The process of 3D-printing hygroscopic materials inherently creates directional strands within each material layer along the extruded path. When exposed to drying or wetting, a single material layer will undergo corresponding shrinking or drying perpendicular to the 3D-printed strands; combined with a non-reactive material layer printed in the perpendicular direction, the material layer shrinks or swells on one side, leading to an overall bending of the resulting bilayer structure.

By connecting opposing bilayers with a flexible curved hinge, both bilayers will bend in opposite orientations – causing a fold to emerge along the curved hinge. These principles allow the 4D-printed structures to be initially fabricated in a flat state and then dynamically self-shaped based on the interplay between their curved hinge geometry, bilayer material structure, and the surrounding humidity.