Artificial kidney devices

As an example, let us consider a simple one-compartment model for the prescription of treatment protocols for dialysis by an artificial kidney device (Fig. 1.1). While fire blood irrea concentration (BUN) in the normal individual is usually 15 mg% (mg% = milligrams of the substance per 100 mL of blood), the BUN in irremic patients could teach SO mg%. The purpose of the dialysis is to bring the BUN level closer to the normal. In the artificial kidney, blood flows on one side of the dialyzer membrane and dialysate fluid flows on the other side. Mass transfer across the dialyzer membrane occurs by diffusion due to concentration difference across the membrane. Dilysate fluid is a makeup solution consisting of saline, ions, and the essential nutrients that maintains zero concentration difference for these essential materials across the membrane. However, during the dialysis, some hormones also diffuse out of the dialyzer membrane along with the urea molecule. Too-rapid dialysis often leads to depression in the individual because of the rapid loss of hormones. On the other hand, too-slow dialysis may lead to unreasonable time required at the hospital. 

Medical Syringes, blood aspirators, intravenous connectors and valves, petri dishes, and artificial kidney devices... 

Fig. 43. Schematic of a hoUow-fiber artificial kidney dialyser used to remove urea and other toxic metaboUtes from blood. Several million of these devices...

In terms of membrane area used and doUar value of the membrane produced, artificial kidneys are the single largest appHcation of membranes. Similar hoUow-fiber devices are being explored for other medical uses, including an artificial pancreas, in which islets of Langerhans supply insulin to diabetic patients, or an artificial Uver, in which adsorbent materials remove bUinibin and other toxins. 

While it would be difficult to enumerate all of the efforts in the area of implants where plastics are involved, some of the significant ones are (1) the implanted pacemaker, (2) the surgical prosthesis devices to replace lost limbs, (3) the use of plastic tubing to support damaged blood vessels, and (4) the work with the portable artificial kidney. The kidney application illustrates an area where more than the mechanical characteristics of the plastics are used. The kidney machine consists of large areas of a semi-permeable membrane, a cellulosic material in some machines, where the kidney toxins are removed from the body fluids by dialysis based on the semi-permeable characteristics of the plastic membrane. A number of other plastics are continually under study for use in this area, but the basic unit is a device to circulate the body fluid through the dialysis device to separate toxic substances from the blood. The mechanical aspects of the problem are minor but do involve supports for the large amount of membrane required. 

Medicine has made major advances in the past 50 or so years partly by the use of devices to improve patient health. These devices include artificial hearts and pacemakers, machines for artificial kidney dialysis, replacement joints for hips, knees, and fingers, and intraocular lenses. These devices need to survive in sustained contact with blood or living tissue. 

The hemodialyzer, also known as the artificial kidney, is a device that is used outside the body to remove the so-called the uremic toxins, such as urea and creatinine, from the blood of patients with kidney disease. While it is a crude device compared to the exquisite human kidney, many patients who are unable to receive a kidney transplant can survive for long periods with the use of this device. 

Six developed and a number of developing and yet-to-be-developed industrial membrane technologies are discussed in this book. In addition, sections are included describing the use of membranes in medical applications such as the artificial kidney, blood oxygenation, and controlled drug delivery devices. The status of all of these processes is summarized in Table 1.1. 

In this chapter, the use of membranes in medical devices is reviewed briefly. In terms of total membrane area produced, medical applications are at least equivalent to all industrial membrane applications combined. In terms of dollar value of the products, the market is far larger. In spite of this, little communication between these two membrane areas has occurred over the years. Medical and industrial membrane developers each have their own journals, societies and meetings, and rarely look over the fence to see what the other is doing. This book cannot reverse 50 years of history, but every industrial membrane technologist should at least be aware of the main features of medical applications of membranes. Therefore, in this chapter, the three most important applications—hemodialysis (the artificial kidney), blood oxygenation (the artificial lung) and controlled release pharmaceuticals—are briefly reviewed. 

Now the major application of dialysis is the artificial kidney and, as described in Chapter 12, more than 100 million of these devices are used annually. Apart from this one important application, dialysis has essentially been abandoned as a separation technique, because it relies on diffusion, which is inherently unselec-tive and slow, to achieve a separation. Thus, most potential dialysis separations are better handled by ultrafiltration or electrodialysis, in both of which an outside force and more selective membranes provide better, faster separations. The only three exceptions—Donnan dialysis, diffusion dialysis and piezodialysis—are described in the following sections. 

As mentioned already, the artificial kidney is a classic example of chemical engineering prowess. The proper design of such devices requires a description of both water and solute transport to and from blood, across membranes, and to and from an adjacent fluid known as the dialysate. Variations on this theme include hemodilution, hemoconcentration, and hemofiltration. Applications of these same principles have been used to examine continuous ambulatory peritoneal dialysis. Oxygenation of blood,... 

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. 

The application of polymeric materials in medicine is a fairly specialized area with a wide range of specific applications and requirements. Although the total volume of polymers used in this application may be small compared to the annual production of polyethylene, for example, the total amount of money spent annually on prosthetic and biomedical devices exceeds 16 billion in the United States alone. These applications include over a million dentures, nearly a half billion dental fillings, about six million contact lenses, over a million replacement joints (hip, knee, finger, etc.), about a half million plastic surgery operations (breast prosthesis, facial reconstruction, etc.), over 25,000 heart valves, and 60,000 pacemaker implantations. In addition, over AO,000 patients are on hemodialysis units (artificial kidney) on a regular basis, and over 90,000 coronary bypass operations (often using synthetic polymers) are performed each year (]J. 

Kidney. The device called an artificial kidney is actually an external hemodialysis system, first developed in the early 1940s, that washes the blood and removes waste products from the body. Over 40,000 patients are maintained by this device each year in the United States, and there are over 100,000 people worldwide undergoing routine dialysis. In addition, many others are placed on the hemodialysis unit for short-term treatment. 

Originally hemodialysis had to be performed in a hospital, but in recent years home units have been developed, which reduces the cost to a great extent. Much research has centered on wearable artificial kidneys (WAK) which enable the patient to have fairly great mobility compared to the conventional units (59. 60). Only a limited amount of research is being done, however, to develop an Implantable device that could truly be termed an artificial kidney. 

Several other reactors for immobilized heparinase have been designed (53,54). The initial reactor (47) caused no more blood damage than conventionally used extracorporeal devices such as the artificial kidney machine (54a). By controlling the mode of immobilized enzyme bead suspension, all blood damage can be essentially eliminated (54). The FDA... 

Extracorporeal medical machines (e.g., artificial kidney, pump-oxygenator) perfused with blood have been an effective part of the therapeutic armamentarium for many years. These devices all rely on systemic heparinization to provide blood compatibility. Despite continuous efforts to improve anticoagulation techniques, many patients still develop coagulation abnormalities with the use of these devices (1-3). Even longer perfusion times may occur with machines such as the membrane oxygenator. In such cases, the drawbacks of systemic heparinization are multiplied (4). A number of ap-... 

At present, synthetic blood filters are routinely placed at the effluent of extracorporeal devices such as the pump-oxygenator or artificial kidney to remove clots or aggregates formed during the perfusion. The filters used in oxygenators can be as large as 2 L, whereas those used in renal dialysis are only several milliliters. With further development, heparinase could be immobilized to polymers in these filters. In this case, the filter could remove both clots and heparin. 

As early as 1955, Kolff and Balzer" described a device patterned after an early renal dialysis unit (the Inouye artificial kidney) wherein polyethylene tubing was used in a coil configuration. While the concept was sound, the membrane material choices available at that time were limited. 

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