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Polymer Gels
Yoshihito Osadaa; Jian Ping Gonga; Yutaka Tanakaab
a
Division of Biological Sciences, Graduate School of Science, Hokkaido University, Sapporo, Japan b
Faculty of Engineering, Department of Materials Science and Engineering, University of Fukui,
Bunkyo, Fukui, Japan
Online publication date: 09 February 2004
To cite this Article Osada, Yoshihito , Gong, Jian Ping and Tanaka, Yutaka(2004) 'Polymer Gels', Polymer Reviews, 44: 1,
87 — 112
To link to this Article: DOI: 10.1081/MC-120027935
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JOURNAL OF MACROMOLECULAR SCIENCEw
Part C—Polymer Reviews
Vol. C44, No. 1, pp. 87–112, 2004
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Polymer Gels#
Yoshihito Osada,* Jian Ping Gong, and Yutaka Tanaka‡
Hokkaido University, Sapporo, Japan
ABSTRACT
An overview was given on polymer gel with respect to its structure, gelation process,
properties, and applications. In a structural analysis it is necessary to have a different
scale depending on the gel structure observed. Properties and responses of polymer gels
were widely described with the explanation for solvent properties and phase transition,
thermoresponse, chemical response, and electric properties. The potential application
of gels has also been shown concerning biomedical use, drug delivery system, and
selective separation.
Key Words:
Applications.
Preparation; Properties; Thermoresponses; Chemical responses;
I.
INTRODUCTION
A polymer gel consists of an elastic cross-linked polymer network with a fluid filling
the interstitial space of the network. The network of polymer molecules holds the liquid in
place and so gives the gel what rigidity it has. Gels are wet and soft and look like solid
#
Reprinted from Functional Monomers and Polymers, Takemoto, K.; Ottenbrite, R. M.; Kamachi,
K., Eds.; Marcel Dekker, Inc.: New York, 1997; 497– 528.
*Correspondence: Yoshihito Osada, Division of Biological Sciences, Graduate School of Science,
Hokkaido University, Sapporo, Japan; E-mail: osada@sci.hokudai.ac.jp.
‡
Current address: Faculty of Engineering, Department of Materials Science and Engineering,
University of Fukui, Bunkyo, Fukui, 910-8507 Japan.
87
DOI: 10.1081/MC-120027935
Copyright # 2004 by Marcel Dekker, Inc.
1532-1797 (Print); 1532-9038 (Online)
www.dekker.com
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Osada, Gong, and Tanaka
material, but are capable of undergoing large deformations. This is in contrast with most
industrial materials, such as metals, ceramics, and plastics, which are dry and hard. Living
organisms are largely made of gels. Except for bones, teeth, nails, and the outer layers of
skin, mammalian tissues are highly aqueous gel materials that are largely composed of
protein and polysaccharide networks in which the water contents range up to 90% (blood
plasma). This makes it easier for the organism to effectively transport ions and molecules
while keeping its solidity.
There are a variety of ways to classify gels, such as natural gel or synthetic gel,
according to the source; hydrogel or organogel, according to the liquid medium in the
polymer network; and chemical or physical gels, according to their cross-linkage.
A hydrogel is a polymeric material that exhibits the ability to swell in water and
absorb a significant fraction (about 3000 times) of water within its structure, but that will
not dissolve in water. A wide variety of natural materials of both plant and animal origin,
materials prepared by modifying naturally occurring structures, and cross-linked synthetic
polymeric materials are hydrogels.
Typical natural gels can be formed simply by cooling aqueous solutions of biological
proteins or polysaccharides, such as gelatin, pectin, agarose, carrageenan, and agar.[1]
Fibrin clots are typical biological gels that are formed by the polymerization of fibrinogen
monomers through a series of enzymatic reactions.[2] Several organic systems prepared by
synthetic methods exhibit similar properties. Dilute solutions of poly(vinyl chloride) in
di(2-ethylhexyl)phthalate form gels on cooling.[3]
Typical examples of covalently cross-linked networks are a styrene – divinylbenzene
copolymers swollen in an organic solvent and ion-exchange resins made from cross-linked
polystyrene sulfonate, poly(p-aminostyrene), or 2-hydroxyethyl methacrylate –ethylene
Table 1.
Swelling medium
Classification of polymer gels.
Solid– liquid
Solid– gas
Solid– solid
Constituent
polymers
Natural gel
Synthetic gel
Hybrid gel
Cross-linkage
Covalent bonding
Molecular
interaction
Hydrogel
Organogel
Liogel
Alcogel
Xerogel
Aerogel
Polymer – gel polymer
Gel – gum
Protein gel,
polysaccharide gel
Organic polymer gel,
inorganic gel
Polysaccharide and
synthetic polymer
Protein and synthetic
polymer
Coulombic interaction
Hydrogen bonding
Coordinate bonding
Water
Organic solvent
Oily solvent
Alcohol
Air
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glycol dimethacrylate copolymer swollen in water, which is the typical material used for
soft contact lenses. Table 1 gives a classification of gels.
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II.
PREPARATION
A polymer gel is a network of flexible cross-linked chains. Structures of this type can be
obtained by chemical or physical processes. Some gels are cross-linked chemically by
covalent bonds (chemical gel), whereas other gels are cross-linked physically by weak forces,
such as hydrogen bonds, van der Waals forces, or hydrophobic and ionic interactions
(physical gel). Physical gelation processes are usually reversible and are called sol–gel
Figure 1.
Schematic representation showing the chemical and physical cross-linking.
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Osada, Gong, and Tanaka
transitions. The final gel structures and properties are sensitive to the preparation methods.
Some examples of chemical cross-linking and physical cross-linking are shown in Fig. 1.[4]
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A.
Chemical Gelation
A first approach to gel synthesis uses additive polymerization. For example, if
we start with a vinyl monomer and react the double bond by a free radical reaction, we
generate (mainly) linear chains. However, if we add a fraction of a divinyl derivative to the
mixture CH255CH–R2 –CH55CH2, the two double bonds will participate in the construction
of two distinct chains, and –R2 – will become a cross-linking bridge in the structure.
A second method of gel preparation is based on the condensation of polyfunctional
units. A typical example would be the condensation reaction between a trialcohol and
diisocyanate. The reaction leads to branched chains; each trialcohol becoming a branch
point when its three functions are reacted.
These polymer networks swell in appropriate solvents, but do not dissolve. The
degree of swelling strongly depends on the degree of cross-linkage. The lower the degree
of cross-linkage, the more the gel swells.
B.
Physical Gelation
The cross-links need not be produced by a chemical reaction. Any physical process
that favors an association between certain points on different chains may also lead to a gel.
Many examples of this are found with biological molecules, such as proteins and certain
polysaccharides. The following are some examples:[5]
1.
2.
3.
Formation of hydrogen bonds: This leads to the formation of helical structures
with two (or more) strands or the formation of a microcrystal.
Formation of coulombic bonds: If the counterions are multivalent, they behave as
a cross-link between two or more polymer chains through ionic interactions.
Polymer – metal and polymer –polymer associates are examples.
Formation of nodules with block copolymers: If the chains are made of three
blocks (BAB) in a solvent that is good for A and poor for B, the B blocks will tend
to coalesce into nodules (or alternatively into sheets or rods). Depending on the
temperature and other similar variables, the B block, inside the nodules, may be
either in a solid state (crystalline or glass) or in a fluid state (micelles).
The foregoing approaches are typical, but not exhaustive. For example, another
important class of gels is made with polyamides, polyesters, polyurethanes, and other
polycondensation polymers.
III.
PROPERTIES AND RESPONSES OF POLYMER GELS
Polymer gels can easily be deformed by external physical and chemical stimuli and
can generate a force or execute work to the exterior. If such responses can be translated
from the microscopic level into a macroscopic scale, a conversion of chemical free energy
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into mechanical work should be realized. These energy-conversion systems have great
potential that may be applied to actuators, sensors, chemical valves, delivery controllers,
and permselective separators. These kinds of developments are very active and practical
utilization is in progress.
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A.
Solvent Property and Phase Transition
Gels may be assumed to be constructed of a macromolecular network consisting of
several small contiguous reservoirs. Water molecules, especially in hydrophilic and
polyelectrolyte gels, are restricted in their motion, compared with that in common water.
The motion restriction of water arises from the extensive association of the water molecules
with the network. The existence of an ordered structure at the water–macromolecular
interface has been generally accepted. These water molecules possess certain preferred
orientations and cannot move independently of their neighboring molecules. In this
sense, the water molecules that are near the macromolecular interface are structured (bound
water); those sufficiently far from the macromolecule have a bulk water structure (bulk
water), and those in between have decreasing orderliness as a function of the distance from
the macromolecular interface.[6] The bound water in the gel does not freeze even if the
temperature of the gel sample is below the freezing point of the bulk water.[7] The presence
of bound solvent molecules has also been observed for organic gels.[8] Phase transitions in
polymer gels were theoretically predicted on the basis of coil–globule transitions that were
observed in solution by Dusek[9] and later experimentally confirmed by Tanaka.[10]
The discrete volume transition is understood as a manifestation of the competition
among the three osmotic pressure mechanisms acting on the polymer network: positive
pressure of counterions, negative pressure owing to the affinity among polymers, and
the rubber elasticity that keeps the network moderately flexible. The effect of these three
pressures determines the equilibrium volume. Temperature, pH, and salt ions affect both
the positive and negative pressures, whereas solvent composition influences only the
negative pressure. Changing one of these factors creates an excess nonzero pressure,
which results in a new equilibrium volume. The phase properties were theoretically
deduced from the Flory– Huggins[4] derivation:[11]
"
1=3 #
DF
ny
f
f
2 2 ln (1 f)
¼
t¼1
(2f þ 1)
2
þ1þ þ
2
kT
f0
f0
f
Nf
f2
(1)
where t is the reduced temperature; N the Avogadro’s number; k the Boltzmann constant; T
the absolute temperature; y the molar volume of the solvent; f the volume fraction of the
polymer network; DF the free-energy decrease associated with the formation of a contact
between polymer segments; f0 the network volume fraction at the condition that the
constituent polymer chains have random configurations; n the number of constituent chains
per unit volume at f ¼ f0; and f the number of dissociated counterions per effective
polymer chain. Thus, the equilibrium network volume fraction is determined as a function of
the reduced temperature. For certain reduced temperature values, Eq. (1) is satisfied by three
values of the polymer network: the volume fraction f, corresponding to two minima, and
one maximum of free energy. The value of f, corresponding to the lower minimum
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Osada, Gong, and Tanaka
Figure 2. Theoretical phase diagram of gels for various values of f, the degree of ionization of the
network. (From Ref.[11].)
represents the equilibrium value. A discrete volume transition occurs when the two freeenergy minima have the same value. Figure 2 is a theoretical phase diagram for gels.[11]
B.
Thermoresponse Polymer Gels
Polymer gels capable of varying their shapes or volumes in response to change in
temperature, appeared during the 1970s.[12] The principle of such a mechanical response
was, for example, insolubilization – solubilization, which came from the change in the
solubility of the polymer chain, or complex formation between polymer chains
accompanying change in temperature. The solubility of the solute molecules changes
drastically with the change in temperature. Also, the behavior of a polymer in a given
medium reflects the balance of interactions among its own segments and the surrounding
solvent molecules.[13,14] Poly(N-isopropylacrylamide) (PNIPAM) is known for its novel
thermal behavior in aqueous media.[15,16] In aqueous solution, PNIPAM has inverse
solubility on heating and shows a lower critical solution temperature (LCST) at about
318C. At the same time, the macromolecular transition from a hydrophilic to a
hydrophobic structure takes place.
Figure 3 shows the state of water in the thermosensitive gel as a function of
temperature.[17] Here, the polymer network collapses with an increase in temperature. As
shown in Fig. 2, drastic changes in the state of the gel can be brought about by small changes
in the external conditions. Under some conditions, swelling or shrinking is discontinuous;
therefore, a minute change in temperature can cause a large change in volume.
A PNIPAM gel can be obtained by the addition of a cross-linking agent to the
polymerization recipe. N,N0 -Methylene-bisacrylamide (MBAA) is the major choice for
this component, which is probably a consequence of its structural similarity to PNIPAM.
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Figure 3. States of water in poly(NIPAAm) hydrogel as a function of its equilibrium
temperature. (From Ref.[17].)
The initial impulse for the research on PNIPAM gels came from work on phase
transitions observed with polyacrylamide (PAM) gels.[18] In 1979, Tanaka noted that PAM
gels possess a collapse transition that is dependent on the composition of an acetone –
water mixed solvent. Later work on gels uncovered early ambiguities and established that
the presence of ionized groups on the polymer introduced by copolymerization of carboxyl
monomers or hydrolysis of esters produced a discontinuous transition compared with a
continuous change in volume when the ionized groups are absent. Subsequently, an
anomaly was discovered: when NIPAM substituted for acrylamide, even a nonionized gel
exhibits a discontinuous transition in aqueous media on heating. According to the
literature, the sharpness of the volume transition varies as different comonomers or
initiators are introduced. When ionic sodium acrylate is added as a comonomer, the
transition of the PNIPAM copolymer gels switches from continuous to discontinuous.[19]
The ability to construct PNIPAM gels with various geometries has led to a number of
applications that exploit the change in gel dimensions to modulate the differential
diffusion of species in a medium. Thus, it is possible to selectively remove[20 – 23] and
deliver[24 – 29] cosolutes with thermal-switching control. These topics will be presented in
other sections.
N-Isopropylacrylamide is one of the few materials that produce microparticle gels
owing to its unique property and the convenience of polymerization. Particles organized
by cross-linked polymer networks have various uses that are different from noncrosslinked particles, such as ink, paints, abradants, cosmetics, medicines, and catalysts. If
the degree of cross-linking is low, the particles are highly swellable and are often
called microparticle gels, which have remarkable fluid properties when the particles are
suspended in a solvent media. The NIPAM particle hydrogels can be obtained by
vigorously stirring an aqueous solution of NIPAM monomer and cross-linking agent
with an initiator at polymerization temperatures (708C). Because the polymerization
temperature is higher than LCST, the PNIPAM produced will separate out. The reasons
PNIPAM does not aggregate in the course of precipitation polymerization are
considered to be due to the rigidity of PNIPAM particle, a balance of hydrophilicity –
hydrophobicity, and electrostatic repulsion that comes from the residual initiator.[30]
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Table 2.
Microparticles of poly(N-isopropylacrylamide) gels.
Initiation systema
Crosslinking
agentb
APS – TEMED
MBAA
APS – TEMED
KPS – sodium
metabisulfite
or 4,40 -azobis(4-cyanovaleric
acid)
Olephilic peroxide
a
Polymerization medium
Swollen
size (mm)
References
0.4
[31]
MBAA
MBAA
Sorbitan monolaurate–
water
Water in paraffin oil
Various surfactants
such as Triton770
aqueous emulsion
100– 1000
0.2– 0.5
[32,33]
[34,35]
Various
Ethyl accetate emulsion
Polymeric suspension
agents
ca. 100
[36]
APS, ammonium peroxydisulfate; TEMED, N,N,N0 ,N0 -tetramethylethylenediamine;
potassium peroxydisulfate.
b
MBAA, N,N0 -methylene-bis-acrylamide.
KPS,
The polymerization systems for PNIPAM particle gels are summarized in Table 2.[16]
In precipitation polymerization, the diameter of the particle obtained is about 0.3–
0.5 mm. If the emulsifier is added to the polymerization system, smaller-sized particles
can be obtained.[37] Pelton et al.[38] reported that some network structures can form
without any cross-linker.
Recently core shell microspheres of NIPAM copolymer gels with styrene (St) have
been prepared.[39] Because St –NIPAM copolymer microspheres prepared by soap-free
emulsion polymerization have an imperfect core shell structure, seeded polymerization of
NIPAM was carried out using the St –NIPAM microspheres as seeds to prepare uniform
core shell microspheres. The particles obtained are thermosensitive gels and the adhesion
between these particles and leukocytes has been investigated. Adsorption of proteins on
the particles was also studied using PNIPAM gels.[40]
Poly[vinyl methyl ether) (PVME) also has a LCST of about 378C in aqueous media
and is a thermosensitive polymer. PVME molecules dissolve in water owing to the
formation of hydrogen bonding between methoxyl groups and water molecules at low
temperatures. When the solution is heated, PVME molecules are dehydrated and
associated through hydrophobic interactions. Hirasa and co-workers prepared PVME gels
using g-ray irradiation methods and studied the thermomechanical response of PVME
hydrogels in detail.[41,42] Irradiation with g-rays of an aqueous PVME solution brings
about gelation. The gel shows thermosensitivity similar to the solution.[43] Swelling occurs
below 378C and shrinking above this temperature. The appearance of the gel formed
is different for different conditions of irradiation. A transparent homogeneous gel forms on
irradiation at temperatures below LCST. In contrast, with irradiation at temperatures
higher than the LCST, gels form with various pore sizes that are controlled by the heating
rate, PVME concentration, and the dose rates. When the PVME concentration is low, the
pore size of the gel is in the order of millimeters, and the gel exceeds the space of the
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Figure 4. Thermal response of PVME hydrogels: L0, gel length at 408C. The swollen gel is 1 cm3.
(From Ref.[42].)
vessel used for irradiation. The pore sizes are smaller with increased PVME concentration.
The pore size occasionally reaches micrometers at higher concentrations. The
thermomechanical responses of these gels are shown in Fig. 4. Both Gel-A and Gel-B
were prepared from 30 wt% PVME solution irradiated with 100-kGy g-rays. A
homogeneous Gel-A was formed at 238C, whereas the Gel-B, formed during heating from
238C to 508C, was sponge-like. The micropore size was about 10 mm.
Macromolecules undergo drastic conformational changes as a result of mutual
interaction in solution and, sometimes, even insoluble complexes are formed. Complex
formation has been obtained in many systems of synthetic polymers with and without
charges.[44 – 47]
A reversible complexation – dissociation was found for poly(ethylene glycol)
(PEG) (Mw, 2000) and poly(methacrylic acid) (PMAA) in an aqueous medium by a
change in temperature.[44] Figure 5 shows the temperature dependence of the viscosity
of an aqueous solution of PMAA (see curve 1) and equimolar PMAA – PEG mixtures
containing equal concentrations of the repeating units of the two components (see
curves 2 and 3).[48] A pronounced decrease in the viscosity at the temperature range
25 –458C in curve 2 demonstrates complexation between PEG (Mw, 2000) and
PMAA. The viscosity recovers completely on lowering the temperature. On the other
hand, a solution mixture prepared from the same PMAA and PEG with a Mw of
20,000 exhibited low viscosities over the entire temperature range (see curve 3),
indicating the formation of a stable complex. The use of PEG with an Mw lower than
1000 resulted in no viscosity change. The endothermic complexation, favored by
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Figure 5. Temperature dependence of reduced viscosity for PMAA and PMAA– PEG complexes
in water: (1) PMAA; (2) PMAA – PEG (Mw ¼ 2000); (3) PMAA– PEG (Mw ¼ 20,000); [PEG]/
[PMAA] ¼ 1.0 (repeating unit), PMAA ¼ 0.05 g/100 mL. (From Ref.[48].)
raising the temperature, is due to hydrophobic interactions between the a-methyl
groups of PMAA and the ethylene backbone of PEG.[45]
This thermoreversible complexation has been examined as a means of transforming
chemical energy into mechanical work.[49] Figure 6 shows the contraction of the PMAA
membrane observation on addition of PEG and a subsequent temperature change (isotonic
contraction). It is seen that the membrane in solutions of PEG contracts sharply with rising
temperature, especially in the region of 20–308C. A 4.7-mg dry membrane, loaded at 100
times its weight, underwent a reversible contraction–expansion by over 70% in the
temperature range of 10–408C. The work done per contraction of 1 g of contractile
substances is 5 1023 cal. The internal stress of the membranes was measured at constant
length for the corresponding system (isometric contraction).[50] The stress made in the PMAA
membrane was 4–6 kg cm22, which is almost the value found in natural muscles. The
temperature coefficient of the membrane dimension can be reversed by changing the property
of the embedding solution. Some applications of this volume change of PMAA gel were
considered to be as selective permeation membranes or chemical valves.[51,52]
C.
Chemically Responsive Polymer Gels
Chemomechanical gel systems that charge in response to change in the environment,
such as pH or formation of chelate compounds, are widely known.[53] Suzuki et al. prepared
composite films consisting of poly-(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA) and
poly(allylamine) (PA1Am) by repetitive freezing and thawing, and demonstrated the
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Figure 6. Temperature dependences of chemomechanical behavior of PMAA membranes with
various embedding fluids: (1) 70 mL of pure water; (2) 70 mL of 0.015 Umol/L PEG solution. Dry
membrane, 10 mm wide, 23 mm long, 4.7 mg weight; loaded with 490 mg, PEG Mw ¼ 2000.
Ordinate is expressed in percentage of the length of the dry membrane. (From Ref.[49].)
shrinkage of these PVA–PAA–PA1Am films by alternating solvent changes between 95%
ethanol and 0.01 N NaOH (Fig. 7).[54] Shrinkage and extension was complete within 40 and
20 sec. The rate of shrinkage was independent of the load applied.
Recently, a spontaneous gel motion was discovered. Amphilic copolymer gels
immediately undergo spontaneous translational and rotational motion when the gels,
swollen in ethanol, are immersed in water.[55,56] The copolymer gels are made of acrylic
acid (AA) and hydrophobic acrylates, such as stearyl acrylates and 12-acryloyl dodecanoic
Figure 7. Change in length for PVA –PAA – PA1Am films with different loads of 1 – 3 g by
alternative solvent change between 95% ethanol (poured at t ¼ 0) and 0.01 N NaOH (poured at
t ¼ 100). (From Ref.[54].)
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acid (ADA). These polymer gels swell in water-soluble organic solvents, such as ethanol
or tetrahydrofuran (THF), by as much as 5 to 20 times their original volume, whereas they
shrink in water to form partially crystalline-layered structures of ordered long alkyl
chains.[57,58]
The velocity, duration, and mode of gel motion are associated with its size, shape, and
chemical nature. Disk-shaped gels exhibit translational motion, with occasional abrupt
turns. For example, a 100-mg ethanol –swollen cylindrical poly(AA –ADA) gel (7 mm in
diameter and 4 mm thick, AA 75 mol%, ADA 25 mol%) underwent translational motion,
with a maximum velocity of 5 cm/sec. When the gel is triangular or square, it exhibits
rapid rotation. A triangular gel 10 mm on its sides and 4 mm in thickness exhibits six to
eight rotations per second for the first 20– 30 sec, and then slows down to four to five
rotations per second over the subsequent 20 min. The duration of gel motion is directly
proportional to size: the larger the gel size, the longer the gel motion.
Gels obtain kinetic energy by the release of organic solvent through an organized
surface layer, whereupon two driving forces for the release of the organic solvent exist:
one is osmotic pressure and the other is hydrostatic pressure.[55] Researchers have
emphasized that the motion induced by such an osmotic pumping has several advantages
and unique characteristics:
1.
2.
No noise is produced and no unnecessary exhaust products, such as those from
combustion or other chemical reactions, are produced.
Motion can be induced only by dilution of the organic solvent, which can be
recovered and reused.
D.
1.
Electric Properties
Hydrogel
When a water – swollen polyelectrolyte gel is interposed between a pair of electrodes
in the air and a DC current is applied, the gel undergoes electrically induced contraction at
one electrode and concomitant water exudation at the other electrode.[59] Electrically
induced contractions are attributed to electrokinetic mechanisms.[60] The applied electric
field induces the migration of hydrated counterions toward the oppositely signed electrode
(electrophoresis) together with the water, thereby, transporting water to the electrode. For
example, in a sulfonic gel that is a strong acid polymer with fully ionized sulfonic groups
as macroions and Hþ as counterions, hydrated Hþ ions (H3Oþ) migrate toward the cathode
and are reduced, liberating H2. The water migrates together with Hþ ions and exits the gel
near the cathode. During this course, the gel contracts and deforms.
Several studies on hydrogels responsive to electric stimulation have been made. For
example, De Rossi et al. reported that PVA – PAA composite films shrink because of the
electrode reaction in aqueous NaCl solution.[61] Norman and Grodzinsky also studied the
kinetics of oscillatory tensile forces generated by sinusoidal electric fields in a collagen
membrane.[62] A comparison of the experimental and theoretical data suggested that
electrodiffusion is the dominant rate-limiting process in this electromechanochemical
transduction.
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Chemomechanical weight lifting was investigated using PMAA gel under the influence of
direct current.[62,63] The lifting rate of the weight attached to the bottom end of the cylindrical
strip of the gel and the power generation initially decreased with an increase in the load, but
then increased with the load (Fig. 8). The efficiency of the work done by the gel with a 22-g
load was 24 times larger than that with 5.5-g load. A polymer–polymer complex[44,48,64 – 66]
prepared from PMAA and PVP or PAA and PVA showed similar behavior.
These lifting behaviors can be explained in terms of spontaneous ionization of
ionizable groups that give rise to an increase in electric current by stretching (reverse
chemomechanical reaction); that is, a stretching of the polymer network will induce an
additional ionization of ionizable groups and, consequently, raise the rate of contraction of
PMAA gel. In fact, ionization of the carboxylic group (i.e., decrease in pH) was observed
when the load was applied to the gel.
This anomalous behavior may well provide the basis for an automatic or inherent
control sensor that spontaneously adjusts the energy absorbed by a synthetic muscle
system commensurate with that required to do the work; that is, the heavier the load the
more energy absorbed without external stimulus, similar to the mechanical devices with a
sensor-feedback control system. One can speculate about the similarity of biological
muscle systems, wherein the force applied to accomplish a given work is proportional to
that required. Without this inherent control, the force applied to any work will be the same,
despite the weight of the objects.
Recently, an attempt was made to design a polyelectrolyte gel as a mechanoelectrical
conversion system based on the spontaneous ionization induced by mechanical deformation that produces an electric potential of several millivolts. Actually, the generation of
an electromotive force has been confirmed for the deformation of a weak polyelectrolyte
gel such as PAA.[67] When a piece of gel was compressed, the pH of the gel changed, and
when the gel was unloaded the pH value quickly recovered to the original value. Because
there was no water outlet in the experimental course of the deformation, the pH change
Figure 8. Rate of contraction and power generation vs. load applied (Wl) per weight and length
of the PMAA gel sample (Wg). Sample dimensions: length ¼ 60 mm, diameter ¼ 17 mm, wet
weight ¼ 12 g, electric field ¼ 5 V DC. (From Ref.[63].)
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should be associated with an enhanced ionization under deformation. On the basis of this
phenomenon, a soft pressure-sensor system or a tactile-sensing device was proposed. This
mechanoelectrical system made of polymer gel is considered to be similar to the tactile
perceptions in the living organism. Both of them are dynamic processes in which the
macroscopic deformation induces the ionic rearrangement, which gives rise to a certain
amount of transmembrane potential.
A response of microparticle gel to an applied electric field was investigated for a
particle of cross-linked sodium PAA perpared by inverse emulsion polymerization.[68] The
PAA microparticles (about 200 mm) contracted when an electric field was applied, and a
90% volume change was reached within 50 sec. Thus, it was confirmed that microgel is
very effective for constructing a system with a minimized response time. Moreover, the
reversible contraction – expansion behavior was observed with and without applying an
electric field. The swelling process after switching off the electric field was almost
complete within a few minutes.
2.
Organogel
Preparation of electroconducting polymer gels is another interesting subject.[8,69 – 74]
Studies have recently been undertaken involving conducting gels swollen in nonvolatile
organic solvents. One involved a conducting gel based on alkyl thiophenes, and another was
based on a polymeric charge-transfer (CT) complex. The organogels have an advantage
because there is no evolution of hydrogen or oxygen gases compared with hydrogels that are
produced by the decomposition of water in an applied electric field. The H2 or O2 gases
produced will make the electrochemical reaction unstable and decrease the efficiency.
Gel formation can occur for 3-alkylthiophenes, with alkyl chain lengths shorter than 12,
through polymerization.[69] The gels prepared showed high conductivity when doped with
iodine. Poly(3-octylthiophene) gel swells in a chloroform solvent, whereas it shrinks in
ethanol. Figure 9 shows the conductivity of the gel doped with iodine at various
concentrations in chloroform. Although the conductivity of the gel without iodine was
10212 S cm21, after doping, the conductivity increased to 1021 S cm21. Stretched gel films
have anisotropic properties. Although the ratio of volume change perpendicular to stretching
was almost independent of the solvent composition of the chloroform–ethanol mixture,
the parallel volume change ratio was highly dependent on the solvent composition.
The electric conductivity was larger for the parallel than for the perpendicular one in the
poly(3-decylthiophene) gel formed by g-ray irradiation and doped with iodine.
Another type of conductive gel consists of an electrodonating polymeric network and
a low molecular weight acceptor, which is subsequently doped in the gel.[73] For example,
7,7,8,8-tetracyanoquinodimethane (TCNQ), an electron acceptor, was cross-linked with a
polymeric donor poly [N-[3-(di-methylamino)propyl] acrylamide] (PDMAPAA) in DMF
(Fig. 10). When TCNQ was doped, a significant swelling and coloration caused by the
formation of a CT complex occurred.
The overall reaction between PDMAPAA and TCNQ can be written as:
1: PDMAPAA þ TCNQ0 ! PDMAPAAHþ þ TNCQ:
2: 2TCNQ: ! TCNQ2 þ TCNQ0
3: TCNQ2 þ O2 ! O C;; þ N þ DCTC
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Figure 9. Dependence of electric conductivity in poly(3-octylthiophene) gel on iodine
concentration in chloroform solution. (From Ref.[70].)
An attempt to associate the swelling in the CT polymer gels as a function of the degree of
cross-linking and the ionic density of the network was made using Flory’s theory. The values
showing the degree of ionization calculated in this way are almost the same as those for r (the
molar ratio of added TCNQ to the PDMAPAA gel), indicating that practically all of the
TCNQ had reacted with PDMAPAA to give cation radicals within the polymer network and
TCNQ anion radicals.[73] Also, the gel obtained was sufficiently electroconductive to cause
electrodriven shrinkage. The rate and efficiency were higher than those for hydrogels.[74]
Figure 10. Chemical structures of DMAPAA used for network preparation and TCNQ doped as an
acceptor molecule in the network.
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IV.
APPLICATIONS
Applications of gels span several fields, including the food industry, medical,
biotechnology, chemical processing, agriculture, civil engineering, and electronics. Some
of these applications are listed in Table 3.
A polymer gel can absorb solvent up to several thousands times its original weight,
depending on the chemical structure of the gel. Diapers, feminine napkins, and perfumes, used
in everyday life are typical examples of this highly water-absorbing property of hydrogels.
There is considerable interest and activity in the application of synthetic and
biological polymer gels in medicine. Interest has focused on the use of the bulk or the
surface properties of hydrogels for biomedical applications. The bulk property of swelling
is of particular interest for “swelling implants”; namely, implants that can be introduced in
a small dehydrated state through a small incision and then swell to fill a body cavity or to
exert a controlled pressure. The swelling of synthetic and natural gels may also help
elucidate swelling and osmotic mechanisms in biological tissues.
Several biomedical applications for hydrogels mentioned in the literature are listed in
Table 4.[75] The wide range of biomedical applications for hydrogels can be attributed
to both their satisfactory performance on in vivo implantation in either blood-contacting or
tissue-contacting situations and to their ability to be fabricated into a wide range of
structural forms.
A modulation of swelling forces in gels by chemical or physical stmulienables
dynamic control of the gel hydration and, thereby, effective diffusion and permeability of
solutes can be obtained. Drug delivery systems (DDS), permselective membranes for
selective extraction, and chemical valves are examples of stimuli-responsive polymer gel
applications.
A.
Biomedical Use
Gels are expected to possess a self-control function that in living organisms is known as
homeostasis. That gels are open to surrounding materials may contribute to this function: for
example, gels always keep solvents, solutes, and other species coming and going between the
surroundings and gel. On the other hand, the gel can shut off these species by its network
Table 3.
Field
Consumer
Food
Agricultural
Industrial
Chemical
Biological
Environmental
Electronics
Applications of polymer gels.
Products
Diapers, napkins, perfumes, cosmetics, toys, air fresheners
Gelatins, food protein isolates, flavor release
Pesticides, herbicides, fungicides, water absorbents
Oil dewatering, mineral dewatering
Fertilizers, cleaning chemicals, adhesives
Cell culturing, templates, electrophoresis, fermentation broths
Metal removal, fire extinguisher, regenerable sorbents
Electrodes, sensor, actuator
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Table 4.
103
Potential and actual biomedical applications of synthetic hydrogels.
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Coatings
Sutures
Catheters
IUDs
Blood detoxicants
Sensors (electrodes)
Vascular grafts
Electrophoresis cells
Cell culture substrates
“Homogeneous” materials
Electrophoresis gels
Contact lenses
Artificial corneas
Estrous-inducers
Burn dressings
Bone ingrowth sponges
Dentures
Eardrum plugs
Synthetic cartilages
Hemodialysis membranes
Breast or other soft-tissue substrates
Particulate carriers of tumor antibodies
Vitreous humor replacements
Devices
Artificial organs
Drug delivery systems
structure and the electric field of a macroion. From these perspectives, gels, particularly
hydrogels, are valuable as biomedical materials, for which the functions desired must include
recognition, judgment, and action. Applications include soft contact lens, artificial skin,
immobilization of bioactive substances, and DDS. In each one, the material is composed of a
gel and a biological system that function under a controlled environment.
1.
Soft Contact Lens
Soft contact lenses (SCL) are one of the most famous application of synthetic
polymer gels. The development of SCL began with the invention of poly(hydroxyethylmethacrylate) by Wichterle in 1960, who improved the material to a soft and
transparent substance. The SCLs were first manufactured by Bausch – Lomb Inc., but
are now found worldwide.
The materials used for SCL are summarized in Table 5.[76] An SCL must allow
oxygen to permeate into the eye. Because this permeation and transport of oxygen
occurs through water molecules, the thickness and water content in the SCL are often
important factors as well as the materials. Figure 11 shows the relation between water
content and the permeation coefficient of oxygen for the SCLs on the market.[76] The
data follow straight lines that do not pass zero. The reason for this is associated with
the nature of water; water molecules bound to the SCL polar groups (bound water) do
not contribute to the permeation of oxygen or ions. Thus, it is important to increase the
content of free water to raise the oxygen permeation while keeping the required
mechanical toughness of SCL.
2.
Artificial Skin
One of the more active research and development fields, among the mechanical
materials using hydrogels, is that of artificial skin.[77 – 79] The pioneering work for a
cultured skin substitute is being done by Rheinwald and Green,[80 – 82] who demonstrated
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Table 5.
Classification
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Moist SCL
Moistless SCL
Materials for SLC.
Examples of polymer materials
Low moisture content: less than 40%
Poly(hydroxyethylmethacrylate)
Middle moisture content: 40– 60%
Copolymer of hydroxyethylmethacrylate, N-vinylpyrrolidone,
methacrylic acid, and methylmethacrylate
Copolymer of methylmethacrylate and glycerol methacrylate
High moisture content: more than 60%
Copolymer of N-vinylpyrrolidone, dimethylacrylamide, and
methylmethacrylate
Silicone elastomer
Acrylic elastomer
the possibility of growing epidermal keratinocytes as layered sheets from single-cell
suspensions, and proved that the resulting multilayered sheets are very effective in the
management of burns.[83] Most of the materials currently used as artificial skin are as
covers for wounds or burns that are capable of inhibiting infection or of absorbing
exudates from the body. The materials are collagen, chitin, poly(amino acids), and others
that are compatible with the human body. The functions demanded are as follows:
(1) prevention of infection; (2) antitoxicity, anti-inflammatory, and antiantigenic;
(3) airtightness to the wound surface, flexibility, and noncontraction; (4) inhibition
of absorbability of exudates; and (5) permeability of moisture and air.
Future development will focus on promotion of natural epidermal tissues’
reconstruction by artificial skin.
Figure 11. Permeation coefficient of oxygen and sodium ion plotted against moisture percentage.
(From Ref.[76].)
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3.
105
Immobilization of Bioactive Substances
A variety of methods and materials have been proposed and tested to immobilize
bioactive substances such as microorganisms, enzymes, and drugs.[84 – 87] Gels, in
particular, have long been used for immobilization of substances (e.g., agarose culture
medium). The immobilization gives us several advantages. They are: (1) bioactive
substances can be recovered and reused; (2) shaping and molding, such as beads or
membranes, are possible according to each reaction system; (3) thermal resistance and pH
resistance increase; and (4) bioactive substances occasionally become stable.
Conventional immobilization methods are generally classified into four categories:
covalent bonding, chemical cross-linking, adsorption, and entrapment.[88] The first two
processes generally provide a small release of the substances from the support matrix, with
a high degree of inactivation owing to chemical modification of the substances. The latter
two processes provide a larger release, but less inactivation, of the substances. In general,
covalent binding and chemical cross-linking methods are used for biosensors, whereas
adsorption and entrapment processes are used for large-scale production in industry.
Large-scale industrial purposes often require the use of sheets, fibers, and rigid beads.
B.
Drug Delivery System
The use of polymer gels as drug carriers has become practical in formulations for
mucous membrane administration. Here, the gels carry drugs on the surface of the mucous
membrane and the supply remains until the drugs are absorbed. This method gives quick
and specific absorption compared with absorption through the skin.
Recently, a new DDS was designed[89] for delivery to targeted sites (site-specific
delivery), for release when temporal control of a drug is required, to maintain effective
drug concentrations over longer periods for maximal efficacy, and to minimize side
effects. To realize such delivery systems, a system in which the drug carrier itself senses an
environmental stimuli and responds by appropriate drug release is needed (Fig. 12).[89]
Yoshida and colleagues have studied “on –off” regulation of drug permeation and
release using a hydrogel made of a copolymer of NIPAM and alkyl methacrylates
(RMA)[90,91] or a hydrogel of interpenetrating polymer networks composed of poly(AAMco-RMA) and PAA.[92] In this system, the release of drug from of membranes is controlled
by changing temperatures. Figure 13 shows the results of the permeation experiments with
indomethacin. At 208C, indomethacin permeates through the poly-(NIPAM-co-RMA)
membrane. However, when the temperature is raised from 208C to 308C the permeation or
release of drug is blocked by the dense surface layer formed on the gel immediately after
the temperature is raised. By lowering the temperature, the deswollen surface recovers its
equilibrium swelling and allows drug permeation. Thus, the gel surface acts as an on –off
switch. If the alkyl chain length and methacrylate become longer, the initial permeability
during the second 208C period is greater than the final permeability during the first 208C
period. This result suggests that the drug concentration profile in the membrane changes
during the off period. However, complete “0 – 1” control is nearly achieved. This surfaceregulating mechanism results in a quick response. These researchers also investigated, in
detail, effects of skin surface as a switch for pulsatile drug release.
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106
Figure 12.
Osada, Gong, and Tanaka
A schematic diagram of an autofeedback drug delivery system. (From Ref.[89].)
Electrically controlled drug release, including that of insulin, has also been
investigated for polymer gels of DMAPAA.[93] When the electric field is applied to the
PDMAPAA gel, insulin leaks from the gel at a constant rate, whereas the leakage ceases
by stopping the applied field.
C.
Gels for Selective Separation
Recently, a separation process using a volume phase transition of gel was investigated
and a general depiction of the process was proposed by Cussler et al. (Fig. 14).[94] In this
process a collapsed gel is introduced into an aqueous solution that needs to be
concentrated. The gel swells absorbing the small species, but excluding the large species,
in the solution. When the gel reaches equilibrium, the unabsorbed raffinate is physically
separated from the gel. Subsequently, the gel is placed in a second environment, which
induces the collapse. The gel is then ready to be introduced into a fresh aqueous solution or
into the raffinate solution again if further concentration is desired. Such gel condensation
processes require less energy than evaporation and can be operated under mild conditions
that will not damage the solutes. The important point is to recycle the gel.
Cussler et al. actually demonstrated the condensation for various kinds of solutions
using PNIPAM gel or copolymer gel of N,N0 -poly(diethyl-acryl-amide) and sodium
methacrylate. Because both are thermosensitive polymer gels, dilute solutions can be
condensed with a small difference in temperature by the repetition of the collapsed and
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Figure 13. Diffused amount of indomethacin through copolymer gels of NIPAM and RMA (RMA
3.74 wt%) membranes in response to stepwise temperature changes between 208C and 308C. W,
RMA ¼ butylmethacrylate; A, RMA ¼ laurylmethacrylate; 4, RMA ¼ hexylmethacrylate. (From
Ref.[90].)
Figure 14. A schematic diagram of a gel separation process: the gel is alternately swollen and
collapsed to produce a concentrated raffinate and dilute retentate from a feed solution. (From
Ref.[94].)
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Osada, Gong, and Tanaka
Table 6. Selective separation using thermosensitive gels.
Gel efficient
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Solute
Urea
Sodium
pentachlorophenolate
Vitamin B12
Ovalbumin
Polyethylene oxide
Gelatin
Blue dextran
Polystyrene latex
Polyethylene glycol
Molecular
weight
60
267
1,355
45,000
600,000
—
2 million
—
400
3,400
8,000
18,500
Poly(Nisopropylacrylamide)
(1%)a
Copolymer of N,Ndiethylacrylamide
and sodium
methacrylate (4%)
Copolymer of N,Ndiethylacrylamide
and sodium
methacrylate (4%)
2
18
3
51
2
32
97
96
98
97
95b
10
30
56
80
15
84
89
97
99
96b
5
19
25
61
7
—
92
—
96
96c
—
11
16
48
a
The number in parentheses after the gel indicates the percentage of cross-linking in the gel.
This latex had a diameter of 0.06 mm.
c
This latex had a diameter of 1.2 mm.
b
Figure 15. Dependence of nonionic surfactant (n ¼ 20) adsorption on temperature and
concentration: W, 20 mg/L; 4, 40 mg/L; S , 80 mg/L; 5, 160 mg/L; A, 400 mg/L. (From Ref.[43].)
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109
Figure 16. Chemical structures of poly(oxyethylene nonylphenyl ether) and poly(vinylmethylether): n denotes the number of repeating units of ethylene oxide.
swollen states. The effectiveness of the separation can be quantified by the efficiency,
which is defined as the actual increase in raffinate concentration divided by the increase
expected from the gel volume change. The efficiency primarily scales how well the solute
is excluded from the polymer network of the gel. The results of selective extraction
are summarized in Table 6.[20] On the whole, the efficiency becomes higher as the solute’s
molecular weight increases. They also examined the relation between separation
efficiency and the degree of cross-linking, and showed that the efficiency increased
monotonously with the degree of cross-linking for a solution of vitamin B12 and
poly(ethylene glycol) (Mw ¼ 3400). Therefore, gels can be made that do not absorb
macromolecules owing to their network structure.
Ichijo et al. tried to develop a separation technology using porous PVME gels.[43] The
PVME has the LCST at 378C, and its water – swollen gel contracts at temperatures
higher than the LCST. Figure 15 shows a thermoreversible adsorption of nonionic
surfactants, poly(oxyethylene nonylphenyl ether) to PVME gel in response to changes in
temperature. The gels adsorb the surfactants above LCST and desorb them below it by
changes in the hydrophilic – hydrophobic balance. The amount of adsorption depends on
the concentration and the additional number of ethylene oxides, n (Fig. 16), on the
surfactants. The smaller the additional number is, the larger the amount of adsorption.
They suggested that separations with polymer gels make efficient use of unused waste
energy and may contribute to environmental protection.
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