This Wireless Brain Implant Is Smaller Than a Grain of Salt

As its whiskers flitter, the mouse’s brain sparks with activity. A tiny implant records the electrical chatter and beams it to a nearby computer.

Smaller than a grain of salt, the implant is powered by and transmits data with light. Unlike most implants, it moves with the brain to reduce scarring. Dubbed MOTE, the device reliably captured electrical signals for a year in mice—about half their lifespan—without obvious damage.

“The long-term recording of neural activity could be used to understand complex behaviors and disorders,” Sunwoo Lee at Nanyang Technological University, Alyosha Molnar at Cornell University, and team wrote in a recent paper describing the implant.

Penny for Your Thoughts

Brain implants are helping decode—and restore—the neural signals behind our thoughts, memories, movements, and behavior.

Most devices rely on arrays of microelectrodes inserted into the brain, though some sit on the brain’s surface to minimize damage. From translating neural activity into computerized speech to restoring movement in people with paralysis, these devices have already transformed lives.

But there’s a major drawback. Most implants use wires plugged into a port embedded in a person’s skull to transfer signals, requiring extensive surgery. Implanted electrodes, although small, are like fixed pins inside a wobbly Jell-o block. They can’t move with brain tissue. Over time, scarring reduces the implants’ efficiency, and the hardware triggers inflammation.

Scientists have been tackling these roadblocks with clever ideas like wireless implants that transmit data on radio frequencies, a bit like walkie-talkies. “Neurograin” implants, for example, record and stimulate the brain wirelessly and transmit data to a thin electrical patch on the scalp. Other devices use ultrasound for power and to send signals to a controller.

But most wireless implants are still bulky, “equivalent to a sizable fraction of the mouse brain,” wrote the team.

Then there’s the ultimate enemy: Time. The brain is bathed in fluid for nourishment and waste removal, but this soupy concoction eats away at electrical components. Although some methods can capture neural activity over many months with a microscope and implanted light probes, they only work in genetically engineered mice with glow-in-the-dark neurons.

A durable wireless implant for living brains has so far escaped scientists’ grasp.

“Our goal was to make the device small enough to minimize that disruption while still capturing brain activity faster than imaging systems, and without the need to genetically modify the neurons for imaging,” Molnar said in a press release.

Power of Light

The new MOTE device, smaller than a grain of salt, combines electronics and LEDs for wireless recording and communication.

Red and infrared light penetrate the scalp, skull, and brain with minimal distortion, making them useful energy sources. The device has a diode that turns those wavelengths of light into electrical energy—a bit like those inside solar panels—to power the device. Once the implant captures electrical signals from the brain, it sends them to a computer on short pulses of light.

Like morse code, the exact timing and duration of the pulses reflect neural activity. This technology is widely used in satellite communication, wrote the team, and requires very little power to operate.

MOTE’s onboard electronics are like computer chips. Each packs 186 transistors, which form the basis of three main circuits. One circuit boosts recorded brain signals, another recodes them into light pulses, and the third drives LEDs for transmission to a computer.

These components are protected by a custom sheath made by coating the implant one atomic layer at a time. The ultra-thin sheath protects MOTE from the brain’s corrosive environment. Each fabrication step can be done in parallel, making nearly 100 devices at the same time.

“As far as we know, this is the smallest neural implant that will measure electrical activity in the brain and then report it out wirelessly,” said Molnar.

Live Long and Prosper

In a first test, the devices reliably captured electrical activity from heart muscle cells in petri dishes, suggesting they worked as intended.

The team next implanted the device into a unique part of the mouse brain. Mice heavily depend on their whiskers to navigate the world. These signals are processed in the barrel cortex. A range of electrical patterns capture sensations and generate twitches in each whisker.

Some mice received the implant on top of their brains, instead of penetrating into the delicate tissue. But most had the device implanted using a nanoinjector. Over the next year, the device faithfully transmitted data from the barrel cortex when scientists tickled the mice’s whiskers. It detected activity from single neurons and neural network activity associated with behavior.

MOTE seemed mostly harmless. None of the mice experienced seizures or other neurological issues sometimes seen in larger implants. They skittered around and chowed down on food as usual. There was also very little scarring around the implant, even after a year.

The devices aren’t just for decoding mouse brains. They could one day pick up electrical signals from organoids—so-called mini-brains. Organoids loosely mimic the early stages of brain development. Although tiny, they’re densely packed with multiple types of brain cells and connections, making it difficult for bulkier implants to record signals without damage.

Upgraded with better detection and light-emission hardware, MOTEs could theoretically work up to six millimeters deep in the brain, enough to record from the entire mouse brain and in organoids, wrote the team.

They’re still far from clinical use, but making implants wireless means they’re more compatible with brain imaging technologies, such as fMRI (functional magnetic resonance imaging), which could paint a wider picture of brain activity during tasks. Outside the brain, MOTE could tap into the spinal cord, heart, or other tissues and record dynamic movies of their health.