Environment–Stimuli Response of Chitosan-Based Hydrogels - Chitosan Based Hydrogels: Functions and Applications - page 242

5.3.1 Chitosan/Polyol Thermosensitive Hydrogels
Polyol can stabilize certain compounds in aqueous solutions and promote the formation of
a shield of water around some macromolecules or polymer chains [30]. Therefore, when
hydroxyl composites are incorporated into a chitosan network, chitosan chains can main-
tain a certain stability and cannot build up a 3D network structure at low temperature
because of the difficulty of creating contacts between the junction chains. However, at high
temperature, water molecules have enough energy from the shackles of chitosan chains.
The dewatered chitosan chains associate with each other via hydrophobic interaction.
Thus, many hydroxyl groups, composites, or polymers are introduced into the chitosan
network via chemical modification, grafting, or blending to obtain chitosan-based thermo-
sensitive hydrogels.
5.3.1.1 Hydroxybutyl Chitosan Derivative Thermosensitive Hydrogels
Hydroxybutyl chitosan, a promising thermoresponsive polymer, is synthesized by conju-
gation of hydroxybutyl groups to the hydroxyl and amino reactive sites of chitosan. It can
be easily applied as a mildly viscous solution without spatial restriction, but quickly trans-
forms into a pliable and durable hydrogel when the temperature is increased. Moreover,
this process is reversible. At temperatures below LCST, hydrogen bonds exist not only
between the OH group of hydroxybutyl groups and the OH and NH 2 groups of the chito-
san chains but also between the hydroxybutyl groups and water. That is to say, hydroxy-
butyl chitosan chains are surrounded by water molecules. When the temperature is higher
than LCST, the intermolecular hydrogen-bonding interactions decrease and the energized
water molecules surrounding the hydroxybutyl chitosan are removed. Hydrophobic inter-
actions between interchains of hydroxybutyl chitosan become more and more strong,
which results in hydrogel formation. Moreover, the hydrogels can transform into solution
when the temperature decreases to below LCST [31].
LCST of hydroxybutyl chitosan depends on the molecular weight (MW) and SD of
hydroxybutyl. The higher the MW the lower the LCST. The lower the SD the higher the
LCST [32]. Gel transformation time is related to temperature. Therefore, the gel is formed
in <100 s when the temperature is 25°C, which is higher than the LCST (20°C, MW
900 kDa, and SD 1.23). However, when the temperature is elevated to 37°C, gel is
formed quickly in <50 s [31]. For hydroxybutyl chitosan hydrogel films, the film surface
appears to be rough and is filled with small agglomerates with different heights
when the temperature is higher than LCST ( cf. Figure 5.13) . The result is likely caused by
the increase of hydrophobicity of the polymer chain and the subsequent dewetting of the
chitosan films. The film surface is significantly smoother when the temperature is below
LCST [33].
5.3.1.2 Chitosan/PEG Thermosensitive Hydrogels
PEG is also used in many biomedical applications due to its outstanding physicochemical
and biological properties such as hydrophilicity, biocompatibility, and lack of toxicity. PEG
solutions are known to become less soluble and precipitate at higher temperatures in
aqueous solutions due to a conformational transition to a less polar form. Chitosan/PEG
composite hydrogels exhibit excellent thermosensitivity. For chitosan-g-PEG, at low tem-
peratures, chitosan chains are covered with water molecules attached by hydrogen bonds
between hydrophilic groups of PEG and water molecules. Thus, direct association between