They treat 180 liters of blood daily, which they process to remove waste products: our kidneys. These two complex organs filter breakdown products of protein and amino acid metabolism – especially urea – from the blood, and these waste products leave the body with the urine. When the kidneys stop doing their job, the waste products accumulate in the body and have to be removed from the blood artificially – a process called dialysis.

In the 1940s, Dutchman Willem Kolff developed the first successful membrane for the dialysis of blood, based on cellophane. This was the start of the large-scale application of membranes in the medical world. This way, the foundations were laid for today’s renal dialysis, in which the bloodstream of a kidney patient is connected to an artificial kidney several times a week. Inside this artificial kidney, blood containing waste products flows along a polymer membrane. A dialysis fluid containing blood components such as potassium, sodium, calcium, glucose and hydrogen carbonate (with the same concentrations as in blood) flows along the other side of this membrane. Because the concentrations of these components are virtually the same on both sides of the membrane, there is no net transport of these components through the membrane. However, the difference in concentration of waste substances over the membrane causes these substances to diffuse from the blood into the dialysis fluid. Blood cells and proteins are too large to pass through the membrane and remain in the blood. The blood is pumped through the artificial kidney for several hours until the waste products are removed.

The polymeric membranes in an artificial kidney are shaped like many thin and long ‘straws’, known as capillaries. This has the advantage that a large membrane surface area is compacted into a small volume, so that the patient does not have to miss too much blood during dialysis.