Researchers make a robotic fish with a battery for blood

Many experimental robots involve a little cheating. Rather than contain all the necessary electronic components and energy sources, they have fasteners and cables that provide power and control without overloading the robot or taking up too much internal space. This is especially true for soft-body robots, which typically pump air or fluids to drive their movement. Incorporating a power source, pumps and a gas or liquid tank would greatly increase the weight and complexity of the robot.

A team from Cornell University has now demonstrated a clever trick that reduces the weight and density of all this by determining how to get one of the materials to perform two functions. Like other virtual robot models, it pumps a fluid so that its structure expands and contracts, generating motion. But in this case, the fluid is also the key element of a flow battery that powers the pumps. This allows them to integrate all critical components into their creation.

Go in the direction of the current

So, what is a battery of streams? The batteries work by having different reactions that take place at their two electrodes. In the case of a lithium-ion battery, the intermediates of these reactions – electrons and ions – flow immediately from one electrode to another, and the essential chemicals spend almost all their time on the electrodes. In flow batteries, chemical reactions always occur at the electrodes, but the chemicals reside in solution rather than being confined to the electrodes.

Since solutions have limitations in their concentration, this approach tends to limit the energy density of discharge batteries, placing most chemicals well below what we can achieve with lithium-ion batteries. But they have a big advantage: the energy storage is limited only by the amount of liquid you can store. This makes the Flow batteries ideal in situations where the energy density does not matter, such as renewable energy storage for use on the network.

In this case, the authors would certainly have liked to have the energy density provided by the lithium-ion. But the liquid component of a flow battery allowed them to increase the energy density in another way. Soft-body robots, as their name implies, do not have the kind of hard skeletons against which typical robotic actuators can grow. Instead, their locomotion depends on the coordinated expansion and contraction of different segments of the body. By enlarging the top surface while contracting the bottom surface, a flat robot can move like a thumb worm, to give an example.

This is managed by pumping a kind of fluid – liquid or gaseous – into or out of the flexible body compartments; pressure changes can cause the necessary dilations and contractions. This makes the flexible body robot perfectly suited to the fluid used in flow batteries. The Cornell team draws an badogy between fluid and blood, as it can travel all segments of the robot and bring in energy, both in the form of hydraulic force and chemical source. ;electricity.

The details

This is the idea at least. How was it implemented? In order to reduce the weight problems badociated with the circulation of a fluid against gravity, the researchers decided to make a swimming robot in the fish image. But not just any fish – a lionfish, which has elaborate dorsal and pectoral fins and can perform displays using them.

The dorsal fins were not articulated in this robot; instead, they are widely used for storing the battery / hydraulic fluid. The pectoral fins (one on each side of the robot's body) are able to move and have a pump dedicated to fluid circulation. A second pump causes swimming by filling and emptying sections of the robot's body. This body also contains embedded electronic components (which communicate via Bluetooth) and enough empty space for researchers to gain weight to prevent the robot from moving to the surface.

The chambers that fill with the battery / hydraulic fluid to move the fish and the pectoral fins also contain the electrodes that allow the battery to operate. It is a simple carbon "felt" with nickel cabling, separated by a membrane that prevents the two liquid components from mixing, while allowing the ions to cross each other. The chemical composition used for the battery is zinc iodide, which was chosen for a combination of high energy density (in theory, up to half that of current lithium-ion batteries ), neutral pH and low viscosity (important for a hydraulic fluid).

All this put together, and you have a fish that moves very slowly. In a low current tank, the fish can move for a minute and a half to the minute, propelled by its tail. Although it's slow enough, it has stamina. The authors estimate that he could swim more than 35 hours in a row without having to pay any charge.

This is a first attempt and the authors have identified several design issues. For example, the battery's capacity begins to fade after only 10 cycles, in part because part of the battery solution begins to be absorbed by the fish's silicon body. The design being too floating, it should be possible to add some extra power by storing more battery fluid in the robot. The researchers also suggested that it would be more effective to avoid reversing the pump and redirecting the flow of fluid through a valve in the plumbing.

And of course, the design could be simplified and made more efficient if the screen of the pectoral fin and all the material that supported it were removed. But that's not really the point; after all, the whole thing could be made much more efficient by replacing the whole design with a lithium-ion battery and a propeller. But it may be that we specifically want robots of this type, and this material is a clever way to get better performance.

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