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DOI10.19817/j.cnki.issn.1006-3536.2025.03.009
In the past few decades, significant advancements have been made in rigid sensor technology, which has been widely applied in industrial production, medical diagnostics, environmental monitoring, aerospace, and other fields. However, rigid sensors have some limitations, such as high costs, difficulty in integration into flexible electronic devices, and inability to adapt to complex deformation environments, which severely restrict their applications. As a new type of sensor technology, flexible sensors have shown great potential in wearable devices, medical monitoring, electronic skin, and smart robotics due to their obvious stretchability, high water absorption, and biocompatibility.
Flexible pressure sensors are an important component of flexible sensors and can be mainly divided into three types based on their sensing mechanisms: piezoresistive, capacitive, and piezoelectric. With the continuous in-depth research on new materials (such as graphene, carbon nanotubes, molybdenum disulfide, and two-dimensional transition metal carbides Mxene) and micro-nano processing technologies, flexible pressure sensors have achieved new breakthroughs in detection limits and ranges. However, they still have limitations and struggle to meet diverse pressure measurement needs. In recent years, inspired by the sensing mechanisms of human skin, various ionic flexible pressure sensors based on ion migration have received widespread attention. Ionic flexible pressure sensors have advantages such as high sensitivity, fast response time, and wide testing range, making them promising for applications in health monitoring, artificial electronic skin, and wearable electronic devices. This article summarizes three typical working mechanisms of flexible pressure sensors, focusing on the current research status and application progress of ionic flexible pressure sensors. It discusses how to optimize the sensing performance of ionic flexible pressure sensors from the perspective of the ionic active materials used in the sensing layer and microstructure design, and points out the challenges faced by ionic flexible pressure sensors in the future.
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Working Mechanisms of Flexible Pressure Sensors
Under the action of external stress and strain, the resistance, capacitance, and surface potential of the material in flexible pressure sensors change. By analyzing these electrical signals, the deformation or motion state of the monitored object can be determined. Depending on the signal conversion mechanism, flexible pressure sensors can be mainly classified into piezoresistive, capacitive, and piezoelectric flexible pressure sensors.
1.1 Working Mechanism of Piezoresistive Flexible Pressure Sensors
The sensing mechanism of piezoresistive flexible pressure sensors is based on the piezoresistive effect. Under the stimulation of external stress or strain, the sensitive material deforms, causing changes in the size and shape of the resistance element, which in turn leads to changes in resistance value. Piezoresistive flexible pressure sensors have the advantages of simple structure and high sensitivity.
1.2 Working Mechanism of Capacitive Flexible Pressure Sensors
Capacitive flexible pressure sensors monitor external pressure based on changes in internal capacitance. These sensors are highly regarded for their mechanical flexibility, high sensitivity, strong repeatability, and low power consumption.
1.3 Working Mechanism of Piezoelectric Flexible Pressure Sensors
The working principle of piezoelectric flexible pressure sensors is mainly based on the piezoelectric effect. Under external stress, the dipoles within the material rearrange, leading to the occurrence of the piezoelectric effect. This polarization phenomenon generates charges on the crystal surface, with the charge magnitude proportional to the pressure applied to the sensor. Piezoelectric flexible pressure sensors exhibit excellent detection sensitivity and response speed, showing great potential in dynamic pressure sensing applications such as electronic skin, health monitoring, and precision instruments.
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Types of Sensing Layer Materials and Microstructure Design of Ionic Flexible Pressure Sensors2.1 Working Mechanism
Ionic flexible pressure sensors infer pressure changes based on the diffusion of ions within the ionic sensitive layer. When pressure is applied, the ionic sensitive layer deforms, causing changes in the diffusion path and rate of ions. This change affects the conductivity of ions within the sensor, allowing pressure magnitude to be inferred by measuring changes in conductivity. Ionic flexible pressure sensors have high sensitivity, fast response, and flexible deformability, making them widely used in medical, health monitoring, smart wearable devices, and human-computer interaction fields.
2.2 Types of Sensing Layer Materials
In 1992, Sadeghipour et al. prepared ionic flexible pressure sensors using perfluorosulfonic acid-based polymers (Nafion), introducing ionic sensing into the sensing domain. To develop ionic flexible pressure sensors with high sensitivity, good stability, and wide sensing range, researchers have made various attempts using different types of ionic active materials and designing diverse micro-device structures.
The pressure sensing layer of flexible pressure sensors is one of its most important components, typically composed of materials with good pressure-sensitive performance. These materials can sense and respond to external pressure based on changes in electrical or mechanical properties. Compared to traditional inorganic materials, ionic active materials with pressure-sensitive properties have mechanical properties that are closer to the soft skin of the human body. Commonly used sensing layer materials include polymer films, nanomaterials, ionic liquids, and ionic gels.
In recent years, ionic conductors have gradually replaced traditional dielectrics and are widely used in flexible sensors. Common ionic conductors, such as hydrogels and ionic gels, provide effective ionic transport channels and exhibit strong competitive advantages.
2.3 Microstructure Design
In addition to the sensing layer materials, the contact area between the electrode layer and the ionic polymer matrix is also one of the important factors affecting sensing performance. In practice, high sensitivity and wide pressure response range are typically achieved by designing and optimizing the microstructure of pressure sensors. The microstructure of pressure sensors can be mainly divided into surface structures and internal structures, both of which ensure that ionic flexible pressure sensors have excellent flexibility and compressibility.
Optimizing the surface structure refers to designing the surface microstructure of the ionic polymer matrix to create gaps between the ionic polymer matrix and the electrodes, increasing the contact area during compression, thereby improving the sensor’s sensitivity and response range. Compared to microstructures such as conical structures, columnar structures, and wrinkled structures, high aspect ratio surface microstructures have height differences. When the sensor is subjected to external mechanical forces, the electrode layer first contacts the highest layer of the microstructure. As pressure increases, the lower layers of the microstructure gradually come into contact with the electrode layer, providing more space for further compression. Therefore, ionic flexible pressure sensors with high aspect ratio surface microstructures can maintain ultra-high sensitivity over a wide pressure range.
Unlike surface structures, internal structures mainly refer to changing the internal structure of the ionic polymer matrix to achieve good compressibility.
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Applications of Ionic Flexible Pressure Sensors3.1 Used in Electronic Skin
The skin, as one of the largest organs of the human body, can sense external stimuli such as touch, pressure, temperature, and pain. Tactile perception allows us to perceive the shape, texture, and temperature of external objects, enabling fine manipulation and hand coordination. Inspired by human skin, electronic skin technology has rapidly developed to simulate the tactile perception functions of human skin.
3.2 Used in Health Monitoring
With the increasing attention to personal health, health monitoring has gained widespread attention. Health monitoring includes real-time monitoring of heart rate, respiratory rate, pulse, human motion behavior, blood pressure, etc. Due to the very small absolute pressure range of pulse rates and the small pressure amplitude, the requirements for the sensor’s sensitivity and low-pressure response limits are even stricter.
3.3 Used in Motion Detection
Ionic flexible pressure sensors are widely used in wearable flexible electronic devices due to their excellent flexibility and electrical performance.
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Conclusion
Ionic flexible pressure sensors, as a new type of pressure sensor, exhibit higher mechanical flexibility, sensitivity, and biocompatibility. In recent years, ionic flexible pressure sensors have been widely applied in electronic skin, medical health, and motion detection fields. With the continuous in-depth research on ionic flexible pressure sensors, their excellent sensing performance lays the foundation for further development of sensor devices with good biological characteristics. However, there are still some issues that need to be addressed: firstly, due to production cost limitations, most sensors cannot achieve mass production; secondly, many sensors have specific requirements for application scenarios, and variations in water content, environmental temperature, and humidity can cause instability in sensor output signals. Additionally, most sensors have a relatively single type of perception for external stimuli, lacking multimodal tactile sensing capabilities. These challenges will drive the rapid development of ionic flexible pressure sensors, providing direction for innovation and breakthroughs in this emerging field.
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Source: “New Chemical Materials”, Issue 3, 2025
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