Biomedical Engineering Reference
In-Depth Information
4 Hydrogels Crosslinked by Ionic Interactions
Ionic interactions between polymer chains provide an alternative to covalent
crosslinking due to their high binding constants and the unique swelling proper-
ties of the resulting hydrogels [
122
,
38
]. Gels can be obtained through these inter-
actions by combining cationic and anionic polymers, with the ability to be fully
degraded by change of pH [
123
,
124
] or salt concentration in the medium [
125
].
A well-investigated example is chitosan, a polycationic copolymer composed of
2-acetamido-2-deoxy-
d
-glucopyranose and 2-amino-2-deoxy-
d
-glucopyranose.
It can be obtained by alkaline deacetylation of chitin, one of the most abundant
biopolymers and main component of the exoskeleton of crustaceans and insects.
Chitosan arose interest due to its biocompatability, biodegradability, and high nat-
ural abundance in form of chitin [
126
,
61
]. Hydrogels based on chitosan have been
investigated for topical ocular applications [
127
], wound healing [
128
], implanta-
tion [
129
], and injection [
130
].
An example of an anionic polyelectrolyte is xanthan. It is composed of a cellu-
lose backbone that has a negatively charged trisaccharide side chain on every other
glucose unit; this side chain is composed of
ʱ
-mannose,
ʱ
-glucoronic acid, and
ʲ
-mannose with a pyruvic acid diketal moiety located on the terminus of the side
chain [
131
]. Expressed by
Xanthomonas campestris
, a bacterium found on cabbage
plants, xanthan is the first biopolymer produced on an industrial scale, serving for
applications as food additive [
132
], in oil recovery [
133
], and in cosmetics [
134
].
In the mid 1990s, Dumitriu and coworkers reported a polyionic hydrogel
based on chitosan and xanthan [
135
]. Since then, numerous publications and
patents were released, pushing the applicability of this system from immobiliza-
tion of enzymes to dermocosmetics [
136
-
140
]. However, little was known about
how the composition of the precursor polymers influences the properties of these
hydrogels. To close this gap, Dumitriu and coworkers investigated the swelling
properties, microstructure, and mechanical traits of chitosan-xanthan hydrogels
[
141
]. Mixing aqueous solutions of anionic xanthan and cationic chitosan forms
hydrogels in a multistep process: after mixing, a common solution-pH value is
established, causing structural changes of the polymer chains. The next step is for-
mation of ionic interactions of the ammonium moities of chitosan with the carbox-
ylate functions of xanthan. These two steps lead to phase separation, often referred
to as coacervation [
142
]. In one phase, often called the coacervate phase, the poly-
mers are concentrated, whereas the other phase is a dilute phase. In the coacervate
phase, water molecules orient towards the polymer chains, leading to formation of
hydrogen bonds. In the dilute phase, water molecules are not orientated. A hydro-
gel is formed when all water molecules have formed hydrogen bonds between the
polymer chains, provided that the polymer chains are distributed properly, as com-
piled in Fig.
11
. Dumitriu and coworkers investigated the effect of coacervation on
the properties of these hydrogels by quantification of their viscolelastic properties
and swelling degrees.
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