Ultra-High Filling Ratio of Non-Percolative Rapeseed-Shaped Liquid Metal Fiber Mats for Pressure Sensors via Electrospinning Aided Inhomogeneous Dispersion
Background Introduction
Flexible capacitive pressure sensors have broad application prospects in intelligent robotics, medical monitoring, and human-machine interaction due to their high sensitivity, fast response, and excellent mechanical flexibility. However, traditional dielectric elastomers typically have low dielectric constants, limiting the range of capacitance signal variation. To enhance the performance of capacitive sensors, researchers often incorporate high-dielectric inorganic ceramics or conductive materials into elastomers. However, these fillers are usually rigid, which can lead to hardening of the elastomer, reduced flexibility, and percolation under high pressure, causing the material to transition from dielectric to conductive and lose its capacitive sensing function.
Liquid metal (LM), with its inherent fluidity and high dielectric constant, is considered an ideal material to address this issue. However, achieving high LM filling ratios while avoiding percolation remains a significant challenge. To tackle this, researchers have proposed a strategy of inhomogeneous distribution, where LM is locally concentrated to form isolated clusters, effectively increasing the percolation threshold.
Source of the Paper
This paper was jointly authored by Yanlin Chen, Tangfeng Feng, Mengyue Peng, and Faxiang Qin from Zhejiang University, published in the journal Advanced Fiber Materials in 2025. The research was supported by the National Natural Science Foundation of China (NSFC) and the National Key Research and Development Program.
Research Process and Results
1. Research Design
This study utilized electrospinning technology to fabricate a liquid metal/thermoplastic polyurethane (LM/TPU) fiber mat with a unique rapeseed-shaped structure. The LM was locally concentrated within the fibers to form isolated clusters, effectively increasing the percolation threshold. The primary goal was to achieve an LM filling ratio of up to 90% while maintaining the material’s dielectric properties and flexibility.
2. Materials and Preparation
The research first dissolved LM (EGaIn, a gallium-indium alloy) and TPU in an acetone/dimethylformamide (DMF) mixed solvent, dispersing the LM into nanoscale droplets via ultrasonic treatment. The mixed solution was then processed into fiber mats using electrospinning. During electrospinning, the LM formed rapeseed-shaped clusters within the TPU fibers, while the connecting parts between the fibers contained minimal LM. This unique structure not only increased the percolation threshold but also preserved the flexibility and mechanical properties of the fiber mats.
3. Performance Characterization
The morphology and structure of the fiber mats were characterized using scanning electron microscopy (SEM), X-ray tomography (Micro-CT), and energy-dispersive spectroscopy (EDS). The results showed that as the LM content increased, the rapeseed-shaped structure became more pronounced, and the size of the LM clusters gradually grew. Additionally, the dielectric and conductive properties of the fiber mats were tested using an impedance analyzer and a four-probe resistivity tester. The results indicated that even at a 90% LM filling ratio, the fiber mats maintained dielectric properties without percolation.
4. Mechanical and Sensing Performance
The mechanical properties of the fiber mats were further tested. The results demonstrated that the addition of LM significantly improved the stress-strain performance of the fiber mats while maintaining a low elastic modulus. The stress of the fiber mats at 70% compressive strain was 142.8 kPa, showcasing excellent flexibility and fatigue resistance. The flexible capacitive sensor fabricated from these mats exhibited a high relative capacitance change (maximum δC/C0 = 6.32) and pressure sensitivity (55 MPa⁻¹), with stable performance across a pressure range of 0–550 kPa. Cyclic stability tests revealed minimal capacitance signal attenuation after 6000 loading-unloading cycles, demonstrating outstanding durability.
5. Application Demonstrations
The study also showcased the sensor’s potential in intelligent sorting with robotic grippers, pressure distribution monitoring, and remote keystroke monitoring. By attaching the sensors to robotic grippers, the researchers successfully identified and sorted objects of different shapes. Furthermore, a 4×4 sensor array was developed for real-time pressure distribution monitoring, with data transmitted to a mobile app via a Bluetooth module, enabling remote keystroke monitoring.
Conclusions and Significance
This study successfully fabricated a high LM filling ratio fiber mat with a rapeseed-shaped structure using electrospinning technology, effectively addressing the percolation issue in LM-filled dielectric elastomers. The fiber mats not only exhibit high dielectric constants and excellent mechanical properties but also demonstrate temperature insensitivity, waterproofing, and recyclability. The flexible capacitive sensors based on these mats hold great promise for applications in intelligent sorting, pressure monitoring, and human-machine interaction.
Research Highlights
- High LM Filling Ratio: Achieved an unprecedented 90% LM filling ratio, surpassing the theoretical percolation threshold prediction of 83%.
- Unique Rapeseed-Shaped Structure: Realized inhomogeneous LM distribution through electrospinning, effectively suppressing percolation.
- Exceptional Sensing Performance: The sensor exhibits high sensitivity, a wide pressure range, and long-term cyclic stability, making it suitable for various complex environments.
- Multifunctional Applications: Successfully demonstrated the sensor’s potential in intelligent robotics, pressure monitoring, and remote keystroke monitoring.
Additional Valuable Information
The study also explored recycling methods for the fiber mats, achieving successful reuse through dissolution and re-spinning processes. Additionally, finite element simulation (FEM) validated the superiority of the rapeseed-shaped structure in electric field distribution, further supporting its role in inhibiting percolation.
Through this innovative design, the performance of LM-filled dielectric elastomers has been significantly enhanced, providing new insights and methods for the development of flexible capacitive sensors. Future research can further optimize the structure and performance of the fiber mats, expanding their applications in more fields.