Abstract
Autonomous shape transformation is key in developing high-performance soft robotics technology; the search for pronounced actuation mechanisms is an ongoing mission. Here, we present the programmable shape morphing of a three-dimensional (3D) curved gel structure by harnessing multimode mechanical instabilities during free swelling. First of all, the coupling of buckling and creasing occurs at the dedicated region of the gel structure, which is attributed to the edge and surface instabilities resulted from structure-defined spatial nonuniformity of swelling. The subsequent developments of post-buckling morphologies and crease patterns collaboratively drive the structural transformation of the gel part from the "open" state to the "closed" state, thus realizing the function of gripping. By utilizing the multi-stimuli-responsive nature of the hydrogel, we recover the swollen gel structure to its initial state, enabling reproducible and cyclic shape evolution. The described soft gel structure capable of shape transformation brings a variety of advantages, such as easy to fabricate, large strain transformation, efficient actuation, and high strength-to-weight ratio, and is anticipated to provide guidance for future applications in soft robotics, flexible electronics, offshore engineering, and healthcare products.
Highlights
Soft matter-based biosystems widely exist in nature to support lives such as octopus, starfish, caterpillars, etc., by fulfilling adaptive shape changes and responsive motions to allow them to survive in complex environments.[1−5] Inspired by these features, a number of soft actuator concepts[6−11] have been developed to mimic dedicated actuations/motions, e.g., soft grippers actuated by inflation of a pneumatic network to manipulate fragile and irregular objects,[12] a humidity- and light-driven liquid crystal network actuator[13] to mimic selfshape morphing of flowers, and bellow-like actuators[14] with origami structures enabled various motions
A 3D curved gel structure with the shape of a “semi-cylindrical shell” is designed and fabricated by synthesizing the poly(acrylamide-co-sodium acrylate) hydrogel (PAAm-co-NaAc) in a 3D printed mold, as shown in Figure 1c
Morphological developments are observed on the specific locations: buckling (Figure 1d,e) occurs at the axial edges and reticulated creases (Figure 1f,g) appear on the circumferential outer surface
Summary
Soft matter-based biosystems widely exist in nature to support lives such as octopus, starfish, caterpillars, etc., by fulfilling adaptive shape changes and responsive motions to allow them to survive in complex environments.[1−5] Inspired by these features, a number of soft actuator concepts[6−11] have been developed to mimic dedicated actuations/motions, e.g., soft grippers actuated by inflation of a pneumatic network to manipulate fragile and irregular objects,[12] a humidity- and light-driven liquid crystal network actuator[13] to mimic selfshape morphing of flowers, and bellow-like actuators[14] with origami structures enabled various motions. The discovery of superior bio-inspired robotic structure/mechanisms with desired working capacity, efficient actuation, high strength-toweight ratio, on-demand shape programmability, and low cost is highly desired for frontier engineering applications. Substantial efforts have been devoted to developing a planar structure with a multilayer,[48,49] different responsiveness,[50] or density gradient[51,52] to improve the controllability and efficiency of instability-induced 3D shape transformations or preparing homogeneous hydrogel structures upon constrained swelling[53] (Figure 1a,b). The developments of post-buckling geometries and crease patterns enable the directional releasing of strain energy, drive the shape transformation of the gel structure from the “open” state to the “closed” state, and realize the function of gripping. By optimizing the inputs from the gel composition and geometrical design, we achieve swelling-driven programmable shape morphing and demonstrate the potential application as an autonomous gripper
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