Hemodialysis module

The use of X-ray tomography is relatively new in the membrane field. The first experimental use of SRpCT was reported by Remigy et al. [5, 6], although Frank et al. used X-ray tomography in 2000 to observe a hemodialysis module [7]. They presented 3D reconstructed structures of UF and MF hollow fiber membranes. Yeo et al. published a paper in 2005 using X-ray microimaging (XMI) to observe the deposition of ferric hydroxide inside the fiber lumen [8] and later Chang et al. observed the flow characteristics in a hollow fiber lumen [9]. 

Frank et al., in 2000, published a paper dealing with the visualization of concentration field in hemodialyzers [7]. Using a medical X-ray scanner, they observe a hemodialysis module (1.4 m ) containing about 10000 fibers (outside diameter 255 pm). To enhance the contrast, they add some sodium iodide (Nal, 0.1 M) to the water. The voxel size is approximately 5 mm in thickness and possesses a cross-section of 0.1875 x 0.1875 mm (note that they do not use the SRpCT but a commercial scanner). That means they cannot directly observe the particle, the fiber or the fouling, all of which are smaller in size. 

Plasmapheresis typically employs a membrane module of similar configuration as a high-flux hemodialyzer. Alternatively, a rotating membrane separation element is used in which the tendency of the blood cells to deposit on the membrane surface is counteracted with hydrodynamic lift forces created by the rotation. The membrane element and the associated plasmapheresis circuitry are shown in Fig. 49. Worldwide, about 6 million plasmapheresis procedures are performed annually using this system, making this one of the largest biomedical membrane applications after hemodialysis. 

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... 

Tokuda, N. et al. (2000) Caldtriol therapy modulates the cellular immune responses in hemodialysis patients. American Journal of Nephrology, 20, 129-137. 

One of the most important fields in which automatic devices can be used successfully is clinical chemistry because of the complexity of the matrix, and because of the number of analyses that must be performed in a very short time. For example, an automatic device is recommended for automation of urea assay to monitor urea in hemodialysis fluids,242 an important measure for human health. The urea analyzer consists of a flow injection system, a signal processing module, and an IBM-compatible PC. The flow system plays a major role in urea assay and can be schematically represented as shown in Figure 7.2. The quality, objectivity, and reliability of the analytical information obtained using this analyzer is good, and the analyzer has the added benefit of making it possible to assay urea without taking blood samples. 

Mainly four types of membrane modules are used plate-and-frame, spiral-wound, tube-in-shell, and hollow fiber. The plate-and-frame module consists of a series of membranes (10-500 pm thick) sandwiched between spacers that act as flow channels (Figure 5.69). (The membranes are often laminated on a porous support that offers no flow resistance.) The feed flows in one set of channels and the permeate, with or without carrier fluid, flows in alternate channels. Plate-and-frame modules find use in ultrafiltration and dialysis applications which include hemodialysis and electrodialysis. 

The most important applications of hollow-fiber modules are in hemodialysis (Figure 5.70), reverse osmosis, and gas separation units. Modules up to 50 cm diameter containing hundreds of thousands of fibers are used in gas separation. 

Several classes of polymeric materials are found to perform adequately for blood processing, including cellulose and cellulose esters, polyamides, polysulfone, and some acrylic and polycarbonate copolymers. However, commercial cellulose, used for the first membranes in the late 1940 s, remains the principal material in which hemodialysis membranes are made. Membranes are obtained by casting or spinning a dope mixture of cellulose dissolved in cuprammonium solution or by deacetylating cellulose acetate hollow fibers [121]. However, polycarbonate-polyether (PC-PE) block copolymers, in which the ratio between hydrophobic PC and hydrophilic PE blocks can be varied to modulate the mechanical properties as well as the diffusivity and permeability of the membrane, compete with cellulose in the hemodialysis market. 

A very common commercial device for hemodialysis is the C-DAK 4000 artificial kidney of Althin CD Medical, Inc. (acquired by Baxter International, Inc. in March, 2000). This disposable, sterilized membrane module, shown in Figure 19.5, resembles a shell-and-tube heat exchanger. The tubes, which number 10,000, are hollow fibers, 200 microns i.d. by 10 microns wall thickness by 22 cm long, made of hydrophilic microporous cellulose acetate of 15 to 100 A pore diameter. Alternatively, fibers of polycarbonate, polysulfone, and other poly-... 

Figure 19.5 Hemodialysis device, (a) single tube (b) ccmqdete module.

Hemodialysis/hemofiltration alone had sales of over US 2200 million in 1998. Reverse osmosis (RO), ultrafiltration (UF) and microfiltration (MF) together accounted for 1.8 billion dollars in sales in 1998. At that time about US 400 million worth of membranes and modules were sold each year worldwide for use in reverse osmosis. About 50% of the RO market was controlled by Dow/FihnTec and Hydranautics/Nitto. They were followed by DuPont and Osmonics. Membranes are apphed during sea-water desahnation, municipal/ brackish water treatment and in the industrial sectors. The market for RO and nanofiltration is growing at a rate higher than 10%/year. The market for desali-... 

Large surface area membrane modules such as hollow fiber units are often used in the production of low-alcohol beer, hemodialysis, or desalination. In all these processes the rate of transfer is believed to be governed by the concentration difference across the membrane, the molecular size, and the permeability characteristics of the membrane. A model that has shown some promise in correlating the amount removed as a function of flow rate is discussed below. 

Initial experiments have been realized for dimensioning of the filter process to be used in the miniplant. Therefore, a conunercial manbiane filter module MF 7 with hollow fibers made of polysulfone for the application of hemodialysis from the firm Meditechlab with a filter area of 1.6 m was tested in batch mode. For measurement of the concentration of ions in the suspension, the electrical conductivity of the suspension was determined. Figure 9 shows the behavior of mass in the source tank of the suspension and the detected conductivity over the filtration process for a suspension with a volume of 2.71 and a particle mass concentration of 2 m%. 

Pereira, B., Snodgrass, B., Hogan, R, and King, A. (1995). Diffusive and convective transfer of c3ftokine-inducing bacterial products across hemodialysis membranes. Kidney Int. AT, 603. Pertosa, G., Gesualdo, L., Bottalico, D., and Schena, F. P. (1995). Endotoxins modulate chronically tumor necrosis factor a and interleukin 6 release by uraemic monocytes. Nephrol. Dial. Transplant. 10, 328. 

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