Natural/biological materials

The sample is a natural biological material. It is not synthesized, and it cannot be controlled. Automation may control the testing process to some degree, but the sample itself cannot be—except perhaps for automatically rejecting a sample that is too small or has been damaged— for instance, hemolyzed (cells ruptured). 

Biological demands of nutrients, biological optimum (cf. Fig. 5.1), maintenance, mutation Impure, inactive, and diluted ( unconventional raw materials cellulose, starch, oil) One-stage possible without intermediary product recovery (e.g., steroid transformation) Potentially better for environment Mainly slow reaction rates Low concentrations High demand in sophisticated apparatus/equipment, supplementary education Demon of nature (biological material, infections, mutations)... 

The use of "natural" biological materials is particularly promising if a combination of biological and synthetic structures, i e, a hybrid approach is chosen to build chemical sensor systems Again the controlled signal transduction across interfaces plays the central role in current research... 

Crystallized silicon is very nonreactive and requires extremely high temperatures to become reactive. It is also known to be a nonbiocompatible material with very poor hemocompatibility [9]. However, in 1995, Canham [10] demonstrated the bioactivity of pSi layers in simulated body fluids (SBFs). Here, the term bioactive refers to silicon as a biomaterial, which is deflned as a nonviable material intended to interact with biological systems when used in a medical device. As noted by Canham, the transition of silicon to a bioactive state via the introduction of pores is consistent with the fact that aU other natural biological materials are porous [77]. In Canham s study, 1 gm-thick pSi layers were incubated in various SBFs for periods ranging from 6h to 6 weeks. While the highly porous Si (porosity >70%) dissolved in aU SBFs tested, the silicon with medium porosity (<70%) was slowly biodegradable. Similar to solid silicon, very low-porosity silicon was shown to be bioinert Thus, porosity is directly related to bioactivity. 

Bioassays are based on the growth response of vitamin-depleted rats or chicks to graded amounts of vitamin B 2 added in the diet. These assays are not specific for vitamin B 2 because factors, other than vitamin B 2 present in biological materials, produce a growth response. Because coen2yme primary form of natural vitamin 2 is light sensitive, assays should be carried out in subdued light. 

Biomimetic materials are modeled after naturally occurring biological materials. 

There are a number of naturally occurring polymers which find technical application, including cellulose and its derivatives, starch, and rubber. In addition, a number of important biological materials, most notably the proteins, are made up of macromolecules. These will be considered briefly in the sections which follow. 

At present, the most promising methods for synthetic colorant analysis seem to be those based on separation approaches such as HPLC and capillary electrophoresis (CE). CE is the method of choice for the determination of synthetic dyes in biological materials while HPLC is generally a more suitable method for the identification and determination of hydrophobic natural pigments, having a better sensitivity and efficiency than CE. 

The same principle is used for the preparative separation of mixtures of biological materials, the extraction of different individual components from these mixtures, and their purification. In this case one uses an electrophoretic method with continued introduction of individual portions of the mixture and withdrawal of portions of pure fractions. There have been reports that such processes were accomplished in spacecraft where, since gravitational forces are absent, the liquid solutions can be used without the danger of natural convection. 

A wide variety of reference materials is now available, covering several different kinds of natural matrix such as food (e.g. milk powder), human tissues (e.g. liver), marine biological materials (e.g. tuna fish) and soils and sediments. The radionuclides of interest cover naturally occurring ones (e.g. Ra), fission products... 

Hundreds of different natural amino acids have been found and isolated. In some cases, they are quite complex and have a variety of functions. The preparation and isolation of amino acids occurs either from biological material or via chemical synthesis. Several amino acids, such as glutamic acid and methionine, are now prepared on a scale of 100,000 tons per year. 

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