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Researchers make a robotic fish with a battery for blood

  Image of a plastic fish

James Pikul

You have to cheat a bit on many experimental robots. Instead of containing all the necessary electronics and energy sources, they have cables and wires that provide power and control without weighing the robot or taking up too much interior space. This is especially true for soft body robots that normally pump air or liquids to propel their movement. Including a source of electricity, pumps, and a reservoir of gas or liquid would greatly increase the weight and complexity of the robot.

A team from Cornell University has now demonstrated a clever twist that reduces the weight and density of all this by finding out how to get one of the materials to perform two functions. Like other soft robot designs, it pumps a liquid to expand and contract its structure and drive movement. In this case, however, the fluid is also the key component of a flow battery that drives the pumps. In this way they can integrate all critical components into their creation.

Stay in the river

So what is a river battery? Batteries work with different reactions that take place at their two electrodes. With something like a lithium-ion battery, the intermediaries of these reactions-electrons and ions-flow immediately from one electrode to the next, and the key chemicals spend almost all of their time at the electrodes. In flow batteries, the chemical reactions still take place at the electrodes, but the chemicals are in solution rather than limited to electrodes.

Because solutions have a limited concentration, this approach tends to limit energy density. This results in most chemicals being far below what we can achieve with lithium-ion batteries. However, they have a big advantage: the energy storage is only limited by how much liquid you can store. Thus, flow batteries are ideal for situations in which the energy density is irrelevant, z. For example, for the storage of renewable energy for use in the network.

In this case, the authors would have loved the energy density of lithium ions without a doubt. The liquid component of a flow battery, however, otherwise enabled them to increase the energy density. Soft body robots, as the name implies, do not have the kind of hard skeleton that typical robot drives can push against. Instead, their locomotion depends on the coordinated expansion and contraction of different body segments. By widening the top and at the same time tightening the bottom, a flat robot can move like an inchworm, just to give an example.

This is accomplished by pumping a type of fluid ̵

1; either fluid or gas – into or out of the flexible body; The changes in pressure can drive the required expansions and contractions. This makes the robot with the soft body perfect for the liquid used in flow batteries. The Cornell team draws an analogy between fluid and blood as it traverses all segments of the robot and can supply them with energy, both as a hydraulic force and as a source of chemical power.

The Details

That's the idea at least. How was it implemented? To reduce the weight problems associated with fluid flow around gravity, researchers decided to make a swimming robot modeled on a fish. But not just any fish – a lionfish, which has sophisticated back and pectoral fins and can perform with it representations.

The dorsal fins were not articulated in this robot; Instead, they are mainly used to store the battery / hydraulic fluid. The pectoral fins (one on each side of the robot) can move and they have a pump designed to move liquid in and out. A second pump drives swimming by filling and emptying parts of the robot body. This body also contains on-board electronics (communicating via Bluetooth) and enough free space for researchers to add more weight to prevent the robot from simply floating to the surface.

The robot goes swimming.

Chambers that fill with the battery / hydraulic fluid to move the fish and pectoral fins also contain the electrodes that the battery can work with. This is a simple carbon "felt" with nickel wiring that is separated by a membrane that prevents the two liquid components from mixing as ions intersect. The chemistry used for the battery is zinc iodide, selected for a combination of high energy density (theoretically up to half of the current lithium ion batteries), neutral pH and low viscosity (important for a hydraulic fluid).

All together this results in a very slow fish. In a low-flow basin, the fish can move about one and a half body lengths per minute, driven by its tail. That's pretty slow, but it has stamina. The authors estimate that it can swim longer than 35 hours without charge.

This is a first attempt, and the authors identified several issues with their design. For example, the capacity of the battery begins to diminish after only 10 cycles, in part because part of the battery solution is absorbed by the silicone body of the fish. Since the design was too buoyant, it should be possible to add extra energy by storing more battery fluid in the robot. Researchers also suggest that it would be more efficient to avoid inverting the pump and instead redirect fluid flow through a valve in the tubing.

And of course, the design could be simplified and made more efficient if the pectoral fins are displayed and any hardware that supports it has been eliminated. But that's not really the point; Finally, the entire design could be made much more efficient by a lithium-ion battery and a propeller. But at some point there may be things we specifically want robots of this type for, and this hardware shows a clever way of getting better performance out of them.

Nature 2019. DOI: 10.1038 / s41586-019 -1313-1 (About DOIs).

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