Mechanics-guided design of shape-morphing composite sheets with hard and soft materials

Nature abounds with structures capable of changing shapes upon exposure to stimuli. These shape-morphing structures (for instance, the conifer pinecone) are often comprised of passive hard phases embedded in active soft materials that are stimuli-responsive. Although enthusiasm in mimicking the shape-changing phenomena in natural systems has induced an impetus to develop various artificial self-shaping structures, the effect of hard phases on the shape transformation and its underlying mechanics have not been systematically investigated. In this paper, we conduct finite element modeling to simulate the shape transformation of hybrid composite sheets consisting of fibers embedded in the hydrogel matrix, which is a widely employed approach to construct self-folding structures. It is revealed that the orientation of the folding axis is dictated by the hard fibers when the fiber modulus is sufficiently larger than that of the hydrogel matrix. Guided by the mechanics analysis, we construct self-folding composite sheets by incorporating 3D-printed stiff polylactic acid (PLA) patterns into soft poly(N-isopropylacrylamide) (PNIPAm) gels. The modulus of PLA exceeds that of PNIPAm by five orders of magnitude. Upon stimulation of elevated temperature, the PNIPAm/PLA composite sheets exhibit directional folding with folding axis parallel to the hard PLA strips, contrasting with most reported hybrid hydrogel sheets in which folding axes were perpendicular to the embedded fibers. Various 3D morphologies, including tubes, helices, scrolls, have been achieved by programming the embedded PLA patterns. We also generate an analog of curled leaves and the results offer mechanistic understandings of the curling process of leaves in nature. We hope this work can provide guidance for designing self-shaping soft machines that are made by integrating hard and soft materials together.