Biomedical Engineering Reference
In-Depth Information
Fig. 10
Creep (
open
symbols
) and recovery (
full
symbols
)
curves
for the
poly(isopropyl lactate diol)-
based polyurethane hydrogels
at 37 °C when a stress of
5 Pa was applied for 60 s.
Reproduced with permission
from [
96
]
Viscoelastic creep data can be presented by plotting the creep modulus (con-
stant applied stress divided by total strain at a particular time) or the strain, as
a function of time. Gradinaru et al. studied the creep response of thermogelling
poly(isopropyl lactate diol)-based polyurethane hydrogels [
96
].
Figure
10
shows the curves that represent the viscoelastic response at an
applied stress of 5 Pa for the three hydrogels obtained at 37 °C, in a creep test fol-
lowed by recovery. The creep curves comprise three parts: the instantaneous strain,
the retardation strain, and the viscous strain. When the applied stress is removed,
the recovery process starts, and first the instantaneous strain is recovered, then the
retardation one, and finally remains the viscous part. The high elasticity of the
hydrogels can be observed, where the reached strain after the stress of 5 Pa was
applied for 60 s is very high, and the recovered strain represents 52 % from the
maximum value reached by the strain in the creep test.
Changes in modulus of thermogelling polymer aqueous solutions can be deter-
mined by dynamic rheometry (Fig.
11
).
• First, standard temperature conditioning at the lower solution temperature 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 at the lower temperature as determined earlier in the judgment of the mate-
rial stability.
• Next, the material is subjected to a temperature ramp at a ixed stress and a
fixed frequency rate.
• The point at which the elastic and loss modulus intersects is deined as the gel
transition temperature.
Jeong et al. reported poly(alanine-co-leucine)-poloxamer-poly(alanine-co-leucine)
(PAL-PLX-PAL) aqueous solution [
62
]. As the temperature increased, the polymer
aqueous solution underwent sol-to-gel transition at 20
−
40 °C in a polymer con-
centration range of 3.0
−
10.0 wt%. The sol-gel transition of the polymer aqueous
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