# Dual Stimuli-Responsive Polysaccharide Hydrogels Manufactured by Radiation Technique
## 3 Results  
### Hydrogels Formation and Swelling  
The formation of HPC and CMC gels can be accomplished by the irradiation of their solutions at high concentration. It was anticipated that the addition of a crosslinker would reduce a dose necessary to create gels and increase the insoluble fraction or gel. Thus, aqueous solutions of two water-soluble polysaccharides at concentrations of 2.5% (only CMC), 5, 10, and 20% (only HPC) were irradiated, also with the addition of the crosslinker. A high dose rate, as provided by electron beam (EB), is advantageous for the creation of HPC gel, whereas CMC gels can be efficiently formed at high or low (1 kGy per hour or less, as delivered by a typical gamma source) dose rate [25; 26]. Therefore, in this study, EB was employed as a means of radiation. The two bottommost lines in Figure 1 represent the gel fraction (GF) of hydrogels manufactured by the irradiation of HPC aqueous solutions at two concentrations of 10 and 20%. The 10% HPC hydrogels formed with lower doses presented a consistency of soft, shape-less gel; on the contrary, the 20% HPC hydrogels formed with high doses appeared as a single piece or several pieces because of their britteness, but after immersion in water and swelling, the gels did not maintain their shape and became more like a slurry or mucous. The gel fraction depends on the concentration and absorbed dose factors, and for irradiated solutions of 10 and 20%, it increases with the increasing dose up to maxima of ca. 65 and 80%, respectively. For the solutions of low HPC concentration, the radiation dose at which the first part of a gel can be obtained is higher in comparison to the dose necessary for it to be delivered to highly concentrated solutions to obtain the gel. Moreover, the GF decreases at the highest applied dose of 100 kGy. It is known that HPC may undergo radiation-induced crosslinking, for instance, HPC with a degree of substitution 3.0 forms gels efficiently--up to 90% GF--when irradiated with a high dose rate at a high concentration, with 20% and over. In such a concentrated system, water acts like a plasticizer allowing for chain mobility [26]. A much lower GF was reported for that material processed with gamma rays--with a lower dose rate, ca. 10 kGy/h. Obviously, a lower degree of substitution of HPC (1.4) in this study generally results in lower GF and its decrease at higher doses, despite the fact that the energy for the initiation of reactions was delivered at a high rate. The mechanism of the crosslinking of polysaccharide derivatives in water, without chemical crosslinkers, is generally known. Upon the radiolysis of water, transient reactive species are formed, namely hydrated electron (\(\mathrm{e_{aq}}^{-}\)), hydrogen atom (\(\bullet\)H), and hydroxyl radical (HO\(\bullet\)), along with molecular products which are mainly those of hydrogen peroxide and the hydrogen molecule. These species, mainly hydroxyl radical but also hydrogen atom (produced with lower yield and reacting with the polymer at lower reaction rate), abstract hydrogen atoms from the polymer to form carbon-centered radicals, i.e., macroradical;other reactions also occur. Macroradicals can also be created by the direct action of ionizing radiation, and the ratio of the direct and indirect formation of macroradicals depends on the concentration of the polymer in water, with irradiation conditions, etc. Consecutive one-radical and bi-radical reactions take place resulting, among others, in the two most important macroscopic effects of degradation, namely scission and crosslinking. Scission causes a reduction in molecular weight but its increase is a consequence of crosslinking reactions. Under specific irradiation conditions, intermolecular crosslinking may outweigh the cleavage of glycosidic linkages in the main chain of polysaccharide derivatives.