Electroactive hydrogel for force-to-electricity conversion: Emerging engineered frameworks for advanced bio-interfaces

During tissue regeneration, the microenvironment functions as a coupled physical, chemical, and biological system that provides essential biophysical cues for regulating cell behavior. Biomaterials that mimic these properties therefore hold significant regenerative potential. Among them, electroactive hydrogels have emerged not only as scaffolds but also as mechanoelectrical transducers that convert mechanical stimuli into biologically relevant electrical signals, which is particularly valuable for excitable tissues. Exogenous electrical stimulation and intrinsic piezoelectricity can modulate a broad range of biological processes, including cell migration, proliferation, differentiation, neural conduction, muscle contraction, and tissue repair. Recent advances in materials science have enabled the development of electroactive hydrogels with improved biocompatibility, tunable mechanics, and force-to-electricity conversion capability. This review first outlines the roles of endogenous bioelectricity and bio-piezoelectricity in tissue development and regeneration. It then focuses on two major classes of electroactive hydrogels designed to mimic microenvironmental mechanoelectrical transduction: piezoelectric hydrogels and piezoionic hydrogels. We compare their distinct operating principles, namely strain-induced polarization and deformation-induced ion redistribution, as well as their mechanical compatibility, signal characteristics, and biological operating windows. Particular attention is given to their use as self-powered biointerfaces for sensing, stimulation, and regenerative modulation. We further provide a comparative framework summarizing their differences in output regime, force sensitivity, frequency response, impedance behavior, conversion characteristics, degradability, and current in vivo validation status. Finally, the opportunities and remaining challenges of both systems are discussed, with emphasis on material design, benchmarking, and translational potential. This review aims to provide mechanistic insight and practical design guidance for next-generation electroactive hydrogels in bio-interfacing and regenerative medicine.