## 3 Results and Discussion
### HPC/23G Hydrogel  
The formation of a double network structure and a nanocomposite with clay has been reported to increase the strength of polymer gels [25; 26]. Another study reported a blend of two types of polymers to improve the properties of hydrogels [27]. The blend gel obtained by irradiation of a mixed aqueous solution of the two polymers had different characteristics. Polyvinyl alcohol (PVA) was selected to improve the mechanical properties of HPC gel by blending with another polymer because the radiation-crosslinked PVA hydrogels exhibited high transparency and excellent mechanical properties and were used in wound dressing materials [28]. The HPC/PVA blend hydrogel was prepared by electron beam irradiation of a blended aqueous solution of HPC and PVA as a preliminary experiment. However, the obtained HPC/PVA hydrogels were semitransparent owing to the microphase separation. Therefore, attempts were made to improve the mechanical properties by adding a water-soluble crosslinker based on polyethylene glycol to the HPC aqueous solution and crosslinking. Figure 4a shows the gel fraction of the hydrogels prepared from a mixed aqueous solution of HPC and 23G. HPC/23G hydrogels with a thickness of 100 \(\upmu\)m, close to commercially available contact lenses, were prepared. The gel fraction of the HPC/23G hydrogels was higher than that of the HPC hydrogels without 23G, especially at low doses. The gel fraction of the HPC/23G (20/0.2) hydrogel reached 80% at 20 kGy. This indicates that the addition of the crosslinker 23G accelerated the gelation and crosslinking reactions of HPC. At the same dose, the gel fraction increased at a higher concentration of 23G. The difference in the gel fraction was remarkable at 10 kGy. At 50 kGy, there was no effect of the concentration of 23G on the gel fraction. It has been reported that the radicals on the side groups of cellulose derivatives generated by irradiation participate in crosslinking reactions, leading to the formation of the gel [29]. In the presence of monomers, the generated polymer radical reacts more quickly with the monomer of low molecular weight than with the polymer radical to form the branched and crosslinked structures. Therefore, in the presence of 23G as the crosslinker, the gel fraction increased sharply, even at low doses. In contrast, the Sw of the HPC/23G hydrogels decreased with increasing dose, as shown in Figure 4b. At the same dose, the Sw of the HPC/23G hydrogels decreased with an increasing concentration of 23G. This was due to an increase in crosslinking density. Figure 3: UV—vis spectra of MC, HPMC, and HPC hydrogels in the visible light region. The tensile strength and elongation at break of the obtained HPC/23G hydrogels as a function of 23G concentration are shown in Figure 5a,b, respectively. The tensile strength of the HPC/23G hydrogel was a maximum at a 23G concentration of 0.2 wt.% and decreased with increasing concentration of the 23G at any dose. Accordingly, the HPC/23G hydrogels became brittle and not easy to stretch because of an increase in the gel fraction and crosslinking density. Therefore, the optimum conditions for obtaining the HPC/23G hydrogel that exhibited a good balance between the tensile strength and elongation at break was a 23G concentration of 0.2 wt.% and at 30 kGy. As a result, the tensile strength and elongation at break of the HPC/23G hydrogel (20/0.2, 30 kGy) prepared under optimum conditions were higher, about 3.0 and 1.5 times, respectively, than those of the pure HPC hydrogel without 23G. Figure 4: (**a**) Gel fraction and (**b**) Sw of HPC/23G hydrogels as a function of dose. The HPC/23G hydrogels were prepared by irradiation of the mixed aqueous solutions, in which the concentrations of HPC and 23G were (\(\triangle\)) 20/0, (\(\Circinus\)) 20/0.2, (\(\square\)) 20/0.4, and (\(\Diamond\)) 20/1.0 wt.%/wt.%. Figure 5: (**a**) Tensile strength and (**b**) elongation at break of HPC/23G hydrogels as a function of 23G concentration.