Plant-like tropisms in artificial muscles.

Helical plants have the ability of tropisms to respond to natural stimuli and biomimicry of such helical shapes into artificial muscles has been vastly popular. However, the shape mimicked actuators only respond to artificially provided stimulus, they are not adaptive to variable natural conditions, thus being unsuitable for real-life applications where on-demand, autonomous operations are required. We here demonstrate novel artificial muscles made of hierarchically patterned helically wound yarns that are self-adaptive to environmental humidity and temperature changes. Unlike shape mimicked artificial muscles, we adopt a unique microstructural biomimicking approach, where the muscle yarns can effectively replicate the hydrotropism and thermotropism of helical plants to their microfibril level using plant-like microstructural memories. Large strokes, with rapid movement, were obtained when the individual microfilament of yarn is inlaid with hydrogel and further twisted into a coil shaped hierarchical structure. The developed artificial muscle provided average actuation speed of ∼5.2%/sec at expansion and ∼3.1%/sec at contraction cycles, being the fastest amongst previously demonstrated actuators of similar type. We demonstrate that these muscle yarns can autonomously close a window in wet climates. The building block yarns are washable without any material degradation, making them suitable for smart, reusable textile and soft robotic devices. This article is protected by copyright. All rights reserved.