Highly Stretchable and Fluorescent Visualizable Thermoplastic Polyurethane/Tetraphenylethylene Plied Yarn Strain Sensor with Heterogeneous and Cracked Structure for Human Health Monitoring

Smart wearable technology has been more and more widely
used in
monitoring and prewarning of human health and safety, while flexible
yarn-based strain sensors have attracted extensive research interest
due to their ability to withstand greater external strain and their
significant application potential in real-time monitoring of human
motion and health signals. Although several strain sensors based on
yarn structures have been reported, it remains challenging to strike
a balance between high sensitivity and wide strain ranges. At the
same time, visual signal sensing is expected to be used in strain
sensors thanks to its intuitiveness. In this work, thermoplastic polyurethane
(TPU) and tetraphenylethylene (TPE) were wet-spun to fabricate flexible
fluorescent fibers used as the substrate of the sensor, followed by
the drop addition of polydimethylsiloxane (PDMS) beads and curing
to produce a heterogeneous structure, which were further twisted into
a plied yarn. Finally, a visualizable flexible yarn strain sensor
based on solidified liquid beads and crack structure was obtained
by loading polydopamine (PDA) and polypyrrole (PPy) in situ. The sensor
exhibited high sensitivity (the GF value was 58.9 at the strain range
of 143–184%), a wide working strain range (0–184%),
a low monitoring limit (<0.1%), a fast response (58.82 ms), reliable
responses at different frequencies, and excellent cycle durability
(over 2000 cycles). At the same time, the yarn strain sensor also
had excellent photothermal characteristics and a fluorescence crack
visualization effect. These attractive advantages enabled yarn strain
sensors to accurately monitor various human activities, showing great
application potential in health monitoring, personalized medical diagnosis,
and other aspects.