Significant Stretchability Enhancement of a Crack-Based Strain Sensor Combined with High Sensitivity and Superior Durability for Motion Monitoring

Abstract

Flexible strain sensors have attracted tremendous interest due to their potential application as intelligent wearable sensing devices. Among them, crack-based flexible strain sensors have been studied extensively owing to their ultrahigh sensitivity. Nevertheless, the detection range of a crack-based sensor is quite narrow, limiting its application. In this work, a stretchable strain sensor based on a designed crack structure was fabricated by spray-coating carbon nanotube (CNT) ink onto an electrospun thermoplastic polyurethane (TPU) fibrous mat and prestretching treatment to overcome the trade-off relationship. Our sensor exhibited combined features of high sensitivity in a greatly widened workable sensing range [a gauge factor of 428.5 within 100% strain, 9268.8 for a strain of 100–220%, and larger than 83982.8 for a strain of 220–300%], a fast response time (about 70 ms), superior durability (>10 000 stretching–releasing cycles), and excellent response toward bending. The microstructural evolution of CNT branches extending from two edges of the cracks and the excellent stretchability of TPU fibrous mats are mainly related to the remarkable sensing properties. Our sensor is then assembled to detect various human motions and physical vibrational signals, demonstrating its potential applications in intelligent devices, electronic skins, and wearable healthcare monitors.

Wearable and Stretchable Strain Sensors: Materials, Sensing Mechanisms, and Applications

“…Although cracks are undesirable for structural designs, the generation of microcracks in conductive thin films has been successfully utilized to develop highly sensitive strain sensors. The opening and enlargement of microcracks have been observed in CNT-based strain sensors, [109,138,173,174] graphene-based strain sensors and its derivatives, [6,57,117] metal NW-and NPs [86,116,[175][176][177][178][179][180] based strain sensors. The rapid separation of nanomaterials at the microcrack edges dramatically limits the electrical conduction paths within the thin films, leading to a significant increase in the electrical resistance of strain sensors under the applied tensile strain.…”

“…Reproduced with permission. [138] Copyright 2019, American Chemical Society. d) Hysteresis loops of a Ti 3 C 2 T x -AgNW-poly(dopamine)/Ni 2þ strain sensor subjected to 5 000 stretching and releasing cycles.…”

Strategies for Designing Stretchable Strain Sensors and Conductors

“…This is the main reason causing the narrow workable strain range. For sensors based on a cracking mechanism, which will be discussed in the next section, the stretchability usually decreases by means that increase the sensitivity, e.g., through introducing large and long cracks …”

“…Apart from these piezoresistive mechanisms, a new cracking mechanism inspired by spider's sensory system basing on slit organ ( Figure a–c) . has recently been identified and used to develop highly sensitive sensors.…”

“…For many nanocomposite‐based sensors, linearity has been found low because the electrical signal changes are attributed to several mechanisms. Some sensors exhibit more than one linear region . This phenomenon makes it challenging to calibrate the sensors and also limits their applications in practice.…”