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A gel is a semi-solid that can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute
cross-link In chemistry and biology a cross-link is a bond or a short sequence of bonds that links one polymer chain to another. These links may take the form of covalent bonds or ionic bonds and the polymers can be either synthetic polymers or natural ...
ed system, which exhibits no flow when in the steady-state, although the liquid phase may still diffuse through this system. A gel has been defined phenomenologically as a soft, solid or solid-like material consisting of two or more components, one of which is a liquid, present in substantial quantity. By weight, gels are mostly liquid, yet they behave like solids because of a three-dimensional cross-linked network within the liquid. It is the crosslinking within the fluid that gives a gel its structure (hardness) and contributes to the adhesive stick (
tack TACK is a group of archaea acronym for Thaumarchaeota (now Nitrososphaerota), Aigarchaeota, Crenarchaeota (now Thermoproteota), and Korarchaeota, the first groups discovered. They are found in different environments ranging from acidophilic the ...
). In this way, gels are a dispersion of molecules of a liquid within a solid medium. The word ''gel'' was coined by 19th-century Scottish chemist Thomas Graham by
clipping Clipping may refer to: Words * Clipping (morphology), the formation of a new word by shortening it, e.g. "ad" from "advertisement" * Clipping (phonetics), shortening the articulation of a speech sound, usually a vowel * Clipping (publications) ...
from ''
gelatine Gelatin or gelatine (from la, gelatus meaning "stiff" or "frozen") is a translucent, colorless, flavorless food ingredient, commonly derived from collagen taken from animal body parts. It is brittle when dry and rubbery when moist. It may also ...
''. The process of forming a gel is called
gelation In polymer chemistry, gelation (gel transition) is the formation of a gel from a system with polymers. Branched polymers can form links between the chains, which lead to progressively larger polymers. As the linking continues, larger branched p ...
.


IUPAC definition

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Composition

Gels consist of a solid three-dimensional network that spans the volume of a liquid medium and ensnares it through
surface tension Surface tension is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension is what allows objects with a higher density than water such as razor blades and insects (e.g. water striders) to f ...
effects. This internal network structure may result from physical bonds (physical gels) or
chemical bond A chemical bond is a lasting attraction between atoms or ions that enables the formation of molecules and crystals. The bond may result from the electrostatic force between oppositely charged ions as in ionic bonds, or through the sharing of ...
s (chemical gels), as well as
crystallite A crystallite is a small or even microscopic crystal which forms, for example, during the cooling of many materials. Crystallites are also referred to as grains. Bacillite is a type of crystallite. It is rodlike with parallel longulites. Stru ...
s or other junctions that remain intact within the extending fluid. Virtually any fluid can be used as an extender including water (
hydrogel A hydrogel is a crosslinked hydrophilic polymer that does not dissolve in water. They are highly absorbent yet maintain well defined structures. These properties underpin several applications, especially in the biomedical area. Many hydrogels ar ...
s), oil, and air (
aerogel Aerogels are a class of synthetic porous ultralight material derived from a gel, in which the liquid component for the gel has been replaced with a gas, without significant collapse of the gel structure. The result is a solid with extremely low ...
). Both by weight and volume, gels are mostly fluid in composition and thus exhibit densities similar to those of their constituent liquids. Edible jelly is a common example of a hydrogel and has approximately the density of water.


Polyionic polymers

Polyionic polymers are polymers with an ionic functional group. The ionic charges prevent the formation of tightly coiled polymer chains. This allows them to contribute more to
viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inte ...
in their stretched state, because the stretched-out polymer takes up more space. This is also the reason gel hardens. See
polyelectrolyte Polyelectrolytes are polymers whose repeating units bear an electrolyte group. Ion#Anions and cations, Polycations and polyanions are polyelectrolytes. These groups dissociation (chemistry), dissociate in aqueous solutions (water), making the pol ...
for more information.


Types


Colloidal gels

A
colloid A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others extend ...
al gel consists of a percolated network of particles in a fluid medium, providing
mechanical properties A materials property is an intensive property of a material, i.e., a physical property that does not depend on the amount of the material. These quantitative properties may be used as a metric by which the benefits of one material versus another ca ...
. The particles can show attractive interactions through osmotic depletion or through polymeric links. Colloidal gels have three phases in their lifespan: gelation, aging and collapse. The gel is initially formed by the assembly of particles into a space-spanning network, leading to a phase arrest. In the aging phase, the particles slowly rearrange to form thicker strands, increasing the elasticity of the material. Gels can also be collapsed and separated by external fields such as gravity. Colloidal gels show linear response rheology at low amplitudes. These materials have been explored as candidates for a drug release matrix.


Hydrogels

A
hydrogel A hydrogel is a crosslinked hydrophilic polymer that does not dissolve in water. They are highly absorbent yet maintain well defined structures. These properties underpin several applications, especially in the biomedical area. Many hydrogels ar ...
is a network of polymer chains that are hydrophilic, sometimes found as a
colloid A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others extend ...
al gel in which water is the dispersion medium. A three-dimensional solid results from the hydrophilic polymer chains being held together by cross-links. Because of the inherent cross-links, the structural integrity of the hydrogel network does not dissolve from the high concentration of water. Hydrogels are highly absorbent (they can contain over 90% water) natural or synthetic polymeric networks. Hydrogels also possess a degree of flexibility very similar to natural tissue, due to their significant water content. As responsive "
smart materials Smart materials, also called intelligent or responsive materials, are designed materials that have one or more properties that can be significantly changed in a controlled fashion by external stimuli, such as stress, moisture, electric or magnetic ...
," hydrogels can encapsulate chemical systems which upon stimulation by external factors such as a change of pH may cause specific compounds such as glucose to be liberated to the environment, in most cases by a gel-sol transition to the liquid state. Chemomechanical polymers are mostly also hydrogels, which upon stimulation change their volume and can serve as
actuators An actuator is a component of a machine that is responsible for moving and controlling a mechanism or system, for example by opening a valve. In simple terms, it is a "mover". An actuator requires a control device (controlled by control signal) an ...
or
sensors A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon. In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
. The first appearance of the term 'hydrogel' in the literature was in 1894.


Organogels

An organogel is a non-crystalline, non-glassy thermoreversible (
thermoplastic A thermoplastic, or thermosoft plastic, is any plastic polymer material that becomes pliable or moldable at a certain elevated temperature and solidifies upon cooling. Most thermoplastics have a high molecular weight. The polymer chains associate ...
) solid material composed of a
liquid A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. As such, it is one of the four fundamental states of matter (the others being solid, gas, a ...
organic phase entrapped in a three-dimensionally cross-linked network. The liquid can be, for example, an
organic solvent A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for p ...
,
mineral oil Mineral oil is any of various colorless, odorless, light mixtures of higher alkanes from a mineral source, particularly a distillate of petroleum, as distinct from usually edible vegetable oils. The name 'mineral oil' by itself is imprecise, ...
, or
vegetable oil Vegetable oils, or vegetable fats, are oils extracted from seeds or from other parts of fruits. Like animal fats, vegetable fats are ''mixtures'' of triglycerides. Soybean oil, grape seed oil, and cocoa butter are examples of seed oils, or fat ...
. The
solubility In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solubil ...
and
particle In the Outline of physical science, physical sciences, a particle (or corpuscule in older texts) is a small wikt:local, localized physical body, object which can be described by several physical property, physical or chemical property, chemical ...
dimensions of the structurant are important characteristics for the
elastic Elastic is a word often used to describe or identify certain types of elastomer, elastic used in garments or stretchable fabrics. Elastic may also refer to: Alternative name * Rubber band, ring-shaped band of rubber used to hold objects togeth ...
properties and firmness of the organogel. Often, these systems are based on
self-assembly Self-assembly is a process in which a disordered system of pre-existing components forms an organized structure or pattern as a consequence of specific, local interactions among the components themselves, without external direction. When the ...
of the structurant molecules. (An example of formation of an undesired thermoreversible network is the occurrence of wax crystallization in
petroleum Petroleum, also known as crude oil, or simply oil, is a naturally occurring yellowish-black liquid mixture of mainly hydrocarbons, and is found in geological formations. The name ''petroleum'' covers both naturally occurring unprocessed crud ...
.) Organogels have potential for use in a number of applications, such as in
pharmaceuticals A medication (also called medicament, medicine, pharmaceutical drug, medicinal drug or simply drug) is a drug used to diagnose, cure, treat, or prevent disease. Drug therapy (pharmacotherapy) is an important part of the medical field and rel ...
, cosmetics, art conservation, and food.


Xerogels

A xerogel is a solid formed from a gel by drying with unhindered shrinkage. Xerogels usually retain high porosity (15–50%) and enormous surface area (150–900 m2/g), along with very small
pore Pore may refer to: Biology Animal biology and microbiology * Sweat pore, an anatomical structure of the skin of humans (and other mammals) used for secretion of sweat * Hair follicle, an anatomical structure of the skin of humans (and other m ...
size (1–10 nm). When
solvent A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...
removal occurs under supercritical conditions, the network does not shrink and a highly porous, low-density material known as an ''
aerogel Aerogels are a class of synthetic porous ultralight material derived from a gel, in which the liquid component for the gel has been replaced with a gas, without significant collapse of the gel structure. The result is a solid with extremely low ...
'' is produced. Heat treatment of a xerogel at elevated temperature produces viscous
sintering Clinker nodules produced by sintering Sintering or frittage is the process of compacting and forming a solid mass of material by pressure or heat without melting it to the point of liquefaction. Sintering happens as part of a manufacturing ...
(shrinkage of the xerogel due to a small amount of viscous flow) which results in a denser and more robust solid, the density and porosity achieved depend on the sintering conditions.


Nanocomposite hydrogels

Nanocomposite hydrogels Nanocomposite hydrogels (NC gels) are nanomaterial-filled, hydrated, polymeric networks that exhibit higher elasticity and strength relative to traditionally made hydrogels. A range of natural and synthetic polymers are used to design nanocomposite ...
or hybrid hydrogels, are highly hydrated polymeric networks, either physically or covalently crosslinked with each other and/or with nanoparticles or nanostructures. Nanocomposite hydrogels can mimic native tissue properties, structure and microenvironment due to their hydrated and interconnected porous structure. A wide range of nanoparticles, such as carbon-based, polymeric, ceramic, and metallic nanomaterials can be incorporated within the hydrogel structure to obtain nanocomposites with tailored functionality. Nanocomposite hydrogels can be engineered to possess superior physical, chemical, electrical, thermal, and biological properties.


Properties

Many gels display
thixotropy Thixotropy is a time-dependent shear thinning property. Certain gels or fluids that are thick or viscous under static conditions will flow (become thinner, less viscous) over time when shaken, agitated, shear-stressed, or otherwise stressed ( ...
– they become fluid when agitated, but resolidify when resting. In general, gels are apparently solid, jelly-like materials. It is a type of
non-Newtonian fluid A non-Newtonian fluid is a fluid that does not follow Newton's law of viscosity, i.e., constant viscosity independent of stress. In non-Newtonian fluids, viscosity can change when under force to either more liquid or more solid. Ketchup, for exa ...
. By replacing the liquid with gas it is possible to prepare
aerogel Aerogels are a class of synthetic porous ultralight material derived from a gel, in which the liquid component for the gel has been replaced with a gas, without significant collapse of the gel structure. The result is a solid with extremely low ...
s, materials with exceptional properties including very low density, high specific surface areas, and excellent thermal insulation properties.


Thermodynamics of Gel Deformation

A gel is in essence the mixture of a polymer network and a
solvent A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...
phase. Upon stretching, the network
crosslinks Crosslinks is an evangelical Anglican missionary society, drawing its support mainly from parishes in the Church of England and Church of Ireland. It was known as the Bible Churchmen's Missionary Society (BCMS) until 1992. The BCMS was created as ...
are moved further apart from each other. Due to the polymer strands between crosslinks act as entropic springs, gels demonstrate elasticity like
rubber Rubber, also called India rubber, latex, Amazonian rubber, ''caucho'', or ''caoutchouc'', as initially produced, consists of polymers of the organic compound isoprene, with minor impurities of other organic compounds. Thailand, Malaysia, and ...
(which is just a polymer network, without solvent). This is so because the free energy penalty to stretch an ideal polymer segment N monomers of size b between crosslinks to an end-to-end distance R is approximately given by F_\text\sim kT\frac This is the origin of both gel and
rubber elasticity Rubber elasticity refers to a property of crosslinked rubber: it can be stretched by up to a factor of 10 from its original length and, when released, returns very nearly to its original length. This can be repeated many times with no apparent de ...
. But one key difference is that gel contains an additional solvent phase and hence is capable of having significant volume changes under
deformation Deformation can refer to: * Deformation (engineering), changes in an object's shape or form due to the application of a force or forces. ** Deformation (physics), such changes considered and analyzed as displacements of continuum bodies. * Defor ...
by taking in and out solvent. For example, a gel could swell to several times its initial volume after being immersed in a solvent after equilibrium is reached. This is the phenomenon of gel swelling. On the contrary, if we take the swelled gel out and allow the solvent to evaporate, the gel would shrink to roughly its original size. This gel volume change can alternatively be introduced by applying external forces. If a uniaxial compressive
stress Stress may refer to: Science and medicine * Stress (biology), an organism's response to a stressor such as an environmental condition * Stress (linguistics), relative emphasis or prominence given to a syllable in a word, or to a word in a phrase ...
is applied to a gel, some solvent contained in the gel would be squeezed out and the gel shrinks in the applied-stress direction. To study the gel mechanical state in equilibrium, a good starting point is to consider a cubic gel of volume V_ that is stretched by factors \lambda_1, \lambda_2 and \lambda_3 in the three orthogonal directions during swelling after being immersed in a solvent phase of initial volume V_. The final deformed volume of gel is then \lambda_1\lambda_2\lambda_3V_ and the total volume of the system is V_+V_ that is assumed constant during the swelling process for simplicity of treatment. The swelled state of the gel is now completely characterized by stretch factors \lambda_1, \lambda_2 and \lambda_3 and hence it is of interest to derive the deformation free energy as a function of them, denoted as f_\text(\lambda_1,\lambda_2,\lambda_3). For analogy to the historical treatment of
rubber elasticity Rubber elasticity refers to a property of crosslinked rubber: it can be stretched by up to a factor of 10 from its original length and, when released, returns very nearly to its original length. This can be repeated many times with no apparent de ...
and mixing free energy, f_\text(\lambda_1,\lambda_2,\lambda_3) is most often defined as the free energy difference after and before the swelling normalized by the initial gel volume V_, that is, a free energy difference density. The form of f_\text(\lambda_1,\lambda_2,\lambda_3) naturally assumes two contributions of radically different physical origins, one is associated with the
elastic deformation In engineering, deformation refers to the change in size or shape of an object. ''Displacements'' are the ''absolute'' change in position of a point on the object. Deflection is the relative change in external displacements on an object. Strain ...
of the polymer network, and the other with the mixing of the network with the solvent. Hence, we write f_\text(\lambda_1,\lambda_2,\lambda_3)=f_\text(\lambda_1,\lambda_2,\lambda_3)+f_\text(\lambda_1,\lambda_2,\lambda_3). We now consider the two contributions separately. The polymer elastic deformation term is independent of the solvent phase and has the same expression as a rubber, as derived in the Kuhn's theory of
rubber elasticity Rubber elasticity refers to a property of crosslinked rubber: it can be stretched by up to a factor of 10 from its original length and, when released, returns very nearly to its original length. This can be repeated many times with no apparent de ...
: f_\text(\lambda_1,\lambda_2,\lambda_3)=\frac(\lambda_1^2+\lambda_2^2+\lambda_3^2-3) where G_0 denotes the
shear modulus In materials science, shear modulus or modulus of rigidity, denoted by ''G'', or sometimes ''S'' or ''μ'', is a measure of the elastic shear stiffness of a material and is defined as the ratio of shear stress to the shear strain: :G \ \stackrel ...
of the initial state. On the other hand, the mixing term f_\text(\lambda_1,\lambda_2,\lambda_3) is usually treated by the Flory-Huggins free energy of concentrated polymer solutions f(\phi), where \phi is polymer volume fraction. Suppose the initial gel has a polymer volume fraction of \phi_0, the polymer volume fraction after swelling would be \phi=\phi_0/\lambda_1\lambda_2\lambda_3 since the number of monomers remains the same while the gel volume has increased by a factor of \lambda_1\lambda_2\lambda_3. As the polymer volume fraction decreases from \phi_0 to \phi, a polymer solution of concentration \phi_0 and volume V_ is mixed with a pure solvent of volume (\lambda_1\lambda_2\lambda_3-1)V_ to become a solution with polymer concentration \phi and volume \lambda_1\lambda_2\lambda_3V_. The free energy density change in this mixing step is given as V_f_\text(\lambda_1\lambda_2\lambda_3)=\lambda_1\lambda_2\lambda_3f(\phi)- _f(\phi_0)+(\lambda_1\lambda_2\lambda_3-1)f(0)/math> where on the right hand side, the first term is the Flory-Huggins energy density of the final swelled gel, the second is associated with the initial gel and the third is of the pure solvent prior to mixing. Substitution of \phi=\phi_0/\lambda_1\lambda_2\lambda_3 leads to f_\text(\lambda_1,\lambda_2,\lambda_3)=\frac (\phi)-f(0) (\phi_0)-f(0)/math>. Note that the second term is independent of the stretching factors \lambda_1, \lambda_2 and \lambda_3 and hence can be dropped in subsequent analysis. Now we make use of the Flory-Huggins free energy for a polymer-solvent solution that reads f(\phi)=\frac frac\ln\phi+(1-\phi)\ln(1-\phi)+\chi \phi(1-\phi)/math> where v_c is monomer volume, N is polymer strand length and \chi is the Flory-Huggins energy parameter. Because in a network, the polymer length is effectively infinite, we can take the limit N\to\infty and f(\phi) reduces to f(\phi)=\frac 1-\phi)\ln(1-\phi)+\chi \phi(1-\phi)/math>. Substitution of this expression into f_\text(\lambda_1,\lambda_2,\lambda_3) and addition of the network contribution leads to f_\text(\lambda_1,\lambda_2,\lambda_3)=\frac(\lambda_1^2+\lambda_2^2+\lambda_3^2)+\fracf(\phi). This provides the starting point to examining the swelling equilibrium of a gel network immersed in solvent. It can be shown that gel swelling is the competition between two forces, one is the
osmotic pressure Osmotic pressure is the minimum pressure which needs to be applied to a solution to prevent the inward flow of its pure solvent across a semipermeable membrane. It is also defined as the measure of the tendency of a solution to take in a pure ...
of the polymer solution that favors the take in of solvent and expansion, the other is the restoring force of the polymer network elasticity that favors shrinkage. At equilibrium, the two effects exactly cancel each other in principle and the associated \lambda_1, \lambda_2 and \lambda_3 define the equilibrium gel volume. In solving the force balance equation, graphical solutions are often preferred. In an alternative, scaling approach, suppose an
isotropic Isotropy is uniformity in all orientations; it is derived . Precise definitions depend on the subject area. Exceptions, or inequalities, are frequently indicated by the prefix ' or ', hence ''anisotropy''. ''Anisotropy'' is also used to describe ...
gel is stretch by a factor of \lambda in all three directions. Under the affine network approximation, the mean-square end-to-end distance in the gel increases from initial R_0^2 to (\lambda R_0)^2 and the elastic energy of one stand can be written as F_\text\sim kT\frac where R_\text is the mean-square fluctuation in end-to-end distance of one strand. The modulus of the gel is then this single-strand elastic energy multiplied by strand number density \nu=\phi/Nb^3 to give G(\phi)\sim \frac\frac\frac. This modulus can then be equated to
osmotic pressure Osmotic pressure is the minimum pressure which needs to be applied to a solution to prevent the inward flow of its pure solvent across a semipermeable membrane. It is also defined as the measure of the tendency of a solution to take in a pure ...
(through differentiation of the free energy) to give the same equation as we found above.


Modified Donnan Equilibrium of Polyelectrolyte Gels

Consider a
hydrogel A hydrogel is a crosslinked hydrophilic polymer that does not dissolve in water. They are highly absorbent yet maintain well defined structures. These properties underpin several applications, especially in the biomedical area. Many hydrogels ar ...
made of
polyelectrolytes Polyelectrolytes are polymers whose repeating units bear an electrolyte group. Polycations and polyanions are polyelectrolytes. These groups dissociate in aqueous solutions (water), making the polymers charged. Polyelectrolyte properties are t ...
decorated with
weak acid Acid strength is the tendency of an acid, symbolised by the chemical formula HA, to dissociate into a hydron (chemistry), proton, H+, and an anion, A-. The Dissociation (chemistry), dissociation of a strong acid in solution is effectively comple ...
groups that can ionize according to the reaction \text\rightleftharpoons\text^-+\text^+ is immersed in a salt solution of physiological concentration. The degree of
ionization Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule i ...
of the
polyelectrolytes Polyelectrolytes are polymers whose repeating units bear an electrolyte group. Polycations and polyanions are polyelectrolytes. These groups dissociate in aqueous solutions (water), making the polymers charged. Polyelectrolyte properties are t ...
is then controlled by the \text and due to the charged nature of \text^+ and \text^-,
electrostatic interactions Electrostatics is a branch of physics that studies electric charges at Rest (physics), rest (static electricity). Since classical antiquity, classical times, it has been known that some materials, such as amber, attract lightweight particles af ...
with other ions in the systems. This is effectively a reacting system governed by
acid-base equilibrium In chemistry, an acid dissociation constant (also known as acidity constant, or acid-ionization constant; denoted ) is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction :HA ...
modulated by electrostatic effects, and is relevant in
drug delivery Drug delivery refers to approaches, formulations, manufacturing techniques, storage systems, and technologies involved in transporting a pharmaceutical compound to its target site to achieve a desired therapeutic effect. Principles related to d ...
, sea water
desalination Desalination is a process that takes away mineral components from saline water. More generally, desalination refers to the removal of salts and minerals from a target substance, as in Soil salinity control, soil desalination, which is an issue f ...
and dialysis technologies. Due to the elastic nature of the gel, the dispersion of \text^- in the system is constrained and hence, there will be a partitioning of salts ions and \text^+ inside and outside the gel, which is intimately coupled to the
polyelectrolyte Polyelectrolytes are polymers whose repeating units bear an electrolyte group. Ion#Anions and cations, Polycations and polyanions are polyelectrolytes. These groups dissociation (chemistry), dissociate in aqueous solutions (water), making the pol ...
degree of ionization. This ion partitioning inside and outside the gel is analogous to the partitioning of ions across a semipemerable membrane in classical Donnan theory, but a membrane is not needed here because the gel volume constraint imposed by network elasticity effectively acts its role, in preventing the macroions to pass through the fictitious membrane while allowing ions to pass. The coupling between the ion partitioning and polyelectrolyte ionization degree is only partially by the classical Donnan theory. As a starting point we can neglect the electrostatic interactions among ions. Then at equilibrium, some of the weak acid sites in the gel would dissociate to form \text^-that electrostatically attracts positive charged \text^+ and salt cations leading to a relatively high concentration of \text^+ and salt cations inside the gel. But because the concentration of \text^+ is locally higher, it suppresses the further ionization of the acid sites. This phenomenon is the prediction of the classical Donnan theory. However, with electrostatic interactions, there are further complications to the picture. Consider the case of two adjacent, initially uncharged acid sites \text are both dissociated to form \text^-. Since the two sites are both negatively charged, there will be a charge-charge repulsion along the backbone of the polymer than tends to stretch the chain. This energy cost is high both elastically and electrostatically and hence suppress ionization. Even though this ionization suppression is qualitatively similar to that of Donnan prediction, it is absent without electrostatic consideration and present irrespective of ion partitioning. The combination of both effects as well as gel elasticity determines the volume of the gel at equilibrium. Due to the complexity of the coupled acid-base equilibrium, electrostatics and network elasticity, only recently has such system been correctly recreated in
computer simulations Computer simulation is the process of mathematical modelling, performed on a computer, which is designed to predict the behaviour of, or the outcome of, a real-world or physical system. The reliability of some mathematical models can be dete ...
.


Animal-produced gels

Some species secrete gels that are effective in parasite control. For example, the
long-finned pilot whale The long-finned pilot whale (''Globicephala melas'') is a large species of oceanic dolphin. It shares the genus '' Globicephala'' with the short-finned pilot whale (''Globicephala macrorhynchus''). Long-finned pilot whales are known as such bec ...
secretes an enzymatic gel that rests on the outer surface of this animal and helps prevent other organisms from establishing colonies on the surface of these whales' bodies.
Hydrogels A gel is a semi-solid that can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady-state, although the liquid phase may still di ...
existing naturally in the body include
mucus Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It is ...
, the
vitreous humor The vitreous body (''vitreous'' meaning "glass-like"; , ) is the clear gel that fills the space between the lens and the retina of the eyeball (the vitreous chamber) in humans and other vertebrates. It is often referred to as the vitreous humor ...
of the eye,
cartilage Cartilage is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck an ...
,
tendons A tendon or sinew is a tough, high-tensile-strength band of dense fibrous connective tissue that connects muscle to bone. It is able to transmit the mechanical forces of muscle contraction to the skeletal system without sacrificing its ability ...
and
blood clots A thrombus (plural thrombi), colloquially called a blood clot, is the final product of the blood coagulation step in hemostasis. There are two components to a thrombus: aggregated platelets and red blood cells that form a plug, and a mesh of cr ...
. Their viscoelastic nature results in the soft tissue component of the body, disparate from the mineral-based hard tissue of the skeletal system. Researchers are actively developing synthetically derived tissue replacement technologies derived from hydrogels, for both temporary implants (degradable) and permanent implants (non-degradable). A review article on the subject discusses the use of hydrogels for
nucleus pulposus An intervertebral disc (or intervertebral fibrocartilage) lies between adjacent vertebrae in the vertebral column. Each disc forms a fibrocartilaginous joint (a symphysis), to allow slight movement of the vertebrae, to act as a ligament to hold t ...
replacement, cartilage replacement, and synthetic tissue models.


Applications

Many substances can form gels when a suitable thickener or gelling agent is added to their formula. This approach is common in manufacture of wide range of products, from foods to paints and adhesives. In fiber optic communications, a soft gel resembling
hair gel Hair gel is a hairstyling product that is used to harden hair into a particular hairstyle. History Analysis of ancient Egyptian mummies has shown that they styled their hair using a fat-based gel. The researchers behind the analysis say that t ...
in viscosity is used to fill the plastic tubes containing the fibers. The main purpose of the gel is to prevent water intrusion if the buffer tube is breached, but the gel also buffers the fibers against mechanical damage when the tube is bent around corners during installation, or flexed. Additionally, the gel acts as a processing aid when the cable is being constructed, keeping the fibers central whilst the tube material is extruded around it.


See also

*
Aerogel Aerogels are a class of synthetic porous ultralight material derived from a gel, in which the liquid component for the gel has been replaced with a gas, without significant collapse of the gel structure. The result is a solid with extremely low ...
* 2-Acrylamido-2-methylpropane sulfonic acid *
Agarose gel electrophoresis Agarose gel electrophoresis is a method of gel electrophoresis used in biochemistry, molecular biology, genetics, and clinical chemistry to separate a mixed population of macromolecules such as DNA or proteins in a matrix of agarose, one of the ...
*
Food rheology Food rheology is the study of the rheological properties of food, that is, the consistency and flow of food under tightly specified conditions. The consistency, degree of fluidity, and other mechanical properties are important in understandi ...
*
Gel electrophoresis Gel electrophoresis is a method for separation and analysis of biomacromolecules ( DNA, RNA, proteins, etc.) and their fragments, based on their size and charge. It is used in clinical chemistry to separate proteins by charge or size (IEF ...
*
Gel filtration chromatography Size-exclusion chromatography (SEC), also known as molecular sieve chromatography, is a chromatographic method in which molecules in solution are separated by their size, and in some cases molecular weight. It is usually applied to large molecules ...
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Gel pack An ice pack or gel pack is a portable bag filled with water, refrigerant gel, or liquid, meant to provide cooling. They can be divided into the reusable type, which works as a thermal mass and requires freezing, or the instant type, which cools ...
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Gel permeation chromatography Gel permeation chromatography (GPC) is a type of size-exclusion chromatography (SEC), that separates analytes on the basis of size, typically in organic solvents. The technique is often used for the analysis of polymers. As a technique, SEC was fi ...
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Hydrocolloid A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others extend ...
* Ouchterlony double immunodiffusion *
Paste (rheology) In physics, a paste is a substance that behaves as a solid until a sufficiently large load or stress is applied, at which point it flows like a fluid. In rheological terms, a paste is an example of a Bingham plastic fluid. Pastes typically consi ...
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Polyacrylamide gel electrophoresis Polyacrylamide gel electrophoresis (PAGE) is a technique widely used in biochemistry, forensic chemistry, genetics, molecular biology and biotechnology to separate biological macromolecules, usually proteins or nucleic acids, according to their ...
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Radial immunodiffusion Radial immunodiffusion (RID) or Mancini method, Mancini immunodiffusion or single radial immunodiffusion assay, is an immunodiffusion technique used in immunology to determine the quantity or concentration of an antigen in a sample. Description ...
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Silicone gel A silicone or polysiloxane is a polymer made up of siloxane (−R2Si−O−SiR2−, where R = organic group). They are typically colorless oils or elastomer, rubber-like substances. Silicones are used in sealants, adhesives, lubricants, medici ...
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Two-dimensional gel electrophoresis Two-dimensional gel electrophoresis, abbreviated as 2-DE or 2-D electrophoresis, is a form of gel electrophoresis commonly used to analyze proteins. Mixtures of proteins are separated by two properties in two dimensions on 2D gels. 2-DE was first ...
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Void (composites) A void is a pore that remains unfilled with polymer and fibers in a composite material. Voids are typically the result of poor manufacturing of the material and are generally deemed undesirable. Voids can affect the mechanical properties and lif ...


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