Water-driven actuation of Ornithoctonus huwena spider silk fibers
Shuyuan Lin1,2, Jia Zhu3, Xinming Li4, Yang Guo5, Yaopeng Fang2,6, Huanyu Cheng3 more...
1State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
2Center for Nano and Micro Mechanics, Tsinghua University, Beijing 100084, China
3Department of Engineering Science and Mechanics, Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, USA
4Department of Electronic Engineering, The Chinese University of Hong Kong, Hong Kong, China
5Institute of Biomechanics and Medical Engineering, Tsinghua University, Beijing 100084, China
6School of Aerospace Engineering, Tsinghua University, Beijing 100084, China
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Spider silk possesses remarkable mechanical properties and can lift weight effectively. Certain kinds of spider silk have unique response to liquid, especially water, because of their hydrophilic proteins, β-sheet characters, and surface structure. The Ornithoctonus huwena (O. huwena) spider is a unique species because it can be bred artificially and it spins silk whose diameter is in nanometer scale. In this work, we report the “shrink–stretch” behavior of the O. huwena spider silk fibers and show how they can be actuated by water to lift weight over long distance, at a fast speed, and with high efficiency. We further rationalize this behavior by analyzing the mechanical energy of the system. The lifting process is energy-efficient and environmentally friendly, allowing applications in actuators, biomimetic muscles, or hoisting devices.
The interaction between matter and liquid contributes to the structural changes of materials and has broad prospects for the applications of actuators, sensors, and flexible devices.1–3 Spider silk is a natural biological material that is highly sensitive to water because of its hydrophilic proteins, β-sheet characters, and surface structure.4 Many previous studies have explored the interaction between spider silk and water, including the mechanism, behavioral performance, and possible applications5–7 (see Table S1, supplementary material). Spider silk such as Major Ampullate silk was found to supercontract when immersed in water,8 i.e., it shrank in length and expanded in volume. It is generally accepted that the supercontraction resulted from the disruption of hydrogen bonding among amorphous proteins and the disordering of protein domains when water infiltrated the silk.9 The Nephila clavipes spider silk was expected to act as high performance biomimetic muscles that could be driven by humidity alone, because it could undergo cyclical contractions as humidity changes and give a power output 50 times greater than that of human muscle of equivalent mass.10 The Uloborus walckenaerius spider silk was found capable of collecting water.11 Within a moist atmosphere, it could quickly and efficiently collect water droplets due to the Laplace pressure differences and the surface energy gradient between its unique “spindle-knot” and “joint” structural features. More recently, the Nephila edulis spider silk with tiny glue droplets on its surface was found to possess a novel solid–liquid behavior.12 This kind of thread was a particularly stretchy solid, but could also shrink without apparent limit into a state that resembled liquid. Buckling of the thread within the droplet induced the shape-switching behavior of the silk.
Here, we report that the Ornithoctonus huwena (O. huwena) spider silk fibers show a similar solid–liquid behavior in the presence of water, even without glue droplets on the surface. That is, the water droplet could trigger the shrinkage of the O. huwena silk fibers. Accordingly, we experimented with its complete shrinkage and recovery cycle driven by water droplet: shrank the silk within the water droplet and let it stretch back into the original shape when the water droplet fell. This kind of spider silk fibers could also serve as lightweight biomimetic muscles that could, upon actuation by water droplet, complete long-distance lifting cycles with fast speed and high efficiency.
The O. huwena spider silk is a kind of flexible, crack-free and lightweight fiber.13 It is extruded from the spinnerets that are usually on the underside of the spider's abdomen (Fig. 1(a)). The fine spider webs created from this proteinaceous silk can be collected with a piece of cardboard (Fig. 1(b)). A bundle of spider silk fibers (diameter: 10–40 μm) is then separated from the spider webs with tweezers (Fig. 1(c)). The three-dimensional (3D) laser scanning confocal microscopy (LSCM) collects a set of real-time scanning images at different depths of the spider silk fibers, and in Fig. 1(d), the 3D fasciculus structure is reconstructed from the obtained images. In addition, the scanning electron microscopy (SEM) images of the O. huwena spider silk fiber in Fig. 1(e) demonstrate the fibrillar structure formed by spider silk threads with smooth surfaces and dimensions at the nanometer scale (232 ± 40 nm).
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