MIT scientists build hair-size batteries that can power cell-sized robots

Researchers have developed a hair-thin battery that can power robots no larger than the dot at the end of this sentence.

The zinc-air battery captures oxygen from its surroundings and oxidizes miniscule amounts of zinc, a reaction that can create up to 1 volt. This energy can then power things like sensors or a tiny robotic arm that can raise and lower to deliver a payload – say, insulin directly into the cells of a person with diabetes.

While cell-sized robots have long been proposed to deliver drugs to specific locations in the body, powering them has been tricky. Many current designs use solar power, which means they must either be exposed to sunlight or be controlled by a laser. But neither penetrates far into the body, limiting how far such robots, nicknamed "marionettes," because they must remain connected to this light source like a puppet string, can travel.

"The marionette systems don’t really need a battery because they’re getting all the energy they need from outside," study senior author Michael Strano, a chemical engineer at MIT, said in a statement. "But if you want a small robot to be able to get into spaces that you couldn’t access otherwise, it needs to have a greater level of autonomy. A battery is essential for something that’s not going to be tethered to the outside world."

The new battery is among the smallest ever invented. In 2022, researchers in Germany described a millimeter-sized battery that can fit on a microchip. Strano and his team's battery is around 10 times smaller, at just 0.1 millimeters long and 0.002 millimeters thick. The average human hair is about 0.1 millimeter thick.

The battery has two components, a zinc electrode and a platinum electrode. These are embedded in a polymer called SU-8. When the zinc reacts with oxygen from the air, it creates an oxidation reaction that releases electrons. These electrons flow to the platinum electrode.

The batteries are made by a process called photolithography, which uses light-sensitive materials to transfer nanometer-sized patterns onto silicon wafers. This method is commonly used to make semiconductors. It can quickly "print" 10,000 batteries per silicon wafer, Strano and his colleagues reported Aug.14 in the journal Science Robotics.

In the new study, the researchers used a wire to connect these itsy-bitsy batteries to cell-sized robots, which Strano's lab also develops. They tested the battery's ability to power a memristor, a circuit which changes resistance based on the amount of charge flowing through it; these memristors can store memories of events based on changes in charge.

They also used the batteries to power a clock circuit, enabling robots to track time, and to power two nano-sized sensors, one made of carbon nanotubes and the other of molybdenum disulfide. Microsensors like these could be released into pipelines or other hard-to-reach places to detect leaks, according to the researchers.

"We’re making the basic building blocks in order to build up functions at the cellular level," Strano said.

The team also used the batteries to move an arm on one of their tiny robots, which are about the size of a human egg cell. These miniscule dynamics could allow for medical robots that work inside the body to release drugs at a certain time or place.

In the future, the team hopes to ditch the wires, and build their batteries into their micro-robots.

"This is going to form the core of a lot of our robotic efforts," Strano says. "You can build a robot around an energy source, sort of like you can build an electric car around the battery."