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Alginates are natural polysaccharides primarily extracted from the cell walls of brown algae (Phaeophyceae) and, to a lesser extent, some bacteria. In commercial use, alginates refer to salts of alginic acid, with sodium alginate being the most common, and they are widely employed as thickeners, stabilizers, and gelling agents.

The polymers are linear copolymers of two uronic acids, β-D-mannuronic acid (M) and α-L-guluronic acid (G), arranged in blocks of G, M, and MG (or GM). The proportion and arrangement of G and M units influence viscosity and gel strength. In solution, alginates are highly viscous; in the presence of multivalent cations such as calcium, they form hydrogels through ionic cross-linking, a process often described by the egg-box model. G-rich regions tend to produce stronger, more brittle gels, while M-rich regions yield softer, more elastic gels. Gelation is affected by pH, temperature, and ionic strength.

Applications span food, cosmetics, and pharmaceuticals as thickeners, stabilizers, and gelling agents. In medicine and biomedicine, alginates are used in wound dressings, drug-delivery systems, and tissue-engineering scaffolds; in dentistry, calcium alginate gels are used for impressions. Additional uses include textiles, paper processing, and oil-drilling fluids.

Production involves alkaline pretreatment of brown seaweed to release alginic acid, followed by conversion to salts such as sodium alginate. Alginates are generally regarded as safe for many applications, though regulatory compliance and individual sensitivities should be considered.