Hydrogel-based wearable and implantable biosensors in health monitoring

Hydrogel-based wearable and implantable biosensors have quickly established themselves as a paradigm-shifting platform for continuous health monitoring. They effectively reconcile the mechanical and chemical discrepancies between conventional rigid electronics and delicate biological tissues. This success is largely attributed to the hydrogel material's inherent advantages: high water content, a precisely tunable elastic modulus, and versatile functional chemistry. Collectively, these features provide structural, biological, and processing benefits that facilitate truly seamless bioelectronic integration. This Review systematically summarizes the latest advancements in hydrogel biosensors, placing emphasis on their distinctive material attributes. These include the synergistic ionic-electronic conduction, sensitive stimuli responsiveness, advanced antifouling surface chemistry, and exceptional tissue-mimicking compliance. We further delve into the fundamental physical and chemical sensing mechanisms, supported by representative applications spanning from the non-invasive, real-time analysis of sweat, tears, saliva, and interstitial fluids to the more complex domains of subcutaneous, cardiac, and neural implants. Finally, we candidly address the crucial hurdles that remain, such as long-term hydration stability, signal fidelity drift, and adverse immune responses. Concurrently, we highlight pioneering strategies to overcome these issues, including the adoption of zwitterionic designs for enhanced biocompatibility, nanocomposite reinforcement for mechanical robustness, and the utilization of transient biodegradability. By critically elucidating the intricate relationships governing hydrogel structure, chemistry, and bioelectronic function, this Review aims to chart the research trajectory toward the next generation of durable, self-healing, and fully biointegrated sensing systems.