Biocompatibility immune response

The first aspect of biocompatibility is a natural immune response. When a foreign object enters the blood stream, it can be attacked by the body s defense system. The first step is protein adsorption on an object surface. It is believed that the amount and type of protein adsorption is one of the most important steps determining whether the object is tolerated or rejected by the body. The next step is cell adhesion, which may cause aggregation and activation of platelets and triggering of the blood coagulation system with resulting thrombus formation. It may not only lead to sensor failure via surface blocking but directly threatens the patient s health. 

The second aspect of biocompatibility is a leaching problem. Ion-selective electrode materials, especially components of solvent polymeric membranes, are subject to leaching upon prolonged contact with physiological media. Membrane components such as plasticizers, ion exchangers and ionophores may activate the clotting cascade or stimulate an immune response. Moreover, they can be potentially toxic when released to the blood stream in significant concentrations. 

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 results by Hetrick et al.32 support the use of NO-release coatings for developing more tissue-compatible sensors. However, the impact of NO on the biocompatibility at a NO-releasing implant is a multifaceted question that is still not fully understood. Further study into the mechanisms by which NO decreases tissue encapsulation and chronic immune response while increasing angiogenesis will aid in optimization of the NO release properties (e.g., flux, concentration, and duration) of an implant coating for sensor applications. 

Elastomers of silicone are widely used as biomaterials. In general, silicone elastomers have excellent biocompatibility, inducing only a limited inflammatory response following implantation. In fact, until very recently, it was assumed that silicones were almost completely inert in biological systems. It is now known, however, that certain silicone polymers can provoke inflammatory and immune responses. The biological response to implanted silicone, and the variability of that response among individuals, is the subject of considerable debate and interest. 

For many water-insoluble polymers the Immune response, if any, is more difficult to evaluate. Recently, Habal and coworkers (]JL) have reported a method to assess the effect of solid implant materials such as silicone, segmented polyether-polyurethane, polyCmethyl methacrylate), and Bioglas using tumor-bearing mice as the experimental model. They have found that the B-cells from the test animals showed a reduced capacity for proliferation when stimulated by mitogens as compared to the controls. The results demonstrate once again that even the relatively biocompatible solid polymers may have a measurable effect on the immune system. 

Intracorporeal applications are far more complex the enzyme should be directed to its target within the patient s body and avoid the immune response. Immobilization to biocompatible supports may reduce the immune response significantly (Klein and Langer 1986). Several systems for enzyme delivery have been envisaged microencapsulation, liposome entrapment (Chen and Wang 1998 Fonseca et al. 2003), microencapsulation (Dai et al. 2005) and artificial red blood cell ghosts (Serafini et al. 2004). An updated review on the subject has been published... 

MWCO may be able to prevent transport of antibodies, but it is not possible to block all components of the immune system while still allowing nutrient transport. Hydrophilic materials tend have a higher biocompatibility as they resist protein adsorption. Protein adsorption can initiate a cascade of events culminating in fibrotic encapsulation of the capsule as discussed in a recent review. The use of natural materials for the capsule increases the risk of triggering an immune response due to antigens in the material or insufficient removal of immunogenic compounds from the material such as endotoxin. 

VandeVord et al. examined the biocompatibility of chitosan in mice. Their data imply that chitosan has a chemotactic effect on immune cells, but that effect does not lead to a humoral immune response. Also, results suggest that the specific responses reported may have been caused by contaminating proteins/polyscaccharides from the source organism. This type of contamination has been reported with other polysaccharides studied for implant use. As discussed earlier with alginates, the need to use highly purified grades of these biomaterials is critical to avoid toxicity and fibrotic encapsulation. ... 

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