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. 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.
Gel: Nonfluid colloidal network or polymer network that is expanded
throughout its whole volume by a fluid .
Note 1: A gel has a finite, usually rather small, yield stress.
Note 2: A gel can contain: (i) a covalent polymer network, e.g., a
network formed by crosslinking polymer chains or by nonlinear
polymerization; (ii) a polymer network formed through the physical
aggregation of polymer chains, caused by hydrogen bonds,
crystallization, helix formation, complexation, etc., that results in
regions of local order acting as the network junction points. The
resulting swollen network may be termed a “thermoreversible gel”
if the regions of local order are thermally reversible; (iii) a
polymer network formed through glassy junction points, e.g., one based
on block copolymers. If the junction points are thermally reversible
glassy domains, the resulting swollen network may also be termed a
thermoreversible gel; (iv) lamellar structures including mesophases
 defines lamellar crystal and mesophase , e.g., soap gels,
phospholipids, and clays; (v) particulate disordered structures, e.g.,
a flocculent precipitate usually consisting of particles with large
geometrical anisotropy, such as in V2O5 gels and globular or fibrillar
Note 3: Corrected from , where the definition is via the property
identified in Note 1 (above) rather than of the structural
characteristics that describe a gel.
1.1 Polyionic polymers
2.4 Nanocomposite hydrogels
4 Animal-produced gels
6 See also
8 External links
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
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.
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See also: Superabsorbent polymer, Self-healing hydrogels, and Hydrogel
Hydrogel of a superabsorbent polymer
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.
A bandaid 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
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. 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
Hydrogel-coated wells have been used for cell culture
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.
Sustained-release drug delivery systems
Providing absorption, desloughing and debriding of necrotic and
Hydrogels that are responsive to specific molecules, such as glucose
or antigens, can be used as biosensors, as well as in DDS.
Disposable diapers where they absorb urine, or in sanitary napkins
Contact lenses (silicone hydrogels, polyacrylamides, polymacon)
EEG and ECG medical electrodes using hydrogels composed of
cross-linked polymers (polyethylene oxide, polyAMPS and
Water gel explosives
Rectal drug delivery and diagnosis
Encapsulation of quantum dots
Granules for holding soil moisture in arid areas
Dressings for healing of burn or other hard-to-heal wounds.
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
Common ingredients include polyvinyl alcohol, sodium polyacrylate,
acrylate polymers and copolymers with an abundance of hydrophilic
Natural hydrogel materials are being investigated for tissue
engineering; these materials include agarose, methylcellulose,
Elastin like polypeptides
Elastin like polypeptides and other naturally derived
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.
See also: 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. (An example of formation of an
undesired thermoreversible network is the occurrence of wax
crystallization in petroleum.)
Organogels have potential for use in a number of applications, such as
in pharmaceuticals, cosmetics, art conservation, and food.
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 are also known as hybrid hydrogels,
can be defined as 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.
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.
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'
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
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
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.
2-Acrylamido-2-methylpropane sulfonic acid
Agarose gel electrophoresis
Gel filtration chromatography
Gel permeation chromatography
Ouchterlony double immunodiffusion
Polyacrylamide gel electrophoresis
Two-dimensional gel electrophoresis
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Wikimedia Commons has media related to Gels.
Look up gel in Wiktionary, the free dictionary.
Routes of administration, dosage forms
Time release technology
Osmotic delivery system (OROS)
Effervescent powder or tablet
Syrup Concentrate for dilution and/or addition of carbonated water
Buccal (sublabial), sublingual
Orally disintegrating tablet
Orally disintegrating tablet (ODT)
Effervescent buccal tablet
Dry-powder inhaler (DPI)
Metered-dose inhaler (MDI)
Oxygen mask and Nasal cannula
Relative analgesia machine
Mucoadhesive microdisc (microsphere tablet)
Pessary (vaginal suppository)
Intrauterine device (IUD)
DMSO drug solution
Electrophoretic dermal delivery system
Contact (rubbed into break in the skin)
Central nervous system
Patient-Controlled Analgesia pump