Reversibly Anisotropic Soft Actuator via a Molecularly Aligned Chitosan‐Templated Ionic Network

ABSTRACT Natural actuation systems achieve adaptive motion by activating internal stress and stiffness only when required, rather than maintaining permanent structural anisotropy. In contrast, most artificial soft actuators rely on embedded fibers or layered architectures that lock anisotropy and generate motion primarily through passive swelling constraints. Here, a reversibly ion‐activated soft actuator that dynamically switches between isotropic and anisotropic mechanical states is reported. The system consists of shear‐aligned stiff chitosan (∼2.4 wt.%) chains dispersed in a poly(acrylic acid) hydrogel, where molecular alignment acts as a latent template that is mechanically inactive in the absence of ions. Upon cation exposure, the aligned chitosan directs the formation of dense ionic bridges orthogonal to the alignment, producing inverse anisotropy with higher strength perpendicular to the alignment (6.0 MPa) than parallel direction (3.0 MPa). This ion‐templated reinforcement drives robust actuation perpendicular to the alignment (67°) while suppressing deformation along the alignment direction (3°), arising from active ionic interactions rather than passive swelling. Ion removal restores mechanical isotropy, enabling repeatable and cyclable transitions. Integration with extrusion‐based 4D bioprinting further enables programmable, cross‐axis shape transformations guided by finite‐element simulations, offering a versatile platform for bio‐inspired soft actuators.