Membranes, alginate-chitosan

The majority of the aforementioned capsules were either not sufficiently mechanically stable or suffered from other surface or matrix related deficiencies. These deficiencies include poor morphology, such as capsule sphericity and surface smoothness, which result from an osmolar imbalance. Membranes are also often leaky (an internal polymer slowly diffuses out through the capsule wall) or shrink in either PBS or in culture media over a period of a few hours. Exceptionally, some capsules are observed to swell excessively and burst. Furthermore, some complex membranes, although stable in water, dissolve over several days upon a contact with culture media. This is true for pectin based capsules (pectin/calcium salt) and for alginate-chitosan membranes and maybe a consequence of the polycation substitution by electrolytes present in the media [10]. In order to improve the existing binary capsules several approaches, both traditional and novel, have been considered and tested herein. These are discussed in the following sections. 

The selective dense layer of hydrophilic membranes is made from different polymers with a high affinity for water. These polymers contain ions, oxygen functions like hydroxyl, ester, ether or carboxylic moieties, or nitrogen as imino or imi-do groups. Preferred hydropilic polymers are polyvinylalcohol (PVA) [32], poly-imides, cellulose acetate (CA) or natural polymers like chitosan [33] or alginates. Organophilic membranes usually consist of crosslinked silicones, mostly polydimethyl siloxane (PDMS) or polymethyl octyl siloxane (POMS). 

Beaumont and Knorr (ID) described the detrimental effect of chitosan on cell viability of Apium graveolens. Later it was found that at chitosan concentrations <250 yg/mL (Beaumont, M. and Knorr, D., Univ. of Delaware, unpublished data), plant cell viability was retained. Development of complex coacervate capsules consisting of alginate chitosan (22) and kappa-carrageenan-chitosan (10) allowed the concurrent release of secondary metabolites while still maintaining reasonable cell viability. Chitosan comprised the outer layer of the gel capsule and chitosan diffusivity could be controlled via capsule membrane permeability. 

Wang, L., Khor, E., Wee, A. and Lim, L. Y, Chitosan-alginate PEC membrane as a wound dressing assessment of incisional wound healing, /. Biomed. Mater. Res., Appl. Biomater., 63, 610, 2002. 

Alexakis, T., Boadi, D. K., Quong, D., Groboillot, A., O Neill, I. K., Poncelet, D., and Neufeld, R. J. Microencapsulation of DNA within alginate microspheres and crosslinked chitosan membranes for in vivo application. Appl. Biochem. Biotechnol. 50 (1995) 93-106. 

Chitosan is a hydrophilic polymer such as cellulose, alginic acid, and pullulan. The characteristics of permeation and separation for an aqueous alcohol solution of the chitosan membrane in pervapo-ration is summarized in Table 34.1, in which separation factors, ag o/ROH was calculated by... 

Kim, S. G., Lim, G. T., legal, J., and Lee, K. H, 2000. Pervaporation separation of MTBE (methyl rcrt-butyl ether) and methanol mixtures through polyion complex composite membranes consisting of sodium alginate/chitosan. J. Membr. Sci. 174 1-15. 

FIGURE 5.3 Performance of sodium alginate-chitosan blend membrane on dehydration of ethanol, effect of feed composition on the polyion complex membrane. (Data from Kanti, P. et al., Sep. Purif. TechnoL, 40, 259-266, 2004.)... 

Plant cell cultures represent a potentially rich source of secondary metabolites of commercial importance and have been shown to produce them in higher concentrations than the related intact plants. However, plant cell cultures often produce metabolites in lower concentrations than desired and commonly store them intracellularly. These limitations can be overcome by product yield enhancement procedures, including immobilization of cultured cells, and permeabilization, or ideally using a combined immobilization/ permeabilization process with retained plant cell viability. Complex coacervate capsules consisting of chitosan and alginate or carrageenan proved to be effective biomaterials for entrapment, controlled permeabilization of cells and to allow control of capsule membrane diffusivity. 

Deposition of polyelectrolytes Lajimi et al. [56] explored the surface modification of nanofiltration cellulose acetate (CA) membranes by alternating layer-by-layer deposition of acidic chitosan (CHI) and sodium alginate (AEG) as the cationic and anionic polyelectrolyte, respectively. The supporting CA membranes were obtained by a phase separation process from acetone/formamide. The permeation rate of salted solutions was found to be higher than that of pure water. The rejection of monovalent salt was decreased, while that of divalent salt remained constant so that the retention ratio increased. Increasing the concentration of feed solutions enhanced this selectivity effect. 

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