A biorobotic approach to understand ferns' spore dispersal mechanism towards the development of fast plant-inspired actuators.

Coupling high performances with limited energy consumption is a goal for the development of technologies that are sustainable and/or exploitable in extreme environmental conditions. This goal is hardly achievable in artificial systems. In nature, however, this is a common requirement for the survivance of several organisms, especially in the plant-kingdom. Plants are thus a source of inspiration in biorobotics, where the replication of some of their peculiar mechanisms is aimed to increase our knowledge of nature and to get hints for technological progress. The fern sporangium, for example, spontaneously ejects spores through one of the fastest movements in nature, resembling that of a catapult. In response to the environmental humidity and temperature, water from within sporangium cells evaporates, while its structure bends and stores elastic energy. The sudden and simultaneous nucleation of several vapour bubbles in the cells triggers the sporangium’s catapult-like return and the consequent spore dispersal. Although part of the sporangium working principles have been explained, the fluidic phenomena underlying this remarkable behaviour are not fully understood. To investigate these phenomena in controlled systems, we developed a biorobotic model: synthetic microchambers that mimic sporangium’s cells. We successfully replicated the cells’ evaporation-driven collapse and their spontaneous, catapult-like return, triggered by nucleation of vapor bubbles. We observed that this peculiar return dynamics is the result of the complex interplay between the bubble’s growth dynamics and the deformation of the surrounding, deformable membrane. We estimated the bubble nucleation pressure with a novel method based on osmosis, corroborating the previous estimation for ferns. We also observed that, to obtain simultaneous cavitation events as in the sporangium, a common hydrostatic pressure is required in contiguous microchambers. These results thus shed new light on the fundamental fluidic phenomena underlying ferns’ spore dispersal, suggesting a plant-like solution to realize fast, spontaneous machines. In the end, we partially applied this new knowledge for the realization of a fern-inspired actuator driven by capillary forces.