Robot fish has battery blood.

Nature powers up with a mechanical fish that runs off a circulating liquid power source:

The roughly 40-centimetre soft robot doesn’t have solid batteries — instead it is propelled by a dual-function fluid that stores energy and moves the fish’s fins. The approach allows the machine to store more energy in a smaller space and operate for longer periods without the need for heavy and cumbersome battery packs.

The innovation is a step towards creating autonomous robots — those that can perform tasks without human intervention or guidance, says Robert Shepherd, a roboticist at Cornell University in Ithaca, New York, who was part of the team that built the robot.

Instead of using conventional hydraulic fluid, which typically circulates around machines to move their parts, Shepherd’s team used a battery fluid that powered the robot and also powered a pump to move the fins, making the fish swim.

The approach increased the amount of energy stored in the robot by 325%, compared with a machine that has a separate battery and hydraulic-fluid system, says Shepherd. The team calculated that the robot would be able to function for 37 hours without requiring recharging.

You can read the research here.