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A dialyzer is an artificial kidney designed to provide controllable transfer of solutes and water across a semi permeable membrane separating flowing blood and dialysate streams. The transfer processes are diffusion (dialysis) and convection (ultrafiltration). There are three basic dialyzer designs: coil, parallel plate, and hollow fiber configurations.

Coil dialyzer: An early design in which the blood compartment consisted of one or two long membrane tubes placed between support screens and then tightly wound around a plastic core. This design had serious performance limitations, which gradually restricted its use as better designs evolved. The coil design did not produce uniform dialysate flow distribution across the membrane. More efficient devices have replaced the coil design.

Parallel Plate Dialyzer: Sheets of membrane are mounted on plastic support screens, and then stacked in multiple layers ranging from 2 to 20 or more. This design allows multiple parallel blood and dialysate flow channels with a lower resistance to flow. The physical size of the parallel plate dialyzers has been greatly reduced since their introduction. There have been major improvements which provide (1) thinner blood and dialysate channels with uniform dimensions, (2) minimal masking or blocking of membranes on the support, and (3) minimal stretching or deformation of membranes across the supports.

Hollow Fiber Dialyzer: This is the most effective design for providing low-volume high efficiency devices with low resistance to flow. The fibers in the device are termed the fiber bundle. The fibers are potted in polyurethane at each end of the fiber bundle in the tube sheet, which serves as the membrane support.

Ultra filtration: All excess fluid must be removed from the bloodstream as the patient's blood flows through the dialyzer. The process of water removal from the bloodstream is called ultra filtration, and the amount of fluid removed is the ultra filtrate.