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An upturned vial of hair gel

Silica Gel

A gel is a solid jelly-like material 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.^[1] By weight, gels are mostly liquid, yet they behave like solids due to 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). In this way gels are a dispersion of molecules of a liquid within a solid in which liquid particles are dispersed in the solid medium . The word gel was coined by 19th-century Scottish chemist Thomas Graham by clipping from gelatine.^[2]

Composition

Gels consist of a solid three-dimensional network that spans the volume of a liquid medium and ensnares it through surface tension effects. This internal network structure may result from physical bonds (physical gels) or chemical bonds (chemical gels), as well as crystallites or other junctions that remain intact within the extending fluid. Virtually any fluid can be used as an extender including water (hydrogels), oil, and air (aerogel). 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 in their stretched state, because the stretched-out polymer takes up more space. This is also the reason gel hardens. See polyelectrolyte for more information.

Types

Hydrogels

Hydrogel of a superabsorbent polymer

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A micropump based on a hydrogel bar (4×0.3×0.05 mm size) actuated by applied voltage. This pump can be continuously operated with a 1.5 V battery for at least 6 months.^[9]

An adhesive bandage with a hydrogel pad, used for blisters and burns. The central gel is clear, the adhesive waterproof plastic film is clear, the backing is white and blue

A short-peptide-based hydrogel matrix, capable of holding about one hundred times its own weight in water. Developed as a medical dressing. The thickness of the fibers was on the order of tens of nm, mimicking the fibrous microenvironment found in the extracellular matrix. Field emission scanning electron microscopy image

Photo of the same short-peptide-based hydrogel, held in forceps to demonstrate its stiffness and transparency.

A hydrogel is a network of polymer chains that are hydrophilic, sometimes found as a colloidal 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.^[10] 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.
The first appearance of the term 'hydrogel' in the literature was in 1894.^[11]
Common uses for hydrogels include:

• Scaffolds in tissue engineering. When used as scaffolds, hydrogels may contain human cells to repair tissue. They mimic 3D microenvironment of cells.^[12]


• Hydrogel-coated wells have been used for cell culture^[13]


• Environmentally sensitive hydrogels (also known as 'Smart Gels' or 'Intelligent Gels'). These hydrogels have the ability to sense changes of pH, temperature, or the concentration of metabolite and release their load as result of such a change.^[14]


• Sustained-release drug delivery systems


• Providing absorption, desloughing and debriding of necrotic and fibrotic tissue


• Hydrogels that are responsive to specific molecules, such as glucose or antigens, can be used as biosensors, as well as in DDS.^[15]


• Disposable diapers where they absorb urine, or in sanitary napkins^[16]


• 
  Contact lenses (silicone hydrogels, polyacrylamides, polymacon)


• 
  EEG and ECG medical electrodes using hydrogels composed of cross-linked polymers (polyethylene oxide, polyAMPS and polyvinylpyrrolidone)


• Water gel explosives


• Rectal drug delivery and diagnosis


• Encapsulation of quantum dots


• Breast implants


• Glue


• Granules for holding soil moisture in arid areas


• Dressings for healing of burn or other hard-to-heal wounds. Wound gels are excellent for helping to create or maintain a moist environment.


• Reservoirs in topical drug delivery; particularly ionic drugs, delivered by iontophoresis (see ion exchange resin).


• Materials mimicking animal mucosal tissues to be used for testing mucoadhesive properties of drug delivery systems^[17][18]

Common ingredients include polyvinyl alcohol, sodium polyacrylate, acrylate polymers and copolymers with an abundance of hydrophilic groups.

Natural hydrogel materials are being investigated for tissue engineering; these materials include agarose, methylcellulose, hyaluronan, Elastin like polypeptides and other naturally derived polymers.
Hydrogels show promise for use in agriculture, as they can release agrochemicals including pesticides and phosphate fertiliser slowly, increasing efficacy and reducing runoff, and at the same time improve the water retention of drier soils such as sandy loams.^[19]

Organogels

An organogel is a non-crystalline, non-glassy thermoreversible (thermoplastic) solid material composed of a liquid organic phase entrapped in a three-dimensionally cross-linked network. The liquid can be, for example, an organic solvent, mineral oil, or vegetable oil. The solubility and particle dimensions of the structurant are important characteristics for the elastic properties and firmness of the organogel. Often, these systems are based on self-assembly of the structurant molecules.^[20][21] (An example of formation of an undesired thermoreversible network is the occurrence of wax crystallization in petroleum.^[22])

Organogels have potential for use in a number of applications, such as in pharmaceuticals,^[23] cosmetics, art conservation,^[24] and food.^[25]

Xerogels

A xerogel /ˈzɪəroʊˌdʒɛl/ 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 size (1–10 nm). When solvent removal occurs under supercritical conditions, the network does not shrink and a highly porous, low-density material known as an aerogel is produced. Heat treatment of a xerogel at elevated temperature produces viscous sintering (shrinkage of the xerogel due to a small amount of viscous flow) and effectively transforms the porous gel into a dense glass.

Nanocomposite hydrogels

Nanocomposite hydrogels^[26][27] 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, and biological properties.^[26]

Properties

Many gels display thixotropy – 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.
By replacing the liquid with gas it is possible to prepare aerogels, materials with exceptional properties including very low density, high specific surface areas, and excellent thermal insulation properties.

Animal-produced gels

Some species secrete gels that are effective in parasite control. For example, the long-finned pilot whale 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.^[28]

Hydrogels existing naturally in the body include mucus, the vitreous humor of the eye, cartilage, tendons and blood clots. 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 replacement, cartilage replacement, and synthetic tissue models.^[29]

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 optics communications, a soft gel resembling "hair gel" 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

• 2-Acrylamido-2-methylpropane sulfonic acid


• Agarose gel electrophoresis


• Food rheology


• Gel electrophoresis


• Gel filtration chromatography


• Gel pack


• Gel permeation chromatography


• Hydrocolloid


• Ouchterlony double immunodiffusion


• Paste (rheology)


• Polyacrylamide gel electrophoresis


• Radial immunodiffusion


• Silicone gel


• Two-dimensional gel electrophoresis


• Void (composites)

References

External links

Wikimedia Commons has media related to Gels.

Look up gel in Wiktionary, the free dictionary.

• Xerogel (definition)