Electrospinning Machine| Stretchable one-dimensional flexible capacitive sensor with high stability and large strain for human vital signal monitoring prepared by water-bath electrospinning technology

With the increasing global aging trend and rising public demand for chronic disease early warning and real-time health monitoring, the importance of flexible wearable sensors in the healthcare field has become increasingly prominent. Compared to rigid electronic devices, textile-based flexible capacitive sensors offer excellent conformability, breathability, and integrability, enabling long-term, non-invasive continuous monitoring. However, most current capacitive sensors adopt three-dimensional sandwich structures or two-dimensional fabric architectures, whose bulky size and structural rigidity limit their adaptability to complex body parts. Additionally, pressure-sensing principles struggle to accurately capture the variable tensile signal characteristics of the human body. Therefore, there is an urgent need to develop a one-dimensional flexible capacitive sensor with a simple structure, high sensitivity, and broad strain adaptability to advance its practical application in precise health monitoring.

Recently, Professor Hong Jianhan's team at Shaoxing University published a research paper titled "Stretchable one-dimensional flexible capacitive sensor with high stability and large strain for human vital signal monitoring prepared by water-bath electrospinning technology" in Materials Today Physics. This work successfully constructed a flexible wearable capacitive sensor (SOFCS) with an integrated electrode-dielectric layer structure through a nanofiber-coated yarn (NFCY) design, combining water-bath electrospinning technology and a double-helix assembly strategy, demonstrating outstanding performance in biomechanical and physiological signal monitoring. Key achievements include:

(i) Using self-developed water-bath electrospinning technology to deposit polyacrylonitrile (PAN) nanofibers in situ on silver-coated nylon yarn (SPN), forming a dense, uniform, and porous dielectric coating to produce NFCY, providing a stable microstructure for the flexible capacitive sensor.

(ii) Constructing an SOFCS with a double-helix electrode structure using NFCY, which maintains stable capacitance response under 3%–70% strain and 6–44 mm/s stretching rates, exhibiting high strain sensitivity (GF up to 2.04) and strong robustness to deformation rates. It also demonstrated high stability and repeatability over 5000 stretching cycles, making it suitable for continuous monitoring in complex dynamic environments.

(iii) The developed SOFCS achieved high-resolution joint motion monitoring, accurately distinguishing intermittent/continuous movements and varying intensities; delivered stable, real-time signals for neck posture and respiration tracking; and recognized speech patterns with high fidelity, showing potential in voice interaction and rehabilitation applications.

Moreover, the sensor features high structural integration, scalable fabrication, and strong mechanical compliance, providing robust support for the practical development of flexible wearable electronics in health monitoring, speech recognition, and smart textiles.

Figure 1: Schematic of water-bath electrospinning setup and SOFCS fabrication.

Figure 2: NFCY morphology and mechanical properties.

By designing a one-dimensional flexible structure with a nanofiber dielectric layer, the research team developed a capacitive sensor combining high stability and a wide strain range. As shown in Figure 1, SPN served as the conductive core, while PAN nanofibers formed the dielectric shell via an improved multi-needle water-bath electrospinning coating process. The self-rotating yarn ensured uniform nanofiber deposition, creating a dense, porous dielectric structure, laying a stable foundation for the electrode-dielectric integrated one-dimensional sensor. Red and white NFCYs were helically wound around a TPU elastic yarn to form the SOFCS, ensuring mechanical stability and consistent capacitance response.

SEM images (Figure 2) revealed a ~20 μm nanofiber coating with fiber diameters ranging from 200–280 nm, exhibiting a uniform porous structure that enhances dielectric performance and strain sensitivity. Mechanical tests showed that NFCY had higher tensile strength and ductility than uncoated SPN, attributed to fiber entanglement and coating reinforcement, significantly improving structural stability under large strain conditions.

Figure 3: Sensor principles, testing setup, and parameter effects.

To investigate structural factors affecting sensing performance, the team systematically adjusted electrode structure, carrier diameter, and winding density. Results (Figure 3) showed that the NFCY+NFCY double-coated electrode structure produced smoother, interference-free signal output curves compared to traditional SPN+NFCY, primarily due to the isolation effect of the coating layer, reducing parasitic capacitance interference. Larger carrier diameters increased capacitance change rates per unit strain, while an 18 loops/cm winding density optimized strain response and sensitivity balance.

Figure 4: SOFCS performance vs. other 1D flexible capacitive sensors.

The SOFCS sensor fabricated under optimal parameters demonstrates outstanding electrical performance. As shown in Figure 4, the sensor maintains stable and reproducible capacitive response curves across a broad strain range of 3%-70%. After 5,000 stretch-release cycles at 30% strain, the sensor exhibits no significant signal attenuation while retaining structural integrity, indicating excellent fatigue resistance. Furthermore, it maintains good linear response at 1% strain with a minimum response time of 50 ms and recovery time of approximately 80 ms. During deformation rate variations (6-44 mm/s), the sensor's response amplitude remains stable, confirming that its output signal is primarily strain-amplitude-dependent rather than being influenced by dynamic loading rates.

Figure 5: Applications in motion and speech recognition.

As illustrated in Figure 5, for practical applications, the sensor was integrated into wearable textiles for dynamic signal monitoring of various body parts including knees, neck, chest, and larynx. In knee tests, whether secured via strapping or sewn into socks, the sensor accurately captured movement frequency and intensity variations during intermittent flexion, sustained bending, and running, delivering clear and reproducible response signals. During neck monitoring, the sensor successfully distinguished different flexion angles in real-time while maintaining stable waveform output, demonstrating good adaptability. In respiratory monitoring tests on the chest region, the sensor clearly reflected periodic capacitance changes corresponding to inhalation and exhalation phases, verifying its high-resolution capability in detecting subtle volume changes. For vocalization tests, the SOFCS effectively differentiated laryngeal vibration patterns associated with distinct speech elements (e.g., letters "ABCDE" or the word "Banana"), with waveform amplitude variations corresponding to sound intensity changes, indicating its potential application value in speech recognition and language rehabilitation scenarios.

In summary, the SOFCS flexible capacitive sensor developed in this study integrates high sensitivity, wide strain response range, excellent dynamic responsiveness, and structural stability. Its core advantages lie in the highly integrated structure, simplified fabrication process, and capacity for low-cost mass production, while demonstrating consistent and reliable performance across various practical wearable applications. This research provides a feasible material and structural design solution with significant potential for engineering scalability in the development of flexible wearable health monitoring devices.

Paper link:https://doi.org/10.1016/j.mtphys.2025.101778