Flexible eutectohydrogel sensor with structural color and stable electrical response under extreme conditions

Flexible strain sensors capable of stable operation in extreme environments are essential for next-generation wearable electronics and soft robotics. However, conventional hydrogels typically fail under harsh conditions because of freezing, dehydration, or hygroscopic swelling. In this work, we developed a flexible eutectohydrogel (EHG) sensor with structural color and stable electrical response by leveraging hydrogen bonding interactions between deep eutectic solvents (DES) and water. Polyvinylpyrrolidone-tannic acid-modified magnetite (Fe₃O₄@TA-PVP) colloid nanocrystal clusters (CNCs) assemble into a photonic crystal array under an oriented magnetic field, facilitating vivid and strain-responsive structural color changes. Concurrently, the PAM/Gelatin network exhibits high transparency, enabling undistorted transmission of optical signals through the gel matrix. The DES-water hydrogen bond network effectively prevents freezing and dehydration, maintaining the flexibility of the EHG and electrical responsiveness even at −40 °C. After seven days of exposure to 30 %-90 % relative humidity, the EHG retains stable optical output without moisture loss or hygroscopic swelling. Furthermore, the sensor exhibits an extremely low detection limit (0.5 %), rapid response time (0.83 s), and durable performance exceeding 1000 cycles. This research provides a reliable strategy for designing hydrogel sensors that adapt to environmental changes with dual-response capabilities, offering insights into developing flexible devices capable of reliable operation under extreme environmental conditions.