Pine cone scale-inspired motile origami

Abstract

Stimuli-sensitive hydrogels have received attention because of their potential applications in various fields. Stimuli-directed motion offers many practical applications, such as in drug delivery systems and actuators. Directed motion of asymmetric hydrogels has long been designed; however, few studies have investigated the motion control of symmetric hydrogels. We designed a pine cone scale-inspired movable temperature-sensitive symmetric hydrogel that contains Fe3O4. Alignment of Fe3O4 along the magnetic force is key in motion control in which Fe3O4 acts like fibers in a pine cone scale. Although a homogeneous temperature-sensitive hydrogel cannot respond to a temperature gradient, the Fe3O4-containing hydrogel demonstrates considerable bending motion. Varying degrees and directions of motion are easily facilitated by controlling the amount and alignment angle of the Fe3O4. The shape of the hydrogel layer also influences the morphological structure. This study introduced facile and low-cost methods to control various bending motions. These results can be applied to many fields of engineering, including industrial engineering. Implanting iron oxide powder in a temperature-sensitive hydrogel produced a material that bends and opens similar to a pine cone. Most approaches to fabricating muscle-like hydrogel actuators rely on combining two materials that have different volume expansions. The approach developed by Kahye Song and Sang Joon Lee of Korea's POSTECH now makes it possible to manipulate single-component hydrogels in a low-cost manner. The team poured thin coatings of iron oxide powder over single layers of poly(N-isopropylacrylamide), PNIPAAm, and then hardened the layers together while applying a permanent magnetic field. By fine tuning the field direction, this procedure generated fiber-like structures that introduced origami-like folds into the PNIPAAm gel. Octagonal folding patterns gave concave structures that resembled closed pine cones; a subsequent temperature stimulus provoked the structures to stretch out and open like flower petals. We designed various bending hydrogels with symmetric structure that responds to temperature gradient, with inspiration from pine cone. We verified the motion changes based on the amount of content, alignment angle and shape of the layer. The developed system can be applied to various fields, because the direction and curvature can be controlled easily.