Dialysis practice art
Types of dialyzer Membrane materials
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The materials most commonly used to make hollow fiber membranes include PSf, PES, cellulose triacetate (CTA), polymethylmethacrylate (PMMA), PEPA, ethylene vinyl alcohol copolymers (EVAL), and polyacrylonitrile (PAN).
PSF MEMBRANES
have the capacity to remove a broad range of uremic toxins, effectively retain endotoxins, and, provide intrinsic biocompatibility and low cytotoxicity.
In addition to its higher sieving capability, increased hydraulic permeability promotes efficient transport through solvent drag (convection).
Although PSf may be the primary polymer, it is blended with other polymers to give each membrane its specific attributes, as in the case of adding the hydrophilizing agent polyvinylpyrrolidone (PVP).
Significant differences among PSf membranes exist because of variations in both the relative amounts of co-polymers used in a particular blend, and the fiber spinning process employed.
PES MEMBRANES
has been developed through an advanced fiber spinning process that creates larger, uniformly sized, and densely distributed pores.
This configuration improves permselectivity by creating a steeper sieving curve for low molecular weight proteins and a sharp cut-off.
PES membranes are therefore known for achieving outstanding middle molecule removal with minimal albumin loss, and both their biocompatibility and endotoxin retaining characteristics adhere to the highest standards.
Previously, a much higher albumin loss was required to achieve a comparable HD treatment efficacy when using dialysis membranes with inferior permselectivity.
These membranes are a blend of hydrophobic base polymers, which favorably determine biocompatibility, while hydrophilic components improve transmembrane solute passage.
CTA MEMBRANES
have a high solute permeability that can remove β2-M by diffusion.
Their diffusive efficiency is very high because their fibers are thin and have a Moire structure, causing the flow distribution of the dialysate to be uniform.
Their reported clinical benefits include high antithrombogenicity, improvement in lipid metabolism, and the reduction of biomarkers such as homocysteine and advanced glycation end products.
Proteomic analysis of CTA membranes has shown high adsorption of albumin, and since the adhesion of thrombocytes to a surface tends to be decreased by albumin adsorption, this would suggest that CTA may offer the potential for a lower activation of the coagulation cascade than PSf membranes, as demonstrated in other studies
PMMA MEMBRANES
have highly adsorptive properties, which may be attributed to a homogeneous structure in which the entire membrane contributes to adsorptive removal, rather than removal through only one membrane layer.
PMMA membranes were found to reduce indoxyl sulphate, p-cresyl sulphate, and 3-carboxy-4-methyl-5-propyl-2-furanpropionic acid (CMPF), which are associated with cardiovascular damage from endothelial dysfunction and reactive oxygen species (ROS) production.
Accordingly, a protein-leaking (superflux) PMMA membrane was found to reduce serum levels of CMPF with improvements in anemia, and to reduce plasma homocysteine, pentosidine and inflammatory cytokines.
PMMA membranes have been shown to adsorb intact PTH and to improve pruritis, enhance the response to the hepatitis B vaccine, and to preserve muscle mass, especially in the elderly
PEPA MEMBRANES
are a combination of PES and polyarylate and have a unique structure: three layers comprise the entire inner surface skin layer; a porous layer lies within the membrane; and, another skin layer covers the outer surface. The permeability of water and solutes is controlled by the skin layer on the inner surface and the outer skin layer can block endotoxin from the dialysate side; thus it can be used as an endotoxin filter.
The amount of albumin loss or β2-M removal can be controlled by the amount of PVP added.
Versions made without PVP have resulted in minimal activation in complement C3a and C5a.
EVAL MEMBRANES
are hydrophilic and uncharged, with a smooth surface that retains water, so they adsorb few plasma proteins and interact weakly with cell components in the blood.
Therefore, there is minimal platelet activation, and little production of ROS and proinflammatory cytokines such as interleukin-6 and monocyte chemo-attractant protein (MCP-1) which may help patients maintain better peripheral circulation.
Accordingly, the longterm use of an EVAL membrane may reduce oxidative stress and inflammation, and thus help reduce the symptoms of vascular disease.
PAN MEMBRANES
are hydrophilic, so they attract water to form a hydrogel structure that confers high diffusive and hydraulic permeability.
The highly specific adsorptive properties are limited on the surface, but favored within the membrane structure and with high specificity for basic, medium sized proteins.
PAN displays high permeability to fluid and a broad spectrum of uremic toxins combined with excellent biocompatibility.
Removal of MCP-1 can only be achieved through specific adsorption which has been demonstrated in a PAN membrane.
Some membranes are coated with polyethylene glycol or vitamin E in order to decrease the activation and migration of monocytes and granulocytes, thus improving biocompatibility.
Such membranes have been found useful in reducing hypotension during HD.
Synthetic membrane surface modifications with heparin have also been developed for heparin-free dialysis for those with increased risk of bleeding
The use of poorly biocompatible, unmodified cellulose dialyzer membranes is discouraged.
Accordingly, most dialyzer membranes are made from synthetic polymers, 93% of which are derived from the parent polyarylsulfone family, with 71% produced as PSF and 22% produced as PES.
All membranes discussed are HPM and confer specific attributes that may be considered in dialyzer selection.
Membranes in the new super high-flux dialyzers are primarily PSf and PES.
