One of the most important functions of the natural kidney, in addition to the filtering of uremic toxins, is the removal of excess water. When the kidneys fail, this function must be taken over by the artificial kidney, which is also known as a dialyzer. The procedure by which plasma water from the patient is squeezed through the dialyzer membrane using pressure is termed ultrafiltration.
In 1947, Swede Nils Alwall published a scientific work describing a modified dialyzer that could perform the necessary combination of dialysis and ultrafiltration better than the original Kolff kidney. The cellophane membranes used in this dialyzer could withstand higher pressure because of their positioning between two protective metal grates. All the membranes were in a tightly sealed cylinder so that different pressure ratios could be generated.
By proving that uremic patients could be successfully treated using the artificial kidney, Kolff sparked a flurry of activity around the world to develop improved and more effective dialyzers. The “Parallel Plate Dialyzer” evolved as the most significant development of this period. Rather than pumping the blood through membranous tubes, this dialyzer directed the flow of dialysis solution and blood through alternating layers of membranous material.
Just as the technology of dialyzers continued to develop, so too did the scientific principles regarding the transport of substances across membranes, and these principles were applied specifically to dialysis. This work enabled scientists to develop a quantitative description of the dialysis process and allowed the development of dialyzers with clearly defined characteristics.