# Dual Stimuli-Responsive Polysaccharide Hydrogels Manufactured by Radiation Technique
## 3 Results
### Stimuli-Responsive Swelling
Although the swelling of HPC hydrogels is reduced upon the temperature increase in a continuous manner, in either case, the biggest change may be observed over 45 \({}^{\circ}\)C, which is higher than the LCST of this polymer between 35 and 40 \({}^{\circ}\)C. The LCST of HPC slightly depends on the degree of substitution [39]. The temperature of conformation change as a function of the substitution degree should also be reflected in the DS of the respective HPC hydrogels. For instance, hydrogels manufactured from HPC of a degree of molecular substitution of 3 (more than twice of the HPC used in this work) in 20% solution with 80 kGy displayed thermoreversible swelling from ca. 17 to ca. 2.5 g/g--representing a reduction of 85% [26]. This is higher than most of the DS changes of hydrogels in this work, particularly those formed with high irradiation dose, as in the abovementioned reference. The peak transition at a higher temperature might have resulted from the size of hydrogel specimens. They were circular slabs with an initial thickness of ca. 1 mm, which could be too big for instant diffusion towards the surface and gel shrinking within a short time of 10 min of incubation at each specific temperature. The swelling kinetics partially depends on the dimensions of the sample, so in these semi-stationary experiments, the gel size exerted its impact. The hydrogels of HPC-CMC composition were expected to demonstrate dual thermresponsive and pH swelling. Initially, the temperature response was investigated. At room temperature, the hydrogels of mixed composition present different swelling, which is controlled by the dose and by the presence of the crosslinker, as demonstrated earlier. The EDS of 5/5 HPC/HPC is approximately 210 and 50 g/g for gels produced by irradiation with 50 kGy of polysaccharides solution containing 2.5 and 5% PEGDA, respectively. The DS of the former decreases nearly constantly to approximately 133 g/g (33%) with an increase in temperature from 25 to 55 \({}^{\circ}\)C, as shown in Figure 8a. In order to induce higher thermo-responsiveness and reduce EDS, making the gels more firm (these are practical aspects), the hydrogels with a lower content of CMC (2.5%) and an increased PEGDA content of 5% produced with two doses were examined. As shown in Figure 8b,c, the initial EDS of approximately 56 and 42 g/g (gels formed with 25 and 50 kGy, respectively) decreased to ca. 32 and 25 g/g at 55 \({}^{\circ}\)C (reductions of over 40% for both). Additionally, in this case, there is a continuous decrease in DS in function of the temperature. The HPC containing hydrogels did not instantly increase their mass upon temperature decrease, however, because the large EDS observed at room temperature, they could nearly return to their initial DS in 7 days. There is an influence of the CMC co-component that combined with the PEGDA crosslinker to reduce the thermo-responsiveness driven by the HPC temperature-sensitive polymer. Upon irradiation, aqueous carboxymethylcellulose solutions generate anionic hydrogels, which are pH-responsive. These deprotonate and swell extensively when the external pH is about and above the pKa of carboxyl groups. This somewhat depends on the substitution degree of CMC, but generally is above pH 4. However, at a pH below 3.5, a considerable number of carboxyls are ionized in their stationary state [40]. Therefore, investigations of the pH dependence of gels were conducted in a wide pH range from ca. 7 down to 1.5. The pH-sensitive swelling of--and an influence of the ionic strength on--CMC hydrogels synthesized by either classical chemical methods and with radiation technique has been reported [41; 42]. Nevertheless, investigations have covered different CMCs in terms of molecular weight or degree of substitution, so in order to compare the swelling properties of mixed, i.e., two polysaccharide hydrogels, to those of the sole one, the actual CMC was examined. Additionally, the macromer crosslinker and the dose factors do influence the properties.