialysis membrane
0.60
0.01
Super-flux dialysis membrane
0.9
0.02±0.05
0
Plasma fractionation membrane
1.0
0.8
0.2
Plasma separation membrane
1.0
1.0
