Implant rejection

Interactions between solid surfaces and solutions are of fundamental importance in many biological systems (joint lubrication and movement, implant rejection, etc.), as well as in mechanics (lubrication and adhesion), in agriculture (soil wetting and conditioning and pesticide application), in communica-... 

In the area of fouling and implant rejection, progress has been made in understanding how proteins interact with these PS latex surfaces and how this interaction can be controlled. Again, the model will be improved as the complexity of the system is increased. The demonstration that the surface of latex particles can be modified either chemically or by adsorption of cell surface constituents opens up new possibilities for the use of such systems in understanding a host of complicated cellular events including the development of colloid-based drug delivery systems. 

Hydroxyapatite constitutes -65% of human bone by weight. There is another 18% collagen fiber which makes the bone flexible and more durable. Then, there is -10% genetic tissue (mostly living bone cells). This tissue carries the genetic code of the person or animal and unless it is in a denatured form, which also kills the bone, it is likely to be rejected in the body as a bone graft. Therefore, it is impossible to successfully implant living bone even from closely related donors. The remainder of bone is composed of capillaries, nerves, and so on. 

Recently, it has been possible to grow cells of the human immune system in special mice. These mice carry a genetic defect called severe combined immunodeficiency (SCID), which leaves them with crippled immune systems, much like those in AIDS patients. Because SCID mice lack functional cellular immunity, it is possible to implant them with human cells without tissue rejection taking place. Researchers have recently developed techniques to implant human fetal tissues containing stem cells for the blood into SCID mice. It is then possible to reconstitute these mice with functional human immune system cells, including T lymphocytes and B lymphocytes. They have also found that if these SCID mice are infected by HIV, the virus will establish infection in the human tissue and destroy the T helper lymphocytes, just as it does in humans. Thus, it may be possible to study some of the mechanisms by which HIV attacks the immune system in these mice. In addition, they may be very useful for testing potential antiviral drugs. 

As discussed in Section 4.7, stem cells have the potential to treat medical conditions beyond the scope that can be offered by drugs alone. However, there are many scientific and ethical hurdles to overcome. On the scientific front, stem cell research activities will intensify over the next decade. These challenges can broadly be divided into (1) determining how to develop stem cells into specific tissues and (2) implanting these tissues into the body without rejection by the recipient s immune system. On the ethical front, it is expected that there will be more debates on the ethical issues of stem cell research. Most scientists consent to therapeutic cloning (stem cell research) but not reproductive cloning. The ethical issue of stem cell research concerns harvesting cells from embryos that are a few days old. This action destroys the embryos. Some questions are ... 

Any material proposed for implantation, whether for cell transplantation or some other application, must be biocompatible i.e. it must not provoke an adverse response from the host s immune system. If this goal is not met the implant may be rejected. To this end it is important that the material be easily sterilized either by exposure to high temperatures, ethylene oxide vapor, or gamma radiation. A suitable material must therefore remain unaffected by one of these three techniques. However, biocompatibility is not simply a question of sterility. The chemistry, structure, and physical form of a material are all important factors which determine its biocompatibility. Although our understanding of the human immune system is advancing rapidly, it is not yet possible to predict the immune response to a new material. This can only be determined by in vivo experiments. 

The increasing demand for synthetic biomaterials, especially polymers, is mainly due to their availability in a wide variety of chemical compositions and physical properties, their ease of fabrication into complex shapes and structures, and their easily tailored surface chemistries. Although the physical and mechanical performance of most synthetic biomaterials can meet or even exceed that of natural tissue (see Table 5.15), they are often rejected by a number of adverse effects, including the promotion of thrombosis, inflammation, and infection. As described in Section 5.5, biocompatibility is believed to be strongly influenced, if not dictated, by a layer of host proteins and cells spontaneously adsorbed to the surfaces upon their implantation. Thus, surface properties of biomaterials, such as chemistry, wettability, domain structure, and morphology, play an important role in the success of their applications. 

Biocompatibility of implanted devices is exponentially more complicated than biocompatibility of topical devices. Rejection of a topical device is not a major issue but it represents the primary difficulty for a device that enters the body. The device will come into contact with fluids and cells that are sensitive to contacts with foreign materials. The success of the biofilters discussed in the last chapters is an implication of a level of biocompatibility. Bacteria are somewhat forgiving because they are able to tolerate contacts that would cause instant death to mammalian cells. Bacteria have evolved to the point where they are able to attach themselves to almost anything. 

Plastic pipes, even when flushed out with the most powerful disinfectants and germicides, have proven to be safe havens for some bacterial strains. Bactena-resistant piping is of major importance in pharmaceutical manufacture, Research is underway to find plastic piping that will reject the adhesion of bacterial slimes. Currently, alloy steels are widely used. The adherence of slimes to plastic pipes permits colonies of bacteria to multiply. A similar problem exists when patients are furnished with plastic implants orprosiheses Hospital water supplies must be continuously monitored. 

Imagine the possibilities, some of which are just now becoming reality Tools, knife blades, and scalpels coated with diamond remain forever sharp. Eyeglass lenses and wristwatches coated with diamond remain scratch-free. High-fidelity loudspeakers coated with diamond give a nearly perfect, undistorted sound at high frequencies. Hip joints and other biological implants coated with diamond are not rejected by the body s immune system. 

Rapid rejection of implantable progestogen-based contraceptives by some users, because of discomfort and local complications (for example because of breakage or migration of the device, or less than expert placement and removal), has attracted much attention in past reviews. 

Ions will not be rejected by electronic gating. Mass analysis of the primary beam Is generally desirable to Insure that no Impurities are present In the beam. Such species will become Implanted Into the sample and preclude their analysis at low concentrations In a sample (2). 

Clarke error grid analysis of a study of 15 diabetic rats showed the percentage of readings that fell into the clinically correct regions (Zones A and B) increased from 92% to 96% when applying the Z-score rejection criteria.38 During the long-term implantation (25 4 days), Z-score calculations removed 32% of the individual sensor data from six fully implanted four-sensor arrays.39... 

Figure 15.5 Schematic of instrumental apparatus. The DT/MH-functionalized AgFON was surgically implanted into a rat with an optical window and integrated into a conventional laboratory Raman spectroscopy system. The Raman spectroscopy system consists of a Ti sapphire laser (Acx = 785 nm), band-pass filter, beam-steering optics, collection optics, and a long-pass filterto reject Raleigh scattered light. All of the optics fit on a 4 ft x 10 ft optical table.

Of all forms of skin grafting, autografting is by far the most likely to be successful. In other forms of grafting, a patient is likely to suffer not only from infections developing from the loss of skin, hut also from immune responses as his or her body begins to reject the "foreign" implant, the skin from some nonself source. 

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