Chemistry Reference
In-Depth Information
7.3.2 Chemical Cross-Linking
Chemical cross-linking results in permanent or chemical hydrogels having covalent
linkages. These hydrogels can attain equilibrium swelling state depending upon the
cross-linking density and intensity of interaction of polymer with water. Chemical
cross-linking may proceed with polymerization of functional groups present on
backbone, polymerization in the presence of cross-linking agent, and
polymer-polymer cross-linking. Polysaccharide-based hydrogels involving chemi-
cal cross-linking can be synthesized by different techniques.
Chemically cross-linked polysaccharide hydrogels can be synthesized in the
presence of cross-linking agent. Different cross-linking agents like
glyceraldehydes, formaldehyde, epichlorohydrin, and
N
,
N
0
-methylenebisa-
crylamide have been reported for the synthesis of polysaccharide-based hydrogels.
Cross-linking involves active reaction sites like -OH groups on the polysaccharide
backbone. Cross-linking on polysaccharide backbone may proceed via condensa-
tion reaction [
29
], Michael addition [
30
], hydrazone bonding [
31
], and enzymatic
cross-linking [
32
]. A carboxymethyl cellulose sodium salt and hydroxyethyl cellu-
lose hydrogels have been prepared using divinyl sulfone as cross-linker to develop
novel system for the body water elimination for treatment of edema [
33
]. Cross-
linked carboxymethyl konjac glucomannan was synthesized by reaction of
monochloroacetic acid, konjac glucomannan, and monochloroacetic acid. Cross-
linked hydrogels were used to absorb Cu
2+
,Pb
2+
, and Ca
2+
ions from aqueous
solution [
34
]. Hyaluronic acid has been cross-linked with
α
-
β
-polyacryl hydrazide
[
35
].
Radiation cross-linking is another technique to synthesize hydrogels. It does not
involve the use of chemical agents, thereby an important method to develop
biocompatible hydrogels. Single-step synthesis and cost-effectiveness are the
other advantages of radiation cross-linking. Radiation cross-linking of hydrogels
may proceed in the presence of high-energy radiations like gamma radiations,
electron beam, or X-rays [
36
]. The cross-linking may occur in dilute solution, in
concentrated solution, or in solid state. In case of radiation-induced cross-linking, in
aqueous state, irradiation of dilute solution results in the absorption of radiation by
water molecules which generate free radicals to activate polysaccharide backbone.
Radiation-induced cross-linking of carboxymethyl cellulose was investigated in
aqueous solution at various radiation doses. Cross-linking was observed in 5 %
aqueous solution [
37
]. Blend hydrogels of carboxymethylcellulose and
carboxymethyl chitosan were prepared by gamma irradiation of high-concentrated
aqueous solution for metal adsorption of Pb and Au [
38
]. Irradiation of
polysaccharides in solid state results in the generation of free radicals directly on
polymer chain. The breakdown of glycosidic bond is the primary reaction involved
in solid state irradiation. It may result in degradation of polysaccharide which
depends upon the concentration of reactants and temperature. Carboxymethyl
cellulose, gum arabic, and dextran have been modified in solid state by high-energy
radiations to synthesize hydrogels [
39
].
