Intelligent Hydrogel Sensors for Health Monitoring and Therapeutic Intervention: From Material Design to Integrated Diagnosis and Treatment.

Intelligent hydrogel sensors are integrated sensing devices employing stimuli-responsive hydrogels as the core functional sensing layers. Featuring tissue-mimetic mechanical flexibility, superior biocompatibility, and dynamic stimulus responsiveness, they fundamentally resolve the mechanical and biological interfacial mismatch between traditional rigid electronics and soft living tissues, providing new avenues for in situ, real-time, and in vivo physiological monitoring and precise biomedical intervention. This review systematically elaborates the full-scale design logic, multifunctional integration strategies, and cutting-edge biomedical applications of engineered intelligent hydrogel sensors. The material selection principles covering natural, synthetic, and hybrid polymer systems, multi-mode crosslinking strategies for three-dimensional network construction, and functional modification approaches for conductivity, biological recognition, self-healing, adhesion, and antifouling performance are comprehensively summarized. The sensing mechanisms that convert physical deformation and biochemical variations into quantifiable electrical or optical signals are classified and analyzed, including resistive/capacitive, piezoelectric, electrochemical, and optical transduction modes. This review strictly focuses on hydrogel-dominant sensing platforms and discusses their versatile applications in wearable physiological monitoring, implantable in vivo sensing, and in vitro biochemical diagnosis. Particular emphasis is placed on the core role of intelligent hydrogels in constructing "perception-feedback-treatment" closed-loop theranostic systems, which enable autonomous on-demand drug release or physical stimulation in response to pathological signal changes, achieving integrated diagnosis and therapy. Despite substantial laboratory progress, the clinical translation of intelligent hydrogel sensors still faces critical challenges, including insufficient long-term in vivo stability, biological contamination, uncontrollable immune responses, large-scale manufacturing difficulties, and poor performance consistency. This review further outlines future development directions based on artificial intelligence-assisted material design, biomimetic structural engineering, advanced 3D/4D printing manufacturing, and interdisciplinary integration. It is anticipated that intelligent hydrogel sensors will evolve into highly integrated, personalized, and autonomous intelligent healthcare units, promoting the paradigm shift from passive universal treatment to proactive, personalized, and predictive precision medicine.