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Flexible pressure sensors hold significant application value across multiple domains, including health monitoring, human motion detection, posture recognition, soft robotics, tactile sensing, and machine vibration sensing. These sensors convert mechanical pressure into electrical signals, enabling real-time monitoring of various physical quantities. Depending on their working principles, pressure sensors can be categorized into four types: piezoresistive, capacitive, piezoelectric, and triboelectric. Among these, piezoresistive pressure sensors are highly favored due to their simplicity, low cost, adaptability, and low power consumption. They respond to mechanical pressure by altering resistance or current and can be adapted to a wide range of materials and manufacturing processes. With the increasing demand for multifunctional sensors capable of detecting various mechanical stimuli (such as pressure and vibration), sensors that can meet these diverse needs have become a research hotspot. In this study, researchers have developed a high-performance pressure sensor based on PVDF/NF ink, manufactured using near-field electrospinning (NFES) technology, to fulfill these requirements.
Near-field electrospinning (NFES) is an advanced manufacturing technique that can precisely control the properties and morphology of materials under low voltage and short working distances, producing three-dimensional porous structures with complex geometries. This technology is particularly suitable for manufacturing high-performance flexible pressure sensors because it enables precise material deposition and structural control. Through NFES, one-dimensional and two-dimensional nanofillers (such as multi-walled carbon nanotubes, reduced graphene oxide, and silver nanowires) can be uniformly dispersed within a polyvinylidene fluoride (PVDF) matrix, forming porous structures with high porosity and evenly distributed nanofillers. This structure not only enhances the sensor's sensitivity and response speed but also maintains good flexibility and scalability. The use of an electrospinning machine in this process allows for the precise control needed to achieve these complex structures.
Material Selection and Ink Preparation: Suitable PVDF powders, nanofillers (such as multi-walled carbon nanotubes, reduced graphene oxide, and silver nanowires), and solvents (such as acetone and dimethylformamide) are selected. Nanofillers are uniformly dispersed within the PVDF matrix through ultrasonic dispersion and magnetic stirring to form PVDF/NF composite ink.
Near-Field Electrospinning Printing: The prepared PVDF/NF ink is loaded into a syringe and precisely deposited onto carbon-coated aluminum foil via NFES technology to form the active layer of the sensor. By adjusting printing parameters (such as the number of printing cycles), the thickness and porosity of the sensor can be precisely controlled. The electrospinning device used in this step ensures the uniform deposition of the ink, contributing to the sensor's high performance.
Sensor Assembly: The printed PVDF/NF active layer is sandwiched between top and bottom electrodes and encapsulated with PET film. Thermal pressing and custom connectors are used to ensure the stability and electrical connection of the sensor. (Fig 1)
1. Pressure Range and Sensitivity
The sensor exhibits a broad pressure range of 0–300 kPa and high sensitivity ranging from 0.014 to 10.67 kPa⁻¹. This wide pressure range allows it to cover various application scenarios from low-frequency human motions (such as finger bending and wrist pulse) to high-frequency machine vibrations (such as vibration monitoring of industrial equipment). Specifically, the sensor demonstrates extremely high sensitivity of 10.67 kPa⁻¹ in the low-pressure range (0–10 kPa), enabling it to detect very minute pressure changes, such as light finger touches or pulse beats. In the medium pressure range (10–100 kPa), the sensitivity is 0.43 kPa⁻¹, which can accurately monitor pressure changes caused by human motion. In the high-pressure range (100–300 kPa), the sensitivity is 0.014 kPa⁻¹, which, although relatively lower, is still capable of effectively detecting larger pressure changes and is suitable for pressure monitoring in industrial applications. The wide pressure range and high sensitivity characteristics give this sensor significant advantages in multiple applications. (See Fig 3(a) Pressure-dependent resistance changes of different PVDF/NF sensors)
2. Fast Response and Low Hysteresis
The sensor has a response time of 16 ms, meaning it can react to pressure changes in an extremely short period. Additionally, the sensor's low hysteresis of 9.62% indicates that its output signal has good repeatability and stability during pressure loading and unloading processes. This characteristic allows the sensor to maintain consistent performance during repeated use, making it suitable for applications that require high precision and reliability. (See Fig. 3(e) Sensor response time test)
3. Durability Test
The sensor demonstrates excellent durability in a pressure cycling test of 1500 cycles, with no significant performance degradation. This result shows that even after prolonged repeated use, the sensor can still maintain stable performance without affecting its measurement accuracy due to material fatigue or structural damage. Durability testing is an important indicator for assessing the practical application value of a sensor, especially in scenarios that require long-term monitoring, such as wearable health devices or long-term operation monitoring of industrial equipment. (See Fig. 3(f) Sensor durability test)
4. High-Frequency Pressure Detection Capability
The sensor is capable of detecting pressure frequencies as high as 500 Hz, making it suitable for dynamic pressure monitoring, such as machine vibrations. The ability to detect high-frequency pressure changes is crucial for monitoring rapidly varying pressure signals, for example, in vibration monitoring of industrial equipment, where it can accurately capture high-frequency vibration signals during equipment operation, thus enabling fault diagnosis and preventive maintenance. Moreover, this high-frequency detection capability also holds potential application value in human motion monitoring, such as monitoring rapid movements of athletes or dynamic responses of robots. Through testing at different frequencies, the sensor exhibits good frequency response characteristics, accurately detecting and recording pressure changes, providing reliable technical support for dynamic monitoring. (See Fig. 4(b) Relative resistance changes of the sensor at different frequencies)
In this study, a high-performance pressure sensor was developed using near-field electrospinning technology and PVDF/NF ink. NFES technology can produce precisely controllable two-dimensional and three-dimensional structures with complex geometries. Sensors manufactured through 2, 4, and 6 printing cycles demonstrate a broad pressure range of 0–300 kPa and sensitivity ranging from 0.014 to 10.67 kPa⁻¹. The sensor also features a fast response time of 16 ms and low hysteresis of 9.62%, enabling real-time pressure monitoring. In terms of durability, the sensor can withstand over 1500 pressure cycles without significant performance degradation. Additionally, the sensor's ability to detect pressure fluctuations up to 500 Hz makes it suitable for applications requiring both low-frequency detection (such as human motion monitoring) and high-frequency detection (such as machine vibrations). The electrospinning machine used in this study played a crucial role in achieving these results.
These results highlight the extensive potential of PVDF/NF pressure sensors in wearable health devices and industrial machinery. Future research could focus on optimizing the material composition of NFES-manufactured porous sensors to enhance key device parameters and develop efficient, multifunctional sensors suitable for a wide range of practical applications.
Article Source: D01: 10.1039/010c052531
