# Dual Stimuli-Responsive Polysaccharide Hydrogels Manufactured by Radiation Technique
## 3 Results
### Hydrogels Formation and Swelling
The hydrolysis of polysaccharides proceeds relatively slowly, however, if the medium contains enzymes, particularly cellulases, the degradation is quick [6]. The incorporation of the crosslinker will not change this character, assuming that it is the network of polysaccharide chains connected with the crosslinker, but not the network of PEGDA itself. Nowadays, biodegradation in nature is a desired feature for materials, and hydrogels based on cellulose fulfill this requirement. Figure 3: Gel fraction of hydrogels manufactured by irradiation of CMC–HPC in aqueous solutions. The swelling phenomenon is characteristic for hydrogels. This is distinct from the behavior of a linear (or branched) polymer that is not inter-connected and dissolves in water. The spatial distribution of macromolecules dissolved in a good solvent is the same in its whole volume. Whether they fill the volume uniformly at the molecular level depends on the concentration of the polymer, which occurs over the so-called critical concentration; below that, however, polymeric chains, typically free coils, fill the space irregularly, leaving substantial parts of the solvent free of polymer. With reference to hydrogels, swelling leads to an increase in volume uniformly occupied by the polymer, however, it is finite. In general, the chains in the gel relax by absorbing and interacting with the solvent and this process is restricted by the network elasticity or stiffness. The attained balanced state is referred to as the degree of swelling in equilibrium or the equilibrium degree of swelling (EDS) [17]. Therefore, the swollen gel does not uniformly fill the whole volume of the solvent being in excess. The EDS of individual polysaccharide hydrogels, produced without and with the crosslinker, is shown in Figure 5. All samples display typical behavior for hydrogels produced by the irradiation of polymer solution (this refers to hydrophilic polymers that are of crosslinking types, or simultaneously undergoing crosslinking and scission with a predomination of the former process). The EDS is the highest at the lowest dose and it gradually decreases with an increase in the irradiation dose that the gel was formed with. Upon the formation of the gel, the part of the polymer comprising the network becomes insoluble and absorbs the solvent. At low irradiation doses, but over the gelation dose, the crosslinking density is the smallest, and therefore water uptake by scarcely connected macromolecules is the highest--and the network stretches broadly. [37]. The EDS is a few hundred for HPC hydrogels and in the case of CMC even several thousand grams of water per a gram of the polymer constituting the gel. The gel ability for water absorption is driven down as the network density increases due to the formation of more and more crosslinks at higher irradiation doses. Table 1 collates the EDS data of hydrogels manufactured with 25 kGy and with the highest applied dose. In general, CMC gels swell in water more extensively than gels made of non-ionic polysaccharides of the same concentration or rather of the same GF. The swelling of CMC hydrogels is imposed by high hydrophilicity and the electrostatic repulsion of this polyion. On the other hand, it is limited, as in HPC gels, by the crosslinking density and stiffness of the network. Moreover, as can be seen in Figure 5a,c and Figure 6a, the swelling of the gels formed with 100 kGy is higher than those formed with lower doses. This is often observed in polymers that partly degrade, in parallel to crosslink formation, as in polysaccharides. The network becomes looser as the interpolymer linking is already limited by either/or movement restrictions or the consumption of all sites able to crosslink, but the degradation occurs gradually. The HPC-CMC hydrogels  
Figure 4: Gel fraction of hydrogels manufactured by irradiation of CMC-HPC-PEGDA in aqueous solutions, with a PEGDA concentration of (**a**) 2.5% and (**b**) 5%.