Chemistry Reference
In-Depth Information
it does not require any cross-linking agent. Blends of starch-carboxymethyl-
cellulose, gelatine-agar form physically cross-linked hydrogels [
7
-
9
]. Chitosan
solution containing glycerol-2-phosphate results in the formation of hydrogel at
37
C due to hydrophobic interactions [
10
].
7.2.1.2 Chemical Hydrogels
These are irreversible and stable hydrogels involving chemical cross-linking
between polymeric networks. Chemical cross-linking involves reaction of polymeric
backbone with cross-linking agent. Polysaccharides have functional groups like
hydroxyl or amine group which on reaction with cross-linking agent form gel
structure. Cross-linking may occur by different mechanisms like condensation,
addition, and vulcanization. These hydrogels having strong covalent bonds can attain
equilibrium swelling state which depends upon the polymer-solvent interactions and
cross-link density. Polysaccharides like cellulose and its derivatives, chitosan, algi-
nate, dextran, guar gum, and starch, can be cross-linked in the presence of chemical
reagents and in the presence of radiations to form chemical hydrogels [
11
,
12
].
7.2.2 On the Basis of Types of Monomer Units Involved
Hydrogels can be categorized into three types: homopolymer hydrogels, copolymer
hydrogels, and interpenetrating polymeric hydrogels.
7.2.2.1 Homopolymer Hydrogels
Homopolymer hydrogels have single specie as basic unit in the polymer matrix.
The single monomer unit cross-links to form three-dimensional network structures.
The cross-linked networks are formed in the presence of cross-linking agent or
without the presence of cross-linking agent. Cellulose hydrogels were reported
through one-step method in which cellulose was dissolved directly in urea/NaOH
solutions in the presence of epichlorohydrin as cross-linker. It resulted in fully
transparent hydrogel with macroporous inner structure [
13
]. Irradiation of polysac-
charide solutions results in the formation of hydrogels in the absence of gelating
agent. Commercial polysaccharides like xanthan, pectin, guar gum, alginate, starch,
xanthan, chitin, and chitosan can be modified in the presence of high-energy
radiations to form hydrogels [
14
].
7.2.2.2 Copolymer Hydrogels
Copolymer hydrogel contains two or more monomeric units in which at least one of
the monomer is hydrophilic. Copolymer hydrogels have different arrangements of
