21: Hydrogels, Including Chemoresponsive Gels, as Smart Materials

Chapter 21 reviews hydrogels, including chemoresponsive gels as smart materials. Hydrogels can be used for many applications, and are increasingly being developed also given their possible biocompatibility. Such smart materials can, depending on suitable chemical components, bind or release for example drugs, pollutants, catalysts, etc. upon interaction with external effectors, and swell or shrink under the influence of different pH, various chemical compounds, temperature, or light. Most hydrogels are amorphous, some are semicrystalline mixtures of amorphous and crystalline phases, or are crystalline. Hydrogels have a water content typically between 80 and 99%, which can be changed by external stimuli; this is the basis of many applications. Natural sources of hydrogels are for example agarose, chitosan, methylcellulose or hyaluronic acid, but most smart hydrogels are based on synthetic polymers or rely on chemical modification of natural systems. Synthetic polymers for gels are usually obtained by copolymerization or cross-linking free-radical polymerizations, reacting hydrophilic monomers with multifunctional cross-linkers. One can produce polymer chains via chemical reaction, through photochemical processes, or by radiation for the generation of free radicals. Alternatively, one can modify existing polymers by chemical reaction.