Abstract:

Objective Flexible sensors, as the core components of intelligent textiles, have become a new research hotspot. Multi-modal sensors have a broader market prospect by virtue of their ability to monitor various external signals. At present, research on multi-modal flexible capacitive sensors mainly focuses on three-dimensional and one-dimensional structures. Three-dimensional flexible sensors are difficult to miniaturize due to their relatively large thickness and area. Long time wearing of one-dimensional flexible capacitive sensors has been reported to cause a sense of compression or friction discomfort to the skin. Therefore, it is of great significance to develop a two-dimensional knitted flexible sensor that is comfortable to wear and well integrated with clothing.

Method Silver-coated polyamide fiber (SCP) was used as the core yarn, on which double-layer polyester (PET) was wrapped, with the inner layer being S-twist and the outer layer being Z-twist, and the twist factor was 1 000 twists/m. PET/SCP yarn was thus prepared. A layer of waterborne polyurethane (PU) was coated on the outer surface of the PET/SCP yarn to obtain PU-PET/SCP composite conductive yarn, which was used as raw material to prepare an all knitted multi-modal flexible capacitive sensor. The influence of coating process on the performance of core yarn was studied. The tensile, pressure and non-contact sensing properties of the sensor were analyzed, and the sensor was applied to human activity monitoring.

Results After PET wrapping and PU coating, the mechanical properties of the composite yarns were significantly improved, with the breaking strength and elongation at break increased by 216.9% and 9.33%, respectively. The composite conductive yarns were knitted into a flexible capacitive sensor with the plain knitted structure on a computerized flat knitting machine, and the sensor was designed to have multi-modal external force detection capabilities. In the tensile sensing tests in both the transverse and longitudinal directions, the capacitance of the sensor gradually increased with the increase of strain. The maximum sensitivity coefficient reached 0.324 3. Under the condition of stretching at a speed of 8.8 mm/s for 250 s at different elongation rates for a total of 1 000 s, the capacitance change of the sensor was relatively stable, demonstrating good repeatability. The sensor had good linearity, with R² values of the fitting equations being higher than 0.97 at different elongation rates, indicating that within a certain strain range, the ΔC/C₀ and elongation rate of the sensor showed a good linear correlation. In the pressure sensing test, by placing weights on the sensor, it was found that the sensor had good recognition ability for weights of different masses, and the capacitance change was stable for weights of the same mass, demonstrating good pressure sensing characteristics. In the non-contact sensing performance test, the sensor demonstrated multi-directional sensitivity. When an object approaches in either the vertical or horizontal direction, the sensor can display a relatively stable capacitance change signal. When an object approached the sensor without contact, the capacitance of the sensor gradually decreased with both the diminishing distance to the object and the increasing area of the object. This sensor demonstrates excellent non-contact sensing capabilities. It can identify the size (from a finger to a palm) and speed of approaching objects, and the corresponding relative capacitance changes are respectively significant. The speed detection was validated at specific vertical and horizontal palm movement frequencies.For human body monitoring, it precisely measures arm and knee bending (by 0°-90°) and detects respiratory rate by capturing the periodic capacitance variations caused by abdominal movements during breathing.

Conclusion A core-spun yarn structure was fabricated using SCP as the core yarn and PET as the sheath, subsequently over-coated with a PU layer to produce PU-PET/SCP composite conductive yarn. The composite conductive yarn exhibited excellent mechanical properties. The knitted capacitive sensors based on the PU-PET/SCP composite conductive yarn demonstrated the capability to monitor three types of external loading, i.e. stretching, pressure, and non-contact interactions. The two-dimensional knitted structure of the sensor features compact dimensions in both area and thickness, ensuring enhanced wearing comfort. Owing to these characteristics, this sensor shows significant application potential in fields such as human motion monitoring, healthcare, and robotics.

Key words: all knitted structure, composite conductive yarn, flexible sensor, capacitive sensor, sensing performance, human motion monitoring, smart texiles