In contrast to most other polysaccharide gels, alginate gels can develop and set at constant temperature. This unique property is particulary useful in applications involving fragile materials like cells or tissue with low tolerance for higher temperatures.
An alginate gel will develop instantaneously in the prescence of divalent cations like Ca2+, Ba2+ or Sr2+ and acid gels may also develop at low pH. Gelling occurs when the divalent cations take part in the interchain ionic binding between guluronic acids blocks (G-blocks) in the polymer chain giving rise to a three dimensional network. Such binding zones between the G-blocks are often referred to as egg-boxes, and consequently alginates with a high content of G-blocks induce stronger gels. Gels made of M-rich alginate are softer and more fragile, and may also have a lower porosity. This is due to the lower binding strength between the polymer chains and to the higher flexibilities of the molecules.
The gelling process is highly dependent on diffusion of gelling ions into the polymer network and there are essentially two main classical methods for the preparation of alginate gels: the dialysis/diffusion method, and the internal gelling method. (NovaMatrix self-gelling technology provides an exciting alternative.)
In the dialysis/diffusion method (diffusion setting) gelling ions are allowed to diffuse into the alginate solution. This method is most commonly used in biotechnology for immobilization of living cells in alginate gel.
An alginate solution can also be solidifed by internal gelation method/internal setting, i.e. in situ gelling. Here a calcium salt with limited solubility, or complexed Ca2+-ions are mixed with an alginate solution into which the calcium ions are released, usually by the generation of acidic pH with a slowly acting acid such as D-glucono-a-lactone (GDL). This method is chosen if the purpose is to create a homogenous, non-syneretic alginate macrogel to fill the space of a given container. The main difference between internal and diffusion setting is the gelling kinetics. Consequently, the gel network will also be different.
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