The word ‘gel’ comes from ‘gelatine’, which is an example of a biopolymer made from peptides. Gelatine can be extracted from animals and used to form a gel in water.
A gel is a solid jelly-like material, that can be either soft and weak, or hard and tough. A gel is made up of large molecules (often polymers) or that have coagulated in a solvent (often water) to form a jelly-like solid. The majority of the weight of a gel is due to the liquid in the system, but the gel behaves as a solid. This is because the gel is made up of cross-linked networks within the liquid, in three-dimensions.
Hydrogels are hydrophilic cross-linked polymers, and are capable of swelling to aborb and hold a large volume of water (up to 1000 times its own weight) within the 3D network of the polymer. These are used in nappies and wound dressings, to absorb liquid, and also in the food industry to alter the texture of food, i.e. jelly.
Gelation occurs when individual biopolymer chains coil together into double or triple helices. These helices act as junction zones within an extended three-dimensional network that traps water into a solid-like gel. In some biopolymers the gelation is driven simply by cooling whereas other liquid-gel transitions can be triggered by a change in pH (e.g. chitosan, alginate) or addition of multivalent metal cations (e.g. alginate, carrageenan). An excellent description of gelation in biopolymers can be found on the London South Bank University website.
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