tissue engineering scaffold, meniscus implants, and dental implants [76–78]. Putra
et al. [79] synthesized a tubular BC gel with proper fibril orientation created by
culturing BC in oxygen-permeable silicone tubes with inner diameter <8 mm. The
fibrils are oriented along the longitudinal axis of the silicone tube, independent of
gravity, oxygen availability, and the morphology of the inner surface of the silicone
tube but dependent on the curvature of the silicone tube. The obtained BC-based
hydrogels with a uniaxially oriented fibril structure have excellent mechanical
properties and hold promise for use as a microvessel or soft tissue material in
medical and pharmaceutical applications.
2.2 Hydrogels from Cellulose Derivatives
The biocompatible cellulose derivatives are most widely applied because of their
water solubility contributing to fabrication of hydrogels, which can be used as
thickener, binding agents, emulsifiers, surfactants, lubricants, and stabilizers, as
well as additives in food, pharmaceutical, and cosmetic industries. Cellulose derivatives including methyl cellulose (MC), hydroxypropyl cellulose (HPC),
hydroxypropylmethyl cellulose (HPMC), and carboxymethyl cellulose (CMC)
have been used to prepare cellulose-based hydrogels through physical cross-linking
and chemical cross-linking. The cellulose derivatives of aqueous solutions can be
transformed into the unique hydrogels featured with thermoset and pH-sensitive
properties by inducing hydrophobic or electrostatic associations [80].
2.2.1 Physical Cross-Linking
Hydrophobic modification provides access to thermo-reversible hydrogels for cellulose derivatives. When hydroxyl groups of cellulose are partly substituted by
methyl groups or hydroxypropyl groups, the formation of intermolecular hydrogen
bonds tends to be limited, and the resultant derivatives become water-soluble. For
example, an elastic and thermo-reversible MC gel was obtained when the MC
aqueous solution was heated above a critical temperature, and this gelation behavior
was demonstrated to be MC concentration-dependent, e.g., the critical temperatures
were 63
C at 0.30–2.5 wt% [81], 42.5
C at ca. 4.7 wt% [80], 32
C at 7.0 wt%, and
27
C at 9.0 wt% [82]. The proposed mechanism of gelation is determined by the
solvent reorganization under heating MC solution. In detail, the originally formed
solvated cage-like MC structure by hydrogen bonds of water along MC chains was
destroyed and thereby exposed their hydrophobic regions, leading to the formation
of hydrophobic aggregates [81]. Similarly, the formation of HPMC gels also resulted
from the hydrophobic aggregates but displayed the slightly higher gelation temperature (e.g., 70
C at 2 wt%) than MC solutions, inferring that the hydroxypropyl
substituents inhibited the gelation [83–85].
326
C. Shao and J. Yang
et al. [79] synthesized a tubular BC gel with proper fibril orientation created by
culturing BC in oxygen-permeable silicone tubes with inner diameter <8 mm. The
fibrils are oriented along the longitudinal axis of the silicone tube, independent of
gravity, oxygen availability, and the morphology of the inner surface of the silicone
tube but dependent on the curvature of the silicone tube. The obtained BC-based
hydrogels with a uniaxially oriented fibril structure have excellent mechanical
properties and hold promise for use as a microvessel or soft tissue material in
medical and pharmaceutical applications.
2.2 Hydrogels from Cellulose Derivatives
The biocompatible cellulose derivatives are most widely applied because of their
water solubility contributing to fabrication of hydrogels, which can be used as
thickener, binding agents, emulsifiers, surfactants, lubricants, and stabilizers, as
well as additives in food, pharmaceutical, and cosmetic industries. Cellulose derivatives including methyl cellulose (MC), hydroxypropyl cellulose (HPC),
hydroxypropylmethyl cellulose (HPMC), and carboxymethyl cellulose (CMC)
have been used to prepare cellulose-based hydrogels through physical cross-linking
and chemical cross-linking. The cellulose derivatives of aqueous solutions can be
transformed into the unique hydrogels featured with thermoset and pH-sensitive
properties by inducing hydrophobic or electrostatic associations [80].
2.2.1 Physical Cross-Linking
Hydrophobic modification provides access to thermo-reversible hydrogels for cellulose derivatives. When hydroxyl groups of cellulose are partly substituted by
methyl groups or hydroxypropyl groups, the formation of intermolecular hydrogen
bonds tends to be limited, and the resultant derivatives become water-soluble. For
example, an elastic and thermo-reversible MC gel was obtained when the MC
aqueous solution was heated above a critical temperature, and this gelation behavior
was demonstrated to be MC concentration-dependent, e.g., the critical temperatures
were 63
C at 0.30–2.5 wt% [81], 42.5
C at ca. 4.7 wt% [80], 32
C at 7.0 wt%, and
27
C at 9.0 wt% [82]. The proposed mechanism of gelation is determined by the
solvent reorganization under heating MC solution. In detail, the originally formed
solvated cage-like MC structure by hydrogen bonds of water along MC chains was
destroyed and thereby exposed their hydrophobic regions, leading to the formation
of hydrophobic aggregates [81]. Similarly, the formation of HPMC gels also resulted
from the hydrophobic aggregates but displayed the slightly higher gelation temperature (e.g., 70
C at 2 wt%) than MC solutions, inferring that the hydroxypropyl
substituents inhibited the gelation [83–85].
326
C. Shao and J. Yang