Environment–Stimuli Response of Chitosan-Based Hydrogels - Chitosan Based Hydrogels: Functions and Applications - page 246

Figure 5.17
Microstructural images of chitosan/GP hydrogels from LSCM; inset: optically magnified 10×. Chitosan composi-
tions: (a) 0.25 wt%/vol%, (b) 0.5 wt%/vol%, (c) 0.75 wt%/vol%, (d) 1.0 wt%/vol%, (e) 1.25 wt%/vol%, and (f)
1.5 wt%/vol%. White areas represent the polymer-rich phase; scale bars represent 50 μm, and 5 μm in the inset.
high-MW chitosan is optimal for hydrogel preparation; (3) chitosan solution with high
concentration are not practically useful to obtain thermosensitive hydrogel owing to their
high viscosity. In general, the concentration of chitosan solution ranges from 1.0% to 2.0%
and this is beneficial for the transition hydrogels. Two lengths are found for chitosan
chains in the chitosan/GP hydrogel, which are highly concentration dependent. The
polymer-rich hydrophobic domains of these particles have a characteristic length of 35 nm,
which decreases with increasing chitosan concentration, and the polymer-poor more
hydrophilic regions have a correlation length of 12 nm, which increases with increasing
chitosan concentration [49]; (4) the gel transition temperature decreases with increasing
amounts of GP [50]. Moreover, if the gels returned to a liquid solution, they were called
thermoreversible hydrogels, whereas those which remained as gel at low temperature
were called thermoirreversible hydrogels. To some extent, chitosan/GP is a thermorevers-
ible hydrogel, which depends on the amount of GP and chitosan in the solution. The
hydrophobic interaction is reversible under temperature stimulation, but hydrogen bonds
are not influenced by temperature. High amounts of GP make chitosan/GP irreversible. At
high concentration of GP salt, the hydrogen bonds between chitosan chains and chitosan-
water molecules predominate due to the neutralization effects of GP. Since hydrogen bonds
are not temperature dependent, cooling the hydrogel does not affect the gel structure.
Ganji et al. [51] found that as the GP salt concentration increases from 0.33 to 0.40 M in a
solution composed of 2% w/v chitosan, a thermoreversible hydrogel becomes an irrevers-
ible hydrogel. (5) The gel transition temperature is also influenced by the medium.
Chen and coworkers [52] found that the viscosity of chitosan/GP hydrogels prepared with