Hydrogels have drawn considerable attention in the field of flexible chemical sensors due to their unique 3D structure, high permeability, ion-conductivity, and tissue-like mechanical properties. These structures and properties allow them to be functionalized into diverse sensing components and respond to chemical signals in complex environments. Herein, an overview of functional hydrogel-based flexible chemical sensors is provided. First, the representative hydrogel materials are introduced and the operating principles for flexible chemical sensors are discussed. Then, state-of-the-art functional hydrogel-based flexible chemical sensing applications are highlighted including gas sensors, humidity sensors, pH sensors, glucose sensors, wound monitoring, and others. Finally, major challenges and opportunities in this field are provided. 

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/adsr.202300021

A Review of Functional Hydrogels for Flexible Chemical Sensors

Lanthanide coordinated multicolor fluorescent polymeric hydrogels for bio-inspired shape/color switchable actuation through local diffusion of Tb3+/Eu3+ ions

Protective cleaning of Chinese paper artworks with strong hydrogels: An interfacial adhesion perspective

Skin mimicking-sweating evaporation polyimide cooling film for electronic devices

Flexible humidity sensors show great potentials in the fields of smart apparels, biomedicine, human-machine interaction, etc., and they have attracted much attention in recent years. Herein, strip-type flexible capacitive humidity sensors (SFCHS) were facilely prepared by intertwining copper wires covered with soluble polyimide (PI) and poly(glycidyl methacrylate) (PGMA). The "pine nut" microstructure on copper wire surface and network structure between PGMA and PI strengthened the adhesion between the copper wire electrode and the polymeric sensitive materials, leading to better stability and sensing performance. The SFCHS with a double helix configuration showed good sensitivity (capacitance change of 111.5% between relative humidity (RH) of 11–98%), moderate hysteresis (∼6.6% RH) and fast response (tadsorption/tdesorption: 10.1 s/5.2 s) at room temperature. Furthermore, the self-standing SFCHS revealed stable capacitance under bending and good long-term stability. In addition, the flexible sensor could monitor real-time change of water content in various liquids (transformer oil, hydraulic oil, ethanol and n-dodecane), suggesting potentials for detecting water content in liquids. The work provides references for developing advanced flexible capacitive humidity sensors for measuring moisture in both air and liquids. 

Strip-type flexible capacitive humidity sensor based on composite of polyimide and poly(glycidyl methacrylate): Fabrication, humidity sensitive performance and potential for detecting water content in liquids

In this paper, we propose a simple organohydrogel based capacitive humidity sensor for noncontact artificial sensation applications. The sensor is simple in design and consists of a transparent polyacrylamide organohydrogel thin film attached on a flexible inter-digit electrode layer. The process of water absorption and desorption is reversible, thus the dielectric of the organohydrogel film as well as the overall capacitance are dependent on environmental humidity. The water absorption capacity and structural reliability of the device have been largely improved by adding glycerol in the organohydrogel network. By optimizing both the glycerol concentration and organohydrogel film thickness, the sensor can respond to cyclic humidity changes in a period of 300 ms. In addition, this sensor achieves a high relative capacitance increase (by 20 folds) in a wide relative humidity range (12%–95%). The sensor also exhibits high stability under different bending curvatures (up to 6.81 mm), wide temperature changes (20°C–40°C) and external pressures (0–8 N). To demonstrate the applications in wearable electronics, we found that the sensor was successful in detecting respiration intensity and rate as well as the difference in moisture content in various objects, i.e., human skin and leaf surface. This sensor is highly sensitive and can be useful in the detection of the widerange of humidity changes.

A flexible organohydrogel-based humidity sensor for noncontact artificial sensation

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A flexible organohydrogel-based humidity sensor for noncontact artificial sensation