Double-Layered Conductive Network Design of Flexible Strain Sensors for High Sensitivity and Wide Working Range

Abstract

For flexible strain sensors, the optimization between sensitivity and working range is a significant challenge due to the fact that high sensitivity and high working range are usually difficult to obtain at the same time. Herein, a breathable flexible strain sensor with a double-layered conductive network structure was designed and developed, which consists of a thermoplastic polyurethane (TPU)/carbon nanotube (CNT) layer (as a substrate layer) and a Ag nanowire (AgNW) layer. The TPU/CNT layer is made of electrospinning TPU with CNTs deposited onto the surface of TPU fibers, and the flexible TPU/CNT mat guarantees the integrity of the conductive path under a large strain. The AgNW layer was prepared by depositing different amounts of AgNWs on the surface of the TPU/CNT layer, and the high-conductivity AgNWs offer a low initial resistance. Benefitting from the synergistic two-layer structure, the as-obtained flexible strain sensor exhibits a very high sensitivity (up to 1477.7) and a very wide working range (up to 150%). Besides, the fabricated sensor exhibits fast response (88 ms), excellent dynamical stability (7000 cycles), and excellent breathability. The working mechanism of the strain sensor was further investigated using various techniques (microscopy, equivalent circuit, and thermal effects of current). Furthermore, the as-fabricated flexible strain sensors accurately detect the omnidirectional human motions, including subtle and large human motions. This work provides an efficient approach to achieve the optimization between high sensitivity and large working range of strain sensors, which may have great potential applications in health monitoring, body motion detection, and human–machine interactions.

Highly Stretchable, Highly Sensitive, and Antibacterial Electrospun Nanofiber Strain Sensors with Low Detection Limit and Stable CNT/MXene/CNT Sandwich Conductive Layers for Human Motion Detection

“…The fibers with diameters of 400–900 nm tend to be randomly aligned due to van der Waals forces during the spinning, forming a disordered and porous isotropic membrane structure with breathability, flexibility, and skin-friendliness. Subsequently, LBL assembly strategy is used in the preparation of conductive network, and the use of a layered structure in crack-based sensors can fully utilize the structural advantages of each layer itself . The sandwich structure of the CNT/MXene/CNT conductive network is coated on the TPU nanofiber by vacuum filtration.…”

“…In addition, electrical stability of the CNT/MXene/CNT/TPU strain sensor is further characterized by the I – V curve under different stretching (Figure S6) and the Δ R / R 0 -strain curve at different stretching rates (Figure S7), which enable the sensor to accurately and timely transmit the signals captured in human motion detection to back-end processing equipment. Additionally, the performances of the CNT/MXene/CNT/TPU strain sensor and recent crack-based strain sensors are compared in terms of sensing range and sensitivity (Figure h), ,,− and the detailed parameters are shown in Table S1. It can be found that crack-based strain sensors are generally unable to combine a wide sensing range and high sensitivity.…”

“…In contrast, the CNT/MXene/CNT/TPU strain sensor achieves both a wide sensing range and high sensitivity through the design of a conductive layer structure. Meanwhile, the performances of CNT/MXene/CNT/TPU strain sensors and recent strain sensors made by CNTs and MXene are compared in terms of sensing range, sensitivity, and minimum detection limit (Figure S8), ,,− and the detailed parameters are presented in Table S2. Maintaining excellent sensitivity and the lowest detection limit while having the widest sensing range proves that the sandwich structure increases the stability of the conductive layer while not reducing its sensitivity to strain response.…”

“…With the rapid rise in wearable electronic devices, flexible sensors are growing by leaps and bounds and are showing substantial applications in various fields, such as human motion monitoring, − artificial intelligence robotics, − and human–computer interaction. − Crack-structured strain sensors with conductive materials coated on a flexible substrate have become a widely used strategy ,, with advantages such as easy fabrication and short response time. However, it is still a challenge to combine high sensitivity and high tensile range due to the resistance change of crack-based sensors based on the breakage and reversion of the conductive network. , High sensitivity requires the sensor to undergo violent fracture or recovery of the conductive network at a weak stretch, which makes it impossible to maintain the conductive network at large strains . Conversely, the large sensing range requires the sensor to maintain the conductive network over a large range, with the consequent inability of the sensor to effectively identify small changes in the conductive network resulted from the small range of strain .…”

“…However, it is still a challenge to combine high sensitivity and high tensile range due to the resistance change of crack-based sensors based on the breakage and reversion of the conductive network. 14,15 High sensitivity requires the sensor to undergo violent fracture or recovery of the conductive network at a weak stretch, which makes it impossible to maintain the conductive network at large strains. 16 Conversely, the large sensing range requires the sensor to maintain the conductive network over a large range, with the consequent inability of the sensor to effectively identify small changes in the conductive network resulted from the small range of strain.…”