Highly cationic chitosans

A number of articles considered the association of chitosan with polylactic acid or similar compounds [47-49] another group of articles presented new data on highly cationic chitosans [ 50 - 55]. More data have also been made available on the delivery of growth factors [56] and ophthalmic drugs [57,58], on the activation of the complement, macrophages [59-61] and fibroblasts [62], on mucoadhesion [63] and functionalization of chitin [64]. The development of new carriers for the delivery of drugs, and the interactions of chitosans with living tissues seem therefore to be major topics in the current research on chitosan. Therefore, this chapter will place emphasis on these aspects. 

While Nafion , a perfluorinated polymer developed by DuPont, is the most commonly used proton conductive polymer electrolyte membrane it is an insufficient solution in a number of areas. It has high cationic transport (approximately 9.56 5/cm) [8] but also has high levels of methanol fuel crossover, slow anode kinetics and very high cost [12]. Fuel cell membrane performance can be estimated from the ratio of proton conductivity (a) to methanol permeability (P). The higher the value of a/P, the better the membrane performance would be [13]. Chitosan has been shown to have a much lower methanol permeability than Nafion [14], and as such, a great deal of attention focused on developing chitosan membranes with high levels of ionic conduction and low methanol permeability as delineated in Table 3.1. 

Sticky removal with different natural polymers has been compared [69]. Tailored starches, with benzyl as well as with ethyl substituents, from very low to high cationic starch density, were investigated as well as benzylated chitosan. 

A key physiochemical trait of chitosan is its unique molecular structure, which contains abundant reactive hydroxyl and amino functional groups. These functional groups provide anchor sites for further conjugation of functional molecules to chitosan. The abundant amino functional groups present in the molecular structure of chitosan also ensure a net cationic character, which enables electrostatic com-plexation with anionic drugs and biotherapeutics. Combined, these functionalities render chitosan highly amendable to further modification, and a desirable polymer platform for a broad range of biomedical applications. 

Precipitation of proteins by the addition of anionic or cationic polymers occurs through the neutralization of charges and formation of high molar mass aggregates. Polymers commonly used for this purpose are carb-oxymethylcellulose and chitosan. Neutral polymers, such as PEG and methylcellulose can also be employed. In such cases, the mechanism is... 

In an alternative route, starch is complexed with a polyamine, poly(vinyl amine), polyethylene imine) or chitosan to render it cationic. Although stronger, these complexes may also separate with time and lead to an undesirable accumulation of a low molecular weight, high-charge polyamine fraction and interference in the retention process. 

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