Alginates

Are a family of copolymers of polysaccharides consisting of α-L-guluronic acid blocks (G) and β-D-mannuronic acid (M). They may bind to certain ions (e. g. Ca⁺²) through guluronate residues, forming three-dimensional hydrogels. Alginates rich in G blocks bind to a larger number of ions, producing more rigid and resistant gels, whereas alginates rich in M blocks are more flexible and enable higher diffusion rates of solutes through the gel. Alginates may be produced from natural sources (brown algae), by extraction and purification by sterile filtration. But they may also be produced by fermentation of various microorganisms. Chemically modified alginates are also synthesized: by esterification with propylene oxide (for beers and salad dressings), alkylation with alcohols (drug delivery systems), alkylation or allylation of binder groups (in order to obtain photo-crosslinkable gels). There are several methods to fabricate globules, microcapsules, fibers, films and membranes. Alginates are susceptible to degradation: by cleavage of glycosidic linkages through hydrolysis in acidic or basic environments, by free radical oxidation, and by enzymatic degradation.