Future - Chitosan Based Hydrogels: Functions and Applications - page 470

prevented from dissolving due to the chemical or physical bonds formed between the poly-
mer chains. Water penetrates these networks causing swelling, giving the hydrogel its form.
Fully swollen hydrogels have some physical properties common to living tissues, includ-
ing a soft and rubbery consistency, and low interfacial tension with water or biological
fluids. The elastic nature of fully swollen or hydrated hydrogels has been found to mini-
mize irritation to the surrounding tissues after implantation. The low interfacial tension
between the hydrogel surface and body fluid minimizes protein adsorption and cell adhe-
sion, which reduces the chances of a negative immune reaction [26].
Despite these many advantageous properties, hydrogels also have several limitations.
The low tensile strength of many hydrogels limits their use in load-bearing applications
and can result in the premature dissolution or flowing away of the hydrogel from a tar-
geted local site. This limitation may not be important in many typical drug delivery appli-
cations (e.g., subcutaneous injection). More important, perhaps, are problems relating to
the drug delivery properties of hydrogels. The quantity and homogeneity of drug loading
into hydrogels may be limited, particularly in the case of hydrophobic drugs. The high
water content and large pore sizes of most hydrogels often result in relatively rapid drug
release, over a few hours to a few days. Ease of application can also be problematic; although
some hydrogels are sufficiently deformable to be injectable, many are not, necessitating
surgical implantation. Each of these issues significantly restricts the practical use of hydro-
gel-based drug delivery therapies in the clinic.
Recently, researchers have developed other hydrogels using chitosan copolymers in
combination with poly( N -isopropyl acrylamide) and poloxamers whose hydrophobic
group interactions dominate at elevated temperatures. These polymers have been recog-
nized as good candidates for in situ , reversible hydrogel formation [27].
10.4 Challenge and Adaptability for Chitosan-Based Gels
10.4.1 Multiple layers for Soft-Tissue regeneration
Soft-tissue implant attempts to replace or augment most of the soft tissues in the body,
such as artificial skin, ligament, tendon, cartilage, blood vessels, heart valves, and so on.
Rives et al. [28] proposed a layering approach to fabricate plasmid-releasing scaffolds that
provide localized transgene expression following implantation into intraperitoneal fat, a
model site for cell transplantation. In our previous research, a novel absorbable scaffold
composed of chitosan and gelatin was fabricated by freezing and lyophilizing methods,
resulting in an asymmetric structure [29]. This bilaminar texture is suitable for preparing
a bilayer skin substitute. The chitosan-gelatin scaffolds were more wettable and adsorbed
more water than did chitosan alone. Keratinocytes were cocultured with fibroblasts in
chitosan-gelatin scaffolds to construct an artificial bilayer skin in vitro . The artificial skin
obtained was flexible and had good mechanical properties. Moreover, there was no
contraction observed in the in vitro cell culture tests. The results suggested that chitosan-
gelatin scaffolds were suitable for skin tissue engineering goals.
10.4.2 Vascularization and Structure for Hard Tissue regeneration
Although tissue engineering has made significant progress in culturing large amounts of
cells in vitro and in the design and usage of support materials to deliver the cells in vivo , the