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In this work, a polyelectrolyte complex (PEC) fiber of alginate (ALG) and poly­(diallyldimethylammonium chloride) (PDDA) is prepared by restricting polyelectrolyte complexation with salt to fit wet-spinning protocol. The ALG/PDDA PEC fiber adsorbs water from the environment fast and shows a quick mechanical response to humidification. When the environment humidity is lower than 65%, the fiber performs like a plastic material, with a modulus of ∼109 Pa. As the humidity increases above 65%, the fiber exhibits a sharp plastic–rubber transition with a significant modulus drop to ∼106 Pa. After the rubber plateau is reached, the modulus continues to decline when further increasing humidity. Furthermore, the ALG/PDDA fiber presents a humidity-induced shape memory effect. The humidity-adaptive behaviors of ALG/PDDA have potential for different applications, such as actuation and sensing.

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