Introduction to In Situ Forming Hydrogels for Biomedical Applications - In-Situ Gelling Polymers: For Biomedical Applications - page 22

Fig. 12 Variation of
viscosity of chitosan-
ammonium hydrogen
phosphate solution with
time as measured using an
oscillatory rheometer at a
fixed frequency of 1 Hz and
fixed temperature of 37 °C.
Reproduced with permission
from [ 97 ]
Sharp increases in both the storage modulus (G′) and loss modulus (G″) of
PAL-PLX-PAL aqueous solutions were observed as the temperature increased
(Fig. 11 ). G′ and G″ are an elastic component and a viscous component of the
complex modulus of a system, respectively. When G′ is greater than G″, the sys-
tem is considered to be a gel, and the crossover point was defined as the sol-to-gel
transition temperature. The sol-to-gel transition temperatures defined by the test
tube inverting method coincided with those defined by dynamic mechanical analy-
sis of G′ and G″ within 2 to 3 °C. By varying the polymer concentration, not only
sol-to-gel transition temperature but also modulus of the gel could be controlled.
The control of gel modulus (G′) has a significant effect on 3D cell culture as well
as the differentiation of the stem cell. In the case of chondrocytes, the modulus of
300-2,500 Pa showed a cytocompatible microenvironment for proliferation of the
cells. The gel prepared from 10.0 wt% aqueous solution of PAL-PLX-PAL formed
a gel with a G′ of 380 Pa at 37 °C, thus being recommendable as a 3D culture
matrix for chondrocytes.
By raising the temperature above the gelation temperature, the time required
for the gelation can be determined. Nair et al. demonstrates the thermogelation
of chitosan-ammonium hydrogen phosphate solution determined as a function of
time using oscillatory rheometers [ 97 ]. The viscosity of the chitosan-ammonium
hydrogen phosphate solution was found to increase after 8 min and showed a sig-
nificant increase within 15 min of incubation at 37 °C, demonstrating the sol-gel
transition (Fig. 12 ).
6 Biomedical Applications
In situ forming hydrogels have been increasingly studied for various biomedical
applications such as drug delivery, gene delivery, wound healing, tissue engineer-
ing, and microfluidics [ 83 , 98 - 105 ]. To use hydrogel systems as drug or gene
delivery systems or tissue regeneration matrices, (1) drugs, genes, and/or cells