Introduction to In Situ Forming Hydrogels for Biomedical Applications - In-Situ Gelling Polymers: For Biomedical Applications - page 10

of the gel is an important consideration for cell growth scaffolds. The stiffness
of hydrogels has been reported to direct the differentiation of different cell types
[ 79 - 81 ]. For drug delivery, hydrogels should preferentially reduce in viscosity
upon injection and undergo rapid recovery upon removal of the stress to form the
drug release gel depot. This design principle has been the basis of several in situ
thermogelling polymeric networks [ 5 , 82 - 84 ]. Finally, rheological measurements
allow for the understanding of the different gelation mechanisms which can be uti-
lized in the optimization of the properties of the hydrogels for tissue reconstruc-
tion and drug delivery applications.
The flow and strain properties of soft materials have been extensively investi-
gated since the 17th century. In the 1830s, scientists discovered that many materi-
als possess time-dependent mechanical properties under various conditions, which
cannot be explained by the classical theory of Newtonian fluid. For example, in
1835, Weber observed the phenomenon of elastic hysteresis when he studied the
uranium filament. In 1865, Lord Kelvin discovered the viscosity behavior of zinc,
and that its inner impedance was not proportional to the strain rate. Two years later,
Maxwell proposed a model for viscoelastic materials having properties both of vis-
cosity and elasticity. The Maxwell model can be simply represented by the series
connection of a purely elastic spring and a purely viscous damper. At the same time,
scientists also found many fluids, which were all called non-Newtonian fluid later
due to the nonlinear relationship between the shear stress and shear rate. Based on
the known constitutive equation, people proposed the concept of stress relaxation
time, suggesting the viscosity to be the product of the elastic modulus and the stress
relaxation time. In 1874, Boltzmann developed the linear viscoelasticity theory,
suggesting that the stress in a given time is not only related to the strain in the given
time, but also dependent on its previous deformation. In 1940s, Reiner pointed
out that in order to eliminate the Weissenberg effect (The Weissenberg effect is a
phenomenon that occurs when a spinning rod is inserted into a solution of liquid
polymer. Instead of being thrown outward, the solution is drawn towards the rod
and rises up around it), a stress proportional to the square of the spinning speed
needs to be applied [ 85 ]. Almost at the same time, Rivlin's study on the torsion of
a rubber cylinder helped to solve the problem of Poynting effect [ 86 ]. The intrin-
sic significance of these two studies is to further apply the generalized approach
regarding the nonlinear constitutive equation, which brought in flourishing progress
in the field of rheology. With the advance in rational mechanics, from small defor-
mation theory to finite deformation theory, from linear theory to nonlinear constitu-
tive theory, from classical object model to microstructure theory, rheology rapidly
advanced after 1965, moving from phenomenological theory, which describes phe-
nomena only into the ontology, which considers the internal structure. The term
“rheology” was first coined by Bingham and Reiner in 1929 when the American
Society of Rheology was founded in Columbus, Ohio [ 87 ]. This term was inspired
by a Greek quotation, “panta rei”, “everything flows”. In the same year, Journal
of Rheology started its publication. In 1932, the Committee on Viscosity of the
Academy of Sciences at Amsterdam was founded, which was later renamed The
Dutch Rheological Society in 1951. The British Society of Rheology was founded