Prof. Nishuang Liu/Prof. Yihua Gao's team—recent advances in self-powered sensors based on ionic hydrogels

Research Background

Although substantial advances in electronic sensors consisting of electronic components including conductors, semiconductors, and dielectrics have been achieved, there remains a technical challenge posed by the intimate communication between these devices and biological systems. This is because biological signaling is mediated by ions and molecules rather than electrons. The introduction of ionic sensing has brought new vitality to bionic soft electronics. Recently, ionic hydrogel possessing favorable ductility and adjustable conductivity, as a new type of conductive material, has been extensively applied in flexible sensors. Conventional ionic hydrogel sensors typically require an external power source to operate, which limits the miniaturization, lightness, and wearable comfort of the sensor. In recent years, researchers have continued to explore diversified energy harvesting strategies and have developed ionic hydrogel self-powered sensors that eliminate the need for the external power supply. Although significant progress has been made in the application of ionic hydrogels in the field of self-powered sensing, the design and application of ionic hydrogels in this field has rarely been more comprehensively and systematically reviewed.

Research Progress

This review focuses on the latest research progress of ionic hydrogels in self-powered sensors and provides a more detailed and comprehensive categorization and summary in terms of self-powered mechanisms, structural configurations, and performance design of the sensors, which is more targeted and systematic (Fig. 1). First, the self-powered mechanisms of ionic hydrogel self-powered sensing systems are highlighted, including piezoelectric, triboelectric, ionic diode, moist-electric, thermoelectric, potentiometric transduction, and hybrid modes. Next, the structural engineering of ionic hydrogel self-powered sensors is discussed, especially the configuration and performance design of the ionic hydrogel sensing layer, including microstructure design, environmental stability, mechanical properties, and self-healing design. Subsequently, the latest applications of ionic hydrogel-based self-powered sensors, such as wearable electronics, human-machine interaction (HMI), environment monitoring, and medical diagnostics, are comprehensively presented. Lastly, the challenges and prospects for the development of ionic hydrogel self-powered sensors are discussed. This review aims to enhance readers' understanding of ionic hydrogel-based self-powered sensors and assist them in keeping abreast of cutting-edge research developments in this field.

Ionic hydrogels have been widely regarded as the preferred material in fabricating flexible sensors on account of their mechanical performance and physical properties that match biological tissues, as well as their customizable conductivity. However, most existing hydrogel sensors require additional power sources, greatly limiting the potential application fields. In order to break through this limitation, the self-powered sensing principle has become a key exploration direction to solve the problem of autonomous operation of sensors. With the innovation of energy conversion technology, the development prospects of ionic hydrogel self-powered sensors with sustainable energy supply are becoming increasingly clear. Unlike other sensors, ionic hydrogel self-powered sensors convert various external stimuli (energy inputs) into electrical signals (energy outputs) via ion movement, which generates charge separation and potential difference. Over the years, various kinds of ionic hydrogel self-powered sensors have been derived. This review highlights the different self-powered mechanisms of ionic hydrogel self-powered sensors, including piezoelectric, triboelectric, ionic diode, moist-electric, thermoelectric, potentiometric transduction, and hybrid modes (Fig. 2).

Ionic hydrogels offer unique advantages in structural and performance design due to their excellent flexibility, ease of preparation, and processing. Researchers have focused on carefully crafting the ionic hydrogel sensing layer in order to improve sensing performance and increase the application range of ionic hydrogel self-powered sensor devices. A number of important features, such as sensitivity, sensing range, response time, stability, and multifunctionality, are intended to be improved by this design optimization. This review provides a detailed presentation of the microstructural design as well as the performance design of ionic hydrogel self-powered sensors (Fig. 3), which includes mechanical properties, self-healing design, environmental stability, conductivity, self-adhesion and biocompatibility design (Fig. 4).

Ionic hydrogel self-powered sensors enable sensory functions akin to human senses while also imitating the biological mechanism of ion migration. Its exceptional flexibility and sensitivity are further highlights of this technology. Numerous hydrogel sensors with distinct self-powering mechanisms have been developed and widely used in domains like wearable electronics, environment monitoring, and HMI by utilizing the various energy harvesting technologies previously mentioned (Fig. 5). These sensors not only eliminate the economic burden and environmental concerns associated with frequent battery replacements but also harness previously untapped clean energy. By doing so, they effectively mitigate the current energy crisis and reduce environmental pollution, making a substantial contribution to sustainable development. The promotion and application of this innovative technology will further drive scientific and technological progress, improving people's quality of life.

Future Prospects

Despite the substantial advancements made in ionic hydrogel self-powered sensor devices in recent years, they are in the early stages of research, many challenges still need to be overcome, and there is room for the development of new technologies (Fig. 6). The main focus of upcoming research and development should be on improving the sensitivity and energy conversion efficiency of ionic hydrogel self-powered sensor devices. The improvement is crucial for advancing their performance and expanding their practical applications. Currently, the energy conversion efficiency in ionic hydrogel self-powered sensor devices is relatively low, which hampers the full utilization of environmental energy sources, resulting in short operation time and poor stability. For the application of ionic hydrogels as electrodes in triboelectric sensors, optimizing electrode design, material selection, and surface treatment can enhance sensor sensitivity and energy harvesting efficiency. The molecular and structural design of ionic hydrogels should be the main focus of efforts for self-powered sensors that use thermoelectric, piezoelectric, and ion diode mechanisms. This will increase the concentration gradient differences and improve ion directional transport. Self-powered sensors will function and operate more efficiently as a result of these advancements. It is crucial to take into account the Faradaic reactions that take place at the interface between the ionic hydrogel and the electrodes when dealing with potentiometric sensors. Furthermore, the development of new energy conversion mechanisms and superior sensing performance is one of the main directions for the future advancement of ionic hydrogel self-powered sensor devices. This requires an adequate combination of novel mechanisms, new materials, advanced structures, and effective fabrication methods. Moreover, future ionic hydrogel self-powered sensor will not be limited to single sensing functions but will integrate multiple sensing functions, such as monitoring temperature, pressure, humidity, and light, to meet the demands of various fields. Achieving the capability to utilize different sensing principles simultaneously to detect and distinguish complex external stimuli, similar to the functionality of human skin, is highly anticipated.

The stability and durability of materials are vital for ensuring the performance and longevity of sensors. Ionic hydrogel materials may experience issues such as swelling, shrinking, or aging over prolonged use, which need to be addressed to enhance the reliability and lifespan of the sensors. Thus, advanced encapsulation techniques should be further developed to minimize the impact of the external environment on ionic hydrogel self-powered sensors. The physical and chemical factors that affect the lifetime of the device should also be considered in future designs to improve stability and durability. To enhance the convenience and functionality of hydrogel devices, future applications of ionic hydrogel self-powered sensors should focus on integrating wireless information transmission technologies, enabling wireless capabilities and broader adoption. It is expected that the development and synthesis of new ionic hydrogel materials will enhance sensor functionality and performance, given the ongoing progress in materials science and nanotechnology. Additionally, the integration with other sensing technologies will bring more application opportunities and innovative breakthroughs for ionic hydrogel self-powered sensors. With continuous advancements in science and technology, ionic hydrogel self-powered sensors are expected to introduce greater convenience and innovation to human life and health.