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Sun Yat-Sen University's Wu Jin group "Mater. Horiz.": Highly stretchable, self-healing and anti-drying dual-network organic hydrogel humidity sensor
2020-01-07 Source: Polymer Technology

At present, wearable electronic devices are booming, and their performance requirements have increased from flexibility to stretchability, self-healing, and transparency. Humidity is an important variable that is closely related to our health and daily comfort, but few studies have reported it in stretchable sensing systems. Compared with traditional nano-materials or complex micro-machined structures, seeking intrinsically stretchable materials is undoubtedly a breakthrough. Hydrogels have natural advantages such as super-stretchability, rich systems, good biocompatibility, and low preparation costs. They have become research hotspots in the field of stretchable electronic devices, but they have been plagued by the problem of water loss. The long-term use stability of rubber-based electronic devices is an important problem that needs to be solved urgently.

The team of associate professor Wu Jin of Sun Yat-sen University used the characteristics of the hydrogel to easily lose water and change its conductivity, and prepared an intrinsically highly stretchable humidity sensor. Using a solvent replacement strategy, a water-retaining, antifreeze ethylene glycol / glycerol was introduced into the hydrogel to convert the synthesized hydrogel into an organic hydrogel to form a polypropylene with a water-alcohol binary solvent system. The amide / carrageenan dual-network organic hydrogel gives the hydrogel sensor anti-drying properties, effectively solving the problem of hydrogel dehydration and drying , extending the service life of the device and expanding the use environment of the device. In addition, a large number of hydroxyl groups on ethylene glycol / glycerol molecules are used to form hydrogen bonds with water molecules to promote the adsorption and condensation of water molecules in the air on the hydrogel, which greatly improves the sensitivity of the humidity sensor.

Figure 1. Solution replacement method for the preparation of ethylene glycol / glycerol-dual network organic hydrogels. Hydrogen bonding between polyhydric alcohols and water molecules is used to improve the hydrogel's moisture retention and humidity detection sensitivity

Figure 2. Transparency, self-healing, and stretchability of organic hydrogels

The polyacrylamide / carrageenan dual network hydrogel itself is highly transparent. After being replaced by an alcohol solution, the transparency does not significantly deteriorate; however, the mechanical strength is enhanced ; thanks to the unwinding and reorganization of the carrageenan double helix structure, In the case of complete fracture, the repaired hydrogel can be obtained after one hour of heat preservation ; in addition, the dual-network hydrogel of this system has super stretchability and has not broken even after undergoing 1225% tensile strain.

Figure 3. Comparison of anti-drying before and after modification of glycol / glycerol modified hydrogels

The unmodified hydrogel loses water and dries within a day, losing its conductivity. Glycol / glycerol-modified hydrogel remained good flexibility and stretchability after being left for 50 hours at 40 C and 55% relative humidity. After a series of temperature gradient and humidity gradient environmental tests, the modified hydrogel still has conductivity. Through comparative research, it is found that under mild (normal temperature, high humidity) environmental conditions, the glycol-modified hydrogel has better moisture retention; while under harsh environment (high temperature, dry), glycerol-modified water The gel has good moisture retention.

Figure 4. Hydrogel humidity sensor response, application, and mechanism

The introduction of ethylene glycol and glycerol greatly improved the sensitivity of the hydrogel humidity sensor. The sensitivity was increased by nearly 40 times , the detection range was wide (4% ~ 90% RH), and the response time and recovery time were as fast as 0.27s and 0.30s , and long-term (one month) use can maintain stable performance. Because the response speed is extremely fast, the humidity sensor can be used to continuously monitor the human respiratory rate for human health monitoring.

The detection mechanism of the hydrogel humidity sensor was revealed for the first time: the polymer chain hinders the movement of ions , free water molecules in the gaps between the polymer chains provide ion transport channels, and changes in the environmental humidity cause the hydrogel polymer chain density and channel volume Changes, which affect the speed of ion transport, leading to changes in conductivity. This study expands the application range of hydrogels and provides new material options for more comfortable and high-performance wearable electronic devices.

The above results were published in the journal Materials Horizons under the title "An intrinsically stretchable humidity sensor based on anti-drying, self-healing and transparent organohydrogels". The first author and corresponding author of this article is Associate Professor Wu Jin of Sun Yat-sen University.

This research work was supported by the National Natural Science Foundation of China, the Natural Science Foundation of Guangdong Province, and the Guangzhou Science and Technology Plan.

Paper link: http://doi.org/10.1039/C8MH01160E

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