All dializers now in clinical use are of the hollow fiber type with membranes of cellulose, modified cellulose or synthetic polymers. The function of a dialyzer is to return the internal milieu to a more normal state through water removal and solute exchange between blood and dialysate. Water removal depends on the hydraulic permeability of the membrane. Automated ultrafiltration control systems are a standard feature of current dialysis machine and have eliminated the potential for excessive, uncontrolled ultrafiltration. Thus, the hidraulic permeability of the membrane is no longer an issue in the clinical use of modern dialyzers. With hydraulic permeability no longer a concern, membrane manufacturers have been free to engineer membranes to maximise solute exchange.
Solute transfers in hemodialysis is determined by the diffusive and convective permeability of the membrane-defined by the mass transfer coefficient and the sieving coefficient, respectively-the membrane surface area, and the blood and dialysate flow rates. The mass transfer coefficient is a composite parameter that incorporates the resistances to mass transfer associated with the membrane and the fluid boundary layers on the blood and dialysate sides of the membrane