Superabsorbent polymers: A review on the characteristics and applications of synthetic, polysaccharide-based, semi-synthetic and 'smart' derivatives

The current review provides an overview of different types of superabsorbent polymers (SAPs) together with appropriate strategies elaborated to enable their synthesis. The main focus will be on polysaccharide-based, semi-synthetic and 'smart' SAPs along with their derivatives. SAPs have already shown their use in a plethora of applications including diapers, the biomedical field, agriculture, etc. The different polymer classification possibilities are discussed, as well as the classification of the constituting building blocks. The main part of SAPs still has a synthetic origin. However, as they are often not biocompatible, biodegradable or renewable, natural SAPs based on polysaccharides have gained increasing interest. Due to the low solubility of synthetic polymers, purification problems or the need for organic solvents, a trend has emerged towards combining polysaccharides with synthetic monomers to create semi-synthetic, hybrid SAPs for specialized applications with fine-tuned properties including wound dressings, fertilizers or self-healing concrete. These specialized, semi-synthetic SAPs offer strong potential for a series of applications in the future. However, future research in this respect is still needed to optimize homogeneity and to increase gel fractions. A final part of this review includes 'smart' SAPs such as SAPs with a T-, electro- and pH-sensitivity. These 'smart' SAPs are especially becoming useful for certain biomedical applications such as drug release for which an in vivo location can be targeted. The use of 'smart', semi-synthetic SAPs with fine-tuned characteristics combining the best characteristics of both synthetic and natural SAPs, offer the greatest potential for the future.