A comparison of the swelling ratio data for different pH values showed that the swelling ratio at pH 3 was highest, and pH 5 produced the lowest swelling ratio. The swelling ratios then gradually increased as the pH was increased to 11. Gelatin is amphoteric as it has both carboxyl and amino groups on the side chains. Therefore, different swelling behaviors at different pH values depend on whether gelatin is acting as an acid or as a base. The isoelectric point of gelatin is observed at pH 4.9. At this value, the number of positive charges is equal to the number of negative charges on a gelatin hydrogel. At the isoelectric point for gelatin, the electrical attraction between these opposite charges will lead to network collapse. 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)