This story begins thousands of feet up in the air with migratory birds, and ends with a robotic fish swimming through the water below. To prepare for their journeys, birds fatten up big time, perhaps doubling their weight, essentially turning themselves into feathered batteries. Over many days and many miles, they burn that energy reserve to power their wings and keep themselves from starving and freezing. Eventually they reach their destinations emaciated.

A fine idea—thought engineers from Cornell and the University of Pennsylvania—for a new energetic system for machines. It got them thinking: fat is a cool battery, but not necessarily feasible to replicate in a robot. But blood? In a human, blood distributes oxygen and energy for cells throughout the body. And fluid, in the form of hydraulics, already powers some robots. So why not modify that fluid to carry energy, as our blood powers our own muscles?

Matt Simon covers cannabis, robots, and climate science for WIRED.

What they’ve landed on is not a robot bird (way too complicated and energy-intensive) but a robot lionfish that uses a rudimentary vasculature and “blood” to both energize itself and hydraulically power its fins. This technology is still in its very early days—and indeed this fish is exceedingly slow—but perhaps some machines of tomorrow could ditch clunky batteries and wires and power themselves like biological organisms. Think machines made more like Cylons than toasters.

The robots of today are stubbornly segmented. They’ve got a lithium ion battery, which distributes energy by way of wires to motors in the limbs, known as actuators. This new robotic lionfish does indeed have batteries, but they are sprinkled throughout its body and operate in conjunction with two pumps—one for powering the pectoral fins and the other for the tail. Together, the batteries and pumps act more like biological hearts than a lithium ion in a traditional robot.

James Pikul

The first component is the “blood,” essentially a charged hydraulic fluid with dissolved ions, which gives it a chemical potential for powering electronics. “Hydraulic fluid transmits force, and only force,” says Cornell roboticist Robert Shepherd, coauthor on a new paper in Nature describing the system. “In our fluid, we're transmitting force and we're transmitting electrical power.”

This charged fluid flows through battery cells in the fish’s abdomen and fins. Each cell has two opposing pieces of metal: a cathode and an anode. As the fluid flows past these, it creates a charge imbalance, or a voltage that causes electrons to flow through the electronics that power the two pumps. These in turn keep the fluid pumping. Eventually the battery cells will die, as the fluid loses ions, and the fluid will stop circulating. At that point you can recharge the fluid to keep the fish going. “You could actually drain the fluid and inject more charged fluid,” says Shepherd, “sort of like filling your gas tank at the gas station.”