Hemodialysis device

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. 

Several key steps in the design procedure are presented next, beginning with the estimation of the mass-transfer coefficient for transport of urea across the membrane. Next, pressure drops are estimated 

The rate of mass transfer of urea from the blood plasma, through the membrane, and to the dialysate is given by 

The overall mass-transfer coefficient, which must consider the resistances of the blood plasma, the membrane, and the dialysate, is given by 

A = arithmetic mean of A, and A , cm l/u = membrane wall thickness, cm P/if = membrane permeability, cmVmin 

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Figure 12.4 Solute permeability relative to the permeability of a film of water for various solutes in a regenerated cellulose membrane (Cuprophan 150). This type of membrane is still widely used in hemodialysis devices...

Hemoperfusion differs from hemodialysis in that the blood is passed over a resin or charcoal column. The drug becomes bound to the column and the clean blood returned to the body. Hemoperfusion units have adsorptive surface areas of several thousand square meters while hemodialysis devices have an effective dialysis surface limited to several square meters. Obviously, relatively sophisticated technology is required for these procedures and there is the need to prevent clotting in the circuit, which can produce complications. 

In hemodialysis devices, urea analysis makes it possible to monitor the effect of the medical treatment continuously. For this purpose, the ET feasibility in urea monitoring was investigated. In comparison to other transducers, the ET... 

Thrombotic complications are frequently encountered when blood is exposed to the surfaces of hemodialysis devices, heart-lung machines, arterial grafts, artificial heart components and other prosthetic devices. The blood platelets are particularly vulnerable to these adverse effects which may include a decrease in platelet count, shortened platelet survival and attendant higher platelet turnover, and altered platelet function. However the interaction of platelets with an artificial surface exposed to blood must be preceded by the interaction of the molecular components of plasma, particularly the plasma proteins, with the surface (1,2). This is due to the prepon-... 

The first hemodialysis devices utilized natural cellulose (cuprophan) membranes, which possessed predominantly small pores. These membranes permitted the removal of excess fluid, ions, and small molecules, but prohibited the removal of substances above approximately 1200 Da in size. Larger molecules, such as P2-microglogulin (P2M, ll.SkDa), accumulated in the blood and were thought to contribute to many of the additional health problems and high mortality of patients on dialysis. This idea, coined the middle molecule hypothesis by Bapp et al. [342], led to the development of new synthetic polysulfone or polyacrylonitrile dialysis membranes that possessed larger pores and, in combination with equipment to control transmembrane pressure, permitted more efficient elimination of middle molecules. 

Alternative 2. Design a peritoneal dialysis device (which uses the serous membrane that lines the cavity of the abdomen of a mammal) having similar attributes as the hemodialysis device. ... 

Be familiar with the design of a representative array of nine chemical products. These products include small hemodialysis devices, solar desalinators, hand warmers, fuel cells to power automobiles, and thin silicon films that coat microelectronic devices. 

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

Develop a design procedure for a hollow-fiber hemodialysis device of the type shown in Figure 19.5. Base the design on a blood flow rate of 200 ml/min and a dialysate flow rate of 500 ml/min. Assume the design will be controlled by mass transfer of one of the blood plasma components to be removed, for example, urea. The blood wiU flow through the hollow fibers, while the dialysate will flow past the outside siuface of the fibers in a direction countercurrent to the flow of the blood plasma. A typical patient wUl require hemodialysis when the blood reaches a urea nitrogen level (BUN) of 100 mg/dl. A target for the hemodialysis device is to reduce the BUN to 30 mg/dl within 4 hr. 

J Consider the hemodialysis device in Example 19.1. Examine the effect on the rate of urea removal of changing the hemodia-lyzer geometry, the blood and dialysate flow rates to the dialyzer, the rate of waste withdrawal, the volume of the dialysate tank, and the sensitivity of the rate of urea mass transfer to the mass-transfer coefficient. In particular, the above estimate of the coefficient on the shell side may be low because the entry to and exit from the hemodialyzer of the dialysate is normal to, rather than parallel to the fibers. This should enhance the shell-side coefficient. 

Polyurethane medical devices such as central venous catheters (CVCs) [139-141] and hemodialysis devices [142-144] require goodhemocompatibihty [145], hi Section 2.6, we know that when a device is implanted in the human body protein adsorption occurs in just a few seconds, hi contact with blood, the protein layer interacts with platelets, which leads to thrombus formation and eventually to device failure. There are several tests available to evaluate hemocompatabihty. Visualization by SEM is one semiquantitative method, which is covered in Section 2.5. More accurate quantitative methods are described below. 

Hemofiltration is similar to hemodialysis, but uses convective transport across high cutoff point membranes to clear toxins. Clearance is slower and requires up to 24 h of continuous use in the clinical setting, which can he advantageous because this reduces the impact the device has on hemodynamic stability, unlike standard hemodialysis devices that can cause hypotension during treatment. Fluid is lost with this method and must be replaced in the patient, typically with a sterile isotonic solution. Hemofiltration can be used in combination with hemodialysis and is called hemodiafiltration. 

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