# Dual Stimuli-Responsive Polysaccharide Hydrogels Manufactured by Radiation Technique
## 3 Results
### Stimuli-Responsive Swelling  
Thermo-reversible swelling of HPC-containing hydrogels was investigated. Figure 7a,b show the dependence on the temperature, from 25 to 55 \({}^{\circ}\)C of the DS of hydrogels formed from 20% HPC solution. Typically, hydrogels manufactured with various radiation doses present different initial degree of swelling (here, EDS) at ambient temperature, which are approximately 85 and 21 g/g at 50 kGy and 100 kGy, respectively. The degree of swelling for both hydrogels decreases with an increase in temperature, to approximately 19 g/g (reduction of ca. 78%) and 6.6 g/g (ca. 69%) for the gels manufactured with lower and higher irradiation doses, respectively. When the temperature increases, the hydrophobic interactions between the hydrophobic moieties of a polymer displaying a lower critical solution temperature (LCST) prevail over the hydrophilic ones and consequently, the hydrogel will shrink or collapse [38]. The hydrogel manufactured with 100 kGy is fully reversible, and it returns back to the initial DS upon temperature decrease (compare red triangles here and afterwards). The hydrogel produced with 50 kGy returned back to its original mass, over 80 g/g, after 7 days at room temperature (not shown). This means that hydrophobic interaction becomes strong enough to expel free water out of the network within minutes upon increasing the temperature, however, relaxing the gel after returning to room temperature, when hydrophilic groups take control by binding water, requires a longer amount of time. Besides the polymer nature, the relaxation time depends on the network density, and is controlled by the maximum amount of water to be absorbed. However, the swollen/shrunk ratio is only a little less than that for the 100 kGy gel, however, what makes the difference is the amount of water to be absorbed--whilst the EDS factor for 50/100 kGy is 4. This and the following changes in swelling are collected in Table 2.  
Figure 7: Temperature-dependent swelling of HPC hydrogels containing PEGDA; HPC%/PEGDA% composition: (**a,b**) 20/0; (**c**) 20/2.5, (**d**) 20/5, (**e**) 10/2.5, and (**f**) 10/5. Gels produced with 50 kGy, except (**b**): 100 kGy. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{2}{c}{**Hydrogel Composition, \%**} & \multicolumn{3}{c}{**DS \% Decrease**} \\ \multicolumn{2}{c}{**Dose (kGy)**} & \multicolumn{2}{c}{**in Response to Change in**} \\ \hline
**HPC** & **CMC** & **PEGDA** & & **Temperature** & **pH** \\ \hline
0 & 2.5 & 2.5 & 25 & & 67.3 \\ \hline
0 & 5 & 2.5 & 25 & &  
Hydrogels comprising 10 and 20% HPC and 2.5 and 5% PEGDA, respectively, manufactured at doses of 50 and at 100 kGy, were also examined. Figure 7c,d show the swelling dependence on the temperature of 20% HPC--PEGDA hydrogels. As previously noted, an increase in PEGDA concentration results in a reduction in water uptake, however, the thermal response does not differ significantly. At room temperature, the EDS is approximately 19 and 17 g/g--which changes to approximately 5.3 (72%) and 5.8 g/g (66%), respectively, at 55 \({}^{\circ}\)C. As mentioned above, the transition is slightly bigger for the gel without PEGDA (Figure 7a), obviously due to a significantly higher DS, resulting from the lower crosslinking density. As for the 20% HPC hydrogel manufactured with 50 kGy, the networks with PEGDA are fully reversible after 2 and 3 days. The EDS of 10% HPC hydrogels is approximately 140 g/g (data not shown), 26 and 14 g/g for the composition without and containing 2.5 and 5% PEGDA, respectively (Figure 7e,f). Consequently, the DS decreases with an increase in temperature, respectively, for PEGDA containing compositions of approximately 6.9 and 4.5 g/g (a reduction of ca. 70% for both). HPC gel without the crosslinker was somewhat difficult to evaluate in this experiment involving multiple weighting, which is due to very low gel fraction. The 10% HPC hydrogels with 2.5 and 5% PEGDA are fully reversible after a few days.