Ion-responsive chitosan hydrogel actuator inspired by carrotwood seed pod.

Inspired by the gradient hygroscopic structure of carrotwood seed pod, patterned anisotropic structure was created in polysaccharide hydrogel by an anodic electrical writing process. Locally released Fe2+ was oxidized to Fe3+ and chelated with chitosan chains in the written area, resulting in a gradient structure in the hydrogel. The asymmetrical stress generated by the different swelling of the gradient structure enables the hydrogel to bend autonomously. The hydrogel shows opposite bending in deionized water and NaCl solution. The physicochemical properties of the hydrogel are characterized by tensile test, SEM, EDS, XRD, TGA, DTG and FT-IR. SEM and EDS show that the written hydrogel has a structural gradient and a concentration gradient of Fe3+ vertically. Moreover, anodic electrical writing increases the flexibility of chitosan hydrogel due to decreased crystallinity. This controllable electrical writing technique is convenient to create patterned anisotropic structure and provide a novel design concept for natural hydrogel actuators.