Building Tiny Batteries to Power Cell-Sized Robots

The zinc-air battery is 0.1 millimeters long and 0.002 millimeters thick. (Image: Courtesy of the researchers)

A tiny battery designed by MIT engineers could enable the deployment of cell-sized, autonomous robots for drug delivery within the human body, as well as other applications such as locating leaks in gas pipelines.

The new battery, which is 0.1 millimeters long and 0.002 millimeters thick — roughly the thickness of a human hair — can capture oxygen from air and use it to oxidize zinc, creating a current with a potential of up to 1 volt. That is enough to power a small circuit, sensor, or actuator, the researchers showed.

“We think this is going to be very enabling for robotics,” said Michael Strano, the Carbon P. Dubbs Professor of Chemical Engineering at MIT and the senior author of the study. “We’re building robotic functions onto the battery and starting to put these components together into devices.”

Here is an exclusive Tech Briefs interview, edited for length and clarity, with Strano.

Tech Briefs: What was the biggest technical challenge you faced while developing the battery?

Strano: The battery is a solution to an even greater technical challenge that few people really know about. If you want to shrink down a robot to the size of a single cell, one of the most surprising things that happens is that the energy you have available to you becomes punishing; it doesn't scale.

The way the math works is if you shrink a device down to a very small scale you don't have enough energy to, seemingly, do anything. Energy becomes very punishing at the micron scale, and that's the single biggest hurdle to making a robot the size of a single cell.

Why do we want to do that? What's the vision of the field? We want to bring electronics to areas that are inaccessible otherwise; there are countless applications for that. But, the first thing that you need is to build a robot around an energy source. We set out to build the highest power density battery that you can make for a micron-scale robot.

What we came up with is what's called an air-breathing battery. We didn't invent that concept; people are working on laptop batteries that are air breathing. What does it mean to be air breathing? The idea is that the energy has to come from a chemical reaction. If you can find a chemical reaction that uses oxygen as one of the reactants, then part of your chemical energy now comes from the environment. That means your little battery can effectively store a lot more energy, because some of it is coming from the oxygen.

We decided to go with an air-breathing battery for the microrobot because a lot of the applications we're thinking of, there's oxygen around. It allows you to pack that much more energy into the small space. The first rule of microrobotics is, if you can be air breathing, you want to be air breathing.

“We think this is going to be very enabling for robotics,” said Michael Strano, the Carbon P. Dubbs Professor of Chemical Engineering at MIT. (Image: Courtesy of the researchers)

Tech Briefs: Can you explain in simple terms how it works?

Strano: The battery is a zinc element that we put in close proximity to a platinum element; they make up the anode and cathode. Then, you connect those with a load, and they function as a battery.

In the paper we look at a couple of different scenarios. One is that it can react to zinc phosphate. As it reacts to zinc phosphate, it releases electrons, and those electrons can flow into your load. It can function like a battery in that way. So, the zinc stores the energy, and it flows through the system.

In making a battery of that type, we put it onto a small square about the size of a cell that we could lift off and have it float around in solution. In the paper we show a couple different micro loads that you can connect to it — showing that the battery's able to power the load.

So, it can power timing circuits. It can power two different kinds of nanosensors. It can power an actuator. The battery can be a central hub around which to build other robotic functions. We've started doing that, and, in our paper, we showed that; we're continuing that now — building around this little battery.

Tech Briefs: Do you have plans for further research work, research work, etc.?

Strano: There's a lot that we're working on. Right now, we're working on basic functions, we're putting together the functional Legos, if you will. We're putting together the functional building blocks that could go into making a robot. Things like movable legs; sensors that can turn the flow from this battery off and on; propulsion; memory; data processing; all of that stuff. We're putting the pieces together.

My lab, in the next few months and years, is going to be showing more and more sophisticated devices. Now is an exciting time. It's a time in the field where we're trying to hook these functional parts together and get them to do more sophisticated things. We have an effort in the lab that's working on steering, working on homing, sensing plus turning the battery on, memory. Communication is a big thing, small particles that can communicate. But, none of that is possible without a central energy source. Even with living things — you start off with an energy source.

Tech Briefs: The article I read says, “…Designing tiny robots that could be injected into the human body where they could seek out a target site and then release a drug such as insulin for use in the human body. The researchers envisioned that the devices would be made of biocompatible materials that would break apart once they were no longer needed.” Have you done any work on that, or have you plans for that?

Strano: We have, we've done some work. We've done some simulations. We also come at it from a couple different angles. We work on responsive therapeutics, which are a new generation of drugs that can measure some therapeutic target in the body. A good example would be glucose responsive insulin. So, pharmaceutical companies and researchers have made insulin that can detect glucose, and, depending on its concentration, can activate or change the potency of the insulin.

We've done a lot of theory and modeling of the human body. You can think of them as like proto-therapeutic robots. We're doing that strategically to understand how those work. And then maybe as a platform to build on. We've done simulations in the literature, where we've looked at devices. We're able to model and map the response of the human body pretty well using computational models. And all of this is sort of gearing toward: can we start to put tools and devices of this kind inside the human body?

Tech Briefs: The article also says that the researchers are working on increasing the voltage of the battery, which may enable additional applications. Do you have any updates you can share?

Strano: The zinc air-breathing battery that I described has a voltage of about one volt. One volt is decent. There are things you can do with one volt, and we show it. You can power a sensor; you can move an actuator; you can access memory; and you can communicate. But, there's more that you can do if you can get that voltage higher. In the devices we have around us, anything that has more than one battery is, in part, trying to boost the voltage. That's one reason why you have two batteries in series in your remote control.

You can do a little bit of that, but you can't do that forever. So, we are looking deeper into the chemistry and trying to access higher voltages. A higher voltage means more devices that you can power, right?