Introduction The materials of interest

Introduction
The materials of interest in this brief review are primarily hydrogels, which are polymer networks extensively swollen with water. Hydrophilic gels that are usually referred to as hydrogels are networks of polymer chains that are sometimes found as colloidal gels in which water is the dispersion medium [1].
Researchers, over the years, have defined hydrogels in many different ways. The most common of these is that hydrogel is a water-swollen, and cross-linked polymeric network produced by the simple reaction of one or more monomers. Another definition is that it is a polymeric material that exhibits the ability to swell and retain a significant fraction of water within its structure, but will not dissolve in water. Hydrogels have received considerable attention in the past 50years, due to their exceptional promise in wide range of applications [2–4]. They possess also a degree of flexibility very similar to natural tissue due to their large water content.
During last two decades, natural Hydrogels were gradually replaced by synthetic hydrogels which has long service life, high capacity of water absorption, and high gel strength. Fortunately, synthetic polymers usually have well-defined structures that can be modified to yield tailor able degradability and functionality. Hydrogels can be synthesized from purely synthetic components. Also, it is stable in the conditions of sharp and strong fluctuations of temperatures [5].
Hydrogels may be synthesized in a number of “classical” chemical ways. These include one-step procedures like polymerization and parallel cross-linking of multifunctional monomers, as well as multiple step procedures involving synthesis of polymer nucleoside analogs having reactive groups and their subsequent cross-linking, possibly also by reacting polymers with suitable cross-linking agents. The polymer engineer can design and synthesize polymer networks with molecular-scale control over structure such as cross-linking density and with tailored properties, such as biodegradation, mechanical strength, and chemical and biological response to stimuli [6].

Classification of hydrogel products

Hydrogel product sensitive to environmental conditions
They may perform dramatic volume transition in response to a variety of physical and chemical stimuli, where the physical stimuli include temperature, electric or magnetic field, light, pressure, and sound, while the chemical stimuli include pH, solvent composition, ionic strength, and molecular species (Fig. 1).
The extent of swelling or de-swelling in response to the changes in the external environment of the hydrogel could be so drastic that the phenomenon is referred to as volume collapse or phase transition [12]. Synthetic hydrogels have been a field of extensive research for the past four decades, and it still remains a very active area of research today.

Utilization of hydrogel products
With the establishment of the first synthetic hydrogels by Wichterle and Lim in 1954 [13], the hydrogel technologies may be applied to hygienic products [14], agriculture [15], drug delivery systems [14,16], sealing [14], coal dewatering [17], artificial snow [14], food additives [18], pharmaceuticals [19], biomedical applications [20,21] tissue engineering and regenerative medicines [22,23], diagnostics [24], wound dressing [25], separation of biomolecules or cells [26] and barrier materials to regulate biological adhesions [27], and Biosensor [28].
In addition, the ever growing spectrum of functional monomers and macromeres widen their applicability. They were used in early agricultural water absorbents based on biopolymers through grafting of hydrophilic monomers onto starch and other polysaccharides [29,30]. Hydrogel products for hygienic applications are mainly based on acrylic acid and its salts. Acrylamide is a main component employed for preparation of agricultural hydrogel products [14].
Various publications on this subject have discussed in detail synthetic methods and applications of hydrogels. For example, a comprehensive review of the chemistry and various synthetic schemes employed for hydrogel preparation can be found in various chapters of a compilation edited by Peppas [31]. More recently, hydrogels produced by radiation polymerization and grafting have been published by Khoylou [32]. Mi-Ran Park [33] described the preparation and chemical properties of hydrogels employed in agricultural applications. Vijayalakshmi and Kenichi have reviewed the potential of hydrogels in sensor utilizations [34]. Dimitrios et al. [21] discussed the tailoring of hydrogels for various applications of medical interest.