Biomedical Engineering Reference
In-Depth Information
Biocompatibility is more of a concern for extracorporeal circuits used for
longer term lung assist. 22 The survival of extracorporeal membrane oxygenation
(ECMO) as the last resort treatment for severe lung failure continue to be poor at
clearly less than 50% for adults and not too much above 50% for infants. Most
of this may be attributed to the extreme morbidity of the patients treated with
ECMO, but nevertheless this mirrors the extreme bio-incompatibility of a
prolonged cardiopulmonary extracorporeal support with the large setting as in
ECMO. Better perspectives exist for patients with arterio-venous carbon dioxide
removal, such as with the new Novalung Õ device of Novalung GmbH, Germany.
These devices have been used for periods of some weeks. 23 However, here the
main clinical concerns do not focus on biocompatibility (such as cell adhesion,
complement activation) but rather on the performance of the device,
anticoagulation, general inflammation and the prevention of organ failure.
1.2.8 Medical device considerations
To create an artificial lung of capillary membranes, the membranes need to be
arranged in a repeating, symmetrical manner by either winding capillaries on a
core, by winding capillary membrane mats on a core or by piling capillary
membrane mats to a stack (see Section 1.2.6). When mats are used, the capillary
ends need to be sealed to prevent potting agent from entering the capillary
lumen. These membrane bodies are then inserted into a housing and potted: a
liquid potting agent such as freshly mixed but not yet polymerised polyurethane
(PUR) or silicone is used to glue the body into the housing. After hardening of
the potting agent, the ends of the potting are cut with a saw or sharp blades, so
that the very end of the capillaries is cut away. The capillaries are thus open; in
the device oxygen or oxygen-enriched air will flow into the capillary lumina.
The outside of the capillaries is sealed to the housing; blood will enter the
housing through a blood inlet port and flow around the capillaries.
Blood film thickness and thus the oxygenator gas exchange performance
depend on blood velocity, and hence the packing density: the lower the void
volume, the higher the blood velocity will be, the better the gas exchange
capacity will become and low surface areas can be employed. The downside of
high packing density is the high blood side pressure drop, which indicates
pressure and shear stress to the cells.
Flow through the bundle can be either longitudinal (i.e. in the same direction
as the capillaries), across (i.e. perpendicular to the capillaries) or anything in
between. Most oxygenators have manifolds that distribute the blood into and
through the wound body. All types of flow can be found.
Blood oxygenators additionally incorporate a heat exchanger to adjust the
patient's blood temperature. Blood commonly flows through the heat exchanger
first and then through the gas exchanger. Heat exchanger material can be metal
or polymeric material. Metal heat exchangers provide superior heat exchange
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