Performance Evaluation of a Sandwich-Structured CNT/Graphene–TPU Nanofiber Strain Sensor for Wearable Deformation Monitoring

Flexible strain sensors require both a responsive conductive network and a mechanically compliant supporting structure. In this study, a sandwich-structured carbon nanotube (CNT)/graphene–thermoplastic polyurethane (TPU) nanofiber strain sensor was fabricated by electrospinning, spray-coating, pre-stretching, and hot-pressing. Field-emission scanning electron microscopy was used to examine the nanofiber and coated-fiber morphology, while energy-dispersive X-ray spectroscopy was used only to describe elemental distribution. The electrical response was quantitatively evaluated under tensile deformation. The sensor exhibited piecewise gauge factors of approximately 47.3, 269.6, and 613.8 over strain ranges of 0–20%, 20–45%, and 45–60%, respectively. The response and recovery times were approximately 120 and 180 ms, and the electrical response remained observable over 5000 loading–unloading cycles at 20% strain. Qualitative demonstrations involving finger-bending, elbow-bending, pulse, grasping, walking, and repeated pressing produced distinguishable resistance-time patterns. These results indicate the potential of the CNT/graphene–TPU device for wearable deformation monitoring.