Introduction to In Situ Forming Hydrogels for Biomedical Applications - In-Situ Gelling Polymers: For Biomedical Applications - page 14

Fig. 4 Typical graph
showing storage and loss
modulus

G', Storage modulus

G”, Loss modulus

Strain amplitude γ 0

material. If tan ʴ > 1 (G″ > G′), the sample behaves more like a viscous liquid
while, conversely, when tan ʴ < 1 (G′ > G″), the sample behaves more like an
elastic solid (Fig. 4 ).
For gel samples, these parameters are often measured as a function of time,
strain and frequency. Observation of the gelation process can be achieved by mon-
itoring the temporal evolution of G′ and G″. The linear viscoelastic region within
which G′ and G″ are independent of shear strain can be determined by monitoring
the moduli of the material as a function of the strain.
The behavior of the hydrogel at short and long timescales can be studied by
measurement of the moduli of the material as a function of frequency. The fre-
quency dependence of the moduli is a critical hydrogel parameter since a single
material can look quite solid-like (G′ G″) at a high frequency (short timescale)

but behave much more liquid-like (G″ > G′) at low frequency (long timescale).
Gelation kinetics and final gel stiffness are critical material properties that directly
impact the application of the material.
Besides small perturbation measurements, creep and creep recovery tests are
also employed to investigate the time-dependent evolution of compliance. This
aids in the critical understanding of the long-term viscoelastic behavior of hydro-
gels. Different mammalian cells exert different stress levels on the hydrogel scaf-
folds and they behave differently in response to the compliance of the gel material.
In typical experimental setups, creep and creep recovery tests are performed con-
secutively. For this experiment, there is an instantaneous increase in the stress
from 0 to ˄ 1 . This is kept constant from t 0 to t 1 in the creep phase to subject the
material to a prolonged period of stress. Then the stress is completely removed
in the subsequent recovery phase. The resulting strain is recorded as a function
of time (t 0 < t < t 2 ) in both tests. The creep compliance is defined as J(t) = ʳ (t)/
˄ 0 which has a unit of reciprocal modulus (Pa − 1 ). Within the linear viscoelastic
region, the creep compliance is independent of applied stress and all J(t) curves
obtained under various stresses should overlap with each other. Sometimes creep

≫