Application of Chitosan-Based Biomaterials in Tissue Engineering - Chitosan Based Hydrogels: Functions and Applications - page 449

with surgery, and immune suppression. Liver tissue engineering, where one seeks to
augment liver function by implanting functional hepatocytes, offers an attractive alterna-
tive. The difficulty of liver regeneration lies in the vast complexity of tissue [199]. It is a
highly organized structure and the 3D arrangement of hepatic cells is integral to its func-
tions. Primary hepatocytes rapidly lose tissue-specific functions once they are removed
from the living organism. In contrast to other simple structural tissues, such as bone and
cartilage, the liver must carry out complex metabolic functions, such as biosynthesis,
biotransformation, and excretion. Unlike the bone and cartilage, liver tissue has no inher-
ent mechanical function. That is to say, it is very difficult to mimic the structure of nature.
The field of liver regeneration remains one of the biggest challenges for tissue engineer-
ing. The chitosan-based biomaterials can be considered as a potential scaffold to support
liver regeneration. At present, however, research on liver tissue engineering is at the ini-
tial stage.
In the human liver, hepatic cells are arranged in an intricate manner, enabling optimal
communication and attachment among cells. Hepatocytes in vivo survive in a 3D system
that is formed by various kinds of ECMs such as collagen, proteoglycan, fibronectin, and
laminin. Chitosan-based biomaterials can provide an appropriate microenvironment
for the growth of hepatocyte due to its various properties. It is evident that several liver-
specific functions, such as albumin secretion and urea synthesis, could be enhanced when
culturing hepatocytes in porous chitosan scaffolds in vitro [200]. In order to provide a bet-
ter microenvironment, collagen or gelatin was introduced into the chitosan-based net-
work. Hepatocytes maintain viability and perform biological functions in chitosan-gelatin,
and the chitosan-gelatin is more efficient in inducing fibrin formation and vascularization
at the implant-host interface [201,202].
Asialoglycoprotein receptors (ASGPRs) are expressed on the surface of hepatocytes.
Therefore, some ligands (galactose and fructose) that can recognize ASGPR are usually
combined into the chitosan scaffold. The scaffold with ligands can provide a new synthetic
ECM for hepatocyte attachment through the specific interactions between ASGPR on
hepatocytes and ligands. Moreover, they can improve hepatocyte attachment and main-
tain viability via the specific interactions between ASGPR on hepatocytes and galactose
ligands. For example, alginate-galactosylated chitosan scaffolds have the potential ability
to improve hepatocyte attachment for short-term culture. Galactose ligands facilitate hepa-
tocyte aggregation in alginate-galactosylated chitosan scaffolds, resulting in the mainte-
nance of high cell activity by the intercellular adhesion molecules in the 3D coculture
system [203,204]. Meanwhile, fructose-modified chitosan porous scaffolds induced the for-
mation of cellular aggregates with enhancing the liver-specific metabolic activities and cell
density to a satisfactory level [205].
Pore size plays important roles in liver regeneration. Microspores (1-100 μm) signifi-
cantly improve hepatocyte attachment and albumin secretion [206]. This character can be
realized via freeze-drying technology. However, artificial liver manufacture requires a
careful interplay of many design parameters owing to the complexity of liver tissue. Precise
control of internal pore architecture parameters as well as the ECM components is essen-
tial to maintain the liver functions. Li and coworkers [9,207] prepared 3D chitosan-gelatin
scaffold using RP technology. The scaffold possesses multilevel organized internal mor-
phologies including vascular systems (portal vein, artery, and hepatic vein) and paren-
chymal component (hepatocyte chamber) ( cf. Figure 9.39). The smallest channels are
approximately 150 μm in width and the smallest distance between channels and chambers
is about 170 μm. The scales for the hepatocyte chambers are 200 μm in width and 580 μm
in length. Moreover, the volume between the blood vessels and the hepatic chambers is a