Biomedical Engineering Reference
In-Depth Information
separation. While cardiac surgery today is an established procedure, where
improvements over generations have built a gold standard, long-term lung
assistance and plasma separation and plasma treatment are rather in their
infancy. It is still a matter of debate how to achieve the best results, who should
perform the procedures, which patients can effectively and efficiently be treated
and last, but not least, who will pay for these procedures.
Membranes as biomaterials form the basis for the procedures of lung
substitution and plasma separation. These membranes are readily available and
look very simple: white capillaries, wound to bundles or finished with rather
basic textile technologies. But it is not as simple as that: they represent the
highest standards of biomedical technology.
1.2 Membranes for blood oxygenation
1.2.1 History
To perform delicate surgery on the human heart, for many interventions it is
essential to stop the heart beating (e.g. for any surgery on the heart valves), and
for all procedures the stoppage greatly facilitates the work of the surgeon. The
heart with its four chambers pumps the blood through body and brain, which
consume oxygen and produce carbon dioxide, and through the lungs simul-
taneously, where carbon dioxide is exhaled and oxygen taken up. If during a
cardiac surgery procedure the heart is still then the lungs are also at rest. This
means that the blood not only needs to be pumped through body and brain to
continue their supply, but also needs to be oxygenated artificially. Therefore, for
any cardiac surgery procedure where the heart is stopped, a device to oxygenate
the blood and to remove the carbon dioxide is also needed.
A good overview of the development of the blood oxygenator is given by
Leonard.
1
The first heart±lung machines have worked with film or bubble
oxygenators.
2,3
In film oxygenators, blood was passed over sieves, plates or
discs in pure oxygen or an oxygen-rich atmosphere. Oxygen and carbon
dioxide were exchanged on this large surface. The second generation of
oxygenators, bubble oxygenators, simply consisted of a bubble chamber where
oxygen was dispersed into the patient's blood and a defoamer which removed
the majority of gaseous residuals from the blood. Both procedures were a
major insult to the patient and their blood, and therefore a more gentle
procedure had to be found.
In the 1940s, Kolff observed that the blood in his rotating drum kidneys
turned to a brighter red during treatment. This was simply due to take up of
oxygen through the cellophane membrane he used to separate the blood from the
dialysis solution, which was exposed not only to the dialysate, but also to the
open air. In a way, the first clinical application of an artificial kidney also was
the first application of a membrane artificial lung. From this starting point, Kolff
