# Dual Stimuli-Responsive Polysaccharide Hydrogels Manufactured by Radiation Technique
## 1 Introduction  
Hydrogels are the three-dimensional structures of crosslinked polymer(s) and water, where both components form interpenetrating continuous phases. The polymer network is responsible for maintaining dimensional integrity and mechanical properties, whereas the liquid phase provides a medium for diffusion and greatly contributes to the biological compatibility of hydrogels [1, 2]. Originally, hydrogels have been formed from hydrophilic synthetic polymers, but nowadays, nature-originated macromolecules are often involved. The formation and stabilization of the network may be of two kinds, i.e., physical interaction, such as ionic or hydrophobic, or chemical covalent bonding, e.g., the most common carbon-carbon bonds. The gels driven by the former are reversible, i.e., they may change into solution, or sol, upon the alteration of physical factors, e.g., pH or temperature, whereas the latter ones may not transit into sol unless some covalent bonds are ruptured, which is why they are called chemical gels. Typically, chemical hydrogels are prepared by the crosslinking polymerization of suitable mono- and multifunctional monomers or by the crosslinking of hydrophilic polymers using chemical crosslinkers, photocrosslinkers, orionizing radiation. Some other methods are infrequently employed and only for specific applications, e.g., using enzymes. Particularly, the synthesis of hydrogels for biomedical applications using radiation technology was successfully developed and commercialized. Undoubtedly, manufacturing hydrogels using radiation has a weighty advantage, since this technology may combine synthesis and sterilization in a single process [3]. With regard to the application of radiation technology for gels manufacturing, hydrophilic polymers, or alternatively monomers (principally avoided in biomedical applications) are irradiated, typically in aqueous solution, to form a network by crosslinking between chains or by simultaneous polymerization and crosslinking, respectively. Upon irradiation in solution, water radiolysis transient species, especially hydroxyl radicals, react with macromolecules by hydrogen atom abstraction, leading to the formation of polymer radicals. Radicals created at the mainchain or side-groups (if they are present) initiate further chemical reactions, e.g., crosslinking, disproportionation, chain scission, radical shift or addition to the double bonds of other molecules present in the investigated composition. The single-radical reaction of the scission of bonds in the main-chain results in degradation, that is, a reduction in molecular weight, whereas intermolecular crosslinking--involving the recombination of two radicals localized at different macromolecules--contributes to the increase in molecular weight and may eventually lead to the formation of macroscopic gel [4]. A variety of synthetic polymers are applicable for radiation processing, however, some successful trials and examples of gel formation with those of natural origin, particularly polysaccharides and their derivatives, have been reported [5; 6]. Basic radiation-induced processes in polysaccharides are fairly well understood, and there is a common agreement that the degradation of the main chain mainly occurs as a consequence of glycosidic linkage scission. This can instantly proceed or be a consequence of the cyclic structure rupture caused by the transformations of radicals created at various locations [7; 8; 9]. Nevertheless, the application of specific conditions at radiation processing was also demonstrated to induce opposite reactions, leading to the formation of crosslinking bonds. The probability of two competing pathways, that is a shift in the balance between inherently present glycosidic bonds scission and crosslinking, is controlled by various factors chiefly related to the polymer used, the presence of additives, and irradiation conditions. At least five synthesis methods of hydrogels based on polysaccharides can be remarked.