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Keratinous materials found in the feather shafts of flying birds possess impressive mechanical attributes, combining excellent strength-to-weight balance, toughness, and more. In this study, we investigate the shape memory effect in bird feather shafts, examining its underlying design principles as templates for bioinspired shape memory composites. Through analytical and computational analysis, we aim to uncover the underlying rules and design guidelines based on stimulus-induced softening (pertaining to strength and/or stiffness) and swelling (pertaining to expansion in volume). More specifically, we study a one-dimensional case to examine the synergistic relationship between the matrix and fibers inside the feather structure. We propose three distinct micro-mechanical modeling approaches to evaluate the contribution of each hydration-induced effect-softening, swelling, and the combined action of both. In all models, the matrix is considered to be an elastic-perfectly plastic material that is sensitive to hydration, while the fibers are treated as purely elastic and unaffected by hydration. The findings of the study provide informative insights into the nuanced nature of swelling within the material, highlighting that its desirability is dependent on specific conditions and circumstances. Furthermore, we find that the softening component plays a large pivotal role in driving the process of shape recovery. Using the proposed analytical framework and design principles, we develop a conceptual feather shaft-like composite, followed by demonstrating its tunability in degree of shape recovery and its versatility in selecting constituent base material components. This research offers valuable core framework for exploring and designing advanced bioinspired shape memory materials while eliminating the need for traditionally active shape memory components, holding promising potential for actuation, deployment, and morphing purposes. STATEMENT OF SIGNIFICANCE: This study investigates the shape-memory effect in bird feather shafts, offering bioinspired strategies for designing advanced shape-memory composites. Unlike conventional materials, which often rely on external stimuli or active components, our research focuses on hydration-driven mechanisms-specifically, matrix softening and swelling. Through micro-mechanical modeling, we demonstrate that softening is the key driver of shape recovery, while swelling plays a secondary role under specific conditions. These insights provide new, passive design principles for creating tunable shape-memory composites without the need for traditional active components. The findings have broad implications for applications in actuation, morphing, and reconfigurable systems, where material adaptability is crucial., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2025 Acta Materialia Inc. Published by Elsevier Inc. All rights reserved.)

Natural fibers have some advantages in comparison to synthetic fibers, especially because they are more environmentally friendly. For this reason, using them as a reinforcement for polymeric matrices is growing exponentially. However, they present the disadvantage of having the hydrophilic nature, which strongly reduces the interface interaction. Sedge fibers have been investigated when reinforcing an epoxy matrix in terms of ballistic properties and mechanical performance. Aiming to enhance the fiber-matrix interface, an alkali treatment was proposed. The group conditions were divided into three NaOH concentrations (3%, 5%, and 10%), as well as the three periods of immersion (24, 48, and 72 h). Therefore, nine different conditions were investigated in terms of their thermal behaviors, chemical structures, physical structures, and morphological aspects. Based on TGA curves, it could be noticed that treatments related to 3% NaOH for 24 h and 48 h exhibited better thermal stability properties. For the time of 48 h, better thermal stability with for a decay of the thermal DSC curve was shown for all treatment conditions. The FTIR spectra has shown a reduction of waxes for higher NaOH concentrations. The XRD diffractogram exhibited an increase in the crystallinity index only for 5% NaOH and an immersion time of 48 h. The morphological aspects of fibers treated with 5% and 10% of NaOH have shown that the treatments have damaged the fiber, which highlighted the crystallinity index reductions.