Natural polymers, gels

The only body part that has transparent materials in the human body is the eyeball. The cornea, lens and vitreous humor consist mainly of collagen and acidic mucopolysaccharides, which makes them gels. The attempt to use natural polymer gels as a cataract cure has met with limited success due to biocompatibility problems and long-term stability. On the otiier hand, if vitreous humor substances are replaced by artificial materials made of PVA hydrogel, the properties (transparency and refiactive index) are very like those of the vitreous humor. Hence, it is ideal as a replacement material [12]. 

Models of regular structures, such as zeolites, have been extensively considered in the catalysis literature. Recently, Garces [124] has developed a simple model where the complex pore structure is represented by a single void with a shell formed by n-connected sites forming a net. This model was found to work well for zeolites. Since polymer gels consist of networks of polymers, other approaches, discussed later, have been developed to consider the nature of the structure of the gel. 

The commercial samples of pectins mainly used as food additives represent modified forms of the natural polymers due to the conditions of extraction. Nevertheless, it is usually recognized two categories of pectins the high methoxyl pectins (HM) with a degree of methylation DM>50% forming gels at low pH in presence of saccharose to reduce the water activity and the low methoxyl pectins (LM with DM<50%) forming gel in presence of calcium [4]. 

A related class of gels are those formed by extensive hydrogen bonding. An example is the polyethylene oxide)-poly(methacrylic acid) complex [18]. Spontaneously gelling natural polymer solutions are frequently of this type, including gelatin and native starch. 

A Suzuki, T Tanaka. Phase transition in polymer gels induced by visible light. Nature 346 345-347, 1990. 

IC Kwon, YH Bae, SW Kim. Electrically erodible polymer gel for controlled release of drugs. Nature 354 291-293, 1991. 

It is generally thought that the ER effect happens only in nonconducting oils. Here an ER effect in solid-like matrices such as polymer gels will be discussed. The nature of the ER effect in polymer gels will be explained using the point dipole model in [44],... 

There are several reports on the coating of bone-like hydroxyapatite onto natural polymer substrates. Kawashita et at. [57] reported that carboxymethylated chitin and gellan gum gels, which have carboxyl groups, can form hydroxyapatite on their surfaces in SBF if they are treated with a saturated Ca(OH)2 solution in advance, while curdlan gel, which has no carboxyl group, does not form hydroxyapatite in SBF, even if it is treated with Ca(OH)2 solution. These results support the hypothesis that carboxyl groups induce hydroxyapatite nucleation. Kokubo et at. [58,59] reported that non-woven fabrics of carboxymethylated chitin and alginate fibers also form hydroxyapatite on their surfaces in SBF if they are treated with Ca(OH)2 solution. 

The features common to reversible polymer gels of many types are identified suid discussed. The nature of the gel state is carefully defined, and a novel classification scheme based on morphology, rather than chemical or mechanistic considerations, is proposed. The article also serves as an overview to some of the more commonly used techniques used in the study of gels, and as an introduction to some of the current trends in reversible gel research. Some speculations regarding future trends in reversible gel research are presented. 

Wang P, Zakeeruddin SM, Moser JE, Nazeeruddin MK, Sekiguchi T, Gratzel M (2003) A stable-quasi-solid state dye-sensitized solar cell with amphiphilic ruthenium sensitizer and polymer gel electrolyte. Nature Mater 2 402-406... 

Walstra, P., van Vliet, T., Bremer, L.G.B. (1991). On the fractal nature of particle gels. In Dickinson, E. (Ed.). Food Polymers, Gels and Colloids, Cambridge, UK Royal Society of Chemistry, pp. 369-382. 

Another interesting aspect of gels is that a gel can be a single polymer molecule . The term single polymer molecule means that all the monomer units in a one piece of gel are connected to each other and form one big molecule on a macroscopic scale. Because of this nature, a gel is a macroscopic representation of single polymer behavior, which will be introduced and discussed later. 

We present a review of theoretical and experimental results on the swelling behavior and collapse transition in polymer gels obtained by our group at Moscow State University. The main attention is paid to polyelectrolyte networks where the most important factor is additional osmotic pressure created by mobile counter ions. The influence of other factors such as condensation of counter ions, external mechanical force, the mixed nature of low-molecular solvents, interaction of network chains with linear macromolecules and surfactants etc. is also taken into account Experimental results demonstrate a good correlation with theoretical analysis. 

Polymer gels have found wide applications in various fields medicine, the nutritive and petrochemical industries, agriculture, biotechnology, etc. They are also used in scientific research for example, in the separation and extraction of natural macromolecules such as DNA and proteins. 

The strong points of this technique are its absence of interaction with the stationary phase, rapid dilution and the potential to recover all of the analytes. Because the technique permits the separation of nominal masses ranging from 200 to 107 Da, its main applications are in the analyses of synthetic and natural polymers. The choice of stationary phase for a given separation is made by examination of the calibration curve of various columns. The column of choice is that which provides a linear range over the masses of the compounds found in the sample. The calibration has to be conducted with the same type of polymers because macromolecules can have various forms ranging from pellet-like to thread-like. The data presented in Table 7.1 show the domains of application of the three gels shown in Fig. 7.3, depending on the standards that are used. 

The natural polymers frequently used for the preparation of pharmaceutical gels include tragacanth, pectin, carrageenan, agar, and alginic acid, as well as semisynthetic polysaccharides such as methylcellulose, hydroxymethylcellulose, and carb-oxymethylcellulose. 

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