*2.8. Swelling Test*

The swelling test was performed by soaking four hydrogel samples (400 mg) in DI water for di fferent time periods. The swelling ratio (SR) was defined as [13]:

$$SR = \frac{Ws - Moo}{Wo} \times 100\% \tag{4}$$

where *Ws* and *Wo* represent the weight of hydrogels after swelling in DI water in di fferent time periods and the weight of hydrogels before swelling, respectively.
