Biomedical Engineering Reference
In-Depth Information
branes were used in blood oxygenators until about 2004 but are no longer in
large-scale use.
Other
A minor share of the oxygenators available today contain flat sheet silicone
membranes. To ensure the wall is thin for increased transport performance, these
membranes are composite membranes with a very thin silicone deposit on a
support mesh. Other membrane production technologies such as casting or
etching do not play a role in commercially available oxygenators.
1.2.4 Membrane structural implications for use
The different production schemes of course produce membranes with different
structures. A closer look into the pore structures reveals the differences (Fig.
1.6). Membranes produced by TIPS exhibit pores with very high tortuosity and
extremely long pore paths. From this they are very resistant against plasma
breakthrough, as the hydrophilic and amphoteric components of blood plasma
will hardly ever make it through the long hydrophobic pore. Membranes pro-
duced by dry stretch have a very solid backbone as can be seen in the figure.
This backbone makes them resistant against mechanical stress and thus easy to
handle and process.
In the 1980s, plasma breakthrough was a problem in blood oxygenation.
However, owing to improvements in the production process and consecutive
narrowing of the pore size range, plasma breakthrough today is no longer an
issue for blood oxygenator applications in cardiac surgery, where oxygenators
are used for no more than six hours.
If an oxygenator is to be used for a prolonged time, wetting of the membranes
may also occur with the improved microporous membrane types used today. A
wetting phenomenon after ten or more hours commonly is not a plasma
breakthrough in the classical sense, in that plasma starts to creep through the
pores, facilitated by the blood proteins, but is often a mixture of a condensation
process with such plasma creeping. Usually, the blood will have a temperature
clearly above room temperature. Therefore, water vapour from the blood may
enter the hydrophobic pores and then condense in the capillary lumen, where
colder gas flows. Partial condensation may also occur in the pores and help
plasma breakthrough.
To prevent this wetting for periods of many hours and even days or weeks,
one needs to go back to fully diffusive membranes, with additional safety
against the entry and penetration of water or plasma. With a TIPS process, a
membrane with a microporous backbone and a thin outer skin can be produced.
As mentioned on pages 8±10, the polymer for this membrane must be permeable
to carbon dioxide and oxygen. SEMs of such a membrane (trade name
