Multimodal Analysis of Surface States Underlying Hydrogel Adhesion

Hydrogel adhesion is of critical importance in a wide range of applications, including biomedical materials and soft devices; however, the mechanisms underlying hydrogel adhesion remain incompletely understood. While previous studies have predominantly discussed adhesion in terms of the chemical design of intermolecular interactions, the role of surface states—such as hydration, polymer chain orientation, and local chain density near the interface—has not been systematically examined experimentally. Moreover, because hydrogel surface states emerge from the coupled evolution of multiple properties, conventional approaches based on single parameters are insufficient to fully describe adhesive behavior. Here, we investigate hydrogel adhesion using a multimodal analytical framework that combines attenuated total reflection Fourier-transform infrared spectroscopy (ATR–FTIR), heterodyne-detected vibrational sum-frequency generation (HD-VSFG) spectroscopy, and complementary measurements of wettability and dewetting/rewetting behavior. To isolate the role of surface states, we employed an experimental strategy in which only the surface state was modulated through drying while maintaining an identical chemical composition. The results suggest that progressive water loss induces cooperative polymer chain reorientation and densification at the interface, and that adhesion strength reaches a maximum during this surface-state transition. Multimodal analysis further supports the view that adhesion cannot be described by a single measured parameter, but instead arises from the coupled evolution of multiple surface factors. These findings provide an experimentally grounded framework for understanding hydrogel adhesion in terms of surface states and highlight the importance of multimodal approaches for capturing the complexity of soft interfacial phenomena.