Conductive hydrogels provide many opportunities for the application of flexible strain sensors due to their good conductivity, high self-healing efficiency and strong stretchability. However, most of the reported conductive hydrogels still have some drawbacks, such as poor adaptability to extreme temperatures, easy to lose water, and not recyclable, which cannot meet the needs of long-term stable sensing applications. In addition, large-scale preparation is costly and environmentally unfriendly. Therefore, the preparation of conductive hydrogels with multiple functional properties using degradable and recyclable raw materials remains a challenge.
As a national intangible cultural heritage, flour sculpture is a handicraft technique that uses wheat flour, glutinous rice flour, water and glycerin as the main raw materials, adds different colors, and can be molded into various vivid images with simple tools. With the development of the times, face plastic handicrafts keep pace with the times, and are still full of vitality in the new era, with the characteristics of moisture-proof, storage-resistant, not easy to mold, not easy to crack, bright colors, and beautiful shapes. Inspired by the production process of face molding, in view of the problem that traditional electronic materials cannot be recycled, Associate Professor Chen Wei, Associate Professor Li Nan and graduate student Qiu Liyuan of Qufu Normal University, together with Professor Ji Xingxiang, State Key Laboratory of Bio-based Materials and Green Paper of Qilu University of Technology (Shandong Academy of Sciences), combined traditional face molding with cutting-edge scientific research, and prepared a new type of multifunctional "ionic dough" using flour, water and choline chloride/glycerol low-eutectic solvent (ChCl/Glycerol DES) as raw materials (Ionic Dough), the ionic dough prepared has high electrical conductivity (3.7 mS·cm−1), good low temperature resistance, long-lasting moisture retention (80% weight after 24 days), reliable self-healing efficiency (95%), excellent antibacterial and biodegradability (completely degradable within 30 days), which provides a new strategy for the preparation of a new generation of green electronic devices.
In this work, a low eutectic solvent (DES) was prepared by using choline chloride as the hydrogen bond acceptor and glycerol as the hydrogen bond donor. DES, water, wheat flour and glutinous rice flour are then mixed with water, wheat flour and glutinous rice flour through traditional dough molding production processes such as mixing, steaming, and kneading to prepare a multifunctional ionic dough (Figure 1).
Figure 1 Design and preparation process of ionic dough
The research team evaluated the frost resistance and water retention properties of ionic dough (Figure 2). The freezing point of ionic dough with different low eutectic solvent content was tested by DSC, and it was proved that the addition of DES could effectively prevent the formation of ice crystals, and with the increase of DES content, the frost resistance of ionic dough increased, and the ionic dough remained soft after 3 days at −20 °C. In addition, when left at room temperature (20 °C, 50% RH) for 24 days, conventional dough splits into pieces due to moisture loss, while ionic dough still retains 85% of the initial water content.
Figure 2 Freeze resistance and water retention of ionic dough
Figure 3 shows the self-healing properties of ionic dough. Due to the interaction of intermolecular hydrogen bonds, ionic dough can quickly self-repair after injury by recombining non-covalent hydrogen bonds. The self-healing process of the ionic dough is observed under a light microscope, and in the absence of external action, the ionic dough can be completely repaired within 1 hour. The self-healing behavior of ionic dough was verified by real-time resistance change and continuous step strain test during cutting-repair process, and the results showed that ionic dough had excellent self-healing performance.
Figure 3 Self-healing properties of ionic dough
By using E. coli (E. coli) and Staphylococcus aureus (S. aureus), the team evaluated the antimicrobial activity of ionic dough (Figure 4). The results showed that compared with traditional dough, ionic dough showed significant antibacterial activity and could inhibit bacterial growth. This is mainly due to the fact that choline chloride in DES, as a quaternary ammonium salt, is able to destroy the negatively charged bacterial outer membrane, causing its contents to leak, resulting in bacterial death. In addition, the dough flakes are buried in moist soil, which can gradually degrade under the action of microorganisms and completely disappear after 30 days, confirming the good degradable properties of ionic dough.
Fig. 4 Antibacterial and degradable properties of ionic dough
Due to its excellent electrical conductivity, ionic dough can be used as a conductor to light LED bulbs, and can be further applied to flexible Cu-Zn batteries and electroluminescent devices. The authors measured the effect of water content on the conductivity of ionic dough: when the water content increased from 0 to 50 wt %, the resistance of ionic dough decreased from 355 Ω to 45 Ω, and the conductivity increased from 0.54 mS·cm−1 to 3.7 mS·cm−1. The effect of temperature on conductivity was further studied: when the temperature was increased from −40 °C to 100 °C, the conductivity of ionic dough increased from 2.8 mS·cm−1 to 4.4 mS·cm−1, indicating the feasibility of ionic dough as a flexible electronic device at extreme temperatures (Figure 5).
Figure 5 Conductivity of ionic dough
The authors assembled the ionic dough into a wearable strain sensor to monitor human activity (Figure 6). Due to their excellent sensitivity and long-lasting stability, wearable strain sensors based on ionic dough can accurately record large and small strain signals in the human body, even at extreme temperatures. Therefore, this study is expected to provide a new, environmentally friendly and promising fabrication strategy for multifunctional electronic and sensing devices.
Figure 6 Strain sensor based on ionic dough
The study was published in the international high-level journal Chemistry of Materials under the title "Engineering Ionic Dough with a Deep Eutectic Solvent: From a Traditional Dough Figurine to Flexible Electronics". Professor Wang Xiaohui of University of Science and Technology Beijing gave a lot of constructive guidance. We would like to thank the National Natural Science Foundation of China, the Natural Science Foundation of Shandong Province, and the State Key Laboratory of Bio-based Materials and Green Paper of Qilu University of Technology (Shandong Academy of Sciences) for their support of this work.
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Original link:
https://doi.org/10.1021/acs.chemmater.3c01761 Source: Frontiers in Polymer Science - Early View