Introduction to In Situ Forming Hydrogels for Biomedical Applications - In-Situ Gelling Polymers: For Biomedical Applications - page 18

Another aspect that has to be ascertained is the stability of the material prop-
erties over the time of testing. For this experiment, an oscillatory time sweep of
about 15 min, with oscillation shear stress/torque within the LVR and a frequency
of 1 Hz can be carried out. The material can be pre-sheared at a shear rate beyond
the 1st Newtonian plateau determined in the previous step. The experiment is
allowed to run and a plot of modulus against time is obtained. The point where the
modulus plateaus off is judged to be the minimum time required for the recovery
of the material structure. This was applied by Moura et al. to understand the gela-
tion kinetics and gel properties upon crosslinking the hydrogel. Both the compo-
nents G′ and G″ moduli, was monitored. Figure 9 shows the time sweep profiles
of elastic (G′) and viscous (G″) moduli near the gel point for pure chitosan solu-
tion (A) and for 0.10 % (B) and 0.15 % (C) genipin concentration networks. At
the beginning, G″ was larger than G′, which was expected because the samples
were still in a liquid state and, thus, viscous properties dominated. As the solutions
began to turn into a gel-like state due to the formation of the cross-linked network,
both moduli increased. However, the rate of increase of G′ ( ʔ G′/ ʔ t) was higher
than that of G″ because the elastic properties started to dominate. This difference
in the rates leads to a G′ and G″ crossover. The time required to achieve this cross-
over is, as mentioned above, the gelation time. From the figure, it is can be seen
that higher genipin concentrations lead to lower gelation times. It should also be
stressed that the gelation time decreases from about 8 min to about 2 min when the
genipin concentration is increased from 0 to 0.15 %.
The creep test probes the time-dependent nature of a sample. Creep and recov-
ery tests allow the differentiation between viscous and elastic responses when the
viscoelastic material is subjected to a step constant stress (creep) and then the
applied stress is removed (recovery). A standard creep experiment provides criti-
cal parameters such as zero shear viscosity ( ʷ o ) and equilibrium compliance (J eo ),
which measures the elastic recoil of a material.
After a sample is allowed to creep under load, the material's elastic behavior
can be obtained by abruptly relieving the imposed stress and measuring the extent
the sample recovers. A creep/recovery test can be carried out as follows.
• First, standard temperature conditioning and pre-shearing beyond the 1st
Newtonian plateau is performed.
• The sample is then equilibrated for a set time necessary to obtain a stable struc-
ture as determined earlier in the judgment of the material stability.
• Next, for the retardation step, a shear stress is again selected from within the 1st
Newtonian plateau and performed for about 15 min or enough time for slope to
be constant.
• Then the recovery of the sample is affected by setting the shear stress to zero
and duration for the sample to recover is examined.
During the creep test, the stress causes a transient response, including the elastic
and the viscous contributions. By following the recovery phase after the release
of the applied stress, one can separate the total strain into the instantaneous elastic
part, the recovered elastic part, and the permanently viscous part.