Implants, artificial problems

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. 

We are able to construct mechanical arms that reproduce movements quite close to those performed by the human arm. The problem in implanting these arms is that movements have to be coordinated with all the other body movements under the brain s direction. There is one possibility for connecting the electronic systems of the artificial arm to the nervous signals (Fig. 33) coming from the brain in order to obtain coordinated movements separate those signals into different components and amplify every component to drive an artificial muscle or electric motor. 

While the mechanical performance of artificial materials in the human body can be predicted with some rehabihty, forecasting their biological performance is difficnlt. The problem of interactions at surfaces has already been mentioned. Research frontiers also include developing ways to simulate in vivo processes in vitro and extending the power and apphcability of such simulations to allow for better prediction of the performance of biomedical materials and devices in the patient. Fundamental information on the correlation between the in vivo and in vitro responses is limited. Chemical engineers might also make contribntions to the problem of noninvasive monitoring of implanted materials. 

The first ever injectable crude biomaterial, that is a dental implant, appeared early in ad 6oo (Fig. 12.1). During those times, Mayan people trimmed seashells into artificial teeth to replace missing teeth (Michael, 2006 Ratner et al., 2004). Early biomaterials also led to problems, including sterilization, toxicity, inflammation, and immunological issues. Since the Mayan s initial use of artificial teeth, biomaterials have evolved to be used in modem artificial hearts, hip and knee pros-theses, artificial kidneys, and breast implants. Materials used in these applications include titanium, silicons, polyurethanes, teflon, polybiodegradable polymers, and most recently bio-nanomaterials (Pearce et al., 2007)... 

The artificial hip has been used to replace the human hip because the hip is easily worn out over a lifetime of mechanical stress resulting from normal activity. The first artificial hip implant was made by Thermistokles Gluck in 1891 in Berlin. This implant made use of a femoral head of ivory fixed with plaster of paris and glue (Gluck, 1890, 1891). The results were not good due to severe infection problems and adverse foreign body tissue reactions. To develop better hip replacement prosthesis, many materials and procedures were examined between 1925 and 1953. 

As promising as Yannas s original formulation was, it suffered from one major problem. In a patient who is severely burned, there may not he enough healthy skin tissue available to harvest the epidermis needed to replace the silicone coating on the artificial graft. Yannas s solution to this problem was to develop an artificial epidermis that could he implanted at the same time as the artificial dermis. 

Dacron, however, continues to he the most popular material used in the production of artificial blood vessels. When first used with experimental animals, treated Dacron appeared to induce the regeneration of interior walls of blood vessels (called neointima) in essentially the same way as the natural process. When implanted into humans, however, the same treated Dacron has a somewhat different effect, resulting instead in the formation of a thin (about 1 mm thick) layer of fibrin (or a pseudointima) on the inner lining of the blood vessel. While this thin layer of fibrin is of relatively modest concern in larger blood vessels, it can be a serious problem in vessels less than 5 mm in diameter. In such cases, fibroblasts from the blood stream attach themselves to the fibrin, and the blood vessel gradually becomes blocked. 

Several implanted biosensors have been developed and evaluated in both animals and humans (see Chapter 4). Detection systems are based on enzymes, electrodes, or fluorescence. The most widely studied method is an electrochemical sensor that uses glucose oxidase. This sensor can be implanted intravenously or subcutaneously. Intravenous implantation in dogs for up to 3 months has demonstrated the feasibility of this approach. Alternatives to enzymes are being developed, including artificial glucose receptors. Less success has been achieved with subcutaneous implants. Implantation of a needle type of sensor into the subcutaneous tissue induces a host of inflammatory responses that alters the sensitivity of the device. Microdialysis with hoUow fibers or ultrafiltration with biologically inert material can decrease this problem. 

In-Vivo Percutaneous Implant Experiment. The principle of percutaneous attachment has extensive application in many biomedical areas, including the attachment of dental and orthopedic prostheses directly to skeletal structures, external attachment for cardiac pacer leads, neuromuscular electrodes, energy transmission to artificial heart and for hemodialysis. Several attempts to solve the problem of fixation and stabilization of percutaneous implants(19) have been made. Failures were also attributed to the inability of the soft tissue interface to form an anatomic seal and a barrier to bacteria. In the current studies, the effect of pore size on soft tissue ingrowth and attachment to porous polyurethane (PU) surface and the effect of the flange to stem ratio and biomechanical compliance on the fixation and stabilization of the percutaneous devices have been investigated.(20)... 

Total Artificial Heart. Although heart transplants have been performed since the pioneering work of Christian Barnard in 1967, this procedure always requires a donor heart, which may not always be available. In addition, the body does tend to reject any implanted organs as undesired foreign material, and close matching of the tissues is difficult at the best. Although various immunosuppressant drugs can minimize this problem, the patient becomes more susceptible to infectious disease. A total artificial heart (TAH) would offer at least a partial answer to both of these problems. 

The adhesion molecules (i.e., gelatin, fibronectin, coUagen, Min3) were covalently coupled to an artificial blood vessel made from PTEE. Due to unspecific adhesion of fibrin and collagen to the implant surfaces, thrombus formation and occlusion may occur in clinical use. To overcome this problem, the concept of EC cultivation on modified implant material was developed to mimic the physiological surface of blood vessels [36]. The expiration of these ceU layers strictly depends on the cell-surface interaction, which can be improved by modifying the graft surface with adhesion molecules. 

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