U.S. patent number RE35,068 [Application Number 08/219,320] was granted by the patent office on 1995-10-17 for collapsible gel compositions.
This patent grant is currently assigned to Massachusetts Institute of Technology. Invention is credited to Izumi Nishio, Shao-Tang Sun, Toyoichi Tanaka.
United States Patent |
RE35,068 |
Tanaka , et al. |
October 17, 1995 |
**Please see images for:
( Certificate of Correction ) ** |
Collapsible gel compositions
Abstract
Ionized crosslinked polyacrylamide gels are provided that are
capable of drastic volume changes in response to minor changes in
solvent concentration, temperature of pH or salt concentration of
the solvent. The gels can contain a metal ion.
Inventors: |
Tanaka; Toyoichi (Wellesley,
MA), Nishio; Izumi (Tokyo, JP), Sun; Shao-Tang
(Newark, DE) |
Assignee: |
Massachusetts Institute of
Technology (Cambridge, MA)
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Family
ID: |
27541703 |
Appl.
No.: |
08/219,320 |
Filed: |
March 29, 1994 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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846645 |
Mar 31, 1986 |
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712459 |
Mar 15, 1985 |
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658075 |
Oct 5, 1984 |
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473161 |
Jan 28, 1983 |
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Reissue of: |
470977 |
Jan 26, 1990 |
05100933 |
Mar 31, 1992 |
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Current U.S.
Class: |
523/300; 204/456;
204/470; 205/688; 524/364; 524/365; 524/379; 524/389; 524/401;
524/434; 524/436; 524/439; 524/555 |
Current CPC
Class: |
C08J
3/075 (20130101); C08J 3/28 (20130101); C08J
2333/26 (20130101) |
Current International
Class: |
C08J
3/28 (20060101); C08J 3/075 (20060101); C08J
3/02 (20060101); C08J 003/28 () |
Field of
Search: |
;523/300
;524/379,555,364,365,389,401,434,436,439 ;204/131 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Tanaka et al, Phys. Rev. Lett., 38: 771-774 (1977). .
Tanaka, T., Phys. Rev. Lett., 40(12): 820-823 (1978)..
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Primary Examiner: Reddick; Judy M.
Attorney, Agent or Firm: Choate, Hall & Stewart
Government Interests
The Government has the rights in this invention pursuant to grant
Number DMR-78-24185 awarded by the National Science Foundation and
grant Number NIH-2-RO1-EY01696-04 awarded by the National
Institutes of Health.
Parent Case Text
This application is a continuation of application Ser. No.
06/846,645, filed Mar. 31, 1986 now abandoned, which is a
continuation of Ser. No. 06/712,459, filed Mar. 15, 1985 now
abandoned, which is a FWC of Ser. No. 06/658,075, filed Oct. 5,
1984, now abandoned which is a continuation of Ser. No. 06/473,161,
filed Jan. 28, 1983, now abandoned, designated in PCT Application
No. PCT/US81/0073, filed June 8, 1981, the contents ar hereby
incorporated by reference.
Claims
We claim:
1. A method of causing a discontinuous volume change in a gel, said
gel including a polymer network defining an interstitial space and
a gel fluid filling the interstitial space, comprising the steps
of:
a) ionizing the gel sufficient to cause the gel to exhibit a
discontinuous volume change upon exposure to sufficient
electromagnetic radiation; and
b) exposing the gel to electromagnetic radiation sufficient to
cause the gel to exhibit a discontinuous volume change.
2. A method of claim 1 wherein the electromagnetic radiation is
supplied by conducting an electric current through the gel.
3. A method of causing a discontinuous volume change in a gel, said
gel including a polymer network defining an interstitial space and
a gel fluid filling the interstitial space, comprising the steps
of:
a) ionizing the gel sufficient to cause the gel to exhibit a
discontinuous volume change upon exposure to metal ions; and
b) exposing the gel to metal ions, whereby the gel exhibits a
discontinuous volume change.
4. A method of causing a discontinuous volume change in a gel, said
gel including a polymer network defining an interstitial space, a
concentration of metal ions dispersed within the interstitial space
and a gel fluid filling the interstitial space, comprising the
steps of:
a) ionizing the gel sufficient to cause the gel to exhibit a
discontinuous volume change upon sufficiently changing the metal
ion concentration to which the gel is exposed to cause the gel to
exhibit a discontinuous volume change; and
b) changing the metal ion concentration to which the gel is exposed
sufficient to cause the gel to exhibit a discontinuous volume
change. .Iadd.
5. A method of causing a discontinuous volume change in a gel
including a polymer network defining an interstitial space and a
gel fluid filing the interstitial space, comprising the steps
of:
a) providing a gel ionized in an amount sufficient to cause the gel
to exhibit a discontinuous volume change upon exposure to
sufficient electromagnetic radiation; and
b) exposing the gel to electromagnetic radiation sufficient to
cause the gel to exhibit a discontinuous volume change..Iaddend.
.Iadd.6. A method of causing a discontinuous volume change in a
gel, said gel including a polymer network defining an interstitial
space and a gel fluid filing the interstitial space, comprising the
steps of:
a) providing a gel ionized in an amount sufficient to cause the gel
to exhibit a discontinuous volume change upon exposure to metal
ions; and
b) exposing the gel to metal ions, whereby the gel exhibits a
discontinuous volume change..Iaddend. .Iadd.7. A method of causing
a discontinuous volume change in a gel said gel including a polymer
network defining an interstitial space, a concentration of metal
ions dispersed within the interstitial space and a gel fluid filing
the interstitial space, comprising the steps of:
a) providing a gel ionized in an amount sufficient to cause the gel
to exhibit a discontinuous volume change upon sufficiently changing
the metal ion concentration to which the gel is exposed to cause
the gel to exhibit a discontinuous volume change; and
b) changing the metal ion concentration to which the gel is exposed
sufficient to cause the gel to exhibit a discontinuous volume
change..Iaddend.
Description
BACKGROUND OF THE INVENTION
This invention relates to novel gel compositions which are capable
of discontinuous volume change of several hundred times induced by
infinitesimal changes in environment.
Gel is a form of material between the liquid and solid state. It
consists of a crosslinked network of long polymer molecules with
liquid molecules trapped within the network. Gels play important
roles in various aspects of our everyday life.
In chemistry and biochemistry, gels are used extensively as
matrices for chromatography and electrophoresis--analytical methods
that separate molecules according to their molecular weights and
charges. In these techniques, the pore size of the crosslinked
polymer network plays an essential role in its sieving effects.
Gels also are important intermediate products in polymer products
such as rubbers, plastics, glues and membranes.
In 1973, a new technique of light scattering spectroscopy was first
introduced to gel studies. It was demonstrated that by measuring
the intensity and the time dependence of fluctuations of laser
light scattered from a gel, it is possible to determine the
viscoelastic properties of the gel, that, is, the elasticity of the
polymer network and the viscous interaction between the network and
the gel fluid. Recently, with the help of this powerful technique,
very interesting phenomena in permanently crosslinked gels gave
been found: as the temperature is lowered, the polymer network
becomes increasingly compressible, and at a certain temperature it
becomes infinitely compressible. At the same time, the effective
pore size of the network increases and diverges. It is also
observed that the volume of polyacrylamide gels ranges reversibly
by a factor as large as several hundred by an infinitesimal change
in external conditions such as solvent composition or temperature.
Tanaka, Physical Review Letters, Vol. 40, No. 12, pgs. 820-823,
1978 and Tanaka et al., Physical Review Letters, Vol. 38, No. 14,
pgs. 771-774, 1977. While these gels can be useful as a switching
device or artificial muscle due to their ability to undergo
discrete volume changes caused by minute environmental changes, it
would be desirable to provide gels which undergo greater volume
changes in order to maximize the efficiency of the functions.
SUMMARY OF THE INVENTION
This invention is based upon the discovery that ionized acrylamide
gel compositions are capable of more drastic volume change as
compared to the acrylamide gel composition of the prior art. The
gel compositions comprise a corsslinked partially ionized
polyacrylamide gel wherein between up to 20% of the amide groups
are hydrolyzed to carboxyl groups. The gel includes a solvent of a
critical concentration at which a slight change in temperature, pH
or, salt concentration causes the gel to shrink or swell
drastically. The particular critical concentration utilized in the
gel composition depends upon the solvent employed, the temperature
of the gel and the degree of hydrolysis of the gel. The gel also
can contain a positive metal ion such as sodium or magnesium which
has the effect of increasing the change in gel volume caused by
change of solvent concentration, temperature, pH or, salt
concentration.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows the swelling ratio of polyacrylamide gels as a
function of acetone concentration and degree of hydrolysis.
FIG. 2 shows the swelling ratio of a polyacrylamide gel as a
function of temperature at a given solvent concentration.
FIG. 3 shows the osmotic pressure of a polyacrylamide gel as a
function of the degree of swelling of the gel.
FIG. 4 is a phase diagram of a polyacrylamide gel as a function of
degree of ionization and temperature or solvent concentration.
FIG. 5 is a graph of the swelling ratio of polyacrylamide gel as a
function of pH.
FIG. 6 is a graph showing the effect of sodium ion or magnesium ion
concentration on the swelling ratio of a polyacrylamide gel.
DESCRIPTION OF SPECIFIC EMBODIMENTS
The chemical structure of the network plays as essential role in
determining the collapse and other equilibrium behavior of gels.
Acrylamide gel consists of linear polymer chains of acrylamide
molecules crosslinked by bisacrylamide molecules, which consists of
two acrylamide molecules connected together. The interstitial space
of the polymer network is filled with water. The gel can be
prepared from acrylamide monomers and bisacrylamide monomers that
are dissolved in water. Thereafter a polymerization initiator such
as ammonium persulfate and TEMED (tetramethyl-ethylene-diamine) are
added to the solution. First, a reaction between amonium persulfate
and TEMED occurs which produces free radical electrons on the TEMED
molecules. These molecules of TEMED with free radicals are the
nuclei of the polymerization. The free radical on the TEMED
molecule attacks and opens one of the double bonded carbon atoms of
acrylamide and bisacrylamide monomers. One electron of the
acrylamide or bisacrylamide double bond pairs with the odd electron
of the free radical to form a bond between the free radical and
this carbon atom; the remaining electron of the double bond shifts
to the other carbon atom, which then becomes a free radical. In the
way, the active center shifts uniquely to the newly added monomer,
which then becomes capable of adding another monomer. The
acrylamide monomers, having one double bond, are therefore
polymerized into a linear chain. The bisacrylamide molecule,
consisting of two connected acrylamide molecules have two double
bonds and serve as crosslinks. This chain reaction continues until
a network consisting of practically an infinite number of bonded
monomers if formed. Both polymerization and crosslinking take place
almost instantaneously when the first free radical appears. The
infinite polymer network is thus formed at this instant.
The gels are then taken out of the container. With syringe and
needle the gel can be carefully separated from the container wall
by forcing water between the gel and the wall. Each gel sample is
then soaked in water so that all the residual acrylamide,
bisacrylamide monomers and the initiators are washed away. The gels
are then immersed in a basic solution of TEMED (4% in volume)
having pH of 12 for a time period of up to 60 days. During this
immersion period the acrylamide groups of the network, --COHN.sub.2
are hydrolyzed into carboxyl groups, --COOH, a quarter of which are
automatically ionized into carboxyl ions, --COO--, and hydrogen
ions, H.sup.+. The polymer network becomes negatively charged
having positive hydrogen ions, H.sup.+, in the interstitial space.
The longer the immersion time, the more charges the polymer network
becomes. After the hydrolysis the gels are soaked in water to be
washed. At this final stage of sample preparation the gel is
largely swollen in water. A fully hydrolyzed gel, for example,
swells 30 times form the original volume. By this procedure, gel
samples having different degrees of ionization can be prepared.
Each gel sample then is immersed in a large volume of a solvent
such as acetone, acetone and water mixture, ethanol and water,
methanol and water. For example, when using a mixture of acetone
and water, the water inside the network and the acetone-water
mixture outside are exchanged within 30 minutes by diffusion. The
equilibrium state, however, is not yet reached. Depending on the
acetone concentration, some of the gels shrink. As shown in FIG. 1,
the degree of swelling as a function of acetone concentration for
various hydrolysis ties at room temperature. The degree of swelling
is defined as the ratio of the final volume, V to the original
volume V.sup.+ of the polymer network when the gel is first
polymerized. At low acetone concentration, the gel is swollen. As
the acetone concentration is increased, the gel gradually shrinks.
The degree of swelling changes continuously with the acetone
concentration. For a gel with two days of hydrolysis, the swelling
curve shows an inflection point at which the swelling curve has a
zero slope. For a gel with 4 days at hydrolysis, the swelling curve
has a discrete transition. At low concentrations of acetone, the
gel swells. As the acetone concentration increases, the gel shrinks
a little; but at 42% acetone concentration of the network suddenly
collapses. Above this concentration, the network remains shrunken.
This phenomenon is entirely reversible. When a collapsed gel is
transferred into a mixture having an acetone concentration lower
than 42%, the gel swells until it reaches the network volume
indicated by the swelling curve. For gels hydrolyzed longer than 4
days, it can be seen that the size of the collapse becomes larger
with hydrolysis. For 60 days of hydrolysis, the volume change at
the transition is more than 350 fold. Therefore, by changing the
acetone concentration infinitesimally a reversible volume change of
several hundred-fold can be effected.
As shown in FIG. 2, the collapse is also observed when for a fixed
acetone concentration the temperature is varied. The illustration
shows the swelling ratio curve of the gel (hydrolyzed for 8 days)
immersed in the mixture of 42% acetone concentration. At
temperatures higher than room temperature the network swells, and
below it, it shrinks. There is a discontinuous volume change at
room temperature.
The total pressure that acts either to expand or to shrink the
polymer network of a gel consists of three types of pressures: the
rubber elasticity, the polymer-polymer affinity, and the pressure
of hydrogen ions. The rubber elasticity is attributed to the
elasticity of individual polymer chains constituting the gel
network. Consider a single polymers chain consisting of freely
joined segments but with both its ends fixed. Due to the random
thermal motions of each segment, there are forces which pull the
fixed ends. When the polymer is in a compressed configuration, the
forces act outward. When the polymer has an extended configuration,
on the other hand, the forces act inward. Such forces create the
elasticity which acts to keep the polymer chain at a particular
degree of expansion at which there are no forces to expand or
shrink the polymer chain. The more active the motions of the
segments, the stronger the elasticity. The elasticity is therefore
proportional to the absolute temperature.
It is essential to take the interactions among polymer segments and
solvent molecules into account in order to understand the collapse
of the gels. In the acrylamide gels with an acetone-water mixture
as the gel fluid, the polymer segments have greater affinity to
themselves than to the solvent molecules. Such affinity among
polymer segments creates a pressure acting to shrink the network.
Such negative pressure is independent of temperature, but does
depend on the solvent composition. Since the acrylamide molecules
are less soluble in acetone than in water, this negative pressure
increases with acetone concentration.
Finally, the ionization produces a positive pressure to expand the
network. In the gel the hydrogen ions, H.sup.+ are freely moving
without experiencing much repulsive force among themselves because
the negative charge background of the polymer network shields the
electric repulsion. The hydrogen ions, however, cannot leave the
network since this would violate the charge neutrality condition in
the gel. Thus, the hydrogen ions behave like a gas confined within
a small volume and produce a gas type positive pressure. Naturally,
this pressure is proportional to the absolute temperature.
The total pressure on a gel network (i.e., the osmotic pressure of
a gel) consists, therefore, of a pressure of rubber elasticity, a
pressure of polymer-polymer affinity, and a pressure of hydrogen
ions. The equilibrium properties of a gel are determined by their
balance, which can be varied by changing whether the temperature or
the solvent composition. This is because the pressures of the
rubber elasticity and the hydrogen ions are proportional to the
absolute temperature, while the polymer-polymer affinity is not. It
is also because the latter depends on the solvent composition
whereas the former two do not.
The osmotic pressure of a gel network varies with the degree of
swelling at various temperatures, or equivalently, at various
acetone concentrations. Such curves of the osmotic pressure vs the
degree of swelling are called isotherms. At high temperatures, when
a gel is shrunken, its osmotic pressure is positive and the network
tends to swell. If a little amount of solvent is added, the gel
absorbs it and swells. As the network swells, the osmotic pressure
monotonically decreases. At a certain network volume, the osmotic
pressure becomes zero, and the gel will not absorb additional
solvent. As the temperature is lowered, an isotherm shows an
inflection point, then follows a wiggle having a miximum and a
minimum. In between these points, the isotherm has a positive
slope. This appears because the polymer-polymer affinity becomes
more important at lower temperatures and this pressure has a
positive slope. Such a positive slope implies a negative
compressibility, that is; given more pressure, the network expands.
Such a condition is clearly unstable. When thermal agitations cause
an infinitesimal local swelling in the network, the osmotic
pressure of that region then increases, which leads to further
expansion of that region in the network. Therefore, once the local
network swelling occurs, it swells further. In the same way, once
it starts to shrink locally, it shrinks further. The gel is
unstable and separates into two regions, one swollen from the
original state and one shrunken. They are shown in FIG. 3 as the
two end points of the horizontal line replacing the positive slope.
The line is drawn so that is produces equal areas in the loop.
All the elements required for a complete description of the
equilibrium properties of the gel are contained in the isotherms.
The swelling curve is defined by the condition of zero osmotic
pressure, which assures there is equilibrium between the gel and
the surrounding fluid. The osmotic pressure is zero at the
intercept of each isotherm with the volume axis. The swelling curve
is shown in the lower diagram as the boundary. The curve indicates
a discrete transition at a temperature at which the horizontal lien
meets the zero osmotic pressure line. This explains the discrete
transition observed by the acrylamide gel.
As shown in FIG. 3 (upper diagram), the osmotic pressure of a gel
is plotted as a function of the degree of swelling of the network
volume for various temperatures or acetone concentrations. The
regions 1 and 2 represent positive osmotic pressure, whereas the
region 3 represents negative osmotic pressure. In the region 1, the
curves of osmotic pressure have a positive slope, which indicates
unstable states for the gel. The gel, then separates into two
domains of gels having two different degrees of swelling
represented by the end points of the horizontal line which produces
two equal areas (shown as shadowed) in the curve. The volume of a
gel in equilibrium with the surrounding fluid is achieved at zero
osmotic pressure. The zero osmotic pressure is shown as the upper
boundary of the region. The discrete transition occurs when the
horizontal line in the osmotic pressure curve touches this
boundary.
As shown in FIG. 3 (lower diagram), each point in this phase
diagram represents a state of a gel. A gel state represented by a
point in the region 1 is stable. In the region 2 a gel is unstable
and separates into two domains of gels having different degrees of
swelling. In the region 3, a gel shrinks until its degree of
swelling reaches the boundary of the region 3. On this boundary,
the osmotic pressure of a gel is zero; it represents the degrees of
swelling of a gel in equilibrium with the surrounding fluid.
Two conditions are required for a gel to be stable. First, the
osmotic pressure of the gel must be zero or positive. It is
negative, the network shrinks excluding the pure fluid. Second, the
gel state should be outside the horizontal line. The phase diagram
obtained by such consideration is shown in FIG. 3 in the lower
graphs of the illustration. The horizontal axis shows the degree of
swelling of the network. The vertical axis indicates either
temperature or solvent composition. Each point on the graph
uniquely defines the state of a gel. The regions indicated by
region 3 represent states of a gel having negative osmotic
pressures. Gel in this region is unstable and shrinks. In the
region 1, a gel is not stable and separates into two regions. Some
part of the gel shrinks and some other part swells, and the domain
is, therefore, called the coexistence curve. The two separated gel
phases become identical at the maximum of the coexistence curve,
and the maximum is called the critical point. A gel having a state
represented by a point in the region 1 is stable.
Experimental results show that the volume change at the transition
can vary depending on the degree of ionization of the polymer
network. The volume change at the transition is observed to range
from zero continuous change) to 350 fold.
From FIG. 4, it can be seen that the volume change of a gel at the
transition is determined by the relative position of the critical
point and transition temperature (or solvent composition). If the
critical point is well above the transition point, the volume
change at the transition is large. As they come closer, the volume
change becomes smaller, and eventually it becomes continuous when
the critical temperature coincides with the transition temperature.
When the polymer network is first polymerized it may be expected
that there is no ionization in the network. During immersion in a
basic solution the polymer network is gradually ionized. The
ionization of the polymer network produces an excess positive
pressure due to the hydrogen ions. In order for the collapse to
occur, this excess positive pressure must be matched by the excess
negative pressure of polymer-polymer affinity. This is achieved
either by lowering the temperature or increasing the acetone
concentration. The network collapses at lower temperatures or at
higher acetone concentrations. The transition point is, therefore,
located below the critical point. With less ionization the critical
point and the transition point become closer. At a particular
degree of ionization they coincide and the swelling curve becomes
continuous. For smaller ionization, the critical point becomes
buried in the region of negative osmotic pressure. In other words,
the critical point becomes unstable and ceases to exist. This final
point is called the critical end point for the critical point.
Thus, the volume change at the transition is determined by the
degree of hydrolysis of the network.
A gel collapse also occurs when, for given temperature and acetone
concentration, the ionization is changed by varying the pH of the
solvent or by adding ions to the solvent. The equilibrium gel
volume is given by a intercept line of the zero-osmotic-pressure
plane shown in FIG. 4 and the horizontal plane which represents a
fixed temperature and acetone concentration. When the horizontal
plane is above the critical end point, the intercept is continuous.
When the plane is below it, the intercepts has discontinuity. A
continuous volume change is observed upon changing the pH, when a
solvent with low acetone concentration is used, and a discontinuous
curve when a higher acetone concentration is used. As shown in FIG.
5, gel collapse changes with pH of the solvent. The swelling curve
shows a continuous change when water is used as a solvent, whereas
the swelling curve is discrete when 50% acetone-water mixture is
used.
The effective ionization can also be modified by adding metal ions
such as by adding salts such as NaCl, KCl, MgCl.sub.2, or the like,
since the positive ions shield the negative charges of the polymer
network. The transition concentration for the divalent ion,
Mg.sup.++, is 4000 times smaller than that for the monovalent ion,
Na.sup.+. Surprisingly, the large difference can be explained by
the fact that only half as many divalent ions are needed to
neutralize the network as monovalent ions. It is interesting to
note that divalent ions have an essential role in the contraction
of muscle, a process that is some respects seems to resemble the
collapse of a gel.
Finally, the transition can be induced by applying an electric
filed across the gel in a 50% acetone-water mixture. As shown in
FIG. 7, when switch 10 is closed to cause current flow between
anode 12 and cathode 14, gel cylinder 16 contracts. When an
electric field (.about.0.5/cm) is applied across the electrodes,
the negative charges on the polymer network are pulled toward the
positive electrode. This produces a pressure gradient along the
electric field. The pressure is strongest on the gel end at the
positive electrode and zero at the other end of the gel. There is a
critical pressure above which the gel collapses and below which it
is swollen. The gel, therefore, has a bottle shape as shown in FIG.
7. As shown in FIG. 8, the swollen portion becomes shorter as the
electric field becomes stronger. The collapsed part swells again
when the electric field is turned off.
The drastic volume change upon infinitesimal variation of external
conditions such as temperature, solvent, and an electric field
across the gel may have a variety of applications. Various chemical
engines may be designed using gels. A gel having such
mechanico-chemical amplification will be useful as an artificial
muscle. The time taken for swelling or shrinking is proportional to
the square of a linear of a gel. For example, for a cylindrical gel
of 1 cm diameter, it takes several days to reach a new equilibrium
volume. For cylindrical gel of 1 .mu.m in diameter, which is the
size of muscle filament, it would take only a few thousandths of a
second. This is quick enough to be used as an artificial muscle.
The discrete change in volume of a gel may be used as a
mechanico-chemical memory or a switch.
* * * * *