An interpenetrating gel and a method of forming an interpenetrating gel which undergoes a significantly large volume change in response to a physical or chemical stimululs is disclosed.

BACKGROUND OF THE INVENTION 
Gels can exhibit phase transition, or significantly large volume change, in 
response to variation of the surrounding conditions. For example, the 
fluids supporting a gel can be modified to cause a significantly large 
contraction or expansion of a gel such as by changing the pH, solvent 
composition, relative concentration of solvents, solvent temperature, or 
the ion concentration of the fluid. 
However, phase transition of gels generally has been dependent upon 
generating an interaction between a gel polymer network, comprising a 
single polymer and a phase-transition-modifying agent, or upon 
intramolecular interactions between like-strands of the polymer network. 
Also, phase transition caused by changes in fluid temperature have 
heretofore been limited to significantly large contraction during an 
increase in temperature and, conversely, a significantly large expansion 
during a decrease in temperature. 
SUMMARY OF THE INVENTION 
The present invention relates to phase-transition gels and to methods of 
forming phase-transition gels which undergo a significantly large volume 
change at a desired phase-transition condition in response to a stimulus. 
The invention is an interpenetrating polymer network gel that undergoes a 
phase transition in response to a specific stimulus. The interpenetrating 
polymer network is gelled with a liquid medium. 
A method of forming a phase-transition gel which undergoes a significantly 
large volume change at a desired phase-transition condition in response to 
a stimulus includes forming an interpenetrating polymer network, having a 
first polymer and a second polymer interpenetrating the first polymer. The 
interpenetrating polymer network is designed to undergo a significantly 
large volume change in response to a chemical or physical stimulus. The 
interpenetrating polymer network forms a gel with a liquid medium. 
This invention has many advantages and uses. Importantly, the gels of the 
present invention exhibit a significantly large increase in volume by 
increasing the temperature of the phase-transition gel and exhibit a 
significantly large contraction by lowering the temperature of the 
phase-transition gel. Also, the phase-transition gels of the present 
invention can exhibit a phase transition in response to hydrogen bonding 
between interpenetrating polymers of the gel. 
Gels of the present invention can also be used for chemical separation. For 
example, components of a solution can be separated from a solution based 
upon hydrogen bonding, ionic interaction, van der Waals forces, 
hydrophobic interaction, and/or any other kind of chemical bond. The 
chemical bond can be between the component to be separated from the 
solution and any of the polymers of the interpenetrating polymer network, 
to thereby cause a phase transition of the polymer network and, 
consequently, cause a phase transition of the gel. Also, interaction 
between a component in a liquid medium within the interpenetrating polymer 
network and the interpenetrating polymer network can cause an interaction 
between the polymers of the interpenetrating polymer network which, in 
turn, causes a phase transition of the gel. 
Whether the phase transition of the gel is caused by one or more of the 
polymers of the interpenetrating polymer network, the component can be 
separated from the solution by contraction of the gel during phase 
transition by entrapment of the component in the gel or by binding to the 
polymer network in the gel. Alternatively, small components can be 
separated from large components that pass through the gel by adjusting the 
pore size of the network to exclude large components. The pore size of the 
network is adjusted when the polymer network undergoes a phase transition. 
Consequently, gels of the present invention can be formed which trap 
specific compounds, including: potentially physiologically harmful agents, 
such as fat and cholesterol; microbial toxins; abnormally high levels of 
phosphate, sodium or potassium; etc. 
Also, the gels of the present invention can be designed to release 
chemicals in response to exposure of the gel to a chemical trigger or a 
physical trigger which interacts with one or more of the polymers of the 
interpenetrating polymer network to thereby directly cause phase 
transition of the gel or to indirectly cause phase transition by 
initiating interaction between the polymers. Elimination of the chemical 
trigger can cause the phase transition of the gel to be reversed.

DETAILED DESCRIPTION OF THE INVENTION 
The features and other details of the phase-transition gels and methods of 
forming the phase-transition gels of the invention will now be more 
particularly described and pointed out in the claims. It will be 
understood that the particular embodiments of the invention are shown by 
way of illustration and not as limitations of the invention. The principal 
features of this invention can be employed in various embodiments without 
departing from the scope of the invention. 
"Phase-transition" of gels, as that term is used herein, means a 
significantly large volume change of gels between an expanded phase and a 
contracted phase or vice-versa. "Phase-transition gels," as that term is 
used herein, are gels which exhibit a phase transition at a 
phase-transition condition. The difference in volume between the expanded 
phase of phase-transition gels and the contracted phase of the 
phase-transition gels can be orders of magnitude. Examples of 
phase-transition gels are disclosed in Tanaka et al., U.S. Pat. No. 
4,732,930 and U.S. patent application Ser. Nos. 07/425,788, 07/470,977 and 
07/558,733, the teachings of which are incorporated herein by reference. 
The phase-transition gels of the present invention undergo a significantly 
large volume change at a desired phase-transition condition in response to 
a stimulus. The phase-transition gel includes an interpenetrating polymer 
network gelled with a liquid medium. 
A liquid medium is suitable if the interpenetrating polymer network can be 
gelled with the liquid medium to thereby form a phase-transition gel. 
Examples of suitable liquids include water, aqueous solutions, organic and 
inorganic liquids. The liquid medium also can be a solution or a mixture 
of liquids. An example of a suitable organic liquid is a dioxane. 
In one embodiment, the interpenetrating polymer network of the present 
invention includes a first polymer and a second polymer, wherein the 
second polymer interpenetrates the first polymer. Suitable first and 
second polymers include polymers which can interact during exposure to a 
phase-transition condition to thereby cause a significantly large volume 
change of the gel. It is to be understood, however, that the 
interpenetrating polymer network can include more than two polymers. For 
example, additional polymers can be included which interpenetrate the 
first and/or second polymers. 
The distinct polymers interpenetrate each other, in that strands of each 
polymer are maintained in relation to strands of at least one other 
polymer, whereby an interaction between the strands of the polymers causes 
a gel, formed of the polymers which are gelled with a liquid medium, to 
exhibit a phase transition between a contracted phase and expanded phase, 
and vice versa. In one embodiment, the polymers overlap each other, such 
as by being tangled within one another. In another embodiment, any one or 
more of the polymers can be crosslinked to form a network of interstices 
whereby the other polymers are distributed through the interstices of the 
network. In a particularly preferred embodiment, the interpenetrating 
polymer network is formed by first polymerizing and crosslinking a first 
monomer. Thereafter, a second monomer is disposed within the interstices 
formed by crosslinking of the polymerized first monomer. The second 
monomer is then polymerized and crosslinked, thereby forming interstices 
through with the strands of the first polymer extend. 
In a preferred embodiment, the first and second polymers of the 
interpenetrating polymer network form a "polycomplex." "Polycomplex," as 
that term is used herein, means an interpenetrating polymer network which 
includes a first polymer and a second polymer interpenetrating with the 
first polymer, and wherein an interaction occurs between the first polymer 
and the second polymer sufficient to cause alignment between polymer 
strands of the first and second polymers in an amount sufficient to cause 
a significantly large volume change of the phase-transition gel 
"Polycomplexation," as that term is used herein, means alignment by 
interaction between strands of the first and second polymers in an amount 
sufficient to cause a discontinuous volume change of the phase-transition 
gel. Examples of the interactions which can occur between the first and 
second polymers of the interpenetrating polymer network include oxidizing, 
reducing, complexing, ionizing, chemically reacting van der Waals forces, 
hydrophobic interaction, hydrogen bonding and ionic interaction. 
The polymers of the interpenetrating polymer network can comprise natural 
polymers, synthetic polymers, or cross-linked synthetic and natural 
polymers. Also, the polymers can be block copolymers. Examples of 
synthetic polymers include poly(N-isopropylacrylamide), poly(acrylamide), 
poly(acrylic acid), etc. In one embodiment, the interpenetrating polymer 
network includes poly(acrylic acid) as one polymer, poly(acrylamide) as 
another interpenetrating polymer, and water as the liquid medium. In 
another embodiment, the interpenetrating polymer network includes two 
interpenetrating polymers of poly(acrylic acid), and water as the liquid 
medium. In both embodiments, the phase transition can occur by 
polycomplexation due to hydrogen bonding between the interpenetrating 
polymers of the interpenetrating polymer network. 
Examples of phase transition gels which are formed of interpenetrating 
polymer networks including synthetic polymers and which can form 
polycomplexes by interaction of positive and negative charges between the 
interpenetrating polymers include: poly(acrylic 
acid)/poly(methacrylamidopropyl-trimethyl-ammonium-chloride(MAPTAC))/water 
; poly(acrylic acid)/poly(allylamide)/water; 
poly(styrene-sulfonate)/poly(allylamide)/water; etc. 
Examples of natural polymers suitable for forming interpenetrating polymer 
networks include: poly(glutamic acid); chitosan; poly(lysine); gelatin; 
deoxyribonucleic acid; and agarose. Examples of phase-transition gels 
which can exhibit a phase transition by interaction of positive and 
negative charges between interpenetrating polymers include: poly(glutamic 
acid)/chitosan/water; poly(glutamic acid)/poly(lysine)/water; etc. 
Examples of phase-transition gels which can exhibit a phase transition by 
hydrogen bonding between the interpenetrating polymers include: 
gelatin/gelatin/water; deoxyribonucleic acid(single 
strand)/deoxyribonucleic acid(single strand)/water; agarose/agarose/water; 
etc. 
The polymers of the interpenetrating polymer network can be suitably 
treated to cause a significantly large volume change of the gel at a 
desired phase-transition condition in response to a stimulus. For example, 
the interpenetrating polymer network can be ionized in an amount 
sufficient to cause a significantly large volume change at a desired 
phase-transition condition. Other examples of treatment of the 
interpenetrating polymer network include oxidizing, reducing, complexing 
and chemically reacting the interpenetrating polymer network to cause a 
phase-transition at a desired phase-transition condition. An example of a 
phase-transition gel which can exhibit a significantly large volume change 
by reduction and oxidation includes an interpenetrating polymer network 
having interpenetrating polymers of poly(vinyl hydroquinone) and an acid 
solution as the liquid medium. 
In a preferred embodiment, one or more of the polymers is at least 
partially ionized. For example, at least a portion of the acrylic acid 
groups in an interpenetrating network of poly(acrylimide) and poly(acrylic 
acid) are ionized. In a particularly preferred embodiment, a poly(acrylic 
acid) is ionized in an amount in the range of between about 0.01 percent 
and 10 percent of the acrylic acid groups of the poly(acrylic acid). 
Phase-transition conditions at which the phase-transition gels exhibit a 
significantly large volume change can include physical conditions, 
chemical conditions, or combinations of physical and chemical conditions. 
Examples of physical phase-transition conditions include: temperature; 
electromagnetic radiation, such as infrared energy, visible light and 
ultraviolet light; etc. Examples of chemical phase-transition conditions 
include: concentration of ionic species, such as hydrogen and water, i.e. 
pH; crosslinking agents, such as cross-linking agents which crosslink the 
polymer network of the phase-transition gel; inorganic and organic 
solvents; specific chemicals; etc. Phase-transition conditions at which 
the phase-transition gels exhibit a significantly large volume change can 
include combinations of physical conditions, combinations of chemical 
conditions, or combinations of physical and chemical conditions. 
The phase-transition gels of this invention can exhibit a significantly 
large volume change at a desired condition by causing a change in the 
binding of one polymer network of the interpenetrating polymer network in 
an amount sufficient to cause a significantly large volume change of the 
gel at a desired phase transition condition. For example, the amount of 
ionization of an ionized polymer within the interpenetrating polymer 
network can be selected to cause a significantly large volume change of 
the gel at a temperature that is different from the temperature that the 
gel would exhibit a significantly large volume change in the absense of 
ionization of the polymer. 
As illustrated in FIG. 1, polycomplexation of an interpenetrating polymer 
network 10 of a phase-transition gel comprising of poly(acrylic acid) 12 
and poly(acrylamide) 14 can occur by the formation of hydrogen bonds 16 
between acrylic acid groups 18 of the poly(acrylic acid) 12 and acrylimide 
groups 20 of the poly(acrylimide) 14, whereby an alignment, or 
polycomplexation, between the polymers occurs, thereby causing a 
significantly large volume change of the phase-transition gel. 
It is to be understood that the interpenetrating polymer network can be 
designed to exhibit a phase transition in response to various types of 
interactions between a component in the liquid medium in the gel and 
either or both of the polymers of the interpenetrating polymer network. 
Examples of interactions between a component in a liquid medium and an 
interpenetrating polymer network or a polymer of an interpenetrating 
polymer network include van der Waals interaction, hydrophobic 
interaction, hydrogen bonding and ionic interaction. Interaction between 
the component and a polymer of the interpenetrating polymer can cause the 
polymer to exhibit a phase transition which, in turn, causes the 
interpenetrating polymer network to exhibit a phase transition. For 
example, a phase-transition gel including an interpenetrating polymer 
network of poly(acrylic acid) and poly(acrylamide) gelled with water, or 
an interpenetrating polymer network of interpenetrating polymers of 
poly(acrylic acid) gelled in water respond to changes in pH, ionic 
composition or solvent composition. 
In another example, individual polymers of an interpenetrating polymer 
network can exhibit a phase transition in response to a physical change in 
the medium, such as temperature. The phase transition of the polymer can 
be caused by, for example, van der Waals forces, hydrophobic interaction, 
hydrogen bonding or ionic interaction within the polymer. Phase transition 
of the polymer, in turn, causes the interpenetrating polymer network to 
exhibit a phase transition. 
FIG. 2 is a plot of phase transitions for acrylamide gels in a solution of 
acetone and water wherein the gels have been ionized in varying degrees 
and wherein the phase transitions are a result of van der Waals 
interaction within the gels. FIG. 3 is a plot of phase transitions for 
N-isopropylacrylamide gels in water wherein the gels have been ionized in 
varying degrees and wherein the phase transitions are a result of 
hydrophobic interaction within the gels. FIG. 4 is a plot of phase 
transition for acrylamide-sodium 
acrylate/methacrylamidopropyltrimethylammonium chloride gel in water 
caused by ionic interaction in the gel in response to change of the pH of 
the water. FIGS. 2, 3 and 4 all represent examples of interactions within 
polymers of gels which cause the gels to exhibit phase transition. 
Formation of interpenetrating polymer networks which include these 
polymers can allow the consequent interpenetrating polymer network to also 
exhibit a phase transition. An example of one such gel includes a 
interpenetrating polymer network of poly(N-isopropylacrylamide) and 
poly(acrylamide) gelled with water. 
In another illustration of the invention, interpenetrating polymer networks 
can be formed wherein either or both of the polymers of the 
interpenetrating polymer network interact with a component of the liquid 
medium in the gel by van der Waals forces, hydrophobic interaction, 
hydrogen bonding or ionic bonding. The interaction with the component 
causes, in turn, interaction between the polymers of the interpenetrating 
polymer network by, for example, van der Waals forces, hydrophobic 
interaction, hydrogen bonding or ionic interaction. The interaction 
between polymers can be different than the interaction with the component 
in the liquid medium. For example, hydrogen bonding between a component of 
the liquid medium and a polymer of the interpenetrating polymer network 
can cause an ionic interaction between the polymers of the 
interpenetrating polymer network. The interaction between the polymers of 
the interpenetrating polymer network causes the interpenetrating polymer 
network to exhibit a phase transition. 
In one embodiment, the interpenetrating polymer network includes 
poly(acrylic acid) as one polymer and poly(acrylamide) as another, 
interpenetrating, polymer, and water as the liquid medium. In another 
embodiment, the interpenetrating polymer network includes two 
interpenetrating polymers of poly(acrylic acid) and water, as the liquid 
medium. Other examples include: poly(acrylic 
acid)/poly(methacrylamidopropyl-trimethyl-ammonium-chloride(MAPTAC)/water; 
poly(acrylic acid)/poly(allylamide)/water; 
poly(styrene-sulfonate)/poly(allylamide)/water; etc. Examples of 
phase-transition gels including interpenetrating polymer networks formed 
of natural polymers include: gelatin/gelatin/water; deoxyribonucleic 
acid(single strand)/deoxyribonucleic acid(single strand)/water; 
agarose/agarose/water; etc. 
In still another illustration of the invention, an interpenetrating polymer 
network can be formed which exhibits a phase transition in response to a 
change in the liquid medium, such as a temperature change. The temperature 
change causes an interaction between the polymers of the interpenetrating 
polymer network, such as van der Waals forces, hydrophobic interaction, 
hydrogen bonding or ionic interaction. The interaction between the 
polymers causes the interpenetrating polymer network to exhibit phase 
transition. FIG. 5 is a plot of phase transitions for an interpenetrating 
polymer network gels of poly(acrylic acid) and poly(acrylamide) in water 
wherein the gels have been ionized in varying degrees and wherein the 
phase transitions are a result of hydrogen bonding between the 
interpenetrating polymers. 
The present invention has many applications. For example, the gel can 
interact with a body surface, such as the gut or skin. The gel can 
selectively absorb or release materials in the gut in response to specific 
environmental changes. Hormones, enzymes and other biotechnology products 
could be protected from destruction in the stomach if they were contained 
in a gel that was coated with an interpenetrating polymer network that was 
collapsed at low pH. For instance, oral insulin can be protected from 
being dissolved in the stomach by swelling insulin into a gel or a 
suitable medium which is then coated with an interpenetrating polymer 
network phase-transition gel that responds to pH. If the interpenetrating 
polymer network is designed to be in a contracted phase at low pH, then 
the insulin is protected when the polymer passes through the stomach. When 
the polymer and insulin reach the intestine, where the pH is relatively 
high, the interpenetrating polymer network coating exhibits a phase 
transition wherein the coating expands, thereby allowing the insulin to 
dissolve out of the gel. Also, physiologically active proteins, such as 
erythropoeitin, can be protected in this manner. It is to be understood, 
however, that the body surface does not have to be the gut. Further, any 
type of molecule, not just pharmaceutical compounds, can be released this 
way. 
In another application of the present invention, a phase-transition gel can 
be formed which, when disposed in the gastrointestinal tract, absorbs 
cholesterol from digesting food. The presence of a high level of 
cholesterol causes an interaction between the cholesterol and the 
phase-transition gel to thereby cause the gel to exhibit a significantly 
large expansion. Expansion of the gel, in turn, causes the gel to absorb 
the cholesterol, so that it is not absorbed through the gut wall. 
An example of an interpenetrating polymer network phase-transition gel 
which would absorb cholesterol includes an interpenetrating polymer 
network of poly(acrylic acid) gelled with bile salts and water. In one 
embodiment, obesity could be controlled by the interpenetrating polymer 
network phase-transition gel by absorption of fat from the intestinal 
tract. Since these gels can be programmed to undergo a significantly large 
volume change, thirty grams of dietary fat could be removed with a small 
amount of material. By incorporating bile salts, cholesterol can be 
selectively absorbed and filtered from the system. Hydrogen bonding of the 
interpenetrating polymer network changes by absorbing cholesterol. 
In still another example of the invention, a phase-transition gel can be 
formed which can selectively separate a chemical from a liquid medium 
passing through an interpenetrating polymer network of the 
phase-transition gel. The presence of the chemical to be separated from 
the liquid medium can cause an interaction with the interpenetrating 
polymer network which, in turn, causes a significantly large contraction 
of the interpenetrating polymer network. The chemical is thereby trapped 
by the phase transition gel and separated from the liquid medium passing 
through the gel. An example of a phase-transition gel which selectively 
separates a chemical from a liquid medium is a gel having an 
interpenetrating polymer network of poly(N-isopropylacrylamide) and 
poly(acrylic acid) gelled with water. This gel can selectively absorb 
surfactants and expand. 
Interpenetrating polymer networks can be formed which exhibit a phase 
transition for absorbing liquids, such as urine and blood, containing 
relatively high concentrations of ions. For example, the gels can 
selectively absorb osmotic fluids having an ion concentration of up to 
about 150 mM. Such gels could be used in disposable diapers and sanitary 
napkins. At least one polymer of the interpenetrating polymer network is 
an acidic (or anionic) polymer network and at least one other polymer of 
the interpenetrating polymer network is a basic (or cationic) polymer. 
Incorporation of acidic and basic polymers into the interpenetrating 
polymer network causes ions in the absorbed liquid, such as sodium ions 
and chloride ions, to become bonded to the polymers, thereby reducing 
osmotic pressure caused by the ions and enabling phase transition for 
absorption of fluids containing relatively high concentrations of ions. 
One example of such a gel includes an interpenetrating polymer network of 
an anionic poly(acrylic acid) and a cationic 
poly(methacrylamidopropyl-trimethyl-ammonium-chloride) (poly(MAPTAC)) in 
water. This gel swells about tenfold in salt solutions having 
concentrations greater than about 100 mM. Another example of a suitable 
gel includes an interpenetrating polymer gel of poly(styrene sulfuric 
acid) and poly(MAPTAC). It is to be understood, however, that any 
combination of acidic and basic polymers can be used. 
An interpenetrating polymer network phase-transition gel can also be formed 
which is salt-selective. An example of such a gel is a gel having an 
interpenetrating polymer network including poly(N-isopropylacrylamide) and 
poly(acrylic acid) gelled in valinomycin and water, which selectively 
absorbs potassium ions and undergoes phase transition. A clinically more 
significant enhancement would involve polymerizing the ionophore into the 
polymer. The gel could be swallowed after a meal and would absorb a 
fraction of its weight. A patient that had adrenal insufficiency and high 
serum potassium might benefit by taking such a product. Likewise, a 
patient with high blood pressure who is on a salt-restricted diet could 
benefit from a sodium-absorbing gel. Also, a patient who has renal failure 
and high phosphate could benefit by a phosphate-absorbing gel. 
Also, since interpenetrating polymer network gels can be designed to 
recognize hydrogen, hydrophobic interactions, ions, and van der Waals 
forces, it is conceivable that custom-engineered interpenetrating polymer 
network gels can be created to recognize specific molecules that have 
given sequences. An application of this would be a gel to absorb microbial 
toxins from a bacteria, toxin, virus or other microorganism that is 
causing diarrhea. By absorbing the toxin, the toxin would not be available 
to irritate the gastrointestinal tract. 
The invention will now be further and specifically described by the 
following examples. All parts and percentages are by weight unless 
otherwise stated. 
Exemplification 
Three different gels were formed comprising interpenetrating polymer 
networks of poly(acrylamide) and poly(acrylic acid). The first gel, Gel 1, 
was formed without ionization of either polymer in the interpenetrating 
polymer network. Three percent and six percent respectively, of the 
acrylic acid groups in the poly(acrylic acid) of Gels 2 and 3 were 
ionized. 
The interpenetrating polymer networks of all three gels were prepared by 
first preparing three poly(acrylamide) gels. Five grams of acrylamide and 
0.133 grams of N,N'-methylenebisacrylamide, commercially available from 
Bio-Rad Laboratories, and 120 microliters tetramethylenediamine 
(accelerator), commercially available from Bio-Rad Laboratories, were 
dissolved in 100 ml of water to form an aqueous solution. One ml of four 
percent aqueous ammonium persulfate solution (initiator), commercially 
available from Mallinkrodt was combined with the degassed aqueous solution 
to form a reaction solution. 
The reaction solution was transferred to a glass capillary tube having a 
length of twenty centimeters and an internal diameter of 0.1 mm. The 
reaction solution gelled in the capillary tube, whereby the monomer and 
crosslinking agent reacted to form three poly(acrylamide) gels. Following 
gellation, the poly(acrylamide) gels were removed from the capillary 
tubes, washed with water and dried. 
Five grams of distilled acrylic acid, 0.133 grams of 
N,N'methylenebisacrylamide and 40 mg of four percent ammonium persulfate 
solution were dissolved in 100 ml of water to form an aqueous solution. 
Three glass capillary tubes having a length of twenty centimeters and an 
internal diameter of 0.1 mm were filled with the aqueous solution. An 
amount of sodium hydroxide was added to two of the glass capillaries in an 
amount sufficient to ionize three percent of the acrylic acid groups in 
one of the capillary tubes and six percent of the acrylic acid groups in 
the other capillary tube. 
The three dried poly(acrylamide) gels were inserted into the capillary 
tubes containing the aqueous solutions. The poly(acrylamide) gels were 
thereby swollen with the aqueous solution of acrylic acid. Radical 
polymerization was then initiated by raising the temperature of the 
poly(acrylamide) gels and aqueous solutions to a temperature of about 
60.degree. C., thereby forming interpenetrating polymer networks of 
poly(acrylimide) and poly(acrylic acid). Gel 1 included poly(acrylic acid) 
which was not ionized. In Gel 2, the poly(acrylic acid) was three percent 
ionized. In Gel 3, the poly(acrylic acid) was six percent ionized. The 
three gels were removed from their respective capillary tubes and washed 
with water. All three gels were disposed in separate glass micropipettes 
having an internal diameter of one millimeter. 
The size and shape of the gels in the micropipettes were monitored at 
different temperatures using a Model C1966 AVEC image processor, 
commercially available from Hamamatsu Photonics, Inc. Each gel was 
submerged in water in a micropipette. The temperature of the water in the 
gels was controlled to within 0.01.degree. C. using circulating water at 
temperatures in the range of between 4.degree. C. and 50.degree. C. 
FIG. 6 is a plot of the volume of Gel 1, Gel 2, and Gel 3 relative to their 
volumes in a contracted phase during expansion of the gels in response to 
an increase in temperature. Curve A represents the volume of Gel 1. As can 
be seen in Curve A, Gel 1 exhibits a continuous transition at a 
temperature in the range of between about 20.degree. C. and 40.degree. C. 
The volume change of Gel 2 and Gel 3, wherein the poly(acrylic acid) has 
been partially ionized, are represented by Curves B and C, respectively. 
As can be seen from Curves B and C, Gel 2 and Gel 3 undergo phase 
transition during an increase in temperature. Gel 2 expands 
discontinuously at a temperature of about 20.degree. C. Gel 3 exhibits a 
significantly large expansion at a temperature of about 18.degree. C. The 
phase transition is a signficantly large expansion of the gels during a 
rise in temperature of the water in which the gels are submerged. 
Also, as can be seen in Curves B and C, Gel 3 exhibits a phase transition 
at a temperature slightly lower than the phase transition of Gel 2. The 
temperature at which phase transition occurs therefore decreases with an 
increasing amount of ionization of the poly(acrylic acid) polymer in the 
interpenetrating polymer network of the phase-transition gel. 
FIG. 7 is a plot of the volume of Gel 1, Gel 2, and Gel 3 during 
contraction of the gels as the temperature of the water in which the gels 
are submerged is lowered. Gel 1, in which the poly(acrylic acid) polymer 
is not ionized, exhibits a continuous volume change in a temperature in 
the range of between 40.degree. C. and about 20.degree. C. Curve D 
represents the volume change of Gel 1, relative to the volume of Gel 1 in 
a contracted phase. 
Curves E and F represent the volume change of Gel 2 and Gel 3, 
respectively, during contraction of Gel 2 and Gel 3. Gel 2 exhibits a 
significantly large contraction at a temperature of about 15.degree. C. 
Gel 3 exhibits a significantly large contraction at a temperature of about 
12.degree. C. As with expansion, the temperature of significantly large 
contraction decreases with an increasing amount of ionization of the 
acrylic acid groups of the poly(acrylic acid) in the interpenetrating 
polymer network of the phase-transition gel. 
FIG. 8 is a plot of the volume of Gel 1 and Gel 3 during expansion and 
contraction of the gels during a change of temperature of the water in 
which the gels are submerged. The volume of the gels are measured relative 
to the volume of the gels in the contracted state. Curve G represents the 
volume of Gel 1 during expansion of the gel as the temperature of the 
water is increased. Curve H represents the volume of Gel 1 during 
contraction of the gel as the temperature of the water is lowered. As can 
be seen from Curves G and H, Gel 1 exhibits hysteresis, wherein the volume 
of Gel 1 is different at given temperatures according to whether the gel 
is expanding or contracting. 
Curve I represents the volume of Gel 3 during expansion of the gel. Curve J 
represents the volume of Gel 3 during contraction of the gel. As can be 
seen from Curves I and J, hysteresis of Gel 3 is exhibited during phase 
transition. The amount of hysteresis is more pronounced in Gel 3, which 
exhibits a phase transition than in Gel 1 which exhibits only a continuous 
volume change over the same temperature range. 
Equivalents 
Those skilled in the art will recognize, or be able to ascertain using no 
more than routine experimentation, many equivalents to the specific 
embodiment of the invention described specifically herein. Such 
equivalents are intended to be encompassed in the scope of the following 
claims.