A Pre‐Strain Strategy for Stretchable, Stable, and Washable Conductive Fibers With a Synergistic Soft‐Hard Core‐Sheath Structure

The development of wearable electronics demands stretchable conductive fibers with exceptional electromechanical stability and durability under cyclic deformation and washing. Although polyurethane (PU)/carbon nanotubes (CNT) composites show promise, their practical application is hindered by poor interfacial compatibility and conductive network instability. This work presents a synergistic strategy integrating rational material design and microstructural engineering: (1) A heterogeneous sheath‐core architecture is constructed using mechanically tailored PUs. It comprises a rigid isophthalic dihydrazide (IPDH)‐based PU sheath for mechanical robustness and 3,5‐diaminobenzoic acid (DABA)‐based PU core matrix. The benzamide groups in DABA promote enhanced π–π interactions with MWCNT, thereby facilitating their dispersion. (2) A pre‐straining treatment primarily reduces core‐layer defects, resulting in a denser conductive core structure. The obtained fibers are characterized by ultra‐high cyclic stability, enduring 10,000 stretching cycles at 50% strain with minimal signal drift. They are capable of sensing over a 0%–700% strain range with tunable gauge factors from 5.39 to 2041.71. Moreover, the fibers exhibit excellent washability, as their sensing performance remains stabilized after 30 washing cycles even with a slight decrease of conductivity. This study establishes a materials‐structure–property relationship paradigm for next‐generation durable stretchable conductors.