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Cite this: Soft Matter, 2012, 8, 8006
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Mechanics and physics of hydrogels
DOI: 10.1039/c2sm90083a
Hydrogels are a special class of polymeric gels that usually contain water as
the solvent, namely the dispersion
medium for the polymer network as well
as other solutes. The polymer network of
a hydrogel is crosslinked either chemically by covalent bonds or physically by
hydrogen bonds, electrostatic interactions, van der Waals forces, or physical
entanglements. Due to the hydrophilicity
of the networks, hydrogels are often
highly absorbent, capable of absorbing
up to several hundred to a thousand
times the dry weight of water.1 The
swelling caused by the absorption of
water can be triggered by various types
of environmental stimuli, such as the
changes in temperature, humidity, pH,
salt concentration, and even specific
molecules.2 The stimuli sensitivity of
hydrogels makes them ideal candidates
for applications in bioseparation,3 drug
delivery,4 microfluidics,5 and artificial
muscles,6,7 and thus the name ‘‘smart
gels’’ or ‘‘intelligent gels’’ has been gained
for hydrogels. The high water content
and biocompatibility also makes hydrogels perfect materials for tissue scaffolding8 as well as contact lenses. On the
other hand, some biological tissues and
other natural materials can also be regarded as hydrogels.9 These applications
aside, the intriguing phenomena of hydrogel systems and the complex mechanisms behind them have attracted even
more interest among scientific communities. This themed issue focuses on
bringing together recent advances in
experimental observation and characterization, theoretical analyses, and
numerical modeling of hydrogel materials and structures, as well as new
applications that utilize the unique
mechanical properties of hydrogels.
Despite the limited scope as it appears in
the title, most of the research presented
in this issue is multidisciplinary and
8006 | Soft Matter, 2012, 8, 8006–8007
involve coupling between thermal,
mechanical, and chemical fields.
Standing in a state between the
conventional regimes of fluid and solid, a
hydrogel shares some similarities with
solids and some others with fluids. The
solid-like features of a crosslinked polymer network enable it to retain its shape,
and to respond to stress or strain
through a change in shape and volume.
While simple free-swelling experiments
are often performed on gels, the swelling/
deswelling of a gel under mechanical
constraints or stress exhibits the
uniqueness of a gel in contrast to a noncrosslinked polymer solution, as reviewed in this themed issue (DOI:
10.1039/c2sm25359c). On the other
hand, the fluid-like features of a gel
enable models originating from fluiddynamics to be applied to hydrogel
systems, such as the study on the
dynamic electromechanical response of a
hydrogel-loaded microchannel (DOI:
10.1039/c2sm07467b). The peculiar lowfriction property of hydrogel surfaces
has recently attracted great interest.10 A
model that captures the effects of multilengthscale roughness on fluid lubrication between two hydrogels is presented
in this issue (DOI: 10.1039/c2sm25414j).
While common synthetic hydrogels are
often soft and brittle, making them less
useful as structural materials, some
tough hydrogels have recently been
developed by engineering their molecular
conformation and microstructures.11–14 It
is believed that the macroscopic
mechanical properties of hydrogels,
weak or strong, are correlated to their
microscopic structures, just as common
solids. In this issue, experimental
evidence through small-angle neutron
scattering has been reviewed to elucidate
the microstructural origin of the strength
of tetra-PEG gels (DOI: 10.1039/
c2sm25325a). A new material model
that better describes the behavior of
tetra-PEG gels is proposed (DOI:
10.1039/c2sm25340b). By varying the
preparation temperature, the structure of
the microcrystalite-crosslinked network
of a poly(vinyl alcohol) gel is tuned, and
the mechanical testing results demonstrated a strong correlation between the
macroscopic properties and the microstructure (DOI: 10.1039/c2sm25513h).
The mechanical performance of a triblock copolymer gel is investigated
through specially designed cavity growth
measurements
(DOI:
10.1039/
c2sm25458a). While most theories
describe a hydrogel by presuming affine
deformation, non-affine deformation of
hydrogels are reviewed in this themed
issue (DOI: 10.1039/c2sm25364j), on
both experimental and theoretical
aspects, to emphasize the effects of
microscopic heterogeneity on macroscopic properties.
Besides improved fracture resistance,
some hydrogels are made to be capable
of self-healing.15 A constitutive model is
developed to capture some mechanical
properties of a type of self-healing gel
(DOI: 10.1039/c2sm25367d). It is suggested through numerical simulations
that the concurrent solvent migration
and stress redistribution at the crack tip
of a gel may improve its mechanical
integrity via self-healing (DOI: 10.1039/
c2sm25399b), as well as delay the fracture of a gel under some circumstances
(DOI: 10.1039/c2sm25553g). A gap often
exists between experiments and theories
of hydrogels: nonlinear theories have
been developed to have high fidelity,
while some linear approximations are
usually adopted to interpret experimental data for simplicity. Through a
numerical approach, the predictions of
linear and nonlinear theories of swelling
kinetics of polymeric gels are compared
and a new procedure is suggested to fit
This journal is ª The Royal Society of Chemistry 2012
the experimental data to the nonlinear
theory (DOI: 10.1039/c2sm25467k).
Modifying the microstructures could
not only improve the mechanical properties of hydrogels, but also help in
developing hydrogels with multifunctionality. Some examples are highlighted in this themed issue. By
introducing a uniaxially aligned lamellar
structure, a highly anisotropic hydrogel
with enhanced strength and toughness
can be used as a photonic crystal (DOI:
10.1039/c2sm25670c).
A
hydrogel
synthesized with interpenetrating poly(acrylic
acid)
and
poly(N-isopropylacrylamide) networks is reported
to have sensitivity to both pH and
temperature, and the results indicate
further hydrogen-bonded complexation
(DOI:
10.1039/c2sm25389e).
The
synthesis of a novel thermosensitive
triblock-copolymer–ionic-liquid
gel
that exhibits a peculiar low-temperaturesol–high-temperature-gel transition is
also reported in this themed issue
(DOI: 10.1039/c2sm25375e). Furthermore, recent advances in photodeformable gels are reviewed together
with crosslinked liquid-crystalline polymers (DOI: 10.1039/c2sm25474c).
The large deformation associated
with the swelling or drying processes of
hydrogels may also cause surface instabilities. It is well known that the
swelling-induced
compression
may
cause creases and wrinkles on a hydrogel,16 and the resulting structure may be
used in surface patterning.17 In a polyelectrolyte gel with semi-rigid molecules,
it has been found that the mechanical
creasing during swelling could induce a
long-range
periodic
birefringence
pattern through a non-equilibrium
coupled
process
(DOI:
10.1039/
c2sm25814e). It has also been observed
that wrinkles may also form during the
drying process of a hydrogel, possibly
due to the glass-transition of the surface
layer, as reported in this issue (DOI:
10.1039/c2sm25480h). A capillary-force
induced instability that changes the
shape of the periodically distributed
holes, and modifies the surface
morphology of a porous hydrogel
membrane is also reported (DOI:
10.1039/c2sm25393c).
On the application side, the actuating
performance of a single hydrogel nanofibre was investigated using AFM, with
the results compared to that of
skeletal
muscle
(DOI:
10.1039/
c2sm25387a).
Porous
hydrogels
have long been used in cell biology.
A tough double network gel was found
to
induce
spontaneous
cartilage
regeneration in vivo.18 By using a hydrogel
with tunable stiffness as a host, the influence of mechanical signals on the biological behavior of stem cells could be
systematically studied, as reported in
this themed issue (DOI: 10.1039/
c2sm25501d). Cornea gel is a representative example of mechanically tough hydrogels in biological tissues. The
mechanical properties of cornea gel were
tested during dehydration, and the
results, which are clearly correlated to its
microstructure are reported in this
themed
issue
(DOI:
10.1039/
c2sm25370d).
Jian Ping Gong, Hokkaido University,
Sapporo, Japan.
References
1 T. K. Mudiyanselage and D. C. Neckers,
Highly absorbing superabsorbent polymer,
J. Polym. Sci., Part A: Polym. Chem.,
2008, 46, 1357–1364.
2 K. Park and H. Park, Smart Hydrogels, in
Concise Polymeric Materials Encyclopedia,
ed. J. C. Salamone, CRC Press, Boca
Raton, 1999, pp. 1476–1478.
3 C. Park and I. Orozco-Avila, Concentrating
cellulases from fermented broth using a
temperature-sensitive hydrogel, Biotechnol.
Prog., 1992, 8, 521–526.
4 Y. Qiu and K. Park, Environment-sensitive
hydrogels for drug delivery, Adv. Drug
Delivery Rev., 2001, 53, 321–339.
5 D. J. Beebe, J. S. Moore, J. M. Bauer,
Q. Yu, R. H. Liu, C. Devadoss and B.H. Jo, Functional hydrogel structures for
autonomous
flow
control
inside
microfluidic channels, Nature, 2000, 404,
588–590.
6 K. Kajiwara and S. B. Ross-Murphy,
Synthetic gels on the move, Nature, 1992,
355, 208–209.
7 Y. Osada, H. Okuzaki and H. Hori, A
polymer gel with electrically driven
motility, Nature, 1992, 355, 242–244.
8 M. D. Buschmann, Y. A. Gluzband,
A. J. Grodzinsky, J. H. Kimura and
E. B. Hunziker, Chondrocytes in agarose
culture
synthesize
a
mechanically
functional extracellular matrix, J. Orthop.
Res., 1992, 10, 745–758.
9 S. A. Wainwright, W. D. Biggs,
J. D. Currey and J. M. Gosline,
Mechanical design in organisms, Princeton
University
Press,
Princeton,
NY,
1986.
10 J. P. Gong, Friction and lubrication of
hydrogels—its richness and complexity,
Soft Matter, 2006, 2, 544–552.
11 Y. Okumura and K. Ito, Adv. Mater., 2001,
13, 485–487.
12 K. Haraguchi and T. Takehisa, Adv.
Mater., 2002, 14, 1120–1124.
13 J. P. Gong, Y. Katsuyama, T. Kurokawa
and Y. Osada, Adv. Mater., 2003, 15,
1155–1158.
14 T. Sakai, T. Matsunaga, Y. Yamamoto,
C. Ito, R. Yoshida, S. Suzuki, N. Sasaki,
M.
Shibayama
and
U.
Chung,
Macromolecules, 2008, 41, 5379.
15 D. Y. Wu, S. Meure and D. Solomon, Selfhealing polymeric materials: a review of
recent developments, Prog. Polym. Sci.,
2008, 33, 479–522.
16 T. Tanaka, S.-T. Sun, Y. Hirokawa,
S. Katayama, J. Kucera, Y. Hirose and
T. Amiya, Nature, 1987, 325, 796.
17 M. Guvendiren, S. Yang and J. A. Burdick,
Swelling-induced surface patterns in
hydrogels with gradient crosslinking
density, Adv. Funct. Mater., 2009, 19,
3038–3045.
18 K. Yasuda, N. Kitamura, J. P. Gong,
K. Arakaki, H. J. Kwon, S. Onodera,
Y. M. Chen, T. Kurokawa, F. Kanaya,
Y. Ohmiya and Y. Osada, A novel
double-network hydrogel induces spontaneous articular cartilage regeneration
in vivo in a large osteochondral defect,
Macromol. Biosci., 2009, 9, 307–316.
Wei Hong, Iowa State University, USA.
This journal is ª The Royal Society of Chemistry 2012
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