Vapor selective and controlled actuation of gelatin-based soft actuators

Abstract

Biopolymer-based soft actuators, responsive to vapor stimuli, show promise in the fields of soft robotics, smart textiles, and energy harvesting, effectively addressing the limitations of their synthetic polymer-based counterparts while offering advantages in biocompatibility and sustainability. We report rapid and fully reversible actuation characteristics of biopolymer gelatin-based soft actuators upon exposure to water and ethanol vapors. The repeatability of the actuation performance is established via extensive testing involving over 1200 vapor exposure cycles. Notably, these gelatin-based actuators display distinct responses to water and ethanol vapors. For instance, they bend upwards in water vapor and downward in ethanol vapor. By tuning the ratio of water to ethanol vapor in binary solutions, the magnitude and direction (upward or downward) of the actuation of the gelatin films can be precisely controlled. The actuation performance of gelatin film is fine-tuned with added salts and saccharides, while also exploring the effects of crystalline structure and post-annealing procedures on its properties. Furthermore, the tunability of the bending axis of the soft actuator is achieved by imprinting oriented periodic patterns onto the surface of the gelatin films. These periodic patterns not only enhance the actuation performance significantly but also aid in generating rectilinear motion of these gelatin film-based actuators. The gelatin-based soft actuators produce a lifting force of around 7 millinewtons when exposed to water vapor and can lift approximately 25 times their weight in response to ethanol vapor. Leveraging the unique vapor-responsive characteristics of the gelatin film, several proof-of-concept applications, including ethanol vapor sensors, intelligent curtains, adaptive lifts, wave-like motion, and voltage generation systems, are demonstrated.