Implantable hydrogel sensors: sensing mechanisms and biomedical applications

Implantable hydrogel sensors, owing to their high-water content, mechanical compliance, and functional versatility, can reduce mechanical mismatch between conventional implants and soft tissues while providing a favorable platform for in vivo sensing and biointerface engineering. This review provides a critical overview of the major sensing mechanisms of implantable hydrogel sensors and recent advances in their biomedical applications. The major sensing mechanisms and characteristics of piezoresistive, piezoelectric, electrochemical, and optical sensors are discussed. It further summarizes their applications in gastrointestinal monitoring, tumor microenvironment monitoring, skeletal muscle function monitoring, neurological and psychiatric disease monitoring, and metabolic biomarker tracking. Implantable hydrogel sensors are gradually evolving from single-signal detection platforms into dynamic biointerfaces with capabilities for long-term monitoring, physiological state assessment, and localized intervention. This is expected to profoundly transform in vivo disease monitoring and therapeutic strategies, while advancing the development of implantable hydrogel sensor technologies.