Biomedical Engineering Reference
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strain, at a frequency of 1 Hz), are distributed homogeneously throughout the chitosan-
collagen scaffold. Moreover, the localizations of ECM proteins, such as vimentin, fibronec-
tin, and collagen type I, are consistent with the distribution of cellularity. The application
of a cyclic strain to fibroblast-scaffold constructs may increase cell survival and integra-
tion of the artificially engineered dermis at the graft site [111]. Third, various growth fac-
tors, such as FGFs, encoding VEGFs, and EGFs, function in the process of wound healing.
They can not only modulate the growth behaviors of fibroblasts but also enhance angio-
genesis of the engineered constructs. Chitosan-based biomaterials-loaded FGFs can
improve the attachment, proliferation, and production of the GAG capacity of fibroblasts
[112]. For example, fibroblasts in chitosan-gelatin scaffolds with bFGF-loaded chitosan-
gelatin microspheres achieve a relatively homogeneous distribution over time, which in
turn resulted in a relatively homogeneous ECM accumulation (
cf.
Figure 9.21)
[113]. More
importantly, the transcript level of laminin, which is one of the most important biological
noncollagenous glycoproteins that participate in angiogenesis, is markedly upregulated
owing to the incorporation of FGFs or VEGFs. For example, the
N
,
N
,
N
-trimethyl chitosan
chloride-pDNA-VEGF complexes-loaded chitosan-collagen scaffold could accelerate the
angiogenesis in which the numbers of newly formed and matured vessels were all
increased.
During the formation process of new dermis, a temporary epidermal layer on the
chitosan-based scaffold is required for the dermal equivalent to be used practically, which
can play a role in controlling water loss and inhibiting bacterial entry until an ultrathin
epidermal autologous graft is applied [114]. The bilayer structure of chitosan-based film and
scaffold has been designed to realize this function. It was processed successively via the for-
mation of a dense chitosan film by the casting method and a porous chitosan sponge by
lyophilization. The dry thickness of the film layer was 19.6 μm and that of the scaffold layer
was controlled at 60-80 μm. The film layer acts as an obstacle to prevent bacterial infection
and to control the loss of body fluid, while the scaffold layer of chitosan allows the ingrowth
of dermal fibroblasts on a wound bed [115]. Bilayer collagen-chitosan scaffold covered with
a silicone film layer is prepared by Gao and coworkers (
cf.
Figure 9.22)
[116]. Comparing
with the commercial product Integra® (composed of a biodegradable collagen scaffold and
a silicone membrane), the bilayer chitosan-based scaffold can remain stable in shape and
size during cell culture. Moreover, the chitosan-based scaffold indeed has the ability to
regenerate a damaged dermis with a similar structure as the normal skin [117].
9.5.2.3 Full-Thickness Replacement
The skin, which contains the epidermis and the dermis, is the body's barrier against
external harmful factors. Therefore, tissue-engineered skin should contain epidermal and
dermal structures to realize the physiological function of the skin. The main objective of
tissue engineering skin is to achieve permanent skin regeneration with both dermal and
epidermal tissues. Keratinocytes and fibroblasts exhibit different cell behaviors on the
chitosan-based scaffold. Keratinocytes prefer a more hydrophobic surface than fibroblasts.
The relative viability values for both the attachment and the proliferation of keratinocytes
on both chitosan scaffolds ranged between 77% and 140% (relative to those on TCPS)
and that of fibroblast ranged between 35% and 62% (relative to those on TCPS) [117].
Therefore, asymmetric chitosan-based hydrogels or scaffold should be designed to obtain
the epidermal-dermal replacement.
Yao and coworkers [8] prepared bilayer chitosan-gelatin and chitosan-gelatin-HA scaf-
folds via the freeze-drying technique. First, fibroblasts are seeded in chitosan-gelatin and
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