# Polysaccharide Hydrogels Based on Cellulose and Chitosan for Drug Sustained-Release Applications
## 3 Results and Discussion
### Stability Testing of CAA-CS Polysaccharide Hydrogel  
Hydrogels prepared by blending different CAA concentrations (1.5%, 2 w%, 2.5 t%) with 2% CS were placed in PBS solution to analyze their swelling and degradation behaviors. The swelling property, which is the ability of hydrogels to absorb water and expand, is closely related to their crosslinking density and network structure. The swelling ratio of the CAA-CS composite hydrogels declined with the increase in CAA concentration, as depicted in Figure 4a. This phenomenon can be attributed to the fact that the increase in CAA concentration results in an elevated crosslinking density of the hydrogels. A higher concentration of CAA provides more acetoacetyl groups, which react with the amino groups in CS to generate more enamine bonds, thereby forming a denser three-dimensional network structure. Such a dense network structure inhibits the permeation of the PBS solution medium into the interior of the hydrogel, reducing the swelling extent of the hydrogel. Consequently, increasing the CAA concentration can simultaneously enhance the mechanical strength of the hydrogel and decrease its swelling property, enabling it to maintain superior morphological stability in a humid environment.  
The degradation performance pertains to the ability of hydrogels to gradually disintegrate in a specific milieu, which directly influences their practical efficacy in applications. Figure 4b reveals that the degradation rate of CAA-CS hydrogels exhibits a downward trend with the escalation of CAA concentration. The experimental outcomes suggest that the degradation rate of hydrogels is negatively correlated with the CAA concentration. The hydrogel with a 2.5% CAA content demonstrates the lowest degradation rate, whereas the 1.5% CAA system undergoes degradation at the fastest pace. This phenomenon is in accordance with the results of the swelling performance, further validating the impact of CAA concentration on the crosslinking density and structural stability of hydrogels.  
Figure 2: **(a)** Infrared spectrum of cellulose acetoacetate (**b**) nuclear magnetic spectrum of cellulose acetoacetate. “a” represents the methylene peak on the acetoacetyl group, and “b” represents the methyl peak on the acetoacetyl group.  
Hydrogels formed by higher concentrations of CAA possess a denser crosslinking network, thereby manifesting a lower degradation rate and higher stability.  
Figure 4: (**a**) Swelling degree of different polysaccharide hydrogels in PBS solution, (**b**) degradation degree of different polysaccharide hydrogels in PBS solution, (**c**) stability test of polysaccharide hydrogels in PBS (pH = 7.34) aqueous solution at 37 \({}^{\circ}\)C for 72 h and at 100 \({}^{\circ}\)C for 2 h.  
Figure 3: Electron microscopy image of polysaccharide hydrogel; (**a**) 1.5% CAA + 2% CS; (**b**) 2% CAA + 2% CS; (**c**) 2.5% CAA + 1.5% CS; (**d**) 1.5% CAA (20 wiu IL-2) + 1.5% CS; (**e**) 2% CAA (20 wiu IL-2) + 2% CS; (**f**) 2.5% CAA (20 wiu IL-2) + 1.5% CS; (**g**) the porosity of CAA-CS hydrogels; (**h**–**k**) freeze-dried CAA-CS hydrogel image.  
A further analysis was conducted on the stability of the CAA-CS hydrogel. The hydrogel was placed in PBS buffer for a certain period. After 72 h of observation, it was noted that the gel merely underwent slight expansion without collapsing. Subsequently, the gel was transferred to an oven and heated at 100 \({}^{\circ}\)C for 2 h (Figure 4c). During this period, no distinct morphological changes were observed in the gel. This indicates that the CAA-CS hydrogel possesses high structural stability and heat resistance. This stability is primarily attributed to the stable enamine bond (-C=N-) formed between CAA and CS through the Schiff base reaction, which can maintain a strong cross-linking intensity even in high-temperature and humid environments.