Durable coaxial fiber‐based underwater strain sensor with reversible dry–wet transition

Abstract Underwater strain sensors are crucial for marine exploration, amphibious robotics, and aquatic dynamic monitoring. However, frequent dry–wet transitions in practical applications can lead to structural degradation and sensitivity loss, limiting their long‐term stability. Traditional designs relying on waterproof or hydrophobic layers isolate the core structure from water but suffer from interface delamination and performance decline during dry–wet cycles. Additionally, these layers increase weight, restricting lightweight and flexible applications. Herein, we developed a novel fiber‐based underwater strain sensor by coaxially spinning cuprammonium rayon (CR) and Ti 3 C 2 T x . A “water‐compatible” strategy was introduced to overcome the limitations of traditional “water‐repellent” approaches by leveraging molecular‐level material design. Ammonium ions in the cuprammonium spinning solution induce MXene gelation, forming a compact core–shell interface. CR's amorphous regions' hydroxyl and amino groups establish dynamic hydrogen bonds with water, enhancing interfacial bonding, mechanical strength, and wet sensitivity. During dry–wet cycles, the water network stabilizes the wet structure and facilitates rapid water release upon drying, restoring molecular interactions to maintain mechanical strength and conductivity. This sensor combines high strength, excellent wet sensitivity, and stable dry conductivity with exceptional adaptability to cycling. It offers a lightweight, high‐performance, multifunctional solution for underwater sensing in low‐latitude high‐humidity environments, ensuring broad applicability. image