and huanyu.cheng@psu.eduDOI: http://dx.doi.org/10.1063/1.4974350ABSTRACTSpider 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, -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 shrinkstretch 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.13 Spider silk is a natural biological material that is highly sensitive to water because of its hydrophilic proteins, -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 applications57 (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 solidliquid 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 solidliquid 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