Hence, the hydrogel shows the lowest swelling ratio at pH near 5. When the pH was different from the isoelectric point, the charge density in the network increased due to the increase in the degree of ionization. The increase of ionization caused the hydrogel to swell significantly. Above the isoelectric point (pH < 4.9), the gelatin network formed a cationic gel. Below the isoelectric point (pH > 4.9), the gelatin network formed an anionic gel, as shown in Eqs. (3) and (4). Hydrogels that are positively charged (a cationic network) will have a higher swelling ratio than those that are negatively charged (an anionic network)