Amino Acid-Based Hydrogel with Interpenetrating Gelatin and Cross-Linked by Metal Ions, Providing High Stretchability and Motion Sensitivity

A double network structure with metal ions was created to enhance the mechanical stability of the hydrogels and increase their low conductivity. For this purpose, the P(AM_AcOr_Gelatin) hydrogel was synthesized by combining gelatin, a biocompatible polymer, <i>N</i>-δ-acryloyl-ornithine (AcOr), an amino acid derivative, and acrylamide (AM). Because the amino acid-based monomer added charged groups to the hydrogel network, the hydrogel exhibited improved conductivity and motion sensitivity properties compared with polyacrylamide (PAM) hydrogels. Furthermore, we altered the P(AM_AcOr_Gelatin) hydrogel by introducing the Fe³⁺ and Cu²⁺ ions, resulting in the formation of the P(AM_AcOr_Gelatin)-Fe³⁺ and P(AM_AcOr_Gelatin)-Cu²⁺ hydrogels. The hydrogels containing metal ions had coordination bonds between the ions, gelatin, and AcOr. Additionally, there were other noncovalent bonds present, resulting in further increased conductivity (approximately 95% improvement) and stretchability (more than double). The conductivity and resistance of the hydrogels changed, depending on the bending position and strain applied to the hydrogel layer. The results demonstrated that the hydrogel layer had good strain sensitivity, with an enhanced gauge factor (GF) of approximately 1.7 (at 250% strain) and a conductivity ranging from 3355 to 4387 μS·cm^-1.