Hollow fiber dialyzer bundles consist of 7–17 x 10 3 semipermeable hollow fibers that allow the transfer of solutes and fluids between blood and dialysate. Typical fibers have an inner diameter of 180-200 microns and a wall thickness of 30-40 microns, resulting in a surface area of 1.0-2.5 m2. The fibers may have features such as undulations to evenly distribute the dialysate flow through the fiber bundle.
The fiber bundles are encapsulated in a housing that forms the dialysate compartment. The header is the space surrounded by end caps and polyurethane potting material to hold the hollow fibers and form a barrier between the blood and dialysate compartments. The headers guide blood from the dialyzer inlet into the membrane fibers and from the membrane fibers to the dialyzer outlet. The end caps can be removed from some dialyzers.
In these cases, an O-ring is used to create a seal between the end cap and the potting material. Blood and dialysate flow in opposite directions (countercurrent) to maximize diffusive solute transfer.
Non-synthetic membranes are derived from natural materials such as cotton and are less biocompatible than synthetic membranes. Biocompatibility can be improved by substituting hydroxyl groups, which reduce the ability of the cellulose membrane to activate the body and cause leukopenia. The cellulose-substituted moieties include acetate, diethylaminoethyl (DEAE), benzyl, polyethylene glycol, and vitamin E. The resulting films are referred to as modified cellulose films.
In the United States, only cellulose diacetate and cellulose triacetate membranes are still widely used clinically. Modified cellulose membranes can be high or low flux.
The primary mode of removal of small solutes, such as urea, by hemodialysis is down diffusion along the concentration gradient between plasma water and dialysate. Transfer of small solutes (eg, HCO3-) from dialysate to plasma water also occurs primarily by diffusion. The diffusion rate is a function of the thickness and porosity of the membrane and the diffusivity of the solutes in the membrane. It is expressed as the membrane diffusion coefficient for a given solute. The diffusion rate of small molecules is the largest, and the diffusivity of solute in the membrane decreases logarithmically with increasing solute size. As the film thickness increases and the porosity decreases, the diffusion rate also decreases.
The primary mode of removal of large solutes by hemodialysis is convection, as water containing these solutes flows from plasma to dialysate in response to a hydraulic gradient. Convective rate is a function of ultrafiltration rate, solute size and membrane pore size. The ability of a solute to pass through the pores of a membrane is expressed as the sieving coefficient of the membrane for a given solute. Solutes with a sieving coefficient of 1.0 can pass freely through the membrane, while solutes with a sieving coefficient of zero cannot pass through the membrane.
Convection is better at removing large solutes than diffusion because the reduction in sieving coefficient is not as pronounced as the reduction in diffusion coefficient with increasing solute size. Dialyzer manufacturers typically provide sieving coefficients for albumin, beta-2 microglobulin, myoglobin, and lysozyme as parameters for albumin leakage and convection performance.