Application of Chitosan-Based Biomaterials in Tissue Engineering - Chitosan Based Hydrogels: Functions and Applications - page 418

hydrogels are poor. (3) Cell death usually occurs inside the hydrogel because of limited
exchange of nutrition and cell metabolism products. (4) The hydrogels can be hard to han-
dle and may be difficult to load cells. (5) The hydrogels may be difficult to sterilize.
In general, cell/hydrogels were formed by incorporating cells into thermosensitive and
covalent cross-linking chitosan-based hydrogels. For example, Wang and coworkers [66]
constructed cell/hydrogels by mixing sheep chondrocytes with a chitosan-glycerophos-
phate hydrogel. The chondrocytes remained >90% viable in the chitosan matrix after being
cultured for 1 day in vitro . On the other hand, cell encapsulation could be prepared by
combining cells in the chitosan-based polyelectrolyte complex and cross-linking the
microcapsules. For example, chitosan-alginate microcapsules have been used to encapsu-
late mammalian cells. Some of these groups used chitosan as the main matrix entrapping
the cells, while alginate was used as the coating polymer [67,68]. Other studies have used
chitosan as the coating membrane of alginate microcapsules when entrapping cells [69,70].
The remarkable long-term mechanical properties provided by the chitosan outer membrane
and the long-term viability achieved are of great advantage when considering this system
for in vivo cell-based therapy [71].
9.4 Incorporation of Growth Factors in Chitosan-Based Biomaterials
Growth factors are part of a large number of polypeptides that transmit signals affecting
cellular activities. They are not always promoting cell growth, but in many instances they
could produce a variety of products for their composition. They have very important roles
in tissue engineering. Growth factors have been introduced in many tissue-engineered
systems, by various methods, for example, (1) by addition to culture, (2) by genetically
engineering cells to overexpress growth factors, or (3) by constructing polymeric systems
that provide for the controlled release of growth factors [72]. It is known that the half-life
of growth factors is very short. Moreover, growth factors added directly to the cell culture
media or injected in vivo may be inhibited by binding proteins or the ECM before reaching
the desired target cells. The most important concern regarding the delivery of growth
factors is whether or not the released protein actually retains its biological activity [73].
Therefore, the best way to enhance the efficacy of growth is by achieving sustained release
and by maintaining a suitable concentration over an extended time period via the third
strategy. Chitosan possesses the intrinsic advantages of being used as a carrier for loading
with growth factors in order to get special bioactivity functions. Chitosan requires only
mild processing conditions, and thus can avoid growth factor inactivation under harsh
processing conditions [74]. Chitosan could substantially prolong the biological half-life
time of fibroblast growth factor (FGF) and could protect the FGF activity from inactivation,
such as heat, proteolysis, and acid. Table 9.2 summarizes chitosan and chitosan-based
growth factor release systems for tissue regeneration. Growth factors can be loaded in the
chitosan-based scaffold via physical adsorption, chemical immobilization, or incorporat-
ing the polymer microsphere containing growth factors.
In general, a biologically active chitosan-based scaffold for tissue engineering can be
prepared by soaking a chitosan-based scaffold in growth factor solution. In this case, the
growth factor can be considered only on the surface of the scaffold and so behaves as free
growth factor. The binding efficiency of growth factors on the scaffold is low and is not
easy to control. Moreover, the stability and release behaviors of growth factors are also