In Situ AFM Elucidation of Moisture-Responsive Expansion Behavior of Chitosan/Alginate Nanofibers and Enhanced Moisture-Driven Actuation Performance

Abstract

Moisture-driven actuators are intelligent materials that convert abundant ambient moisture into mechanical deformation. High-performance moisture-driven materials have been developed through material hybridization and structural design. However, the complexity of components and structures influences macroscopic characterization through multiple factors, limiting in-depth research on the intrinsic moisture-driven mechanisms. In this study, an in situ controllable-humidity AFM system was constructed to investigate the moisture-responsive behavior of chitosan/sodium alginate (CSSA) nanofibers. This method enables tracking of the same nanofiber during continuous humidity variations, allowing direct visualization and quantification of nanoscale moisture-responsive expansion. A hypothetical core–shell model was proposed based on the observed correlation between fiber diameter and expansion behavior. The thickness of the expansion layer in this model is approximately 100 nm, and the saturated expansion ratios of CSSA nanofibers at 30%, 50%, and 70%RH were calculated to be 12%, 16%, and 26%, respectively. To further enhance the actuation performance, graphene oxide (GO) was introduced to form an interconnected CSSA/GO network that provides additional moisture-adsorption sites and pathways for moisture transport. The assembled actuators exhibit excellent deformation capability in practical demonstrations, including microgrippers, microcranes, and biomimetic structures. This study establishes a clear structure–mechanism–performance relationship and offers valuable insights into the design and optimization of high-performance moisture-driven actuators.