Formation of Chitosan-Based Hydrogels Network - Chitosan Based Hydrogels: Functions and Applications - page 212

6.9%) self-assembled nanoparticles due to the driving force of hydrophobic interaction.
These nanovesicles can be used to modify the biodistribution of PTX in vivo , which is
advantageous for enhancing the therapeutic index and reducing the toxicity of PTX [212].
Yao and coworkers [213] prepared the pH-sensitive chitosan/ovalbumin nanogels
through self-assembly; in the system chitosan chains are supposed to be partly trapped in
the nanogel core upon heating because of the electrostatic attraction between chitosan and
ovalbumin, and the rest of the chitosan chains form the shell of the nanogels. The nanogels
do not change the size distribution after long-time storage and do not dissociate in the pH
range of 2-10.5. The dispersibility, size, and hydrophobicity/hydrophilicity of the nanogel
are pH dependent. The nanogels are good candidates for cosmetic and pharmaceutical
applications. They can absorb positively charged drugs at alkaline pH through electro-
static attractions and release them at acidic pH, where nanogels also carry positive charges.
The nanogels can absorb molecules with low polarity at acidic pH, where the core of the
nanogels is relatively hydrophobic, and release them at neutral pH, where the nanogels are
more hydrophilic.
4.5.2 Self-Assembly Multilayer
The electrostatic layer-by-layer (LBL) self-assembly is based on the attraction between pos-
itively and negatively charged molecules, which seem to be a good choice as a driving
force for the multilayer's buildup [214]. Compared with the classic chemical immobiliza-
tion method, the LBL technique has the least demand for chemical bonds. The multilayers
built by the LBL method afford a more stable coating than those prepared by physical
adsorption because of the electrostatic attractions between layer and layer and between
layer and substrate. One important feature of this method is the adsorption at every step
of the polyanion/polycation assembly, which results in recharging of the outermost layer
during the film fabrication process. The overcompensating adsorption, more than equal
charge, allows for charge reversal on the surface. It has two important consequences: first,
repulsion of equally charged molecules and thus self-regulation of the adsorption and
restriction to a single layer, and second, the ability of an oppositely charged molecule to be
adsorbed in a second step on top of the first one [214]. The LBL self-assembly of polyanions
and chitosan into multilayered coatings is a versatile, inexpensive yet efficient technique
to “build” a biologically active surface. Electrostatic LBL assembly can be used to prepare
the nanoscale bioactive coating on chitosan-based biomaterials [215]. Chitosan LBL self-
assembly multilayer films open many new opportunities for us to achieve the ideal model
surface whose properties are controllable [216]. The polyelectrolyte multilayer (PEM) depo-
sition has also become a well-established methodology for clinical applications that require
biomaterials with finely engineered surfaces, such as implant materials, stents, prostheses,
and artificial organs. In these systems, the PEM mediates the cellular responses upon the
device implantation, controls processes such as inflammation and tissue regeneration, and
modulates the adhesion, migration, and proliferation of cells. The chemical composition,
surface topography, and rheological characteristics of PEMs control their functions and
their interactions in the biological milieu.
4.5.2.1 Chitosan/Protein Self-Assembly
As is known to all, collagen fibrillogenesis in vivo is a complex process that includes intra-
cellular and extracellular compartments. Collagen molecules undergo fibril assembly in