Bilayer Actuator with Overload Protection on the Basis of Semicrystalline Polyurethane

Response to external stimuli plays a significant role in the environmental adaptation of living matters and intelligent devices. Most stimulus-response systems in nature can respond to appropriate stimuli, and inhibit the response under excessive stimuli, such as excessive heat or water, which can be called overload protection. However, even though various responsive materials have been developed for different stimuli, most of them are not protective against the overload stimuli. In this work, a bilayer actuator based on semicrystalline polyurethane is designed, which can respond differently to proper stimuli and excessive stimuli, i.e., water. This actuator can bend gradually under the proper stimulation of water, but will straighten and even bend reversely with excessive stimulation. The mechanism behind the reversible and adjustable actuator with overload protection is investigated both experimentally and theoretically, and the competition between dynamic factors and thermodynamic stability in the swelling process is considered the main cause.