Energy Storage in Stiff Hydrostatic Skeletons

The multifunctionality of energy components allows versatility and compactness in biology. For example, fat, as energy storage in many animals, doubles as insulation and as a structural element. Multifunctional energy storage can also improve the operating time of robots. In this work, we demonstrate that a liquid energy source can be used to create a controllably stiff “hydroskeleton” type structure for use in robots. This hydroskeleton type structure can be combined with tendon actuation for fast, efficient motion. We characterize this method of stiffening for a generalizable structure, and use it in a demonstration soft marine creature inpired robot’s motion. We adopt a flow battery with this scheme, where the battery electrolyte is used as the fluid in the hydroskeleton, optimizing materials and methods for incorporating a flow battery into a soft robot, and improving lifetime and performance. The power system and actuation would interact symbiotically: the motors would be powered by the flow battery to make the electrolyte flow inside the robot, which provides controllable stiffness to help the robot swim. Simultaneously, the electrolytic hydraulic fluid increases energy density and reduces system resistance. We show why flow battery soft robots are most competitive in swimming applications and discuss more possibilities for multifunctional fluid energy storage in soft robots.