Coordination of Covalent Cross-Linked Gelatin Hydrogels via Oxidized Tannic Acid and Ferric Ions with Strong Mechanical Properties

The design of gelatin-based hydrogels with high mechanical strength, high gelation temperature, and a rapid self-healing property still presents a challenge to researchers. In the present study, single cross-linked gelatin–oxidized tannic acid (SC-GT/OTA) hydrogels were fabricated through covalent cross-linking between gelatin and tannic acid (TA) oxidized by using sodium periodate (NaIO4). Double cross-linked gelatin–OTA–FeCl3·6H2O (DC-GT/OTA/FeIII) hydrogels were also created using metal coordination bonds formed between the catechol groups present in OTA and FeIII in ferric chloride. As a result, the maximum gelling temperature of the SC-GT/OTA hydrogel (37 °C) was considerably higher than that of the pure gelatin hydrogel (15.4 °C). Moreover, the maximum values of compressive stress of SC-GT/OTA hydrogels increased significantly by almost seven times the original value as the molar ratio of NaIO4 to TA increased from 3:1 to 5:1. When the molar ratio of NaIO4 to TA was maintained at the constant of 4:1, the storage modulus values of DC-GT/OTA/FeIII hydrogels with the FeIII-to-TA molar ratio of 1.5:1 were three to 4 orders of magnitude higher than those of SC-GT/OTA hydrogels in the whole angular frequency range. The double cross-linked gelatin hydrogels developed in this research can be used widely in agriculture and material science fields.