Biomedical Engineering Reference
In-Depth Information
1.7 SEM of PMP membrane with outer skin.
300m.
8,16,19
Most of these diameters are a relict from times where oxygenators
were constructed with blood flowing inside the capillaries. To ensure a proper
flow of blood through the tiny capillaries at the haematocrit of around 30% that
is commonly employed during cardiac surgery, an inner diameter (i.d.) of 200±
280 m became the preferred dimension. Many oxygenators were then
developed and approved with capillaries with i.d. of 200±280m and outer
diameter (o.d.) of 300±380m. With the trend to blood flow outside the
capillaries, these dimensions were not changed. As new developments and new
approvals in the medical device industry are time consuming and costly, the
`standard' dimensions have remained the same.
Of course, priming volume can be reduced with smaller membranes, and
therefore small membranes are the preferred choice for baby and infant
oxygenators to reduce the use of foreign blood products. However, with small
dimension membranes manufacturers need more kilometres of membrane for the
same active exchange surface, so that for the more cost-sensitive products such
as adult oxygenators for cardiac surgery the standard dimensions with o.d. of
300 or 380m are the best compromise.
1.2.6 Membrane make-up
For devices with blood flow outside the capillaries, the make-up (arrangement)
of the capillaries is of great importance. The simplest construction of a capillary
membrane device would be a bundle of parallel capillaries in a cylindrical
housing, as has been produced by Terumo (Japan) in the SX series. With such a
construction, the fluid around the capillaries is prone to create shunt flows in
some parts of the bundle and stagnant regions in other parts. Therefore, the
membranes need to be properly arranged to yield a uniform flow around the
capillaries and good mixing of the blood.
This can be achieved by winding the capillaries on a core in a symmetrical
fashion, similar to a spool of thread. By using a defined angle between the layers
and a suitable blood inlet and distribution, good mixing and thus high per-
