Biomimetic Polysaccharides and Derivatives for Cartilage Tissue Regeneration - Biomimetics: Advancing Nanobiomaterials and Tissue Engineering - page 9

in aqueous solutions above pH 7, due to its stability and crystalline struc-
ture. However, in dilute acids, the free amino groups are protonated and
the molecule becomes fully soluble below ~pH 5. Viscous solutions can
be extruded and gelled in high pH solutions or in baths of non-solvents
such as methanol. Such gel fi bers can be subsequently drawn and dried to
form high-strength fi bers. This polymer has been extensively studied for
industrial applications for fi lm and fi ber formation, and their preparation
procedures and their mechanical properties have been reviewed exten-
sively in the past [13, 14].
The most promising feature of CS is its excellent ability to be processed
into porous structures for use in cell transplantation and tissue regenera-
tion. Porous CS structures can be formed by freezing and lyophilizing of
CS acetic acid solutions in suitable molds [12, 24]. During the freezing
process, ice crystals nucleate from solution and grow along the lines of
thermal gradients. Lyophilization generates a porous structure with con-
trolled pore sizes through the variation of the freezing rate, i.e., the varia-
tion of ice crystal size. Pore orientation can also be directed by controlling
the geometry of thermal gradients during freezing. The mechanical prop-
erties which are critically important for any tissue engineering applica-
tions and properties of CS scaffolds are mainly dependent on the pore
sizes, pore orientations, and molecular weight of CS.
In order to improve physical, chemical, biological and mechanical
properties of chitin and CS, a variety of techniques can be applied. For
example, chemical derivatization of CS provides a powerful means to
promote new biological activities and modify its mechanical properties.
The primary amino groups on the molecule are reactive and provide a
mechanism for side group attachment using a variety of mild reaction
conditions. Generally, the addition of a side chain alters the structure of
the material and often increases the solubility of the fi nal compound, and
allows for a wide range of scaffold processing opportunity. Of course,
the precise nature of changes in chemical and biological properties
depends on the nature of the side group. The variety of groups which
can be attached to CS is almost unlimited, and side groups can be cho-
sen to provide specifi c functionality, alter biological properties, or modify
physical properties. Another example for modifi cation of CS is the physi-
cal blending approach. CS has been combined with a variety of materi-
als, such as poly(ethylenimine), poly( e -caprolactone), poly(L-lactic acid)
(PLLA), poly(2-Hydroxyethyle methacrylate) (PHEMA), poly(ethyelene
oxide) (PEO) and poly(vinyl acetate) (PVAc) for potential application
in orthopaedics and cell-based TE for cartilage regeneration [12, 34-36].
The blended CS form 3D hydrogel scaffold via hydrogen bonding
interaction, and such a system provides an excellent environment for
fetal skeletal cells growth and which is guided towards chondrogenic
differentiation [12].