Recent insights into polysaccharide-based hydrogels and their potential applications in food sector: a review

2. Preparation of hydrogels 
Hydrophilic properties of hydrogels with well-explored 3D structures 
decide the preparatory method for them. Using initiators, hydrogels are 
synthesized from monomer units that are linked together with the help 
of a cross-linker (
Fig. 1
). 
However, their specific applications could also attract the hydro-
phobic monomers for deciding their preparation and regulation. For the 
hydrogel preparation, polymers (natural or synthetic) as the starter are 
required. Any method incorporating cross-linking in the hydrogel, 
which in turn provides elasticity to hydrogel is used. Moreover, back-
bone bond strength or the cross-links bond strength determines the 
hydrogel biodegradability under chemical or enzymatic conditions. 
Classification of Hydrogel is the prime focus of many researchers 
(
Fig. 2
). 
To sum up, the methods involved in hydrogel preparation are 
determined by the various agents used in physical or chemical cross- 
linking (
Table 3
). 
The hydrogel structure has an innate stiffness in both acidic and basic 
environments, which is connected to the production of hydrogen bonds 
at low pH and electrostatic repulsive forces at high pH. Even though 
both chemical and physical crosslinks cooperate to cause hydrogel 
shrinkage, it appears that hydrogen bonds stabilize the structure more 
efficiently than electrostatic interactions, according to the experimental 
findings. The protonated form of the carboxylic groups, which is 
responsible for hydrogen bonds at low to medium pH, or the deproto-
nated form, which is responsible for electrostatic forces, can stiffen the 
structure 
[114]
. 
2.1. Physical 
The various mechanisms involved in physical cross-linking are 
hydrogen bonding, crystallization, hydrophobized polysaccharides, and 
ionic interaction. The basic and easy mechanism, ionic interaction oc-
curs under optimal pH and temperature and in the absence of ionic 
groups. However, hydrogels with appreciable strength result in the 
presence of metallic ions during the polymer cross-linking process 
[115]
. Comparatively, enhanced water uptake and swelling occur in the 
hydrogels of various polysaccharides including carboxymethyl curdlan, 
pullulan chitosan, and dextran constituted by hydrophobic interactions 
[116]
. 
The physical interaction of crystallization occurs between the Pro-
Lastins (blocks of elastin) and silk-like units in protein interaction and 
the ProLastins (fluid in nature) forms a gel by the process of