Biomedical Engineering Reference
In-Depth Information
the longer time of exposure to this foreign surface, the stronger the reaction
of the body must be.
From this it is obvious that an artificial lung, like other similar biomedical
devices, must find a difficult compromise: to reduce insult to the blood, both
surface and contact time should be minimised. Today's oxygenators have
exchange surfaces of 0.25m
2
(devices for neonates) to 2.5 m
2
(for large adults).
In order to make up for this lack of exchange surface, according to Fick's law,
one must thus use high activity gradients, longer contact times, low diffusion
distances and materials with high diffusion constants (high permeabilities to the
relevant gases).
Interfaces
The interface of blood and gas in an artificial lung consists of (at least) five
phases (Fig. 1.1). In the example of oxygen, oxygen is first transported towards
the membrane from the bulk gas by convection. On the gas side of the
membrane, there is a diffusion layer, where oxygen is no longer transported by
convection, but can only diffuse. Then follows the membrane as one or possibly
more distinct phase(s) (more on that below). Next follows a diffusion layer on
the blood side of the membrane, before the oxygen finally reaches the bulk of
the blood flow and can be transported to the organ of the patient, again by
convection. The overall permeability of a given gas from the gas side to the
blood side of an artificial lung is thus a combination of different permeabilities.
In the end, the overall permeability can only be as high as the permeability with
the lowest single value.
1.1 Blood±membrane±gas interface.
