Characterizations of Polyion Complex Formed at Hydrogel Adhesion Interface

Hydrogels exhibit numerous outstanding characteristics, making them a next-generation material with promising applications in the medical and industrial fields. For the practical implementation of hydrogels as products in various domains, it is essential to fix hydrogels firmly due to low friction. Therefore, in recent years, the adhesion of hydrogels has been attracting much attention. In order to effectively control hydrogel adhesion, it is critical to understand the structure and physical properties of the adhesion interfaces. However, conventional methods have faced difficulties evaluating the local structure near the adhesion interface in situ. In this study, the microelectrode technique (MET) was newly applied to hydrogel adhesion interfaces to observe the polymer density near the adhesion interface. Through electric potential measurements near the adhesion interface between two hydrogels with opposite charges, we successfully achieved the direct observation of a network structure (polyion complex, PIC) due to electrostatic interactions. Furthermore, we confirmed that the cross-linking density of the hydrogels influenced the stiffness of the PIC. We can widely apply this method for the structural evaluation of hydrogel adhesion interfaces, and it is expected to contribute as a novel analysis technique for adhesion interfaces.