Smart fire safety wearable electronic devices based on ion gel microneedle arrays

Ion gels provide innovative applications and future prospects for flexible electronics, covering areas such as wearable electronics, soft robots, and intelligent systems. Ion gels used in flexible electronics need to maintain good conductivity, structural stability, processing compatibility, and sensitivity when affected by factors such as pressure, temperature, humidity, and solvents. Although the miniaturization, integration, and wireless operation of flexible sensor devices have improved their sensitivity and increased the selectivity, comfort, and convenience of the devices, they have also brought manufacturing challenges. Therefore, the key issues currently facing ion gel sensors are to develop ion gels with excellent performance, clarify the intrinsic sensing mechanism of ion gel sensors, and ultimately promote the innovative application of micro-integrated flexible sensor devices.

Recently, the team of Professor Gui Zhou and Professor Zhu Jixin from the University of Science and Technology of China published a research result entitled "Unlocking Intrinsic Conductive Dynamics of Ionogel Microneedle Arrays as Wearable Electronics for Intelligent Fire Safety" in the journal Advanced Fiber Materials. The study reported in detail a series of high-performance ion gel materials, using density functional theory calculations, molecular dynamics simulations, and in situ temperature-dependent spectroscopy to reveal their intrinsic sensing mechanisms. In addition, the research team also prepared a series of bionic microneedle ion gel devices with customizable structures. Based on these results, they also developed a real-time intelligent monitoring system, which achieved dual-effect early warning of intelligent fire safety through the integration of the Internet of Things, short-range wireless communication and smart mobile application technologies. Doctoral student Zheng Yapeng from the University of Science and Technology of China is the first author of this article, and Professor Gui Zhou and Professor Zhu Jixin are co-corresponding authors.

Figure 1 shows the structure and morphology of ion gels. Their molecular structure contains a variety of dynamic bonds, especially hydrogen bonds and ionic bonds. These non-covalent interactions form spontaneous changes and reversible processes, showing extraordinary application potential in wearable electronic devices. The research team successfully prepared a series of ion gels with excellent mechanical properties, including high tensile strength (elongation of about 794%), high pressure resistance (deformation rate of about 90%), high mechanical strength (tensile strength and compressive strength of about 2.0 MPa and about 16.3 MPa, respectively), and excellent transparency (transmittance> 91.1%) by using ultraviolet (UV) curing technology for in-situ photopolymerization. In addition, the research team selected representative solvents for swelling tests to evaluate and confirm the tolerance of ion gels under different environmental conditions.

Figure 1 Structure and morphology of ion gel

Figure 2 Mechanical and electrical properties of ion gel

研究团队通过微电子印刷技术在银电极上实现离子凝胶的印刷，制备了微型柔性电子器件。离子凝胶器件的电阻变化受温度范围影响，随着温度的升高，柔性器件的电阻显著降低。在30℃~ 45℃的温度范围内，离子凝胶器件的热敏系数（Tcr = -2.074%，R²= 0.993）较高；经历连续循环多次升降温后，其电阻变化基本保持一致，表明其具备优异的温度传感稳定性。

Figure 3 Mechanical and electrical properties of ion gel

Inspired by the microneedle patch technology, the research team combined digital light processing (DLP) 3D printing, mold casting and in-situ photocuring technology (Figure 4a) to develop a series of customizable ion gel devices, such as micro-cylindrical, micro-hemispherical, micro-spherical and micro-pyramidal microneedle array structures, and confirmed the thermal behavior and mechanical stability by finite element distribution simulation (Figure 4b ~ 4i). By combining digital electrodes with bionic microneedle structure ion gel, a flexible and soft sensing system was prepared.

In order to verify the feasibility of the flexible sensing system in practical applications, the research team developed a real-time intelligent monitoring system based on wearable safety devices, which can realize real-time signal acquisition, processing and temperature transmission, and wireless monitoring through the integration of Internet of Things, short-range wireless communication and intelligent mobile application technology. As shown in Figure 4j ~ 4p, the system can wirelessly detect and monitor the thermal stimulation generated by the external environment or human activities in real time. The wireless wearable monitoring system includes: signal acquisition module, control circuit module, central processor, i.e. single-chip microcomputer, wireless Bluetooth transmission module and smartphone APP real-time receiving module. The system obtains the change of the resistance of the safety device and converts it into an output voltage signal, which is transmitted to the microcontroller; the microprocessor processes the collected signal and transmits it through the Bluetooth module; the Bluetooth receiver converts the digital signal of the sensor into data, which is analyzed and displayed in real time through the smartphone APP. The research team successfully demonstrated the feasibility of the system in practical applications (Figure 4), including real-time monitoring of the temperature of the external flame contact point and the effect of the intelligent audio-visual dual-effect alarm system in different states (normal, warning, and alarm states).

Figure 4 Real-time intelligent security monitoring system

In summary, this work uses a simple and universal method to prepare a series of ion gel materials with high transparency, excellent mechanical and electrical properties, outstanding electrochemical stability, environmental tolerance, and good strain and temperature sensing capabilities. In addition, the research team demonstrated the potential of microneedle array structure ion gel devices for customized wearable safety thermosensitive devices, and developed a real-time intelligent monitoring system for smart fire safety warning and temperature indication.