We report the unique actuation characteristics of moisture-driven, fully reversible soft biopolymer films fabricated from Bombyx mori silk. The instantaneous actuation is driven by the water vapor induced stress gradient generated across the thickness of the film, and it possesses subsecond response and actuation times. The excellent durability and consistent performance of the film without any noticeable fatigue are established by subjecting it to more than a thousand continuous actuation cycles. The weight-lifting capability of the film is fascinating, where a few tens of micrograms of water generate a colossal force required to lift hundreds of milligrams of weight. Several other potential uses of silk fibroin based soft actuators, such as an intelligent textile layer with the crescent-shaped windows that open on perspiring skin and an autonomous crawler, are also demonstrated. Interestingly, even moisture emanating from the human palm triggers the ultrafast actuation process. These silk films are fabricated using a simple facile solution-casting technique, which can be scaled up with relative ease.
shape morphism observed in nature inspires the development of stimuli-responsive soft actuators. Vapor-responsive actuators are promising among the different stimuli-responsive materials due to their capability to produce macroscale movements in response to a minuscule amount of specific chemical vapor. Here, we report unusual multiple vapor-responsive bidirectional macroscale actuation behaviors of single-layer regenerated silk fibroin films. The vaporresponsive silk fibroin actuator exhibits antagonistic actuation characteristics in a reversible manner to both water and ethanol vapors. For instance, it produces an upward bending in the presence of water vapor and downward bending in ethanol vapor, which demonstrates the chemical vapor-specific actuation. However, the actuation characteristics remain largely invariant upon changing the polarity of alcohol molecules. The silk fibroin actuators effectively utilize the vapor-induced minuscule expansion and contraction of the film surface to produce large-scale actuation, which is fully reversible. The intrinsic water content of the films and the vapor pressure of the stimulants are exploited to control the actuation performance. Further, we demonstrated the 3D shape morphing ability of the actuator by generating an undulating wavelike motion via preprogrammed water and ethanol vapor exposure conditions. The change in the actuation direction is instantaneous, which ensures the sensitivity and rapid response of the fabricated actuators.
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