Ions-induced gelation of alginate: Mechanisms and applications

Alginate is an important natural biopolymer and has been widely used in the food, biomedical, and chemical industries. Ca²⁺-induced gelation is one of the most important functional properties of alginate. The gelation mechanism is well-known as egg-box model, which has been intensively studied in the last five decades. Alginate also forms gels with many other monovalent, divalent or trivalent cations, and their gelation can possess different mechanisms from that of Ca²⁺-induced gelation. The resulted gels also exhibit different properties that lead to various applications. This study is proposed to summarize the gelation mechanisms of alginate induced by different cations, mainly including H⁺, Ca²⁺, Ba²⁺, Cu²⁺, Sr²⁺, Zn²⁺, Fe²⁺, Mn²⁺, Al³⁺, and Fe³⁺. The mechanism of H⁺-induced gelation of alginate mainly depends on the protonation of carboxyl groups. Divalent ions-induced gelation of alginate show different selection towards G, M, and GM blocks. Trivalent ions can bind to carboxyl groups of uronates with no selection. The properties and applications of these ionotropic alginate gels are also discussed. The knowledge gained in this study would provide useful information for the practical applications of alginate.