Abstract
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.
Highlights
Helical structures behave as building blocks in Nature across several length scales ranging from nanoscale DNA macromolecules and bacterial flagella [1] to millimeter-scale plant tendrils [2] and sea snail shells [3]
Recent progress on designing programmable self-morphing low-molar-mass chemicals that have revolutionized the networks (LCNs) or liquid crystal elastomers (LCEs) materials can be and contraction along the molecular alignment. This anisotropic response is inherent in LCNs or LCEs microstructures without the introduction of additional reinforcement or the integration of different material compositions, which is a distinct advantage compared to hydrogel systems
In the following sub-sections, we focus on the helical structures made of LCNs or LCEs that mimic Bauhinia variegata’s pod opening in terms of the responsive stimuli ranging from the most widely used ones, heat and light, to less-frequently used ones including chemical, humidity, and helical shape in response to stimuli, and this actuation behavior resembles seedpod chiral opening with many similar behaviors
Summary
Helical structures behave as building blocks in Nature across several length scales ranging from nanoscale DNA macromolecules and bacterial flagella [1] to millimeter-scale plant tendrils [2] and sea snail shells [3]. One example is the chiral opening process of Bauhinia variegata’s pods [11], where an initially flat pod valve changes its shape into a 3D helix via a hygroscopic process. Another example is the coiling of a plant’s tendrils such as. We provide a brief review on recent progress in the synthesis of helical structures relying on shape transition from a 2D rod-like or ribbon-like configuration to a 3D helix mimicking Bauhinia variegata’s pod opening and tendril coiling. Carbon-nanotube-based helical fibers that consist of hierarchical chiral building blocks are introduced as examples following the coiling mechanism before the tendril finds the support. We close with an outlook on the potential opportunities and future challenges in the field
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