# Dual Stimuli-Responsive Polysaccharide Hydrogels Manufactured by Radiation Technique
## 3 Results
### Hydrogels Formation and Swelling
Higher irradiation dose rates are beneficial for the crosslinking of polymers because recombination requires the presence of two radicals at the same time--simultaneously at neighboring chains--and bi-radical reactions are likely possible. This increases the probability that they can recombine to form a crosslink. Conversely, if the radical creation rate is low (and/or their decay is fast), i.e., the temporal number of radicals is low, they rather tend to react by breaking bonds, and then the degradation of polymer chains occurs. In some polymers, the dose rate is a critical factor for crosslinking [5]. The handling and mechanical properties of polysaccharide-based hydrogels may be an issue with regard to applications. The performance of the product could be improved by several approaches, for instance, selecting a proper composition of the admixture of a plasticizer, typically synthetic polymer, and the manufacturing of interpenetrating polymer networks, where one of the components is the polysaccharide and the other is a synthetic polymer, by using the internal (polymerizable side groups) or external crosslinker and/or an application of the most advantageous dose. Moreover, the crosslinking reactions in polysaccharide derivatives are driven by the recombination of side group radicals; therefore, a higher degree of substitution causing an increase in the probability of radical creation at the side groups is advantageous. HPC hydrogels formed from the polysaccharide of the degree of substitution 1.4 change their appearance from their previous shapeless form, through very weak consistency to brittle in the order of an increasing absorbed dose. Therefore, the improvement was proposed by the employment of a crosslinker. It is known that crosslinking agents--typically bi- or multi-functional monomers or macromers, inevitable in chemical methods of crosslinking--may be used in radiation processing in order to reduce the necessary dose or enhance the properties of the resulting modified  
Figure 1: The gel fraction of hydrogels manufactured by the irradiation of HPC and HPC–PEGDA in aqueous solutions. materials, which are also hydrogels. Nevertheless, one should be aware that some doubts and inconsistencies exist in the literature on the rather of polysaccharides [27]. Poly(ethylene glycol) diacrylate is a water-soluble prepolymer or macromer that undergoes instant polymerization and crosslinking when irradiated in aqueous solution [28]. Upon the consumption of all the double bonds of acrylate, the formed polymer or network lose its toxicity which is characteristic of the PEGDA macromer. Therefore, PEGDA may be used in biomedical applications [29; 30; 31]. This is in line with the anticipated applications of polysaccharides that are biocompatible and biodegradable. The solutions of HPC and of CMC and subsequently of both polysaccharides were irradiated in the presence of PEGDA to form hydrogels. The GF of HPC-PEGDA gels increases with an increase in the absorbed dose, as depicted in Figure 1. A general feature can be remarked, which is that the more PEGDA there is in the composition, the higher the GF is, e.g., as is evident in a comparison of 2.5% and 5% PEGDA gels for every concentration of HPC. On the other hand, an increase in HPC concentration from 5%, through 10%, and up to 20% reduces the GF. Gels formed in the middle range of the absorbed dose, 20-50 kGy, are relatively strong, easy to handle, and not brittle. The crosslinking of HPC is less efficient than the polymerization and crosslinking of PEGDA, however, HPC undoubtedly contributed to the gel formation--GF is over the minimum of 50 and 20%, respectively, for the HPC /PEGDA initial percentage compositions of 5/5 and 20/5, which are required to confirm the participation of the polysaccharide in the network. This was further demonstrated in thermo-responsive swelling experiments. Similarly to HPC, concentrated aqueous solutions of carboxymethylcellulose form gels with a yield depending on the concentration and the degree of substitution.