Abstract:

Objective To address the high energy consumption and environmental burden of traditional actuators, this study aims to develop sustainable, humidity-driven smart fibers based on viscose, a biodegradable material with excellent hygroscopicity. Two types of humidity-responsive fiber bundles were designed, which are a “rotatable” type that performs torsional actuation through moisture-induced untwisting, and a “contractile” type that exhibits axial contraction under humidity stimuli.

Method Viscose fibers were thermally stretched at 60 ℃ first to improve molecular orientation and mechanical strength. The "rotatable" humidity-responsive fiber bundles were fabricated by twisting three viscose fibers with varying twist levels (10-20 twists/cm) and folding them to form double-helix structures. The "contractile" humidity-responsive fiber bundles were prepared by helically winding the "rotatable" humidity-responsive fiber bundles onto steel rods and thermally setting them at 95 ℃. Morphological, structural, and mechanical properties were characterized using optical microscopy, X-ray diffraction (XRD), and tensile testing. Humidity-induced torsion and contraction behaviors were recorded under controlled water mist concentrations using optical and video analysis.

Results Research results demonstrated that thermal stretching significantly enhanced the hygroscopic expansion and mechanical performance of the viscose fibers. The swelling ratio of thermally stretched fibers increased by 64.47%, and their tensile strength improved by 12.5%. XRD results revealed a rise in molecular orientation factor of the thermally stretched fibers from 0.82 to 0.86, confirming enhanced structural order.For "rotatable" humidity-responsive fiber bundles, torsional performance strongly depended on the twist level. The optimal actuation was achieved at a twist of 18 twists/cm, producing a maximum rotation angle of 1 075.5(°)/cm and a maximum rotational speed of 65.1(°)/(cm·s) under a water mist flux of 0.11 g/s. These fibers also exhibited excellent reversibility, completing full forward and reverse rotations upon humidity cycling. The actuation mechanism was explained by a geometric model linking fiber swelling and untwisting dynamics. For "contractile" humidity-responsive fiber bundles, both twist and coil pitch significantly influenced contraction performance. The best contraction occurred at a twist of 18 twists/cm and a pitch of 0.15 cm, achieving a maximum contraction ratio of 76.7% and a contraction rate of 23.3 %/s. Increasing water vapor concentration accelerated actuation speed without affecting maximum deformation. Notably, these bundles demonstrated high humidity sensitivity. When stimulated by body-temperature vapor from evaporating water droplets, a contraction of 62.22% was witnessed within 82 s. These findings demonstrate that viscose-based fiber bundles can serve as efficient, reversible, and green actuators for low-power soft systems. A proof-of-concept "smart window" driven by the "rotatable" humidity-responsive fiber bundles illustrated their potential in adaptive, energy-free environmental control devices.

Conclusion This work developed two types of humidity-responsive viscose fiber bundles successfully with distinct actuation modes. Thermal stretching effectively improved molecular orientation and hygroscopic expansion, providing a structural foundation for enhanced actuation. The "rotatable" humidity-responsive fiber bundles exhibited superior torsional performance, while the "contractile" humidity-responsive fiber bundles achieved remarkable linear contraction and high sensitivity to ambient humidity. Structural parameters such as twist density and coil pitch were identified as key factors influencing actuation efficiency. The study provides a new design strategy for sustainable, humidity-driven soft actuators that convert environmental water vapor energy into mechanical motion without external power. Such fiber-based actuators hold great promise for applications in smart textiles, adaptive ventilation systems, and self-regulating wearable devices, offering a low-cost and eco-friendly alternative to conventional energy-consuming actuators.

Key words: viscose fiber, thermal stretching, twisting, smart fiber, humidity-responsive, driving performance