Helical structures are ubiquitous in natural and engineered systems across multiple length scales. Examples include DNA molecules, plants’ tendrils, sea snails’ shells, and spiral nanoribbons. Although this symmetry-breaking shape has shown excellent performance in elastic springs or propulsion generation in a low-Reynolds-number environment, a general principle to produce a helical structure with programmable geometry regardless of length scales is still in demand. In recent years, inspired by the chiral opening of Bauhinia variegata’s seedpod and the coiling of plant’s tendril, researchers have made significant breakthroughs in synthesizing state-of-the-art 3D helical structures through creating intrinsic curvatures in 2D rod-like or ribbon-like precursors. The intrinsic curvature results from the differential response to a variety of external stimuli of functional materials, such as hydrogels, liquid crystal elastomers, and shape memory polymers. In this review, we give a brief overview of the shape transformation mechanisms of these two plant’s structures and then review recent progress in the fabrication of biomimetic helical structures that are categorized by the stimuli-responsive materials involved. By providing this survey on important recent advances along with our perspectives, we hope to solicit new inspirations and insights on the development and fabrication of helical structures, as well as the future development of interdisciplinary research at the interface of physics, engineering, and biology.

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Motile plant structures such as Mimosa pudica leaves, Impatiens glandulifera seedpods, and Dionaea muscipula leaves exhibit fast nastic movements in a few seconds or less. This motion is stimuli-independent mechanical movement following theorema egregium rules. Artificial analogs of tropistic motion in plants are exemplified by shape-morphing systems, which are characterized by high functional robustness and resilience for creating 3D structures. However, all shape-morphing systems developed so far rely exclusively on continuous external stimuli and result in slow response. Here, we report a Gaussian-preserved shape-morphing system to realize ultrafast shape morphing and non-volatile reconfiguration. Relying on the Gaussian-preserved rules, the transformation can be triggered by mechanical or thermal stimuli within a microsecond. Moreover, as localized energy minima are encountered during shape morphing, non-volatile configuration is preserved by geometrically enhanced rigidity. Using this system, we demonstrate a suite of electronic devices that are reconfigurable, and therefore, expand functional diversification.

The programmable shape transition of a two-dimensional sheet to a three-dimensional (3D) structure in response to a variety of external stimuli has recently attracted increasing attention. Among the various shape changing materials, shape memory polymers (SMPs) can fix their temporary shape and/or their length and recover under proper thermal treatment. In this work, we create a bilayer composite by bonding one layer of elastomer with one layer of stretched SMPs, which can undergo a series of shape transitions via the storage and release of internal stresses. The programed shapes are achieved by adjusting the orientation and elongation of the SMPs. Meanwhile, the 3D structures exhibit tristability and can transit between hemihelical, left-handed helical, and right-handed helical shapes. Both theoretical analysis and finite element simulations were conducted to understand the mechanism of shape transformation and used to predict the deformed configuration by adjusting preprogramming parameters. Our work provides a new strategy and design space for fabricating smart reconfigurable structures and paves way for the design and development of bioinspired four-dimensional active matter for a broad range of applications in intelligent materials.

Bistable structures featuring two stable states have been widely applied in designing fast and high-force-output actuators under various types of stimuli, such as mechanical force, swelling, thermal expansion, and so on. In this paper, we designed a magneto-actuated mechanism to realize the reversible shape transition between two curved stable configurations of a buckled beam using magneto actuation. The beam is composed of a silicone elastomer matrix with embedded micro-sized iron particles. The magnetic response of these iron particles endows the composite beam with the ability to snap from one stable shape to the other when the magnitude of the surrounding magnetic field exceeds the threshold value. By separately analyzing the electric-magnetic field and the magnetic-mechanical field, we formulate a simple and efficient computational method to numerically predict the critical current on the onset of snap-through. The computational and experimental critical currents show good agreement for different material and geometrical parameters, including the thickness of the beams, iron particle mixing ratios of the material, and the distances of the beam to the electromagnet. The proof-of-concept design is demonstrated to be efficient in the application of a magneto-responsive soft switch and a catapult for ejecting small objects, providing new insights into designing contactless, low-voltage-actuated bistable structures.

Controlling adhesion on demand is essential for many manufacturing and assembly processes such as microtransfer printing. Among various strategies, pneumatics‐controlled switchable adhesion is efficient and robust but currently still suffers from challenges in miniaturization and high energy cost. In this paper, a novel way to achieve tunable adhesion using low pressure by inducing sidewall buckling in soft hollow pillars (SHPs) is introduced. It is shown that the dry adhesion of these SHPs can be changed by more than two orders of magnitude (up to 151×) using low activating pressure (≈−10 or ≈20 kPa). Large enough negative pressure triggers sidewall buckling while positive pressure induces sidewall bulging, both of which can significantly change stress distribution at the bottom surface to facilitate crack initiation and reduce adhesion therein. It is shown that a single SHP can be activated by a micropump to manipulate various lightweight objects with different curvatures and surface textures. Here, it is also demonstrated that an array of SHPs can realize selective pick‐and‐place of an array of objects. These demonstrations illustrate the robustness, simplicity, and versatility of these SHPs with highly tunable dry adhesion.

Morphogenesis, commonly found in leaves, [1-3] flowers, [4,5] cones, [6] seed pods [7,8] and other biological systems, is typically driven by differential growth, swelling, or shrinkage [6,9,10] that occurs within multilayered components of species. For example, the opening and closing of pine cones are attributed to the tissue's self-bending, which undergoes three states of humidity-driven deformation. [6] As these morphological changes in nature result from the variation of the surrounding environment, it is desirable to mimic these natural examples to fabricate multilayered structures that can spontaneously respond to various external stimuli, such as temperature, pH, biochemical enzymes, magnetic fields, and solvent composition, [11-14] which can find a variety of applications, such as semiconductor nanotubes, [15-18] soft robotics, [19-22] snapping surface, [23] and micro/ nanoelectromechanical systems. [24-26] For a multilayer structure, the misfit strain across layers can lead to some interesting phenomena such as multistability, where more than one stable state exists with the same boundary conditions or control parameters of the system. One specific case is called neutral stability, in which the system can stay stable at each point in a continuous path of shape change. In such a case, the system is said to have zero stiffness because the potential energy of the system keeps unchanged during the shape change and theoretically no external force is needed. Various cases that incorporate neutrally stable (zero stiffness) systems have been studied. Guest et al. [27,28] discovered neutral stability of a heated copper beryllium strip, the shape of which depended on the residual stresses. The strip has zero stiffness for finite deformation along

Currently soft robots primarily rely on pneumatics and geometrical asymmetry to achieve locomotion, which limits their working range, versatility, and other untethered functionalities. In this paper, we introduce a novel approach to achieve locomotion for soft robots through dynamically tunable friction to address these challenges, which is achieved by subsurface stiffness modulation (SSM) of a stimuli-responsive component within composite structures. To demonstrate this, we design and fabricate an elastomeric pad made of polydimethylsiloxane (PDMS), which is embedded with a spiral channel filled with a low melting point alloy (LMPA). Once the LMPA strip is melted upon Joule heating, the compliance of the composite structure increases and the friction between the composite surface and the opposing surface increases. A series of experiments and finite element analysis (FEA) have been performed to characterize the frictional behavior of these composite pads and elucidate the underlying physics dominating the tunable friction. We also demonstrate that when these composite structures are properly integrated into soft crawling robots inspired by inchworms and earthworms, the differences in friction of the two ends of these robots through SSM can potentially be used to generate translational locomotion for untethered crawling robots.