Gels,originating from the Latin gelatus– frozen, are defined as fine dispersedsystems, consisting of solid and liquid or gaseous phases.
Gels are defined as a substantially dilutecrosslinked 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-dimensionalcrosslinked network within the liquid. This internal network structure mayresult from physical or chemical bonds. Physical bonds may be electrostatic,hydrogen or van der Waals interactions and the chemical bond is mainly ofcovalent in nature. Gels are classified into organogel, xerogel and hydrogel. Organogel isa non-crystalline, non-glassy thermos-reversible solid material composed of a liquid organic phase entrapped in a three-dimensionallycross-linked network and used in pharmaceuticals, cosmetics, and foodindustries.A xerogel isa solid formed from a gel by drying with unhindered shrinkage.
Xerogels usually retain high porosity and enormoussurface area along with a very small pore size. Hydrogelsare three dimensional cross-linked covalent networks those imbibe largequantity of water without dissolving themselves. By definition waterconstitutes at least 10% of total weight (or volume) of a hydrogel and when thecontent of water exceeds 95% of the total weight (or volume), then, hydrogel iscalled a superabsorbent. To qualify a polymer as hydrogel, in practical sense, the dry materialshould spontaneously imbibe about twenty times its own weight of aqueous fluid.While undergoing this phenomenon of volume change of 2000 percent, the swellingmaterial retains its original identity. Hydrogelsare 3D lattice polymers that, due to the presence in them of hydrophilicfunctional groups, swell up to several times of their original volume inaqueous media. The most common definition of hydrogel is a water-swollen;cross-linked polymeric network produced by the simple reaction of one or moremonomers or by hydrogen bonds and strong van der Waals interactions betweendifferent chains.
It is also defined as ‘polymer based three dimensionalnetwork systems capable of imbibing large amount of water and biological fluidswithout getting them dissolved’. The soft, rubberyconsistency of swollen hydrogels minimizes frictional irritation of surroundingcells and tissue and may be fabricated in a variety of shapes and geometries.Hydrogel networks are useful for applications requiring a material that hasgood compatibility with aqueous fluids, without dissolution. The ability of a hydrogel to hold significant amount of waterimplies that the polymer chains should have moderate to high hydrophiliccharacter. The ability of hydrogels to absorb water arises fromhydrophilic functional groups attached to the polymeric backbone, while theirresistance to dissolution arises from cross-links between network chains. Hydrogels could be both naturaland synthetic. Naturally occurring polymers both in untreated or treated formsare used as hydrogels. The synthetic gels areprepared either by crosslinking the water-soluble polymers or by converting hydrophobic polymers into hydrophilic one andthen by crosslinking.
The water content in the hydrogels affect differentproperties like permeability, mechanical properties, surface properties, andbiocompatibility. Hydrogels have similar physical properties as that of livingtissue, and this similarity is due to the high water content, soft and rubbery consistency,and low interfacial tension with water or biological fluids. Hydrogel may have differentphysical forms, which includes: solid molded forms ( e.g. soft contact lenses),pressed power matrices (e.
g. pill or capsules for oral insertion), microgels(e.g. as bio-adhesives carrier or wound treatments), coatings ( e.g.
on implantsor catheters, on pill or capsules or coating on the inside capillary wall incapillary electrophoresis), membranes or sheets ( e.g. as a reservoir in atransdermal drug delivery patch or for 2D electrophoresis gels), encapsulatedsolids( e.g.
in osmotic pumps) and liquid (e.g. that form gels on heating orcooling). Hydrogelsare used in many potential applications due to their excellent high water absorbency,water retention and environmental sensitive nature. These hydrogels are widelyused in many products such as disposable diapers, feminine napkins, soil for agricultureand horticulture, gel actuators, water-blocking tapes for biomedical applications,and as absorbent pads. 1. PROPERTIESOF HYDROGELS: The characteristic features of an idealhydrogel can be listed as follows:1) The highest absorptioncapacity (maximum equilibrium swelling)2) Desired rate of absorption (preferred particle size andporosity) depending on the application requirement.
3) The highest absorbency under load (AUL).4) The lowest soluble content and residual monomer.5) The lowest price.6) The highest durability and stability in the swellingenvironment and during the storage.
7) The highest biodegradability without formation of toxicspecies following the degradation.8) pH-neutrality after swelling in water9) Colorlessness, Odorlessness, Non-toxicity, Photostability.10) Re-wetting capability (if required e.
g., in agriculturalor hygienic applications) In general the common and the most importantproperty of hydrogels is the water uptake property. The water uptake propertyof some hydrogels depend on temperature, pH, ionic strength of theswelling medium, chemical architecture of the gel, types of solvent or even inthe presence of electric field, magnetic field, mechanical forces, ultra violetlight and presence or absence of specific molecules. The polymers whichpossess this type of surrounding environment sensitiveness due to the presenceof certain functional groups along the polymeric chains are referred to as”intelligent” or “smart polymer”.
The water containing hydrogels have otherimportant biophysical properties such as softness, rubbery texture, resemblanceto the living organs and permeability to various biomolecules. Oneof the unique properties of hydrogels is their ability to maintain originalshape during and after swelling due to isotropic swelling.