Hydrogel-Based Iontronic Sensing

Hydrogel-based iontronic sensing (HBIS) is emerging as a compelling frontier at the interface of soft matter, electrochemistry, and bioelectronics, driven by the unique ability of hydrogels to communicate with living systems through hydrated ionic networks rather than electronic conduction in rigid solids. This ionic mode of signal transduction enables intrinsically compliant interactions with biological tissues, electrolytes, and dynamic fluidic microenvironments, making HBIS highly attractive for flexible bioelectronics. As the field evolves from material optimization toward integrated and intelligent systems, a unifying understanding of material design, ion transport, and device function remains lacking. Here, a multiscale quantitative framework is established to bridge structure, transport, and sensing performance in HBIS. Representative material platforms, including polyelectrolytes, ionogels, and nanocomposite hydrogels, are examined alongside key transport mechanisms, such as diffusion, electromigration, convection, electroosmotic flow, adsorption-site hopping, and Grotthuss-type conduction. Quantitative modeling approaches based on electrochemical impedance spectroscopy and Poisson-Nernst-Planck theory are further highlighted, together with emerging applications in iontronic skins, soft robotics, human-machine interfaces, and energy conversion. This review provides a design blueprint for next-generation hydrogel iontronic sensors with improved predictability, adaptability, and operational stability across biological interfaces.