Machines operated by hydraulic systems typically require an electrical connection or a battery pack to provide power for their pumps, increasing the devices’ weight and limiting their design flexibility. Now, researchers at Cornell University and the University of Pennsylvania have made a hydraulic system that doubles as an energy storage system. They demonstrated the concept with a small soft robot modeled after a lionfish (Nature 2019, DOI: 10.1038/s41586-019-1313-1). The robot moves forward by swishing its tail from side to side—motion provided by hydraulics—and the hydraulic fluid serves as the electrolyte of a flow battery. The team envisions the technology being used to power wearable devices of the future.