A device that is an artificial substitute for a body part, whether it is a limb or a heart valve, is called a prosthesis. When the prosthesis replaces all or part of an organ, it is called an artificial organ. Though replacement of organs from donor transplants is a more straightforward and reliable method, the supply of donor organs and thus their use is lim­ited. Artificial organs have been designed because they can be produced in sufficient quantities to meet demand and they eliminate the possibility of transferring infections, for exam­ple, HIV and hepatitis, from the donor to the recipient. When designing an artificial organ, function is of primary concern and can result in a device that bears little resemblance to its natural counterpart. Typically, artificial organs are made from synthetic materials not found in nature and use mecha­nisms different from those of the natural organ to achieve the same function. Disadvantages of artificial organs include the relative inability to adapt to growth, which limits their use in children, the mechanical and chemical wear due to use, and the body’s environment, which can limit the life of the device. Recently the design of artificial organs has included combin­ing biological material, such as organelles, cells, or tissues, with synthetic, engineered devices. These hybrids are called bioartificial organs (60).

Artificial hearts are primarily used as a “bridge-to-trans – plant,’’ that is, a temporary replacement used until a donor organ donor is transplanted. Research continues in devel­oping long-term, completely implanted heart replacements. The heart-lung machine is a short-term artificial organ used for patients undergoing transplant operations. It allows the patient to survive the removal of the heart until the replace­ment organ is surgically implanted. Common prostheses for the circulatory system are cardiac valve prostheses and vas­cular grafts. Concerns with these prosthetics include the for­mation of fibrous blood clots inside the circulatory system (thrombi), tissue overgrowth, hemorrhage from anticoagu­lants, and infection (61).

The artificial lung must provide a mechanism for the up­take of O2 by the blood and the removal of CO2. It can be used to completely replace the function of the lung temporarily during surgery or to assist with gas exchange temporarily un­til the lung can heal. Artificial lungs also replace or assist lung function permanently, if necessary. Typically, artificial lungs are not placed where the natural lung is located so the blood in the pulmonary system must be diverted to the artifi­cial lung and pumped to return it to the heart and systemic circulation. Gas is commonly exchanged by using membrane oxygenators. Difficulties in design include developing mem­branes as thin as the walls of the alveoli and finding a blood distribution method that mimics the branching achieved in a short distance by the lung (62).

One kidney can sustain function for a lifetime which makes live kidney donation possible: however, donors are typ­ically cadavers. The artificial kidney provides a common in­termittent treatment for renal failure during diminishing function of the kidneys or for patients who are waiting for a donor kidney. Dialysis, the mechanism of the artificial kidney, performs the necessary functions of the kidneys. These in­volve regulating (1) the volume of the blood plasma (contrib­uting significantly to the regulation of blood pressure), (2) the concentration of waste products in the blood, (3) the concen­tration of electrolytes (Na+, K+, HCO3, and other ions) in the plasma, and (4) the pH of plasma (63). More aggressive dial­ysis of the peritoneum, the membrane surrounding the body cavity and covering some of the digestive organs, is a recently developed treatment for irreversible end-stage kidney fail­ure (64).

The main concern with the loss of liver function is loss of the ability to detoxify the blood. Therefore, devices which aug­ment liver function focus on methods of detoxification. Some procedures currently in practice or under investigation in­volve dialysis, filtration, absorbent materials, and immobi­lized enzymes to convert specific toxins to less harmful sub­stances. Currently, temporary replacement of the liver involves systems with mammalian hepatocytes (liver paren­chymal cells which remove most of the carbohydrates, amino acids, and fat from the digestive products absorbed from the intestines by the blood) attached to a synthetic sup­port, where input from the host is separated from the device by a semipermeable membrane. Bioartificial livers using functional hepatocytes in a device immersed in body fluids are being investigated as an alternative to organ replace­ment (65). Partial or complete removal of the pancreas can occur due to polycystic disease, trauma, or tumors. The replacement ar­tificial pancreas focuses on the hormonal or endocrinal activ­ity of the pancreas (i. e., insulin and glucagon secretion), which regulates the uptake and release of glucose. Devices have not yet been developed that can replace the exocrine function of the pancreas, namely, the secretion of proteolytic and lypolytic enzymes in the gastrointestinal tract. Other ar­tificial organs for the digestive system include trachea re­placements, electrical and pneumatic larynxes, which replace only the phonation function of the larynx because a complete artificial organ that restores respiration and protection of the lower airway during swallowing has yet to be designed, and extracorporeal and intraesophageal stents (66).

Skin replacement following loss from events, such as a fire or mechanical accident, or through conditions, such as skin ulcers, is achieved by using autographs of the patient’s skin, allographs from cadavers, xenographs from animals, or arti­ficial skin. The risk of viral infection and rejection are con­cerns when using allographs and xenographs. Artificial skin is a bilayer membrane whose top layer is a silicone film that controls moisture and prevents infection and whose bottom layer consists of a porous, degradable copolymer. The top layer is removed and replaced by an autograph after about two weeks, and the bottom layer is removed by complete deg­radation after it induces the synthesis of new dermis. Clinical studies have shown that autographs take better than artifi­cial skin, but donor sites in which the top layer has artificial skin instead of silicone film heal faster and appear more like the patient’s skin than donor sites that used autographs (67).

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