Super-Flexible Water-Proof Actuators

Humidity-responsive materials hold broad application prospects in sensing, energy production, and other fields. Particularly, humidity-sensitive, flexibility, and water resistance are pivotal factors in the development of optimized humidity-responsive materials. In this study, hydrophobic linear polyurethane and hydrophilic 4-vinylphenylboronic acid (4-VPBA) form a semi-intercross cross-linking network. This copolymer of polyurethane exhibits excellent humidity-sensitive, mechanical properties, and water resistance. Its maximum tensile strength and maximum elongation can reach 40.56 MPa and 543.47%, respectively. After being immersed in water at various temperatures for 15 days, it exhibited a swelling ratio of only 3.28% in water at 5 degrees C and 9.58% in water at 70 degrees C. While the presence of 4-VPBA network imparts humidity-sensitive, reversible, and multidirectional bending abilities, under the stimulus of water vapor, it can bend 43 degrees within 1.4 s. The demonstrated material surpasses current bidirectional humidity actuators in actuating ability. Based on these characteristics, automatically opening waterproof umbrellas and windows, as well as bionic-arms, crawling robots, and self-propelled boats, are successfully developed. The reversible conditions of boron-oxygen bond cleavage and formation, triggered by humidity, endow the prepared material with favorable humidity-responsive capabilities. When subjected to humidity stimulation, the material undergoes minor volume changes, enabling it to exhibit actuation behavior. Additionally, the material exhibits robust mechanical performance under normal humidity conditions. The material simultaneously possesses favorable tensile strength and elongation at break. image