Super-flexible and thermochromic core-sheath fiber sensors for body temperature visualization and strain sensing

The rapid development of wearable electronics has driven demand for flexible, multifunctional fiber-based materials that can enable real-time physiological monitoring and adaptive responses. We report the design and fabrication of a core-sheath structured, dual-responsive fiber sensor featuring a conductive core and a thermochromic sheath via coaxial wet-spinning. The core comprises a conductive styrene-ethylene-butylene-styrene/carbon nanotubes-Ti 3 C 2 T x (SEBS/CNTs-Ti 3 C 2 T x ) composite optimized for strain sensing, while the sheath incorporates thermochromic microcapsules (TMs) within a SEBS matrix for temperature sensitivity. This integrated architecture enables simultaneous and independent detection of mechanical deformation and thermal variation, which demonstrates sharp and reversible color transitions within a 25–40 °C range, offering visual feedback to physiological temperatures. Furthermore, the conductive CNTs-Ti 3 C 2 T x hybrid network within the core facilitates sensitive strain-resistance behavior, with a maximum gauge factor of 214 at 100 % strain and reliable electromechanical performance over 3000 cycles. When applied to joints and vital regions (e.g., wrist, throat, knees), the fibers provide stable and high-fidelity signals during motion, pulse detection, and vocalization. Collectively, the integrated mechanical compliance, thermal responsiveness, and strain-sensing capacity position these fibers as promising candidates for next-generation wearable electronic systems, enabling smart diagnostics, physiological monitoring, and adaptive interfacing.