Dialyzer
Dialyzers are so-called biocompatible polymer membranes and, as a central exchangeable blood purification unit, represent the heart of the dialysis machines , which are used for renal replacement therapy through hemodialysis or hemodiafiltration . In medical jargon they are also called "filters" for short and are used once.
In principle, a mass transfer takes place in dialyzers in such a way that the blood and the dialysis fluid (a mixture of electrolyte concentrate and treated water) flow through them, the two media being separated from one another by a semipermeable (= semi-permeable) membrane.
Capillary dialyzer
The so-called capillary dialyzer is the most frequently used type of dialyzer. Other theoretically possible designs, such as B. the plate dialyzer, however, are practically no longer used due to various disadvantages.
Similar to the principle of the tube bundle heat exchanger, the capillary dialyzer consists of a housing in which a large number of up to 18,000 hollow fibers (length from 20 to 30 cm) are arranged in parallel. Filter areas of up to 2.3 m² are achieved. The blood flow is guided in the fibers. The dialysate flows through the outer space largely parallel to the hollow fibers. This flows in countercurrent to the blood to increase the effectiveness of the mass transfer.
The wall of the hollow fibers, which have a diameter of 190 µm to 300 µm with a wall thickness of 30 µm to 40 µm, is formed by the semipermeable membrane, which is permeable to water and substances dissolved in it up to an average molecular size for the blood cells and larger protein molecules, such as B. albumin however not.
The substance transport of the dissolved substances in blood and dialysate takes place via the semipermeable membranes, either by selective diffusion or convection. The ultrafiltration, i.e. the transport of water from the blood to the dialysate circuit, is created by applying a pressure difference across the membrane. This pressure difference, the so-called transmembrane pressure, is made up of the static pressure difference between blood and dialysate as well as an osmotic pressure, which essentially arises from the concentration differences of albumin on both sides of the membrane.
The methods of numerical flow simulation (CFD = Computational Fluid Dynamics) offer a detailed insight into the three-dimensional flow and material transport processes in the dialyzer. With the "virtual dialyzer" based on this, in addition to the blood and dialysate flow, the distributions of the substance concentrations of all substances involved in the entire device are calculated and used for optimization tasks.
Membrane material
An optimal material for the production of dialysis membranes has to meet many requirements. Typical requirements are:
- Biocompatibility
- not blood damaging
- semi-permeable
- robust and pressure-resistant
Typical semipermeable materials for capillary membranes ( hollow fibers ) are based on polysulfone , polyethersulfone or cellulose acetate .
Web links
Individual evidence
- ↑ Amitava Majumder, Anne Paschen: Medical working techniques. In: Jörg Braun, Roland Preuss (Ed.): Clinic Guide Intensive Care Medicine. 9th edition. Elsevier, Munich 2016, ISBN 978-3-437-23763-8 , pp. 29–93, here: pp. 62–66 ( dialysis method ), in particular p. 62.
- ↑ Amitava Majumder, Anne Paschen: Medical working techniques. 2016, p. 62.