Liquid metal microparticles enabled low-cost, compact, and sensitive humidity sensors for in situ moisture monitoring.

Moisture-sensitive materials require reliable moisture-proof packaging to maintain quality throughout manufacturing. In situ humidity monitoring inside moisture-proof packaging is essential for an early detection of moisture barrier failures. Although moisture testing kits like humidity indicator cards are available, they are often toxic or slow, and technological advancements in developing faster, safer, and more affordable alternatives have remained stagnant for years. Here, we report a liquid metal microparticle-enabled humidity sensor (referred to as LM²H) which is passive, low-cost, highly sensitive, and ultra-compact. The core functionality of the sensor lies in the aluminum core-liquid metal shell microparticles (Al@LM-MPs), which possess a distinctive ability to react with surrounding water molecules and generate heat in an extremely sensitive way. The LM²H has a measurement range of 30-90% relative humidity (RH) and provides a fast response of ∼4 s, representing a >1700-fold improvement over current commercial humidity indicator cards. The LM²H utilizes the color change to indicate the humidity and integrates full sensing and display functions within an ultra-compact size (45 × 20 × 0.96 mm), a lightweight design (∼2.1 g), and a low cost (0.66 USD per unit). These features empower the LM²H to revolutionize humidity monitoring in desiccant packaging across various industries, and applications under extreme conditions or for intermittent measurements.