Formation of Chitosan-Based Hydrogels Network - Chitosan Based Hydrogels: Functions and Applications - page 210

Amphiphilic chitosan-based biomaterials are developed by conjugating hydrophobic
groups and/or hydrophilic groups into chitosan molecules. Amphiphilic chitosan can
form nanosized self-assembly particles with a hydrophobic core and a hydrophilic shell
due to the intra- and/or intermolecular interactions of hydrophobic segments in the aque-
ous media. The hydrophobic microdomains are formed by the association of hydrophobic
groups. Chitosan backbones coil to form the hydrophilic shells outside these hydrophobic
microdomains. Furthermore, inter- and/or intramolecular hydrogen bondings among
tightly packed chitosan backbones will promote the self-assembly of hydrophobically
modified polysaccharides ( cf. Figure 4.17) [203]. These self-assemblies have potential
applications in biotechnology and medicine due to their unique supramolecular
structures.
Grafting an amphiphilic biblock copolymer on chitosan molecules can also endow the
chitosan with amphiphilicity. Amphiphilic N- phthaloylchitosan-grafted mPEG (PLC-g-
mPEG) is a good model to fulfill the conditions for colloidal phenomena where the nano-
spheres are induced. The stability of the appearance of the milky solution is enhanced in
protic solvents where hydrogen bonds are accomplished. And the nanosize can be con-
trolled by the length of the mPEG chain. mPEG with M n = 5 kDa gives spheres with sizes
about 80-100 nm, whereas mPEG with M n = 550 Da provides spheres with an average
size of 400-500 nm [204,205]. The favorable conditions for the guest molecules to incor-
porate into chitosan nanosphere are high hydrophobicity of the guest itself and positive
charge. An effective guest incorporation is achieved by dispersing the PLC-g-mPEG
nanosphere in a guest-isopropanol solution, or so-called the heterogeneous system ( cf.
Figure 4.18) [206,207].
PLC-g-mPEG forms a core-shell micellar structure after dialysis of the polymer solutions
in dimethyl sulfoxide or dimethylformamide (DMF) against water. Camptothecin (CPT)
can load into the inner core of the micelles by the dialysis method with high stability and
with a sustained release against PBS buffer, pH 7.4. Phthaloylchitosan units of the hydro-
phobic chain of the polymers and drug contents are important factors for the incorporation
Hydrophilic chitosan
backbone
Hydrophobic
micro-domain
Hydrogen bonding between the polar groups
Figure 4.17
Schematic representation of amphiphilic chitosan self-assembly nanoparticles. (From Wang, Y. S. et al. 2007.
Eur Polym J 43: 43-51. With permission.)