Introduction to In Situ Forming Hydrogels for Biomedical Applications - In-Situ Gelling Polymers: For Biomedical Applications - page 11

as an informal British Rheologists' Club in 1940. In the following years, Society
of Rheology/Group of Rheology was founded in countries like Germany, Austria,
Belgium, Sweden, Czech Republic, France, Italy, Israel, Japan and Australia. Many
of these societies/groups became members of the European Society of Rheology
later, greatly promoting the development of rheology. Basically, rheology is defined
as the science of the deformation and flow of matter, thus it mainly focuses on
the investigation of the time-dependent strain and flow properties of soft materi-
als under conditions like stress, strain, temperature, humidity and radiation [ 88 ].
Rheological parameters include the physic-mechanical properties of liquids and sol-
ids which describe strain and flow behavior. When external forces are exerted on the
materials, strain can be measured and experimentally studied.
The viscoelastic properties of hydrogels can be determined by rheometry. The
basics and theories of rheology, its measurements and the types of equipment can
be found in several seminal publications [ 89 , 90 ].
In rheology, the variable shear stress, ˄ , is defined as the ratio of the force F
applied on a sample area A to cause the disruption of the material between the two
plates. The strain, ʳ , is defined as the ratio of the deviation of x of the sample to the
height of the sample, h or more simply defined as tan ʱ . The velocity of the move-
ment at the applied force is controlled by the internal force acting within the material.
The mechanical properties of hydrogels are determined by small perturbation
rheology experiments on hydrogels. When the hydrogel is subjected to a small
perturbation, the material particles are displaced relative to each other result-
ing in strain. When external anisotropic forces are exerted on elastic bodies, they
undergo elastic strain. A spontaneous full recovery of the original form of the
material results when the external force is removed. On the contrary, the strain
on viscous bodies is irreversible once external anisotropic forces are exerted. The
input energy is transformed and this causes the material to flow. Hydrogel materi-
als are neither completely viscous nor elastic; instead it exhibits a behavior known
as viscoelasticity. These small perturbations of the hydrogels are meant to ensure
that the rheology experiment is carried out within the linear viscoelastic region
(LVR) of the material, hence ensuring the measured properties of the hydrogels
are independent of the magnitude of imposed strain or stress. In addition, the lin-
ear viscoelasticity region is when the magnitude and stress are related linearly.
When small deformation is applied sufficiently slowly, the molecular arrange-
ments of the polymers are still close to equilibrium. The mechanical response is
then just a reflection of dynamic processes at the molecular level, which go on
constantly, even for a system at equilibrium (Fig. 1 ).
Small amplitude oscillatory shear measurements, creep and creep recovery tests
are examples of small perturbation tests carried out on hydrogels. The principle of
small amplitude oscillatory shear measurements is shown in Fig. 2 .
By applying shear stress, a laminar shear flow is generated between the two plates.
The uppermost layer moves at the maximum velocity V max , while the lowermost
layer remains at rest. The shear rates of typical actions are summarized in Table 1 .

= DV
DH

SHEAR RATE ˙

γ