A hydrogel construction can be described as a three-dimensional jetty is made up of linear polymer chains with covalent connections, which are in turn connected together by further cross-connections. These cross-connections could be covalent, ionic grafts or crystal sections [2]. Hydrogels form due to polarity and hydrophilic nature of polar groups existing between the polymer chains cross-connections that render it insoluble.

There are several known methods of synthesizing hydrogels, some of which are explained below.

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A simple method of constructing hydrogels is by crosslinking water-soluble polymers (with functional groups like -OH, -COOH, -NH2) in solution. In solution co-polymerization reactions, ionic or neutral monomers are mixed with a multi-functional crosslinking agent. The reaction is initiated thermally using UV-light, or by a redox initiator system. The solvent serves as heat sink and minimizes temperature control problems. The reaction is carried out in an organic solvent to prevent water from reacting with the crosslinking agent. Once crosslinked, the resultant hydrogels are washed with distilled water to remove any unreacted monomers, crosslinking agent, and the initiator. Equation 1 shows one such solution polymerization reaction conducted on 2-hydroxyethylmethacrylate monomers to form a hydrogel.

This solution crosslinking method is often advantageous since the starting material used can be a well-characterized, purified polymer, and the crosslinking conditions required are mild enough to be carried out in the presence of an active agent. For example, poly (2-hydroxyethyl methacrylate) hydrogels are prepared from hydroxyethyl methacrylate by this method, using ethylene glycol dimethacrylate as the crosslinking agent [3]. The hydrogels synthesized can be made pH- sensitive or temperature-sensitive as required by incorporating methacrylic acid or N-isopropylacrylamide [4] as monomers.

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