Coconut pollen paper soft actuator: tunable shape morphism driven by vapor

The development of innovative devices capable of producing substantial mechanical responses from minimal environmental stimuli is crucial for energy conversion strategies. Owing to pressing environmental concerns, pollen grains, renowned for their natural durability and widespread availability, have emerged as a promising candidate for such applications. In this study, an eco-friendly, flexible, reversible and multi-vapor-responsive paper-like soft actuator is constructed from monocolpate pollen grains derived from <i>Cocos nucifera</i>. Driven by the natural evaporation of water and alcohol vapors, the direction of bending shifts with the type of vapor, morphing from a "smiley-faced" to a "sad-faced" shape. Altering the geometry of the actuator or adjusting the ambient humidity tunes the shape morphism of the actuator. Furthermore, the direction of the bending axis in response to vapor is controlled through a simple, periodic surface structuring technique. A comprehensive finite element analysis is employed to elucidate the influence of these surface corrugations on the directional bending behavior of the actuator. The mechanical integrity of the pollen paper under prolonged exposure to water vapor is also thoroughly investigated. This work highlights the complex interplay between pollen-based materials, environmental stimuli and structural design in ridged soft actuators, underscoring their potential in the development of responsive and adaptive systems.