Bio-inspired paper plant robots: artificial heliotropism and nyctinasty through transpiration

Self-regulation is a fundamental feature of all living systems, which maintains their metabolic activities through biochemical feedback loops. Those feedback loops respond to environmental stimuli, and therefore the organism displays ‘embodied intelligence’. Despite the complexity of these feedback loops, using them as a source of inspiration can open new endeavors in soft robotics to design self-regulating systems, and ultimately to fuse embodied intelligence into soft robots. Here we show some simple systems, in which plant-inspired soft robots display heliotropism (tracking the sun) and nyctinasty (opening and closing its leaves) through material feedback and artificial transpiration for self-regulation – all being examples of embodiment of intelligence. First, materials feedback is adapted to a hard robotic system behaving similar to a soft robot, displaying heliotropism and nyctinasty by using the phase transition of shape memory alloy springs. Then, artificial transpiration is integrated into a soft robotic system, using swelling/deswelling of hydrogels on the paper body by using origami/kirigami strategies. Both material and transpiration feedback involve stabilization with cycles of traction and contraction (of shape memory alloys, or hydrogels) events that keep the robots at a metastable state, maximizing of solar flux on the leaves (decorated with solar panels) for higher efficiencies of harvesting light. Moreover, the feedback mechanism of the hydrogel-based system can be advanced by using hybrid hydrogels (e.g. an addition of thermoresponsive hydrogels), or doping the gel with light-absorbing chemicals, or altering the geometrical design of the systems. Finally, achieving self-regulation in soft robots through material/transpiration feedback is important to attain the embodiment of intelligence in them, and this may have implications extending from energy efficiency to adaptability in autonomous soft robots.