Bidirectional Actuation of Silk Fibroin Films: Role of Water and Alcohol Vapors.

Three-dimensional (3D) 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.

Penetrant-Induced Glass-like Transition in Thin Chitosan Films.

We present the water vapor-induced swelling and the emergence of a penetrant-induced glass-like transition in the substrate-supported glassy chitosan thin films. The time evolution of the film thickness under different levels of relative humidity conditions is measured in real-time using a spectroscopic ellipsometer equipped with a humidity cell. In a dry film, the network of chitosan chains is in a glassy state, and upon exposure to water vapor, initially, the film swells by Fickian diffusion of water molecules, which triggers the structural relaxations of the chains.

Rapid Moisture-Responsive Silk Fibroin Actuators.

We report the unique actuation characteristics of moisture-driven, fully reversible soft biopolymer films fabricated from 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.