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A Review of Flexible Force Sensor Technology: Development History, Working Principles, Innovative Techniques, and Applications
2025-11-15
With advancements in smart materials, bionics, and robotics, flexible force sensors capable of detecting normal forces and multi-axis forces are gaining increasing attention for applications such as health monitoring, prosthetics, and robotic manipulation. These force sensors employ diverse sensing mechanisms, including resistive, magnetic, and optical methods, while simultaneously needing to strike a balance between high spatiotemporal resolution, stretchability, and robustness against electromagnetic and thermal interference.
Flexible force sensors can detect and measure external forces—such as pressure, shear force, and tensile force—under various deformation conditions, including bending, stretching, and torsion. Compared to traditional rigid force sensors, flexible force sensors offer advantages like being lightweight, thin, stretchable, and highly adaptable. These unique features enable them to seamlessly conform to complex curved surfaces, making them indispensable in applications such as wearable devices, robotic tactile sensing, medical rehabilitation, electronic skin, and intelligent interaction systems.
The development of force sensors can be traced back to the early 20th century, marking a crucial milestone in the advancement of measurement technology. With continuous progress in mechanical, electronic, and materials technologies, force sensors have gradually evolved from purely mechanical and electrical designs into miniaturized, flexible, and intelligent systems. As shown in Figure 1, the figure illustrates the historical progression of force sensor development.
Figure 1: The Evolution of Force Sensors—(a) Early 20th Century, (b) Mid-20th Century, (c) Late 20th Century, (d) 21st Century, (e) Present Day
Currently, force sensors are evolving toward intelligent and multi-functional integration, leveraging the convergence of artificial intelligence (AI), wireless communication, and cloud computing technologies to enable smart data processing and remote monitoring. Moreover, force sensors are increasingly being combined with the detection of multiple physical parameters—such as temperature, humidity, and strain—thereby further expanding their potential applications in areas like industrial automation, aerospace, medical devices, and bionic electronics.
Flexible Force Sensor Working Principle and Sensing Technology
The working principles of flexible force sensors are diverse, including resistive, capacitive, magnetic, and optical types, as well as Triboelectric Sensing mechanisms. Among these, flexible force sensors based on resistance and capacitance principles have attracted more research attention due to their relatively simple structural design and compatibility with readout electronic devices. In contrast, sensors utilizing magnetic and fiber-optic principles place higher demands on the associated hardware, often involving more complex structural designs and manufacturing processes.
Resistive Flexible Force Sensor
A resistive flexible force sensor is a type of sensor that detects external forces by monitoring changes in electrical resistance. It typically consists of a combination of flexible materials and resistance-sensitive elements. This sensor can measure deformations or applied forces on flexible structures, generating corresponding sensing signals based on the variation in resistance values.
The structure of resistive flexible force sensors typically consists of a conductive sensing layer, comb-shaped electrodes, and non-conductive encapsulation materials (such as PDMS) on the upper and lower layers. Generally, these sensors are limited to detecting unidirectional normal forces. In this design, the configuration of the conductive sensing layer is particularly critical, as it directly determines the sensor's overall sensitivity, stability, and mechanical adaptability. Common design approaches for the conductive sensing layer include curved and wrinkled structures, interlocking microstructures (as shown in Figure 2c), biomimetic microstructures, and foam-like architectures. Microcolumn array structure (2b) and pyramid array structures (Figure 2a), among others.
Capacitive Flexible Force Sensor
Capacitive flexible force sensors are a type of sensor that detects force signals by measuring changes in capacitance. These sensors typically feature a parallel-plate structure, using the capacitance between the upper and lower plates to determine the magnitude of the force signal. By embedding conductive materials into a flexible substrate, various structural designs can be created, enabling the sensors to detect both normal forces and shear forces exerted from external sources.
Capacitive flexible force sensors have a relatively simple structure, typically involving the integration of microcapacitors into a flexible substrate material. These microcapacitors consist of an electrode layer and a dielectric layer. The dielectric layer is usually positioned between the upper and lower electrodes—or encapsulates the electrode materials—and plays a critical role in capacitive flexible force sensors. The choice of its material and the design of its microstructure directly determine key performance parameters of the sensor, such as its measurement range and sensitivity.
Flexible Magnetic Force Sensor
The working principle of flexible magnetic force sensors relies on various magnetoresistive effects, which convert changes in the magnetic field into electrical signals and then transform these signals—via signal processing—into force readings. The primary operational mechanisms of flexible magnetic force sensors include the Hall effect (Figure 4a) and the giant magnetoresistance effect (Figure 4b). Tunneling Magnetoresistance Effect (Figure 4c) and the anisotropic magnetoresistance effect (Figure 4d). Typically, their structure consists of a soft substrate material combined with magnetic materials, enabling the detection and reflection of the flexible structure's state by monitoring changes in the magnetic field.
Flexible Optical Force Sensor
Flexible optical force sensors are designed to measure external forces—including pressure, shear force, and tensile force—by detecting changes in optical signals. Their fundamental working principle primarily involves optical phenomena such as intensity modulation, interference effects, wavelength shifts, scattering variations, or resonant responses. Based on these optical measurement principles, the sensors can be categorized into types like fiber Bragg grating-based, intensity-modulation-based, and interference-based designs. Typically, the overall architecture of most modern flexible optical force sensors includes components such as optical fibers, a light source and detector, and a flexible substrate.
Applications of Flexible Force Sensors
Flexible force sensors have been widely applied in various fields, including wearable health monitoring, robotic manipulation, intelligent prosthetics, human-machine interaction, industrial automation, and biomedical implants. Among these applications, wearable monitoring and robotic manipulation stand out as particularly representative, showcasing a rapidly evolving trend. Therefore, this review focuses on these two key application scenarios, offering in-depth analysis and highlighting notable case studies to underscore their significance and emerging development trends.
Figure 6: Application of Flexible Force Sensors in Health Monitoring
Figure 7: Application of Flexible Force Sensors in Robotic Manipulation
Summary
In summary, flexible force sensors demonstrate tremendous potential in a variety of fields, including wearable electronics, human-machine interaction, robotics, and biomedical monitoring. This review categorizes these sensors based on their sensing mechanisms—such as resistive, capacitive, magnetic, optical, triboelectric, and hybrid types—and elaborates on their working principles, design advancements, and key applications. Although significant improvements have been made in terms of sensitivity, response speed, flexibility, and integration capabilities, challenges still remain in areas like signal stability, scalable manufacturing, and adaptability to specific applications. Moreover, the trade-off between performance and manufacturing complexity continues to hinder their practical implementation. Future research should focus on developing sensors that are multifunctional, self-powered, biocompatible, and capable of long-term reliability. Integrating flexible sensors with cutting-edge technologies such as artificial intelligence, edge computing, and wireless communication will be crucial for advancing the next generation of intelligent sensing systems. This review provides a clear technical overview and serves as a valuable reference for future research and development efforts.
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