Artificial Lungs: More than just hot air?

19. September 2011

An implantable artificial lung is one of the great visions of bionics, the science of artificial humans. Scientists from Cleveland are now one step closer to that goal: they have taught an artificial lung to satisfy itself by taking in air.

Artificial organs are one of those tricky things. The idea is compelling: (simply replace) replacing a failed kidney via an implantable artificial kidney or a failed heart via an implantable pump. In reality, of course, it always appears more complicated than in theory. Problems with power supply and frequent suboptimal tissue compatibilities of the employed materials are the two main stumbling blocks which work against the bionic person. The consequence is that the portability of artificial organs isn’t everything it’s cracked up to be. The artificial kidney is a recognised reality. The only thing that one can’t do with a dialysis machine is go jogging. The initial stages of the artificial heart also brought us monsters-on-wheels. In this case, efforts have meanwhile advanced things somewhat more.

These lungs live on air alone

The artificial lung as well would in medicine be highly timely: More than 200 million people worldwide suffer from severe lung disease. For many of these people, at some point in time medicine can offer nothing aside from a lung transplant. Similar to the kidney, the lung can be technically replaced in various ways: a partial substitution works via ‘classical’ respiration. Extracorporeal membrane oxygenation is virtually a complete bypass. Neither of the two methods is an artificial lung in the narrower sense. True artificial lungs attempt to mechanically recreate the physiological gas exchange of lungs – that’s the great goal – in order to incorporate it one-to-one into the thorax.

None of the varied lung-replacement options is currently particularly mobile. One of the biggest problems with artificial lungs is that they don’t simply just “breathe” air, but have to be fed by oxygen bottles. And these have to be moved first. Scientists led by Joseph Potkay of the Department of Electrical Engineering and Computer Sciences at Case Western Reserve University in Cleveland, Ohio, now report in the journal Lab on a Chip on an artificial lung that does not require supplemental oxygen, but gets by on completely ordinary breaths of air.
This is possible because the gas exchange in the replacement lung developed by Potkay works much more effectively than conventional replacement lungs: Tests using pig’s blood have shown that oxygen consumption is three to five times higher. If Potkay’s artificial lung were ventilated with quite normal breaths of air, an oxygen content of the blood could be reached which the contemporary common artificial lungs only achieve with the use of pure oxygen. “Based on current performance, we estimate that a device for use in humans would have to have a size of six inches by six inches by four inches,” according to Potkay. That means 15 by 15 by 10 centimetres, which would just make it fit in a big chest. “Therefore the device could be connected to the heart and would not need its own pump,” says Potkay.

Silicone rubber plus nano technology equals alveoli

The fact that this has been achieved essentially lies in the fabrication technique. Potkay’s artificial lung is made entirely of silicone rubber, which is formed, using microtechnology and nanotechnology-based manufacturing methods, into a type of artificial capillary system. Both the size of the capillaries and the thickness of the membrane that separates the blood compartment from the air compartments are similar to the natural ratios in the human body. The diameter of the smallest silicone rubber “capillaries” for example is less than a quarter of the diameter of a human hair.

The result is both a very short diffusion distance and, at the same time, an extremely large ratio of surface to volume. Both are qualities that also distinguish the natural lung. About the gas exchange itself the manufacturers of artificial lungs have no need to be concerned: Oxygen and carbon-dioxide diffusion through similarly fine membranes occurs at the same respective partial pressures as in the natural model.

The problem of the artificial lung is admittedly not yet finally solved with the Potkay prototype. Potkay himself also does not make this claim and estimates that at least eight to ten years will pass before an artificial lung based on his technology could be used (in the US) in clinical trials in humans. The connection of the capillary-tubing to the circulatory system should be the minor problem. There is now quite a bit of experience in this area, not least because of artificial heart research.

More difficult would be the mechanical fitting of the lungs into the thorax. An artificial lung which breathes air is all well and good, but such a construct will only become mobile when it appears to be possible to connect it to the respiratory mechanisms. Only then can the oxygen-tank stay home. Using the respiratory muscles, the artificial lung has to be so stretchable in the hermetically-sealed pleural cavity that sufficient oxygen can be absorbed past the trachea. It should also be stretchable in such a way that the thing does not collapse completely subsequent to that, but rather to the extent that the re-opening does not require an endless amount of breathing work. Biocompatibility is also still an issue, when it comes to the real possibility of it being in the thorax.
There really are a few things that still have to be done on the way to fully bionic breathing.

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