Pressure Sensors Based on Densely Structured Graphene Fibers for Motion Monitoring

Nanoscience Electrical Engineering Chemistry Materials Science Engineering Life Sciences Bioengineering
graphene fibers wet spinning dense structure pressure sensor motion monitoring

Academic Background

With the rapid development of smart wearable devices, pressure sensors, as core components, have garnered extensive attention in fields such as health monitoring, human-machine interaction, and artificial intelligence. Based on their sensing principles, pressure sensors are primarily categorized into capacitive, piezoelectric, triboelectric, and piezoresistive types. Among these, piezoresistive pressure sensors have become a research hotspot due to their simple structure, high sensitivity, and low manufacturing cost. However, achieving high sensitivity while expanding the detection range remains a significant challenge for piezoresistive sensors in practical applications.

Graphene has demonstrated outstanding performance in the sensor field due to its excellent electrical conductivity, high specific surface area, and superior mechanical strength. However, the mechanical and electrical properties of graphene often fall short of ideal levels in practical applications, affecting its durability and performance consistency. Graphene fibers, as macroscopic assemblies of graphene, inherit the excellent properties of graphene and possess good weavability and wearability due to their fibrous form. Nevertheless, balancing stress, strain, and electrical properties during the preparation of graphene fibers remains a challenge. By optimizing spinning processes and post-treatment techniques, the performance of graphene fibers can be enhanced, thereby improving the performance of fiber-based pressure sensors.

Source of the Paper

This paper was co-authored by Yifan Zhi, Honghua Zhang, Lugang Zhang, Qianqian Li, Xiangtian Kuang, Wen Wu, Qingqing Zhou, Ping Li, Wei Li, and Huanxia Zhang, from Donghua University and Jiaxing University. The paper was published on November 19, 2024, in the journal Advanced Fiber Materials, with the DOI 10.1007/s42765-024-00502-9.

Research Process

1. Material Preparation

In this study, graphene oxide (GO) was first prepared using the modified Hummers method, and amino-modified Fe₃O₄ nanoparticles were synthesized based on the authors’ previous work. Other reagents such as acetic acid, N-hydroxysuccinimide (EDC), and 1-ethyl-(3-dimethylaminopropyl) carbodiimide hydrochloride (NHS) were purchased from commercial sources.

2. Preparation of Spinning Doping Solution

Different ratios of large and small GO sheets were mixed with Fe₃O₄ nanoparticles and ultrasonically dispersed in an ice-water bath for 2 hours to prepare a 25 g/L spinning doping solution. By adjusting the ratio of large to small GO sheets (10:0, 9:1, 8:2, 7:3, and 6:4) and the ratio of GO to Fe₃O₄ nanoparticles (10:0, 9.5:0.5, 9:1, 8.5:1.5, and 8:2), the performance of the spinning doping solution was optimized.

3. Preparation of Graphene Fibers

The spinning doping solution was extruded at a flow rate of 25 mL/h through a flat spinneret into an acetic acid coagulation bath, with a magnetic field applied in the coagulation bath. Fibers were collected at different draw ratios (1.0:1.3) and vacuum-dried at 85°C for 12 hours to obtain magnetic graphene oxide fibers (MGOFs). Subsequently, the fibers were reduced using HI vapor for 4 hours, and after repeated washing and drying, magnetic graphene fibers (MGFs) and graphene fibers (GFs) were obtained.

4. Preparation of Piezoresistive Sensors

The prepared MGFs were woven into a plain weave fabric consisting of six fibers, and five layers of fabric were stacked and connected using conductive tape. Finally, the MGFs fabric was combined with a PET flexible interdigitated electrode and encapsulated with a PDMS film.

5. Material Characterization

The morphology and structure of the materials were characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman spectroscopy, and small-angle X-ray scattering (SAXS). Tensile and repeated compression tests were conducted using a universal testing machine and a source meter.

Research Results

1. Morphological and Structural Analysis of MGFs

SEM images showed that the drawn GFs were highly oriented along the long-axis direction, with interlayer spacing ranging from 70 to 497 nm and an average value of 183 nm. The addition of small GO sheets intensified the grooving phenomenon on the surface of GFs, enhancing interlayer forces. The incorporation of Fe₃O₄ nanoparticles caused the particles to enter the spaces between graphene sheets, reducing the density of the sheets, with an average interlayer spacing of 80.2 nm. AFM analysis revealed that the height difference on the fiber surface decreased from 60 nm to 45 nm after drawing, indicating that drawing effectively improved the flatness of the fiber surface. XPS and XRD analyses confirmed the reduction effect of HI and high temperature on MGFs, while Raman spectroscopy showed a reduction in defect structures after reduction. SAXS analysis indicated that drawing and Fe₃O₄ nanoparticle doping had varying impacts on the ordered arrangement and densification of graphene fibers.

2. Sensing Performance of the MGFs Pressure Sensor

The MGFs pressure sensor exhibited high sensitivity (0.233 kPa⁻¹) in the low-pressure range (0–40 kPa), with sensitivity decreasing to 0.048 kPa⁻¹ in the high-pressure range (>40 kPa). The sensor demonstrated good repeatability and stability in repeated compression experiments at different pressures (10, 20, 30, and 40 kPa). The response and recovery times were 121 ms and 158 ms, respectively, and the sensor maintained a stable current signal after 1300 compression cycles, showing excellent durability.

3. Applications of the MGFs Pressure Sensor

The MGFs pressure sensor was successfully applied to monitor human physiological activities, including finger pressing, elbow bending, finger bending, pronunciation, breathing, and pulse detection. The sensor accurately identified signals under different pressures, demonstrating its broad application potential in wearable devices.

Conclusion

This study proposed a strategy for preparing MGFs by channel-confining GO and adding Fe₃O₄ nanoparticles, successfully constructing a fiber-based wearable pressure sensor. The MGFs exhibited excellent electrical and mechanical properties, and the sensor demonstrated a wide detection range, high sensitivity, fast response/recovery time, and outstanding durability. The sensor has significant application value in physiological monitoring, human-machine interaction, and smart wearable devices.

Research Highlights

  1. Highly Dense Graphene Fibers: By optimizing spinning processes and post-treatment techniques, MGFs with high tensile strength (58.6 MPa), strain (5.3%), and electrical conductivity (1.7 × 10⁴ S/m) were prepared.
  2. Multilayer Fabric Sensing Layer: The sensor adopted a multilayer fabric structure, significantly improving the detection range and sensitivity.
  3. Fast Response and Durability: The sensor had a response time of 121 ms and a recovery time of 158 ms, maintaining stable performance after 1300 compression cycles.
  4. Broad Application Potential: The sensor was successfully applied to monitor human physiological activities, demonstrating its wide application prospects in wearable devices.

Other Valuable Information

This research was supported by multiple projects, including the Zhejiang Natural Science Foundation, the National Key Research and Development Program of China, and the Jiaxing Science and Technology Plan Project. Data are available from the corresponding author upon request.