Hollow fibers hemodialyzers

An artificial kidney is a device that removes water and waste metabolites from blood. In one such device, the hollow fiber hemodialyzer, blood flows from an artery through the insides of a bundle of hollow cellulose acetate fibers, and dialyzing fluid, which consists of water and various dissolved salts, flows on the outside of the fibers. Water and waste metabolites—principally urea, creatinine, uric acid, and phosphate ions—pass through the fiber walls into the dialyzing fluid, and the purified blood is returned to a vein. 

Topical microporous-membrane materials used in dialysis are hydrophilic, including cellulose, cellulose acetate, and various acid-resistant polyvinyl copolymers, typically less than 50 pm thick and with pore diameters of 15 to 100 A. Dialysis membranes can be thin because pressures on either side of the membrane are essentially equal. The most common membrane modules are plate-and-frame and hollow-fiber. Compact hollow-fiber hemodialyzers, which are widely used, typically contain several thousand 200-pm-diameter fibers with a wall thickness of 20-30 pm and a length of 10 to 30 cm (Seader and Henley, 2006). 

Similar set-ups have been conceived for other organs devoted to toxin removal from the blood flow the bioartificial kidney, precisely, is a modification of the common hollow-fiber hemodialyzer, which has widespread use in the clinical practice of hydrosoluble toxin removal upon kidney failure. 

Nevertheless to avoid the risk of serious damaging the chemical modification on hollow fibers hemodialyzers has to be performed in the absence of organic solvents and consequently the choice of the activation procedures is very limited. 

For example, Ronco [48] suggested the possibility of mixing the biomaterial of a specific membrane with a sorbent material based on a significant enhancement of internal and backfiltration in hollow-fiber hemodialyzers. Thus, the single membrane will have both characteristics, i.e., diffusion and adsorption for removal of uremic acid, and is called mixed matrix membrane (MMM) [49,50]. The MMM concept had been proposed earlier as an alternative to traditional chromatographic column [51]. 

Hollow fiber refers to a membrane tube of very small diameter (e.g., 200 pm). Such small diameters enable a large membrane area per unit volume of device, as well as operation at somewhat elevated pressures. Hollow-fiber modules are widely used in medical devices such as blood oxygenators and hemodialyzers. The general geometry of the most commonly used hollow-fiber module is similar to that of the tubular membrane, but hollow fibers are used instead of tubular membranes. Both ends of the hollow fibers are supported by header plates and are connected to the header rooms, one of which serves as the feed entrance and the other as the retentate exit. Another type of hollow-fiber module uses a bundle of hollow fibers wound spirally around a core. 

The relative magnitudes of the three terms on the right-hand side of Equation 15.25 vary with the diffusing substance, the flow conditions of both fluids, and especially with the membrane material and thickness. With the hollow-fiber-type hemodialyzers that are widely used today, membrane resistance usually takes a substantial fraction of the total resistance, and the fraction increases with increasing molecular weight of the diffusing component. 

In a hollow-fiber-type hemodialyzer of the following specifications, 200 cm min of blood (inside fibers) and 500 cm min of dialysate (outside fibers) flow countercurrently. 

Calculate the overall mass transfer coefficient ffp (based on the hollow-fiber inside diameter) and the dialysance of the hemodialyzer for urea, neglecting the effect of water permeation. 

Hollow Fibers. The general configuration of the hollow-fiber apparatus is similar to that of hemodialyzers and blood oxygenators. Hepatocytes or microcarrier-attached hepatocytes are cultured either inside the hollow fibers or in the extra-fiber spaces, and the patient s blood is passed outside or inside the fibers. A bioartificial liver of this type, using 1.5 mm o.d. hollow fibers with 1.5 mm clearance between them, and with tissue-like aggregates of animal hepatocytes cultured in the extra-fiber spaces, can maintain liver functions for a few months [20]. 

In a hoUow-fiber-type hemodialyzer of the total membrane area (based on o.d.) A = 1 m , 200 cm min of blood (inside fibers) and 500 cm min of dialysate (outside fibers) flow countercurrently. The overall mass transfer coefficient for urea (based on the outside diameter of the hollow fiber) is 0.030 cm min . Estimate the dialysance for urea. 

Parallel-plate hemodialyzers using flat membranes, with several compartments in parallel, separated by plastic plates, are now only available from Hospal Co (Crystal and Hemospal models). Blood circulates between two membranes and the dialysate between the other side of membrane and the plastic plate. These parallel-plate dialyzers have a smaller blood-pressure drop than hollow-fiber ones and require less anticoagulants as flat channels are less exposed to thrombus formation than fibers, but they are heavier and bulkier and thus less popular. A recent survey of the state-of-the-art in hemodialyzers is given in [13]. 

In dialysis, size exclusion is the main separation mechanism, while osmotic pressure and concentration difference drive the transport across two typically aqueous phases. While dialysis is used in some analytical separations, dialysis for the removal of toxins from blood (hemodialysis) is the most prominent application for hollow fiber technology in the biomedical field. The hemodialyzers are used to treat over one million people a year and have become a mass produced, disposable medical commodity. While the first hemodialyzers were developed from cellulosic material (Cuprophane, RC, etc.), synthetic polymers such as polyacrylonitrile, poly(ether) sulfone, and polyvinyl pyrrolidone are increasingly used to improve blood compatibility and flux. Hemodialyzer modules consist of thousands of extremely fine hollow fibers... 

The dialysate solution is recirculated through the hemodialyzer system. In hospitals where multiple patients are treated, central dialysate supply systems are normally used. The flow rates of blood and dialysate through a hollow-fiber-type dialyzer are approximately 200-300 ml min-1 and 500 ml min-1, respectively. The more recently developed hemodialyzers have all been disposable that is, they are presterilized and used only once. Normally, a patient will undergo dialysis for 4—5 h per day, for three days each week. 

In a hollow-fiber-type hemodialyzer, 200 ml min-1 ofblood (inside fibers) and 500 ml min-1 of dialysate (outside fibers) flow countercurrently. The urea concentrations of the inlet blood, outlet blood, and outlet dialysate are 100 mg dl x, 80 mg dl-1 and 32 mg dl-1, respectively. Calculate the clearance for urea. 

Researchers at Oregon State University have demonstrated the advantages of microchannel architecture in improving the hemodialysis process. Using microchannel architectare, they were able to show 70-80% reductions in the necessary transfer area relative to commercial hollow fiber systems for the clearance of creatinin (Fig. 7.23) and urea (Fig. 7.24) from a simulated blood stream [285]. The microchannel advantage, as has been seen in other applications, comes in the form of well-defined and narrow channels that facilitate rapid mass transfer into and out of the fluid media. This approach is expected to change the current paradigm in hemodialysis from clinical treatment to at-home use, and may allow for the creation of a wearable hemodialyzer [286]. 

Because the blood flow through the hollow fibers of the hemodialyzer is laminar, the pressure drop is computed from the Hagen-Poiseuille equation ... 

Hollow liber membranes are numerous small hollow fibers with semi-permeable walls, and are assembled within a cylindrical shell/jacket to function as a bioreactor. One of the clinical applications of hoUow fiber bioreactors is the hemodialyzer. These hollow fiber membranes are produced by solution-based processing method by solvent phase separation. This process has been used to produce filtration membranes in the past [11], and is now being used to produce tissue engineering scaffolds [12-14]. 

To reduce these side-effects the extracorporeal use of enzymes has been developed by linking enzymes to the different biocompatible polymers "". Enzymes have been, as an alternative immobilized to hollow fibers of commercial hemodialyzer without altering the dialyt-ical properties of the fibers and their hemocompatibility ". ... 

To overcome this problem,we developed a simple technique,which involves two steps 1. chemical activation of hollow fibers and their characterization 2. assessment of the functionalized hollow fibers in conventional hemodialyzers and their use for coupling several nucleophiles,such as aminoacids,peptides and enzymes. 

In conclusion, the proposed method based on the chemical activation of the hollow fibers followed by their assessment in the bioreactor form,may be a solution to the problems normally encountered during the activation of commercial hemodialyzers. 

Use the following data to calculate the urea concentration exiting from a hemodialyzer with a total surface area of 1 m and 100 hollow fibers of length L = 17 cm. 

Careful consideration should be given to each test article to determine an appropriate approach to the testing. For example, a medical device may be a thin, sealed titanium can containing electronics. Extracting this device with the electronics contained within would not be appropriate, because there is little to no likelihood that a patient will be exposed to the electronics, and it would add mass to the test article and therefore increase the amount of extraction vehicle the can is exposed to. The result is a dilution of extractants leaching from the can, which can potentially mask toxic responses. In the case of hemodialyzers, the hollow fibers and housing are often tested separately, because of the different amount of extraction vehicle required. 

Figure 19.3 shows a typical hemodialyzer. Device properties such as the fiber length, membrane surface area, number of fibers, hollow fiber packing density, and header design aU affect solute clearances. 

Ronco, C., Brendolan, A., Lupi, A., Metry, G., and Levin, N. W. (2000). Effects of a reduced inner diameter of hollow fibers in hemodialyzers. Kidney Int. 58, 809. 

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