Weave-pattern-dependent fabric piezoelectric pressure sensors based on polyvinylidene fluoride nanofibers electrospun with 50 nozzles

Piezoelectric materials efficiently convert mechanical energy into electrical energy. While piezoelectric power generation devices produce energy only upon stimulation, they offer a unique energy source for low-power electronics. Material selection, device structure, and processing are critical for efficient sensing devices. While inorganic materials have high piezoelectricity, flexible organic materials are attractive for wearable applications. Poly(vinylidene fluoride) (PVDF) is a promising piezoelectric polymer due to its availability, processability, and strong piezoelectricity. PVDF nanofibers, often produced by electrospinning, are particularly suitable for flexible devices due to their high flexibility and good air permeability. Nanofiber structures have been extensively studied for piezoelectric energy harvesters. For wearable applications, fabric-type devices are ideal, offering adaptability and resilience. Previous studies have used PVDF fibers processed by melt-spinning, requiring extra poling and offering less dimensional control. Electrospinning to create twisted yarns for continuous processing is less common. Limited studies have achieved high sensitivity without post-treatment. This research proposes a fabric generator using PVDF nanofiber yarns produced by a sequential process involving 50-nozzle electrospinning, drawing, and twisting. Commercial PET yarns were used for mechanical integrity, creating 1/1, 2/2, and 3/3 weft rib patterns. The PVDF yarn was woven in the weft direction, and PET in the warp direction. The performance was optimized, with the 2/2 weft rib weave achieving a sensitivity of 83 mV N⁻¹. The fabric effectively detected signals under various physiological mechanical sources, and a pressure sensor array demonstrated potential for wearable applications.

Extensive research exists on piezoelectric materials and their applications in energy harvesting and sensing. Studies have explored various inorganic and organic materials, focusing on improving efficiency and flexibility. Piezoelectric polymers, especially PVDF and its derivatives, have gained significant attention for wearable applications. Electrospinning is a widely used technique for producing PVDF nanofibers with controllable diameters and alignment. The integration of PVDF nanofibers into fabric structures has been investigated for both energy harvesting and sensing applications. However, prior work often utilized melt-spinning, limiting dimensional control and requiring additional poling. Furthermore, achieving high sensitivity without post-treatment remains a challenge. This study builds upon existing literature by addressing the limitations of previous methods, focusing on a novel 50-nozzle electrospinning process and various weave patterns to enhance sensitivity and adaptability for wearable sensor applications.

A PVDF solution was prepared by dissolving PVDF powder in a mixed solvent of DMSO and acetone. Electrospinning was performed using a customized multinozzle electrospinner with 50 spinnerets. The nanofibers were continuously ejected onto a conveyor belt moving at 20 m/min under a 20 kV electric field. Parallel Cu rods were placed on the belt to align the fibers. The resulting nanofiber mat was drawn and twisted to form a single filament yarn, and four of these were twisted together to create a four-ply yarn. These yarns were woven with PET yarns into 1/1, 2/2, and 3/3 weft rib fabrics using a commercial weaving machine. The morphology of the yarns and fabrics was analyzed using SEM, fiber alignment was evaluated using FFT, and the crystalline phase of the PVDF yarn was determined using XRD and FTIR. Tensile strength and rupture strain of the yarn and fabrics were measured using a universal testing machine. Piezoelectric pressure sensors were fabricated by sandwiching a 2 cm × 2 cm fabric sample between ITO/PET and Ag-coated nylon fabric, encapsulated with a PI film. Output voltage and current were measured using a nanovoltmeter and galvanostat, respectively, while applying compressive force with a force sensor. All-fabric pressure sensors were assembled using Ag-coated nylon fabric as electrodes and covered with cotton fabric. These sensors were tested under bending, twisting, crumpling, and various human motions. A large-area pressure sensor array with 4 × 3 touch pixels was also fabricated and tested.

The 50-nozzle electrospinning method produced well-aligned PVDF nanofibers. The four-ply PVDF yarn exhibited optimal tensile strength at 300 TPM (twists per meter). The XRD and FTIR analyses confirmed the presence of the β-phase, crucial for piezoelectricity. The 2/2 weft rib fabric showed the highest sensitivity (83 mV N⁻¹) among the three weave patterns, significantly higher than the 1/1 pattern (24 mV N⁻¹). The sensitivity difference was attributed to the balance between the number of contact points and the magnitude of compressive strain in the different weave structures. The all-fabric sensors effectively detected various physiological signals, including finger bending, walking, and running, demonstrating a wide detection range (0.02 N to 694 N). The sensor showed good durability, retaining 81.3% of its original output after five washes. The large-area sensor array showed uniform and independent responses from each pixel, indicating the absence of crosstalk.

The results demonstrate the effectiveness of the 50-nozzle electrospinning and the 2/2 weft rib weave pattern in creating high-sensitivity piezoelectric pressure sensors. The significantly higher sensitivity of the 2/2 pattern compared to the 1/1 and 3/3 patterns can be explained by the optimal balance between the number of contact points and compressive strain. The ability to detect a wide range of forces, from subtle finger movements to the impact of running, highlights the versatility of this sensor for various wearable applications. The excellent durability and the absence of crosstalk in the sensor array further enhance its practicality for real-world use. This technology could significantly advance wearable health monitoring and human-computer interaction.

This study successfully demonstrated high-sensitivity, wearable, all-fabric pressure sensors using piezoelectric PVDF nanofiber yarns. The optimized 2/2 weft rib pattern provided superior sensitivity compared to other weave patterns. The sensors effectively detected various physiological motions and demonstrated good durability. Future work could explore different polymer combinations, advanced weave patterns, and miniaturized sensor designs for broader applications.

The current study focused on a specific set of weave patterns and PVDF concentrations. Further research could investigate a wider range of parameters to further optimize sensor performance. The long-term stability of the sensors under extended use and various environmental conditions also warrants further investigation. The scalability of the 50-nozzle electrospinning process for mass production needs to be addressed.