Natural tissues

Biomaterials for Cardiovascular Devices. Perhaps the most advanced field of biomaterials is that for cardiovascular devices. For several decades bodily parts have been replaced or repaired by direct substitution using natural tissue or selected synthetic materials. The development of implantable-grade synthetic polymers, such as siHcones and polyurethanes, has made possible the development of advanced cardiac assist devices (see... 

Polylactic Acid. Polylactic acid (PLA) was introduced in 1966 for degradable surgical implants. Hydrolysis yields lactic acid, a normal intermediate of carbohydrate metaboHsm (23). PolyglycoHc acid sutures have a predictable degradation rate which coincides with the healing sequence of natural tissues. 

This is the most polar group of lipids in natural lipid samples. When developed in a nonpolar solvent system, phospholipids remain at the origin and more polar solvent system should be used to elute and separate individual phospholipids. The most popular system is the Wagner system, which consists of chloroform metha-nohwater (65 25 4) [51] for the separation of common phospholipid species in natural tissue samples. 

One of the widely used categories of polymers for biomaterials design is that of homo-or copolymers, which could generate hydrogels. Hydrogels are three-dimensional polymer networks that could swell in water without dissolution and that, due to their high water content and rubbery nature, are very similar to natural tissues and could be considered... 

Literature on the effect of guanethidine on other natural tissue amines is scanty. In rats, up to about 50 per cent depletion of 5-hydroxytryptamine has been reported in the brain [211], thyroid [333], duodenum [211] and small intestine [334, 335], but no depletion has been observed in the spleen or ileum [21 1,334]. Guanethidine does not affect rat-heart histamine [336]. 

The best known isoprenylated quinones are the benzoquinone ubiquinone (157) and the naphthoquinone menaquinone (158), which normally occur in natural tissues as a mixture of homologues (isoprenylogues) with different chain lengths. Analysis of the mixtures of ubiquinone or menaquinone isoprenylogues present in various species has been used in the chemotaxonomic characterization of bacteria. 

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. 

In the case of drying foodstuffs, this is a complex combination of coupled heal and mass transfer through natural tissues The need to take... 

In natural tissues, carotenoids generally occur as complex mixtures of many substances in orange juice, for instance, more than 50 carotenoids were identified. A survey of the carotenoid content in some fruits and vegetables is given in Refs. 19 and 20. Differences due to variety, site, and maturity must be considered. 

Fig. 2.4 The cross-sectional view of the elongation of the tissue (a) by applied lateral stress to demonstrate the change of shape in natural tissue, and the cross-sectional view of skin and soft tissue is shown in (b) with a barrier dressing that is liquid applied over the tissue. The barrier dressing must conform to the surface contours of the tissue...

Chimyl alcohol (Structure 7.79), isolated from the mantle tissues of the nudibranch Tritoniella belli,44 caused rejection by O. validus of shrimp-treated disks at natural tissue-level concentrations. T. belli sequesters this defensive chemistry from its diet, the stoloniferan coral Clavularia franklin-iana. Sequestration of chimyl alcohol from the diet, however, seems to be opportunistic, and T. belli also provisions its mucus with other yet undescribed deterrent natural products.49 Chimyl alcohol is reported from other non-Antarctic molluscs209 and has been demonstrated to function as an antibacterial and a fish antifeedant at levels found in the tissues of the dorid nudibranchs Archidoris montereyensis and Aldisa sanguinea cooperi.210... 

Under normal conditions, the activity of MMPs is very low and is strongly regulated by natural tissue inhibitors (TIMPs). The TIMPS are a family of four structurally related proteins (TIMP-1, 2, 3, and 4), exerting dual control of the MMPs by inhibiting both their active forms and their activation. In addition, the proteolytic activity of MMPs is inhibited by non-specific protease inhibitors, such as alpha2-macroglobulin and alphai-antiprotease. 

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