Biomedical Engineering Reference
In-Depth Information
et al. developed a membrane oxygenator, the first clinical application of which
was described in 1956.
4
The first membrane oxygenators incorporated various membrane materials,
among them cellophane, polyethylene (PE) and Teflon, all of which were com-
paratively impermeable to oxygen. In the 1960s, the first oxygenators with
silicone membranes were developed.
5
For these, thin films of polysiloxane
(silicone) were supported by meshes or sieves, then wrapped to a roll or stacked
to a pile to incorporate sufficient surface in a volume that could still be handled.
Those oxygenators had large priming volumes and large surfaces and
consequently resulted in considerable blood dilution and blood damage.
In the 1980s, the first oxygenators with microporous membrane sheets were
produced. The preferred material for microporous membranes in blood
oxygenation was and remains polypropylene (PP).
6
Owing to the presence of
pores, the permeability of these membranes to oxygen was much higher than
that of any other material before, so that the limiting factor for gas transfer was
no longer the membrane, but the gas transport within the blood films directly
adjacent to the membrane (see also page 10).
The next breakthrough in oxygenator performance was the introduction of
capillary membranes. Those had the same pore size and same polymer (PP) as
the flat sheet microporous membranes introduced. Similar to the development
with haemodialysers, the introduction of capillary membranes reduced the
priming volume of the oxygenator and turned a bulky space consumer into a
handy device. The first capillary membrane oxygenators worked with blood
flow inside the capillaries, but soon the trend to blood flow outside the
capillaries prevailed: blood side pressure drop always used to be a limiting factor
for `blood inside' oxygenators, and the `blood outside' models were smaller and
more handy, and demanded less priming and thus less haemodilution. Finally,
this was also welcomed from the manufacturer's perspective, as it saves about
half the amount of membrane.
These microporous membranes provide excellent performance, but compared
with the older silicone membranes are more prone to plasma breakthrough.
Around the year 2000, the first oxygenators with a membrane from polymethyl-
pentene (PMP) with a very thin dense outer skin on a microporous body were
introduced. These provide performance on the same level as the microporous PP
membranes, but do not show the phenomenon of plasma breakthrough.
Today, almost all commercially available blood oxygenators incorporate
capillary membranes and have the blood flow outside and the gas flow inside the
capillaries. Most of the oxygenators have microporous membranes of PP, and a
growing share is equipped with the dense PMP membrane type. Only one
silicone membrane oxygenator is still commercially available.
