Biomedical Engineering Reference
In-Depth Information
OH
OH
OH
O
O
O
O
O
HO
O
HO
+NH
3
HO
+NH
3
OH
+NH
3
OH
OH
O
n
O
O
SO
2-
O
O
SO
2-
HO
O
SO
2-
SO
2-
HO
NH
3
+
HO
NH
3
+
+NH
3
NH
3
+
+NH
3
n
OH
+NH
3
OH
O
OH
O
O
O
O
OH
O
OH
OH
Figure 4.5
Ionic cross-linking of the chitosan using H
2
SO
4
. (From Cui, Z. et al. 2008.
Carbohydr Polym
73: 111-116. With
permission.)
and cell hypertrophy without exogenous factors. Many researchers have studied the
sulfuric acid-cross-linked chitosan network. Firstly, H
2
SO
4
protonates chitosan amine
groups, and then the SO
2−
anions slowly interact with chitosan NH
3
+
groups to form ionic
bridges among the polymer chains (
cf.
Figure 4.5) [73]. After cross-linking, the surface
topography becomes more homogenous and relatively flat, and the change of nanoto-
pology can affect the surface wettability and surface reactions, for example, capillary
reaction, and so on [74].
4.2.3 Covalent Cross-linking Network
Compared with PEC networks, the functions of covalent cross-linking networks are easier
to be controlled because their formation and application are not limited by pH value.
However, a high cross-linking density may produce some side effects. For example, Picart
et al. [75] found that chitosan/HA films cross-linked by a high 1-ethyl-3-(3-dimethylamin-
opropyl) carbodiimide (EDC) concentration lead to a pronounced inflammatory response
in vivo
and also to the formation of fibres. Therefore, a moderate cross-linking seems to be
of greater interest in terms of biodegradability and application. One could control the
cross-linking density through many ways: (1) changing the number or activity of cross-
linked sites; (2) adjusting the cross-linked agent concentration; as the initiator concentra-
tion increases, the chain length between cross-links decreases and leads to stiff; (3) using
different cross-linking agent types; the cross-linking degree of chitosan-based biomateri-
als cross-linked using EDC is lower compared with that using dialdehyde agents [76]; and
(4) using different cross-linking methods (surface cross-linking or bulk cross-link).
Covalent cross-linking treatments have been used to modulate the physical properties and
biofunctions of chitosan-based biomaterials. Usually, the hydrophilicity-hydrophobicity,
swelling behavior, biodegradation performance, mechanical properties, and biocompati-
bilities of cr-CS have a good relationship with the network structure as is illuminated in
the following.
1.
Hydrophilicity-hydrophobicity
: The hydrophilicity of chitosan comes from the
hydrophilic groups as -OH and -NH
2
of the chitosan chain. The reaction of a dial-
dehyde cross-linker with primary amino groups results in the formation of Schiff
bases. Therefore, the decline in the -NH
2
group content enables the enhancement
of the hydrophobicity of the chitosan network. The hydrophilic cross-linking agent
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