Hemodialysis membranes requirements

As outlined earlier, hemodialysis and hemofiltration require the removal of solutes smaller than albumin from blood. Solute mass transfer rates across hemodialysis membranes cannot exceed the diffusivity of the solute In water. Solute diffusivity decreases with Increasing molecular diameter (Stokes-Einstein relationship) consequently, solute mass transfer rates for hemodlalyzers intrinsically decrease with increasing molecular size. In addition to limitations Imposed by diffusion In solution, mass transfer is further limited by diffusion resistance in the membrane as well as boundary layer effects resulting from laminar flow both of these effects are also functions of molecular size. The quantitation of mass transfer In hemodlalyzers has been reviewed extensively (22). 

For hemodialysis membranes, biocompatibility is the primary requirement. It is known that surface properties such as surface roughness play important roles in determining membrane biocompatibihty. It has also been reported that for a given material, smoother surfaces are more biocompatible [64]. Hence, the sinfaces of three different commercial hollow fibers were studied by AFM to compare their roughness parameters. Figures 4.42 and 4.43 show AFM images of inner and outer surfaces, re-... 

Dialysis involves diffusion of solutes and fluids (e.g., water) through a semipermeable membrane separating out larger molecules and solid particles. Hie membranes employed are generally similar to the reverse osmosis or the nano-/ ultrafiltration type. Historically, dialysis has heen employed for various laboratory separations and commercially employed for NaOH or dilute sulfuric add recovery. The primary com-merdal application involves hemodialysis employed in artifidal kidney machines. Initially, cellulose and cellulose acetate membranes were employed, but polysulfone and poly (ether sulfone) are now in use with biocompatibiUty being a key membrane requirement. 

With either type of dialysis, studies suggest that recovery of renal function is decreased in ARF patients who undergo dialysis compared with those not requiring dialysis. Decreased recovery of renal function may be due to hemodialysis-induced hypotension causing additional ischemic injury to the kidney. Also, exposure of a patient s blood to bioincompatible dialysis membranes (cuprophane or cellulose acetate) results in complement and leukocyte activation which can lead to neutrophil infiltration into the kidney and release of vasoconstrictive substances that can prolong renal dysfunction.26 Synthetic membranes composed of substances such as polysulfone, polyacrylonitrile, and polymethylmethacrylate are considered to be more biocompatible and would be less likely to activate complement. Synthetic membranes are generally more expensive than cellulose-based membranes. Several recent meta-analyses found no difference in mortality between biocompatible and bioincompatible membranes. Whether biocompatible membranes lead to better patient outcomes continues to be debated. 

Radiation Induced Reactions. Graft polymers have been prepared from poly(vinyl alcohol) by the irradiation of the polymer-monomer system and some other methods. The grafted side chains reported include acrylamide, acrylic acid, acrylonitrile, ethyl acrylate, ethylene, ethyl methacrylate, methyl methacrylate, styrene, vinyl acetate, vinyl chloride, vinyl pyridine and vinyl pyrrolidone (13). Poly(vinyl alcohols) with grafted methyl methacrylate and sometimes methyl acrylate have been studied as membranes for hemodialysis (14). Graft polymers consisting of 50% poly(vinyl alcohol), 25% poly(vinyl acetate) and 25% grafted ethylene oxide units can be used to prepare capsule cases for drugs which do not require any additional plasticizers (15). 

Physical or physico-chemical capability (Table 1), including mechanical strength, permeation, or sieving characteristics, is another important requirement of biomaterials. Cuprammonium rayon, for instance, maintains its dominant position as the most popular material for hemodialysis (artificial kidney). Thanks to its good mechanical strength, cuprarayon can be fabricated into much thinner membranes than synthetic polymer membranes as a consequence, much better clearance of low-molecular-weight solutes is achieved. 

In contrast to hemodialysis that uses ultrafiltration membranes, plasma separation (also called plasmapheresis) requires microfiltration membranes with a pore size from 0.2 to 0.6 pm, in order to transmit all proteins and lipids, including LDL cholesterol (2000kDa) and retain completely platelets (2 pm diameter), red blood cells (8 pm diameter) and white blood cells. Thus, membrane plasmapheresis can yield high-quality platelet-free plasma and red cells can be either continuously returned to the donor or saved in another bag for blood transfusion. But it is important, in the case of plasma collection from donors, to minimize the membrane area, in order to reduce the cost of disposable hollow-fiber filters and to avoid the risk of hemolysis (free hemoglobin release) due to RBC damage by contact at the membrane if the pressure difference across the membrane is too high. 

A review of 15 other available reports of renal insufficiency and proteinuria in patients with chronic myeloid leukemia or other malignancies confirmed that the histological spectrum of renal lesions associated with interferon alfa is varied, and includes membranous glomerulonephritis, minimal change glomerulonephritis, acute interstitial nephritis, hemolytic-uremic sjmdrome, and thrombotic microangiopathy. Renal comphcations were reversible in nine patients three patients had persistent proteinuria, and four had persistent renal dysfunction, of whom three required chronic hemodialysis. Two-thirds of the patients developed renal comphcations within 1 month of treatment with interferon alfa, and one-third had received a relatively low dosage of interferon alfa (9-15 MU/week). 

The membrane separates the blood from a dialyzing solution, or dialysate, that is similar to blood plasma in its concentration of needed substances (e.g., electrolytes and amino acids) but contains none of the waste products. Because the concentrations of undesirable substances are thus higher in the blood than in the dialysate, they flow preferentially out of the blood and are washed away. The concentrations of needed substances are the same on both sides of the membrane, so these substances are maintained at the proper concentrations in the blood. The small pore size of the membrane prevents passage of blood cells. However, Na and Cl ions and some small molecules do pass through the membrane. A patient with total kidney failure may require up to four hemodialysis sessions per week, at 3 to 4 hours per session. To help hold down the cost of such treatment, the dialysate solution is later purified by a combination of filtration, distillation, and reverse osmosis and is then reused. 

Membrane separation is a relatively new and fast-growing field in supramolecular chemistry. It is not only an important process in biological systems, but becomes a large-scale industrial activity. For industrial applications, many synthetic membranes have been developed. Important conventional membrane technologies are microfiltration, ultrafiltration, electro- and hemodialysis, reverse osmosis, and gas separations. The main advantages are the high separation factors that can be achieved under mild conditions and the low energy requirements. 

As in hemodialysis, the clearance of urea, a product of protein catabolism, is measnred with Kt/V. Kt/V is a unitless value that correlates the patient s peritoneal membrane urea clearance (K) with the dnration of dialysis (t) and the volume of distribution (P) of urea. Calculation of Kt/V for PD requires that the total volume of drained effluent per day be determined (this value is the volume instilled plus volume of water ultraflltered). The dialysate to plasma (D/P) urea concentration is determined, and Kt is estimated as ... 

For the design of the C-DAK 4000 artificial kidney, and the many similar hemodialysis devices (Daugirdas and Ing, 1988), rates of permeation of the species through the candidate membranes are necessary. Estimates for the permeability of pure species in a microporous membrane can be made from the molecular diffusivity, and pore diameter, porosity, and tortuosity of the membrane (Seader and Henley, 1998), as shown in Example 19.1. For this reason, considerable laboratory experimentation is required when selecting membranes in the molecular structure design step. 

Hemodialysis units are usually hoUow-fibre devices with a membrane area of 0.5-1.5 m. The classical membrane material is regenerated cellulose, closest to the natural material. Other membranes include polyether sulphone (PES) and polysulphone (PS), which are made somewhat hydrophilic by blending with PVP, a necessary requirement to address the problems of biocompatibUity and fouhng by proteins [39]. The membranes are asymmetric (10—100 pm thick) with a narrow pore size distribution and the pore diameter less than 10 nm [17]. 

Appiications battery separator, faucet components, fibers, hot water fittings, medical applications which require resistance to hot water and sterilization, membranes (hemodialysis, water treatment, bioprocessing, food and beverage, and gas separation), microwave cookware, plumbing manifolds, printed circuit boards, tubing, solar hot water applications, utrafiltration membrane ... 

Although red blood cells (erythrocytes) play only a minimal role in wound healing and blood-biomaterial interactions, the contact of red blood cells with the material can lead to hemolysis. Hemolysis is the breakage of the erythrocyte s membrane with the release of intracellular hemoglobin. Normally, red blood cells live for 110-120 days. After that, they naturally break down and are removed from the circulation by the spleen. Some diseases and medical devices cause red blood cells to break too soon requiring the bone marrow to accelerate the regeneration of red blood cells (erythropoesis). Medical devices for hemodialysis, heart-lung-bypass machines or mechanical heart valves induce more hemolysis than smaller implants like stents or catheters [201]. 

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