Hydrogel and Organogel-Based Flexible Wearable Sensors: Synthesis, Sensing Mechanisms, Performance Optimization, and...

Hydrogel-based flexible sensors have emerged as a promising platform at the intersection of materials science, flexible electronics, and biomedical engineering, owing to their tissue-like mechanical compliance, tunable ionic conductivity, and versatile chemical functionalizability. To translate laboratory-scale hydrogel devices into durable, reliable wearable systems, this review critically summarizes recent progress in functional hydrogels and organogels, covering literature reported up to September 2026, and organizes the discussion along the complete chain from material synthesis and characterization to wearable deployment. We first outline the synthesis principles of gel network architectures, including double networks, semi-interpenetrating networks engineered with nanosheet crosslinkers, organohydrogels employing mixed solvent systems, and mechanically interlocked topologies, followed by MXene-based composite gel materials and advanced 3D printing fabrication strategies. Six representative sensing mechanisms, namely piezoresistive, capacitive, triboelectric, thermoelectric, electrochemical, and multimodal decoupling, are then qualitatively and, where possible, quantitatively compared regarding their performance characterizations. We also discuss signal processing and readout electronics for each sensing mechanism, which are critical for wearable deployment. Dedicated sections analyze performance enhancement strategies aimed at mechanical robustness and sensitivity, and summarize self-healing chemistries along with environmental adaptation approaches, such as solvent replacement and deep eutectic solvent engineering, which are essential for long-term operational stability. We further propose a minimum reporting checklist for standardization and benchmarking and provide a systematic evaluation of long-term stability, toxicity, degradation data, leachables, . Finally, we highlight application frontiers of these gel materials in health monitoring biosensing, electronic skin, and human-machine interaction, and conclude with a critical assessment of key challenges in scalable manufacturing, biocompatibility, multimodal crosstalk, energy autonomy, and environmental sustainability. A dedicated discussion of environmental impact and sustainability, covering bio-based polymers, biodegradability, recycling, and life-cycle assessment.