Ionic crosslinking
Ionic cross-linking is a physical crosslinking that occurs when a water-soluble and charged polymer crosslinks with an ion of opposite charge.
Alginate is the well-known example of a polymer that can be crosslinked by calcium ionic interaction.
Alginate gel is usually produced by dripping sodium alginate solution into aqueous CaCl2. However, the resulting hydrogels are not uniform and mechanically weak. The gelation time is also hard to control.
Another approach to generate ionic crosslinked alginate is to use CaCO3-GDL system. CaCO3 acts as a source of calcium ions. It provides a slower gelation time that produces more uniform and mechanically stronger gels. The gelation rate is also more tunable.
Figure 1. Ionic crosslinked alginate. The alginate is shown in grey. The egg-box structure can be seen around the calcium ions.
D-glucono-d-lactone (GDL) is used to initiate the gelation. This molecule is hydrolyzed slowly to gluconic acid, which provides sustained release of protons, which in turn dissolve the calcium carbonate. The calcium released is chelated by the alginate, resulting in gel formation.