A soft-bodied robotic fish runs on a synthetic circulatory system that pumps a liquid with electrolytes through the body, the developers described in Nature yesterday (June 19). The robot swam against a current and moved its pectoral fins. The circulatory system and electrolyte liquidfuels a battery that allows the robot to operate for up to 36 hours.
C.A. Aubin et al., “Electrolytic vascular systems for energy-dense robots,” Nature, doi:10.1038/s41586-019-1313-1, 2019.